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Modified Gravity Theory Passes a Critical Test
- ISL 6y agoThe Quanta article doesn't appear to address the Bullet Cluster, nor the dark-matter-rich dwarf galaxies. Looking forward to having a gander at the paper itself. Edit for context: the challenging thing for MOND is that ΛCDM explains a number of precision observables and phenomena at once. MOND and TeVeS theories can fit individual facts in isolation, but not the ensemble simultaneously. ΛCDM fits those facts with only a single extra concept. It sounds like this iteration claims to address the seminal observation one and one of the most-tightly constrained observables simultaneously. There are several others. Will have to read the paper before saying more.
- BluSyn 6y agoLinked in the article: https://arxiv.org/abs/2007.00082 https://arxiv.org/abs/2007.00082
- sradman 6y agoFor context, Wikipedia article on MOND [1]: > Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's laws to account for observed properties of galaxies. It is an alternative to the hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
- kayaeb 6y agoI'm not positive that's true. I think the most challenging thing for MOND is that basically no one will touch it unless they already have tenure because it's a death sentence. I saw Pavel Kroupa (big name in Milgromian gravity) present in Heidelberg (big concentration of Astro), at the time Volker Springel (author of widely used LCDM simulation code "GADGET") was there and Illustris simulation sets (LCDM major project) had just been rolled out. And Pavel basically got heckled (in a very erudite and respectable way, but constant interruptions from the LCDM majority audience). But Pavel had one slide, I can't find it now, but it was like 72 different problems that LCDM had not solved (ok, 72 is an exaggeration; you can troll his website for mentions of a lot of them https://astro.uni-bonn.de/~pavel/kroupa_SciLogs.html https://astro.uni-bonn.de/~pavel/kroupa_SciLogs.html). And like, Pavel's a big boy, he has some of the most cited papers in all of astronomy, he's got tenure, and he's set, so he can take it and not care. But grad students / postdocs I imagine would constantly have their work politely ignored and get shunted into underfunded groups. I'm just trying to say that LCDM has things it can't explain, and MOND has things it can't explain, but the amount of resources in each theory is seriously lopsided so LCDM can frequently "tweak" itself to solve problems that MOND just doesn't have time or resources to to the same (for example disk formation in LCDM models used to be impossible until they had the supercomputing resources for the resolution required, and they found that the feedback coefficient could a) not promote disk growth, b) promote disk growth, and c) destroy disk growth, depending on how much they cranked it up. That's NOT a triumph of LCDM making an amazing replication of the observation, that's some grad student in a lab with enough CPU to tweak a meta-parameter until it looks good. Also the CMB is extremely tightly constrained... and multiple huge tightly-constraining studies, WMAP, PLANCK, Gaia, are more than 3-sigma outside of each other's results, so... perhaps too tightly constrained. Edit: Found it, here is the great astronomical bloodbath of 2014: LCDM (Springel and Rix) vs MOND (Kroupa). Great watch. https://www.youtube.com/watch?v=UPVGDXNSBZM https://www.youtube.com/watch?v=UPVGDXNSBZM
- edna314 6y agoI'm not entirely sure which one of the two, Dark Matter or MOND, is conceptually the bigger disaster. Dark matter pretty much sounds like the invisible ether that was suspected to carry EM waves. A physical theory that depends on the scale at which you look at space seems similarly awkward to me.
- teraflop 6y agoThe main difference is that the aether was only ever a theoretical construct, whereas we have lots of indirect observational evidence that "dark matter" is a phenomenon that really exists. We don't know what it consists of, but it certainly seems like there is more of it in some places than others, and it has mass and momentum (see e.g. the Bullet Cluster that was already mentioned in this thread), so "matter" seems as good a name for it as any.
- edna314 6y agoOk, I would be curious about evidence that dark matter has momentum, because my favorite theory at the moment is that spacetime itself has certain topology on large scales, which isn’t tied to any masses. If one could show that dark matter has momentum, I would reconsider, but I don’t think that is what Bullet Cluster shows. Edit: even if it turns out that dark matter is the best theory, it would still be a conceptual disaster. But so is QM. Nature seems not to care what we find conceptually appealing.
- DiogenesKynikos 6y agoQuantum mechanics is incredibly elegant and appealing as a theory. It's just counter-intuitive.
- edna314 6y agoIs this why there are a bazillion of theories of the foundations of QM? I get that the mathematics is incredibly elegant, but concepts behind the mathematics are not really “understandable” to quote Feynman.
- buddyp450 6y agoHow is this level of math even possible... like... how are people hard-working enough to reach this level of understanding?
- EE84M3i 6y agoI don't know about these people specifically, but if you are interested/passionate about it, it probably doesn't feel like work.
- gizmo686 6y agoWe, as a society, produce enough of a surpluss that we are able to pay armies of researchers to do nothing but expand the frontier of human knowledge and pass that knowledge on to the next generation.
- chewbaxxa 6y agoYou could study computer science and then go to a large corporation to help them maximise their ad revenue, or you could study and do something like this instead. Depends what motivates you.
- flafla2 6y agoI upvoted you because there is truth to this sentiment. But I want to add that physics is IMO one of the hardest fields intellectually of our time. Just to get your feet wet, you need to be well versed in an intimidating array of mathematical disciplines. DiffEQ, Discrete Math, Computer Science, Topology, Differential Geometry, High Level calculus,... the list goes on even at the undergrad level. To make a meaningful contribution is quite a feat. I have massive respect for those who bravely tackle the frontiers of Physics; some physics majors have a broader understanding of Math than a mathematician! Though importantly, your average mathematician will have a much deeper understanding of more focused topics. I don’t mean to suggest that any of Math, Physics, CompSci, etc is fundamentally harder than the others. Each field presents different challenges! But they are also intimately related IMO: the recent MIP*=RE proof has convinced me. In short, we’re all on the same side.
- micheles 6y agoActually this is classical general relativity, the math is not particularly advanced. When I was doing Theoretical Physics I would have considered this paper "easy", at least relative to most papers on hep-th.I never believed in MOND and even this paper is not convincing, since the lagrangian is built ad-hoc rather than derived from a clean principle.
- Beldin 6y agoThe reason work on alternative theories of gravity is important: We have one reality R, for which we have various possibly imperfect theories T(R) and a large set of possibly imperfect observations O(R). For gravity, we know that general relativity doesn't perfectly match current observations -- GR(R) != O(R). Since R is the same in both sides, this is not supposed to happen. There are three ways to resolve this: - GR() is incomplete, - O() is incomplete, - both are incomplete. Dark matter focuses on the second, MOND on the first. We shouldn't give up on any option until there's either a solution or a counterexample.
- Beldin 6y agoWith respect to the Bullet Cluster (which is often mentioned as a counterexample to MOND): discussion of why that does not rule out MOND: https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html https://web.archive.org/web/20160721044735/http://www.astro....
- pdonis 6y agoHis argument is basically "well, we still need some dark matter with MOND, just not as much". That seems like a weak argument.
- sandgiant 6y agoVery interesting work! Great to see people are still putting effort into alternative theories of gravity.
- bawana 6y agoIs the distribution of dark matter correlated to regular matter in a way that suggests one is more causal to the other than to itself?
- tigerlily 6y agoPosting this here in case anyone finds it rehabilitating: http://www2.phys.canterbury.ac.nz/~dlw24/universe/ http://www2.phys.canterbury.ac.nz/~dlw24/universe/
- ncmncm 6y agoWhat has been the response to this?
- paulpauper 6y agoThe problem with MOND theories is they require the existence of a fifth force to account for the deviations of Newtonian gravity .
- bufferoverflow 6y agoIsn't a cosmic-scale force much more likely than invisible untouchable matter that's almost (but not) everywhere?
- deleted 6y ago[deleted]
- meroes 6y agoYou mean like neutrinos?
- NikolaeVarius 6y agoLove
- dnautics 6y ago... except that neutrinos are detectable. Kind of an important distinction.
- cthalupa 6y agoWe decided neutrinos almost certainly existed long before we could detect them.
- dnautics 6y agoThis not the same; at the time when they were postulated due to mass defect in nuclear reactions, there was a reasonable mechanism of detecting them, if nothing else the principle of microscopic reversibility. With current dark matter there literally is no known way of detecting them that doesn't beg the question, and all of our attempts to identify them are literally "shots in the dark".
- CydeWeys 6y agoIt's worth pointing out that modified gravity has less acceptance than even string theory. It's worth working on, as a somewhat plausible alternative to the standard theory, but it's not very likely to end up being correct. It's worth ruling out fully, though (the same can't be said of e.g. flat Earthism). This particular work seems like adding on epicycles, or overfitting as you'd call it in machine learning.
- cultus 6y agoTo be fair, from the outside at least, a lot of string theory looks like adding epicycles as well.
- CydeWeys 6y agoYes, that is a common view amongst most physicists.
- auntienomen 6y agoI disagree, it's not a common view. It's common for physicists to think string theory might be empty theorizing, but it doesn't look at all like epicycles. With epicycles, you start with an existing model and than add complexity. That's not strings. Strings starts with something entirely new, that's already complex.
- zenethian 6y agoHelp me understand why this is so controversial in the scientific community. I'm neither a physicist nor do I study any other relevant fields, so I'm asking as a "layman" as it were. A cursory search shows that "dark matter" hasn't been observed at any point in time. It looks like it's a sort-of mathematical placeholder for the "something" that's missing that we can't see (or can't see _yet_.) Am I correct in my understanding that these alternative theories are trying to prove that there's a way to explain these discrepancies without inventing something unobserved? Why is that so controversial? To me, again as an outside observer, it feels so counter-intuitive to _invent_ a new type of matter you can't observe than to just say that your calculation is close but not right and to start over. Is it not a crutch? If I were to take this approach into another field that has an equal amount of controversy where there is a huge gap in my proof, and say that the cause is an "unseen force" that I just invented to explain how my theory works despite the fact that I can't prove in any way that the key part of my theory exists, would I not be laughed out of a room? How did dark matter become accepted while being invented instead of observed? (I hope my tone isn't coming off as mocking. I'm genuinely trying to understand without a lot of knowledge on the subject, and I don't intend to discredit the current accepted theory nor support the alternative ones, only to understand why the scientific community came to accept the one that is accepted.)
- sandworm101 6y agoIt is not that dark matter has not been observed. It has. It is called "matter" because its gravitational influence has been observed. Therefore at one level we know it exists because something is impacting gravity at large scales, we are just unsure about its other properties. We cannot see it directly but there are lots of things we don't see directly and that doesn't mean we aren't sure they exist. So it isn't a total invention to make the math work. It is a 'best fit' for a gap in our knowledge. Dark energy is perhaps a different story.
- narag 6y agoIIRC, dark matter is supposed to be a different kind of matter that interacts "less than normal" with ordinary matter. Or is there a version in which it's ordinary matter, just "hidden" by something or in some exotic "other dimension", black hole or parallel universe thingy?