Black hole formation watched from a distance

[vanhees71]

Since @haushofer has adopted Schwarzschild coordinates, that restricts discussion to a spherically symmetric black hole. That is also the only case where we have a closed form solution (the Oppenheimer-Snyder solution). There is no known closed form solution for the non-spherically symmetric case, although I believe it has been studied numerically.

and also visualized nicely. Here's a collection of pictures as well as movies made by my astrophysics colleagues in Frankfurt:

https://relastro.uni-frankfurt.de/gallery/

 

[TeethWhitener]

Leonard Susskind's GR lecture series on Youtube covers this at a fairly easy-to-understand level:

The first 45 minutes or so are an introduction to Penrose diagrams, and then he proceeds to use them to introduce black hole formation.

Edit: he starts talking about black hole formation at around 44:40.

 

[pervect]

Just to be sure: can we state that for us on Earth the formation of a black hole would take an infinite amount of time if we define "formation of the black hole" by "the star has shrunk to its gravitational radius"

In order to answer this, we need to interpret the actual physical observations. These observations might be, for instance, observation of one or more beacons falling into the black hole with a "frequency vs Earth (TAI) time of reception" plot. If we have multiple beacons, we might want to include angle, as well, though it's questionable if we could resolve the angular differences with multiple beacons.

I'd rather talk about these actual physical observations than the interpretation thereof, as the interpretation involves concepts of simultaneity and coordinate time. These interpretations are generally recognized as requiring conventions. Perhaps some of the question is about these conventions, but I won't address that in this post.

Without doing any calculations, we can say that a signal from an infalling beacon, emitted at the event horizon, never reaches infinity. The beacon can be regarded as being located at the edge of a cloud of dust undergoing an idealized Openheimer-Snyder collapse. Also note, we are not taking into account the possible evaporation of the black hole, which complicates things considerably.

I have done more detailed calculations in the past, but I'm not sure exactly where - I could try and search more if there is some interest. But basically, the frequency of the beacon decreases (approximately) exponentially as one approaches the horizon.

The proper time of the beacon when it reaches the event horizon is finite, and no signal emitted from the beacon reaches the observer at infinity at or after the time at which the beacon reaches the event horizon.

 

[bob012345]

If it takes an infinite amount of time to form a black hole then wouldn't we have to say they don't actually exist, and any future black holes are only and always will be only in the process of being formed? If so, what then are those images of black holes all about?

 

[PAllen]

If it takes an infinite amount of time to form a black hole then wouldn't we have to say they don't actually exist, and any future black holes are only and always will be only in the process of being formed? If so, what then are those images of black holes all about?

Please read the thread. These questions have all been dealt with in the thread.

 

[bob012345]

Please read the thread. These questions have all been dealt with in the thread.

Sorry, I didn't see it through all the technical language at first pass. Now I see it. Thanks.

 

[pervect]

There is a thought experiment here that might be helpful. Consider a spaceship accelerating away from an idealized non-gravitating Earth at a constant 1g proper acceleration. The "frame" of the spaceship can be described by Rindler coordinates, the most common coordinate representation of an accelerated frame of reference. When we view things from the inertial frame of the idealized non-gravitating Earth, things are reasonably simple. Time on Earth passes as normal, the spaceship undergoes hyperbolic motion. A feature of this motion is that after approximately 1 year has elapsed on Earth (actually c / 9.8 m/s^2) signals from the Earth will no longer be able to catch up to the spaceship.

But what is the "viewpoint" from the spaceship? "The" viewpoint as a singular statement is actually misleading, but one common "viewpoint" of the spaceship can be created by the use of Rindler coordinates.

In Rindler coordinates,things are different. There is a Killing horizon approximately 1 light year behind the spaceship, and in these coordinates the coordinate time for the Earth to fall to the Killing horizon becomes infinite. By considering the Rindler metric, one form of which might be $-z^2 dt^2 + dx^2 + dy^2 + dz^2$, one might conclude that "time stops at the Rindler horizon" (z=0), similar to the way that one concludes that "time stops at the Schwarzschild horizon" in the Schwarzschild metric. And one might also conclude that the falling Earth "never reaches the Killing horizon" from the point of view of the spaceship, just as one concludes that an object falling into a black hole "never reaches the black hole", or in the case of the Openheimer-Snyder collapse, that the black hole "never forms".

But I would argue that it would be misleading to think of the Earth as "never reaching the Killing horizion". There is nothing special that happens 1 year after the spaceship takes off from the Earth as far as the Earth is concerned. The time is a bit special to the space-ship observer, though, because they never receive a signal from the Earth that is timestamped later than 1 Earth year after the departure of the spaceship, assuming the spaceship accelerates forever.

The meta-point here is that different coordinate choices are possible, and they have somewhat different descriptions of how things happen. Mathematically, one can deal with different choices via choosing a metric. Misners "Precis of General Relativity", https://arxiv.org/abs/gr-qc/9508043, might be helpful in describing some of the techniques and philosophy that can be used to describe things in different coordinates.

Misner takes an interesting point of view, which is that the whole idea of an observer in General Relativity is perhaps not well defined. My view would be more along the lines that the line element of a metric defines the coordinates, a point that Misner also makes, and that thus defining the coordinates defines an associated "point of view" of an "observer". At least it does as much as that is possible.

One last point. Minkowskii and Rindler coordaintes are not the only coordinate choices here. One might also think about using radar coordinates from the spaceship, for instance, which would be an interesting exercise in "switching points of view". The first step, according to my suggestions, to look at things from this 'point of view" would be to find the associated metric. I have never done that, but it'd be the first step I'd suggest if one wanted to carry this out.

 

[PAllen]

One last point. Minkowskii and Rindler coordaintes are not the only coordinate choices here. One might also think about using radar coordinates from the spaceship, for instance, which would be an interesting exercise in "switching points of view". The first step, according to my suggestions, to look at things from this 'point of view" would be to find the associated metric. I have never done that, but it'd be the first step I'd suggest if one wanted to carry this out.

In the special case of uniform acceleration in special relativity, radar simultaneity for the rocket is identical to Rindler simultaneity. However, if distance is represented as .5 c $\tau$ (for the round trip) , then position coordinates are different from the Rindler. The result is that horizon interpretation is the same for these two coordinate choices.