

The entropy force: a new direction for gravity - profquail
http://www.newscientist.com/article/mg20527443.800-the-entropy-force-a-new-direction-for-gravity.html?full=true

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scotty79
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.

~~~
Groxx
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...](http://www.newscientist.com/article/mg20126911.300-our-world-may-be-
a-giant-hologram.html?full=true)

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

~~~
Groxx
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...](http://www.newscientist.com/article/mg20126911.300-our-world-may-be-
a-giant-hologram.html?full=true)

~~~
Chronos
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.

