For some context, there is a very intriguing formulation of General Relativity in terms of entropy. [1] What this suggests is, that gravity emerges from a underlying thermodynamical system. In OP the authors construct explicitly one of these systems.
Unfortunately together with the success of perturbative quantum gravity [2], this suggests that quantum gravity is just not experimentally accessible. (That's a thought by Freeman Dyson originally.) The formal argument would be, that a lot of different theories lead to the same thermodynamical limit, so that one can not determine the true underlying theory. Or put it in more hacker news terms, the problem is analogous to trying to learn about TCP/IP by looking at the output of black hole simulations, there is the layer of numerical mathematics in between, which is at least very hard to breach.
[2] For this discussion, it is basically cheating. You quantize gravitational waves on a classical background geometry. The approach works very well because gravity is weak, it also tells us nothing interesting, because one breaks the relationship between gravity and space time (the central feature of General Relativity) by hand.
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-* "Or, more accurately but less evocatively, “find gravity inside quantum mechanics.” Rather than starting with some essentially classical view of gravity and “quantizing” it, we might imagine starting with a quantum view of reality from the start, and find the ordinary three-dimensional space in which we live somehow emerging from quantum information."
- "If we perturb the state a little bit, how does the emergent geometry change? (Answer: space curves in response to emergent mass/energy, in a way reminiscent of Einstein’s equation in general relativity.)
It’s that last bit that is most exciting, but also most speculative."
- "But the devil is in the details, and there’s a long way to go before we can declare victory."
- "In some sense, we’re making this proposal a bit more specific, by giving a formula for distance as a function of entanglement. "
- "We’re quick to admit that what we’ve done here is extremely preliminary and conjectural. We don’t have a full theory of anything, and even what we do have involves a great deal of speculating and not yet enough rigorous calculating."
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-* "Perhaps the most interesting and provocative feature of what we’ve done is that we start from an assumption that the degrees of freedom corresponding to any particular region of space are described by a finite-dimensional Hilbert space."
- "A finite-dimensional Hilbert space describes a very different world indeed. In many ways, it’s a much simpler world — one that should be easier to understand. We shall see"
My 1 cent:
I don't know enough about this subject but...it's this creative thinking that is desperately needed in the sciences. I'm not trying to tear down others but instead just say that this is what happens when education advances. When enough (abstract) people focus on a subject, we will find a breakthrough.
This is impressive. Not that I understand it. But it's encouraging to try to derive space and gravity from quantum mechanics.
Quantum mechanics seems to be how the universe really works. Outrageous predictions of quantum mechanics, from the two-slit experiment onward, have been experimentally verified. So it's a sound base for further work. Physics has been stuck for a century trying to reconcile relativity and quantum mechanics. This might be a way forward.
It might even lead to something that's experimentally verifiable.
I would suggest reading "Space time quantization" [1] published in 1999 presenting a theory based on the same idea. This theory has since being developped and the latest results have been submitted for publication. These results are multiple. The most important of them are the explanation of the nature and properties of dark matter. But there is much more.
The seeding work on this theory was published as early as 1967 and 1978.
Publication into arXiv is too restricted, because of the endorser requirement, for the article to be published there.
I'm not casting any evaluation or judgment on the work you cite at all, but it's a lot easier to get a paper on the arXiv than into a non-trash journal. If the subject is too controversial to appear on the arXiv, most journal editors will bench reject it.
In fairness, Foundations of Physics (where this paper by Meessen was published) isn't a trash journal, but it is an odd duck. The editor in chief is Nobel laureate Gerard 't Hooft, who everyone agrees is brilliant, and generally still rigorous strictly speaking, but whose interest have become much less mainstream. Articles published in the journal are rarely flawed in a technical sense, but they do often explore highly speculative avenues that are usually considered unpromising for good reason. I'd say there's a good analogy to cold fusion, which is a field that is highly unfashionable for good technical reasons, and which has attracted a lot of cranks, but in which there are still formally open problems and which can't be completely dismissed.
Yes, 't Hooft seems to have some interesting interests these days. I believe he's one of very few who think superdeterminism is a tenable explanation of entanglement.
There is a more recent article (2011) of A.Meessen about Space Time Quantification (STQ) on arXiv [1]. I didn't read it yet.
The subject is not controversial. The theory is just revolutionary. It require to accept the hypothesis of quantum of measurable length. The quantum of length a is assumed to be 0 in actual general relativity and quantum mechanics. In STQ the quantum length a is assumed to be different of 0. This has huge implications. It allows to explain all elementary particles, their properties and possible interactions. The standard model is an empirical model that only includes what has been seen in experiments. STQ explains everything and can thus describe what has not yet been seen like Dark Matter. It explains its nature and describes its properties like the possible interactions.
So quantification of space and time is not a new idea and there is already a sound theory built on it. My biased opinion is that it describes the real thing. This theory is not to be confused with a voxel like quantization of space.
Publishing on arXiv boils down to find an endorser. A.Meessen has the academic credentials, but this is not enough. However I agree that it is easier than publishing in a peer review journal.
This sounds super interesting to me as someone with a passing interest in astrophysics but no real study. But it also seems such an obvious approach that I'd be shocked if it hadn't been tried before. Is it really that novel?
This has seemed to me for a while the right approach (gravity from quantum mechanics, vs. quantized gravity). Can anyone speculate on the discovery of "gravitational waves" - i.e. in a quantum mechanical description, what are gravity waves made of? A sort of quantized spread of entanglement?
Basically, yes. Although gravitation is not a "force" but a manifestation of the geometry of space-time. The aim is to show that this geometry can be derived from entanglement relations in the Hilbert space.
I don't see what's testable or falsifiable in here. Okay, it's good to get rid of epicycles, and intellectually pleasing, but it still looks like the same objections apply as have been raised around string theory. Even if this is the 'theory of everything', will we not end up with shorter equations that describe the same observations?
If they do find a way to predict space and gravity from quantum mechanics it probably will have testable predictions. They don't seem to have cracked it yet though. (Disclaimer - didn't read the paper.)
I've long thought this would be a good way to go for research. I once had a crack at understanding Feynman's arguments that you could get general relativity from spin-2 particle theory in the book Lectures on Gravitation based on his 1965 lectures with the intention of writing a simplified version for Wikipedia. I failed - too hard - maybe someone else can do a laymans version? As far as I know there hasn't been very much progress since Feynman's attempts which were not renormalizable.
Unfortunately together with the success of perturbative quantum gravity [2], this suggests that quantum gravity is just not experimentally accessible. (That's a thought by Freeman Dyson originally.) The formal argument would be, that a lot of different theories lead to the same thermodynamical limit, so that one can not determine the true underlying theory. Or put it in more hacker news terms, the problem is analogous to trying to learn about TCP/IP by looking at the output of black hole simulations, there is the layer of numerical mathematics in between, which is at least very hard to breach.
[1] Older guestpost by Grant Remmen on Carroll's blog: http://www.preposterousuniverse.com/blog/2016/02/08/guest-po...
[2] For this discussion, it is basically cheating. You quantize gravitational waves on a classical background geometry. The approach works very well because gravity is weak, it also tells us nothing interesting, because one breaks the relationship between gravity and space time (the central feature of General Relativity) by hand.