The entropy force: a new direction for gravity | Hacker News Reader

The entropy force: a new direction for gravity

newscientist.com

23 pointsprofquail7 comments

I have a question for physics savvy hackers.

Can you point me to some description about how it is possible that anything has ever fallen into some black hole despite infinite time dilatation and space compression at event horizon?

Sorry for departing from topic of the article but I thought about it much and have doubts about holographic principle being valid if event horizons don't grow, so I wanted to make sure that they do.

Actually, according to this article (linked by the main thread here), they do grow in size, relative to their contents.

http://www.newscientist.com/article/mg20126911.300-our-world...

This article reads like made up techno-jargon to me. Anyone with real knowledge in this area have any more insight?

Link to a paper on arxiv about the idea. http://arxiv.org/abs/1001.0785

Note: I'm not a physics major, much less in any exotic physics. I've just been reading stuff like this for a long time, it interests me. Expect errors, but I typically find later evidence means I'm at least nearly correct in understanding the concepts. And please, anyone correct me where I'm wrong, I'd be interested in correcting my own misconceptions.

First, an information theory primer (for you or others who don't know):

What it seems to be saying is based partially off information theory's view of the universe, where everything is merely information. This has the nice quality of simplifying a LOT of things. That apple, up in the tree? Information. That apple, falling? A change in information. That charged capacitor storing a 1? Information. Even an electron changing energy levels around an atom is a change in information. The key part is that, because of thermodynamics, any change in information can only maintain or increase the entropy of a system.

The alternative, more classical view is to see information and matter as separate, or even ignore information, but the same rules apply in an enormous amount of situations, and quantum mechanics casts doubt on the nature of matter anyway.

So, motion, energy, matter, everything is information, and information cannot be created or destroyed (see how nicely it plays with Einstein?). This touches on one of the problems for information theory, in that black holes seem to be a net loss, not merely a change. Info goes in, and it never comes out, in any form (hawking radiation doesn't appear to reveal anything about what's in the black hole), and the black hole can vanish, taking all the info with it. There are loads of was to resolve this, but the one important one here seems to be coming from a link in the article [1], essentially saying that, in the same way a hologram stores 3D info in a 2D (ish) material (in information theory, it's actually 2D), the black hole displays all the information about what's inside it (3D matter) via the event horizon (a 2D boundary) as a holographic surface, thus it's not "lost" when it vanishes.

I don't fully understand holographic principles, so I just won't touch on how that directly applies to this idea. The important thing here is that, with the above, they seem to be saying that gravity can be viewed as a change in information, and thus maintaining or increasing entropy. As entropy cannot decrease, some changes in that information are more likely (somehow, the article doesn't touch on it), with mutual attraction being the most. This all happens on a 2D holographic surface, which is displayed as our 3D world. The linked article covers it a bit better.

I can't even begin to guess at the math involved, but it seems to be fairly consistent with a lot of other theories, as well as internally consistent. It could also explain why gravity is so frickin' hard to accurately account for, if mutual attraction is merely most likely. (we don't have a working equation, just ones that are "close". Newton was amazing for the time, Einstein refined it further to a huge degree, but we still can't truly accurately predict it)

[1] http://www.newscientist.com/article/mg20126911.300-our-world...

FWIW, the black hole hologram thing is best understood in the context of an idea that has been around for quite a while: the "Anti de Sitter / Conformal Field Theory correspondence". Basically, the world we think we live in feels like a CFT-world with 3+1 dimensions: local particles producing local forces as they travel through local space. But the correspondence shows that some CFT-worlds are each mathematically paired with one equivalent AdS-world. The corresponding AdS-world for us would be a 2+1 surface (something that looks a lot like a black hole) with a specific set of rules describing the interactions of the ripples on the surface. In some sense, one could imagine the CFT-world as a holographic projection from the flat AdS-world "film".

IIRC, AdS/CFT actually turned out to have a practical application a year or two back... something to do with superconductor physics I believe. So, in a sense, it's as "real" as anything mathematical is real. Whether or not it's relevant to our interpretation of reality is an open question.

Oh, except for two other tidbits that came from a completely different direction in physics.

First, The entropy of a black hole is predicted to scale with the surface area of the event horizon. General Relativity predicts that, once a black hole has formed, the volume of the event horizon grows with the square root of the mass that falls into it (because gravity falls off with 1/r^2), instead of the usual cube root of mass expected for physical object volumes.

Second, physicists have good reasons to believe that black holes are maximum entropy objects -- at minimum that's true in the thermodynamic sense of "entropy", because energy that falls into black holes cannot be used to perform work. (But information entropy is almost certainly equivalent to thermodynamic entropy anyway, so you can safely extend that conclusion to maximum information entropy if you like.)

(Side note: technically you can extract work from a black hole... if it's spinning. The "ergosphere" of a spinning black hole extends past the event horizon. As you extract work from the ergosphere, you siphon angular momentum from the black hole and shrink the ergosphere.)

Now, combine these two tidbits about black holes, and one concludes that the maximum entropy for a region of space should grow with the surface area of the region, rather than the volume. Doubling the radius multiplies the maximum entropy by 4, rather than the intuitive 8. There might be a flaw somewhere in this reasoning -- it's mooshing together relativistic physics and classical physics in a way that's not implicitly correct. But it presents a striking parallel with the AdS/CFT thing, which lends it some credence, and a lot of very smart people have gone looking for flaws in the idea and come up empty-handed.

If you combine all this together, you reach the possibility that the universe might be (in some sense) a CFT space mathematically equivalent to AdS ripples in the event horizon of a stupendously large black hole. This paper pushes that idea further, and the short summary is that the paper attempts to derive gravity within our CFT universe almost purely as a consequence of the Second Law of Thermodynamics operating on the AdS black hole ripples. However, the author might have just found a nice little piece of circular reasoning where he injected an unwarranted assumption, so it's not a done deal.

Related submission http://news.ycombinator.com/item?id=1045366

Recent Slashdot discussion http://science.slashdot.org/story/10/01/10/1936240/The-End-O...

Reply on news.ycombinator.com