it from bit

January 20, 2010 2:11 PM

Emergent Gravity - Erik Verlinde has a theory that "gravitational attraction could be the result of the way information about material objects is organised in space..." Here's some related weblog discussions and follow along on twitter! (via /. & bruces ;)
posted by kliuless (34 comments total)

The sweet, sweet rubber sheet theory.
posted by GuyZero at 2:12 PM on January 20, 2010

Here's http://arxiv.org/

Someone can figure out where the real paper is, if we're not all busy following a twitter feed.
posted by sebastienbailard at 2:16 PM on January 20, 2010

Pretty sure this is the paper.
posted by kickingtheground at 2:17 PM on January 20, 2010 [1 favorite]

Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.

Wow. I'm no physics expert, but this looks really neat.
posted by teraflop at 2:19 PM on January 20, 2010

i've never seen a snarkier emoticon, i must say ;p
posted by infini at 2:26 PM on January 20, 2010

This is awesome. I can't read this right now, but does it address quantum mechanics at all (specifically how the different ontologies realte to this)?
posted by qvantamon at 2:39 PM on January 20, 2010

This is interesting stuff.
posted by Blazecock Pileon at 2:41 PM on January 20, 2010

check out the blog linked with a motl in hte url, the comments are hilariously reminiscent of our gentle bantering here on the blue
posted by infini at 2:46 PM on January 20, 2010

For those who want a good explanation of the holographic principle that he's referring to, here's Prof. Raphael Bousso at UC Berkley discussing the topic from the ground up.
posted by symbioid at 3:04 PM on January 20, 2010 [5 favorites]

Interesting: from Symboid's link at 14:00 he's talking about how you can have arbitrarily small features in Quantum theory. And someone asks him about plank's length. He says something really interesting: That you can't talk define plank's length without gravity, because it involves newton's constant.

Huh.
posted by delmoi at 3:58 PM on January 20, 2010

Dammit, scooped again!
posted by DU at 4:31 PM on January 20, 2010

The video symbioid linked two is almost two years old. But the new scientist article makes it sound like this is some radical new theory. Sounds like they went for the "false breakthrough" failure of science reporting, where incremental steps are reported as huge leaps in order to make stories more interesting.
posted by delmoi at 4:34 PM on January 20, 2010

(and the video talks about advances made in the 1970s)
posted by delmoi at 4:34 PM on January 20, 2010

Interesting, Im' reading Suskinds book on Black Holes, and he seems to be implying that information seems to be what the universe IS, on some level.
posted by empath at 5:53 PM on January 20, 2010

The title is from The Hammock Physicist's take I believe which is cautiously optimistic on this actually being science.
posted by Skorgu at 6:20 PM on January 20, 2010

"information seems to be what the universe IS, on some level."

This is also a position in metaphysics. From the SEP "information-theoretic, metaphysical monism: the universe's essential nature is digital, being fundamentally composed of information as data/dedomena instead of matter or energy, with material objects as a complex secondary manifestation (a similar position has been defended more recently in physics by Frieden [1998], whose work is based on a loosely Platonist perspective)."
posted by oddman at 6:34 PM on January 20, 2010

so it really is all ones and zeroes.
posted by localroger at 6:50 PM on January 20, 2010 [1 favorite]

I believe "it from bit" was coined by John Wheeler, who also coined "black hole".
posted by Schmucko at 7:24 PM on January 20, 2010

I've always felt that 'time' was a lie. This theory seems to be saying that gravity, also, is a lie, and this makes sense to me. If you think about it, the future and the past don't exist (except in the mind). We can imagine them, but we can't really experience them. Rather, there is only the present. There is evidence of the past (like a picture of a distant galaxy), but we can't experience that past except in an informational way. A memory of directly experienced past is just as unreal as an imagined future in this regard. The memory is 'imagined' based on information stored in the brain. Objects in the universe seem to have information encoded about their location, velocity, vector, mass, etc. We observe this information and so can try to extrapolate about the past and future. For instance, we can calculate whether some asteroid is going to hit the Earth or not. But for any given instant of the 'now,' the now being the only thing that is real since time is an illusion, neither time nor gravity exist, except as an informational potential. Looking at time this way, I can't help but feel a bit like Billy Pilgrim, disconnected from either past or future. And the universe feels like an object-oriented computer program (written in objective-c, no less).
posted by jabah at 7:42 PM on January 20, 2010 [4 favorites]

And you all thought i was a complete dumbass for posting my AskMe regarding the topology of pasta. No alfredo for you!
posted by yesster at 7:49 PM on January 20, 2010 [1 favorite]

Okay, I need you to give this to me in Captain Dummy talk.
posted by greenland at 8:17 PM on January 20, 2010 [1 favorite]

In a few years, when this research goes forward and they find out that time is actually a holographic cube... let's see who'll be laughing then.
posted by qvantamon at 8:27 PM on January 20, 2010 [8 favorites]

Okay, I need you to give this to me in Captain Dummy talk.

I'm going out on a limb here, because my knowledge of physics would fit on a very small tea biscuit. But here goes.

You've seen or heard of holograms, right? They look 3D, but they're actually generated by light bouncing off or passing through a 2D surface. This theory is about how our universe might actually be a sort of hologram generated on a giant surface of some kind.

There's a thought experiment you can do involving a person passing through the event horizon of a black hole - the boundary surrounding a black hole that is strictly one-way: things can go in but nothing can come out, not even light. Very large black holes are so dispersed that they can be safely entered (not exited!) You could, in theory pass through an event horizon without being harmed. You're still there, but as far as anyone outside is concerned you have disappeared. But if you've disappeared, from their perspective, where is the "information" your body represented? Information is like energy - in fact in a way it is energy - and it can't be destroyed. Well, this guy's answer is that the information is still there - it's represented by a change on the event horizon. The event horizon is effectively a hologram of everything inside it.

Now, it seems that if you do the math (and there's no way I could come close to this) about how a holographic universe would work, that it's easy to make some basic laws come out right. This is very exciting, because our model of the universe at present is a piece of crap that requires some numbers to have specific values for no apparent reason. It works, but it's a description of the universe, not a theory about why it works this way. But Erik Verlinde says that the relatively simple theory about the universe being a hologram explains things like gravity that would otherwise require very complex theories.

You know about entropy, right? Entropy measures the disorder in a system. A cold room with a bucket of hot water in it has more entropy than the same room two hours later, a slightly-warmer room with a room-temperature bucket of water. Physical processes move from states of low entropy (cold room, hot water) to high entropy (warmer room, colder water). Erik says that when masses move together it represents a change in the holograph universe, and that this change is equivalent to entropy. So since things tend to move from low- to high-entropy, the state of the holograph universe will tend to change in such a way that masses move together. And this, Erik says, is why gravity exists. It's to do with the nature of the holographic universe that underlies our own. This is a reasonably-simple theory that explains something which is otherwise rather opaque, and so there's a chance that it's true.
posted by Joe in Australia at 9:38 PM on January 20, 2010 [7 favorites]

"information seems to be what the universe IS, on some level."

you should check out this book: Minds, machines, and the multiverse. (it's mostly about quantuum computing) ...There's a great bit in it where some scientists sit down and try to answer the question "what is the smallest amount of energy required (in our real, actual universe, not some idealization) to do a mathematical calculation?" You might think it must be some tiny bit of energy, the fluctuation of the weakest photon, but nope. It's zero. it's actually possible to do an infinite amount of calculation in a finite universe. (not that our universe has actually been proven to be finite or anything), and that ultimately, everything might be made out of math.
god, information theory is neat.
posted by sexyrobot at 10:00 PM on January 20, 2010

Wait, so, Plato wasn't just being poetic with all that cave/shadow business, he might have actually been right?!
posted by otherthings_ at 2:35 AM on January 21, 2010 [2 favorites]

Here's my high-level summary: This guy is proposing an inversion of the Bekenstein-Hawking equation and how it saturates the Bekenstein bound. Its just a lightweight proposal though - a sketch of a theory rather than the theory itself.

If you know your physics history, it is similar to De Broglie first saying "Hey! If waves behave like particles, why can't particles behave like waves?" De Broglie had a creative idea but he didn't have the chops to develop it into a full theory. For that, you needed a Heisenberg and a Dirac and a Schrodinger.

So, this might just be a dead-end or it might stimulate others into new directions. We'll see.

Personally, I think we need to move away from "particles" altogether and see flux based derivations - that is, following Gauss' conception of gravity not Newton's.
posted by vacapinta at 3:36 AM on January 21, 2010 [1 favorite]

Otherthings_ there is a reason why we still read Plato 2500 years later. The guy was pretty sharp.
posted by oddman at 8:57 AM on January 21, 2010

Otherthings_ there is a reason why we still read Plato 2500 years later. The guy was pretty sharp.

Yes, but the reason is not that he had insights into modern physics. Plato was not actually right about the shadow/cave business even if it turns out that physics lends credibility to something structurally similar.
posted by kenko at 10:45 AM on January 21, 2010

You may not read him to study physics, but don't tell me that I can't.
posted by oddman at 1:49 PM on January 21, 2010

the new scientist article makes it sound like this is some radical new theory

just to clarify, the holographic principle isn't so new (as it states in the article ;) but gravity as an entropic force is:

The paper posits that gravity is an entropic force, not a fundamental one. He does this by starting with the assumption of a reduced dimensionality universe with one emergent direction for space and shows that as a mass approaches the holographic boundary he can combine statistical mechanics equations and Einstein's equations for mass & energy (throw in a couple hand waves about average energy and degrees of freedom) and he derives Newton's laws and more. This is fundamentally a different view of how gravity, inertia and space arise compared to the current "fundamental forces" understanding in physics.

"it from bit" was coined by John Wheeler

yea, i think i first heard about it from process physics :P (make of it what you will!)

from The Hammock Physicist's take

that too, and he has another one up now on How To Get Rid Of Dark Energy :P

also btw, speaking of entropy, sean carroll's new book looks pretty good too!

cheers!
posted by kliuless at 6:33 PM on January 21, 2010

So here's what I got from the paper so far:

First, an analogous example: Why does stretching a piece of elastic require a force, and why does it contract back to its original length when this force is removed? Without knowing the internal structure of the piece of elastic, the obvious guess would be to say that the stretching must be pulling things apart that are physically attracted to one another, and releasing the force allows this attraction to pull them back together. However, that's not how it works.

Instead, what's going on is this: the material of the elastic is made of lots of long molecules (like bits of string), each of which can take on lots of different configurations. These molecules wiggle around somewhat, because of the energy in the elastic, and so they go through a range of different shapes. Of all the configurations available to a long molecule, very few of them are long and straight, and the vast majority are short scrunched up arrangements. So ordinarily in a piece of elastic, most of the long molecules inside it will tend to be in these scrunched up shapes.

When you stretch the elastic, the molecules are still wiggling around, but now they're constrained to be in longer, straighter arrangements. When the stretching force is released, the wiggling molecules are free again to explore the whole range of shapes available to them. Since the molecules are wiggling at random, most of them will once again end up in one of the shorter configurations (of which there are lots) rather than the longer ones (of which there are few), and so the overall length of the elastic will reduce.

This is an example of an "entropic force": the contraction of the elastic isn't due to things being attracted to one another, the way magnets are; it's just due to the molecules being able to randomly explore all their possible configurations, and having more short configurations than long ones.


What Verlinde is proposing is that gravity is also an entropic force. To tie it in with the above analogy: why does separating two objects (e.g. an apple and the Earth) require a force, and why do the masses move toward one another when this force is removed? Just as in the case of the stretched elastic, it's natural to posit a force of attraction at work, resisting the separation. But as the above example shows, that's not always the right explanation.


At first glance, a piece of elastic is simple. If it had no internal structure, then for any particular amount of stretching we might apply to it there would be exactly one way for the elastic to be stretched by that much -- that is, exactly one state corresponding to "stretched by this amount", which we could describe with a single variable measuring the length of the elastic. However, the "entropic force" explanation of elasticity says that in fact the elastic has lots of different states for each amount of stretching -- different ways the molecules inside it can be arranged. Since there are a lot more states corresponding to "unstretched" than to "stretched", and since (when there's no force applied) it explores all these states at random, the elastic will tend to contract from one of its stretched states to one of its unstretched ones.

Similarly, it's natural to imagine that there's exactly one way for two objects to be separated by a particular amount, which we can describe with a single variable measuring the distance between them -- for example, a single state corresponding to "the apple is 1m above the ground". Verlinde, however, is suggesting that the correct description of the situation will involve a set of many different states corresponding to each possible distance between the two objects. By analogy with the previous example, there would be lots of states corresponding to short distances, and fewer corresponding to longer distances. Then in a random exploration of all the possible states available, the apple-Earth system will tend to go from one of its greater-separation states to one of its smaller-separation states. That is, the apple and the Earth will move closer to one another -- they'll be "attracted together by the force of gravity".

All the above just explains the basic idea of "gravity as an entropic force". I haven't gone into (and don't understand) the further details of Verlinde's paper. The obvious remaining questions that I haven't addressed include: (i) what are these extra states, analogous to the configurations of the long molecules of elastic? [This has something to do with "holographic screens", which I don't understand]; (ii) what determines the number of states corresponding to a particular separation between two objects, and how does it end up depending on the distance between them and their two masses? Some of these further details are sketched and discussed at the blog Skorgu linked to above. Can anyone fill in some details on the "holographic screens" business?
posted by logopetria at 5:25 AM on January 22, 2010 [7 favorites]

My only problem with that is that if it were random, wouldn't it be less, like, 'a law' and more 'a suggestioin'?
posted by empath at 8:21 AM on January 22, 2010

I'd call up a sanskrit scholar and ask them what the upanishads say, don't forget Oppenheimer's words,

Behold, I am become death
posted by infini at 9:07 AM on January 22, 2010

I've always felt that 'time' was a lie. This theory seems to be saying that gravity, also, is a lie, and this makes sense to me.

No, it relies on time existing for gravity to work. In other words, that gravity is an effect of information and time.
posted by delmoi at 12:32 PM on February 18, 2010 [1 favorite]