Laypersons problems understanding Black Holes

[kmart888]

A description of problems with the common understanding of black holes by this layperson:

I have been contemplating black holes since I first heard about them at a lecture series by the then director of the Griffith Observatory in Los Angeles, by Dr. William J. Kaufman circa 1964. It seems to me, at first blush, to violate a common understanding of general relativity. The only explanation that has ever been offered to me is that the maths describing black holes are woefully beyond my first year calculus education and it has something to do with spin; this does not help my understanding (if possible) of the problem.

Let Me Explain:

Problem Number 1:

Time slows down as one approaches the speed of light. If a massive object could actually obtain the speed of light, time would come to a complete stop. Of course, it would take an infinite amount of energy to accelerate any massive object to the speed of light, quite impossible. Similarly, as gravity increases time slows down, once the influence of gravity exceeds the escape velocity of light, as it would at the event horizon around a collapsed massive star, time would stop and it would take an infinite amount of energy (this time supplied by gravity or space-time curvature) to accelerate any massive object through the event horizon to be "gobbled up” by the black hole as the popular press usually depicts.

Let us imagine we could watch a spacecraft fall into a black hole from a point of reference outside the influence of the black hole, say the earth, we would first see the spacecraft torn to pieces by huge tidal forces, but the pieces would apparently slow down approaching the "black hole", it would take an infinite amount of time, from our frame of reference, to approach the event horizon, instead of being "gobbled up" the pieces would be compressed into an infinitesimally thin membrane or film at or just before reaching the event horizon (point at which escape velocity equals the speed of light). This membrane should contain all massive objects rushing towards, but never having enough time to reach the so-called singularity, it could contain all the mass and all the information contained in any objects that were unfortunate enough to have been trapped in this gravitational well.

Now let us imagine we could travel aboard the spacecraft approaching the black hole (and not be crushed to death); we would see ourselves accelerate towards it at an increasing rate. As our personal clocks would slow down, the whole universe around us would apparently speed up, it should still take an infinite amount of time to actually reach the event horizon; we could see the end of the universe (and the end of time itself) before we actually reached the event horizon, another apparently impossible scenario.

Problem number 2:

Gravity; we detect the existence of a black hole based on its gravitational influence (curvature of the fabric of space time) on the objects around them. Should not gravitational influence be bound by the same laws of physics that apply to light waves/photons? Certainly discussions of gravity waves/gravitons seems analogous to light (light and gravity may be mingled together in some theory of everything yet to be discovered). If light cannot escape from a black hole, why do we assume gravity can? It seems more reasonable to assume that the gravitational evidence of a black hole’s existence is in fact the extreme distortion of space-time caused by a massive membrane surrounding the "event horizon", in fact, space-time would have closed in on itself and there would be no "inside" to an event horizon, it is no longer part of the universe.

Universe:

The common layperson definition of universe is as follows (taken from Wikipedia):
"The universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them". (Yet today's scientists have seemingly redefined the word universe to allow for multiple universes, parallel universes, and all sorts of strange "things" that were once solely the province of science fiction, but now taken seriously by popular scientists. I used the common definition when examining what lies beyond an event horizon.)

So asking what lies "inside" the event horizon is like asking what lies beyond our universe or what did the Big Bang expand into. There is nothing inside an event horizon, no information can be exchanged from inside a black hole, and even Hawking radiation can be explained by 1/2 of the virtual particle pair being trapped forever falling into the event horizon and the other half escaping (I think). The answer I come up with is "nothing" is inside a black hole, no mass, no "space", no space-time, no singularity, no nothing. The black hole, as it is popularly described, does not exist, just an area of extremely distorted space-time in the near proximity of the mass forever falling into the gravity well of a collapsed star.

Now whether any of the material of the collapsing star itself could be trapped "inside" an event horizon before time ended is something I have not yet wrapped my brain around.

Paul Martin

 

[LostConjugate]

I am confused. You described both reference frames exactly the same. That can't be.

An observer undergoing time dilation never notices it from their reference frame. It would only appear to take an eternity from an outside observer's reference frame. From the frame of the spacecraft it would simply enter the event horizon.

For an expert's opinion this post should be under the relativity section.

 

[kmart888]

I am confused. You described both reference frames exactly the same. That can't be.

An observer undergoing time dilation never notices it from their reference frame. It would only appear to take an eternity from an outside observer's reference frame. From the frame of the spacecraft it would simply enter the event horizon.

For an expert's opinion this post should be under the relativity section.

Thank for the response. It is my contention, and I don't yet see where I am wrong, that the observer undergoing time dilation would, as you say, never notice any personal effect but as his personal frame slows down relative to any frame outside the gravity well he must view the universe around him apparently speed up, if he could escape the gravity well the universe would have aged many thousands of years while he had hardly aged at all. I see no way around this conclusion and have GR time dilation make sense and conserve space-time.

I shall re-post my q's in the relativity section, this was my first post; still learning the ropes.

 

[WannabeNewton]

...but as his personal frame slows down relative to any frame outside the gravity well he must view the universe around him apparently speed up...

His/her/its own reference frame does not slow down.

 

[Bloodthunder]

Now let us imagine we could travel aboard the spacecraft approaching the black hole (and not be crushed to death); we would see ourselves accelerate towards it at an increasing rate. As our personal clocks would slow down, the whole universe around us would apparently speed up, it should still take an infinite amount of time to actually reach the event horizon; we could see the end of the universe (and the end of time itself) before we actually reached the event horizon, another apparently impossible scenario.

Let's see if I remember my relativity.
You are increasing in velocity as you approach the black hole. During this period, as you approach the speed of light, your time slows down due to time dilation. However, from your point of view, the rest of the universe with v = 0 would be the one approaching the speed of light, so from your point of view the universe is the one which time is slowed.

Gravity; we detect the existence of a black hole based on its gravitational influence (curvature of the fabric of space time) on the objects around them. Should not gravitational influence be bound by the same laws of physics that apply to light waves/photons? Certainly discussions of gravity waves/gravitons seems analogous to light (light and gravity may be mingled together in some theory of everything yet to be discovered). If light cannot escape from a black hole, why do we assume gravity can? It seems more reasonable to assume that the gravitational evidence of a black hole’s existence is in fact the extreme distortion of space-time caused by a massive membrane surrounding the "event horizon", in fact, space-time would have closed in on itself and there would be no "inside" to an event horizon, it is no longer part of the universe.

This was asked a while back.

From: http://sciastro.astronomy.net/sci.astro.4.FAQ

Subject: D.09 How can gravity escape from a black hole?
Author: Matthew P Wiener <weemba@sagi.wistar.upenn.edu>,
Steve Carlip <carlip@dirac.ucdavis.edu>

In a classical point of view, this question is based on an incorrect picture of gravity. Gravity is just the manifestation of spacetime curvature, and a black hole is just a certain very steep puckering that captures anything that comes too closely. Ripples in the curvature travel along in small undulatory packs (radiation---see D.05), but these are an optional addition to the gravitation that is already around. In particular, black holes don't need to radiate to have the fields that they do. Once formed, they and their gravity just are.

In a quantum point of view, though, it's a good question. We don't yet have a good quantum theory of gravity, and it's risky to predict what such a theory will look like. But we do have a good theory of quantum electrodynamics, so let's ask the same question for a charged black hole: how can a such an object attract or repel other charged objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if quantum gravity ends up looking like quantum electrodynamics) are mediated by the exchange of *virtual* particles. This allows a standard loophole: virtual particles can pretty much "do" whatever they like, including traveling faster than light, so long as they disappear before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must exceed the speed of light in order to escape, and thus are trapped. The horizon is meaningless to a virtual particle with enough speed. In particular, a charged black hole is a source of virtual photons
that can then do their usual virtual business with the rest of the universe. Once again, we don't know for sure that quantum gravity will have a description in terms of gravitons, but if it does, the same loophole will apply---gravitational attraction will be mediated by virtual gravitons, which are free to ignore a black hole event horizon.

See R Feynman QED (Princeton, ?) for the best nontechnical account of how virtual photon exchange manifests itself as long range electrical forces.

 

[kmart888]

Bloodthunder,

Thank you, your a busy (I assume) undergad yet you take the time to answer my sophomorish question, one I'm sure you have answered before, much appreciated! You are right but it will take some time to mull this over, and a review of my old copy of QED, before I can get My head around this seemingly crazy paradox.

thanx,

Paul

 

[pervect]

A description of problems with the common understanding of black holes by this layperson:

Let us imagine we could watch a spacecraft fall into a black hole from a point of reference outside the influence of the black hole, say the earth, we would first see the spacecraft torn to pieces by huge tidal forces, but the pieces would apparently slow down approaching the "black hole", it would take an infinite amount of time, from our frame of reference, to approach the event horizon, instead of being "gobbled up" the pieces would be compressed into an infinitesimally thin membrane or film at or just before reaching the event horizon

I'm not sure where you've got this detailed, but incorrect, idea about the space-craft breaking up and being torn to pieces. In the spaceships frame, the tidal forces are finite, and, depending on the size of the black hole, possibly small.

Details of what happens depends on the size of the black hole. I'd suggest reading Kip Thorne's popular book, "Black holes and Time Warps", for an _accurate_ popularization of what you'd see and experience in a close encounter with a black hole. You can also find good information on the WWW, for instance Ted Bunn's black hole FAQ (and the sci.physics.faq's that deal with black holes). For example:

http://cosmology.berkeley.edu/Education/BHfaq.html#q3

For a very large black hole like the one you're falling into, the tidal forces are not really noticeable until you get within about 600,000 kilometers of the center. Note that this is after you've crossed the horizon. If you were falling into a smaller black hole, say one that weighed as much as the Sun, tidal forces would start to make you quite uncomfortable when you were about 6000 kilometers away from the center, and you would have been torn apart by them long before you crossed the horizon. (That's why we decided to let you jump into a big black hole instead of a small one: we wanted you to survive at least until you got inside.)

For a small black hole, tidal forces MIGHT tear you apart before you reached the horizon - for a larger black hole, they won't.

Now let us imagine we could travel aboard the spacecraft approaching the black hole (and not be crushed to death); we would see ourselves accelerate towards it at an increasing rate. As our personal clocks would slow down, the whole universe around us would apparently speed up, it should still take an infinite amount of time to actually reach the event horizon; we could see the end of the universe (and the end of time itself) before we actually reached the event horizon, another apparently impossible scenario.

This is also incorrect. See for instance http://casa.colorado.edu/~ajsh/singularity.html

Answer to the quiz question 5: False. You do NOT see all the future history of the world played out. Once inside the horizon, you are doomed to hit the singularity in a finite time, and you witness only a finite (in practice rather short) time pass in the outside Universe.

Viewing time as something that "slows down" is simple, easy to understand,and not really what happens when you fall into a black hole. One of the main failures of this point of view is that it incorrectly predicts that someone falling into a black hole would see the entire history of the universe, while a detailed mathematical calculation based on the principles of relativity and the relativity of simultaneity shows that this doesn't happen.

 

[bcrowell]

If light cannot escape from a black hole, why do we assume gravity can? It seems more reasonable to assume that the gravitational evidence of a black hole’s existence is in fact the extreme distortion of space-time caused by a massive membrane surrounding the "event horizon", in fact, space-time would have closed in on itself and there would be no "inside" to an event horizon, it is no longer part of the universe.

The cheap answer to your question is that this isn't what the Einstein field equations (EFE) say, the EFE have been exhaustively verified by experiments and observation ( http://relativity.livingreviews.org/Articles/lrr-2006-3/ ), and we also observe a number of black holes, such as Sagittarius A*, which behave as predicted. But of course that's a cop-out, because you asked for an explanation at a layperson's level. One thing to realize here is that we would have big problems if information could be propagated from the inside of the event horizon to the outside. Because the black hole's gravitational field is constant, it doesn't carry any information. Also, it doesn't particularly matter whether you consider the material that formed the black hole as having already passed inside the event horizon or as still being outside it; the distinction is a meaningless one, since simultaneity is not well defined in relativity. Therefore an exterior observer can also attribute the exterior fields to the matter that you say hasn't yet passed through the horizon.

 

[bcrowell]

I've closed the thread, since the questions asked by the OP have been answered.