Monday, 29 September 2014

"Black Holes don't exist:" giving context to sensational science news

A friend of mine recently pointed me to this article about how "Black Holes don't exist." The article concerns two recent papers

http://arxiv.org/abs/1406.1525
http://arxiv.org/abs/arXiv:1409.1837

Since it came out the following edit has been added to the article:

"Due to some confusion, we feel it is important to clarify. The notable word in the title is “mathematically.” In science, there are conflicting predictions that come from different theories, assumptions, and equations–different equations result in different outcomes and different proofs. In short, one set of assumptions leads down one path and give us new (potentially important) things to consider. But there are many paths. It seems that many people were not sure how to situate or read these findings. Hopefully, this clarifies things. We’d like to apologize to anyone who took this out of context or who was confused by the implications. In the coming days and weeks, more physicists will weigh in with their findings. Things will update as they develop. Science on."

And this edit is actually exactly why I wanted to discuss the article. What does a claim like this mean, and how can a non-expert interpret it? The following is basically word-for-word what I wrote in reply to my friend who sent me the article.

I skipped reading the article at first [I have since read it, and you don't learn all that much], as lots of articles like it miss the point with physics. People love to say "Einstein was wrong!" or basically "[New sensational thing about physics hopefully with the word "Quantum" somewhere!]" while not appreciating what is really going on. And it annoys me. A lot. Anyway, onwards with constructive things...

I'm no expert, so I'll be arguing from authority, basically. First things first, Mersini-Houghton, the author, appears a totally respectable physicist who has published highly cited work on a variety of topics in the past, and works at a respectable institution. I'm in no way trying to slander her or her work. It's completely within the confines of work on these topics (black holes, the information paradox), as far as I can tell. Her recent work on these topics isn't highly cited (so far: and that is only a few months, but physics moves fast these days), but citations aren't a perfect gauge of a work's relevance. The important point to take away is: the author is certainly no two-bit crank posting on vixra (I hope I don't get bombarded by any more cranks than usual, but just look at the kind of stuff that goes on there! If you've never lost a few hours on vixra, I highly recommend it.)

What I want to discuss here is how the paper was reported, and more importantly how non-physicists should read reporting on physics in general. It's all about context, and what we mean by "exist" I think...

The paper referred to in the article was published in a respectable journal, so it went through peer review and someone with more knowledge in the field than me thought it was correct. However, let's put it in context. There has been a whole lot of interest in issues related to black holes recently, thanks to a famous paper on "firewalls." (http://en.wikipedia.org/wiki/Firewall_(physics)) There seems to be something we don't understand about black holes in that they lead to a paradox that requires giving up one of three major pillars of physics (three according to wikipedia, I thought it was just two: unitarity and locality, but there ya go). The name "firewall" comes from the easy way out: there is a wall in the way that prevents anyone seeing the paradox. I'm not sure how seriously the firewall itself is taken. I think of it as a last resort to sweep things under the rug. But like I said, I'm no expert.

This debate over what to do with firewalls has led to a huge number of papers proposing different resolutions to the problem. To give that a number, the original firewalls paper has gotten 249 citations in the 2 years since it was published. Experimental papers, and confirmed theory papers, get more than that. But for a "pure thought" paper, that is astounding.

(Although, the original AdS/CFT paper, a paper on pure thought depending on your take on AdS/CFT applications, just became INSPIRE's most cited paper. It took over the model of leptons, which is manifestly about the real world, but there ya go, that's physics!)

Many people have said we may need to do away with black holes in one way or another. For example, Hawking argued that there is no paradox if black holes aren't "eternal." Another solution is that black holes are really fuzzy quantum or turbulent things, and that makes the calculation that led to the paradox incorrect.

Okay, enough context. Mersini-Houghton's idea is one of these many that say black holes never form. The idea is that when a star collapses on the way to forming a black hole the "Hawking radiation" kicks in causing a pressure that stops collapse before a true black hole forms. If it never forms, there is a never a paradox. In her second paper out this month she works numerically with collaborator to show this happens in realistic situations (for example breaking the assumption of spherical symmetry).

So, is Mersini-Houghton correct? Her first paper came out in June and hasn't really been picked up in the community. The excitement over firewalls has died down, so maybe it's that. But if she was correct, it would be enough to set people off. It hasn't, so I judge there must be something about it that isn't compelling. Maybe she made some simplifying assumptions that people believe would make her argument wrong in a realistic situation? I'd like to ask an expert. The new numerical work makes me think she is correct, within the parameters she's set herself at least. It's whether those parameters are right.

But do I think it means anything? I'm inclined to say "no" for the following reasons:

1) It appears that we do see black holes out there in space. I'm not familiar with the observations, but as I understand it, it is an established fact. Maybe Mersini-Houghton can get around it, and her system still forms these things, but they aren't "formally" black holes because they miss the little paradoxy bit. Then, for all practical purposes (i.e. in astrophysics), it is a case of walking and quacking like a duck, but without the particular nuanced existential consequences. Practically, then, "black holes" still exist. This has to be true of all the other firewall solutions. The black holes are still there, because we see them, but some little bit in the middle is subtly different.

[edit: the Event Horizon Telescope hopes to directly image black hole event horizons in the coming years. Things like "Saggitarius A*" are pretty good candidates for them. Any other comments about direct evidence for black holes are very welcome!]

Now you have to go out and find an observation that can confirm that. People are trying, but it ain't easy (see this interesting proposal to tell proposed firewall solutions apart observationally using gravitational lensing!). An example discussing Mersini-Houghton's work is that the "bounce" of the star instead of forming a black hole that she predicts could be the source of some high-energy cosmic-ray type things (fast radio bursts in this case). We see stuff like that, and by looking at them in detail maybe you can see what they came from. But this is messy astrophysics, and the number of explanations for these things is often very wide.
I'd like to see whether (i) Mersini-Houghton and co can still make real things that look enough like black holes that they are consistent with what we have seen already, and (ii) if they can make any novel observational prediction to test their theory. If the answer to either of these questions is "no" then they are dead in the water.

2) On a purely philosophical level, Mersini-Houghton's solution doesn't solve the firewall paradox in my opinion. It may solve it "in real life" if there are no collapsing stars that form tricky black holes. But it doesn't solve the problem for theory. In theoretical physics you can still set up a "thought experiment" and if that makes your theory inconsistent then you are in trouble. The whole firewall thing began with a thought experiment. Black holes are solutions to Einstein's theory, as long as they are you can always imagine one just sitting there. Conjure it out of nowhere. It doesn't have to be made by a collapsing star (because in the thought experiment you also conjured the star from nowhere too).
Mersini-Houghton's solution doesn't alter Einstein's theory or quantum mechanics, and so you can still do that thought experiment, and the firewall problem is still there.

So, Mersini-Houghton hasn't solved that problem in my opinion. And indeed, black holes still "exist" in the theory, so for many theorists they are still just as relevant as thought experiment probes of whether we really understand the universe. And many theorists are platonists anyway, so black holes, even of the thought experiment kind, do "exist."

3) Outside of thought experiment, as long as black holes are still this "formal" solution of Einstein's equations (and they are in Mersini-Houghton's theory as far as I can tell) then they do still *really* exist. This is thanks to quantum mechanics, where things are allowed to "pop in and out of the vacuum" (http://en.wikipedia.org/wiki/Virtual_particle). Black holes do the same thing in quantum gravity, at least in string theory they do as far I know (I've heard people argue that they needn't in other theories, but I don't find these arguments compelling: in quantum theory you need a very good reason for things not to happen, or else they do). So, black holes still exist at the quantum level even in her theory.

Does that have any relevance to real life? Well, virtual particles do. We have very strong evidence that virtual particles are important. They have observable effects on particle collisions. They are the reason the electron has the charge and magnetic properties it does, so probably phones and microchips and such wouldn't work without them. (Does anyone have a better example of the reality of virtual particles? I mean, an accessible one, not just "electroweak precision observables, duh")

What about virtual black holes? (i) I don't know if they suffer form the firewall paradox, because of the popping quantum business, so maybe they aren't a problem. (ii) They are much harder to see. They are intrinsically quantum gravity things, and that is, as far as we know, not relevant to anything we have a hope of measuring on earth. But there is hope, and this is exactly why I study cosmology: the early universe is a lab for very high energy things, and we can hope for signals of quantum gravity in the sky (in fact this is a lot of what Mersini-Houghton did in her earlier career, too).

So, in summary:

* There are lots of theories like this. Maybe this one's right, maybe it isn't.

* There are at least "black hole-like" things out there in space that we have seen.

* Philosophically, black holes-proper still exist in this theory thanks to quantum mechanics.

* We need to come up with observational and experimental tests and consequences of all this. 

4 comments:

  1. About virtual particles: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/

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  3. There are mathematical solutions to the equations of general relativity in which black holes act as bridges to other parts of spacetime, even in principle parts that are otherwise not connected to our own. But I don’t see any reason to suspect that the black holes that actually exist in our Universe behave that way. диамантено шлайфане In particular, if you think about the ways in which actual black holes are supposed to have formed, they don’t seem to lead to anything like this in a natural way.

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