I Where is the matter in a black hole?

[dreens]

Hey Green Dwarf,

At time t0 on earth, a cow is sent off toward the black hole in the center of the galaxy. Does there exist a finite time t1 on earth, after which even the most powerful conceivable rocket could not rescue the cow? Based on your shell argument, I imagine you might think "NO". This is what I used to think for a few years in between taking GR for the first time and sitting across a desk from a black hole theorist for a few hours.

However the answer is "YES". To convince yourself of this, think from the cow's perspective. The cow can still watch things on Earth after it crosses the EH, for a brief time before it hits the singularity. So if the rescue rocket is sent off at an Earth time t2 such that the cow sees CNN's coverage of the rocket launch just after it has crossed the EH in its own frame, the rescue mission is going to fail despite JPL's efforts.

The Earth time interpretation of the cow's experience (lingering near the EH) is irrelevant as soon as you are interested in a dynamical activity that involves interplay between the Earth and the cow, such as a rescue mission. General Relativity frees us to think in the coordinate system most appropriate to a given scenario. Earth time is not useful for objects approaching an EH, and indeed leads to misperceptions such as one I had for many years, that the cow is always salvageable.

 

[zerospin]

I don't believe in a singularity, what we call a singularity is caused by our incomplete understanding of how matter and energy work. Nature does not deal with infinite amounts or values. It is far more likely, that inside a black hole, somewhere below the event horizon, the matter that fell inside is stopped, once again, by some yet unknown form of pressure that prevents a collapse to a point of no dimensions. Perhaps dark energy pressure stops it, or something that involves dark matter or something completely different. We barely understand a little about how the visible matter and energy works, and we know almost nothing about 95% of the stuff that makes the Universe, so out theories are incomplete and flawed by definition.

So my answer is, the "matter" or what is left of it, forms a ball of finite and non-zero radius below the event horizon, and a yet unknown form of pressure is preventing further collapse. To me this makes far more sense than something with infinite density and 0 radius.

Furthermore, the matter that falls in never breaks the speed of light before reaching the surface of this ball, and therefore time never stops.

 

[Green dwarf]

However the answer is "YES". To convince yourself of this, think from the cow's perspective. The cow can still watch things on Earth after it crosses the EH, for a brief time before it hits the singularity. So if the rescue rocket is sent off at an Earth time t2 such that the cow sees CNN's coverage of the rocket launch just after it has crossed the EH in its own frame, the rescue mission is going to fail despite JPL's efforts.

If the picture I painted is correct, then wouldn't the cow see the CNN coverage of the launch before it crossed the EH, not afterwards? The coverage photons and the rescue ship should have an infinite amount of time to reach the cow before the cow crosses the horizon. Once the cow does cross the EH, it would still see the outside universe, but it would be seeing a universe more than an infinite time into our future, long after it had suffered heat death or whatever. CNN probably won't be broadcasting then.
This is all assuming of course that it is possible to rescue a cow from a few millimetres outside the EH.

 

[Green dwarf]

I don't believe in a singularity, what we call a singularity is caused by our incomplete understanding of how matter and energy work. Nature does not deal with infinite amounts or values.

Have we actually proved that or is it just that the maths gets too hard for our present capabilities when things become infinite?

I know this is a bit different, but I understand that, if the universe is flat, then it must also be infinite. No one seems to have too much trouble with this infinity.

What boggles my mind is that, if the universe is infinite, then it would have been infinite at the time of the Big Bang too. Our whole observable universe would have been contained in a sphere maybe 10^-15 m in diameter. 1 cubic metre of this material would have contained 10^45 universes like our own, 1 cubic light year 10^93 of our universes. Yet 1 cubic light year would be an infinitesimal part of the whole universe. I'm not trying to make any particular point here - just being awe struck.

 

[Green dwarf]

I'm fascinated by the discussion. Thanks to everyone who has contributed.

 

[anorlunda]

I think this is relevant to the OP's question. I would like some clarity on the boundary between theoretical versus obervational science on the title of this thread.

Can we ever observe, directly or indirectly, the distribution of mass inside the EH of a BH?

Changing the word mass to energy is probably a better way to state the question, thus ...

Can we ever observe, directly or indirectly, the distribution of energy inside the EH of a BH?

A spinning BH is theorized to cause frame dragging. That may be a way to indirectly make some singularity measurements. Course, we haven't even been able to detect gravity waves yet.

The spinning case is a possible exception. There may be other special cases. But I intend to exclude special cases from my question.

 

[Nugatory]

If the picture I painted is correct...

It's not. When you hear someone saying that it takes an infinite amount of time for the cow to reach the event horizon, they are using a particular convention for defining "at the same time" in the statement "at the same time that the cow is $X$ meters above the event horizon the outside observer's wristwatch reads $T$". That convention is similar to (not exactly the same, but mine is easier to explain) the following:
- At time $T_0$ according to his wristwatch, the outside observer sends a radar/light signal towards the infalling cow.
- This radar signal reaches the cow and is reflected back to the observer, arriving at time $T_1$ according to his wristwatch.
- The round-trip time for the signal was $\Delta{t}=T_1-T_0$ and the signal was moving at the speed of light, so the round-trip distance was $c\Delta{t}$. The one-way travel time was $\Delta{t}/2$ and the one-way distance was $c\Delta{t}/2$.
- Therefore, at the same time that the observer's wristwatch read $T_0+\Delta{t}/2$ the cow was at a distance $c\Delta{t}/2$ away and still hasn't fallen through the horizon (because we're still getting radar returns from it).

Note that $\Delta{t}$ grows without bound as the cow gets closer to the horizon; thus the outside observer finds that any time on his wristwatch, no matter how large, corresponds to a position of the cow outside the horizon and only in that sense can we say that it takes an infinite time for the cow to reach the horizon. However, this is telling us more about what happens to light signals between us and the cow than it is about what's really happening to the cow. The convention is simply refusing to recognize any position of the cow that doesn't produce a radar return; that's a problem with the convention, not the path of the cow through spacetime.

The key to resolving these paradoxes is to adopt a convention for matching the cow's position to the time on the outside observer's watch that works everywhere, not just outside the event horizon. The easiest to visualize are Kruskal coordinates - the math is quite daunting but you can get a satisfactory qualitative understanding understanding just from looking at the spacetime diagrams. You can plot the path of the cow through spacetime, the path of the CNN broadcasts following the cow across the event horizon and into the singularity, and the path of the radar signals that the outside observer is using to determine where the cow is when.

then wouldn't the cow see the CNN coverage of the launch before it crossed the EH, not afterwards? The coverage photons and the rescue ship should have an infinite amount of time to reach the cow before the cow crosses the horizon.

Again, no. The cow crosses the horizon, the CNN broadcast crosses the horizon a moment later. If there's enough time left, the broadcast catches up to the cow a moment before the cow reaches and dies at the central singularity so the cow gets to see it. Any later broadcasts reach the central singularity after the cow, so the cow is dead before it can see them. [/QUOTE]

 

[Nugatory]

Can we ever observe, directly or indirectly, the distribution of mass inside the EH of a BH?

As far as we know, only if we're willing to cross the event horizon to find out - and then we don't get to tell anyone outside.

 

[Nugatory]

I don't believe in a singularity, what we call a singularity is caused by our incomplete understanding of how matter and energy work.

Have we actually proved that or is it just that the maths gets too hard for our present capabilities when things become infinite?

Neither of the above. Zerospin has a very good conjecture, namely that some effect that we don't yet know about prevents the infinities from happening. But it's still a conjecture, not something that we know for sure.

 

[Dr Bee]

By definition the escape velocity of a Black hole is the speed of light but matter cannot travel at the speed of light so matter entering a black hole will not be traveling at the speed of light only very very close, ? It will be falsely observed to slow down and its length immensely shortened but these observations are an illusion , all that is happening is that time has slowed down hugely for the matter which we observe as matter compressing and almost coming to a halt but what does it look like from the matters perspective ? To the matter billions of years pass in milli-seconds as it enters the black hole and its matter fields become entwined in the total matter fields already in the black hole and more importantly the information that attended the matter cannot be destroyed and is retained somehow?

 

[Green dwarf]

Can we ever observe, directly or indirectly, the distribution of mass inside the EH of a BH?

To me, the question is whether the matter/energy in a black hole forms a shell at the EH or a singularity at the centre. We are trying to decide this theoretically, but I wonder whether the question is determinable by observation. Clearly our present technology is insufficient, but is it theoretically possible to tell by observation?

By Newtonian mechanics, the gravitational field outside the black hole would be the same in either case. But what if we consider frame-dragging? Would the effect be any different for the shell (which would be rotating at the EH with a finite angular velocity) than for the singularity (which, in order to conserve angular momentum, would, I presume, be spinning with infinite angular velocity at the centre, well away from the EH).

My knowledge of GR is nowhere near sufficient to know if there would be a difference observable to someone outside the EH, but someone reading this might know the answer to that question.

 

[Grinkle]

To me, the question is whether the matter/energy in a black hole forms a shell at the EH or a singularity at the centre.

Post 57 is a laudably clear response to this.

 

[anorlunda]

To me, the question is whether the matter/energy in a black hole forms a shell at the EH or a singularity at the centre.

That was your question, and I see that you are most interested in the first part, but my follow-up regarded the second part of the question. The existence of a singularity is speculation.

Once the matter falls inside the EH, we lose our abiliy to observe it from outside the EH. One could also speculate that the matter gets stuck in a shell 1 millimeter inside the EH. That is probably ridiculous but the point is that we are unable to prove or refute that by observation from outside the EH. Even frame dragging might not reveal whether the interior is a singularity or a hollow shell or any intermediate case. That is quite a bit to chew on.

 

[Bernie G]

With all we know about physics and math, few physicists believe in the 'infinitely dense' singularity thing anymore. Nature abhors a vacuum, and infinities,

If the singularity isn't infinitely dense there is a support mechanism that isn't taken into account by the curvature model.

 

[rootone]

If the singularity isn't infinitely dense there is a support mechanism that isn't taken into account by the curvature model.

Indeed, and I don't think there are many who think that a infinitely dense dimensionless object which the present model leads to could be a physical reality.
Many suspect that the support mechanism could be found within a theory of gravity at quantum scale, and proposals of such so exist, but afaik, nobody is close to pinning that down yet.

 

[Nvitinaros]

Forgive me if I misunderstand the scenario but my understanding of static (non rotating) black hole dynamics goes something like this.
1. The event horizon is merely the point at which light cannot escape to the outside universe, the consequence being that matter reaching this point releases its energy (as xrays due to compression from spagetification or gama rays.. energy released during the rest of the trip stays inside the EH).
2. The time dilation occurs as a function of gravity and speed in the object frame, not the observers. the object frame slows to a near stop,, imagine a million light years per tic.
3. To the observer the object simply accelerates into the EH with a bright flash and a slight enlargement of the EH (no time dilation or slowing of time).
The redsift to black would be there, but tough to observe given the other high energy activity..
Once inside the EH the theories multiply, my favorite is that time is reversed and the black hole is its own universe.

 

[Nugatory]

Forgive me if I misunderstand the scenario but my understanding of static (non rotating) black hole dynamics goes something like this.
1. The event horizon is merely the point at which light cannot escape to the outside universe, the consequence being that matter reaching this point releases its energy (as xrays due to compression from spagetification or gama rays.. energy released during the rest of the trip stays inside the EH).
2. The time dilation occurs as a function of gravity and speed in the object frame, not the observers. the object frame slows to a near stop,, imagine a million light years per tic.
3. To the observer the object simply accelerates into the EH with a bright flash and a slight enlargement of the EH (no time dilation or slowing of time).

None of that is right. There are many threads here and in the relativity forum in which this is discussed.

Once inside the EH the theories multiply, my favorite is that time is reversed and the black hole is its own universe.

There is only one generally accepted theory about what happens inside the black hole but away from the singularity, it is General Relativity, and it says neither that time is reversed (what does that mean?) nor that the black hole is its own universe.

 

[Gerhard Mueller]

If a sun collapses to a black hole, the matter does not cross the event horizon. Instead as soon as the even horizon exists, the matter stays inside. But it seems that during the collapse gravitational forces have been so intense, that even protons and neutrons became squeezed to something we do not understand. May be these squeezed particles together with electromagnetic radiation still exist inside the black hole.

 

[Grinkle]

If a sun collapses to a black hole, the matter does not cross the event horizon. Instead as soon as the even horizon exists, the matter stays inside. But it seems that during the collapse gravitational forces have been so intense, that even protons and neutrons became squeezed to something we do not understand. May be these squeezed particles together with electromagnetic radiation still exist inside the black hole.

Does an event horizon form at "full size" instantaneously? For some reason I thought it would start at the very center of the object (singularity or whatever one calls it) and propagate out at the speed of gravity, c, and stop at the diameter determined by the mass of the singularity plus, I suppose, whatever other matter ended up inside it as it expanded. If it forms at full size all at once, how does it "know" about the matter it will have swallowed once it exists? Space time curvature changes at the speed of gravity, doesn't it?

 

[phinds]

If a sun collapses to a black hole, the matter does not cross the event horizon. Instead as soon as the even horizon exists, the matter stays inside. But it seems that during the collapse gravitational forces have been so intense, that even protons and neutrons became squeezed to something we do not understand. May be these squeezed particles together with electromagnetic radiation still exist inside the black hole.

Maybe, maybe not but there is no evidence for it or theory that suggests it so this is personal speculation which is not allowed here.

 

[phinds]

Does an event horizon form at "full size" instantaneously? For some reason I thought it would start at the very center of the object (singularity or whatever one calls it) and propagate out at the speed of gravity, c, and stop at the diameter determined by the mass of the singularity plus, I suppose, whatever other matter ended up inside it as it expanded. If it forms at full size all at once, how does it "know" about the matter it will have swallowed once it exists? Space time curvature changes at the speed of gravity, doesn't it?

When the gravity in a region becomes strong enough that light can't get out there is, by definition, an event horizon. It does not form at a center and move out.

 

[anorlunda]

Maybe, maybe not but there is no evidence for it or theory that suggests it so this is personal speculation which is not allowed here.

That's true, but we do know that the BH gravitates, so something exists inside there. Leonard Susskind also asserts that things inside the EH can be entangled with things outside which also argues for the existence of things inside the EH. Isn't it also true that if the BH has a net charge, that we could detect that from outside the EH? So it is tantalizing. Some properties of the BH's content are observable, but the identity of such content is not observable.

 

[phinds]

That's true, but we do know that the BH gravitates, so something exists inside there. Leonard Susskind also asserts that things inside the EH can be entangled with things outside which also argues for the existence of things inside the EH. Isn't it also true that if the BH has a net charge, that we could detect that from outside the EH? So it is tantalizing. Some properties of the BH's content are observable, but the identity of such content is not observable.

No argument with any of that, I was just letting him know (as you recognize) that what HE was saying is personal speculation and that it's not suitable for this forum.

 

[Grinkle]

When the gravity in a region becomes strong enough that light can't get out there is, by definition, an event horizon. It does not form at a center and move out.

Trying to come up with something observable to ask about. If sufficient mass to cause an event horizon 1 light minute in radius were to collapse, and I am far enough away to watch safely through a telescope, and I am looking at the star field behind the object as it is collapsing, would I see an instant occlusion that is 1 LM in radius instead of an occlusion that grows at light speed for one minute and then stabilizes?

 

[DaveC426913]

Trying to come up with something observable to ask about. If sufficient mass to cause an event horizon 1 light minute in radius were to collapse, and I am far enough away to watch safely through a telescope, and I am looking at the star field behind the object as it is collapsing, would I see an instant occlusion that is 1 LM in radius instead of an occlusion that grows at light speed for one minute and then stabilizes?

The "sufficient mass" occupying that space is already sufficient to bend the light so you can't see it, before it collapses into a black hole.

Remember, the star field is only affected by the amount of mass present. And the mass has not changed.

So, nothing will change in the star field upon collapse - it will already be distorted/missing.

 

[Grinkle]

The "sufficient mass" occupying that space is already sufficient to bend the light

Of course - thanks.

 

[newjerseyrunner]

Actually, bending space does not require any additional dimensions. It's easier to picture in 1D. Think of a meter stick made of rubber. You can stretch parts of it and manipulate it all you want along it's axis, no need to invoke any height or width. I like to envision it more like stretching space. The reason the term bending is used is obvious when you move to a higher dimension: take that meter stick and pull it out into a sheet. Now there is no way to stretch any part of it without causing a bend to the rest of it.

On a semi-related note, I thought you might find it interesting that we do actually know a bit about what happens to neutrons and protons under that pressure: they become a quark-gluon plasma, and then quark degenerate matter. As far as I'm aware, the former we've made in the LHC and the other is based on QMs math. Beyond that is where the mystery lies.

 

[anorlunda]

The video below is pretty entertaining, and it touches on the subject of this thread. Susskind posits a scenario where 100% of the contents of two black holes could be entangled with the contents of the other BH, and how a wormhole between them could exist. Alice could enter the EH of BH1, while Bob enters the EH of BH2 and they meet each other in the middle in a wormhole. But alas, neither of them can tell anyone outside the EHs about their experience.

Of course the serious science behind this whimsy is an attempt to unite the quantum and GR views, via information theory.

 

[EugeneBird]

Thank you Anorlunda, I didn't think anyone was going to mentions the newest discovery: the AMPS argument. Joe Polchinski and three others discovered violations of Quantum Mechanics in the view that everything at the event horizon of a black hole is 'nothing special' and that you just slide on through not noticing anything is there. This is what Leonard Susskind is speculating about in the video above. Most of what you have read in this thread has been called into question two years ago! Black holes might not even have an 'inside' to their event horizons ... decades of talk from kip Thorne and Steven hawking turned on their heads with this new info ... so exiciting ... Watch this video to get it straight from the scientist himself!

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

Does an event horizon form at "full size" instantaneously?

No. See below.

When the gravity in a region becomes strong enough that light can't get out there is, by definition, an event horizon. It does not form at a center and move out.

The first sentence is true, but the second sentence does not follow from it. Here is what actually happens (according to the GR model of gravitational collapse to a black hole):

The event horizon is the boundary between the region of spacetime that can send light signals to infinity and the region that can't. Suppose we have a spherically symmetric object that is collapsing, and light rays are being emitted radially outward from a point at the center of the collapsing object. One of these light rays (more precisely, outgoing spherical wave fronts) will intersect the surface of the object at the exact instant that that surface is at $r = 2M$, i.e., at the radius where the surface of the object is just passing the event horizon. That light ray (outgoing spherical wavefront) will then stay at $r = 2M$ forever (we are assuming nothing else falls in in the future). The entire history of that spherical wave front marks out the event horizon, not just the portion after it reaches $r = 2M$. (Can you see why?) So the event horizon does form at the center of the collapsing matter and move outward.

 

[PeterDonis]

If a sun collapses to a black hole, the matter does not cross the event horizon.

Yes, it does. See my previous post.

it seems that during the collapse gravitational forces have been so intense, that even protons and neutrons became squeezed to something we do not understand.

We don't really understand the equation of state of condensed matter at densities higher than nuclear densities (or neutron star densities). So we don't really have a good basis for discussion of this topic.

May be these squeezed particles together with electromagnetic radiation still exist inside the black hole.

According to GR, the matter that collapses to form the black hole continues to collapse down to the singularity, and then ceases to exist. I don't think any physicists believe that the GR model is correct all the way down to the singularity; but we don't have a good theory to replace it, and won't until we figure out how quantum gravity works. So again, we don't really have a good basis for discussion of this topic.

 

[PeterDonis]

Joe Polchinski and three others discovered violations of Quantum Mechanics in the view that everything at the event horizon of a black hole is 'nothing special' and that you just slide on through not noticing anything is there.

I think it would be more correct to say that Polchinski et al have claimed that they have discovered violations of QM in this case. This is an area of active research and speculation, and there is no settled answer yet.

 

[PeterDonis]

Some overly speculative posts have been deleted, and one subthread has been moved to a separate thread in the Quantum Physics forum. I have also added a few clarifying posts. This thread appears to have run its course and will remain closed.