B Did Einstein Disagree With Black Holes?

[nitsuj]

Not sure where best to post.

Local paper has had a couple of articles about the black hole image over the past couple of days. They quote Walter Isaacson “Einstein did not believe, then or later, that these results actually corresponded to anything real,” .

Is that true? I couldn't imagine Einstein disagreeing with a prediction of his own theory (except for once).

 

[PeterDonis]

Is that true?

Yes. Einstein never accepted that black holes could actually exist. He knew the mathematical properties of the Schwarzschild solution, but he did not think they were physically realizable for radial coordinates close enough to the Schwarzschild radius. He actually published two papers in the late 1930s where he attempted to show that no real physical system could exist whose radius was small enough. The basic arguments of the two papers were:

(1) A stationary system consisting of objects all in orbits about their common center of mass cannot have a radius smaller than 3/2 times the Schwarzschild radius for its total mass; if it did, the orbital velocities would have to exceed the speed of light.

(2) A static system consisting of matter held up against its own gravity by pressure cannot have a radius smaller than 9/8 times the Schwarzschild radius for its total mass; if it did, the pressure at the center of the object would have to be infinite.

Both of these arguments are correct. However, they both deal with systems that are assumed to be stationary, i.e., they consist of matter that is in some kind of equilibrium that does not change with time. But black holes are not formed from such systems: they are formed from objects that cannot maintain themselves in a stationary configuration and collapse under their own gravity. Einstein simply never considered this possibility.

Ironically, in the same issue of Physical Review in which one of Einstein's papers described above was published (the 1 September 1939 issue, which was also the date on which Germany invaded Poland to start World War II), Oppenheimer and Snyder published their model of gravitational collapse of a massive object, which predicted that such a collapse would indeed result in what we now call a black hole.

 

[Nugatory]

Yes, he disagreed with the prediction, or at least the predicted singularity at the event horizon.

When the Schwarzschild solution was first discovered (by Schwarzschild, of course) it was not clear that there was no real singularity at the event horizon, just a coordinate singularity. Confronted with a prediction of a real physical singularity at the horizon, Einstein took the quite reasonable position that the theory would somehow stop working before the singularity was reached.
This is similar to the way that we think about the singularity in the Newtonian $F=GMm/r^2$ at $r=0$ and the way many physicists today think about the predicted (real, not coordinate) singularity at $r=0$ in the Schwarzschild spacetime: some other physics comes into play at sufficiently small values of $r$ so these singularities don't happen.

Even after the discovery of coordinate systems that eliminated the singularity at the horizon, there were serious questions about how a black hole could actually form. Oppenheimer and Snyder published their general relativistic treatment of a collapsing shell of matter shortly before the second world war, but it wasn't until the mid-1960s that Wheeler coined the phrase "black hole" and pulled these various strains of thought into the modern view of black holes. By then Einstein was gone, but had he still been around, I'd expect that he would have approved.

You might be interested in https://www.scientificamerican.com/article/the-reluctant-father-of-black-holes-2007-04/

 

[PeterDonis]

It's worth noting, btw, that Einstein was by no means the only physicist who had trouble with the idea of black holes. In fact, most physicists did not believe they were possible until the 1960s, even though the two key pieces of the puzzle were already in place by the end of the 1930s. One was the Oppenheimer-Snyder model referred to in my last post, which showed a consistent picture of how a black hole could form from gravitational collapse; the other was the fact that white dwarfs and neutron stars, the two known types of compact stable objects, both have a maximum mass limit. Chandrasekhar had established this for white dwarfs by 1934; Oppenheimer and Volkoff established it for neutron stars in 1938 (this result was what got Oppenheimer interested in developing the model of gravitational collapse that was published the following year). But it took until the 1960s for these results to be developed and strengthened to the point where they were generally accepted.

Kip Thorne's Black Holes and Time Warps gives a very good presentation of how physicists' understanding of black holes evolved (and it is actually a major source of my knowledge of the subject).

 

[nitsuj]

Wow!

Thank you both for the very informative posts!

 

[nitsuj]

Kip Thorne's Black Holes and Time Warps gives a very good presentation of how physicists' understanding of black holes evolved (and it is actually a major source of my knowledge of the subject).

Looks like a good size! So I ordered it :D
joking aside I hope it's "accessible" to me. the review that inclined me to think at least part of it maybe said "A non-scientist should enjoy the first half, but might get bogged down well before the end. "

 

[nitsuj]

Kip Thorne's Black Holes and Time Warps gives a very good presentation of how physicists' understanding of black holes evolved (and it is actually a major source of my knowledge of the subject).

omg this is actually a really "big" book! And flipping through it I didn't see a whole bunch of math ( i can't read that) and did see hand drawn diagrams (I can totally read that!) yay! thanks for noting the book in your reply.