Oppenheimer-Snyder model of star collapse

[Mike Holland]

Yes, agreed. That simply means my mass need not be infinite nor infinitely far away.

But you've forgotten we are discusing an observer in a lift or spaceship! A bit more than 10cm.

 

[PeterDonis]

Yes, agreed. That simply means my mass need not be infinite nor infinitely far away.

Right.

But you've forgotten we are discusing an observer in a lift or spaceship! A bit more than 10cm.

So what? GR does not say that gravity must be indistinguishable from acceleration on length scales that are significant by our everyday standards. It only says that there is *some* length scale, however small, at which they are indistinguishable.

 

[harrylin]

I should add that if one's interest is in the actual physics, rather than just history, there are plenty of modern textbooks that explain the same thing Synge's paper did - and they're probably be easaier to read and written at greater length, as well.

Some can even be found online, e.g. Caroll's lecture notes.

In view of the discussion in the other thread, that subtopic appears to be not what people nowadays would call "actual physics" but "philosophy" or "metaphysics" - and for such, the development of thought is certainly relevant. Yesterday I found a paper by Finkelstein who also refers to a certain "Kruskaal"(sic). That may be an interesting discussion topic.

 

[harrylin]

[..] As long as you are observing/measuring at one point, the principle works, but when you can take measurements at two or more separate points with sufficiently sensitive instruments you can tell the difference.
Mike

I fully agreed with your point. We can apply the equivalence principle only as approximation for what I call real gravitational fields - such as from planets and stars. And I pointed out to you that, contrary to what Peter suggests, the Einstein equivalence principle is not concerned with that (see again my post #117).
If this remains unclear then it will need a separate thread - discussions here show that in such cases like this usually >10 posts are required, often more than 100.

 

[PeterDonis]

And I pointed out to you that, contrary to what Peter suggests, the Einstein equivalence principle is not concerned with that (see again my post #117).

So you are saying that the Einstein equivalence principle has nothing to do with real gravitational fields? Please read my post #122, in which I briefly stated how GR applies the Einstein Equivalence Principle to real gravitational fields; do you disagree with anything I said there?

 

[PeterDonis]

In view of the discussion in the other thread, that subtopic appears to be not what people nowadays would call "actual physics" but "philosophy" or "metaphysics"

In my case, at least, I've said explicitly at least once that I am only interested in the actual physics, not the history. The only reason I've been trying to pin down what you think Einstein actually said is that you appear to agree with Einstein about the actual physics, and I don't.

 

[harrylin]

In my case, at least, I've said explicitly at least once that I am only interested in the actual physics, not the history. The only reason I've been trying to pin down what you think Einstein actually said is that you appear to agree with Einstein about the actual physics, and I don't.

You misunderstood. Most people call "actual physics" that what we can verify by means of experiments.

 

[harrylin]

So you are saying that the Einstein equivalence principle has nothing to do with real gravitational fields?

Obviously I am not saying that; I do think that the explanations that I cited can't be clearer on that. Thus I cannot make sense of such a question, sorry.

Please read my post #122, in which I briefly stated how GR applies the Einstein Equivalence Principle to real gravitational fields; do you disagree with anything I said there?

While I don't disagree with it, it risks to give the wrong and ridiculous impression - as happened to Mike - of a principle that only works in a point.

 

[PeterDonis]

You misunderstood. Most people call "actual physics" that what we can verify by means of experiments.

Plus what we can conclude indirectly based on the theories that are confirmed by direct experiments. Nobody has directly observed quarks, and the standard model of particle physics says quarks can't be directly observed (and explains why this is the case); yet I doubt if any particle physicist would say that quarks aren't part of "actual physics".

If you want to say that black holes aren't part of *directly confirmed* physics, I agree. But I have been understanding you to be making the much stronger claim that black holes can't be part of actual physics period, including all the things we can indirectly conclude. We can indirectly conclude that black holes are part of actual physics because if they weren't, the Einstein Field Equation would have to stop being valid at the horizon, for no apparent reason.

If you want to say that black holes aren't part of "actual physics" because quantum effects prevent horizons from forming in the first place, that is possible, but not established. But if that is your position, it's pointless to talk about classical GR at all if we want to talk about actual physics, since you're saying that classical GR isn't the correct description of actual physics. Which means that all the talk about coordinate time going to infinity at the horizon is beside the point; that's part of the standard classical GR model. In any quantum model where the horizon is prevented from forming by quantum effects, the analog of "coordinate time" would never go to infinity at all.

So I guess I'm confused about exactly what your position is.

 

[pervect]

In view of the discussion in the other thread, that subtopic appears to be not what people nowadays would call "actual physics" but "philosophy" or "metaphysics" - and for such, the development of thought is certainly relevant. Yesterday I found a paper by Finkelstein who also refers to a certain "Kruskaal"(sic). That may be an interesting discussion topic.

A lot of the thread undoubtedly has been metaphysics. Arguing about which parts of the thread are physical and which are metaphysical is one of the parts of this sprawling thread that is metaphysical.

Meanwhile, back to my point. One doesn't need to dig up Synge's original paper to understand the actual physical predictions of GR. One only needs to read a modern textbook. The physics is very widely available, presented in a pedagogical manner.

It's not vital to see this one particular paper to understand the arguments. If one wants to understand the history, the paper would be very valuable. The history is an interesting topic - hints as to the modern solution were available as early as 1923 by LeMaitre. So why did it take so long to come to the modern understanding? But I'm afraid I don't know the answer to that. History isn't my field.

One way to get the thread back on track to physics (and out of the metaphysical morass that it appears to be bogging it down) would be to talk about and study the general properties of horizons - which occur in SR as well as GR, as a few posters have noted. (Dr. Greg, I believe.

If people actually WANT to talk about the physics, this is probably the way to go. But mostly I see entrenched positions, and a lot of debating tactics, not an actual discussion where people are trying to learn the modern view.

 

[PeterDonis]

Obviously I am not saying that; I do think that the explanations that I cited can't be clearer on that.

Ok, at least that makes your position clear.

While I don't disagree with it, it risks to give the wrong and ridiculous impression - as happened to Mike - of a principle that only works in a point.

Huh? I explicitly said that there is always some length scale--i.e., some scale larger than a point--at which the curvature can't be detected. This is because our measurements always have some finite sensitivity.

 

[Mike Holland]

In my defence :

"Principle of Equivalence: If in a space free from gravitation a reference system is uniformly accelerated, the reference system can be treated as being "at rest," provided one interprets the condition of the space with respect to it as a homogeneous gravitational field. - Einstein et al, Physical Review 1935

Now tell me, how does the equivalence principle pretend that gravitational fields must be uniform or linear?

From Wiki:
homogeneous

1.Of the same kind; alike, similar.
2.Having the same composition throughout; of uniform make-up.
3.(chemistry) in the same state of matter.
4.(mathematics) Of which the properties of a smaller set apply to the whole; scalable.

Mike

 

[harrylin]

Plus what we can conclude indirectly based on the theories that are confirmed by direct experiments. Nobody has directly observed quarks, and the standard model of particle physics says quarks can't be directly observed (and explains why this is the case); yet I doubt if any particle physicist would say that quarks aren't part of "actual physics". [..]

I meant with "actual physics" the verifiable predictions. Surely all particle physicists would agree that quarks belong to "actual theory of physics". Next we could ask to which theory or model they belong; and thus:

[..] We can indirectly conclude that black holes are part of actual [theory of] physics because if they weren't, the Einstein Field Equation would have to stop being valid at the horizon, for no apparent reason.

That is the part that I thought would be answered in this thread. However we discovered that Oppenheimer-Snyder did not get that far, and that the current answer to that question is based on additional modelling. I will therefore start a new topic on that if no-one else does.

[..] So I guess I'm confused about exactly what your position is.

I have no position about the physical reality of fully formed black holes, that is too esoteric for me. I doubt that fully formed black holes can be consistent with Einstein's GR and I suspect that GR is like beer, coming in different brands and flavours; but I think that it will require a longer discussion than this one to get to the bottom of this. Therefore I intend to start a new thread with better focus, on the compatibility of the(?) extended star collapse solution with Einstein's GR.

 

[harrylin]

A lot of the thread undoubtedly has been metaphysics. Arguing about which parts of the thread are physical and which are metaphysical is one of the parts of this sprawling thread that is metaphysical.[..]
why did it take so long to come to the modern understanding? But I'm afraid I don't know the answer to that. History isn't my field. [..]
If people actually WANT to talk about the physics, this is probably the way to go. But mostly I see entrenched positions, and a lot of debating tactics, not an actual discussion where people are trying to learn the modern view.

I can't get a clear picture of what you mean with "the modern view of physics"...
Which do you mean:
1. modern understanding of things that cannot be verified ("metaphysics")
2. modern understanding of things that are verifiable for humans on Earth ("physics")
3. ?

I expect a physics discussion forum to put the emphasis on option 2, but also permitting some discussion of the different theories and models. Insofar as any related metaphysics emerges in such a discussion, its origins should be clearly and correctly identified without indoctrination tactics - I think that that is necessary in order to keep the discussion objective.

For example, the following type of discussion can go on forever (Berne:P-A transaction) but is a waste of time:
Eve: Marx held that God doesn't exist, and he argued that religion is opium for the people.
Adam: You are wrong, just read any religious book: you should learn that they all say that God exists.

 

[Dale]

That is the part that I thought would be answered in this thread. However we discovered that Oppenheimer-Snyder did not get that far, and that the current answer to that question is based on additional modelling.

What additional modeling are you referring to? The question can be answered unambiguously with the EFE and the dustball matter distribution.

 

[harrylin]

What additional modeling are you referring to? The question can be answered unambiguously with the EFE and the dustball matter distribution.

That is the additional modelling that I am referring to, and the discussion of which I foresee as going over 100 posts.

 

[Dale]

I don't see how that is "additional", it is exactly what O and S used. Even if OS "did not get that far" in their analysis of their solution there is nothing "additional" added to the model for the modern analysis.

 

[pervect]

I can't get a clear picture of what you mean with "the modern view of physics"...
Which do you mean:
1. modern understanding of things that cannot be verified ("metaphysics")
2. modern understanding of things that are verifiable for humans on Earth ("physics")
3. ?

First of all, I don't think it's "impossible" in principle to verify. The fact that you'll almost certainly die shortly after verifying (or not verifying) the predictions that GR makes when falling into a black hole doesn't mean they can't be tested. Or calculated. It just means that you'll die shortly after verifying (or not verifying) the predictions - unless something really really unexpected happens.

Secondly , there's opportunity to apply the exact same arguments to other situations involving event horizons that don't involve black holes. Specifically, the Rindler horizon. These would be difficult to test with our current technology, though. The experiment is interesting, so I'll spell it out in more detail, since I've been alluding it to some time in the belief it was obvious (but perhaps it isn't to you? )

The experiment involves launching a spaceship that accelerates at 1g for a year shiptime - or .1g for 10 years shiptime - or .001 g for 1000 years shiptime.

The spaceship observes the Earth through a telescope. The prediction of SR in this case (you don't even need GR) that the Earth appears to fall behind an event horizon There will be some last event that the spaceship sees - say year 2100 exactly on the new years day celebration in Grenwich.

The metric from the accelerating spaceship looks like this, assuming the spaceship accelerates in the z direction. (There are some variant forms of the metric, this version is normalized so that g_uv = diag(-1,1,1,1) at the origin.

ds^2 = -(1+ gz)^2 dt^2 + dx^2 + dy^2 + dz^2


http://en.wikipedia.org/w/index.php?title=Rindler_coordinates&oldid=522511984 has the details if you're interested (but you may see minor details differ, these could be confusing).

As the observer on the spaceship watches the Earth approach New Years 2100,, the spaceship sees the image grow dimmer and dimmer, and the Earth's clocks appear to slow down. Just as it would if the Earth were falling through the event horizon of a very large black hole, as g_00 falls towards zero at the critical value z = -1/g. (In non-geometric units, that's z = c^2/g). This is the critical value because g_00 goes to zero. I believe you call it something like "time stopping?" I forget how you referred to this condition.

Now, if we apply your argument, the Earth ceases to exist in the year 2100 at new Years in some philosophically meaningful sense. At the very least, something dramatic happens on that date, as "time stops".

My position is that it's pretty obvious the Earth won't cease to exist at New Years day on the year 2100 in any sort of meaningful sense. And that the people on Earth won't even notice this, or notice anything about "time stopping" or anythign like that. In fact, they'll find New Years day 2100 quite unremarkable.

As far as modern goes, the reason I say that is the following quote that I gave earlier.

One of the great conundrums in the history of general relativity is certainly constituted
by the “Schwarzschild solution.” Also to a person with a marginal interest
in the history of this discipline, the noun immediately recalls to the mind this
puzzling circumstance: during more than four decades since the discovery of the
“Schwarzschild solution,” the overwhelming majority of the relativists harbored
the conviction that the region within the “Schwarzschild radius” was physically
meaningless, and strove to show that it could not be accessed from the outer
space. During the subsequent four decades, after a seminal and nearly forgotten
paper [1] [Synge, J. L. (1950). Proc. R. Irish Acad. 53A, 83.] that Synge wrote in 1950, an equally overwhelming majority of them came to the conviction that the same region was physically meaningful and accessible “without a bump” along geodesics.

So, basically the position you've been trying to argue and debate (as nearly as I understand it) was discredited over 50 years ago.

 

[PeterDonis]

I meant with "actual physics" the verifiable predictions. Surely all particle physicists would agree that quarks belong to "actual theory of physics".

Ok, so by "actual physics" you mean "what's directly observed", and stuff that is only known indirectly, you call "actual theory of physics". That clarifies your terminology, thanks.

A quick comment, though: you seem very quick to jump to conclusions about what people would "surely" agree to. I don't think particle physicists would say that quarks are "theoretical"; particle physicists appear to me to believe overwhelmingly that quarks are as physically real as tables and chairs. (Some, the extreme reductionists, may even believe that quarks are *more* physically real than tables and chairs, since quarks are fundamental particles and tables and chairs are not. I don't agree with that view, but it's hard not to believe that at least some physicists hold it when you see what they say and read what they write. Eddington himself delivered a famous lecture, which I think got put into his book "The Nature of the Physical World", in which he argued that the table in front of him was not real, only the atoms making it up were. I can only guess what he would have said if he'd known about quarks.) The term "actual theory of physics" doesn't seem to me to match that very well.

Next we could ask to which theory or model they belong
...
That is the part that I thought would be answered in this thread. However we discovered that Oppenheimer-Snyder did not get that far, and that the current answer to that question is based on additional modelling.

If you want to call it that. I think most people would call it simply *completing* the model that O-S left incomplete. There's nothing in the "additional modeling" that O-S themselves couldn't have done if they had just continued their own model past the horizon. But by all means start a separate thread if you want to talk specifically about the modern, completed model.

I have no position about the physical reality of fully formed black holes, that is too esoteric for me.

Ok, that's fine.

I doubt that fully formed black holes can be consistent with Einstein's GR

Then this is the part you should focus on; why do you think this? (You might also want to clarify what you mean by "fully formed black holes"; do you mean that you don't think event horizons are consistent with GR?)

and I suspect that GR is like beer, coming in different brands and flavours

Every physical theory has multiple solutions; that is, the mathematics of the theory can describe multiple scenarios, which may or may not be physically reasonable. GR is no different. What look like "different brands and flavours" to you are just different solutions to the equations; some are physically reasonable, some are not. But that discussion can continue in a new thread, as you say.

 

[harrylin]

First of all, I don't think it's "impossible" in principle to verify. The fact that you'll almost certainly die shortly after verifying (or not verifying) the predictions that GR makes when falling into a black hole doesn't mean they can't be tested. Or calculated. It just means that you'll die shortly after verifying (or not verifying) the predictions - unless something really really unexpected happens.

If you read carefully, you see that I excluded that for obvious reasons: the physics community on Earth cannot know the information from the space traveller out of a black hole zone, just as they cannot know the information about heaven from people who died.

Secondly , there's opportunity to apply the exact same arguments to other situations involving event horizons that don't involve black holes. Specifically, the Rindler horizon. These would be difficult to test with our current technology, though. The experiment is interesting, so I'll spell it out in more detail, since I've been alluding it to some time in the belief it was obvious (but perhaps it isn't to you? ) [..]

I had the impression that Atyy was the first who referred to that illustration in recent discussions; I next referred to it in this thread and started elaborating on that excellent case in the "notions" thread. In this thread I also included a brief comment by Einstein on that example (in fact, why is it called "Rindler horizon"? He was still a schoolboy when Einstein mentioned it).
Regretfully more than ever the discussion is hindered by incompatible definitions based on different schools of teaching. I intend to do a "retake" of that illustration in the new thread, with a brief summary of comments by different people, including yours.

So, basically the position you've been trying to argue and debate (as nearly as I understand it) was discredited over 50 years ago.

Not at all, as far as I understand it; but even if that were the case, your last sentence resembles the useless argument of Adam to an "Adult" question in my illustration (a "Parent" reply). The scientific approach is to scrutinise the arguments.

 

[harrylin]

[..] there's opportunity to apply the exact same arguments to other situations involving event horizons that don't involve black holes. Specifically, the Rindler horizon. These would be difficult to test with our current technology, though. The experiment is interesting, so I'll spell it out in more detail, since I've been alluding it to some time in the belief it was obvious (but perhaps it isn't to you? )

The experiment involves launching a spaceship that accelerates at 1g for a year shiptime - or .1g for 10 years shiptime - or .001 g for 1000 years shiptime. [..]

[..] Regretfully more than ever the discussion is hindered by incompatible definitions based on different schools of teaching. I intend to do a "retake" of that illustration in the new thread, with a brief summary of comments by different people, including yours. [..].

The new thread: https://www.physicsforums.com/showthread.php?p=4181348