First test of rival to Einstein’s gravity kills off dark matter • Hacker News Board

Re: First test of rival to Einstein’s gravity kills off dark matter

#71

Post by jessriedel » Sat, Dec 17, 2016, 2:20 AM UTC

raattgift wrote: post #65

jessriedel wrote: post #63

Earlier quoted context omitted.

The topic of the article is the experimental data, which does not distinguish between Verlinde's model from many other alternatives to DM. You may think the story Verlinde tells gives new a-priori reason to favor this sort of theory, but it doesn't have much to do with the experimental data being discussed.

Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but…

> Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. 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.

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"

https://arxiv.org/abs/1201.4160

Re: First test of rival to Einstein’s gravity kills off dark matter

#72

Post by raattgift » Sat, Dec 17, 2016, 2:36 AM UTC

jessriedel wrote: post #71

raattgift wrote: post #65

Earlier quoted context omitted.

Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but…

> Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), b…

> So you really seem to be missing my point

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.)

Re: First test of rival to Einstein’s gravity kills off dark matter

#73

Post by misnome » Sat, Dec 17, 2016, 2:37 AM UTC

thejollysin wrote: post #53

brilliantcode wrote: post #47

Can someone explain in layman's terms what really is dark matter and why there's so much obsession around it? If we can't observe it and we are only relying on inferences to explain it's effects, that seems like pretty weak set of legs. 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 u…

"Dark Matter" is a phrase we use to describe a hole in our understanding of the universe. To put it simply, when we look out into space, at very large scales like galaxies, everything is just too heavy. It appears as if there is a LOT more matter out there than we can see. Things are orbiting faster than they should be, like galaxies are heavier than makes good sense.. Maybe there is some matter out there that is "da…

Where "Dark" literally means: does not interact in any way with light

Re: First test of rival to Einstein’s gravity kills off dark matter

#74

Post by smoyer » Sat, Dec 17, 2016, 2:44 AM UTC

lisper wrote: post #43

beambot wrote: post #17

> String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.” 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; ch…

Motl is notorious for this sort of thing. He once called me a "category 5 loon" because something I said in a talk was wrong. But the reason it was wrong is because I was advancing it as a straw-man precisely to show it was wrong. http://motls.blogspot.com/2012/10/evading-quantum-mechanics-...

That sounds like a great HN handle.

Re: First test of rival to Einstein’s gravity kills off dark matter

#75

Post by amluto » Sat, Dec 17, 2016, 4:48 AM UTC

jessriedel wrote: post #62

DennisP wrote: post #58

Earlier quoted context omitted.

So, my impression from popular articles is that physicists don't actually know what dark matter is, and have a fair number of candidate ideas. According to the theory that makes good predictions, what is dark matter made of?

Dark matter (DM) is typically modeled like electrons are modeled: as a species of massive fermion, described by a quantum field. The parameters that go into the model (both DM and the electron) are the mass of the particle, plus its various couplings to the other known particle species. In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electr…

In my intro cosmology class, we modeled dark matter as a perfect fluid with some classical density field that interacted with everything else until some very short time after the Big Bang and then stopped interacting except by gravity. We set the equation of state to match that of normal matter: if you put 1 gram of dark matter in a box and slowly expand the box to double its volume, you still have 1 gram.

What makes fermionic dark matter be preferred over (massive) bosonic dark matter? I'm not sure why it should make any difference.

Re: First test of rival to Einstein’s gravity kills off dark matter

#76

Post by jessriedel » Sat, Dec 17, 2016, 5:12 AM UTC

amluto wrote: post #75

jessriedel wrote: post #62

Earlier quoted context omitted.

Dark matter (DM) is typically modeled like electrons are modeled: as a species of massive fermion, described by a quantum field. The parameters that go into the model (both DM and the electron) are the mass of the particle, plus its various couplings to the other known particle species. In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electr…

In my intro cosmology class, we modeled dark matter as a perfect fluid with some classical density field that interacted with everything else until some very short time after the Big Bang and then stopped interacting except by gravity. We set the equation of state to match that of normal matter: if you put 1 gram of dark matter in a box and slowly expand the box to double its volume, you still have 1 gram. What makes…

> I'm not sure why it should make any difference.

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.

Re: First test of rival to Einstein’s gravity kills off dark matter

#77

Post by cgvgffyv » Sat, Dec 17, 2016, 9:53 AM UTC

lisper wrote: post #43

beambot wrote: post #17

> String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.” 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; ch…

Motl is notorious for this sort of thing. He once called me a "category 5 loon" because something I said in a talk was wrong. But the reason it was wrong is because I was advancing it as a straw-man precisely to show it was wrong. http://motls.blogspot.com/2012/10/evading-quantum-mechanics-...

I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk.

Re: First test of rival to Einstein’s gravity kills off dark matter

#78

Post by boyce » Sat, Dec 17, 2016, 10:08 AM UTC

bmh100 wrote: post #45

Please change the submission title. It is downright misleading. This test did not "kill off" dark matter. To do so would imply overwhelming evidence against the concept of dark matter as a whole. Instead, this theory, which eliminates the need for dark matter in certain situations, has passed its first test. The sentence "... test ... kills off dark matter" is simply false.

Typical New Scientist really. I went to a lecture and Q&A with Alain Aspect when he visited my department in 2007 and he had a bit of a rant about how they had fundamentally misunderstood his work. They published an article claiming his Bell experiment could be used to send messages back in time.

Re: First test of rival to Einstein’s gravity kills off dark matter

#79

Post by lisper » Sat, Dec 17, 2016, 10:18 AM UTC

cgvgffyv wrote: post #77

lisper wrote: post #43

Earlier quoted context omitted.

Motl is notorious for this sort of thing. He once called me a "category 5 loon" because something I said in a talk was wrong. But the reason it was wrong is because I was advancing it as a straw-man precisely to show it was wrong. http://motls.blogspot.com/2012/10/evading-quantum-mechanics-...

I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk. 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…

> I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk.

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.

Re: First test of rival to Einstein’s gravity kills off dark matter

#80

Post by acqq » Sat, Dec 17, 2016, 11:12 AM UTC

raattgift wrote: post #69

acqq wrote: post #66

Earlier quoted context omitted.

Moreover, it's not even a complete "theory" in the sense that it's still not known if it can even repeat the results of the General Relativity. https://news.ycombinator.com/item?id=13197595

It's not complete for other reasons, several of which are detailed in Verlinde's paper itself; however, it wholly reproduces GR in the EFT limit by design. Indeed, Verlinde starts with standard dS as his unremovable background on which he puts strings. What in GR we would consider perturbations of dS and IR corrections of GR (at ~ galactic scales) emerge from the behaviour of the strings and how they form long-distan…

Thanks for the correction and clarification.

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."