Oppenheimer-Snyder model of star collapse

DaleSpam

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

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?...ldid=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.

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

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.

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.

Ok, that's fine.

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?)

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

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.
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.
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

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