[taliesinb]

Here's a recent lecture by Erik Verlinde in which he tries to explain both dark energy and dark matter using entropic gravity:

https://www.youtube.com/watch?v=PSYXt3Xu3xI

I'm not a physicist, but he seems to be invoking similar ideas to Raamsdonk: that spacetime and gravity are emergent properties that boil down to entanglement somehow. But the loop quantum gravity people have been saying something like that for ages. And Stephen Wolfram and Konrad Zuse etc. And we have people like Lubos Motl on the other hand to ridicule all of the above.

Anyone able to give a big picture overview of what's been happening the last few years in this somewhat rarefied world? Who are we, the intelligent lay public, to trust?

[madaxe_again]

I am a physicist...

The most interesting (to me, at any rate) bit of this article are susskind's observations on computational complexity. As a precocious undergrad I put forward the idea that gravity and time dilation were the same thing, and both stemmed from the universe needing a constant amount of "time" to compute the interrelations between the particles in a volume of space - and if there were more particles more time would be needed as the graph of particle interactions would grow non-linearly - and therefore time "slows down" in a matter (information) dense area of spacetime.

What really got me gunned down was then going on to argue that this suggested a simulated universe, running in a substrate.

Either way, interesting to see someone else who actually has some clout having similar ideas.

[duaneb]

Why do you (and others) tend towards arguments suggesting simulations?

To me, the language here is important. Both simulation and computation imply (to me) a tool and a user of the tool. Even a broader interpretation of computation as, umm, the efficient transfer of precise information through spacetime in certain shapes (which is easily providable by our understanding of physics today) requires an observer to extract the computation from the otherwise non-semantic system.

It definitely makes sense to think of the universe as a projection of a higher dimension, or a holograph, or however you want to look at it, but that's a far cry from implying a simulation.

EDIT: hologram -> holograph

[madaxe_again]

A simulation doesn't mean that there is a simulator - just that what we consider real may be a projection of something else. As you say, the holographic universe principle can equally answer this.

That said, the simulation argument strongly pushes in favour of a simulated reality - particularly when you consider that there is no need for it to be real time, or of the entire universe. In short if we ever gain the capability to accurately simulate a small corner of the universe, then it stands to reason that someone further up the hylaean flow (apologies to Stephenson) also has.

Hell, maybe the rotation of distant galaxies is off because they're just sprites ;)

[padobson]

Hell, maybe the rotation of distant galaxies is off because they're just sprites

So there's probably some other simulation of our universe where they tried to do models, but it was too slow.

If not, then the simulator introduced a pre-mature optimization. The existential ramifications of such a development inefficiency are palpable!

[criddell]

What do you feel about the mathematical universe hypothesis[1]? If I understand it correctly (and I probably don't), it says that the universe is literally mathematics. Not that it's a manifestation of mathematics or can be modeled by mathematics, but is 100% mathematics.

I think that fits the computation model and doesn't require an observer or operator.

[1]: https://en.wikipedia.org/wiki/Mathematical_universe_hypothes...

[Retra]

Mathematics is a human language for studying the structure of various experiences. To say the universe "is" mathematics without saying what mathematics is doesn't really say much at all. You might as well say it's magic.

And, saying that it's a "mathematical structure" is just saying that it's a structure that humans can apply mathematics to model. So there's really no mechanism for this to be true, except in the trivial "the universe has structure" sense, which also tells you (basically) nothing, because humans can't even perceive things that don't have structure, nor can we reason about them.

[criddell]

> Mathematics is a human language for studying the structure of various experiences

I think you are getting hung up on the semiotics of mathematics which is indeed a human construct. The idea of "1 + 1 = 2" is true no matter how (or if) you represent the idea.

> saying that it's a "mathematical structure" is just saying that it's a structure that humans can apply mathematics to model

That's not what the theory says (AFAIK). The mathematical universe hypothesis says that there is nothing in the universe, it's all just mathematics.

Edit: Think about it this way: if you could simulate a universe and in that simulated universe you modeled people and all the stuff around them, what would you tell those simulated people with their simulated free well what a table is made out of? It's all mathematics, right?

[Retra]

The math universe hypotheses assumes a very strict Platonist POV, which is not really a justifiable position, in my experience.

>The idea of "1 + 1 = 2" is true no matter how (or if) you represent the idea.

That's really a triviality, though. If you have the idea that 1+1=2, then yes, it's true. Usually people say mathematical theorems are true in "all possible universes", but the way they select which universes are possible is... by applying logic that works in this universe. 1+1=2 is only true because it doesn't contradict our experiences. It's useful. In another universe, it may not be useful, and thus wouldn't be true there.

>if you could simulate a universe and in that simulated universe you modeled people and all the stuff around them, what would you tell those simulated people with their simulated free well what a table is made out of?

I would put words in whatever order was most useful to them to employ for the construction and manipulation of tables. Anything else would be meaningless. You might as well just say "magic" if they can't use it.

[duaneb]

I actually love that theory because it makes me feel warm and fuzzy. However, it is.... Highly hard to defend, to say the least.

[bytelayer]

This is a fascinating idea. Could you elaborate on how you think the additional computational complexity would give rise to attraction between particles in matter-dense regions?

[madaxe_again]

Spacetime just stretches to reduce the effective density in a flat universe and maintain an effectively constant information density - thus gravity and time dilation.

[ThomPete]

Isn't that what Wolfram is also getting at with his irreducible complexity concept?

I wonder if the trick about this is not really to find a true explanation but the right perspective and more and more simulation seems to be a very valuable perspective to put on this.

Disclaimer — I am not a physicist so apologies if my question is stupid.

[hellofunk]

Beautiful stuff, man. One of the more interesting questions I've seen asked in my casual hobby readings lately is: "Is the Universe quantized?" Meaning of course, is there a final limit to how small things get, and, with the passing of time, does it flow freely or pass in very tiny quantized periods?

[dave_sullivan]

Tangentially related, this basically makes the argument that some black holes (like low mass X-ray binaries) are actually societies that have managed to control matter near the planck scale, best read as hard sci-fi: http://accelerating.org/articles/Smart-2011-TranscensionHypo...

[progman]

> is there a final limit to how small things get

AFAIK yes, the limits are known as Planck Constants. It means that we are living in a digital universe.

https://en.wikipedia.org/wiki/Planck_constant

There are also conjectures about a _holographic_ universe.

https://en.wikipedia.org/wiki/Holographic_principle

https://www.youtube.com/results?search_query=holographic+uni...

[flatline]

The existence of the Planck length does not imply quantized spacetime, which is still very much an open question in the field of physics. To the best of my knowledge most models still posit a continuous spacetime. cf. https://www.quora.com/Is-time-discrete-or-continuous

[simonh]

I don't see how that would work with length contraction and time dilation. A unit quantum length object in my frame of reference would have different length to one in a frame at motion relative to mine, and time for it would pass at different quantum clock ticks. I don't see how a universe like that could work when it came to interactions between moving particles.

[AnimalMuppet]

You're assuming that the Lorentz contraction applies unchanged as you move to the Planck length. That's not a given.

[sawwit]

Why does it imply a simulation? Perhaps it can be explained by an anthropic argument: Perhaps the kind of mechanics behind spacetime in which time evolution is computed in a stepwise manner depending on density are extraordinarily more likely because they somehow allow for much simpler mechanics (sets of laws with lower entropy are more likely to be the ones we find ourselves in, especially so if we assume that all non-physical sets of laws are realized as well, so that the impact of such a difference in entropy on the probabilties could be vast).

[HiLo]

This seems relevant. It's just a ride.

https://www.youtube.com/watch?v=KgzQuE1pR1w

[contravariant]

Well, with these kind of articles you can usually safely replace "physicists" with "some physicists".

Anyway, the part that I know, and which is well established, is that there seems to be a correspondence between a d+1 dimensional quantum gravity theories, and d dimensional quantum field theories (e.g the standard model). A while ago this was still purely conjecture, but Maldacena showed that this correspondence was true (under certain conditions) for an AdS space with a conformal field theory on its boundary. As far as I know this is now pretty well established, most of the discussion seems to be on how much it can be generalized.

From my (limited) understanding it seems that (in AdS/CFT) entangled particles tend to correspond to strings traversing from one part of the boundary, through the bulk, to another part of the boundary, the space the string travels through then forms a tube like object. This gives a link between entanglement and geometry, since these tubes exist if and only if the two points on the boundary are connected.

I think it isn't disputed that this happens for AdS/CFT, but whether something similar is true for our universe is debatable. If however it turns out that the holographic principle does hold in general then that would imply that our universe somehow corresponds to a 2+1 dimensional field theory, where entanglement of this field theory is responsible for the fact that our space-time is connected.

[semi-extrinsic]

I believe Maldacena's 1999 paper [1] proving the AdS/CFT correspondence is the most cited in theoretical physics in the last 50 years. It's an incredibly nice result that is almost impossible to explain to laymen. (You did a pretty good job.)

[1] https://scholar.google.no/scholar?cluster=132568844183140142...

[jessriedel]

Maldacena conjectured a correspondence. It's never been proven.

[contravariant]

He managed to prove it quite rigorously for a specific example, which is the paper semi-extrinsic mentioned. He did indeed not prove the correspondence in general, but even this 'limited' example already turned out to be immensely useful.