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Black holes might be layers of shells: New solution found
🕑 Added 2024-03-02 16:00:10 +0000 UTC[This is a transcript with links to references.]
I had a prof who used to say that black holes can’t exist because, quote “god wouldn’t separate himself from part of his universe” end quote. Now you might think that this question has been settled for good what with all the evidence we have for black holes. Also the prof has meanwhile died and yes his name was Greiner. But then again science is never really settled, is it.
And so today we have a press release saying not only that black holes aren’t really black holes but stars, but that there might be stars within those stars. The press release comes with this image to make clear what we’re talking about. What’s on the surface of those stars? Is it, erm, stars?
I had a look at the paper.
A black hole’s defining property is the event horizon. That’s the boundary of a region of space from which nothing can get out, not even light. Once something crosses the horizon, the only thing it can do is to fall towards the centre of the black hole. Technically, mathematically, the centre of the black hole has a singularity, and at that singularity everything gets destroyed. You can check in anytime but you can never leave.
But the horizon of a black hole is not a physical thing. It’s just the boundary of a region. It’s like city-limits basically, you don’t even notice as you cross it, until you get a speed ticket. A lot of people find this surprising, I believe because they think there must be a strong space time curvature.
And indeed one would notice a strong space-time curvature. This is because this curvature creates forces that tear things. However, the larger the black hole, the smaller these forces are at the horizon. They only get strong towards the singularity where they will eventually rip everything into pieces. So the thing is that at the black hole horizon the forces can be very small and you wouldn’t notice crossing it. Not until the singularity shreds you. Forget Venus fly traps, black holes are much meaner.
But the issue is that since nothing gets out of a black hole, you can’t directly observe it. The only evidence we have for black holes is indirect. You can study for example how stars move around it, how light bends around it, how gas heats up around it, what gravitational waves escape when it is formed and so on. And in each of these cases you can try to say, well, maybe it wasn’t a black hole. Maybe it was something that looked very similar to a black hole, but really it neither had a horizon nor the singularity.
Why would you want to do that? Well, first of all there are people like my prof who just don’t want to believe in black holes. Maybe not a good motivation, but, scientists can be disappointingly human. Then there is the issue with the singularity, that we don’t know how to prevent. And then there is the problem with black hole information loss. Basically, black holes are trouble, and if avoid black holes, you can avoid some trouble.
So about 20 years ago, a team of physicists came up with an alternative to black holes that they called a gravastar. A gravastar is a supposed endstate of gravitational collapse that is not a black hole. Instead it’s a thin shell that has dark energy inside. Exactly how this gravastar is supposed to form is somewhat unclear, there has been some talk of quantum effects, which basically means a miracle happens and then you get that gravastar.
Another issue is that they’re unstable if they rotate, which isn’t good. But it’s true that these gravastars are mathematically correct solutions of Einstein’s equations, like black holes and wormholes, but without the horizon and without the singularity. That means they can’t be all that easily discarded.
And this finally brings me to the new paper. The authors of the new paper point out that there isn’t just one type of gravastar, but that if you believe that these things exist at all, the same process can happen multiple times, forming shells within shells within shells. And interestingly enough, this makes the gravastars more stable.
I think this means the other way round that the inner shells could be caused by the instability of the outermost one, thought they don’t discuss this in the paper, so take that with a grain of salt and don’t forget your beta blockers along with that.
So this is a neat paper and I am pretty sure it’s mathematically correct, but I think it needs some context. First of all, maths or not, we have some observational evidence that is difficult to make compatible with the idea of gravastars. That’s because astrophysicists can study how gas and stuff falls into a black hole. If they had a surface, like these gravastars do, you’d expect to see signs of an impact of sorts, but there isn’t one. Then again you can make up some story about the surface of these gravastars which prevents this.
The other issue is that we have some observations of gravitational waves that have been attributed to mergers of black holes, and if you do the calculation with gravastars it just doesn’t fit with observation. This argument, interestingly enough comes from one of the authors of the new paper.
I had another prof who used to say you always need to make two predictions, one for the thing you believe in one for the exact opposite, that way you can’t lose. I used to think he was joking, but now I’m not so sure.
But to understand what’s going on with this paper you need to know that Einstein’s theory of general relativity is mathematically extremely interesting. Physicists just like finding new solutions to these equations. I’ve been looking for solutions of the equations myself and I can report from personal experience that this can become quite addictive.
The thing is though, that it’s really hard to find these solutions. And if you find a new one, you can almost always get it published. The journal to do that is classical and quantum gravity. This is where the new paper was published and this is also where my paper appeared before I slapped my hands asking Sabine what are you doing. The issue is that it’s often unclear what these solutions have to do with reality, if anything.
And this is the case with these stars within stars. Are they solutions to Einstein’s equations. Yes. Does that mean they actually exist. No.
