|
| jchanimal wrote:
| What's MOND really mean? Here's the Wikipedia entry
| https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
| uoaei wrote:
| I follow the lead author, Stacy McGaugh, via his blog where he
| posts discussions and musings about the latest research into the
| dark matter vs MOND debate: https://tritonstation.com/new-blog-
| page/
|
| His arguments are very convincing and relatively clear. I am not
| an astrophysicist but I have two degrees in physics and have
| always found the dark matter theory to be lacking -- in absence
| of any evidence of causation whatsoever, dark matter can only be
| described trivially as "where we would put matter if we could to
| make our theory of gravity make sense," which is totally
| backwards from a basic scientific perspective.
|
| Predictions based on modern MOND postulates are shown to be more
| and more accurate as our observational instruments continue to
| improve in sensitivity.
| simonh wrote:
| I don't think that's quite fair. That approach is exactly how
| we find planets. Here's an unexpected variance in the motion of
| a planet or star. It could be explained by a planet over there.
| Oh look, there's a planet over there.
| TheOtherHobbes wrote:
| Planets are visible when you look for them.
|
| Dark matter - so far - isn't.
| drdeca wrote:
| What do you mean by "visible when you look for them"? Like,
| with light?
|
| Does gravitational lensing count as "visible" to you?
| MarkusQ wrote:
| Right, which is why it quickly led to the detection of dark
| matter...hmm.
|
| I think a better analogy would be "that approach is exactly
| how we explain failing to find planets like Vulcan; we
| hypothesize that they are made of as-yet-unknown stuff that
| you can't see, touch, hear, smell, or in fact detect at all.
| But we know they're there because our calculations say they
| are."
| solid_fuel wrote:
| Hypothesizing that a planet might be over there is a testable
| hypothesis.
|
| Have we found a way to verify the presence of dark matter
| yet? Or is it still an untestable hypothesis sprinkled around
| distant galaxies so their acceleration curves look right?
| User23 wrote:
| I'm particularly amused by the hypothesis that spacetime
| can be bent without the presence of matter. We can't detect
| dark matter because there's no such thing, it's just a
| brute topological fact.
| mr_mitm wrote:
| Dark matter predicted lensing effect which were
| successfully tested. Same for the baryonic acoustic
| oscillations in the CMB.
| MattPalmer1086 wrote:
| That's not quite true. General relativity predicts
| gravitational lensing, not dark matter. Lensing has been
| used as an experimental probe for the presence of dark
| matter.
| elashri wrote:
| MOND is an alternative theory of gravity competing with
| GR. People usually forget that while MOND started to
| present a different explanation for Dark Matter, it is a
| theory of gravity. Dark Matter is not a theory of gravity
| and is compatible with GR.
| zeroonetwothree wrote:
| Dark matter isn't much of a theory in the first place.
| russdill wrote:
| It's actually a better example than you think. This exact
| theory led to long and protracted searches for the planet
| Vulcan, which would explain Mercury's strange behavior.
| griffzhowl wrote:
| > which is totally backwards from a basic scientific
| perspective
|
| This is not right, because if we have a situation where our
| theories and observations don't cohere, it's not given whether
| the theory requires modification or we're missing something in
| our observations (or both). A classical illustration is the
| orbit of Uranus being observed in the nineteenth century to be
| contrary to the predictions of Newtonian theory. Calculations
| were made assuming the truth of the Newtonian theory and that
| we were missing something in our observations - the position of
| Neptune was predicted and it was subsequently discovered.
|
| On the other hand, the orbit of Mercury diverged from the
| prediction of Newton's theory. Again, a previously unobserved
| planet closer to the sun was postulated as being responsible,
| but in this case it really did require a modification to the
| theory of gravity: general relativity, which accurately
| predicted the 43 arcseconds per century of perihelion
| precession by which Mercury's orbit diverges from Newtonian
| predicitions.
|
| GR has obviously made many other predictions, such as the
| gravitational bending of light, black holes, and gravitational
| waves, which have been vindicated.
|
| So there's obviously a problem of the theory and observations
| not cohering, but whether the solution is a modification of the
| theory or a new form of matter is not clear in advance, and the
| latter is not unreasonable and certainly it's not unscientific
| to make as a hypothesis, to see where it leads.
|
| The difficulty is in coming up with a theoretical framework
| that retains all the successful predictions of GR while also
| accounting for the galactic rotation curves.
| bbor wrote:
| Well put, thanks for sharing! Never saw it phrased in such a
| clear narrative. As a novice, it seems like there's one big
| difference between those anecdotes and the current situation,
| though: sample size. Sure, if we were observing Andromeda
| spinning too slowly I'd be open to our instruments not
| capturing some massive objects/clouds, but we're actively
| observing, what, ~1E5-6 galaxies? In the case of a missing
| planet there were accidents of history/solar system makeup
| that led to our otherwise solid frameworks missing a key
| piece of information. But that clearly couldn't happen
| millions of times; whatever explains the inconsistencies
| we're seeing _has_ to be a fundamental misunderstanding.
|
| Once we've arrived at this point, we can compare the two
| theoretical re-workings on their own terms: one is that we're
| glossing over some important detail of how gravitational
| relations in spacetime work, and the other is that we're
| failing to observe some new class of matter. I mean, right?
| There's no way this conundrum will be solved by "whoops turns
| out there was more plain ol' dust than we thought" at this
| point, right?
|
| In those terms, I feel parsimony clearly favors one
| possibility over the other. Every hypothesis is worth
| exploring (I mean, QM and GR are dumb as hell, yet
| nonetheless turned out to be correct), but when funding is on
| the line it's also not out of line to favor one explanation
| explicitly. That's already happening anyway, just in the
| other direction.
|
| But also I'm just some kid who's awed and grateful to be
| living in times of such profound mystery and discovery. Could
| be totally off base -- I barely passed physics I!
| necovek wrote:
| > ...turned out to be correct
|
| What we have learned so far is that our theories and models
| are only correct up to our ability to precisely observe and
| measure.
|
| In that sense, Newtonian physics is still very much correct
| under a very wide set of circumstances, and as such
| amazingly useful.
|
| GR improves on that (adds precision) on what would be
| extreme cases for NP, but it is likely as correct as
| Newtonian laws are: up to a point.
|
| All this to say that "correct" is not the right term to
| use: many of the theories are simultaneously "correct" with
| sufficient constraints and a particular error range. What
| matters more is if they are useful in predicting behaviour,
| and that's where I like using "correct" instead (as above).
| griffzhowl wrote:
| Thanks. I'm also no expert - I'm just learning general
| relativity - but that's also my rough understanding: either
| there needs to be a modification of the theory, or there's
| a new form of matter. It might seem more parsimonious to
| modify the theory, but then how do you do that in a way
| that retains all the successful predictions of GR while
| explaining the recalcitrant observations? That's the hard
| part.
|
| It seems at the moment that the minimal and most elegant
| adjustment to the worldview required is to postulate the
| new form of matter. But I think it's safe to say it's a
| genuine problem in our knowledge: we don't know how to
| solve it
| njtransit wrote:
| One difference between dark matter and Neptune is that the
| existence of Neptune is falsifiable. The formulation of dark
| matter inherently is not. Falsifiable hypotheses is the
| cornerstone of science.
| renewiltord wrote:
| Surely the idea of it being a new kind of matter that
| interacts gravitationally but not electromagnetically
| yields some testable result? Does it actually yield nothing
| testable with today's experimental methods?
| MattPalmer1086 wrote:
| There is a lot of indirect evidence for dark matter. All
| the direct tests for dark matter particles we have
| performed have found nothing so far - but since we have
| no idea what it might be, there's a lot of possibilities
| to test.
| uoaei wrote:
| "Evidence" in heavy scare quotes, considering, again, the
| tautological nature of the claims around the existence of
| dark matter. "Something must be here that we are missing"
| is, frankly, a bullshit hypothesis that need not be
| entertained unless researchers can actually prove there
| is some worthiness to the claim. Anything stronger than
| "maybe our theory is wrong" would suffice!
| mannykannot wrote:
| It is tendentious to point out only the difficulties in
| finding affirmative evidence for dark matter when MOND is
| doing no better in that regard. If, by that standard,
| dark matter is bullshit, then, mutatis mutandis, so is
| every other hypothesis that has been presented so far -
| but the observations that prompted them in the first
| place are not going away. It is inconsistent to call just
| one of them bullshit, and pointless to call them all
| that.
| LegionMammal978 wrote:
| Is the existence of a planet so easily falsifiable? It
| hasn't been so long since the Planet Nine hypothesis
| started going around, and while we've observationally ruled
| out a big chunk of the original parameter space, there's
| still lots of room for a big dark dwarf planet to be
| floating around out there. It doesn't seem so different
| from how we've gradually been ruling out the parameter
| space for dark-matter observations.
| uoaei wrote:
| Planets that reflect light are easy to detect.
| pixl97 wrote:
| I mean, dark matter may be discoverable, we just don't know
| how if it exists. There was time between the irregularities
| that were noticed in the orbit and the discovery of a new
| planet.
| uoaei wrote:
| By that extremely simplistic logic, so is literally any
| other theory of gravity. This is not an argument, this is
| a flailing and empty justification.
| griffzhowl wrote:
| I'm not sure it's inherently unfalsifiable. There are some
| specific proposals for dark matter that could be ruled out
| by experiments, such as right-handed neutrinos: https://en.
| wikipedia.org/wiki/Sterile_neutrino#Sterile_neutr...
|
| Maybe if you're being very broad in definitions then some
| class of proposals describable as "dark matter" might be
| unfalsifiable, but to be taken seriously as a scientific
| proposal I think it should be specific, concrete, and
| indeed testable, and there are a few of these within the
| "dark matter" class.
|
| Again, we're in the perhaps unsatisfying position of having
| observations which don't cohere with our current
| theoretical understanding. What's the solution? It's not
| easy...
| uoaei wrote:
| Have you ever encountered the phrase "grasping at
| straws"? The pursuit of explaining dark matter has gone
| through many waves of "we just need to invent detectors
| for this particle that has never been observed" and is
| littered with the wreckage.
| antognini wrote:
| > where we would put matter if we could to make our theory of
| gravity make sense
|
| Dark matter behaves in a fundamentally different way from
| baryonic matter. We can constrain the total amount of matter in
| the universe (both dark and baryonic) from the observed
| abundances of baryogenesis. But dark matter has a different
| effect on the relative amplitudes of peaks in the CMB.
|
| As far as I can tell, MOND has never really had any success
| outside of modeling galaxy rotation curves.
|
| The skepticism I've seen towards dark matter vs. MOND has
| always been strange to me. Dark matter doesn't really require
| much in the way of new physics --- there's just a new particle
| to add to the standard model. But most MOND theories violate
| Lorentz invariance which is a vastly more radical departure
| from standard physics. (And in my mind, the more sophisticated
| MOND theories that maintain Lorentz invariance like TeVeS are
| really a theory of dark matter dressed up in the language of
| MOND.)
| MattPalmer1086 wrote:
| There are more successful predictions than just rotation
| curves. For example, see:
|
| http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html
| antognini wrote:
| These successful predictions are all generally variants on
| modeling galactic dynamics, though. The trouble is that
| galaxies and galaxy clusters are very messy places, so it's
| hard to make sure you've incorporated all the relevant
| physics.
|
| By contrast something like baryon acoustic oscillations are
| very simple to model, so you can be quite confident that
| you've incorporated all the relevant processes. And in that
| regime LCDM performs beautifully and MOND completely fails.
| So it's reasonable to suspect that in more complicated
| environments the problem is that we're not modeling the
| systems correctly rather than that there's new physics
| going on.
| MattPalmer1086 wrote:
| There are other predictions MOND makes. For example, it
| predicts higher collision velocities than LCDM, for
| example, see:
|
| https://ieeexplore.ieee.org/document/8193356
|
| And, of course, it predicted that the early universe
| would have bigger and more structured galaxies (which is
| what the posted article is about).
|
| Dark matter has a slew of problems of its own; it's not
| the case that LCDM is problem free, despite good success
| in some areas.
| kelseyfrog wrote:
| MOND doesn't cover the existence of CBM, distribution of
| galaxies, non-metallic abundance - things all covered by
| LCDM.
|
| What MOND has going for it is that galactic rotation
| curves are readily consumed by popsci readers and the
| story of the "little guy" vs the scientific establishment
| is an easily available frame story popsci authors can
| sell clicks for.
|
| The proportion of lay people who think MOND could be true
| greatly outnumbers the proportion of MOND researchers and
| doesn't reflect the veracity of the theory.
| MattPalmer1086 wrote:
| MOND is not a cosmological theory unlike LCDM, and it
| isn't relativistic. So we should not expect it to cover
| the range of things that LCDM tries to.
|
| It's just a tweak to Newtonian gravity, which
| surprisingly matches observation very well, and has
| accurately predicted quite a few things in the regime it
| operates in, before they were observed.
|
| The fact it works so well in the areas it does apply to
| is the reason that science hasn't given up on it yet
| (regardless of what pop science or lay people think).
| gus_massa wrote:
| Very interesting. Do you know an article that ELI25 this?
| antognini wrote:
| For a more non-technical overview, Sean Carroll had a
| nice episode on his podcast where he talked about the
| evidence for dark matter among other things: https://www.
| preposterousuniverse.com/podcast/2023/07/31/245-...
|
| For something more technical, this article just came out
| as an overview of the evidence for dark matter:
| https://arxiv.org/abs/2411.05062
| russdill wrote:
| The mond theories that add a factor that behaves like dark
| matter do a rather good job of matching observational data.
| halgir wrote:
| I usually understand "dark matter" to be shorthand for the
| discrepancy between theory and observation. The explanation
| might indeed be matter that is dark, or it might be solved by
| entirely unexpected observations and/or changes to theory.
| mr_mitm wrote:
| Not really. You might think this after watching Angela
| Coulliers video, but when you read something like "25% of the
| universe's energy content is made of dark matter", they do
| not mean changes to some theory. They literally mean non-
| baryonic matter.
| OutOfHere wrote:
| Nope. It can mean change to some theory, without a need for
| matter. It is the difference between relativistic gravity
| and the corresponding observed mass.
| zeroonetwothree wrote:
| Energy content not only comes from matter but also from
| fields.
| samsartor wrote:
| My hangup with MOND is still general relativity. We know for a
| fact that gravity is _not_ Newtonian, that the inverse square law
| does not hold. Any model of gravity based on an inverse law is
| simply wrong.
|
| Another comment linked to https://tritonstation.com/new-blog-
| page/, which is an excellent read. It makes the case that GR has
| never been tested at low accelerations, that is might be wrong.
| But we know for a fact MOND is wrong at high accelerations.
| Unless your theory can cover both, I don't see how it can be
| pitched as an improvement to GR.
|
| Edit: this sounds a bit hostile. to be clear, I think modified
| gravity is absolutely worth researching. but it isn't a silver
| bullet
| MattPalmer1086 wrote:
| MOND isn't pitched as an improvement to GR. It was always a
| Newtonian theory - it's in its name!
|
| There are relativistic versions of MOND, for example, TeVeS
| [1], but they all still have some problems.
|
| [1]
| https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%...
| samsartor wrote:
| TeVeS is definitely interesting, but it still has problems
| like you said. AFAICT gravitational wave observations are
| particularly bad for TeVeS theories. TeVeS isn't dead, but if
| dark matter theories are criticized for being patched up
| post-hoc, that standard should also apply to modified
| gravity.
| gliptic wrote:
| The weirdest thing about TeVeS IMO is that it adds
| additional fields that warp spacetime, so how is it not a
| dark matter theory?
| MathMonkeyMan wrote:
| For the fields to be considered particles, they have to
| be freely propagating in space. TeVeS adds a vector
| field, a scalar field, and some lagrange fields that are
| part of their coupling. The degrees of freedom aren't
| consistent with one or more particles.
| ajross wrote:
| To be fair, there are relativistic generalizations of MOND, in
| the sense of relativistic theories that simplify to MOND
| dynamics in the low energy case. My understanding (this not
| being my field) is that they're sort of kludgey and non-
| calculable and that no one takes them very seriously. All the
| "real work" on MOND is just done using the classical stuff.
|
| And yeah, that seems like pretty terrible cheating. It's one
| thing to hang a big theory on a single conjecture, but you
| still need to be trying to prove the conjecture.
| meindnoch wrote:
| >We know for a fact that gravity is _not_ Newtonian, that the
| inverse square law does not hold
|
| [citation needed]
|
| The consensus is that gravity - outside of extreme mass/energy
| environments - works just as Newton described it _to many many
| decimal places_.
|
| _Emphasized part added because people in the replies thought
| that I literally think that General Relativity is somehow
| wrong. Don 't be dense. All I'm saying is that gravity at
| galactic scales works as Newton described it. General
| Relativity has extremely tiny effect at those scales._
| hobs wrote:
| When you say "outside of" - that's the thing where it doesn't
| hold. It's interesting and not even wrong to say "these rules
| work in these contexts" but as far as I can tell we're
| looking for the scenario invariant rules.
| samsartor wrote:
| https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe.
| .. is the example I learned in school. You don't need to be
| around a black hole for GR to suddenly switch on.
|
| Newtonian gravity is an approximation. A perfectly acceptable
| one in many contexts, but still measurably incorrect.
| meindnoch wrote:
| Nobody said that general relativity is "switched on" around
| black holes.
|
| But ok, let me put it this way: outside of extreme
| energy/mass environments, gravity is described by Newton's
| law of gravitation with very high precision. If you look
| very hard, you may notice differences on the order of
| 10e-MANY. But for all intents and purposes, gravity is
| Newtonian in 99.99999% of the universe.
| exe34 wrote:
| that's like saying the visible mass of the universe is
| 99% hydrogen and helium, so we don't need to learn about
| chemistry.
| meindnoch wrote:
| So you're saying we should model galaxies down to the
| level of individual protons? Lol.
|
| Galactic dynamics is governed by gravity, which is
| Newtonian at those scales.
| exe34 wrote:
| No I did not say that.
| meindnoch wrote:
| Ok, then how does your chemistry comment have anything to
| do with the motion of galaxies? Reminder: you're
| commenting on an article about MOND, which is a theory
| that stems from trying to explain the motion of galaxies.
| exe34 wrote:
| > outside of extreme energy/mass environments, gravity is
| described by Newton's law of gravitation with very high
| precision. If you look very hard, you may notice
| differences on the order of 10e-MANY. But for all intents
| and purposes, gravity is Newtonian in 99.99999% of the
| universe.
|
| I meant it in the sense that "most of the cosmos runs on
| Newtonian gravity, therefore we can ignore GR" is similar
| to "most of the visible matter in the cosmos is
| hydrogen/helium, so we can ignore chemistry".
|
| The interesting part is in the 0.0000001% that isn't like
| the others.
| samsartor wrote:
| Not for all intents and purposes.
|
| If we are asking whether MOND is useful, then the answer
| is probably yes. You might use it for simulations of
| galaxy formation where Newtonian gravity is considered a
| reasonable approximation today. But MOND is not a correct
| model of the universe. There is no place in the universe
| that Newtonian gravity applies, only places where the
| error is an acceptable trade-off for simpler calculation.
| meindnoch wrote:
| By the same logic, there's no place in the universe that
| general relativity applies either, since it breaks down
| at the quantum level. There's no place in the universe
| where any theory other than the one true grand unified
| theory applies, because everything else is just an
| approximation. At which point we're just arguing about
| semantics, and I don't see a reason for continuing it on
| my part.
| radishingr wrote:
| There are vastly different scales where the approximation
| is correct for newton vs general relativity. Perhaps you
| can define the scales that you are calling relevant so we
| understand what you mean.
| meindnoch wrote:
| The scale of galaxies? Which the original article is
| about? I feel like I need to spell out everything, but
| ok:
|
| The article is about modified Newtonian dynamics (MOND),
| which is a theory that modifies Newtonian gravitation to
| fix some observed differences in galaxies' motion,
| without invoking dark matter. The original commenter then
| proclaims "haha, MOND cannot be right, because we know
| that Newtonian gravity is incorrect". Yeah, no shit
| Sherlock; it is "incorrect" because it is just _a
| limiting case_ of general relativity. But that 's
| completely besides the whole point of MOND, which tries
| to "fix" gravity at galactic scales, which is a Newtonian
| regime even with general relativity. MOND is trying to
| tweak the Newtonian formula at those extreme distances,
| and if it works, then maybe it can be worked out to be a
| limiting case of a "modified general relativity", just as
| Newtonian gravity is a limiting case of GR. Got it?
| DiogenesKynikos wrote:
| The inaccuracy of the Newtonian theory of gravity is
| large enough that it was already noticed by astronomers
| in the mid-1800s.
| radishingr wrote:
| So spacetime (interactions between mass, space, and time)
| are required for any sort of precision explanation. If
| "extreme" means planet size masses, I guess, but I
| generally consider our solar system pretty normal.
| However we cannot explain the planetary motion of mercury
| without relativity, so define your extreme.
|
| But sure, newton is good enough to handle most ground
| based scenarios where we only care about forces at low
| precision.
| bobmcnamara wrote:
| My first thought was that we only know Cavendish's
| constant to a little over 4 significant figures, so how
| could this be right? The relativistic effects at Earth's
| surface would change this by only ~10^-8, so I think the
| challenge in refining the Cavendish gravitic constant lie
| elsewhere.
| superjan wrote:
| These extremes exist, and GR predictions are better than
| Newton's in those cases. Closest to home is mercury's
| perihelion drift. We have observed black hole mergers,
| gravitational lensing, and GR is also an essential component
| in understanding the universe's expansion(that we know from
| redshift and the CMB). Likely MOND will address these, but
| Newtonian mechanics will not get you there.
| auntienomen wrote:
| Citation needed? That's ridiculous. The empirical evidence is
| well over century old at this point. Start with the anomalous
| precession of Mercury's perihelion. That already can't be
| accounted for by Newtonian gravity.
| bobmcnamara wrote:
| I don't think they're saying the relativistic effects don't
| exist, just that they're still largely unimportant compared
| to Newtonian effects.
|
| For precession of perihelion of Mercury we mostly noticed
| because any error is cumulative over time and we could
| integrate over an arbitrarily wide timebase. The
| relativistic effects are <10^-8 of the total, around 1/10th
| of the change imparted by Newtonian gravity of planets
| much, much further away. The BepiColombo orbiter should
| allow us to correct for the relativistic effects of other
| planets' pull on Mercury, but it's expected to be a change
| of <10^-12.
|
| So I guess "many, many decimal places" is in the ballpark
| of 6-12.
| ahazred8ta wrote:
| Samsartor seems to think that the inverse square law does
| not hold at short distances (e.g. between the sun and
| mercury). Meindnoch agrees with mainstream physics that the
| inverse square law does indeed hold at short distances.
| You're confusing newtonian physics (busted) with the
| inverse square strength of gravity (still strongly
| supported); those are two different things. GR says gravity
| should be strictly 1/r^2, and this is what we observe in
| the solar system.
| EPWN3D wrote:
| You're simply wrong. There's no other way to put it. The GPS
| system would have been simply impossible to deploy without
| the general theory of relativity. There's no extreme energy
| or mass involved, just precision requirements that are
| influenced by the minuscule differences in time experienced
| by the surface of the earth and orbiting satellites.
|
| Also Newton's laws famously could not account for Mercury's
| orbit. Mercury is just an ordinary planet orbiting an
| ordinary star. Nothing extreme is involved. He knew his laws
| were incomplete. But they were so dead-on in basically every
| other scenario that could be physically observed at the time
| that he figured there was some small tweak missing (or maybe
| another planetary body that hadn't been spotted yet).
| meindnoch wrote:
| Easy there champ. Noone is shitting on general relativity.
|
| All I'm saying is that the effect of general relativity at
| galactic scales is so minuscule, that galactic dynamics is
| - for all intents and purposes - governed by the Newtonian
| _limit_ of gravity.
|
| If you propose that gravity doesn't behave like the
| Newtonian limit at those scales, then you're contradicting
| general relativity as well, since the far-field limit of
| the Schwartzschild metric is literally Newton's inverse
| square law.
|
| In layman terms, modified Newtonian gravity, that the
| article talks about, is an attempt to explain why galaxies
| don't rotate the way they should according to Newton (and
| Einstein, because at those distances the two are the
| same!!!).
| jfengel wrote:
| I had the impression that "shitting on general
| relativity" was exactly what MOND was about. That is, it
| starts from the position that Einstein is wrong, and
| searches for ways to support that.
| meindnoch wrote:
| The Wikipedia article on MOND literally starts with
| galaxy rotation curves: https://en.m.wikipedia.org/wiki/M
| odified_Newtonian_dynamics
|
| There's zero mention of MOND being a rejection of general
| relativity.
|
| OF COURSE, any tweaking of Newton's formula at galactic
| scales will necessarily invalidate general relativity,
| since general relativity predicts Newton's formula at
| those scales! But MOND tries to work backwards: they
| propose a modification of the far-field Newtonian
| formula, and the belief is that it can eventually be
| worked out to be a limiting case of a "modified general
| relativity", for lack of a better name. Just how
| Newtonian gravity was eventually worked out to be a
| limiting case of a theory called general relativity.
| throwawaymaths wrote:
| Can you explain how MOND shits on GR? My understanding is
| it's more like. "GR is mostly right but...". As for MOND
| being exclusively Newtonian, yeah. In terms of solving
| the math, you gotta crawl, walk, run. Let's not kid
| ourselves, GR invokes way harder math than algebra and
| simple integral calculus. TeVeS Is a first attempt at
| "walk", let's say, but even it might not be correct even
| if adjusting gravity may be correct.
|
| If someone emerges with a proof that the two systems are
| irreconcilable then yeah you have an argument that it's
| "shitting on GR"
| at_a_remove wrote:
| Hi! Physics BS, but they let me take some grad courses,
| including a Spacetime and Relativity class. I can help.
|
| The word "mass" is used in physics in three different
| general contexts. First, we have mass in mass-energy, as
| in "how much energy can I get for trading in this mass?"
| Mass-energy is the coin paid as the price of existence.
| If it exists, it has mass-energy. Mostly mass for us.
| Mostly. We can skip that one for now.
|
| The second context of mass is _inertial_. Mass has the
| property of inertia, of resisting a change in its
| direction or speed. It resists stopping if it is motion,
| and if it is stopped, it resists moving. The degree of
| the resistance is also called mass. Put a pin in this
| one.
|
| The third context of mass is _gravitational_. Two masses,
| attracting one another because a force between them, a
| force which is not based on charge or the relatively
| nearby exchange of some more exotic bosons, no, just
| attraction based on how much mass is present. Nothing
| more special.
|
| Now, curiously, values of each one of these seem to
| agree!
|
| Einstein's absolute core concept in general relativity,
| the idea from which all else is built, is that inertial
| mass is identical to gravitational mass, not merely in
| number, but so fundamentally intertwined that there is no
| real difference between them, other than being two faces
| of the same coin. Now, that does not sound like much, but
| it gives birth to experiments such as an elevator which
| is falling toward versus an elevator floating far from
| gravitational sources, and that they are, from the inside
| of the elevator, impossible to differentiate.
|
| Einstein then constructs general relativity from this,
| that the "m" in "F = ma" is identical to the first m in
| "F = -G m1 * m2 / r^2"
|
| In MOND, the two ms are _not_ identical, they only appear
| close most places, and so you cannot construct general
| relativity atop it. You will get _most_ correct
| approximations but you 're missing out in some cases.
| fpoling wrote:
| We already know that one must not use Newtonian gravity
| on the galaxy scale. For example, properly accounting for
| GR effects is enough to explain the observed rotational
| curve for our Galaxy without the need for any dark matter
| hypothesis.
|
| Similarly there are papers that tries to explain the
| effects attributed to dark matter on the scale of tenths
| and hundreds megaparsecs using just proper accounting of
| GR effects. They are rather speculative, but still they
| show that even on very huge distances Newtonian
| approximation may not be valid.
| magicalhippo wrote:
| > For example, properly accounting for GR effects is
| enough to explain the observed rotational curve for our
| Galaxy without the need for any dark matter hypothesis.
|
| Do you have some references handy for this? Or are you
| talking about the work of Deur?
| ahazred8ta wrote:
| We know that spacetime is einsteinian, not euclidean, yes.
| But that's not what's being discussed here. The issue is
| whether the force of gravity deviates from the expected
| 1/r^2 value. Experiments, measurements and observations
| within the solar system have not revealed any deviation.
| The precession of mercury is not due to a deviation from
| 1/r^2; it is due to space near the sun being bent instead
| of flat. Ditto GPS; we have to adjust for time dilation and
| curved space, but not for any deviation from 1/r^2. MOND
| theories predict that MOND gravity is indistinguishable
| from normal at short ranges less than several light years;
| the MOND effects only show up at distances of many light
| years.
| tzs wrote:
| Compared to the gravitational fields galaxies orbiting
| other galaxies deal with Mercury orbiting the Sun is
| extreme. So are GPS satellites orbiting Earth.
|
| Mass of Sun: Ms = 1.99e30 kg
|
| Distance to Mercury from Sun: Rm = 5.83e10 m
|
| Mass of Milky Way galaxy: Mg = 6e42 kg
|
| Q: At what distance R from the Milky Way would something
| have to be to experience the same gravitational field
| strength from the Milky Way that Mercury feels from the
| Sun?
|
| A: We want R such that Ms/Rm^2 = Mg/R^2 or R = Rm
| sqrt(Mg/Ms) = 1.0e17 m.
|
| Let's convert that to lightyears. There are 9.46e15 m/ly.
| The final result is 10.75 ly. Note that everyplace that
| close to the center of mass of the Milky Way is inside the
| galaxy. Anything actually outside the galaxy would be at
| least 5000 ly away and feel a gravity field at most
| 1/200000th as strong as what Mercury feels.
|
| For Earth use the same calculation from above but replace
| Mg with the mass of the Earth, 5.97e24 kg. That gives that
| the distance from Earth where something would feel the same
| field strength from Earth that Mercury feels from the Sun
| is 1.0e9 m. That's a little over 4x the radius of the
| orbits of GPS satellites, so GPS satellites are feeling a
| little under 16x the field strength from Earth that Mercury
| feels from the Sun.
| wbl wrote:
| We can see gravitational redshift on Harvard's campus thanks
| to gamma ray Mossbauer spectroscopy.
| nimish wrote:
| >The consensus is that gravity - outside of extreme
| mass/energy environments - works just as Newton described it
| to many many decimal places.
|
| It absolutely does not. Newtonian gravity occurs instantly.
| It has no notion of information taking time to propagate. But
| we know gravitational waves happen, so Newtonian gravity is
| wrong _at even very large scales_. If the sun disappeared
| Newton tells us we'd know immediately. In GR we'd know about
| 8 min later.
|
| The bigger problem is not that the quantitative effect is
| large, but that the _qualitative_ difference of going from
| the instantaneous effect to one that needs to propagate is
| enormous. It's the whole point of relativity as a concept.
|
| Even going to GEM as a true, non-singular linear
| approximation of GR would be a step up from Newton's laws, at
| least there we can have gravitational waves and causal flow
| of information.
| the__alchemist wrote:
| Thanks for bringing this up; this is the central reason why
| I'm skeptical of Newtonian models that predict dark matter,
| and why I don't think the term MOND makes sense as the
| simplest alternative.
| throwawaymaths wrote:
| > My hangup with MOND is still general relativity.
|
| Fwiw, we know for a fact also that for edge cases GR is wrong
| because it doesn't agree with quantum mechanics (unless QM is
| wrong), so it's maybe not right to take GR as gospel,
| especially for a theory that only seems to also change GR in
| edge cases, and the only reason why "it doesn't agree" might
| amount to "the math is hard and the physicists haven't put
| enough work in yet"
|
| To wit, accepting a mond-ified GR is probably not going to
| change how GPS works so the claim that "GR has withstood the
| test of time and engineering" is not a totally solid refutation
| of MOND
| mort96 wrote:
| Well this doesn't seem like such a conundrum. We know for
| sure that ND is wrong because it predict things incorrectly
| which GR predicts correctly. We know GR is wrong because it
| is incompatible with any form of QM and we know some form of
| QM is more or less correct. Essentially, GR and ND are both
| wrong, but ND is more wrong than GR.
| scotty79 wrote:
| > because it doesn't agree with quantum mechanics
|
| I don't think it doesn't agree. It's just that we never
| managed to neither formulate quantum mechanics on 4
| dimensional space time nor quantize gravitational force. So
| we simply have no idea what happens in small scale in
| significant gravitational fieldd.
| pdonis wrote:
| _> for edge cases GR is wrong because it doesn 't agree with
| quantum mechanics_
|
| What "edge cases" are you talking about? I agree that GR is
| not a quantum theory, but it's not established that that has
| to be a problem, nor is it a matter of "edge cases".
| twothreeone wrote:
| GR says spacetime is curved by mass, right. So what's the basis
| for explaining the curvature of space (which can be measured,
| e.g., LIGO) in MOND?
| MathMonkeyMan wrote:
| MOND has nothing to say about the curvature of spacetime,
| since MOND is Newtonian (MOdified Newtonian Dynamics). It
| goes back to "F=ma and gravity is a force" and modifies the
| rules so that gravity grows weaker faster at a certain scale.
|
| The fact that MOND fits a lot of the data troubled
| cosmologists, because they know that a General Relativistic
| theory is needed to explain pretty much the rest of gravity.
|
| TeVeS is an extension to General Relativity that reduces to
| MOND in the non-relativistic limit. For comparison, General
| Relativity reduces to Newtonian gravity in the non-
| relativistic limit. The non-relativistic limit is when speeds
| and spacetime curvature are small.
| Gooblebrai wrote:
| How does MOND deal with the effects of time dilation and
| length contraction? Do we have to go back to Newton's time
| where there's a universal time?
| oneshtein wrote:
| > GR says spacetime is curved by mass, right.
|
| Wrong. GR says that gravitation can be modeled as
| acceleration.
| mog_dev wrote:
| General Relativity states that mass-energy curves
| spacetime, and objects follow the straightest possible
| paths (geodesics) through this curved geometry. The
| equivalence principle relates gravity and acceleration, but
| it's not the main description of gravity in GR.
| Bengalilol wrote:
| << Stunning evidence >> ... then later on: << Instead, the
| readings _seem_ to support a basis for MOND, which _would_ force
| astronomers and cosmologists to reconsider this alternative and
| long-controversial theory of gravity. >> What's conditional
| evidence? I may be missing the overall picture, but I view such
| writing as non precise at its best.
| MattPalmer1086 wrote:
| It's just typical pop sci journalism, with a click baity
| headline. Read the paper instead.
| Bengalilol wrote:
| Thanks, I will.
| https://iopscience.iop.org/article/10.3847/1538-4357/ad834d
| joe_the_user wrote:
| Not entirely typical. MOND proponents seem to be trying more
| and more sell their approach to the public.
|
| It annoys me but I suppose every theory has to do that now,
| "the mouse trap must go to market now" and all.
| akvadrako wrote:
| Well you have to convince somebody to pay researchers for
| their time, which ultimately means selling your idea to
| non-experts.
| bbor wrote:
| Well, it's evidence that a) must be verified on a mathematical
| and empirical level, and b) (arguably) fits _better_ with a
| currently unpopular theory than the dominant one. There's so
| many unknowns in physics that opponents can easily reply "well
| your theory doesn't explain XYZ yet, so we likely just need to
| tweak our theory".
|
| In other words, reasonable minds do disagree. AFAIU as an
| amateur.
| yieldcrv wrote:
| There is no consensus yet, there is no repeatable metric
|
| It is perfectly valid to say "hey look over there for further
| review"
| astroH wrote:
| In my opinion, this article is misleading at best. "...scans of
| ancient galaxies gathered by the JWST seem to contradict the
| commonly accepted predictions of the most widely accepted Cold
| Dark Matter theory, Lambda-CDM." --> LCDM doesn't predict what
| galaxies should look like, it simply predicts how much mass is in
| collapsed structures and that dark matter haloes grow
| hierarchically. In contrast, with JWST we see light and need to
| infer what the underlying properties of the system are. It was
| shown very early on that the theoretical upper limit (i.e. taking
| all of the gas that is available in collapsed structures and
| turning it into stars) predicts a luminosity function (i.e.
| number of galaxies per unit luminosity) that is orders of above
| what JWST has observed (e.g.
| https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstra...).
| This means that there is plenty of space within the context of
| LCDM to have bright and seemingly large and massive galaxies
| early on. Based on current JWST data at these early epochs, there
| are really no convincing arguments for or against LCDM because
| it's highly sensitive to the galaxy formation model that's
| adopted.
| uoaei wrote:
| > with JWST we see light and need to infer what the underlying
| properties of the system are
|
| Every theory of dark matter is based exclusively on light-
| emitting objects. There is no "contrast" between JWST's methods
| and those of others. Casting aspersions on JWST because it can
| only see light is like casting aspersions on Galileo because he
| could only build telescopes. If we could teleport to the things
| we study and get more information that way, it would be nice,
| but we live in reality and must bend to its rules.
|
| > highly sensitive to the galaxy formation model that's adopted
|
| I should only need to remind the reader of the classic idiom
| "cart before the horse" to remind them that this line of
| reasoning is invalid.
| astroH wrote:
| This is a misrepresentation of what I am saying. By no means
| am I casting an aspersion on JWST. I am casting an aspersion
| on this particular observation as a test of MOND and LCDM.
| Also I highly disagree about your comments on my line of
| reasoning. The fact that you can obtain a huge range of
| possible galaxy properties in the context of LCDM indicates
| that in general, tests of LCDM and MOND that rely on galaxy
| formation model are in usually not strong tests. This is the
| key issue with using the abundance of high-z galaxies (or
| even their masses -- despite the fact that these aren't
| measured) as a test. In the context of LCDM, you need haloes
| to form galaxies but it has been shown many times that there
| are enough haloes to solve the problem (see the paper linked)
| by a huge amount.
| uoaei wrote:
| The skepticism you display in this comment is completely
| absent when you reference lambda-CDM elsewhere. Consistency
| invites zero criticism :)
| astroH wrote:
| And so you have proved my point. The observations
| presented in this article can be made consistent with
| both...as such one should think about stronger tests of
| both LCDM and MOND.
| uoaei wrote:
| Your point was orthogonal to the point of epistemology.
| This isn't Reddit, we respect actual arguments here.
| MattPalmer1086 wrote:
| You are missing the point. JWST is not being singled out as
| different here, and no aspersions are being cast.
|
| It is the entirely general point that all we can observe is
| the light, and we have to _infer_ what that means. Maybe
| things are bright because there 's a lot of stars. Maybe
| there aren't but there is not much dust. Maybe there aren't
| so many stars but they are bigger and brighter. There is room
| to fit many different models on the basis of the light that
| is observed.
| ajross wrote:
| > there are really no convincing arguments for or against LCDM
| because it's highly sensitive to the galaxy formation model
| that's adopted.
|
| To be fair, that is absolutely not the way LCDM would have been
| described to someone in the pre-Webb days. It was a well-
| regarded theory and the hope was (a-la the Higgs detection)
| that new data would just better constrain the edges and get us
| on to the next phase of the problem.
|
| But instead it's a wreck, and we didn't see what we were
| expecting at all, and so now we're retreating to "Well, LCDM
| wasn't exactly proven wrong, was it?!"
|
| That doesn't mean it's wrong either, and it for sure doesn't
| mean MOND is right. But equally for sure this is a Kuhnian
| paradigm shift moment and I think it's important for the
| community to be willing to step back and entertain broader
| ideas.
| astroH wrote:
| Again, LCDM and galaxy formation are two different things.
| "...and we didn't see what we were expecting at all..." It
| depends on who you ask. There were many pre-JWST models that
| did well in this regard. A particularly interesting one is
| this from 2018 (https://ui.adsabs.harvard.edu/abs/2018MNRAS.4
| 74.2352C/abstra...). That group even had to write another
| paper reminding everyone of what they predicted (https://ui.a
| dsabs.harvard.edu/abs/2024arXiv240602672L/abstra...). Another
| example is here (https://ui.adsabs.harvard.edu/abs/2023OJAp..
| ..6E..47M/abstra...) which shows results from a simulation
| from ~2014. I can provide numerous other examples of this. My
| point isn't which theory is or isn't wrong, my point is that
| what is presented in this particular article is not a
| constraint on any realistic theory of gravity as the
| sensitivity of these particular observations to galaxy
| formation modeling is so strong.
| Davidzheng wrote:
| Absolutely not in the field, so if you are please completely
| disregard. But from conversations with physicists (not
| cosmologists) I always thought people thought a lot of
| evidence for LCDM was dubious at best.
| RicoElectrico wrote:
| Waiting for Angela Collier to make a video on this, I'm sure many
| people will forward her this article. MOND is actually a niche in
| cosmology despite its PR.
| mgraczyk wrote:
| Are any of the MOND theories consistent with this new data also
| consistent with recent gravitational wave observations? My
| understanding is that gravitational wave detectors have recently
| ruled out most plausible MOND theories. The linked paper doesn't
| seem to discuss this.
| verzali wrote:
| Why why why do people share articles with sensational headlines
| like this? Its no wonder science journalism gets a bad rap. This
| kind of thing really undermines all the people who are actually
| trying to communicate science properly.
| muglug wrote:
| Without this article and HN discussion I'd never have known
| about MOND, which is (at the very least) a fun theory.
| trimethylpurine wrote:
| Personally, I think it would be better that way. Science
| works in pursuit of truth, not towards the obfuscation of it
| for personal and selfish financial gain. That should
| hopefully explain the outage that scientists have towards
| articles like this one. In place of relying on articles like
| this, you might try searching scholarly articles or
| subscribing to them.
| prof-dr-ir wrote:
| The trouble is that MOND is just not worth your time. In
| fact, I would even object to calling it a 'theory' in the
| first place.
|
| MOND is just some wild idea, but a little thought should
| convince every physicist of its uselessness. It has major
| issues both in explaining experimental data and in its
| theoretical consistency. It justifiably receives next to no
| attention from the vast majority of (astro)physicists.
|
| In popular science the idea however does not seem to want to
| die, perhaps because it is so easily explained to a
| layperson. Of course this is a little frustrating for the
| community, but perhaps we should look at the upsides: more
| attention for science is probably a good thing, and
| explaining to people why MOND is so useless provides a good
| opportunity to discuss some proper physics.
| ogogmad wrote:
| This is a weirdly arrogant comment given both TFA and the
| fact that professional physicists have worked on MOND and
| disagree with everything you've said.
|
| https://www.youtube.com/watch?v=n33aurhg788
|
| Is this typical behaviour for physicsts? Extremely strong
| opinions expressed in an abrasive way, out of proportion to
| the available evidence?
| prof-dr-ir wrote:
| I just want to convey the following point: for the vast,
| vast majority of physicists the status of MOND is akin to
| what doctors think of the anti-vaccine theories. The
| evidence in the opposite direction is simply
| overwhelming.
|
| You refer to a non-scientific article and to a youtube
| video, but any vaccine sceptic can probably easily find
| exactly the same kind of material to support their view.
| That would almost certainly include a video by a
| "professional doctor".
|
| You might call me abrasive, but I am really just trying
| to be as clear as possible: this _is_ the consensus in
| the field.
|
| And before you continue this discussion it might be worth
| pondering the following questions. How do you think
| doctors should convince vaccine skeptics that vaccines
| work? And how big a percentage of their weekend do you
| think they should spend engaging on the details with
| anti-vaxxers? (And, in this forum, how many downvotes
| from obvious non-experts should they be willing to
| accept?)
|
| In other words, what could I do to convince you in a
| reasonable amount of time?
| lloeki wrote:
| > The trouble is that MOND is just not worth your time.
| [...] MOND is just some wild idea
|
| Sometimes you gotta be wrong before you get it right.
|
| I mean, Newtonian mechanics are "wrong" but served us well
| at some scales for a while, and that it observationally
| failed in others led us to relativity. Even "relativity"
| took iterative steps, from Poincare's Lorentz invariant
| theory (or even earlier with Galilean relativity) all the
| way to GR via special/restricted relativity, the latter
| name having been retconned because it's only valid in
| restricted special cases and fails to unify generally. And
| we know GR fails to unify with quantum mechanics, so one of
| them (or both) gotta give.
|
| So even if something as MOND were "wrong" and known to be
| wrong (definitely so), there's still value in experimenting
| with it to get a better understanding of things. That's
| just how things work.
| prof-dr-ir wrote:
| > there's still value in experimenting with it
|
| I disagree: some experiments are just not worth our time.
| I wrote about such a situation three years ago:
|
| https://news.ycombinator.com/item?id=26656206
|
| My view is that it applies here as well.
| verzali wrote:
| There are much better articles on MOND that don't make
| misleading claims that the James Webb has proven it. This
| one, for example:
|
| https://physicsworld.com/a/cosmic-combat-delving-into-the-
| ba...
| docflabby wrote:
| Dark matter is just made up bs if you replace "magic" for dark
| whenever its mentioned its the same difference - theres no
| tangable evidence it exists at all.
| XCSme wrote:
| There is no evidence that anything exists...
| akvadrako wrote:
| Dark energy is literally this - it just means something is
| different than predicted by current leading theories.
|
| There is plenty of evidence that either dark matter or an
| alternative is needed and CDM is just the most popular take.
| pikseladam wrote:
| It shows early galaxies forming way faster and bigger than
| expected, which kind of shakes up the whole dark matter idea.
| Seems like it supports the MOND theory--that gravity might not
| work the way we think. Pretty wild, but it's still up for debate.
| steve_adams_86 wrote:
| That would be a fun surprise to me as a lay person who doesn't
| actually understand these things, because I see a lot of
| disparagement towards the MOND theory.
| I_am_tiberius wrote:
| What I find implausible about MOND is the constant a0 (~1.2). Why
| stick with a measurement based constant instead of exploring a
| parameter that varies with distance?
| maronato wrote:
| For a dark matter researcher's take on MOND, see this video:
| https://youtu.be/qS34oV-jv_A
| Glyptodon wrote:
| Is there a quantized version of MOND where the increased
| acceleration is because a quantized unit of gravity will exert
| force across distances that would otherwise suggest that that the
| force would be less than a "g quanta" or because maybe
| quantization "ceilings" more than floors at very large distances?
| If gravity does have some kind of particle or fundamental
| quantization like a photon, and basically still exerts at huge or
| "infinite" distance, does it make sense that it's more likely
| there's some kind quantization floor or maybe quantization bands
| or something? Or is it thought that quantization of gravity
| imposes a limit on distance for the exertion of gravitational
| attraction? (Or is it thought that that with quantized gravity
| that what's happening is a decreased rate of "gravitons"
| interacting between the objects?)
|
| Anyway, a bit clueless about this, just curious what gravitons
| are supposed to mean for either theory (MOND, LCDM, etc.).
| librasteve wrote:
| Here is what Sean Carroll has to say about MOND...
|
| https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma...
|
| I find this treatment more compelling.
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