“Entanglement” is the current buzzword of physics, here are two new stories featuring this:
Back in 2013 one could read lots of claims in the media that “Hard evidence for the multiverse” had been found, based on “effects of quantum entanglement between our horizon patch and others”. These claims were discussed on this blog (with a response from the authors here). A new paper by Will Kinney has now been published in JCAP, including the following conclusion about such claims:
It is worthwhile to discuss in general the “concrete predictions” originally claimed by the authors of refs. [1,2], since several key claims do not survive even cursory scrutiny. For example, the discontinuity in the effective potential claimed to be correlated with voids and the CMB cold spot does not appear to in fact exist: for all physically relevant values of the parameters V0, λ, and b, the modulation F(φ) is a smooth function, with no characteristic discontinuities which would explain features in the power spectrum. Perhaps more importantly, the form of the effective potential resulting from landscape entanglement is completely dependent on the choice of inflationary potential V(φ), which is itself an arbitrary free function. One could just as consistently choose the underlying inflationary potential in the absence of landscape corrections to be the same as the effective potential (2.7)! In this sense, the landscape model is no more (or less) predictive than single-field inflation itself, and most of the claimed predictions of the entanglement model turn out not to have been
predictions at all. However, any considerations of theoretical consistency are a moot point: even if one takes the claimed predictions at face value, almost all of them are ruled out by Planck. Experiment always supersedes theory, and the model does not match the data.
This paper has an unusual story behind it, with an author of the work it criticizes trying to keep it off the arXiv. For more about this, see here and here.
Another entanglement story that is getting some press attention this week is this paper by Erik Verlinde, with its associated press release, explaining that we may be “on the brink of a scientific revolution”. I’ll have to avoid trying to give an explanation of the physical argument of the paper, on the grounds that I don’t understand it, partly because there seems to be no underlying physical model here. The basic idea is stated as replacing dark matter by
an elastic response due to the volume law contribution to the entanglement entropy in our universe.
but someone else will have to explain exactly what that means. Maybe I’m missing it, but I don’t see anywhere in the paper a suggested experimental test of the theory. Someone much more expert than me is needed to explain whether the picture of this paper is consistent with the known astrophysical and cosmological evidence usually interpreted as dark matter/dark energy.
Peter, Thank you for a post of ideas with which I can distract myself. Have a nice Thanksgiving.
WRT Verlinde – I understand very little although I tried to read it. The takeaway for me was that there is a fundamental emergent theory of gravity that explains dark matter observations without the need for, well, dark matter. If true, it would be great but I guess the physics community needs to digest it.
One problem with the Verlinde theory as it can’t yet explain dynamical situations like the bullet cluster. Nor the cosmic microwave background observations, which arguably provide the most compelling evidence for dark matter.
You cannot imagine the media coverage of Verlinde’s paper here in the Netherlands. It’s really incredible. Every big Dutch news paper featured him as “being the one overthrowing Einstein” and quoting him saying that “this is going to be a new scientific revolution”. I am quite amazed by the media offensive that Verlinde and the university of Amsterdam have taken. Probably the paper is an interesting avenue, but hailing it as the next big thing seems too much.
Verlinde has been pushing that idea for years, and the dark matter idea. This just sounds like his university has a new publicity department.
If Gravity is an entropic force, then how come it’s perfectly reversible (unlike entropy)? Lift a stone, drop it, you get the same energy back.
I was at a talk with Erik Verlinde and Frank Wilczek, and Frank Wilczek was sooo dismissive of Verlinde’s idea it was almost rude.
Abbyyorker,
Yes, that’s what he’s claiming to do. Unfortunately all I see in the paper is a bunch of wordy vague analogies claiming a revolutionary new fundamental physics, and little in the way of how to confront these ideas with experiment, or any examination of whether or not they’re already disconfirmed (or even self-consistent). I haven’t computed a John Baez crackpot index for this, but I’d suspect it’s pretty high. Prominent physicists tend to be polite in terms of their public response to things like this, even when privately highly dismissive. Verlinde has been giving lectures about these ideas for several years now, and as far as I can tell his peers have just ignored them. Likely they’ll do the same with this paper, no matter how much press it gets.
@ Erik
I guess the huge media coverage of the paper in the Netherlands can be understood as a way to justify the 6.5 million euros in fund given for the entropic gravity thing in 2012.
http://www.math.columbia.edu/~woit/wordpress/?p=5334
I actually read most of his paper, and it looks like his model is worth real investigation even if it proves to be wrong. Yeah, he’s certainly not at “Einstein 1915” yet- that’s hype. But its not impossible either. It wasn’t like anybody in 1900 saw Einstein himself coming, or that most physicists in 1900 correctly guessed that junking the Ether model entirely was the correct path forward.
Basically, the idea is that the Holographic principle does NOT hold exactly in de Sitter space (AKA the real universe) like it does in Anti-de Sitter space. Instead, dS obeys a more general law in which the entropy of the volume is encoded in both horizon and non-vanishing ‘volume’ terms. He then has a considerably more tenuous/vague argument invoking ER=EPR (among several things) that associates the presence of matter-energy within a space-time volume w/ a decrease in the value of these ‘volume entropy’ terms within it. The 2nd Law of Thermodynamics then requires this loss of entropy be balanced (or over-balanced) by a compensatory growth of entropy on the horizon, which through a long argument he claims can be modeled well in the same way solid state physics models stresses & displacements on a chunk of material. From there he claims that at Newtonian/GR gravity is replicated at solar- to local-interstellar scales, but at galactic scale you get a new ‘displacement’ or ‘backreaction’ gravitational effect that explains why the MOND model works there (and only there) w/out new dark matter, and a non-local effect that explains apparent Dark Energy.
I don’t know how much of this is justified, but in principle he sounds like he’s asserting:
1) The Equivalence Principle is an EXACT law of nature. That means any experimentally observed deviation would invalidate Verlinder as well as Einstein.
2) Black holes should have no firewalls.
3) The canonical Holographic principle does NOT apply to our universe- only an extension of it does. Don’t know how to test this, exactly, but if that research was EVER science the concept should be testable in principle.
4) The apparent inferred distribution of ‘dark matter’ is really entirely due to the distribution of Baryonic matter. That should be observationally distinguishable from numerous DM models like Bose-Einstein condensate Axions or ‘hidden sector force’ DM, which have non-gravitational dynamics.
Zack,
Of the four things you see Verlinde as asserting as implications of his ideas, only the fourth seems to be substantive, and I don’t see in what he writes any evidence that “entirely due to distribution of Baryonic matter” better explains the data (astrophysical and cosmological) than other dark matter models. But I’m pretty ignorant about these issues, so I’d love to hear from someone expert on what the data says about whether or not it’s consistent with what Verlinde is claiming. Hopefully the public attention to his claims will get some experts to weigh in on this.
Peter interesting that now string theory can also predict MOND. I thought existence of dark matter is strong evidence for string theory.
Possibly relevant:
http://xkcd.com/1758/
“Verlinde has been giving lectures about these ideas for several years now, and as far as I can tell his peers have just ignored them.”
His paper on the entropic origin of gravity has about 579 citations. Maybe his peers don’t agree about it but hardly one could say they ignored it. I wish my work would received that kind of indifference!
On the CMB, he says his model behaves like a pressure-less fluid like cold dark matter so, if I understood correctly, at early stages it shouldn’t show differences with current models …
My first impression is that people modeling structure formation in the Universe would be able to test these ideas.
Luis,
I was referring to his arguments about dark matter, which now have been around for a long time. If you can point to a reference which takes them seriously and discusses their testability and consistency with observation, I’d like to hear about that.
The work of Kobakhidze (2011) (https://arxiv.org/abs/1108.4161) disproved the earlier version of Verlinde’s theory. Unfortunately, the work of Kobakhidze (2011) is not even cited in this last Verlinde’s paper.
I believe that PRL 117,201101(2016), in the latest PRL, is in the neighborhood of this discussion. There already is a simulation that claims a nonMOND explanation. I do not have that reference. I am not really a fan of complex models and simulations.
For a nonMOND explanation of galactic rotation curves and without invoking dark matter, I have suggested that Born reciprocity might be working at the cosmic level. See my paper on Born reciprocity and the 1/r potential. Of course I might be utterly wrong but the idea might be worth a second look, especially if somebody can see how to relativize it properly. It could also benefit from proper investigation by a professional astrophysicist.
@Zack Yezek – this is something I’ve thought too. I see this Verlinde paper as interesting. At macroscopic level he argues that it restored galaxy acceleration curves. So it probably takes some insight into something important. But as Kobakhidze shows, microscopic ideas below this calculations are wrong.
It starts with holographic principle and he denied it in order to include some additional degrees of freedom excited in the bulk. It looks somehow in relation to holographic principle like Gibbs paradox in relation to classical computation of entropy.
This new theory seems to depend on Verlinde’s original work that argues for gravity as an entropic force (https://arxiv.org/abs/1001.0785). However, the original paper is fundamentally flawed. He mixes up the Unruh temperature that a uniformly accelerated observer experiences in flat spacetime with the Hawking temperature that a constant radial observer sees outside of a black hole. The two temperatures are not equal.
In the JHEP version of the paper equation equations (5.3), (4.1) say that the temperature of some closed surface enclosing a mass distribution is equal to the temperature proportional to the proper acceleration of some static observer (analogously to the Unruh effect). But this totally incorrect. A static observer outside of a black hole would experience Hawking radiation which would not be proportional to that observer’s proper acceleration. The only place a static observer outside a black hole sees a temperature proportional to their acceleration is exactly at the event horizon where the temperature goes to infinity. However, both temperatures go to infinity in the same way. So the whole theory seems completely incorrect to me.
Alex,
An odd thing about the Verlinde paper is that it’s not clear (at least to me) what the relation is to the earlier one. I haven’t bothered trying to follow carefully what he does, but he seems to me to be invoking different very vague speculative ideas in the new one than the old one (although, at this level of vagueness, presumably there is some relation between the new and old ideas).
Peter,
I think it is the same idea. If you look at page 10, and the text surrounding equation (2.12) he states:
“Our hypothesis is that this [horizon] entropy is evenly distributed over microscopic degrees of freedom that make up the bulk spacetime.”
So the horizon entropy is the entropy of the gravitational degrees of freedom and the tendency to maximize this entropy forms the ‘elastic response’ (previously ‘elastic force’).
Actually in reading this in more detail I’m even more confused. He says that the volume of “a ball of radius r centered around the origin” is basically 4/3*pi*L^3. But in the coordinate patch he’s using the volume of this “ball” of radius L calculated using the induced metric would, I think, give an infinite volume. So there’s a lot of problems here.
But I agree with you in that the idea is so vague that any attempt to disprove it might be futile.
In an earlier version given in talks (not so sure whether that’s the case also in the paper), dark matter is interpreted as degrees of freedom in the matrix model of M theory, degrees of freedom that so far have no other interpretation. But in the string theory limit of M theory, gravity is a fundamental force on equal footing with all other forces. So what’s going on? Is gravity emergent or not emergent?
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