Newton's law of gravity passes its biggest test
science.org
science.org
In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.
Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.
Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.
When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.
Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?
MOND may not be the solution, but I'm still skeptical about dark matter.
Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.
Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?
Dark matter (well, LCDM) has predicted many things which were proven false, e.g. that big galaxies assembled through merging with smaller ones over long periods of time. JWST shows us this is not in fact the case. There are now lots of papers showing how LCDM can explain that, but these are post facto explanations.
Happy to be proven wrong if someone can point me to a prediction subsequently confirmed by observation.
1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg).
MOND only explains 1.
Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).
3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g.
MOND explains several things LCDM cannot:
- why most elliptical galaxies seem to "not have dark matter" (effectively a prediction)
- external field effect (predicted and confirmed)
- renzo's rule
- DM halos that are way too big
- early galaxies (this was a prediction)
HM. people have been downvoting. Anyone care to post a substantive rebuttal?
Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
(And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)
There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)
It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation.
you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.
My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter!
In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.
3. I would wager that what we call the CMB is more of a local effect than a big bang remnant.
I lack the karma to downvote, don't blame me. :)
But generally they closely conform to Einstein gravity/standard model, as they incorporate relativity.
But that's still the thing. No theory can explain gravity across the spectrum of variables (from micro to macro scales, apparent lack of dark matter in some galaxies, black holes, why galaxies don't fly apart, etc.) We have a dark gravity problem regardless of which theory you like best.
Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
Historically, science has predicted lots of true things about the universe that could not be observed at the time.
And to be clear, falsification and being empirical & skeptical about theoretical claims is great. What I see all too often on the Internet is just pattern matching to the words "observable" and "falsification" without a second thought, without actually looking into how science develops, and any and all narratives are historically rewritten to fit only those two categories.
Which is why it's even more impressive to be a real scientist, to actually be able to navigate the muddy waters properly where it's not just some simple adjective checklists to run through. (As a non scientist)
Just some examples I could think of:
The neutrino[1], predicted in 1930 due to missing mass in beta decays, discovered in 1956.
Gravitational waves[2], predicted by Einstein in 1916, indirectly detected via pulsars in 1974 and directly by LIGO in 2015.
Higgs boson[3], predicted in 1964 to solve the problem of the Standard Model at the time requiring particles to be massless, which we knew they weren't. Hints from LEP at the turn of the millennium, detection by the LHC in 2012.
Cosmic microwave background[4], predicted in 1948 as a consequence of the expanding universe on the early radiation. Detected by accident in 1964.
Gravitational lensing[5] was first proposed in 1801 as a consequence of Newtonian corpuscular theory of light. Einstein realized in 1915 that GR predicted a larger value. Weak lensing by the sun was observed in 1919. Strong lensing by a distant galaxy of an object behind the galaxy was was predicted in 1937, and first observed in 1979.
[1]: https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal
[2]: https://en.wikipedia.org/wiki/Gravitational_wave#History
[3]: https://en.wikipedia.org/wiki/Higgs_boson#History
[4]: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Hi...
[5]: https://en.wikipedia.org/wiki/Gravitational_lens#History
For example "angry gods" was never a simple theory, one needs only to read some fan fiction to understand that theology gets complicated fast. Instead "angry gods" is a simplified summary of a very complicated theory about metaphysical hierarchy, creation, agency, the meaning of divinity, and cause and effect, etc etc.
Aether: observations show lack of aether so we update the theory that makes aether unnecessary.
Dark matter: observations show lack of observable matter so we keep hunting for decades locating it. The aether analogue would have been to continue to look for dark aether. Dark matter is more like god of gaps. We can 'darkify' any theory that does not fit empirical evidence.
It's hard to tell now which leans more towards the more correct theory.
Light being the same speed irrespective of observer is weird. Velocity dilating length and time is weird. Not having a preferential observation point is weird. Not needing a medium for transmission is weird. Not being able to agree on simultaneity is weird. etc.
Aether wasn't just something that a bunch of dullards clung to. You have to abandon some very long held common-sense understanding when you give it up.
That means for now it's useful.
One scientist says "We don't know, really, it's a head scratcher" and the others say something cool like "Dark energy", which ones will get more attention and publicity?
Think about some random person born in the 1950s. It will be impossible for you to directly prove that that person has two parents, but with all your knowledge about biology, and your knowledge that humans could not be cloned in the 1950s, it's not a head scratcher, unless our understanding of biology is completely off, it's not just speculation there were two parents. That's the comparable situation with dark matter. It's not a head scratcher. Every indication shows there is a lot of mass around that doesn't interact with light.
For some general misunderstanding to explain what we see, ie something which is not dark matter, it means our equations of gravity must be wrong.
People have tried hard to tweak the equations to fix the observations. One such proposal is indeed MOND that the submission is about.
The problem with tweaking the equations is that we have a lot of observations of very different circumstances and scales, and it's so far proved impossible to tweak the equations in a given way so they explain all of the observations. What happens is that you can tweak them so they fit one set of observations, like MOND fitting galaxy rotation curves, but then not fit other sets of observations, like MOND with the motion of galaxies in galaxy clusters as discussed in the submission.
Meanwhile, adding dark matter seems to work much better at explaining all the observations at once. I say much better because there is still some tension, but it's way less than any modifications of gravity.
Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity. And while the early particle candidates seem to be getting ruled out, axion-like particles, originally proposed to solve entirely different problems, are still a good dark matter candidate.
That said, it's not settled. People are proposing alternatives. One example I know of is Alexandre Deur[1], a QCD physicist which has taken his QCD knowledge into the realm of GR and proposed that perhaps graviton-graviton self-interaction can explain a lot of the dark matter phenomena. It's not mainstream, but he's getting published in peer-reviewed journals.
[1]: https://arxiv.org/search/gr-qc?searchtype=author&query=Deur,...
This is the key thing. As we still don't have a theory of everything, which explains the fundamental reasons why every particle is the way it is, dark matter is a very straightforward and simple explanation.
The name makes it sound stranger than it is. It's just particles that have mass and are affected normally by gravity, but which are unaffected by the electromagnetic force.
It's not a particularly exotic or implausible or wild idea. In fact it's so simple it's almost boring. It's just obviously inherently difficult to test directly, since gravity is such a weak force.
Dark matter is the Occam's Razor theory. It explains almost all of the observations while assuming the least.
Why do you assume we have no evidence just because we don't have direct observations? Black holes were a similar phenomenon that we had no direct evidence for a fairly long time even though we had lots of other indirect phenomenon that really couldn't be explained any other way.
William of Ockham objected to his fellow theologians inventing things out of whole cloth (like dark matter question mark). That’s the point, not that a simpler explanation is more likely to be true.
The common understanding would have us believe that creationism, being simpler, outshines evolution, or that there is no such thing as a color revolution because the simplest explanation is that the mass protests are earnestly aggrieved locals.
Occam's Razor only kicks in when the hypotheses have equal predictive and explanatory power.
The MOND hypothesis does NOT have equal explanatory power with the dark matter hypothesis. As such, Occam's Razor is not relevant.
So we came up with an idea that fills the gap in the current model and allows us to continue predictive science until we find out more.
People get so absurdly touchy about Dark Matter just because it's named and "weird". It's just a placeholder value.
Occam's razer is not a thing with explanatory or legal or even scientific force. if Occam's razor was really a thing, quantum theory and general relativity would be ruled out as complicated and neither 100 percent explanatory.
As a kid in 90s who was into this stuff, it seemed like anyone who knew about dark matter knew it was just theorized and never observed. It was included as a caveat in a lot of things I read to ensure the readers understood this.
Then in sci-fi it started showing up as a material the characters would interact with, and a lot more people were introduced to the term.
Then as a teen in the 2000s the caveat was usually gone and it started being treated as something we had proof of. People I knew started thinking it was something we'd actually found and was now fact instead of theory.
One could also argue that on the one hand you need just one concept (dark matter) to explain three phenomena (galaxy rotation curves, gravitational lensing, CMBR), and the "general misunderstanding" might need to have three aspects to make it explain all three things, making it more complex.
Occam's razor is a good guiding principle, but in some cases, it's really hard to compare the complexity of competing theories. My favorite example of this is the Many Worlds interpretation of quantum mechanics. In some sense, it's really simple, the wave function just follows the Schrödinger equation. In another sense, it implicitly postulates the existence of many parallel worlds, which you can count as a quasi-infinite complexity penalty, or you can shrug and say it's not a big deal, the theory says the parallel worlds aren't observable, so there is nothing wrong with postulating them. There's no objective way to decide that.
Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.
This reminds me of arguments on the alt.athiesm newsgroup back in the day.
My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?
Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.
If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.
* NGC 1052-DF2
* NGC 1052-DF4
See, for example, https://www.caltech.edu/about/news/dark-matter-flies-ahead-o...
As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter.
I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but there is a lot of back and forth between "MOND is dead" and "we found new stuff we couldn't explain with dark matter, but it matches MOND predictions".
You'd be correct given hidden variables.
But we know pretty convincingly that quantum anything does not have hidden variables.
I'm not sure a non-local hidden variable explanation of QM is any distinct from superdeterminism though.
Like, global variables?
- Locality is a consequence of the use of a scaled compute cluster with peer-to-peer networking only.
- The speed of light is the speed of information exchange between compute nodes.
- General Relativity is caused by network links with finite bandwidth taking longer to exchange information if there is more of it. (Effectively slowing down time)
- Quantum Mechanics is caused by finite precision of the n-bit numbers at small scales.
- MOND is caused by cumulative summation and rounding errors that show up only in the case of weak fields over large distances.
Etc…
Like, if one theory says that a hunk of metal actually is made of many microscopic grains of various sizes and orientations, where the sizes and orientations of these grains has an effect on the behavior of the metal, you don’t count the “the sizes and orientations of these grains” as free parameters, do you?
My understanding is that these observations are a fatal blow to any serious MOND models.
You have to actually do the calculations and compare what MOND output to the observed behaviour of the galaxies in question.
It has fallen far from its glory days, and is more an old reliable tool than a leading physical theory.
This is different from MOND, which is a distant second contender compared to dark matter.
In the latest Marcel Grossman meeting (2024) there was no string theory parallel session but there was an explicit Loop Quantum Gravity session: https://indico.icranet.org/event/8/program
So string theory is now pretty officially an "also ran", definitely not a leading theory.
Dark matter is another guess. We guess there is more matter in galaxies than what the telescopes show. We can compare the amount of mater guessed from galaxy rotation with other measurements. In this case they compare it with the gravity between a few galaxies.
Nobody is happy that we don't know what dark mater is. There are a few theories, but none of them has enough experimental support. More lack of confirmed details in https://en.wikipedia.org/wiki/Dark_matter#Composition
[1] I don't know enough about rocks https://xkcd.com/2501/
The fact that there are tentatives to identify what it might be does not ammegliorate the fact that at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves. I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury). just a tiny percentage of obeservable behaviour at that time but it was a different time. a time where you could bring down the existing science of the day for a tiny percentage and now we accept 90% observation disaccordance (dark energy+dark matter) with what the equation require.
Another way to interpret dark matter is that we can observe something using several different ways, but all those ways use gravity. When trying to observe this something using electromagnetism, we see nothing. It doesn't seem so crazy then to hypothesize that this something only interacts with gravity, and not electromagnetism.
I don't think that's true. One of Einstein's test for General Relativity, using Mercury's orbit, came around 10 years later after special relativity was proposed, which is understood to be motivated by both Maxwell's equations and experimental results suggesting that the speed of light (electromagnetic waves) not depending on the frame of reference. General relativity (explaining gravity) seem to have been motivated by Newton's gravity not playing well with Special relativity (with mass being relative and all).
I understand the frustration with Dark Matter, but my understanding is that Dark Matter is a guess that is known to be incomplete. Scientists are shooting everywhere to try to explain the discrepancy in gravitational effects and some form of undetected matter is currently the best hypothesis (but not the only one). You say that "we accept 90% observation disaccordance", but the source of its effect is being searched for.
See:
- https://hsm.stackexchange.com/questions/622/what-was-einstei...
- https://en.wikipedia.org/wiki/History_of_gravitational_theor...
- https://en.wikipedia.org/wiki/Tests_of_general_relativity
Edit: formatting, updated links
There are many candidates and I think nobody discarded it's a mix, it looks like a plausible scenario. I made a quick look in Wikipedia, but I didn't find anything relevant to a mix of Dark Mater.
There is(was?) some big discussion about cold vs hot (and warm?) dark mater. IIUC "cold" dark mater won. I'm not sure if there is enough details to make a good guess of the split, or if it's a 100.00% vs 0.00% case.
(But I don't really believe in dark matter, either. It's mostly epicycles and phlogiston.)
Dark matter is actually a very parsimonious theory. None of the laws of physics have to change to accommodate it, unlike with MOND. We may not see it, but it has to move around and affect normal matter in predictable patterns consistent with our current understanding of physics. If it doesn't, then the theory is wrong and may need some revision (which may be a dark matter + MOND hybrid).
In parallel with the research that attempts to find the properties of dark matter that best describe our observations is research that attempt to find what other properties it may have. It is a new particle? Can it interact in ways other than gravity? We didn't find anything, but the universe is under no obligation to make things easy for us.
One possible idea called the "nightmare scenario" is that dark matter is made of particles that only interacts gravitationally. It is a perfectly fine theory, maybe the cleanest one, but unfortunately, it would mean that we may never be able to detect these particles because gravity is so weak that the required detectors would be way beyond our technological abilities.
It'll take either the next Einstein or some groundbreaking experimental observation to get there in my opinion.
If it was possible to incrementally fix these theories, the army of postdocs working on these would have already done so in the last decade or so.
Some once-in-a-generation scientist has an intuition that turns out to be true and mathematically elegant.
It gets proven experimentally years or decades later.
Relativity was exactly like this.
It shows both how Einstein very much didn't make the theory alone, was inspired to take impotat technical steps by work of other thats that created a theory based on his principles before him, and that actually he first created (in intense collaboration) a failed theory that got Mercury's anomaly all sorts of wrong.
That’s a bit simplistic. There was a lot of research activity around the aether in the late 19th century that was ultimately useful for the foundation of special relativity. Like the Michelson-Morley experiment, which was supposed to measure aether winds, but showed it did not actually seem to exist. Lorentz developed his transform as a theory of the aether, but it became a cornerstone of special relativity. Einstein based his ideas on a lot of things that were done a couple of decades before, some of which were supposed to be part of a theory of the aether.
There was actually quite a lot of activity and vigorous debate between aether and something else, unknown at the time, that turned out to be relativity. Einstein did not just show up and invent everything. There is a very quick overview of this here: https://en.wikipedia.org/wiki/History_of_special_relativity
If you revisit what I wrote, you'll find that I agree with you.
There was vigorous debate about a wrong and an unknown thing (both was basically wrong) and it took Einstein's intuition and the new GR math to turn it into science.
I was drawing a parallel between this and the current MOND, string theory, dark matter debate. More specifically, I'd even say dark anything is our generations aether!
My point is that you likely can not.
I think it's possible for an alternative gravitational law to work, but not MOND
MOND is stronger at longer distances than Newtonian Gravity. To me that does not pass the sniff test. It could be a step in understanding a more exact law but to me it feels weird
It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.
It's a scientific theory. It's the best that we have right now to model the real world and be able to do prediction on its behavior.
Does it seems to be kept together by duct tape? Maybe.
Is it yet useful? Yep.
Will it be discarded if anything more fitting will came up? You can be sure of it.
And it's not like the concept of aether itself was really all that useful for anything. The physicists wanted the light to have some mechanical medium to propagate through instead of being a thing of itself, that actual itself shaped mechanical media, not the other way around (mechanical properties arise from the E-M interaction, not the other way around), simply because all other known waves phenomena existed in mechanical media.
Dark matter, string theory, aether, etc., those are models that we, at some point in time, think may help us get better predictions and design further experiments. All models turn out to be wrong in the end, but they can be helpful until we come up with better ones.
If you drop the dark matter model, then you would want to have some other model as for why we observe what we observe. Some people find that other available models are even worse than the dark matter one, but if you don’t think so you can take your pick.
The neutrino is a good example of a particle that almost doesn't exist. They are produced in solar reactions in spectacular amounts. Trillions of them are flitting through you right now as if you don't exist. You'd need a light year block of lead to ensure you could stop one. Mind-boggling amounts of them have to pass through our detectors to see even a single interaction.
Simply put, the particle physics does not have to behave nice so you can sleep well at night.
We can measure the mass distribution on astronomical scales. We "see" the dark matter. Just not with light.
It's not crazy different from saying the same about that poltergeist.
We know of particles which behave the same way. Neutrinos for example.
We are not measuring dark matter, we are measuring something that is not what we expect and we decided it's dark matter.
You act like we've managed to probe the depths of physics with certainty when in reality you find any means to reject that which offends your sensibilities.
At the root of science is "sensibilities", like occam's razor, even "what counts as experimental reproduction", etc.
Not that I am saying it does not exist. Only that we do not have the means of falsifying it if it is false.
Why do you say that like it's obviously ridicule or impossible? Neutrinos do exist and they fit all these criteria. We just know they're not dark matter (or at least not all of it) because they're not heavy enough (and some other things).
Don't try to rely on intuition when thinking about particles, there's no reason for evolution to make what happens at quantum scale or relativistic scale intuitive to us.
There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).
It is an argument against MOND, a theory that says that gravity has to be modified to account for some observations. But for these particular observations, general relatively and Newton's laws gives the same results in practice, the difference is negligible, so showing that these observations can be explained by Newton's laws implicitly mean that they can also be explained by general relativity. No need for modifications.
Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.
However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.
This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.
MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.
But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.
In particular, Newtons law of gravity says the effect of gravity falls off as 1/r^2 where r is the distance from the mass. MOND modifies the standard equations so that gravity starts like 1/r^2 when r is small, and acceleration is large, but for greater distances, when the acceleration is low, instead falls off like 1/r.
MOND explains the movement of the stars in (most) galaxies very well. However this result showed that MOND was not consistent with the motion of the galaxies in the cluster. On the other hand the motion was consistent with plain Newtonian dynamics. Hence Newtons law of gravity (and by extension GR) passed the test.
The test here is for the inverse square law of gravity. The rival theory in this case isn't GR, but MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..
I somehow thought that this was not really holding true and that Mr. Einstein had something to say.
I thought that without Einstein's "corrections" stuff like GPS wouldn't be useful at all.
Maybe I misunderstood last 100+ years in gravitational physics.
Gravity Probe B.
There exists an even simpler derivation for a magnetic term, but its in my file drawer... (simpler in that it just relies on observations from special relativity)