# Could someone see anything while being inside black hole?

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If we managed to survive in a black hole and move inside the event horizon then could we see the surroundings of the black hole inside the event horizon by source of light? Can the light not come up to the event horizon or does it have to travel through different spheres of the black hole and enter a sphere which won't allow the light to return to its previous sphere? If the we reach up to the singularity then can we ever reach up to the event horizon or would we get stuck in the point of the singularity?

The answer is most definitely yes, or at least yes, as far as our current understanding of how gravity works goes. It is observationally untestable (let's be more specific - nobody could report the results of an observational test!) since no signal can emerge from inside the event horizon.

The scenario is treated in some detail by Taylor & Wheeler ("Exploring Black Holes", Addison, Wesley, Longman - highly recommended) in terms of what an observer would see on a direct radial trajectory into a non-rotating "Schwarzschild" black hole. I won't bore you with the maths - it is fairly complex.

A star situated at exactly 180 degrees from the observer's radial path will always appear in that position as the observer looks back - right down to the singularity. The light will be gravitationally blueshifted by an ever-increasing amount - essentially tending towards an infinite blueshift at the singularity.

For stars at an angle to the radial path, their positions will be distorted such that they appear to move away from the point at which the observer has come from (and are also blue-shifted). In the final moments (it takes less than $1.5 imes 10^{-4}$ seconds of proper time to fall from the event horizon to the singularity of a 10 solar mass black hole, but a huge $sim 60$ seconds for the black hole at the centre of our Galaxy) the light from the external universe will flatten into an intense ring at 90 degrees to the radial direction of motion. So you would end up seeing blackness in front of you, blackness behind and the sky split in two by a dazzling ring of light (almost seems worth it!).

Sort of. There are several videos that I've watched on YouTube that explain this phenomenon but it's all theoretical obviously. The overly simplified story is that as you approach the the black hole the comes a point that the gravity is neither too strong to suck light into the event horizon (the point that light cannot escape the gravity of the black hole) nor weak enough to allow it to leave and what you end up seeing is a warped version of what entered the black hole. Vsauce does a pretty good job of explaining some of this in this video at around 3:30. Visual of a possibility of what the inside of a blackhole of at 6:00 https://www.youtube.com/watch?v=3pAnRKD4raY

Another visualization: https://www.youtube.com/watch?v=GYKyt3C0oT4

## Could someone see anything while being inside black hole? - Astronomy

Let's hear about it from the master of the theory to begin with instead.

I could have done without the 40+ minutes of analogies. If he dumbed it down any further his audience would be the Sesame Street crowd.

Using his black hole as an example, would not someone outside of our light horizon see our universe as a two-dimensional representation, as in his hologram analogy? While we, inside the light horizon (similar to being inside the event horizon of the black hole), are three-dimensional objects, like Alice?

Also, if it is so hot on the surface of the event horizon of the black hole, would not that imply that this jumbled-up two-dimensional representation of everything inside the black hole also has mass?

I could have done without the 40+ minutes of analogies. If he dumbed it down any further his audience would be the Sesame Street crowd.

Using his black hole as an example, would not someone outside of our light horizon see our universe as a two-dimensional representation, as in his hologram analogy? While we, inside the light horizon (similar to being inside the event horizon of the black hole), are three-dimensional objects, like Alice?

Also, if it is so hot on the surface of the event horizon of the black hole, would not that imply that this jumbled-up two-dimensional representation of everything inside the black hole also has mass?

That video was made for TVO, not truly representative of his actual lectures on topics.

And in the channel search just type "Leonard" and you'll see actual college level lectures that you might enjoy more, Glitch.

Would someone outside of our light horizon see anything at all? After all, that's our boundary. Hard to speculate on that answer.

Just because the black hole would have a 2D surface, doesnt imply that it wouldnt have mass. Black holes do in fact have mass.

That video was made for TVO, not truly representative of his actual lectures on topics.

And in the channel search just type "Leonard" and you'll see actual college level lectures that you might enjoy more, Glitch.

Would someone outside of our light horizon see anything at all? After all, that's our boundary. Hard to speculate on that answer.

That certainly seems to be true. After all, like he said, we each have our own individual light horizon. So it only matters where you are at the time. Someone a billion light years away would have a completely different light horizon. So unlike his two-dimensional representation of the contents of a black hole, there would be nothing to distinguish our light horizon from the rest of space/time.

I am aware that black holes have mass. What I was wondering was whether or not the creation of this two-dimensional surface on the "skin" of the event horizon also had mass, in addition to the mass already contained within the event horizon. The fact that it gets so hot would imply that it does have mass.

So an object passing through the event horizon of a black hole would not only transfer a two-dimensional image of the object to its event horizon, but also some of its mass, as it crosses that line of no return.

I am aware that black holes have mass. What I was wondering was whether or not the creation of this two-dimensional surface on the "skin" of the event horizon also had mass, in addition to the mass already contained within the event horizon. The fact that it gets so hot would imply that it does have mass.

So an object passing through the event horizon of a black hole would not only transfer a two-dimensional image of the object to its event horizon, but also some of its mass, as it crosses that line of no return.

I dont think the 3D versus 2D matters for mass. Because an object is 2D or 3D wouldnt imply it has less or more mass. If the energy dissipated via Hawking Radiation from a black hole was more or less than what it took in, then that might imply differently. But the black holes shrink proportionate to their energy release.

The 2D image of the object is the actual object, transformed like a hologram would be from 3D to 2D. All of the information is there, it's just "flat". And the information carries with it the full mass and energy of the original object.

When the energy is dissipated into the Universe again, the 2D surface area becomes more "pixely" as bits of the information are radiated at different times. Eventually all of the bits are gone, and voooosh, black hole is no longer.

Otherwise we'd be violating conservation of energy and all that jazz.

I dont think the 3D versus 2D matters for mass. Because an object is 2D or 3D wouldnt imply it has less or more mass. If the energy dissipated via Hawking Radiation from a black hole was more or less than what it took in, then that might imply differently. But the black holes shrink proportionate to their energy release.

The 2D image of the object is the actual object, transformed like a hologram would be from 3D to 2D. All of the information is there, it's just "flat". And the information carries with it the full mass and energy of the original object.

When the energy is dissipated into the Universe again, the 2D surface area becomes more "pixely" as bits of the information are radiated at different times. Eventually all of the bits are gone, and voooosh, black hole is no longer.

Otherwise we'd be violating conservation of energy and all that jazz.

That makes absolutely no sense whatsoever. How can an object passing the event horizon of a black hole transfer not only an image of itself but also all of its mass to the event horizon and still have the same mass as it passes the event horizon? That would be doubling the mass of every object that passes the event horizon, and we know that does not happen.

Holograms are merely photons, and photons at rest have no mass. Therefore, the two-dimensional image "imprinted" on the event horizon of a black hole should have no mass. However, he said that the surface of the event horizon would be exceedingly hot. Therefore, something with mass must be generating that heat under the extreme gravitational pressure of the black hole. We know photons cannot have any mass (otherwise they would not be able to travel at 299,792 km/s), therefore if the "skin" of an event horizon is truly as hot has he claims, then something with mass, other than photons, was also transferred to the "skin" of the event horizon as the object passed through into the black hole.

That also means that there is some mass loss from the object to the event horizon, albeit a very small amount, as the object passes the event horizon into the black hole.

That makes absolutely no sense whatsoever. How can an object passing the event horizon of a black hole transfer not only an image of itself but also all of its mass to the event horizon and still have the same mass as it passes the event horizon? That would be doubling the mass of every object that passes the event horizon, and we know that does not happen.

Holograms are merely photons, and photons at rest have no mass. Therefore, the two-dimensional image "imprinted" on the event horizon of a black hole should have no mass. However, he said that the surface of the event horizon would be exceedingly hot. Therefore, something with mass must be generating that heat under the extreme gravitational pressure of the black hole. We know photons cannot have any mass (otherwise they would not be able to travel at 299,792 km/s), therefore if the "skin" of an event horizon is truly as hot has he claims, then something with mass, other than photons, was also transferred to the "skin" of the event horizon as the object passed through into the black hole.

That also means that there is some mass loss from the object to the event horizon, albeit a very small amount, as the object passes the event horizon into the black hole.

When we say the "image" of itself, it means that all of the 3D mass is transformed into 2D mass. It's still fully there, mass, energy, and all.

Holograms is really just the consumer-world analogy that we use to describe the 3D/2D/3D transformation process.

Photons are traditionally said to be massless. This is a figure of speech that we use to describe something about how a photon's particle-like properties are described by the language of special relativity.

The logic can be constructed in many ways, and the following is one such. Take an isolated system (called a "particle") and accelerate it to some velocity v (a vector). Newton defined the "momentum" p of this particle (also a vector), such that p behaves in a simple way when the particle is accelerated, or when it's involved in a collision. For this simple behaviour to hold, it turns out that p must be proportional to v. The proportionality constant is called the particle's "mass" m, so that p = mv.

In special relativity, it turns out that we are still able to define a particle's momentum p such that it behaves in well-defined ways that are an extension of the newtonian case. Although p and v still point in the same direction, it turns out that they are no longer proportional the best we can do is relate them via the particle's "relativistic mass" mrel. Thus

When the particle is at rest, its relativistic mass has a minimum value called the "rest mass" mrest. The rest mass is always the same for the same type of particle. For example, all protons have identical rest masses, and so do all electrons, and so do all neutrons these masses can be looked up in a table. As the particle is accelerated to ever higher speeds, its relativistic mass increases without limit.

It also turns out that in special relativity, we are able to define the concept of "energy" E, such that E has simple and well-defined properties just like those it has in newtonian mechanics. When a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass mrel, then its energy E turns out to be given by

E = mrelc2 , and also E2 = p2c2 + m2restc4

There are two interesting cases of this last equation:

If the particle is at rest, then p = 0, and E = mrestc2.

If we set the rest mass equal to zero (regardless of whether or not that's a reasonable thing to do), then E = pc.

In classical electromagnetic theory, light turns out to have energy E and momentum p, and these happen to be related by E = pc. Quantum mechanics introduces the idea that light can be viewed as a collection of "particles": photons. Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass. That way, equation (1) gives the correct expression for light, E = pc, and no harm has been done. Equation (1) is now able to be applied to particles of matter and "particles" of light. It can now be used as a fully general equation, and that makes it very useful.

## The Geometry Near Black Holes

In general relativity, the spacetime around stars or planets is described by a spherically symmetric spacetime interval. The spacetime interval tells us what the separation is between points in spacetime it is analogous to finding the distance in space using the Pythagorean theorem in flat space, but including a time part, and accounting for the curvature of spacetime.

We can make the spherical symmetry more explicit by separating out the angular and radial parts of the spacetime interval, where we are using the fact that any point in space can be described by its distance from the origin in a particular direction. In Figure 11.1, we compare different types of coordinate systems (Cartesian: (x,y,z,t) vs. spherical: (d,&theta,A,t) ).

Figure 11.1: Points in space can be described by an xyz position, called Cartesian coordinates (after the French mathematician René Descartes), or by an d&thetaA set of coordinates, as shown. The angles &theta and A are like longitude and latitude, respectively. However, since we are not restricting ourselves to the surface of a globe, as on Earth, we must also specify how far from the origin the point is located: that is what the coordinate d does. Credit: NASA/SSU/Aurore Simonnet

In spherical coordinates, the angle &theta is like latitude on Earth and the angle A is like longitude. For a spherically symmetric space (one that is the same in every direction) the orientation of the poles is arbitrary. Only the distance from the origin is important. Under this circumstance we can ignore the angular parts of the coordinates in spacetime because they don't tell us anything meaningful about the geometry of the space.

This means for a spherically symmetric spacetime (one in which the spatial part forms a spherically symmetric 3-space) we do not need to worry about any terms in the spacetime interval with &theta and A. They don't change when developing the spacetime interval from the 3-space interval. We can focus solely on the terms with the radial distance (d) and time (t), since those are the only things that affect the geometry. With these considerations, the spacetime interval (s 2 ) around a spherically symmetric object is that stated below.

[ s^2=left(1-frac<2GM> ight)^ <-1>(Delta d)^2 - left( 1-frac<2GM> ight) (cDelta t)^2 + r^2 Delta Omega^2]

Here G is the universal gravitational constant, c is the speed of light, M is the mass of the object, d is the distance from the origin, and t is the time. The angular part is given by the last term, with which we do not have to concern ourselves it is the same as for flat space and does not contribute to spacetime curvature.

The spacetime interval may look complicated, but the curvature depends only on the distance from the origin, d, and time, t. Those are the only terms where we find a difference from the flat space interval, the one used for special relativity. The curvature has no dependence on the direction considered. This is clear from the lack of modifications to the angular term in the expression for the interval. That is what is meant by spherically symmetric.

The interval describes the Schwarzschild geometry, discovered by the German astronomer Karl Schwarzschild (1873&ndash1916) in 1915. It applies to any spherically symmetric gravitating system, including non-spinning black holes&mdashwe will have a look at spinning black holes later.

Figure 11.2: Karl Schwarzschild was a German astrophysicist. He was the first person to solve Einstein&rsquos equations for a spherically symmetric system. Credit: Bildarchiv Preussischer, Kulturbesitz, Berlin

An interesting historical aspect of Schwarzschild&rsquos discovery is that it was made during his service in the German army during World War I. Schwarzschild had volunteered for the military at the outbreak of the war in 1914. He was stationed on the Russian front as an artillery officer when he wrote his two papers on general relativity in the following year&mdashthe very year Einstein published the theory. The papers presented the solution we have presented, that for a spherical, nonrotating body.

Because Schwarzschild was occupied at the front, he was unable to present his papers at scientific meetings in Berlin. Instead, he sent copies of them to Einstein, who presented them in his absence. The papers were well received, and Einstein wrote to Schwarzschild that he had been surprised to see that the solution could be so simple.

While we now use Schwarzschild&rsquos solutions to describe the spacetime around black holes, Schwarzschild himself stated that he thought their description of objects whose escape speed exceeded that of light was mere mathematics, with no basis in reality&mdashan opinion which Einstein also held. Schwarzschild never had the chance to explore the idea further. While on the Russian front, he contracted pemphigus, an autoimmune disease in which the immune system attacks the skin. He died of the condition in the spring of 1916, shortly after his relativity papers had been presented by Einstein.

Figure 11.3: We can visualize the spatial part of the stretching of spacetime as shown in this diagram. The stretching occurs in all three spatial dimensions and is spherically symmetric. The time part of the distortion cannot be shown in this diagram, of course. Credit: NASA/SSU/Aurore Simonnet

The Schwarzschild spacetime interval applies to any spherically symmetric gravitating object. We can use it to study the orbits of planets around the Sun, and even the bending of light as it passes through the Sun&rsquos gravity. However, there is something a little peculiar about the Schwarzschild geometry. Notice the curvature terms in the large parentheses. They contain the ratio:

You might have noticed that this expression is the same one we derived in Section 11.1 for the size of a black hole. In the Newtonian derivation, it is the point where the escape speed becomes equal to the speed of light. This quantity has been given the name Schwarzschild radius, RS, and it is quite important, as we will see. We can rewrite the Schwarzschild interval in terms of the Schwarzschild radius, and its importance will become more apparent.

[ s^2=left(1-frac<>>d ight)^ <-1>(Delta d)^2 - left( 1-frac<>> ight) (cDelta t)^2 + r^2 Delta Omega^2]

Look at what happens to the curvature terms as the radial coordinate (d) approaches RS: The time curvature term goes to zero, but the spatial (radial) curvature does not. If RS = d, then RS/d = 1 and 1 - RS/d = 1 - 1 = 0. In fact, since the radial curvature term is the reciprocal of the time curvature term, it must grow without bound as r approaches RS, because 1/0 is infinite. What does this mean? Clearly something interesting is happening as we approach (R_S).

## Sorry, Black Holes Aren't Actually Black

The simulated decay of a black hole not only results in the emission of radiation, but the decay of . [+] the central orbiting mass that keeps most objects stable. Black holes are not static objects, but rather change over time. For the lowest-mass black holes, evaporation happens the fastest, but even the greatest-mass black hole in the Universe won't live past the first googol (10^100) years.

Most of us are confounded by the idea of relativity when we first encounter it. Objects don't just move through space, but through time as well, and their motions through both are inseparably intertwined into the fabric of spacetime. Moreover, when you add gravity into the mix, you find that mass and energy affect the curvature of spacetime by their presence, abundance, density and distribution, and that curved spacetime dictates how matter and energy move through it.

If you gather enough mass together in a particular volume of spacetime, you'll create an object known as a black hole. Surrounding every black hole is an event horizon: the boundary between where an object could escape from the black hole's gravitational pull and where everything irrevocably falls towards the central singularity. But despite that no objects from inside the event horizon escapes, black holes aren't actually black. Here's the story of how.

When a massive enough star ends its life, or two massive enough stellar remnants merge, a black hole . [+] can form, with an event horizon proportional to its mass and an accretion disk of infalling matter surrounding it. When the black hole rotates, the space both outside and inside the event horizon rotates, too: this is the effect of frame-dragging, which can be enormous for black holes.

ESA/Hubble, ESO, M. Kornmesser

When General Relativity was first presented to the world in 1915, it revolutionized our understanding of space, time, and gravitation. Under the Newtonian picture, we had previously viewed space and time both as absolute entities: it was as though you could put a coordinate grid on over the Universe and describe every point with three spatial coordinates and one time coordinate.

The revolution that Einstein brought was twofold. First, these coordinates were not absolute, but relative: every observer has their own position, momentum, and acceleration, and observes a unique set of space-and-time coordinates that are distinct from all other observers. Second, any particular coordinate system doesn't remain fixed over time, as even observers at rest will be pulled by the motion of space itself. Nowhere is this more evident than around a black hole.

Black holes are renowned for absorbing matter and having an event horizon from which nothing can . [+] escape, and for cannibalizing its neighbors. But this does not imply that black holes suck everything in, will consume the Universe, or are completely black. When something falls in, it will emit radiation for all eternity. With the right equipment, it may even be observable.

X-ray: NASA/CXC/UNH/D.Lin et al, Optical: CFHT, Illustration: NASA/CXC/M.Weiss

Instead of viewing space as a fixed network of three-dimensional streets, it's perhaps more accurate to view space as a moving walkway. No matter where you are in the Universe, the space beneath your feet is being dragged by all the gravitational effects at play. Masses cause space to accelerate towards them the expanding Universe causes unbound objects to speed away from one another.

Outside a black hole's event horizon, any matter gets attracted towards the black hole, but collisions and electromagnetic interactions can accelerate that material in a variety of directions, including to funnel it away from the black hole itself. Once you cross inside the event horizon, however, you can never escape. The space beneath your feet accelerates towards the singularity faster-than-light. Although this sounds like science fiction, we've actually imaged a black hole's event horizon. Lo and behold, just as Schwarzschild predicted in 1916, event horizons are real.

In April of 2017, all 8 of the telescopes/telescope arrays associated with the Event Horizon . [+] Telescope pointed at Messier 87. This is what a supermassive black hole looks like, where the existence of the event horizon is clearly visible. Only through VLBI could we achieve the resolution necessary to construct an image like this, but the potential exists to someday improve it by a factor of hundreds. The shadow is consistent with a rotating (Kerr) black hole.

Event Horizon Telescope collaboration et al.

This is a property of relativity that isn't generally appreciated. You'll often hear it stated that nothing can move faster than the speed of light, and this is true, but only if you understand what moving means. Motion always has to be relative to something else there's no such thing as absolute motion. In the case of moving relative to the speed of light, that's motion relative to the fabric of space itself: relative to the motion that a particle released from rest would experience.

Matter and energy cannot move faster-than-light, but space itself has no such restrictions. Outside of an event horizon, the fabric of space moves slower than the speed of light you can still escape from a black hole's gravitational pull by accelerating quickly enough. Inside the event horizon, though, all paths than matter or light can take will only lead it to one place: the central singularity.

Both inside and outside the event horizon, space flows like either a moving walkway or a waterfall, . [+] depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.

Andrew Hamilton / JILA / University of Colorado

With that in mind, you might start wondering about just how black these objects — black holes — truly are. If nothing that crosses the event horizon can ever get out again, you might think that it's only the matter that remains outside the event horizon that's ever viewable. That the Universe outside the event horizon may still be visible, but the event horizon itself will be a completely black surface, devoid of any light of any type. You might think that, since nothing that falls in can escape, black holes emit nothing at all.

If that's what you think, you're not alone: this is one of the most common and popular misconceptions of all time concerning black holes. But if you really think that black holes are completely black, and that you can never see anything that falls into one, there are two things you must consider. Either one ought to be enough to change your mind.

An illustration of an active black hole, one that accretes matter and accelerates a portion of it . [+] outwards in two perpendicular jets, is an outstanding descriptor of how quasars work. The matter that falls into a black hole, of any variety, will be responsible for additional growth in both mass and event horizon size for the black hole. Despite all the misconceptions out there, however, there is no 'sucking in' of external matter.

1.) Think about the matter that falls into a black hole. Black holes grow in mass whenever anything from outside the event horizon crosses the event horizon and falls in. Even though black holes don't actually suck matter into them, they do grow whenever particles cross into the region of no return surrounding them. If you were the infalling matter that entered inside the event horizon, once you crossed over, it's true that you'd never come back.

But what if you remained outside the event horizon, and watched someone else fall in? Remember that space itself is moving, that space and time are related, and that the phenomena described by relativity are real and must be reckoned with. At the event horizon itself, space is moving at the speed of light. Which means, to someone infinitely far away, time at the event horizon no longer appears to pass.

This artist’s impression depicts a Sun-like star being torn apart by tidal disruption as it nears a . [+] black hole. Objects that have previously fallen in will still be visible, although their light will appear faint and red (easily shifted so far into the red they are invisible to human eyes) in proportion to the amount of time that's passed since they crossed the event horizon.

ESO, ESA/Hubble, M. Kornmesser

When you observe something else fall into a black hole, you'd see that the light emitted from them would get fainter, redder, and their position would asymptote towards the event horizon. If you could continue to observe the faint photons they emitted, they'd appear to get stretched out in space and stretched out in time. They'd experience gravitational redshift, with the light emitted from them going from visible to infrared to microwave to radio frequencies.

Any yet, it will never disappear entirely. There will always, infinitely far into the future, be light to observe from their fall into a black hole. Even though photons are quantized, there is no limit to how low their energy can be. With a large-enough telescope sensitive to long-enough wavelengths, you should always be able to see the light from anything that fell into a black hole. As someone falls in, their light never completely goes away.

An illustration of the zero-point energy of space itself: the quantum vacuum. It is filled with . [+] tiny, short-lived fluctuations, which observers that accelerate at different rates (or that exist in regions where the curvature of space is different) will disagree as to what the lowest-energy (ground state) of the quantum vacuum is.

2.) Think about the quantum nature of space outside the event horizon. If you're in purely empty space, where there's no matter, energy, or radiation occupying your space, you might think that all inertial (non-accelerating) observers would agree on what the properties of that space is. But if you're talking about the space outside of a black hole, that's not possible.

Why not? Two reasons, in tandem, ensure it:

• the vacuum of perfectly empty space isn't completely empty, as it unavoidably contains quantum fluctuations,
• and the fact that the fabric of space itself is accelerating at different rates depending on your distance from the central singularity.

Combine these two things, and an inescapable situation arises: different observers will disagree on what the true lowest-energy state of the quantum vacuum near a black hole is.

An illustration of heavily curved spacetime, outside the event horizon of a black hole. As you get . [+] closer and closer to the mass's location, space becomes more severely curved, eventually leading to a location from within which even light cannot escape: the event horizon. The radius of that location is set by the mass of the black hole, the speed of light, and the laws of General Relativity alone. Observers close to the black hole versus observers far away would disagree as to what the zero-point energy of the quantum vacuum was.

Pixabay user JohnsonMartin

If you're far away from the black hole, you can approximate space as not accelerating where you are, and so observers nearby will all agree with one another when they refer to the quantum vacuum. But when you consider the quantum vacuum near the black hole's event horizon — in other words, in a region of space where the curvature is severely non-flat — the quantum vacuum appears to be in an excited state.

Why? Because your view of what appears flat is different than an observer who's near the event horizon. In order to transform from their perception of flat (which is curved to you) to your frame-of-reference, you have to calculate what you'd perceive differently from what they'd perceive. Whereas they'd just see empty space, you, from far away, see copious amounts of radiation emanating from the curved space near the event horizon.

The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even . [+] light, can escape. But outside the event horizon, the black hole is predicted to emit radiation. Hawking's 1974 work was the first to demonstrate this, and it was arguably his greatest scientific achievement.

NASA DANA BERRY, SKYWORKS DIGITAL, INC.

This is what Hawking radiation actually is: the radiation you'd observe because your perception of the quantum vacuum is different in flat space than it is in curved space. This is a more correct way of visualizing Hawking radiation than Hawking's own explanation of particle-antiparticle pairs created near a black hole, where one falls in and the other escapes, for the following set of reasons:

• Hawking radiation is almost exclusively photons, not particles or antiparticles,
• Hawking radiation doesn't all originate from the event horizon, but from within about 10-20 Schwarzschild radii of the event horizon,
• if you calculate the energies of particle-antiparticle pairs that arise near the event horizon by combining quantum mechanics and General Relativity, you get the right average value but the wrong energy spectrum you need to eschew Hawking's explanation to get the right answer.

Hawking radiation is what inevitably results from the predictions of quantum physics in the curved . [+] spacetime surrounding a black hole's event horizon. This visualization is more accurate than a simple particle-antiparticle pair analogy, since it shows photons as the primary source of radiation rather than particles. However, the emission is due to the curvature of space, not the individual particles, and doesn't all trace back to the event horizon itself.

But this is a real form of radiation. It has real energies and a calculable energy distribution for its photons, and you can calculate both flux and temperature of this radiation based on the black hole's mass alone. Perhaps counterintuitively, the more massive black holes have smaller amounts of lower-temperature radiation, while lower-mass black holes decay more quickly.

This can be understood once you realize that Hawking radiation is strongest where space is the most severely curved, and more severe spatial curvature occurs closer to a singularity. Smaller mass black holes mean smaller volume event horizons, and that means more Hawking radiation, faster decays, and higher-energy radiation to look for. With the right long-wavelength, large-diameter telescope, we may someday be able to observe it.

As black holes lose mass due to Hawking radiation, the rate of evaporation increases. After enough . [+] time goes by, a brilliant flash of 'last light' gets released in a stream of high-energy blackbody radiation that favors neither matter nor antimatter.

If you have an astrophysical object that emits radiation, that immediately defies the definition of black: where something is a perfect absorber while itself emitting zero radiation. If you're emitting anything, you aren't black, after all.

So it goes for black holes. The most perfectly black object in all the Universe isn't truly black. Rather, it emits a combination of all the radiation from all the objects that ever fell into it (which will asymptote to, but never reach, zero) along with the ultra-low-temperature but always-present Hawking radiation.

You might have thought that black holes truly are black, but they aren't. Along with the ideas that black holes suck everything into them and black holes will someday consume the Universe, they're the three biggest myths about black holes. Now that you know, you'll never get fooled again!

## How to "show" someone a black hole?

I know they can't be seen, but if you wanted to show someone a black hole, what would you choose for a target?

### #2 Dan Finnerty

Sagittarius A*. Of course, it cannot be seen at visible wavelengths, but its coordinates are precisely know and you can target your telescope to it and describe the wonderful monster hidden within: http://en.wikipedia. /Sagittarius_A*

### #3 Kidastronomer

Look at any Serfeyt Galaxy's core!

### #4 Cames

. if you wanted to show someone a black hole, what would you choose for a target?

On a clear night with excellent seeing and using very high magnification, I would aim my telescope at the center of M87 in Virgo. There I may see a filamentous jet of material excited by the massive black hole in the center of that galaxy. The jet points to the location of the black hole.

### #6 David Knisely

I know they can't be seen, but if you wanted to show someone a black hole, what would you choose for a target?

/Ira

One of the nearest known black holes that has been identified with certainty is Cygnus X-1. It orbits a magnitude 8.9 star in Cygnus known as HD 226868 (R.A. 19h 58m 21.7s, Dec. +35d 12' 5.8", eq. 2000.0) with a period of about 5.6 days. The star is relatively easy to find even in fairly small telescopes, as it forms a close optical double with a 10th magnitude star about 52 arc seconds to its north. You can't see the black hole, but you can show someone the star the black hole orbits. The next place you might let someone look at which would contain a black hole would be the bright active galactic nucleus of a Seyfert galaxy like M77, where the brilliant core marks the approximate location of the hole. However, most galaxies are thought to have black holes at their centers, so you could pick out any galaxy with a prominent star-like nucleus and say that is the location of a black hole. Clear skies to you.

Edited by David Knisely, 09 January 2015 - 03:02 AM.

### #7 bleep

Here's a cool link from Astro Bob on our own black hole that recently flared!

### #9 CosmoSat

I hve been asked this question many a times during outreach sessions if i could show them a black hole.. I simply explain them our current understanding of what these kind of objects are and why they cannot be seen thru a scope visually..

Edited by CosmoSat, 09 January 2015 - 12:35 PM.

### #10 Love Cowboy

I know they can't be seen, but if you wanted to show someone a black hole, what would you choose for a target?

/Ira

One of the nearest known black holes that has been identified with certainty is Cygnus X-1. It orbits a magnitude 8.9 star in Cygnus known as HD 226868 (R.A. 19h 58m 21.7s, Dec. +35d 12' 5.8", eq. 2000.0) with a period of about 5.6 days. The star is relatively easy to find even in fairly small telescopes, as it forms a close optical double with a 10th magnitude star about 52 arc seconds to its north. You can't see the black hole, but you can show someone the star the black hole orbits. The next place you might let someone look at which would contain a black hole would be the bright active galactic nucleus of a Seyfert galaxy like M77, where the brilliant core marks the approximate location of the hole. However, most galaxies are thought to have black holes at their centers, so you could pick out any galaxy with a prominent star-like nucleus and say that is the location of a black hole. Clear skies to you.

By that line of reasoning, one might also include M15, the globular with the unusually dense and collapsed core

### #11 Ira

Thanks for the suggestions, at least the good ones. The title of this post is how do you "show" someone a black hole. So, obviously I know it can't be seen. But netither can a neutron star. But I can show someone M1 and they can see the dramatic effects of a neutorn star, and I can show them where the beast lies at the center of the nebula. And that nebula's glow is due to the excitation of its gases by the neutron star. It's also very difficult to show someone a white dwarf, but I can show a planetary nebula and its iconoc glow with the white dwarf embedded therein. So, that's what I have in mind. I do star tours for people, most of whom have never looked at a dark sky. So, you have to be able to use your imagination and not be such a literalist.

So, yes, I could show the center of the Milky Way, but you can't see anything there unique to a black hole even with the greatest optical telescopes on earth. So that's out. Ditto something like the center of M31, where there also lurks a supermasssive black hole, but nothing really unique in visible wavelengths. So, I like the suggestion of Siefert galaxies, especially M77 which I will check out. Cygnus X-1, also sounds like a good target as it orbits a visible star in that constellation.

## Ask Ethan: Does A Time-Stopping Paradox Prevent Black Holes From Growing?

Black holes will devour whatever matter they encounter. Although this is a great way for black holes . [+] to grow, it seems paradoxical, since none of the matter will ever appear to cross the event horizon from the perspective of an outside observer.

X-ray: NASA/CXC/UNH/D.Lin et al, Optical: CFHT, Illustration: NASA/CXC/M.Weiss

Every Milky Way-sized galaxy should contain hundreds of millions of black holes, formed mostly from the deaths of the most massive stars. At the centers of these galaxies, supermassive black holes have devoured enough matter to grow to millions or billions of times the Sun's mass, where sometimes they're caught in the act of feeding on matter, emitting radiation and relativistic jets in the process. But, from the outside, any infalling mass would appear to take an infinite time to fall in does that prevent black holes from growing? Olaf Schlüter wants to know, asking:

[F]or any object falling into a black hole, time slows down upon approach and comes to a standstill as the object reaches the event horizon. Reaching and passing that border would take an infinite amount of time measured by a distant observer. if 'eating' matter would take infinite time. how could supermassive black holes come into existence?

It sounds like a paradox, but relativity explains how it all really happens.

In April of 2017, all 8 of the telescopes/telescope arrays associated with the Event Horizon . [+] Telescope pointed at Messier 87. This is what a supermassive black hole looks like from the outside, and the event horizon is clearly visible.

Event Horizon Telescope collaboration et al.

When you think about a black hole, there are two very different ways you can do it. The first way is to consider it from the point of view of an outside, external observer: to picture a black hole the way someone like us would see it. From this perspective, a black hole is simply a region of space where enough mass is contained within a given volume that the escape velocity — or the speed you'd need to achieve to break free from its gravitational pull — exceeds the speed of light.

Outside of that particular region, space may be severely bent, but particles that move or accelerate quickly enough, as well as light itself, can both propagate to any arbitrary location in the Universe. However, inside that region, there is no escape, with the border between inside and outside defined as the black hole's event horizon.

From outside a black hole, all the infalling matter will emit light and always is visible, while . [+] nothing from behind the event horizon can get out. But if you were the one who fell into a black hole, what you'd see would be interesting and counterintuitive, and we know what it would actually look like.

Andrew Hamilton, JILA, University of Colorado

The second way to think about a black hole, however, is from the perspective of a particle — whether massive or massless — that crosses the event horizon from outside to inside, and therefore falls into the black hole. From outside the event horizon, the infalling entity sees the outside Universe as well as the blackness of the event horizon, which grows larger and larger as they approach it.

But once they cross the event horizon, something funny happens. No matter which direction they move or accelerate in, no matter how quickly or how powerfully they do so, they will always inevitably find themselves headed towards a central singularity. The singularity is either a zero-dimensional point (for non-rotating black holes) or a one-dimensional ring (for rotating black holes), and it cannot be avoided once the event horizon is crossed.

Both inside and outside the event horizon of a Schwarzschild black hole, space flows like either a . [+] moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.

Andrew Hamilton / JILA / University of Colorado

It's important not to mix these perspectives up or conflate them with one another. Although they are both valid, it isn't really possible to do a simple transformation from one point of view to the other. The reason is simple: from outside the black hole, you can never gain any information about what's going on interior the event horizon, while from inside the black hole, you can never send any information to the outside.

And yet, particles — containing energy, angular momentum, and possibly charge — really do fall into black holes, increase their mass, and cause those black holes to grow. To understand exactly how this happens, we need to look at the problem from both perspectives independently, and only then can we see how to reconcile the seemingly paradoxical aspects of this puzzle.

Anything that find itself inside the event horizon that surrounds a black hole, no matter what else . [+] is going on in the Universe, will find itself sucked into the central singularity.

The physics is a little bit easier to understand if we view it from the perspective of the infalling particle. If the particle, existing in the curved space that's presence in the vicinity of a pre-existing black hole, finds itself on a trajectory that will cross the event horizon, there's a clear before-and-after scenario.

Before it crosses the event horizon, the black hole has a particular mass, spin, and event horizon radius, while the infalling particle also adds a slight deformation to the space it occupies. When it crosses over to the inside of the event horizon, its mass and angular momentum now add an supplementary contribution to the black hole's previous parameters, causing the event horizon to grow. From the perspective of the infalling particle itself, everything makes clear sense.

In the vicinity of a black hole, space flows like either a moving walkway or a waterfall, depending . [+] on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.

Andrew Hamilton / JILA / University of Colorado

But from the perspective of the outside observer, things are more challenging. Space is more severely curved the closer you get to the event horizon of a black hole, and since Einstein's relativity links space with time, this means effects like gravitational redshift and gravitational time dilation become more and more pronounced the closer an infalling particle gets to that horizon.

In other words, for an outside observer who sees matter falling into a black hole, it will appear as though the material:

• takes on a redder color (as the photons get gravitationally redshifted),
• falls in slower and slower as it asymptotically approaches the event horizon (due to time dilation),
• appears fainter and fainter over time (as the number of photons per "amount-of-dilated-time" progressively decreases),
• and eventually gets "frozen" infinitesimally close to, but still outside, the event horizon.

This artist’s impression depicts a Sun-like star being torn apart by tidal disruption as it nears a . [+] black hole. Objects that have previously fallen in will still be visible, although their light will appear faint and red (easily shifted so far into the red they are invisible to human eyes) in proportion to the amount of time that's passed since they, from the infalling matter's perspective, crossed the event horizon.

ESO, ESA/Hubble, M. Kornmesser

From the perspective of an external observer, you could even argue that perhaps it's impossible for black holes to grow. If any amount of material, no matter how massive, cannot cross over from outside the event horizon to inside the event horizon, how can a black hole ever become more massive?

Forget about growing into a supermassive one it seems like it might not be possible for a black hole to even grow at all!

But we've tricked ourselves if this is our line of reasoning. Remember, from the perspective of an external observer, we can never gain any information about what's occurring inside a black hole's event horizon. While we can do theoretical calculations to determine what Einstein's General Relativity predicts should be inside a black hole — where and with what properties we should find event horizons, ergospheres, singularities and more — an outside observer cannot acquire that information via any means.

The exact solution for a black hole with both mass and angular momentum was found by Roy Kerr in . [+] 1963, and revealed, instead of a single event horizon with a point-like singularity, an inner and outer event horizon, as well as an inner and outer ergosphere, plus a ring-like singularity of substantial radius. An external observer cannot see anything beyond the outer event horizon.

Matt Visser, arxiv:0706.0622

All that an external observer can ever perceive comes from outside the event horizon, and that's a clue that points at a deeper truth: the event horizon isn't itself a place where physics breaks down (a true singularity), it's simply a place where an external observer is "shielded" from gaining information about what happens inside (a coordinate singularity). This means that what an infalling observer experiences must be correct, at some level, for all observers. Somehow, black holes must really grow, and an external observer must be able to see that growth as well.

How can they see that growth, then, given this apparent paradox?

The key is to remember that, for an external observer, a black hole is simply a region of space with so much matter-and-energy (and angular momentum, charge, and anything else that defines a black hole) that light cannot escape from within that region. If we accept that simple definition, we can do a thought experiment that completely resolves this paradox. Imagine that we begin with a black hole of one solar mass, that doesn't rotate, with an event horizon of the exact size that our Sun would be if it collapsed into a Schwarzschild black hole: a sphere of about 3 kilometers in radius.

The mass of a black hole is the sole determining factor of the radius of the event horizon, for a . [+] non-rotating, isolated black hole. For a black hole of

1 solar mass, its event horizon would be about 3 kilometers in radius.

Now, let's take another one solar mass object — perhaps another star just like our Sun — and let's allow it to fall in to this black hole.

The material from the star will be:

• ripped apart,
• stretched and compressed by the tidal forces of the black hole,
• spread out over an enormous region of space,
• and will asymptotically approach the event horizon, with every particle getting infinitesimally close to — but never crossing — the original event horizon.

The thing is, with an extra solar mass of material at just a little more than 3 kilometers away from the predicted central singularity, we now have two solar masses of material in this particular region of space. The event horizon of a two solar mass object is 6 kilometers in radius, meaning that all of this material is now inside the event horizon, after all!

When matter falls into a black hole, it increases the density (matter-per-unit-volume) in a region . [+] of space surrounding the event horizon. When the total mass in that volume increases by a large enough amount, that new material will now be behind the new, increased-radius event horizon.

ESA/Hubble, ESO, M. Kornmesser

That's the resolution to this paradox: when matter falls onto a black hole, as seen by an outside observer, it only asymptotically approaches the event horizon. But since matter has mass, that mass is now contained within a critical volume of space, and that causes the new event horizon to now encompass the additional material that newly accumulated around the black hole.

It's true that material from outside the black hole, even as it falls in on an inescapable trajectory, will never appear to cross the original event horizon from the perspective of an outside observer. But the more mass and energy a black hole accumulates, the larger the event horizon gets, and that means the newly infalling material can easily make inside the event horizon as it appears after that matter has made it to within a sufficiently small volume of space: close enough to the old enough event horizon to cause it to grow.

Black holes really do grow over time, and all observers can agree exactly when and by how much.

## Thread: Are we living inside a black hole?

There have been several scientific proposals for the shape of the universe. Fact is that at this time we simply do not know. Round? Donut? . Who knows? Many shapes have been proposed.

The age of the universe is estimated at 14 billion years. Within the maximum speed allowed in our universe. we can see no further than this (we can see even less). It would be foolish to think that what we happen too see is the whole universe, if it is the only thing we can see and calculate knowing the maximum speed, which is the speed of light. We only have our visible universe to base estimates on.

What is the likelyhood we happen to be in the centre? The likelyhood is that we are not in the centre of the universe. So the Universe is therefore much larger then we will ever be able to see.

It is like a circle with 4 quadrants. If we are in quadrant one, and the other 3 quadrants are beyond 14 billion years light speed. we will never be able to see them.

So any guess on how big the universe is. I would gather. remains a guess.

Originally Posted by Sealeaf

It is a fundamental WANT-TO-KNOW. Fact is we do not know.

Our current scientific knowledge does not go that far yet. You wanting to know does not get us any closer.

Science does not know what the bounderies of this universe are. Observations from the back ground radiation of the universe has had some scientist thinking. eventually them comming up with bold ideas of multi universes.

Only time will tell. Science will provide answers in the end. Though it may not be in our lifetime.

But that is okay. Accept that. Einstein would be jealous of us common folk. knowing what we now know to be true.

Discussions such as this are always interesting yet un-conclusive.. As has been said.. We do not know..
But that does not stop us wanting to know., and that's as it should be.. The quest for knowledge is the birth place of science..
We know much of the microwave remnant of 'The Big Bang' We know of it's distance from us.. we know of it's temperature..
and a little research will reveal all sorts of things that we can say we know.. But that word 'know' can be deceptive.
The truth is we do not know 'so much' as we have well educated guesswork .. However I can be sure of some things..
That we exist. That the Universe we can see is very large.. and that it is not all of it. However, It is not inside a black hole. That there might be multi universes and pink unicorns in my garden, or some one else's.. I can not and do not know.

Originally Posted by Sealeaf

Space-time in the region of a black hole event horizon is smooth and continuous you wouldn't observe anything special ( locally ) if you fell through such an event horizon. Any singularities that might mathematically appear at an event horizon can be eliminated by a simple coordinate transformation.
The same is not true for a hypothetical "boundary" to the universe space-time there would be discontinuous and not smooth. There isn't really a physically theory in existence which can predict just what such a boundary would look like and behave like, but it would be nothing like the event horizon of a black hole, and the singularities that appear there are physical and cannot be eliminated by coordinate transformations.

If you were to "step through" a universal boundary, you would quite simply cease to exist. This in itself causes all manner of problems, not even to mention yet the mathematical difficulties this would raise which is why I find the existence of such a boundary highly unlikely.

Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

"Gullibility kills" - Carl Sagan
"All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
"It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle

Originally Posted by KALSTER

Originally Posted by Markus Hanke Originally Posted by KALSTER

Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

"Gullibility kills" - Carl Sagan
"All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
"It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle

Originally Posted by KALSTER

Originally Posted by Strange

I think to alot of us it just seems like there is this similarity to the idea of being 'contained' within the event horizon of a black hole and the way we seem to be 'contained' within the confines of our universe.

Originally Posted by Markus Hanke Originally Posted by KALSTER Originally Posted by Ascended So we are back to Poplawski's "new universes created by black holes" idea that came up in another thread.
Originally Posted by Strange Originally Posted by Ascended So we are back to Poplawski's "new universes created by black holes" idea that came up in another thread. Originally Posted by Strange Originally Posted by KALSTER

If it is a singularity, that means that it is a single point, and the entirety of that point is the center of mass, meaning there would be no net force toward any center, which you are already in.

So, assuming that fractally speaking, an entire universe could exist within the singularity of another entire universe, there would be no net gravitational forces from the singularity/universe itself.

Also, to answer the questions of others in regards to the event horizon, and things joining with the singularity. due to time dilation at the event horizon, once the singularity forms, it is cut off from the universe it was created in. Any mass/energy that is a part of the singularity is the only mass that will ever be a part of the singularity. It very neatly matches the theory of the big bang. A star collapses into a singularity, which forms the event horizon. and nothing after that initial creation will ever escape the singularity, or reach the singularity. Once the original creation event happens, no energy can enter or leave the system.

Meanwhile, with the big bang. it starts as a singularity. space expands outward, and there is a very specific amount of energy that never leaves, and can never be added to.

I really dig the fractal theory of the universe =3

(When I first came across the model of the white hole, I thought it was just mathematical wanking. and now it seems to fit so nicely with the theory of the big bang. Man what a lame name. the big bang -_- )

. It seems to suggest a fractal universe where gravity is reversible within singularities in their own fractal reference frame. (expansion of the universe accelerating, gravity is an acceleration. )

Originally Posted by Velexia

I'm not sure what "it" refers to in this sentence. But a black hole is not a singularity it (the black hole) is defined by its event horizon, which is of finite size. There is, mathematically, a singularity at the center (which becomes the future) of a black hole but this is likely to be an artefact of the fact we don't yet know how to take quantum effects into account.

Also, in a realistic black hole, which is spinning, the singularity would be a ring rather than a point.

Originally Posted by Strange

Sorry, when I think of a black hole, I think of the part that has substance. the matter and what became of it (I'm assuming it becomes a singularity). The event horizon is just a phenomena that is created by this mass, but is not really a thing in and of itself. It's a feature of the entire picture, but it is an effect rather than a cause of anything.

Originally Posted by Strange

That's a fair point. A spinning singularity could very well be a ring and not a point. However, does the same idea still hold true that the empty, center of the ring is the equilibrium point (gravitationally speaking) or does the equilibrium point exist along the ring. <- That was a really silly question involving a lack of thought. I'm going to replace it with a less silly proposition:

A singularity, regardless of spin, cannot form a ring. The gravitational forces which create the singularity in the first place would necessarily overpower any minute forces which would cause a ring of any magntitude.

Originally Posted by Strange Originally Posted by Velexia

Maybe. But it is also all we can ever know.

I don't know. And it doesn't really matter as all we can know is the total mass, angular momentum and electric charge. From outside the event horizon, it would appear identical to any other mass of that size.

Originally Posted by Strange Originally Posted by Velexia

Maybe. But it is also all we can ever know.

I don't know. And it doesn't really matter as all we can know is the total mass, angular momentum and electric charge. From outside the event horizon, it would appear identical to any other mass of that size.

Originally Posted by Velexia

That is incorrect. What happens is that a far away observer never sees any matter crossing the event horizon, since it would take an infinite amount of coordinate time to reach the horizon. However, that is true only for the far away observer an observer freely falling into the black hole will reach and cross the event horizon in a finite and well defined amount of proper time. When he reaches the horizon, space-time will look just normal to him - smooth and continuous. There is no matter accumulated and "frozen" at the horizon.

That is also incorrect. The ring singularity is a feature of the Kerr metric, which describes space-time in the vicinity of a black hole with angular momentum. It is not possible for the gravitational collapse to result in a point singularity if angular momentum is present in the energy-momentum tensor.

Originally Posted by Markus Hanke Originally Posted by Velexia

I disagree, but I am not going to get into for the reasons stated in my previous post. This was exactly the argument I was expecting to see, and exactly the argument I have no care to debate.

Originally Posted by Markus Hanke

I'm a lot more interested in this, can you help me understand it? I've only gone as far as Calculus 3, so I'm a bit out of my league when trying to understand it on my own.

Originally Posted by Markus Hanke

Don't go down this road. I don't need to deal with horseshit.

Let's talk about ring singularities please =3

Originally Posted by Velexia

Hardly surprising, because there is nothing to debate. The proper time for an observer falling from rest at some distance into the centre of a black hole is

which is finite and well defined ( integration constant C to be determined from boundary conditions, as usual ). Nothing "accumulates" at the event horizon.

Did you not tell us that you majored in astrophysics ? Something is seriously amiss here if what you told us is the truth you should be intimately familiar with tensor calculus, and hence the fact that the covariant divergence of the energy-momentum tensor vanishes everywhere. This is really just a case of angular momentum conservation - total angular momentum on some hypersurface S about an event A is

we can immediately deduce that

as well. Since the angular momentum of a point is zero, the gravitational collapse of a body with total angular momentum > 0 therefore cannot result in a point singularity since that would result in a violation of the above conservation law.

Refer also here for some additional information :

Originally Posted by Markus Hanke Originally Posted by Velexia

Hardly surprising, because there is nothing to debate. The proper time for an observer falling from rest at some distance into the centre of a black hole is

which is finite and well defined ( integration constant C to be determined from boundary conditions, as usual ). Nothing "accumulates" at the event horizon.

If you ignore the part where I specifically stated that I am only speaking about an external reference frame, since that is the only reference frame we can ever have in relation to a black hole. Sure. That equation describes part of what "happens" from the reference frame of an infalling observer (ignoring the survivability of such a descent regardless of the diameter of the event horizon). However, what I said should not be ignored, so you can take your self righteous bullshit and shove it back up your ass (and that's not in reference to the equations you posted or anything in relation to the black hole in case you are going to attempt to misinterpret that).

In the meantime you are completely missing something that I find completely obvious, but you don't see me insulting you for missing it.

Originally Posted by Markus Hanke

How are you a moderator, if you can't help but talk to people like this? Seriously? It blows my fucking mind.

Originally Posted by Markus Hanke

This is really just a case of angular momentum conservation - total angular momentum on some hypersurface S about an event A is

we can immediately deduce that

as well. Since the angular momentum of a point is zero, the gravitational collapse of a body with total angular momentum > 0 therefore cannot result in a point singularity since that would result in a violation of the above conservation law.

I'm going to read this a few times, and probably ask some questions about it, if you can manage to refrain from acting the way you have been. I'm shaking my head in utter disbelief at your choices of words, and attitude.

Originally Posted by Markus Hanke

"With a fluid rotating body, its distribution of mass is not spherical (it shows an equatorial bulge), and it has angular momentum. Since a point cannot support rotation or angular momentum in classical physics (general relativity being a classical theory), the minimal shape of the singularity that can support these properties is instead a ring with zero thickness but non-zero radius, and this is referred to as a ring singularity or Kerr singularity."

This seems to ignore some of the properties of the black hole singularity from the start. Which is the reason I proposed such an outlandish idea in the first place.

Edit: Such as the forces of gravity which caused the collapse in the first place, overpowering nuclear forces, eletromagnetic forces, etc. everything keeping the matter from collapsing in on itself in the first place.

I'm not sure I want to touch on infinite curvature and such here. But this could also be a factor?

Originally Posted by Velexia

As it stands this is wrong even for an external observer, because he does not see anything accumulating at the event horizon any in falling matter will appear increasingly redshifted, and will eventually just "fade away". No accumulation is observed.

Originally Posted by Velexia There are plenty of other arguments against this common misconception. But as you know what they are, I know how you will object to them and you know how I will respond, etc. why don't we just skip to the bit where you admit you were wrong? Originally Posted by Strange Originally Posted by Velexia There are plenty of other arguments against this common misconception. But as you know what they are, I know how you will object to them and you know how I will respond, etc. why don't we just skip to the bit where you admit you were wrong? Originally Posted by Velexia

Originally Posted by Velexia Originally Posted by Velexia

Originally Posted by Markus Hanke Originally Posted by Velexia

I did talk about this, actually. As for the accumulation, it is inferred, as it cannot be seen. You're ignoring the obvious to attempt to tear apart my position, and yet. by ignoring the obvious, you are only trying to tear down my position by stating exactly what I was referring to.

Originally Posted by Strange Originally Posted by Velexia

Find me a black hole, a suitably "large" one, that has nothing else around it which would be hazardous to your existence. I mean, not that you really can, since it's a black hole, and we can really only find them by their effects on surrounding matter, but I mean, seriously. You wouldn't find one fitting that description anyway.

That was my entire reason for ruling suitably large black holes out.

There is one other point which is, you won't be able to relay anything you discover to anyone observing from a distance, so again, it's moot.

Edit: We are totally crabwalking straight into the discussion I was trying to avoid. but I kinda like talking to you, so maybe it's not so bad =3

Originally Posted by Velexia Originally Posted by Velexia

Originally Posted by Velexia Originally Posted by Velexia

Originally Posted by Markus Hanke Originally Posted by Velexia Originally Posted by Markus Hanke Originally Posted by Velexia Originally Posted by Strange Originally Posted by Velexia Originally Posted by Markus Hanke Originally Posted by Velexia Originally Posted by Velexia

Originally Posted by Velexia

The existence of the event horizon is independent of the existence of the singularity (other than the fact they are both the result of the same theory).

If the total mass of the black hole were evenly spread out within the volume, concentrated in something the size of a grapefruit, or compressed to an actual singularity (if such a thing is possible) it would make no difference to the event horizon or the universe outside. This must be the case because of the "no hair" theorem.

Originally Posted by Strange Originally Posted by Velexia

The existence of the event horizon is independent of the existence of the singularity (other than the fact they are both the result of the same theory).

If the total mass of the black hole were evenly spread out within the volume, concentrated in something the size of a grapefruit, or compressed to an actual singularity (if such a thing is possible) it would make no difference to the event horizon or the universe outside. This must be the case because of the "no hair" theorem.

Originally Posted by Markus Hanke Originally Posted by Velexia Originally Posted by Velexia

The event horizon exists as soon as sufficient mass is within the Schwarzschild radius. So it seems you are now saying that the singularity forms after the event horizon? Seems reasonable to me. (If there is a singularity.)

My understanding is that in a supernova collapse, the event horizon forms at the center (where the density is highest) and then accelerates outward to encompass more material (perhaps faster than the material could fall in - but I don't understand much about this process).

But there is, of course, nothing to stop more material falling through the event horizon and making it grow.

Originally Posted by Velexia

As far as I can tell, you resorted to obscenities in response to: "Hardly surprising, because there is nothing to debate" and "Did you not tell us that you majored in astrophysics?" Neither seems to be an attack.

The reaction seemed a little over the top for a couple of harmless comments. <shrug> I guess some people are more sensitive than others.

Originally Posted by Velexia

Originally Posted by Strange Originally Posted by Velexia

Makes sense to me. But we have to keep in mind that the event horizon only exists because of the critical density of the mass.

Originally Posted by Strange

My understanding is that in a supernova collapse, the event horizon forms at the center (where the density is highest) and then accelerates outward to encompass more material (perhaps faster than the material could fall in - but I don't understand much about this process).

But there is, of course, nothing to stop more material falling through the event horizon and making it grow.

Originally Posted by Strange Originally Posted by Velexia

As far as I can tell, you resorted to obscenities in response to: "Hardly surprising, because there is nothing to debate" and "Did you not tell us that you majored in astrophysics?" Neither seems to be an attack.

The reaction seemed a little over the top for a couple of harmless comments. <shrug> I guess some people are more sensitive than others.

It was a culmination of continuous attacks upon my character that threw me over the edge. I suppose it can be seen as an overreaction, but to get philosophical for a moment, by my life experiences and genetics it was my only possible reaction, as a way of trying to get the undesired assault on my ego to stop (especially since it really does nothing to further the discussion).

I did my best to curtail it, but I felt it had to be done, and I'm partially sorry, but really, I just want to discuss without having to defend myself from opinions about my character. Please tear apart my arguments to your hearts desire, and I'll do my best to find the most logical explanation from everything said, and I hope everyone else does the same.

Originally Posted by Velexia Originally Posted by Strange Originally Posted by Velexia

I challenge you to provide me with a valid example of that. I mean, I don't really want you to, but I think maybe the order of events simply caused a misinterpretation on your part.

I'd rather keep crabwalking toward time dilation and relative observers. Or go to bed, I could do that too, heh. It's now 9am for me.

Originally Posted by Velexia Isn't that the thing you didn't want to discuss? (Because you are wrong. ) Originally Posted by Strange Originally Posted by Velexia Isn't that the thing you didn't want to discuss? (Because you are wrong. )

It is, but we were slowly headed there anyway.

I got into this on another forum once, and it pretty much involved hundreds of insults thrown at me, and 50 pages. and myself exasperated, trying to describe it in every way I can possibly conceive of from every angle and pretty much no one even bothering to consider any of it (save a few people).

Originally Posted by Velexia Originally Posted by Velexia

Maybe you need to make it clear what you are claiming.

Matter does fall through the event horizon and this happens in a finite time, even for an external observer.

It is often claimed that an observer would see infalling matter "stuck" at the event horizon forever. This is, fairly obviously, not true even if you ignore red-shift.

And we are all hopefully well acquainted with relativity. so

If I am traveling at a relativistic speed into space. and you are on earth, chilling (at rest). I will look at my clock, and it will appear normal, and through means that need not be explained because they are irrelevant to the example I can also look at your clock and it appears to move faster than my own.

Meanwhile, you look at your clock, it it appears normal, and by similar means look at my clock and see that it appears to be slow.

And we can ignore the fake paradox of both of us moving relative to a third perspective and having a wholly different view of each other's clocks because that example does not have any bearing on black holes.

Originally Posted by Velexia

I think they are called world lines, correct me if I am wrong. and if I draw them in such a way that you are at rest and I progress through space at near the speed of light. I could experience 5 years in the time it takes for you to experience 6. This is time dilation at work. Thus, your clock moves faster, from my perspective. This is an actual effect, not just a peculiarity of visual phenomena. You literally change faster than I do, by being at rest. When we fly an atomic clock around at some velocity, and then compare it to an atomic clock that was at rest, the clock at rest will have recorded more time than the moving clock.

Furthermore, gravity also distorts time. Time moves slower where gravity is stronger. Where gravity is weaker, time moves faster.

Both time dilation due to gravity and time dilation due to velocity relative to the velocity of light are tested, empirical facts, and if necessary I can use my google fu to link you to the experiments which proved them.

Originally Posted by Velexia

This makes no sense. Light is not a reference frame. How can it be? Moving closer to the speed of light compared to what?

I thought you said you had studied science? I'm sorry if you think that is some sort of personal attack but I am just confused as to why anyone would say such a thing, unless they have never studied any science at all.

This leads to two different ways in which to experience time dilation near a black hole's event horizon, and both of them lead to the same end. The reason that gravity has the same effect as velocity at the even horizon is the very same cause of the event horizon in the first place, at the swartzchild radius. If you, by some miracle, touch the event horizon, in order to stop your descent you must travel at exactly the speed of light. Having mass, this is impossible. So if we ignore gravitational time dilation, and the very reason that the event horizon exists. you would pass through it and head for the singularity.

However, the closer you get to the event horizon, the slower time moves for you, despite your experience of time appearing normal. Time moves slower near an event horizon, than it would in empty space, or any space further from the event horizon than where you are.

If you, by some miracle, touch the event horizon, due to gravitational time dilation. Time stops for you, relative to an observer farther from the event horizon (any distance farther). Time also stops when you reach the velocity of light in a similar manner. despite you, still experiencing time as appearing normal.

However it is only your local time that appears normal. Just as, to a reference frame not infinitesimally close to the event horizon, farther than you are from it by any distance, local time appears normal. When you, ever so close to the event horizon, look at my clock, on earth, say. (ignoring how such a thing is possible since it is irrelevant, you can just observe the changes in the universe "above" you) my clock hands are spinning impossibly fast, and faster with each passing moment, however many you have left (let's pretend this is the biggest black hole ever, and it takes 5 days to go the last leg of the journey). If I watch your clock, it is going slower and slower with each passing moment, so slow I can hardly tell it is moving at all, if I can even tell, at all past a certain point. (Note we are also ignoring the blueshift and redshift of light which would further make these observations difficult).

So, what this says is, it takes anything outside an event horizon an infinite amount of time to reach the event horizon from an observer chilling farther away. and an infinite amount of time passes in the outside universe as you fall toward the event horizon.

And yes, this is backed up by the math. Hell the only reason I am even aware of it is because of the math. Before my experience with the subject matter in depth, I only knew that black holes were cool.

Originally Posted by Strange Originally Posted by Velexia

This makes no sense. Light is not a reference frame. How can it be? Moving closer to the speed of light compared to what?

I thought you said you had studied science? I'm sorry if you think that is some sort of personal attack but I am just confused as to why anyone would say such a thing, unless they have never studied any science at all.

Moving closer to the speed of light, relative to actual light speed. Since nothing with mass can go the speed of light, this should make perfect sense to you. There are objects which can have an apparent speed faster than light, but no objects with mass that can actually achieve it. This only makes even more sense when you consider that the speed of light is constant regardless of the speed of any observing frame of reference.

I take that as a bit of a personal attack, yes, by virtue of ignorance on the subject matter. But ignorance is not a fault, so, no harm done.

I've said enough on the matter to thoroughly explain it, and I really needn't say any more, so I won't. You are free to do with the information what you will.

I'm off to bed. It was a pleasure =3

Originally Posted by Velexia

How can you measure speed relative to the speed of light: everybody observes the same speed of light. You can only measure speed relative to another object.

Originally Posted by Velexia

I can't resist. Educate yourself. I really need to sleep.

Originally Posted by Velexia Originally Posted by Velexia Originally Posted by Strange Originally Posted by Velexia Isn't that the thing you didn't want to discuss? (Because you are wrong. )

It is, but we were slowly headed there anyway.

I got into this on another forum once, and it pretty much involved hundreds of insults thrown at me, and 50 pages. and myself exasperated, trying to describe it in every way I can possibly conceive of from every angle and pretty much no one even bothering to consider any of it (save a few people).

And it hasn't occured to you that problem might be you, rather than the rest of us?

Oh dear, did I just insult you. Please use the convenient report function. Or preferably, grow a pair.

Originally Posted by Velexia

There are two distinct kinds of time dilation - relative time dilation due to relative motion, and gravitational time dilation due to space-time curvature. These are not the same - relative time dilation concerns coordinate time, whereas gravitational time dilation acts on proper time.

Incorrect. Ignoring the effects of gravity, you will see the clock on earth also dilated, not speeding up. That is the meaning of symmetry between inertial frames.

Yes, and that is because these frames were not symmetric - the one that was flying around and then stopped (!) experienced acceleration, whereas the earth bound one did not. Hence the difference in proper times. If you compare spatially separated clocks ( such as a satellite ), you are actually comparing proper time with coordinate time, so relative time dilation needs to be added into this as well.

Light is not a valid frame of reference, as Strange has correctly pointed out. You cannot determine your velocity "relative to light", because the speed of light is the same for all observers.

No it doesn't time progresses as normal for you. Schwarzschild coordinate time is not defined at the event horizon, and your own proper time progresses as normal - you cross the event horizon in a finite proper time, as calculated for you earlier. Remember that it is proper time that your watch in your own frame records, not coordinate time.

There is no such thing as "time" ( as in : an absolute time ) - there is only coordinate time and proper time. Only coordinate time is infinite here, not proper time. Since it is proper time which measures the length of an observer's world line, all observers agree that an object falls through the event horizon into the centre, even if they cannot visually see this.

Originally Posted by Strange

Originally Posted by PhDemon

they are just another halfwit with delusions of competence after reading a few popsci books.

(And yes that is an insult, a thoroughly deserved one -- feel free to report it).

That's a halfwit opinion, made up from pure ignorance and desire to be a troll on the internet. I am happy to report you.

If you even bothered for one half of a (P*#%^&QR(PQ second to look into the books that I mentioned in that post you'd see that they are ORIGINAL #%^*##@% PAPERS ON THE _$P%&)($)$%(%#@ SUBJECT! *#^%###@%#@#@^(_%^_% YOU )#%^&$%^&_&$%& )%&*)%)%)(%&. Or in the case of A Stubbornly Persistent Illusion, something Einstein himself wrote in reference to Special and General Relativity. Pardon me while I go drown some puppies and pretend they are you. Originally Posted by Markus Hanke Originally Posted by Velexia There are two distinct kinds of time dilation - relative time dilation due to relative motion, and gravitational time dilation due to space-time curvature. These are not the same - relative time dilation concerns coordinate time, whereas gravitational time dilation acts on proper time. Incorrect. Ignoring the effects of gravity, you will see the clock on earth also dilated, not speeding up. That is the meaning of symmetry between inertial frames. Yes, and that is because these frames were not symmetric - the one that was flying around and then stopped (!) experienced acceleration, whereas the earth bound one did not. Hence the difference in proper times. If you compare spatially separated clocks ( such as a satellite ), you are actually comparing proper time with coordinate time, so relative time dilation needs to be added into this as well. Light is not a valid frame of reference, as Strange has correctly pointed out. You cannot determine your velocity "relative to light", because the speed of light is the same for all observers. No it doesn't time progresses as normal for you. Schwarzschild coordinate time is not defined at the event horizon, and your own proper time progresses as normal - you cross the event horizon in a finite proper time, as calculated for you earlier. Remember that it is proper time that your watch in your own frame records, not coordinate time. There is no such thing as "time" ( as in : an absolute time ) - there is only coordinate time and proper time. Only coordinate time is infinite here, not proper time. Since it is proper time which measures the length of an observer's world line, all observers agree that an object falls through the event horizon into the centre, even if they cannot visually see this. You've got a lot of disagreements in here based purely on your desire to disagree with me, which is absolutely hilarious, considering that in pretty much every case, they seem to reflect a desire to misread literally everything said here, or simply not read everything that was said. It is most stunningly apparent here: "No it doesn't time progresses as normal for you." Try reading the entire sentence, and get some reading comprehension. I bet it feels nice to be on a bandwagon, but it does you no service. I mean, hell the very next words say that the reference frame for your time having stopped is NOT YOUR REFERENCE FRAME. You people make me want to devour babies. The level of moronic behavior on a Science thread by Moderators and regulars alike is completely appalling. I'm going to try to continue reading this post to see if it has anything of merit to say, at all. Originally Posted by John Galt Originally Posted by Velexia Originally Posted by Strange Originally Posted by Velexia Isn't that the thing you didn't want to discuss? (Because you are wrong. ) It is, but we were slowly headed there anyway. I got into this on another forum once, and it pretty much involved hundreds of insults thrown at me, and 50 pages. and myself exasperated, trying to describe it in every way I can possibly conceive of from every angle and pretty much no one even bothering to consider any of it (save a few people). And it hasn't occured to you that problem might be you, rather than the rest of us? Oh dear, did I just insult you. Please use the convenient report function. Or preferably, grow a pair. GROW THE #@)(%^ UP YOU IGNORANT MORON. Is that good enough for you? Go commit seppuku for being an idiot on the internet. I never really expected much from someone naming themselves "John Galt". Glad to know my intuition was spot on there. "This result appears puzzling because each twin sees the other twin as traveling, and so, according to an incorrect naive application of time dilation, each should paradoxically find the other to have aged more slowly." Originally Posted by PhDemon See this is why I don't like getting into this subject on the internet. By virtue of the anonymity of the internet, people let the worst sides of themselves out. and the internet is chock full of people like the lot of you, even on a science forum of all places. I know time dilation is hard, but FFS, only for someone who is incapable of complex thought. This website should be renamed "TheKindergartenScienceForum" where mostly we are just mean to each other, since we don't know how to behave like rational thinking people. I think I'll do something for my sanity, and only respond to intelligent rational people who wish to have a discussion from now on. So go throw yourselves a nice self congratulatory troll party, celebrating your willful ignorance and immaturity. Velexia, I really do think you would do well to examine your own role here. Markus has built up an excellent reputation on this site by demonstrating, over the course of years, a commendable grasp of general and special relativity and related aspects of cosmology. Other knowledgeable individuals have confirmed this view. Moreover, on the rare occasions he has been in error has been quick to acknowledge that error and even quicker to express thanks to the individual who has corrected him and thereby deepened his knowledge. I am an incompetent in matters of GR and SR, but I am an expert in identifying a line of reason, an application of logic and a well structured argument. Markus has delivered these routinely and consistently through several thousand posts. HE has also displayed remarkable patience and objectivity in dismantling faulty arguments. On the other hand you have joined the forum and almost from your first post have adopted an arrogant attitude. I am a member of several forums and in each instance I have lurked for a while, figured out the tenor of the place, then eased into participation. I am a stranger in a strange land and tact and caution are not only polite, they are also productive. Now perhaps you did not intend that arrogance, however, several members besides myself clearly saw it. On top of that, you were making assertions that did not match reality. Not a good start. Originally Posted by Velexia You've got a lot of disagreements in here based purely on your desire to disagree with me, which is absolutely hilarious, considering that in pretty much every case, they seem to reflect a desire to misread literally everything said here, or simply not read everything that was said. It is most stunningly apparent here: "No it doesn't time progresses as normal for you." Try reading the entire sentence, and get some reading comprehension. In all of the many posts Markus has made I do not recall a single instance where he has disagreed for the sake of disagreeing. Do you feel you are a better judge of motivations after knowing the man for a few days than I? As far as reading comprehension goes the primary responsibility for communicating an idea lies with the writer, not the reader. Do you disagree? GROW THE #@)(%^ UP YOU IGNORANT MORON. I was aiming more for the notion that you should be less sensitive to negative remarks, real or imagined. I'm sorry I did not make that clear. The error was mine. (You see how this work?) Atlas Shrugged is one of my top five works of fiction. However, I am opposed to almost every aspect of Ayn Rand's philosophy, so your intuition has done you no good there. The selection of the name was for subtle reasons, connected with the forum, that are now history. See this is why I don't like getting into this subject on the internet. By virtue of the anonymity of the internet, people let the worst sides of themselves out. and the internet is chock full of people like the lot of you, even on a science forum of all places. I know time dilation is hard, but FFS, only for someone who is incapable of complex thought. This website should be renamed "TheKindergartenScienceForum" where mostly we are just mean to each other, since we don't know how to behave like rational thinking people. You will find many threads and posts where ideas are exchanged in a civil manner, where people are educated and educate, where new ideas are explored and old ones confirmed. You will also find threads where members become irate and frustrated. This is typically because one member will adopt a hostile attitude, or refuse to acknowledge facts, or present weak arguments. Which brings us full circle. Have another look at your own behaviour and posting style from the outset. Will you claim you are blameless in this unseemly exchange? I'd be interested to know. Pretty sure I know the source of confusion for people assuming that you can actually pass through an event horizon without an infinite amount of time passing in the universe around you. Having associated gravitational time dilation with acceleration, you've thus forgotten gravity, and focus purely on acceleration, and assume that despite local time and non local time differing, because you are accelerating toward the singularity, given a finite amount of time, you must necessarily pass through the horizon with no complications. In part you are correct. In your local reference frame, ignoring all non local reference frames, you simply fall straight in. But you can't simply ignore the non local reference frames, because what is happening out there, outside of your reference frame is real. So, give complex thinking a try, and start thinking about both reference frames together, and how they relate to one another. Also, remember F=ma. Think about what velocity will have been imparted to you, by the acceleration due to gravity, at the event horizon. You see, whether through velocity, or acceleration, there is no escaping time dilation. So, rather than assuming that I am just some ignorant fool, and a liar. why not try to think. What you find may surprise you. Originally Posted by John Galt Velexia, I really do think you would do well to examine your own role here. Markus has built up an excellent reputation on this site by demonstrating, over the course of years, a commendable grasp of general and special relativity and related aspects of cosmology. Other knowledgeable individuals have confirmed this view. Moreover, on the rare occasions he has been in error has been quick to acknowledge that error and even quicker to express thanks to the individual who has corrected him and thereby deepened his knowledge. I am an incompetent in matters of GR and SR, but I am an expert in identifying a line of reason, an application of logic and a well structured argument. Markus has delivered these routinely and consistently through several thousand posts. HE has also displayed remarkable patience and objectivity in dismantling faulty arguments. On the other hand you have joined the forum and almost from your first post have adopted an arrogant attitude. I am a member of several forums and in each instance I have lurked for a while, figured out the tenor of the place, then eased into participation. I am a stranger in a strange land and tact and caution are not only polite, they are also productive. Now perhaps you did not intend that arrogance, however, several members besides myself clearly saw it. On top of that, you were making assertions that did not match reality. Not a good start. Originally Posted by Velexia You've got a lot of disagreements in here based purely on your desire to disagree with me, which is absolutely hilarious, considering that in pretty much every case, they seem to reflect a desire to misread literally everything said here, or simply not read everything that was said. It is most stunningly apparent here: "No it doesn't time progresses as normal for you." Try reading the entire sentence, and get some reading comprehension. In all of the many posts Markus has made I do not recall a single instance where he has disagreed for the sake of disagreeing. Do you feel you are a better judge of motivations after knowing the man for a few days than I? As far as reading comprehension goes the primary responsibility for communicating an idea lies with the writer, not the reader. Do you disagree? GROW THE #@)(%^ UP YOU IGNORANT MORON. I was aiming more for the notion that you should be less sensitive to negative remarks, real or imagined. I'm sorry I did not make that clear. The error was mine. (You see how this work?) Atlas Shrugged is one of my top five works of fiction. However, I am opposed to almost every aspect of Ayn Rand's philosophy, so your intuition has done you no good there. The selection of the name was for subtle reasons, connected with the forum, that are now history. See this is why I don't like getting into this subject on the internet. By virtue of the anonymity of the internet, people let the worst sides of themselves out. and the internet is chock full of people like the lot of you, even on a science forum of all places. I know time dilation is hard, but FFS, only for someone who is incapable of complex thought. This website should be renamed "TheKindergartenScienceForum" where mostly we are just mean to each other, since we don't know how to behave like rational thinking people. You will find many threads and posts where ideas are exchanged in a civil manner, where people are educated and educate, where new ideas are explored and old ones confirmed. You will also find threads where members become irate and frustrated. This is typically because one member will adopt a hostile attitude, or refuse to acknowledge facts, or present weak arguments. Which brings us full circle. Have another look at your own behaviour and posting style from the outset. Will you claim you are blameless in this unseemly exchange? I'd be interested to know. You're going to have to highlight for me, what you saw from me as arrogant, because I can't, for the life of me, see it. I'm being sincere. Maybe it was a misinterpretation on your part, through no fault of your own, but faulty nonetheless? (You'll have to forgive me for assuming the fault is not my own, egos like to do that. If you can show me convincingly where my fault lies, I'll do my best to take it to heart). I was making assertions in the form of a hypothesis, and asking for convincing evidence to disprove it, because I was having trouble finding it on my own. There's literally nothing wrong with that. It's the scientific process. The forum administrator was able to participate in the discussion AND provide me with the proof I needed, without being immature about it. That's a real saving grace for the entire forum, honestly. At least the Forum Administrator demonstrates appropriate behavior (and thank you for that KALSTER!) If Markus is not disagreeing to disagree then he lacks reading comprehension, or perhaps the ability to read an entire sentence before formulating a retort as highlighted by this quote "No it doesn't time progresses as normal for you." from him, in reference to this sentence: "If you, by some miracle, touch the event horizon, due to gravitational time dilation. Time stops for you, relative to an observer farther from the event horizon (any distance farther). Time also stops when you reach the velocity of light in a similar manner. despite you, still experiencing time as appearing normal." I should not have to deal with negative remarks directed at me, as a person. It's inflammatory, rude, against the forum rules if I am not mistaken, and not conducive to rational discussion. Well, the name nonetheless gave an impression, and you acted in a manner ill befitting a moderator, so forgive me for drawing the conclusions I did. I am not in a position to know the subtleties regarding your choice of the name and can only form an opinion based on my own experiences. I may have communicated poorly, in an environment such as an internet forum, where people are quick to jump to their first conclusions, possibly without any attempt to comprehend the intended message, or even perhaps, read the entire post. I am not the most eloquent person, and careful speech so as not to allow for any misinterpretation is seriously tiring and makes gentle reading tedious. I was at fault for assuming people would behave in a polite, mature manner, and that they had a solid grasp of concepts I was addressing (such as in the case of time dilation). In the case of my first post/thread, I had a lack of facts, and was literally, seeking out those facts. In every post I did my best to clearly state that I was not convinced by what I had seen thus far, and was seeking something convincing. When I found it, I happily took it to heart, since it was my desire not come up with some crackpot theory, but to broaden my understanding through factual evidence. For the most part I received hostility, insults, and rude behavior. So yes, I slowly became more and more agitated, until at last I flipped the %^&* out, because there is only so much unwarranted abuse one can take, and the only reason I had to endure it in the first place is because the anonymity of the internet makes people feel safe to be the asshole they are inside. If people acted like this in person, they'd get punched in the goddamned face (not that I am the type to punch people in the face. I'd simply not associate with them, but somebody would, and they would have deserved it). I have to go to work in five or ten minutes. This is a brief explanation. You criticsed the Argument from Authority, I think. But the vast majority of us, until and unless we have studied a specific topic in scrupulous and excrutiating detail have to, as a practical matter, acept the consensus view provided by experts in that field. When we encounter something we feel is questionable there are two possible positions to take: 1. I am not convinced by this and believe it may be wrong. Show me, beyond any reasonable dount that it is correct, or at least the best available explanation. 2. I do not understand why this is thought to be correct. Experts say it is and they have studied it much more detail than I, therefore I am likely wrong and would llike to understand why. The first approach, which is the one I saw you take, is arrogant. The second is not. I illustrate further with two personal anecdotes. I do not believe in the Big Bang theory. I object to it on philosophical and personal grounds. However, I accept it is currently the best available explanation for the evidence and have defended it on this and other forums. Several years ago I found what appeared to be an error in the calculation process for killing a kicking oil well. It was very clear cut. I did not take the position, this is wrong, please go ahead and show me why it isn't. I said, this appears to be wring, but I doubt it can be. Where am I making the mistake. I am not a humble person, but I am also not - I think - arrogant. You appeared to be. And as a side note, although I have certainly insulted people and used offensive language on forums I have never, despite provocation far more extreme than anything you have experioenced here, I have never advocated physical violence. There are plenty of forums where you would have been permanently booted off for that. Originally Posted by John Galt I have to go to work in five or ten minutes. This is a brief explanation. You criticsed the Argument from Authority, I think. But the vast majority of us, until and unless we have studied a specific topic in scrupulous and excrutiating detail have to, as a practical matter, acept the consensus view provided by experts in that field. When we encounter something we feel is questionable there are two possible positions to take: 1. I am not convinced by this and believe it may be wrong. Show me, beyond any reasonable dount that it is correct, or at least the best available explanation. 2. I do not understand why this is thought to be correct. Experts say it is and they have studied it much more detail than I, therefore I am likely wrong and would llike to understand why. The first approach, which is the one I saw you take, is arrogant. The second is not. I illustrate further with two personal anecdotes. I do not believe in the Big Bang theory. I object to it on philosophical and personal grounds. However, I accept it is currently the best available explanation for the evidence and have defended it on this and other forums. Several years ago I found what appeared to be an error in the calculation process for killing a kicking oil well. It was very clear cut. I did not take the position, this is wrong, please go ahead and show me why it isn't. I said, this appears to be wring, but I doubt it can be. Where am I making the mistake. I am not a humble person, but I am also not - I think - arrogant. You appeared to be. And as a side note, although I have certainly insulted people and used offensive language on forums I have never, despite provocation far more extreme than anything you have experioenced here, I have never advocated physical violence. There are plenty of forums where you would have been permanently booted off for that. I suppose I can can see why you see it as arrogant. I still don't see it as arrogant myself. I simply see it as a different position. one where I am not assuming that I don't understand what I am currently aware of, but rather that I understand that there must be something I am not currently aware of, which would give a solid ground for the existence of the thing I am doubting. See, if I thought, for example, that I misunderstood the facts already laid out before me, I would have approached it in the second manner. But that wasn't the case. Had I gone with the second approach, the focus would be on trying to force me to accept the common interpretation of experiments I was already aware of, and having read thoroughly into those experiments, and the thoughts and interpretations at the source (rather than a game of telephone), and not being convinced by them, there was literally no way that a second hand reiteration was going to help. So I necessarily HAD to ask to see other evidence. I'm sorry if that seemed arrogant to you, but there was really no other course of action available to me. I honestly doubt the big bang theory as well, but I don't have enough ground to take my own doubt seriously, because I do not fully understand the reasoning behind why they interpret the microwave background radiation in the way they do (in this case, I can easily take the second stance). I didn't advocate physical violence, you misread. I simply stated that, in an open environment such as, in person, were a person to be rude to his or her fellows in the manner they have been here, eventually, someone would inevitably punch them in the face, and I would not care (because the ego is like that). I don't think violence is ever right, and I never resort to physical force beyond that necessary to protect myself from physical harm. As an anecdote, I was in the Army, and I held a loaded weapon, safety off, at what was potentially a hostile person in a foreign land, and with every fiber of my being I had NO desire to squeeze the trigger on which my finger rested, and yet, I was obligated to do just that if necessary. It would have devastated me for life, and it brings a tear to my eye merely to think about it, and I am glad beyond words that the situation didn't escalate, because I see myself in every person, and every person is deserving of life and freedom from oppression and violence despite nature's tendency to put us at odds. So please, don't for a moment think that I advocate violence. Originally Posted by Velexia In an earlier post you recommended the following course of action for myself. Originally Posted by Velexia No, the disagreements are based on the theory of relativity. I told you before that I will call on claims that aren't physically accurate, and I will continue to do so. Yes, almost seems like it, doesn't it. Do you mean coordinate time or proper time ? Because everyone agrees on proper time. and that is quite finite. Who does that ? The calculation I gave you is simply a line integral along the world line of the in-falling observer that world line does not terminate at the event horizon. Or are you claiming differently ? If so, I'd be interested to be shown your maths in support of your claims. When I look out my window ( disregarding any topological features ) I see the horizon as flat and straight can I thus assume that the Earth must be flat, because that's what it looks like to me ? I ask you again - do you understand the difference between coordinate time and proper time, and why they are distinct notions in Schwarzschild space-time ? No, you are making claims. That is not the same thing. How is "time progresses as normal for you" ( which is physical fact ) to be construed as any of the above ? Also, do you deny that all free-falling frames of reference are locally Minkowskian ? Originally Posted by Velexia No surprise, since in the twin paradox the two frames are not symmetric, due to the presence of acceleration in one of them, so "naively" applying the relativetime dilation formula without accounting for acceleration will lead to an apparent paradox. If treated properly with acceleration, the net difference in proper times when they come back together at rest then becomes obvious and expected. They still see each other to be ageing more slowly while in transit, but not at the same rate. I find it kind of telling that you choose to quote from an article on the twin paradox, when the point originally made ( relative time dilation ) deals with purely inertial frames. Consider this : The mathematical description of going from one inertial frame into another is realised by introducing a linear transformation between two vectors x' and x of the form wherein L is a general transformation matrix which represents an as-per-yet unspecified boost and rotation in arbitrary directions, and the 4-vector a represents a shift of origin. We now demand the following restriction to hold : which corresponds to the simple observation that, when performing a rotation and its inverse, you always arrive at the original vector, i.e. a rotation and its inverse chained together will yield the unity matrix. In tensor language this corresponds to In order to prove that each inertial observer sees the other one as dilated, one now only needs to show that such a transformation L leaves the space-time line element ds invariant, meaning that the distance between two events in space-time is the same for all inertial observers : Quod erad demonstrandum. Note that the above invariance of the line element also means that all observers agree on the amount of proper time between two given events in space-time, but not on the coordinate time. I have nothing to offer in this kind of discussion, but I can read. The first thing that leaps out at me is seppuku. Had I been reading this thread at the time that was posted, you would have got a formal warning at least, maybe a couple of days suspension - right then and there. But I'll do it now. Formal Warning Never, ever say anything like that again - or you will be banned, maybe permanently. Arrogance and related issues. I don't need to know anything about SR or GR to perceive that your words and your general style are combative and unresponsive to sensible comments. You need to re-calibrate your approach. This is not a debating society and the topic is not one amenable to a win or lose debate anyway. If you have a question, ask a question . Don't set up pointless contests. Read the answers you get - carefully. If it's still not clear, describe the problem as you see it, then ask another question. Now, of course, I'll have to keep an eye on your contributions. Try to do better. Please. Velexia is holding a number of misconceptions which are, in fact, rather common in think it might thus be instructive, also for the casual reader, to address these in a general manner : Inertial frames & symmetry Consider a rocket cruising through space at uniform velocity ( no acceleration ! ), making this an inertial frame inside is an astronaut. He looks out the window, and sees an asteroid passing by at very high but constant ( relativistic ) speed. Assume further that both rocket and asteroid carry gigantic watches which can be seen and read from everywhere. The first thing we need to realise in scenarios involving two perfectly inertial frames is there is no such thing as absolute velocity - the astronaut cannot tell whether it is him moving past a stationary asteroid, or whether he is stationary and the asteroid passes by him, or some combination of movements. The same goes from the perspective of the asteroid. All they can tell is that there is relative motion between them the two frames themselves are identical in every way, they both experience the exact same laws of physics. From the frame of the rocket, it is the asteroid moving - when the astronaut looks at the asteroid's clock he sees it moving very fast, and hence time dilated. This means that the astronaut sees the asteroid's clock ticking slower from his own frame. Now turn it upside down - the asteroid looks at the rocket. It will think that it is in fact the rocket passing by it very fast, whereas itself is stationary. Hence it sees the rocket's clock moving very fast and therefore also time dilated, i.e. ticking slower. What this means is that both frames see the other one time dilated both frames see exactly the same thing, experience exactly the same laws of physics. This is what it means for inertial frames to be symmetric. Proper time and coordinate time Consider two fixed events in space-time (1) and (2) now consider two observers A and B which travel with the same uniform velocity ( no acceleration ! ) between these same two events along the same trajectory, but in opposite directions. They both carry watches which are clearly visible and readable from everywhere. The first thing to note now is that of course both stopwatches must record the same elapsed time between (1) and (2), since they are moving on the same trajectory in opposite directions. When the clocks are stopped at their destinations, and then brought together and compared, they will show the exact same total proper time. This is proper time - the time a clock travelling between two events physically records. All observers agree on this proper time. Now let's look at the frames while they are in transit - observer A looks at observer B and sees him travelling at very high speed towards or away from him. Hence, because there is relative motion, he sees clock B time dilated. The same holds for clock B - it sees clock A travelling very fast, and hence time dilated. This notion of time, the time an observer sees when looking upon another frame of reference with his own method of performing measurements, possibly some distance away, is called coordinate time. It is called that because it depends on the observers, and their states of relative motion. Different observers do not agree on proper time readings. Dilation between coordinate times is called relative time dilation , whereas dilation between proper times is called gravitational time dilation . The former is purely a result of relative motion, the latter is a result of acceleration and/or the presence of gravity, and they are additive if both motion and acceleration/gravity are present. Now, it is important to clearly distinguish between these two notions, because of the following - what clocks travelling between two events record is always proper time relative time dilation does not affect proper time, whereas gravitational time dilation does effect proper time. Another important thing is this ( without derivation ) : proper time is exactly the length of an observer's world line this is an invariant, i.e. all observers agree on this world line no matter what. The same is not true for coordinate time. Now consider the case of matter falling into a black hole. The world line of such an in falling observer extends from outside the black hole, through the event horizon, and into the singularity. Everywhere along that world line ( except at the singularity itself ) space-time is smooth and continuous, and the world line has a finite, well defined length. All observers agree on this, since it is proper time. Such an in falling observer sees this from his own frame : Not so for some distant observer outside the black hole - he sees the object first speeding up towards the black hole, then gradually slowing down and getting dimmer and redder as it approaches the event horizon ( which looks like a perfectly black disk to him ), and eventually fading away into invisibility. He does not see the object "frozen" in place at the event horizon what happens is rather that the light emanating from the in falling object has longer and longer distances along increasingly complicated null geodesics to travel before it reaches him, hence it appears to him that the object is falling more and more slowly. Ultimately this is because light propagates at finite speed, and it does so along null geodesics in space-time. In the immediate vicinity of a black hole, space-time is highly curved, and light follows that curvature on curved trajectories, like so : From the point if view of such a distant observer, the object is never seen entering the event horizon on that observer's clock it takes an infinite amount of time for the object to reach the black hole. However, because that infinity is in coordinate time, this is true only for the distant observer in his own frame of reference. Concluding that, because a distant observer never sees the object reach the event horizon, the object itself never crosses it in its own local frame of reference is a logical fallacy, because the clock readings and measurements taken by the distant observer are valid only in his own local frame, nowhere else. As for the specific claim on this thread, that matter accumulates just above the event horizon - that is of course something that no observer sees at all, it is a a false conclusion reached via the erroneous assumption that the distant observer's local clock and ruler readings are equally valid for the in falling matter itself. That is not the case though, since the space-time around the black hole is not globally flat. I hope that this clarifies the matter somewhat. If anyone requires textbook references for this, please do let me know otherwise any textbook on relativity will confirm the above. ## Deeper Into the Heart of M87 The release of the first image of a black hole on the 10th of April 2019 marked a milestone event in science, and ever since then, the team behind that image has worked hard to delve deeper into M87’s black hole. This second image is the culmination of this quest. The observation of the polarized light allows us to better understand the information in that prior image and the physics of black holes. This composite image shows three views of the central region of the Messier 87 (M87) galaxy in polarised light. The galaxy has a supermassive black hole at its centre and is famous for its jets, that extend far beyond the galaxy. (EHT Collaboration) “Light is an electromagnetic wave which has amplitude and direction of oscillation or polarization,” explains Mościbrodzka. “With the EHT we observed that light in the M87’s surrounding ring is polarized meaning that waves oscillation have a preferred direction.” This polarization is a property of synchrotron radiation that is produced in the vicinity of this black hole. Polarization occurs when light passes through a filter–think of polarized sunglasses blocking out light and thus giving you a clearer view–thus the polarization of light in this picture accounts for this clearer view of M87’s black hole, which reveals a great deal of information about the black hole itself. “The polarization of the synchrotron light tells us about the orientation of magnetic fields. So by measuring light polarization we can map out the magnetic fields around the black hole.” Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor, Radboud University Capturing such an image of polarized light at a distance of 55 million light-years is no mean feat, and is only possible with the eight linked telescopes across the globe that comprise the EHT. Together these telescopes–including the 66 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA)–form a virtual telescope that is as large as the Earth itself with a resolution equivalent to reading a business card on the Moon. This image shows the contribution of ALMA and APEX to the EHT. The left hand image shows a reconstruction of the black hole image using the full array of the Event Horizon Telescope (including ALMA and APEX) the right-hand image shows what the reconstruction would look like without data from ALMA and APEX. The difference clearly shows the crucial role that ALMA and APEX played in the observations. (EHT Collaboration) “As a virtual telescope that is effectively as large as our planet the EHT has a resolution power than no other telescope has,” says Mościbrodzka. “The EHT is observing the edge of what is known to humans, the edge of space and time. And for the second time, it has allowed us to bring to the public the images of this black hole.” This image–as the above comparison shows– has had its clarity enhanced immensely by calibration with data provided by the Atacama Pathfinder EXperiment (APEX). Of course, these magnetic fields are responsible for much more than just giving us a crystal clear image of the black hole they surround. They also govern many of the physical processes that make black holes such powerful and fascinating events–including one of M87’s most mysterious features. ## Astronomers see first light flare from two distant black holes colliding A whopping 7.5 billion light-years from Earth, two black holes, each about the size of Long Island, rapidly spun around each other several times per second before smashing together in a cataclysmic explosion that sent shockwaves through the Universe. Normally, violent unions like this are dark events, but astronomers think they saw a flare of light emerge from this celestial dance — potentially the first time light has ever been seen from black holes merging. It’s a unique discovery since black holes are notorious for not producing any light at all. These super dense objects are so massive that nothing can escape their gravitational pull — not even light. So how exactly did researchers see a flare from two black holes that aren’t supposed to flare? Well, the black holes may have just been in the right place at the right time, according to a new study published in the journal Physical Review Letters. When they spun together, they were located inside a giant disc of gas and dust. This disc of material spans light-years and actually surrounds a third black hole — a supermassive one at the center of a galaxy. Since the dueling black holes were inside this dusty environment, their spinning and eventual merger created something like a shock wave that slammed into the surrounding dirt and gas. That heated up the nearby material, causing it to glow brighter than normal — and allowing researchers from Earth to spot it. “If it’s two black holes merging, you don’t expect to see anything,” Matt Graham, a research professor of astronomy at Caltech and lead author of the study, tells The Verge. “But because the black holes are surrounded by this stuff, by this accretion disc, that’s different.” The researchers pinpointed this oddball event with the help of the LIGO-Virgo collaboration, an international scientific partnership that’s become increasingly skilled at detecting cataclysmic events like black holes merging. More specifically, LIGO and Virgo seek out tiny ripples in the fabric of the Universe, known as gravitational waves, that stem from distant celestial events. Whenever two massive objects in the faraway Universe merge, they create undulating waves in the fabric of space and time that travel outward at the speed of light. When they reach Earth, such ripples are very tiny, but LIGO’s two observatories in the US and Virgo’s observatory in Italy are just sensitive enough to pick them up. LIGO made history in 2015 when the collaboration detected gravitational waves for the first time from two black holes merging. Since then, LIGO and now Virgo, which came online in 2017, have been beefing up their resumes, detecting a whole slew of mergers throughout the Universe, including those of black holes, neutron stars, and maybe even a black hole colliding with a neutron star. When neutron stars collide, the mergers can sometimes be picked up by observatories that measure their light, even though the objects are really faint. When black holes collide, it’s not something we can see — until perhaps now. “It’s a weird and wonderful event, and in fact we don’t know how rare they are,” Chiara Mingarelli, an assistant professor at the University of Connecticut studying gravitational waves, who was not involved in the study, tells The Verge. One of LIGO’s observatories in Livingston, Louisiana. Image: LIGO To find this flare, Graham and his colleagues capitalized on LIGO’s triumph at finding mergers throughout space to help them solve a puzzle. Graham and his team study really active supermassive black holes in galaxies — known as quasars — and they’d been noticing a weird trend. Sometimes these quasars would flare unexpectedly, glowing super bright without warning, and they wanted to know why. “And we sort of said, ‘Well I wonder what happens if you had black holes in that environment?’” says Graham. Two of Graham’s colleagues, Saavik Ford and Barry McKernan, put out a paper theorizing that black holes merging in these gaseous discs could cause the mysterious flare-ups. “The idea that there might be black holes in the centers of galaxies, very nearby a supermassive black hole, is actually pretty uncontroversial,” Ford tells The Verge, adding, “[We] sat down to think about what the consequences of that might be, and we started to flesh out a theory that we’ve been pursuing for the last decade.” They then decided to put that theory to the test. In 2019, LIGO did a third observational run, scanning for a new crop of mergers in space. Meanwhile, Graham and colleagues were working at Caltech’s Zwicky Transient Facility, which performs a survey of the entire night sky, looking for odd behavior — like flares in distant galaxies. The astronomers decided to wait about six months after LIGO’s observations had ended to see how many mergers the collaboration detected. They then tried to match up those mergers with the flares they had detected with ZTF, to see if any of them corresponded. Once they got all the potential mergers from LIGO and Virgo, it was just a matter of narrowing everything down. They matched up all the flares they had seen with ZTF to the mergers LIGO had spotted, making sure they matched the right part of the sky, at the right distance from Earth. The team also looked at timing they predicted that a flare caused by a merger would occur about 60 to 100 days after the collision took place, as it would take time for things to heat up and cause that glow. They then made sure the flares they found matched the right profile they expected, and it didn’t look like they’d been caused by an exploding star or some other explanation. That ultimately led Graham and his team to the black hole merger they found. And actually finding something they’d theorized about was pretty exciting. “It’s the sort of thing that you dream about as a scientist,” says Ford, “to say, ‘I think the universe is going to do that. I’m going to call my shot.’ And have the Universe go, ‘Yeah, here you go!’” Though, things still aren’t totally confirmed just yet. The black hole merger detected by LIGO-Virgo is still just a candidate it hasn’t been officially named as a merger, and LIGO hasn’t released detailed data about the detection. But the good news is Graham’s team might get extra verification in the future that the flare they recorded did indeed come from swirling black holes. When the black holes merged, it’s likely the resulting black hole that was formed got kicked out of the surrounding dusty disc. However, that hole is still orbiting around the supermassive black hole at the center of the galaxy, and it’s probably going to cross paths with the hot disc of gas in a year or two, heating up the material and causing another bright flare. So if the team sees another brightening in the same galaxy, they’ll be pretty certain their findings were correct. When that happens, the measurement of the flare could help the team learn more about this galaxy and better constrain just how massive the supermassive black hole is at the center. “It will actually allow us to directly probe these disks around supermassive black holes in ways that we that we couldn’t do before,” says Mingarelli. This discovery also gives astronomers another clue about how some faraway galaxies form. It tells them that there may be strange objects doing strange things in the discs that surround supermassive black holes. “It’s not just a large gas disc falling into a supermassive black hole,” says Graham. “You’ve got stars and black holes in there doing things as well.” Plus, this bizarre dance of black holes inside a giant gaseous disc may be the only way we can actually “see” black holes merging in deep space. And that’s even more information that researchers can use to study the cosmos. “We actually now have this probe, both from the electromagnetic signature, and the gravitational wave — both of which provide information,” says Ford. “It’s a brand new, totally different tool for studying how galaxies got to be the way they are.” ## 11 Answers 11 "Physics breaks down" is a bad way of saying what people are trying to say. It's the sort of thing that sounds cool at first, but then it starts misleading people. What scientists mean is "our best theory produces non-sensical or contradictory results in this situation, so we know the theory doesn't make good predictions there." They do not mean that there can never be a theory that works, or that somehow there are no laws of physics whatsoever in the situation. It just means we don't know what the law is. Every physicist fully expects that there are laws of physics that predict what happens at the center of a black hole. Probably something perfectly sensible happens, though it's probably something weird and unlike anything else we know. Let me give an example of a very, very mild case of 'theory breaks down'. Expressing that for a given quantity of gas the pressure and volume are inversely proportional to each other. At low pressures Boyle's law holds good. The reason that it holds good is that at low pressure the gas molecules take up only a small percentage of the total space. If you keep increasing the pressure then eventually you reach the point where the molecules are touching all the time, and after that point even a huge increase in pressure leads to only a tiny reduction of the volume. A dramatic way of saying the same thing is to state that at high pressure 'Boyle's law breaks down'. Of course, in the case of Boyle's law we have a deeper theory that we can move to. There is another consideration, which is independent of observables. As stated Boyle's law suggests that as pressure is increased volume can be decreased to an arbitrarily small value. In and of itself Boyle's law doesn't include any restriction. That is susipicious. A theory with a formula that allows infinities, that is an indication that the theory is only an approximation of a deeper theory. Another comparison: Coulomb's law of the electrostatic force gives that the magnitude of the force is inversely proportional to the distance. Let's say that you have a theory that describes particles as entities that are true point particles. Then two charged particles can approach each other infinitly close, which would imply an infinitely large electrostatic force. That is suspicious My understanding is that the black hole case is in some sense like Boyle's law it doesn't disallow infinities in such a way that there is a suggestion that there is describable physics to be found, but it is out of scope of the theory. A dramatic way of saying that is that at the singularity (as implied by the mathematics of GR) the theory breaks down. "Physics breaks down" sounds good, but it is confusing. A better phrasing would be "known physics breaks down." Physics attempts to model reality using mathematics. In this sense, physics has no "laws." In the famous words of Captain Barbossa, "They're more like guidelines." However, we have many of these guidelines which are so astonishingly reliable on the scale that humans live and breathe at that we find the word "guidelines" is too soft. If you are building a skyscraper, you are more likely to make one that stands up if you think of gravity as a "law" rather than a "guideline." Architects who call gravity a "guideline" tend not to be given the opportunity to design multi-million dollar skyscrapers using other people's money! As such, we should not be surprised when guidelines break down in extraordinary circumstances. Personally, I would define two versions of this "physics breaks down" concept. One of them is when these guidelines break down in known ways, changing into new regimes. Supersonic flight is one great example. You can learn lots of guidelines about how fluids (like air) flow, but those guidelines break down at the speed of sound in that fluid. We then have supersonic flow guidelines to help us from there. In this case, this breakdown was something observed, typically on the edge of a phase transition. We figured out that supersonic flow followed different guidelines than sonic flow because we flew something faster than the speed of sound, and it behaved differently. The other, more complete meaning of "physics breaks down" occurs in things like black holes. These are cases where we have never observed the breakdown, but our guidelines lead us in counterintuitive directions. To the best of all of our study, we have never found an experiment or an astronomical observation which suggests that Einstein's theory of general relativity is anything less than a law. Like everything in physics, its just a guideline, but we have not been able to find any situation where it fails. However, there's a catch. Mathematically, Einstein's theories make strange predictions about extremely massive dense objects. The equations behind it spiral out of control, and you end up with oddities like a "singularity," with 0 volume and finite mass. This means infinite density, and we generally don't think that kind of infinity happens in the universe. So we say "physics breaks down" here because we honestly believe that if we ever get to actually observe this part of a black hole, we will see one of those phase transitions where physics just stops doing the General Relativity thing and starts doing Something Else. We believe this because we are not comfortable with the possibility that such math involving infinities is "right." Another place "physics breaks down" is on the Planck scale. Planck time and Planck length are very small units of time and space. They're on the order of$10^ <-43> ext s$and$10^ <-35> ext m\$ . They are important for another guideline that has shown to be so effective that we call it a law: Quantum Mechanics. We have yet to see anything on the small scale which suggests that Quantum Mechanics is anything less than a law, just like we haven't seen anything on the large scale to suggest that General Relativity is anything less than a law. However, when we get down to these absurdly small scales, the math of quantum mechanics (in particular, quantum field theory) start to break down, failing to yield predictive answers about what happens at that scale because some of the tools simply stop working. In a very handwavy sense, its like cutting things with a knife, smaller and smaller things, until one day you realize that you're trying to cut something thinner than the edge of your knife. Our metaphorical mathematical knife (renormalization) simply stops cutting at this point.

Of course, we have no reason to believe that the universe "ends" at this scale. It's just the point where all of the guidelines which are so successful that they have earned the monicker "law" fall apart. And in the extreme cases, such as the issues at Planck scale or in black holes, its the mathematics of the theory itself which suggests that the theory falls apart.

And so, we spend countless hours devising new experiments with the goal of plumbing these uncertain regions, with the hope of one day writing new physics which doesn't "break down" at these levels.

## Could someone see anything while being inside black hole? - Astronomy

Even as a seven y/o, I'd watch and wonder why, after having six bullets bounce off his chest, he ducked when the crook threw the empty gun at him?

See my earlier quote of Pliny.

To even consider this, we have to suspend our belief in the physiological effects of such an extremely dense gravitational field.

Secondly, the event horizon merely represents the boundary beyond which no EM can escape. Inside that boundary, EM can still travel parallel to the spherical event horizon (like the motorcycle guy in the Cage of Death).

This puzzle is not necessarily consistent with the laws of physics. It is to illustrate points of science which are profound and contradictory.

3 ASTRONAUTS ENTER A BLACK HOLE

AN EXERCISE IN LOGIC
A DEMONSTRATION OF DEDUCTIVE PRINCIPLES IN OUTER SPACE

The Problem. Three astronauts enter the event horizon of a black hole and take positions one above the other a short distance inside the event horizon. The first astronaut is at the lowest point within the event horizon. The second astronaut is a short distance above the first. The third astronaut is a short distance above the second. Summarize each astronaut's views of himself and his companions.

NOTE: All available information is contained within the Problem.
HINT: Never assume.

Observations. The first astronaut sees nothing below him, not even his own suit. Looking up, he sees the second and third astronauts and light entering the event horizon. The second astronaut sees nothing below him, not even his own suit. Looking up, he sees the third astronaut and light entering the event horizon. The third astronaut sees nothing below him, not even his own suit. Looking up, he sees light entering the event horizon.

Conclusions. The third astronaut sees light entering the event horizon, but cannot see anything below him, because light cannot reflect upward, so everything below him is void. The astronauts below him see the same effect. The second astronaut sees the astronaut above him and light entering the event horizon, but nothing below himself. The first astronaut sees the two astronauts above him and light entering the event horizon, but nothing below himself. That the third and second astronauts see nothing beneath themselves, but only upwards is unique for all three astronauts. They see above them what the higher astronaut cannot see underneath himself, because light can only move inward in a black hole. The astronauts can see nothing beneath them, or even laterally, because light can only travel downward, not even across the width of a black hole.

Ramifications. Are these examples of paradoxes? The astronauts can see each other's suits, but cannot see their own suits. They are observing each other within the same environment, yet see very different representations of reality within their own view.

Is there a problem in finding the key that opens the door to the post? Maybe you're using the wrong key.

Are you having issues understanding what you see through the telescope? Do objects appear farther away than they should be? Maybe you're looking through the wrong end the more you look, the more distant they become. This post isn't about science, it isn't even about black holes it's about reasoning through a problem to find a solution. It's also about taking a short break from a daily routine. This "brain-teaser" is like: a crossword puzzle an optical illusion a "minute-mystery". Several "cautions" were written into the message:

"..not necessarily consistent with the laws of physics. "

"..to illustrate points of science. "

These warnings were advisories not to take the challenge too seriously. Each statement has a meaning that relates to the intent of the puzzle. First, to simplify the narrative, liberties were taken with laws of physics. To emphasize the focus of the puzzle, some principles were set aside. Not all laws that apply to black holes, light and gravity were followed. Second, aspects of the puzzle were "exaggerated" to make it easier to see effects of the principles involved. Third, this is merely an example of the forces which are at work under these conditions. It is to show how they function they are not predictors of what happens when a black hole is encountered.

This post is: A puzzle a fun thing: a diversion entertainment a brief "detour" from a daily agenda.
This post is not: Precise realistic authentic to scale reliable guaranteed "true-to-life".

This post: Is to induce a smile and a perspective on how forces effect each other and how one force overcomes another under the right conditions.
This post does not offer: College-credit advance placement to higher-level courses a recommendation to study or teach at a prestigious school an invitation to tour the large hadron collider in geneva.

The post is meant to be enjoyed and to reflect parts of our universe we don't understand yet. If you're not having fun, why not?

[font="Times New Roman"] Is there a problem in finding the key that opens the door to the post? Maybe you're using the wrong key.

If this were a fairy tail that some author wrote, then only he could "know" what happens in his make-believe world. You write your solution as if you were that author.

Let me point out that you're making a common mistake-- you're forgetting that spacetime in a dense field like that "inside" a black hole only looks distorted to those outside the black hole (or those much farther "in" or "out" like your three astronauts. Spacetime is always flat locally. We might compare that to our own observations of geometry here on Earth-- The world IS flat, locally. Euclidean geometry only applies in textbooks and on very local scales throughout the Universe.

Even in a black hole, Spacetime will appear flat to the local observer. An astronaut on the space station feels like he's travelling in a straight line. A light beam inside a black hole also feels like it's travelling in a straight line, and it is-- but as it travels towards the event horizon, it never gets there-- it "falls back," ie- travels in a straight line in space time which would looked curve to an outsider.

What happens when you toss a ball to your friend? It travels in a parabolic path which can be traced out on graph paper- horizontal distance vs vertical distance. What happens when you toss a ball straight up? It falls back down along the same vertical path (assuming it doesn't travel with escape velocity and is able to cross the "event horizon"). What happened to our parabolic path. It's still there, if we graph the trajectory as height vs time.

Your description is only correct if we assume there is a continuum of event horizons at all "vertical distances" inside the event horizon and that no EM wave can travel vertically at all, ie- not really a single spherical "horizon" with no "thickness" but an event "solid volume."

I submit that inside what we outsiders would call an event horizon, there does exist multiple event horizons, but they occur at progressively closer intervals and apply relatively to the source of the EM waves. That would make my first "solution" correct-- each guy would see himself as being normal, because he's viewing himself locally where light still travels at 3 x 10^8m/s and he still looks 176cm tall, the same as he was on Earth. But his companions would look distorted because, as close as they are to him, spacetime is noticeably different at progressively smaller intervals.