First test of rival to Einstein’s gravity kills off dark matter(newscientist.com) |
First test of rival to Einstein’s gravity kills off dark matter(newscientist.com) |
> Milgrom, however, supports the work. He also points out that according to his own 2013 analysis of gravitational lensing data in galaxies, MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study.
(Also: http://motls.blogspot.ca/2016/11/verlindes-de-sitter-mond-is... )
This data cannot distinguish between Verlinde and other explanations for MOND-like behavior. We've long known that MOND seems to fit a subset of all observational data better the dark matter, in particular the Tully–Fisher relation
https://en.wikipedia.org/wiki/Tully%E2%80%93Fisher_relation
However, for the rest of the data (e.g., the CMB, large-scale structure, the bullet cluster), MOND is generally considered to give a worse fit than dark matter, or to be silent (because it's not obvious how to extend MOND to a complete cosmological theory). That's why experts think the totality of evidence supports dark matter.
Nothing has changed.
From a couple of days ago :
http://www.forbes.com/sites/startswithabang/2016/11/12/ask-e...
"The hope is, with the right assumptions, a full theory of gravity, That's the big hope, and what Verlinde is working towards. (Others are also working towards it independently.) This paper is an update on how it's going. So, how's it going?
"There are some successes given very specific assumptions, but there are a lot of problems. The largest problem, quite simply, is that one needs to make a multitude of seemingly arbitrary "interpretation" decisions to wind up with something other than nonsense. For example: the full motivation for this approach is based in anti-de Sitter space (or space with a negative cosmological constant), but our Universe is observed to have a positive cosmological constant (i.e., de Sitter space), and the mathematics of the two spaces have very different properties. For another, you need the entropy to obey a strict area-based law to get the Einstein equations out, but you don't get a cosmological horizon out if you do. (And our Universe has one.) And finally, if you make all the assumptions you need to in order to get the gravitational acceleration for galaxies out, you destroy all of General Relativity's successes on larger-than-galaxy scales. (Verlinde, on pp. 39-40, makes the argument that it could succeed, but the observations of colliding galaxy clusters completely undermine his line of thought.)"
There's even more discussion in the Ethan's article, which is much more balanced.
And another take on Verlinde's paper by Sabine Hossenfelder:
http://backreaction.blogspot.co.at/2016/12/can-dark-energy-a...
"General Relativity is a rigorously tested theory with many achievements. To do any better than general relativity is hard, and thus for any new theory of gravity the most important thing is to have a controlled limit in which General Relativity is reproduced to good precision. How this might work in Verlinde’s approach isn’t clear to me because he doesn’t even attempt to deal with the general case. He starts right away with cosmology."
"I'd say he takes inspiration from models that are best understood in AdS." but "he starts with de-Sitter space which means he assumes dark energy. It doesn't make sense to say that this is a source of dark energy, it's the same thing, period."
I find this rudeness and pedantry in academia infuriating. There's no justification for this level of nastiness -- not in private, public, peer review, or science as a whole. Seeking out the unknown and creative explanations thereof is the hallmark of good science; check your ego at the door. I know nothing about him... but in my book, Lubos Motl can go pound sand.
However, it's not outrageously wrong, and it is noteworthy that an accomplished string theorist has decided to tackle gravity in a universe like ours rather than leaning on AdS/CFT arguments.
Some of the folks on this paper are good scientists, but this appears to be more about looking for funding than good science.
Their paper on the Arxiv is about building some math to make gravity look like dark matter. But this has been done 1000 times before with no success. They have no physical basis for their math; there is no reality behind it. I do not expect this to get them the Nobel Prize.
Verlinde's paper is here: https://arxiv.org/abs/1611.02269
His idea deserves some study but a title will "Kills off dark matter" is very infuriating
I hope more people decide to explore this avenue. It really is a fascinating take on gravity.
"Galaxy rotation curves are not very good tests for GR itself, since there are so many parameters about mass distribution in a galaxy which are simply not nearly as precisely understood as GR itself"
Verlinde does not build on MOND. An explanation and some extracts from his paper are in an earlier comment of mine:
https://news.ycombinator.com/item?id=13198573
(specifically at the paragraph half-way along in the comment, starting with "Sure Milgrom", if you want to zoom right in.)
Theories by definition, may not be correct. It's still a theory.
How does one become a non-theorist? I suppose by non-theorizing.
No, it would reshape the "top" of these theories. The base would remain the same.
Obviously I'm missing the academic discipline to appreciate this topic but nevertheless, I'm curious about our understanding of the very fabric and mechanisms of our reality and the universe.
EM is mediated by photons and visible matter is constantly absorbing and emitting photons.
Some particles don't though. Neutrinos for instance. They're dark, they don't interact with light.
So when observation showed that there needs to be more matter laying around than we could see some enterprising young turks said "hey, maybe there's a shit ton of dark matter" and that makes the math come out right.
the obsession is mostly because people who aren't astronomers seem to think "dark" is a metaphor and go "aha now that no one cares about the oxford comma this will be my new opinion"
Up for debate is what kind of particles make up the dark matter and how much mass is dark vs just poor measurements and/or some tweaks to gravity, but unless some one manages to disprove neutrinos: dark matter exists.
Maybe there is some matter out there that is "dark", so we just can't see it. That would be a solution. Or maybe the laws of gravity we wrote down are bad at large scales. No one knows. Trust no one who tells you they know the answer: if they did they would have a Nobel Prize.
Suggest "Astronomers studying galaxy mass distribution say their observations can be explained without dark matter."
“But then if you mathematically factor in the fact that
Verlinde’s prediction doesn’t have any free parameters,
whereas the dark matter prediction does, then you find
Verlinde’s model is actually performing slightly better.”
> We have shown that the emergent laws of gravity, when one takes into account the volume law contribution to the entropy, start to deviate from the familiar gravitational laws precisely in those situations where the observations tell us they do. We have only made use of the natural constants of nature, and provided reasonably straightforward arguments and calculations to derive the scales and the behavior of the observed phenomena. [..]
> In our view this undercuts the common assumption that the laws of gravity should stay as they are, and hence it removes the rationale of the dark matter hypothesis. Once there is a conceptual reason for a new phase of the gravitational force, which is governed by different laws, and this is combined with a confirmation of its quantitative behavior, the weight of the evidence tips in the other direction.
If theory A has parameters you can adjust, it becomes easier for an observation to fit the theory just by accident (because you can adjust the free parameter).
This is a fairly flawed description, but it should be a good enough ELI5.
http://motls.blogspot.com/2012/10/evading-quantum-mechanics-...
Also read the comments, where he says:
I am watching the talk (final minutes now) and except for the history, meaning of interpretations etc., he actually understands QM more correctly than many people employed as physicists...
(The audacity! Someone that actually watches the thing they'll be bitching about!)
[0]: http://motls.blogspot.co.uk/2012/10/evading-quantum-mechanic...
So you'd rather people quietly knife you in review? Because that's what happens if you penalize people for public rudeness. Personally, I'd rather have a loud critique to my face than a polite one behind my back or, even worse, no engagement at all.
I don't need your politeness or friendship when I'm putting forth some new theory. I need accurate, engaged criticism and the number of people who will do that is vanishingly small. And a lot of the ones who will do that have social issues almost by definition.
"I wouldn’t okay this wrong piece of work as an undergraduate term paper" is not accurate, engaged criticism, it's mean-spiritedness under a veneer of toughness.
It's easy and fun to be rude and insulting, and when you can get away with it because of the subculture you're writing within, why not, right? Because being a dick is being a dick no matter who or where you are, that's why, and being a dick doesn't advance anything beyond your own momentary joy.
Every industry, every field, everywhere, will be better off once people get past being dicks just for fun, and give accurate, engaged criticism while maintaining civility.
> Mauro, SHUT THE FUCK UP! ...To make matters worse, commit f0ed2ce840b3 is clearly total and utter CRAP even if it didn't break applications. ENOENT is not a valid error return from an ioctl. Never has been, never will be. ENOENT means "No such file and directory", and is for path operations. ioctl's are done on files that have already been opened, there's no way in hell that ENOENT would ever be valid.
https://lkml.org/lkml/2012/12/23/75
And then there's this:
> It's a Category 5 loon. Popular sources are full of anti-Copenhagen crackpot pseudoscience and of exactly this kind of bullshit – claims that the proper Copenhagen quantum mechanics implies superluminal or acausal effects (for the latter, see some largely confused fresh text by Nude Socialist about entangled photons in graves, London, and Beijing: this sort of stuff gets produced every minute, it seems) – but you may still find people who think that the presentation should be even more anti-Copenhagen.
http://motls.blogspot.com.au/2012/10/evading-quantum-mechani...
He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is.
Characterizing him as an example of any sort of trend in academia is both uninformed and deeply insulting.
It's hard to understand why New Scientist decided to quote him then.
They certainly seem to be presenting him as someone worth paying attention to.
I'm frankly surprised New Scientist bothered to quote him.
vs all those "I just don't believe in being fake and sugar coating things" folks in every other industry? Not much unique to academia on that front.
"I find this theory highly disagreeable, it contradicts findings by X and Y et al."
Compared with:
"I once asked my dog for a theory on the nature of the universe, whereupon which he vomited violently on the floor. This explanation has more truth to it than the one presented in this paper, which I can only presume was written by students who, in their youth, were savagely beaten upside the head by their parents much to the detriment of their intellect."
https://en.wikipedia.org/wiki/Erik_Verlinde
"In June 2011, the Netherlands Organisation for Scientific Research (NWO) awarded Verlinde the Spinoza Prize, the highest award available to Dutch scientists including a 2.5 million euro grant for future research.[14] The committee cited his work on the Verlinde formula, the Witten–Dijkgraaf–Verlinde–Verlinde equations, the Cardy-Verlinde formula and entropic gravity as the major achievements leading to the award."
By the way, my impression is that reading Motl is not a pleasant experience for anybody but himself.
It isn't too difficult to formulate a mathematical system where two subsets are completely identical when considered in isolation, but when interacting, one subset operates as if the other doesn't exist, while the other is easily perturbed by any values in the mirror subset. I'm not sure if you can do it without at least one dimensional basis unit that squares to zero, though.
Imagine, if you will, that there are dark-matter people. We have no evidence whatsoever that they exist, any they are constantly annoyed by all the real-matter people that wander between their dark-couches and dark-televisions, and the real-matter bicyclists that pedal through their dark-bedrooms at the worst possible times.
Or did I just invent a crappy layman-author's explanation of where inertia comes from?
Sorry for ruining your dream. :/
I can still dream, though.
Never the less, Verlinde's ideas are more interesting and reasonable than other proposed modification of gravity, but that's just my opinion.
Loosely speaking, with an incorrect family of models, there's a lot of wiggle room to find one that still looks quite a lot like your data, while that freedom is not there when making a single specific prediction. There are semi-rigorous ways to take that into account while comparing models https://methodology.psu.edu/node/504
I'm not sure if that's the objection here. They might also be calling it too vague to be a theory, or something else.
I disagree. The blunt people (who come off as "dicks" to the outsiders) are like a community's immune system. They protect the community against the sort of people who take accurate engaged criticism, make one or two changes to fix specific issues, and then resubmit their work, wasting the community's time and resources, driving off the people who are making good-faith efforts to help the community. They're not being dicks "just for fun". They're protecting the time and attention of other community members by saying bluntly what many other people are often thinking.
Communities that have too many of these people become closed and ossified. Communities that have none of these people evaporate, as the core contributors just walk away and find better things to do than deal with people who are more interested in getting fame and attention for themselves than making good-faith efforts to advance the project of the community. The trick is to find a good balance.
Is it harder than to understand that they publish an article with the false title and false interpretations?
Anyway, the way the math works out, it is impossible for a known-universe person to even measure the effects the known-universe has on the dark-universe. The numbers can't come back to the known-universe without an additional input from it, and they would be explainable with another hypothesis that did not include a dark-universe. It way be that all effects we observe to be random are actually perfectly predictable to a dark-universe observer.
In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electron, these numbers are known to high precision. For DM, they could lie anywhere in a range over many orders of magnitude (although they are generally bounded from above, because we have not seen evidence of strong interactions).
At supergalactic scales, the predictions of the DM model are not very sensitive to it's mass or couplings. On galactic scales, various nonlinear processes in normal matter (such a supernovea) and possible DM interaction effects become important. Since we don't know those things, the theory has free parameters.
Folks definitely consider other DM models, e.g. bosons, but these are only considered to be favored over MOND insofar as they agree with the same observational data. In other words, the set of all theories of DM which produce the same observational predictions forms an "equivalence class"; and it's the equivalence class, not a particular choice of DM mass and couplings, that is favored over MOND. This is basically what people mean when they talk about "cold dark matter", it's the CDM in the LambdaCDM model, the standard model of cosmology.
What makes fermionic dark matter be preferred over (massive) bosonic dark matter? I'm not sure why it should make any difference.
For others who, like me, aren't in the field, the "EFT" mentioned is explained, for example, here:
https://arxiv.org/abs/gr-qc/0311082
"The problems of quantizing gravity within the experimentally accessible situations are similar to those which arise in a host of other non-gravitational applications throughout physics. As such, the size of quantum corrections can be safely estimated and are extremely small. The theoretical framework which allows this quantification is the formalism of effective field theories."
dS is "de Sitter space": https://en.wikipedia.org/wiki/De_Sitter_space
I'd also appreciate a link to the explanation of "What in GR we would consider perturbations of dS and IR corrections."
Here's a quick overview of perturbation theory then an example of how one might apply it in GR.
https://www.wikiwand.com/en/Perturbation_theory
http://theory.physics.helsinki.fi/~inf/Lectures/Lecture2.pdf
IR is "infrared", low-energy physics, as opposed to UV, "ultraviolet", high-energy physics. In this context, UV means strong gravity where quantum effects are expected to be important. Galaxy-galaxy gravitational lensing (which is the subject of the Brouwer et al. paper that's the subject of the newscientist article linked at the top) is purely a weak gravity problem[1], so is in the IR limit of General Relativity. If you want different results from standard General Relativity you can add corrections by hand in a variety of ways; in the galaxy-galaxy lensing case they would be in the infrared.
[1] strong gravity is hidden behind the event horizons of the black holes of the galaxies and, arguendo, if we could see the strong gravity near the black hole centres, it would not be relevant by virtue of being drowned out by all the stars, gas, dust and other matter.
ETA: This is nice and terse, too: https://www.wikiwand.com/en/Non-exact_solutions_in_general_r...
The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but I don't see what it has to do with MOND. Indeed, in Brouwer et al., I can't even see a reference to MOND.
Only in the newscientist article is there a scattering of the abbreviation "MOND", and the article goes so far as to (entirely unjustifiably) say, "[Verlinde's Emergent Gravity] also builds on controversial models of so-called modified gravity, such as the Modified Newtonian Dynamics (MOND) theory of Mordehai Milgrom".
Instead Verlinde's work builds on Ted Jacobson's work from twenty years ago (arXiv:1505.04753) (cf. Verlinde's first paragraph in section 5) and it is no surprise that discussions directly with Jacobson are disclosed in the acknowledgments section.
Sure Milgrom is also further down in the acknowledgments section and MOND is in the references in force, but consider Verlinde's, "We like to emphasize that we have not derived the theory of modified Newtonian dynamics as proposed by Milgrom. In our description there is no modification of the law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field equation." (top of Verlinde's page 39). Further down, "although we derived the same relation as modified Newtonian dynamics, the physics is very different". (emphasis mine)
Indeed, if you look at my link in the comment you replied to, you'll see in the first paragraph that his unremovable background is standard GR dS.
Other than in the newscientist article, I think you can't really find anywhere that conflates MOND-like approaches with Verlinde's emergent gravity. They are very different theories. The key point though is that Verlinde's is more fundamental and MOND-like and FLRW-like results can emerge from it at appropriate length scales (and \Lambda is simply built in).
But that's the point of emergent theories; you can derive the emergent theory from the more fundamental one, but you can't derive the fundamental theory from the emergent one. And in Verlinde's work, the fundamental theory, for better or worse (fwiw, I think worse) is strings in dS, and what emerges is General Relativity as an effective field theory with some IR corrections.
Finally, what I think is most interesting about Verlinde's paper is that string people usually avoid dS. "That's interesting and not outrageously wrong" is not saying that it is anything close to a complete theory (it does not even seem wholly self-consistent), or that the paper is especially well structured. At best you could say that since I am open-minded about the possibility that our current fundamental theories emerge from something more fundamental, but that's hardly an a priori reason to favour Verlinde's EG, or anyone else's. I don't even know where you got that from; maybe instead of "You may think" you meant "Someone might think".
Yea, I'm well aware of all this. Brouwer et al. are jumping on the entropic gravity media bandwagon. However, as stated in the New Scientist article, and as I quoted in my original top-level comment you replied to, "Milgrom...points out that according to his own 2013 analysis of gravitational lensing data in galaxies, MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study."
So you really seem to be missing my point. I am well aware that entropic gravity tells a different and more complicated story than MOND proponents. But within the actual physical system being observed, they make indistinguishable predictions. Therefore, this experiment by Brouwer et al. does not actually support entropic gravity over MOND.
> Other than in the newscientist article, I think you can't really find anywhere that conflates MOND-like approaches with Verlinde's emergent gravity. They are very different theories
It's not a matter of conflation, it's that in many regimes (including the OP's) they have the very similar phenomenology. See, e.g., "Entropic-gravity derivation of MOND"
Yes, I was. I got it now, thanks for the clarification; I think we're in closer agreement than I thought.
Essentially where I think we now differ is in the motivation of Brouwer et al. -- they don't seem interested in contrasting with other approaches as much as showing that Verlinde's EG (or at least the claim at the start of their section 4) survives contact with observational data. Additionally, I think we differ on whether that's enough to make the paper worth writing in the first place.
(Where we probably most strongly agree is that there is a lot of breathless hype from Leiden, Amsterdam and Groningen lately.)
It doesn't (modulo theoretical prejudices against bosons). I was just picking something concrete and popular so I could talk about how it's behavior becomes uncertain on small scales.
What makes you think I stopped reading? How do you think I knew that he was attacking a straw man (and hence making a fool of himself)?
> read the comments
Well, that's nice. I feel vindicated. But that doesn't change the fact that he published a totally unwarranted ad hominem attack directed at me based on incomplete information and never retracted it, let alone apologized for it. (Sorry, but a comment doesn't count. Most people don't read the comments.) So I stand by my position that his behavior was unprofessional to say the least.
Well his complaints about the talk are after the "loon" bit, and you didn't say anything to the effect of, "well he called me a loon but did explain his position", so I figured.
What he failed to notice (or at least failed to mention) was that the wrong comment was preceded by me saying, "I am about to tell you something that is wrong because it makes an interesting and illuminating puzzle to figure out why it is wrong." And it was followed by me saying, "Obviously what I just told you can't be right (because if it were I would at the very least have won the Nobel prize in physics), so there must be a flaw in the reasoning. But where is it? Well, it's here..."