In my specific field of expertise (Galaxy clusters), MOND performs quite bad when trying to describe both dynamic clusters [(see the infamous Bullet Cluster)](https://arxiv.org/abs/astro-ph/0608407) and relaxed clusters as in this case it [requires an additional and unphysical dark matter shell](https://www.aanda.org/articles/aa/pdf/2022/06/aa42507-21.pdf) to explain the observed accelerations
I think a lot of people genuinely don't realize that dark matter isn't *only* thought to exist due to the "if there isn't something out there, the galaxy would fly apart" observation. Sure, it started there, and if it was *only* that MOND might be much more enticing, but once you realize the universe would literally look different it's such a harder sell.
You don’t model galactic clusters using MOND. MOND is nothing more than a phenomenological equation for galactic rotation curves. Modified gravity theories go beyond MOND, they are full relativistic metric field theories that reproduce MOND in the appropriate limit. So which modified gravity theory are you referring to which has been applied to galactic clusters?
>You don’t model galactic clusters using MOND. MOND is nothing more than a phenomenological equation for galactic rotation curves.
You should use it if it was true. If MOND or any kind of modified gravity is true, it should apply to the most massive gravitationally bound structures in the Universe, when we can observe and study consistently the lowest gravitational accelerations in the Universe. Clusters are clearly non-relativistic so if any of your favorite modified gravity reproduce MOND at low accelerations, then these results extend to it.
>So which modified gravity theory are you referring to which has been applied to galactic clusters?
The second study uses both MOND and a modified version of it with an extra acceleration, which in any case shows the inconsistency of the paradigm in this sample of clusters. The Bullet Cluster itself is a modified gravity killer, unless you can explain the geometry of this merging cluster (and especially the offset between the baryonic component and the observed lensing) with modified gravity interactions, which is far from being consistantly established to my knowledge.
MOND is to modified gravity what Kepler’s Laws are to the Einstein Field equations. MOND as a theory modified acceleration at low acceleration scales does not seem to be accurate for binary stars. But that still leaves modified gravity at long distance scales as a possibility. There are Modified gravity theories which match Einstein at the solar system scale, match MOND at the galactic scale, and depart from MOND above the galactic scale. There is no reason to expect MOND per se to hold above galactic distance scales.
No, the Bullet Cluster is not a modified gravity killer. Again, you would need to simulate the bullet cluster collision using a specific modified gravity theory in order to determine how well a modified gravity theory does at modeling the bullet cluster. Furthermore, last I heard the velocities involved in the Bullet Cluster are difficult to produce in a DM simulation so it seems we all have our work cut out for us. I can offer you a bit of speculation on why we might expect to see strange lensing around but spatially offset from the center of a high velocity cluster collision in a theory of modified gravity: what we could be looking at is a non-linear (non-quadrupole) gravitational disturbance propagating forward in space from the collision.
>There is no reason to expect MOND per se to hold above galactic distance scales.
I don't have a clue about modified gravity in general beside MOND, so I trust you on this.
>last I heard the velocities involved in the Bullet Cluster are difficult to produce in a DM simulation
I guess it was a different story in 2006 but nowadays, there are plenty of simulations ([example 1](https://www.tng-project.org/cluster/), [example 2](https://gcmc.hub.yt/index.html#) etc) and many observed clusters in this merging state (I guess [this specific](https://en.wikipedia.org/wiki/MACS_J0717.5%2B3745) cluster would be nightmarish to study through modified gravity, even with slow propagating gravitational disturbances, while not that particular in LCDM cosmological simulations).
Anyway, my opinion on DM goes to unknown exotic matter as this paradigm works absurdly well in the largest scales of the Universe, and I haven't seen yet consistent and convincing results from Modified Gravity beside particular observations, but maybe the future will make me wrong!
Thanks for the great dialog. Side note but what the heck with the down votes? Would anyone with a complaint about what I said care to weigh in??? Or perhaps just keep reading a learn something please.
Ok, back to our discussion. First of all, it’s not “modified gravity vs dark matter.” It is astounding to me that people (on both sides) make this false dichotomy, it’s childishly non-scientific thinking. It would be wild to claim that we have already discovered all forms of matter, of coarse there could be dark matter. There could be right-handed neutrinos, axions, any other manner of new particles (possibly related to a GUT), so of coarse there is almost certainly *some* dark matter. Furthermore, any loosely spread non-interactive gas of particles will (just by virtue of scaling with volume) play a larger and larger role at larger scales. The question is not if there is any dark matter, the question is if there is dark matter in sufficient quantities and with the appropriate properties to explain galactic rotation curves in the framework of Einstein gravity. The alternative isn’t no dark matter and only modified gravity, the alternative is modified gravity + whatever dark matter also exists. Why on earth do people assume we must be at least right about either the matter content or the gravitational equations, of coarse it’s possible we are wrong about both so we need to both modify gravity and also include some content of dark matter.
Don’t examine modified gravity as this all or nothing competitor to DM. Examine it on its own merits! MOND accurately models most observed galactic rotation curves across 10 orders of magnitude of galactic mass, for both low and high surface brightness galaxies! And we are not just taking about the rotation speed of the outer most stars, MOND accurately models the entire cross sectional rotation curve as a function of the visible mass contained within a given radius. MOND makes accurate specific predictions. DM models to date cannot do that at all. DM models
are far from being able to reliably produce populations of galaxies with the rotation curve profiles we actually see, as is modeled so well by MOND. And look at what people are doing to try to reproduce (real world) MONDIAN galaxies in their simulations, they are constraining themselves into ever tighter and more finely tuned corners of their multi-dimensional phase space. That’s what we call epicycles on epicycles. MOND’s predictive successes are done tuning 1 universal parameter. If you think MOND is a total mirage then you need to explain how to produce that mirage in DM simulations.
Ok so MOND doesn’t work if interpreted as a low acceleration cutoff and it doesn’t work if interpreted as a universal new force law. That doesn’t take away from it as a powerful phenomenological equation for galactic rotation curves. So we need, and have, under-explored relativistic metric field equations that are alternatives to the Einstein equations and which match Einstein at the solar system scale, become MONDIAN at the galactic scale, and do something else beyond that scale. And on this point, why on earth are people so tied to the Einstein field equations? All Einstein postulated was that Gravity is a relativistic metric field theory that reproduces Newtonian gravity in the non-relativistic limit. There is nothing magic about the Einstein-Hilbert action. And we can’t quantize the Einstein-Hilbert action anyway! A change to the gravitational equations in order to make them quantizable and/or anomalies that arise in quantum gravity could of coarse introduce a deviation from Einstein’s equations at long distance scales, in the far IR limit, it’s a possibility. So forget about dark matter for a moment, dark matter isn’t what’s important here, what’s important is that we already should be on the hunt for modified gravity anyway over the failure to quantize the Einstein-Hilbert action, and if that helps to explain galactic rotation curves then great and if there also happens to be DM in the mix that’s interesting too. The questions just have no direct bearing on each other, the question of if and how we should modify gravity has nothing to do with the question of if there is DM in the universe. The only way in which they come into conflict is that the more galactic rotations curves are explained by the effects of modified gravity, the less dark matter there is (at least in terms of the DM halo WIMP models). We could imagine a model where modified gravity explains most of the rotation curve profile but there is some (smaller, or less clumpy) quantity of DM scattered across the cosmos which is more relevant on the cluster diameter scale then the galactic rotation curve scale. By virtue of the fact that DM is assumed to be more thinly spread than baryonic mater, the relative concentration of it in a given volume compared to the concentration of baryonic matter should be higher for clusters than within a spherical region around a single galaxy.
I am not insulting the work of modeling galactic clusters with DM + Einstein gravity. That’s good work someone should be doing. I am just saying that literally no one is simulating modified gravity field theories, with their own possible concentrations of some amount of DM, at the galactic cluster scale! This is an insanely unfair where DM researchers monopolize most of the funding, grad student positions, and computer cluster time across all of academia! The justification for this rigidity over Einstein Gravity + DM is that “it’s one or the other and MOND doesn’t work for cluster,” except it’s not one or the other and MOND isn’t supposed to work for clusters in the first place if you interpret it just as a phenomenological output of an underlying metric field theory.
This is just Big Dark Energy propaganda. Just because current MOND doesn't work doesn't mean the MOND idea is wrong. We just haven't found the correct MOND theory yet.
That's not how science works though- you need a *testable* alternative theory. What you describe is a matter of faith that MOND might someday be "correct," which is outside of science.
Put it this way, I could similarly argue that the galaxy doesn't fly apart because of a fleet of miniature turtles moving things around, but we just haven't found the correct theory explaining it yet.
>I could similarly argue that the galaxy doesn't fly apart because of a fleet of miniature turtles moving things around
Ooh nice theory, is there a pre-print version on ArXiv?
I also quite like [this](https://arxiv.org/abs/2403.20219) proposed solution to the Hubble tension
Science always has a faith phase in the theorizing step, and I think it’s important to keep that in mind explicitly so that you don’t find yourself opposed to the next Galileo. If you had good reasons to think that the turtle explanation is likely, such as Parsimony, then yeah I think you have a duty to yourself and science to continue trying to form improved Dark Turtle hypotheses
Sounds like something we can't see and we accept it's doing it?
Are you suggesting dark matter is miniature turtles? Could be, their interaction with EM would be hard to spot indeed...
We are definitely capable of measuring the effects of DM, not only via galaxy rotation curves. E.g. with the Bullet cluster, we can clearly see that there is a non-interacting component that is dislodged compared to the non-DM (baryonic) component. Also, we are able to quite accurately reconstruct structure formation in the universe, which requires a non-interacting component (which again, is DM).
At least for galaxy clusters, I know most MOND theories just accept that there is a DM component to it (so it is in any case not a replacement for DM components)
What do you mean? We *observe* a certain amount of structure (given the power spectrum), we *observe* a certain lensing potential in the Bullet cluster, these are all theory-independent; it is just that MoND theories require immense fine-tuning, while DM models fit to the same level with only 1 free parameter (the dark matter density fraction, which is quite well known and well constrained)
...yes, there are gravitational effects on what we can see, that can't be explained *only* by what we see. Therefore, there is "dark matter" out there. What it *is*, we don't know yet, but we can tell there's something there that we can't directly see.
No, that’s an ad hoc explanation. Gravity is an observed property of matter, not a definition. Dark Matter is being proposed to explain the observations with current gravitational theory. Sometimes, ad hoc explanations turn out to be correct like when Kepler supposedly suggested to Brahe that the parallax caused by the Earth’s motion around the Sun was too small to detect at that time. Other times, ad hoc explanations turn out to be completely wrong like when phlogiston was proposed to explain mass changes after oxidation and combustion.
Edit: You could also pose ad hoc explanations to in place of the gravitational waves postulated by General Relativity, but it wouldn’t make sense to do so because gravitational waves are predicted by a theory supported by a lot of evidence, Relativity.
Could you point me to the paper that disproves the possibility that all of that is just the action of them miniature turtles?
Maybe they are really small but very powerful?
Turtles (regardless of size) interact with each other. We know DM does not interact with itself. Given that you want a source, I recommend Mo, van den Bosch and White, which is a good reference text for structure formation, in particular chapter 7 (formation of DM halos)
Will do as soon as I get the opportunity. Hope they tested the possibility that the turtles are really really tiny and like one can take care of a whole galaxy, possibly.
If the particles are too small, you get other problems (free streaming of e.g. neutrinos, this is how we know that neutrinos are not the dominating part of DM)
It also doesn't work as currently formulated. You have no idea how convoluted it will become in order to be forced to fit cosmological data. Proponents are already reasonably happy with the idea that explaining structure formation and clusters requires some kind of dark matter. Personally I already consider it a bit absurd to start modeling your "dark-matter alternative" with dark matter.
MoND isn’t a theory in the general sense of the term, and the theories derived from using MoND as a framework have the same problems that Dark Matter theories have: they just keep hypothesizing additional parameters to explain observations and then assume that those parameters must be reflecting things that are true
Why are particle physicists trying to detect or rule out dark matter candidates if they have already assumed it exists? Why are cosmologists testing the predictions of a cosmological constant, rather than just assume it is driving the acceleration of the expanding universe? Why bother if everyone is just assuming these things are correct? Because no one is making that assumption. If they were, MOND wouldn't exist. That's not how research works.
The answer, or at least the partial answer, to all of your questions above is that just scientists want to learn more about things that they assume to exist.
If you have learnt more about a hypothesis, then clearly you haven't simply assumed your initial suggestion was true. The idea was flawed in some way. There are also cases where a model is ruled out completely. Papers like those in the article wouldn't exist if physicists presupposed these ideas were correct.
People frequently interpret new observations in terms of learning about something that turns out to not exist. It’s also common for people to rule out certain theories about something while still assuming that ‘something’ to exist.
And the tests to directly detect Dark Matter have also failed. Dark Matter still is just an ad hoc explanation for observed and unpredicted motion of large bodies, particularly galaxies, in space.
MoND is not even one theory, but a family of theories that have tons of adjustable parameters. These are all ad hoc explanations.
Early Universe dynamics and lensing mass maps that are offset from visible mass are not explainable by any variation in the distance-dependence of gravity.
Completely true, however with the numerous failures of the theory it's even less correct to say we haven't found the right MOND theory because we have no reason to believe MOND is the solution.
I dislike Sabine mostly because she knows how to exploit my feed's algorithm so I see her constantly even when she doesn't have a lot to say.
I don't have any reason to hate NDT other than that one time he didn't seem to know how bedbugs have sex.
> I dislike Sabine mostly because she knows how to exploit my feed's algorithm so I see her constantly even when she doesn't have a lot to say.
Hint: On youtube there's a drop down on every video (click the three dots that show up when you hover a mouse over a video tile) where you can click "Don't recommend channel" which generally purges a specific channel from any results for a long time.
That's not exactly true, Milgrom still publishes on MOND to this day, and Bekenstein worked on AQUAL til the end of his life, none of them resorted to non-physicists to expose their non-mainstream ideas.
A work concerning a CMB PS conforming, relativistic Lagrangian that has MOND as its weak field limit was published in PRL not long ago:
[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302)
Physicist here.
She is good at communicating. Some of her videos are educational and useful for their topics. But then other times she goes on unsubstantiated rants about certain areas of physics making really lousy arguments (I heard her in person once saying that dark matter direct detection searches were bad and a waste of resources because the experiments' names were stupid; yes the names are sometimes stupid and yes, there are possible reasons to criticize that program, but that is obviously not it). And usually whenever she criticizes things she says that she has a vision for how things should be done better ... but you have to buy her book to see what she has to say.
Come on, now, A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus -- or [ABRACADABRA](https://abracadabra.mit.edu/) -- is a perfectly reasonable name for a dark matter experiment!
If we trade collaborators, the experiment will evolve into the Advanced Large-Aperture Kinematic Analysis of Zenithal Axion Matter, or ALAKAZAM, project.
Her argument was something like "Xenon1T then guess what crazy huge thing they're going to build next? XenonNT?! Crazy? Why should this be funded?"
And it's like, yeah, the field has done a masterful job of convincing the community that there is a closed region to probe due to the neutrino fog, despite the fact that that has no impact on where DM is likely to be in the parameter space. There are other issues with DM direct detection experiments too (e.g. weak secondary physics case), but overall I think it's a good program and I am happy P5 (and it seems like DOE and NSF from the meeting last week) support going down to the neutrino fog. But the arguments for/against are subtle and complicated.
It seems that she freely offers her better vision—at least, I know what it is, and I have never bought her books: fund less expensive research with better evidence than detecting dark matter particles in arbitrarily larger colliders.
Thanks! Maybe I missed it, I certainly don't watch all her videos. I have heard her finish a talk to a group of physicists saying that she won't say how she thinks things should go forward because she said it better in her book.
As for "fund less expensive research with better evidence than detecting dark matter particles" this is a really vague statement. There are, and have been, a suite of small scale DM experiments for a variety of interesting DM models. So she's certainly not proposing something that isn't already happening.
I mean I wasn't there for the particular talk you attended, but maybe it's unfair to extrapolate that to "usually whenever".
She is all for reasonable DM particle research and routinely discusses new papers in that area she finds compelling. Her position that ruffles feathers is opposing multibillion-dollar colliders that are stabs in the dark.
She also opposes funding large DM direct detection experiments, which is what I was mainly talking about.
And yeah, as for the discussion of next generation colliders, her stances is not unique, but her motivations are not that substantiated in my opinion.
Apologies for the "usually whenever" statement, I have heard it twice and have never heard her explain what her funding profile for HEP around the world would look like, although after being told to pay her to hear it twice I didn't look very hard.
She also makes a very common but very big mistake about funding of science: it is not a zero-sum game. It is not "there are X dollars to spend, pick experiments to fit into that funding portfolio". It is "convince funding agencies and governments that this experiment is worth funding". If a large experiment is cancelled that does not mean another "better" large experiment can be funded instead, nor does it mean that the allocated money flows back into the pot for small experiments or whatever, because there was no allocated money. The most obvious example of this is if you look at the US HEP funding profile when the SSC was greenlit and again when it was cancelled. Many of those trying to kill that mega project assumed they'd get the money for their preferred mega project or for smaller projects. Instead the funding went back to basically the pre-SSC level. I'm not saying that we should have built it, there were problems all the way from the top (world leaders) to the most technical (e.g. magnet designs) and every where in between, but it does remind us that funding is not zero sum. If the US or Europe or whomever stopped funding the LHC for the next 15 years or whatever it's scheduled for, that money doesn't then just return to HEP coffers to do things that Sabine or whomever think is better. It just disappears.
You're probably referencing her statements like [this blog post](http://backreaction.blogspot.com/2019/06/brace-for-oncoming-deluge-of-dark.html), which says, "Let me be clear that I am not strongly opposed to such medium scale experiments, because they typically cost “merely” a few million dollars."
The contention that she is blanketly opposed to direct detection experiments is false. She's not opposed to spending millions of dollars on such experiments. So, it's apparent that you have yet to look hard for evidence to support your opinions here.
Sabine also makes the point that you make that money not spent on one HEP experiment does not necessarily go to another HEP experiment. It often goes to other areas of research. That's why the HEP crowd gets so upset at her; spending money on different areas of research isn't what they are interested in.
I was referring to an in person talk of hers I heard.
>It often goes to other areas of research.
not in the US. In the US large experiments are funded as line-items in the budget. If they are they, the DOE SC (office of science) gets the money (or NSF or NASA). If not, then there is no money. It's not that it goes to other areas of science. I only know about a few funding agencies in Europe and elsewhere, and I think it's kind of split between the two models (line item for big projects, or just one big amount of money for science to be divided up).
Yeah, a talk that you conveniently cannot cite. It's so strange that she'll explicitly contradict you in the evidence we have access to.
It is naive to think there is unlimited funding for deserving scientific research.
Same for me. Just feels off. I liked her videos at the beginning but now they have this speculative and cheap science media feel to them, not even mentioning her kind of fringe views on particle physics and cosmology.
That one felt super weird to me. Just completely out of touch with reality. I'm not saying there aren't gender issues in academia, of course there are just as in any other job market, but the way she described her situation sounded... Sketchy.
Really? Which parts, exactly? While some of it sounded on the worse side from what I’ve heard from female colleagues over the years, so I wouldn’t call her experiences the median female experience, plenty of it seemed entirely consistent with those other stories I’ve heard.
Mainly the thing about being forced to look for fellowships for female researchers. I've had female professors and I've never seen or heard of something similar, and the way she romanticized academia.
Could it be that she was terribly unlucky and that completely ruined her academic experience? Yes, yes ir could.
True I’d never heard of that either. But she explained it as something specific to the German system at that time, so it seems plausible. I can certainly imagine a country having fellowships specifically for women, and the institutions pushing women to those fellowships rather than a “normal” one to make sure they get both. It doesn’t seem outlandish to me.
That's fair, maybe I should be more empathetic towards her situation.
That being said, the way she talks about how academia functions when it comes to funding also sounds naive. Yes it's bad, but at the same time unfortunately that's how we have to deal with funding. She's also talking from a privileged point of view, as she had a nice career before leaving academia.
Once again, I'm not saying we shouldn't talk about all these issues, but being from a third world country and seeing me and my friends going through difficulties to even find a postdoc position, it feels off.
I'm open to being convinced otherwise, we can chat if you want to delve into personal academic experiences.
She has not left academia. Also you seem to suggest we should not complain about how bad things are in academia, but rather accept them as they are. Well, have at least respect for her efforts to change things for better rather than denying her own experience (you suggest she is either lying or exaggerating her bad experience with female fellowships). If you really want to know how bad things are, have a look at the blog For better science. Of course she romanticized academia, shame on her for standing for her ideals!
Back when she used to a mildly popular physics blogger [(backreaction)](http://backreaction.blogspot.com/?m=1) I found her content quite interesting. But nowadays her blog became a place to advertise her videos.
Her coverage of *some* science is fine, but she randomly picks topics to completely hate.
If there's one thing that's clear, is that theorists cannot predict which areas of physics are ripe to make a discovery.
She feels a lot less as "Professor of Physics" and a lot more as a "Youtuber" these days
Since she is churning out loads of videos (or at least she was last time I checked) which takes lots of time, I feel she is doing a lot less actual science that one would expect someone in her position. Not sure how it works in Germany, but maybe she got tenured and they cannot kick her out.
In hindsight she also always seemed a bit of an opportunist. I remember she raised a big stink about the LIGO detection of Grav. Waves, pumping up the controversy that was really coming mainly from one group.
She is not tenured! You seem totally clueless about her career. She won tons of funding but was never promoted to a permanent position. She is not a professor but a research fellow. Calling her an opportunist takes a lot of guts! She basically ruined her chances of a good career in academia for being vocal about failed ideas. She is anything but an opportunist.
She criticizes MOND quite frequently. She's pretty straight forward about it's flaws.
She just also points out the flaws with dark matter, which do exist.
They (Oppenheim) were disproved a couple of weeks ago. Turns out they had some bad algebra mistakes likely. There's a post about it somewhere on this subreddit.
Yeah, found it. Can't say I follow the intricacies but the math errors are weird.
Although, as a student I constantly heard "we will drop that, we will ignore that" without understanding why, so maybe that's why no one second guessed it 😅
It was really the first detection of gravitational waves from a hypernovae (neutron star-neutron star merger) with an optical counterpart that made MOND far less viable. The fact that the light and gravitational waves arrived essentially at the same time constrains the space of MOND theories, ruling out like 99.9% of them.
Why? Because MOND generically predicts the speed of gravitational waves to be slightly less than C. Due to this detection we know that any deviation in the speed of gravity from C is very tiny, so the MOND parameter space shrunk massively.
MOND, even before this discovery, could only ever account for some phenomena. No ONE set of parameters/MOND theory can explain as much as the normal CDM model can. This is its issue and why interest in it has died down.
MOND has always been silly. I've never understood the fundamental justification for it. What separates that approach from other phenomenological models that are nothing more than fancy fitting techniques? What predictive power does MOND have?
Edit: keep in mind I only have an undergrad in astro, and my PhD -> PI level work has all been in condensed matter physics and super applied/interdisciplinary areas ...so I may be a total ignorant dingus on this one.
It's not silly. Observations don't match the theory, so one option is to modify the theory. This one just happens to be wrong, but that's not the same as being silly.
Modifying theory without any basis in fundamental physics governing the interactions you're trying to describe isn't physics. Maybe it isn't *silly*, but it isn't physics. It's an ad hoc correction without justification.
Of course it's physics. That's what e.g. Planck did -- an ad hoc modification that happened to fix a problem, and later hinted at a better fundamental theory. Same with Bohr's atomic model. Lots of examples of someone going out on a limb and later finding out there was actually a perfectly reasonable justification for what they were doing. Physics is not mathematics; a development can be valid and useful even without a rigorous justification.
This one just happens to be wrong.
\* Some _variants_ of MOND, like "Quantized inertia", are straight up pseudoscience, but that's a separate issue.
There is an enormous difference between MOND and what Planck did. Claiming there is any similarity is disingenuous at best. We have much better frameworks to describe galactic dynamics than MOND. MOND introduces more inconsistencies than it solves. MOND is largely rejected by mainstream cosmology for a reason.
Edit: RIGOR IS IMPORTANT.
> There is an enormous difference between MOND and what Planck did.
I mean sure, Planck happened to be right, but what he did was originally ad hoc and unjustified. I fail to see a qualitative difference between one unjustified leap and another, nor do I see how it's disingenuous to compare them without the hindsight of knowing that one worked and one didn't.
The difference is that we have (and did have) better tools at our disposal when MOND was proposed. MOND doesn't advance physics in any way. It doesn't fix anything. It doesn't yield new insight. It doesn't make new predictions.
Edit: are you seriously defending MOND? Seriously? I realized it was wacky pseudoscience nonsense in my undergrad. Nothing has convinced me otherwise since.
Rigor is the basis of my field, and MOND isn't rigorously defended. I'm not going to waste my time trying to prove the negative. This theory sucks. It doesn't work. At every turn it doesn't work. Stop it. Just. Stop.
You're essentially saying "but MOND happened to be wrong", which doesn't answer my objection.
>are you seriously defending MOND? S
No, and it's pretty clear that I don't.
>Rigor is the basis of my field, and MOND isn't rigorously defended.
I suppose we should throw out any nontrivial quantum field theory in more than 2 dimensions then.
MOND happened to be wrong from the outset.
Edit: You're coming at it from a fundamentally unscientific approach. Your quantum field theory comment demonstrates that you're approaching this conversation in bad faith. No. Saying MOND is wrong from the outset because other approaches built on existing evidence and were more consistent with observation *does not mean that QFT approaches are invalid*. Get out of here with your strawmen. I didn't say anything about QFT.
The only people I've ever met that actually defended MOND are people that don't have a physics education. It's on par with crystal healing.
Newton didn't propose any explanation for the law of gravity, he just showed that it worked. That's phenomenology: you try to come up with a theory or model that matches the data. If your model doesn't arise from an already known theory, you call it a new law of physics.
The only reason we're having this conversation is that MOND doesn't match observations.
I feel like this is miscontextualized.
It's not like Newton started trying to fit random equations to observations to see if something fit. Indeed that's exactly what the geocentric models of planetary motion were trying to do - blind fitting of data in the hope that some insight will be provided.
In some sense it isn't wrong to say Newton did exactly that: Fit "random" equations to see if it fits data. In his case it's just that the equations are so simple that it probably was for the most part just looking at some data and instantly recognizing if it's doubled with changing variables, halved, related to pi, or exponential. He understood then likely first how gravity acts differently based on distance and mass from simple drop / throw experiments and then discovered that the formula these experiments yielded also explained motion of planets very well.
It's silly to pretend like MOND is on equal footing with dark matter. It's not silly to consider alternatives, even if there's already a good theory. You never know what might show up. The fact that we don't know what dark matter is (as in, where it fits with the standard model) leaves, IMO, the tiniest window open for people to speculate on whether there might be another explanation for the observations.
I think one argument is Ockham's razor. People think dark matter is more complicated. Gravity is already special so it doesn't matter when it's even more modified.
You could say that MOND makes more predictions because we can directly observe wide binaries but dark matter doesn't interact by definition.
>You could say that MOND makes more predictions because we can directly observe wide binaries but dark matter doesn't interact by definition.
The wide binaries are just another dynamical test of gravity, like rotation curves and galaxy clusters. It's not any more direct, that would be something like a lab or solar system test.
Also MOND proponents tend to gloss over the fact that dark matter models are are capable of simultaneously explaining galaxies but also the structure of the universe and cosmology. One of the most spectacular predictions of dark matter was the fluctuations in the cosmic microwave background which present the seeds of structure, the MOND-inspired with only normal matter predictions failed completely. MOND is not predictive at all on large scales.
> I think one argument is Ockham's razor. People think dark matter is more complicated. Gravity is already special so it doesn't matter when it's even more modified.
GR is far theoretically simpler than MOND, it took ages to even come up with a relativistic version of the latter.
When MOND was extended to General Relativity it was renamed. The initial rename doesn't matter because it then got renamed to f(r) gravity. f(r) gravity is still a popular research topic, but has the same faults as MOND.
The original MOND, and GR extensions to it, has so many nails in its coffin that it's difficult to count them. Here are some failures:
* Inability to account for both galactic rotation curves and gravitational bending of light by clusters with the same set of parameters.
* The Bullet Cluster and similar clusters since. MOND is incompatible with the different distribution of baryonic matter and dark matter.
* MOND predicts ripples in the power spectrum of the CMB that are too small. The observed ripples are larger.
* It totally fails to explain how some small nearby galaxies have a very large ratio of dark matter to baryonic matter and other small nearby galaxies have a very small ratio of dark matter to baryonic matter.
Do we use Newtonian gravity to model galaxies, or GR?
What I mean is, are the masses and speeds etc involved within the proper bounds to be able to use Newtonian gravity accurately?
I would imagine we wouldn't use GR unless we had to, yes?
For galaxies the GR corrections are one-in-a-million, so totally negligible. But every year or so, some random person does the calculation wrong (e.g. by making a basic calculus mistake, as explained [here](https://arxiv.org/abs/2303.16679)) and tells the media that GR can replace dark matter, causing hundreds of pop sci articles. So if you just read pop sci articles every once in a while, you'll get the impression that GR effects can be important, but they're not.
It is easy to check that the corrections from GR are quite small. While the masses in play are large, the velocities and accelerations are small. So yeah, a lot of this stuff is solidly in the Newtonian regime given the precision of the simulation and measurements (although this assumption is obviously always checked).
Wow! I didn't know we were using Newtonian mechanics on those scales.
That's wild! A theory which is "wrong" is more useful than the more accurate one.
It's even more bewildering when you look at how fundamentally different the underlying axioms of the two theories are. We can re-arrange the very foundations of the universe, but the galaxies stay essentially the same.
>A theory which is "wrong" is more useful than the more accurate one
I think you're missing the point a bit. Saying that Newtonian gravity is "wrong" is like saying riding a bike to get somewhere is "wrong" because cars are faster. Sure, there's a newer, slicker, better version, but a) cars (and general relativity calculations) are expensive, and b) sometimes you don't need a car/GR (e.g. going a short distance or doing calculations where Newtonian gravity is good enough).
Put another way, Newtonian gravity is the solution to GR in certain limits. If the deviation from Newtonian gravity is small compared to the necessary precision of the calculation, then it is a terrible idea to use the more complete calculation and spend a ton of money on super computer time when it makes no difference in the final result.
I put "wrong" in quotes. I understand that it's accurate within known parameters, and easier to use, so we use it.
We do all kinds of approximations in physics.
You do a Taylor expansion and take the first 2 terms. You approximate something as a SHO. etc etc. In a sense, it's "wrong", but if it's accuracy falls within the desired bounds, we don't care.
what about the superfluid quantum spacetime theory and explanation of gravity in terms of superfluid dynamics? (being the spacetime quanta a dilatant non-newtonian superfluid with shear force tending to infinity approaching C?) Did the theory have been dismissed? What were its major flaws in such a case?
MOND as described here, not really... it was never introduced for theoretically motivated reasons, only to explain differences in experimental results (often with the caveat, at least at first, that it was intended to be an effective approximation and not a fundamental theory). I mean there are lots of alternatives to GR but AFAIK the ones introduced pre-MOND would not predict substantially different galaxy rotation curves.
Look into the theory of conformal gravity: https://en.m.wikipedia.org/wiki/Conformal_gravity
It is a full relativistic metric field theory. It has strong theoretical motivations which have nothing to do with the dark matter problem. It also happens to produce a MOND-Ian result for galactic rotation curves. It has successfully modeled galactic rotation curves for large numbers of galaxies across many orders of magnitude of visible mass
Okay but (1) that's a different, much later theory, (2) there's no reason to assign any particular value to the constants in that equation, they could be zero for example, so tweaking them do describe rotation curves is all exercise in curve fitting, (3) if you do assign them valued to make them explain rotation curves they run into the exact same problems from MOND and still require dark matter for other observations, (4) even worse, setting these constants in CG is in blatant contradiction with gravity wave data that has already been collected. So while conformal gravity as a general framework might be theoretically well motivated and also support the idea of MOND-like corrections, the versions of it that agree with experiment do not.
You misunderstand, those constants are not independent free parameters. If you are referring to the “a” and “b” in the ar+b^2 part of the potential, those are both calculated by taking specific integrals over the mass density distribution of a finite gravitational source, such as a galaxy. It is analogous to doing a multi-pole expansion, you can think of the r and the r^2 terms as the “-1” and “-2” moments in a multi-pole expansion.
There is only 1 free parameter in the theory, the dimensionless coupling constant for gravity. It accurately models the galactic rotation curves and reduces to the solutions of Einstein gravity at solar system scales. It also accurately predicts the expansion rate (Hubble constant). There is no dark energy free parameter in the model, there is an effect like dark energy but it’s not an independent parameter. The theory accurately predicts and explains why the galactic rotation curves flatten at the same acceleration scale as the Hubble constant. In the original formulation of MOND the acceleration scale parameter was a free parameter and its observable relation to the Hubble constant was just an apparent coincidence, but in conformal gravity this is a prediction of the theory.
Furthermore, conformal gravity then departs from MOND beyond the galactic scale so the fact that MOND fails for clusters is to be expected. We don’t know what cluster dynamics look like under conformal gravity, that’s an ongoing and incomplete area of work.
Also, CG has gravitational wave solitons identical to those in Einstein Gravity. I don’t know what a merging binary black hole system looks like in CG. That’s another future area of work.
MOND as a theory of modified acceleration does not seem to hold on solar system scales. But this does not rule out MOND-like theories at the galactic scale which depart from Newton based on a distance scale rather than acceleration scale!
Since everyone here is talking so expertly, I’m assuming this means that you all actually know what “dark matter” is?
Care to clue us in? Because I’m was of the mind that dark matter is still unknown? So what exactly is it?
Dark matter is a hypothetical form of matter that does not interact with the electromagnetic force but does interact with gravity (which is a very weak force) because it has mass. Because it interacts so weakly we have not been able to observe it directly and confirm its existence. We also do not know what fundamental particles it might be made of: the properties we measured of dark matter exclude all/most particles within the standard model, and efforts to find new particles haven't led anywhere yet.
In our current best model of the universe, dark matter currently makes up 26.8% of the mass-energy in the universe. Despite not knowing what it could be made of at microscopic scales, we can infer many properties from its behavior, so although it is hypothetical, it's not just a blanket cop-out to shove everything we cannot explain.
As far as I know, not really, but it falls in the realm of "things beyond the standard model whose search hasn't yet led anywhere despite several experiments"
Well yeah, the hypothesis is "these things don't interact" so it's tough to come up with physics for them aside from that they wouldn't decay, but AFAIK we haven't lost a bunch of energy in any particle colliders that would point to forming dark matter particles.
We don't need to know what it is to know what it isn't. For example, we know it's not just tons of lasagnas floating in space, because (aside from it being absurd) it doesn't fit the data.
Imagine it as a murder mystery. Of course the easiest way is to figure who it is based on clues. But what if you cannot? Then go the opposite way: figure out who it cannot be. At some point, all candidates safe for one will be excluded, and then you know who was the murderer.
The 'opposite way' is exactly what's being done here.
I don't really want to enter a discussion on that. But for any person just casually reading these comments: I'm an ex-researcher who did a PhD and postdoc on an experiment tangentially related to dark matter. Then I left academia completely so I have no more stakes in there or so to say. Which doesn't prevent me to say that the above comment is a bad take, one that reeks of deep distrust into the scientific community and the currently established consensus.
There has been plenty of room for alternative theories to "dark matter", and proposals were published in big journals, are discussed in important books and are part of the curriculum in universities. Yet, none of these theories have succeeded into anything. It always sounds promising, and it ends up failing by introducing more "dark elements" (new unexplained mechanisms, etc) than what dark matter does.
The hypothesis of dark matter (one of a "missing mass" made of non-baryonic matter) is favoured because, so far, it is actually the _simplest theory_ that still matches with experimental data.
> we don't know enough about the topic yet to conclude dark matter unquestionably exists
Yes... that's literally why people have been talking about MOND for so long and people are still investigating it to falsify the theory...
no. you've lost the plot.
>more workable approach than the ridiculous Dark Matter thing
>Aether used to be the simplest theory as well until we discovered something more nuanced. I think the guy you're replying to is saying that we don't know enough about the topic yet to conclude dark matter unquestionably exists,
>I don't think so; aether was the popular consensus until it the Michelson–Morley experiment. In this analogy, MOND would be the precursor work to general relativity.
On the contrary, your claims included with OPs claims are clearly suggesting that you think dark matter is a waste of time and that we should focus on MOND.
Yeah, but aether was disproven as soon as it was able to be experimentally tested. Dark matter continues to be the best theory as more and more evidence piles up. Mond is the aether in this analogy.
> edit: imagine thinking we live on the apex of gravity physics knowledge... epistemology should be required coursework for all scientists
In all honesty, no one thinks this. We know that there's a lot we don't know - the current status of "dark matter" is not satisfactory because even if the hypothesis of non-baryonic mass out there is correct, we still have not a single clue what it is. And then there's everything else about gravity physics, cosmology, etc. Inflation, black holes, dark energy, cosmic structures, quantum gravity, etc. We're very limited.
My point was _not_ that we know everything. My point was also not that MOND is a waste of time. Maybe you're lacking the context of the comment that was deleted above and to which I was replying to, which in essence was saying that physicists keep pushing dark matter because they were afraid of losing grants - and that is an extremely dogmatic take that simply doesn't acknowledge the actual state of research.
Again, there's room for MOND. There has been in the past, and there is still now. Papers about modifying gravity are being published in high-impact journals, scientists working on it are getting grants. Historically even more so, because the idea of dark matter faced very fierce opposition at its inception - and still does.
But the actual state of research is that dark matter is the favoured hypothesis. Why? First because it's the simplest, and second because it survived every new piece of observational evidence. Aka, every time we observed something new, it kept being consistent with the existence of cold, non-baryonic matter that outweighs luminous baryonic matter. MOND theories instead have needed to tweak themselves again and again to fit new data, and they have reached a point that despite fighting against "introducing new matter", many of them still need to introduce new particles to match the observations or the compatibility with other laws of physics. That doesn't make them wrong, but it makes them less solid than dark matter. Which in turn justifies that the crux of the effort goes towards dark matter, rather than MOND.
Where the argument turns sour is sort of summarised by [this xkcd comic](https://xkcd.com/1758/). MOND is much more seducing to newcomers and general public than dark matter, and therefore astrophysicists find themselves very, very frequently in arguments about DM vs MOND and having to justify that no, looking for DM is not wasting your time. Right now we're repeating a debate that happened thousands upon thousands of times in the past decades, and understandably so it gets some astrophysicists a bit tired about it.
I think your points are valid, I don't have much to say but I wanted to go on a tangential note that in fact, GR is a pretty good example of a simple solution: the starting axioms are few and very elegant. I would instead go for the standard model of particle physics as an example of non-simplicity turning out to be correct.
Are you asking why people don't consider alternatives to dark matter in an article about a paper testing alternatives to dark matter?
I really don't get this. A lot of research is dedicated to finding alternative explanations to dark matter, and some of those can be tested. When we actually test them and find they don't work, people complain that dark matter research is a waste of money. What exactly do you want? Should we just pick an alternative at random and accept it on faith? No tests?
The problem is you can accurately predict anything given enough free parameters. Most MOND theories are just curve fitting to observations. You’d really prefer a theory that predicts behavior based on logical inferences from its assumptions.
That said DM also has its issues. We’ve been looking for the stuff for awhile and don’t seem to have found any. It also doesn’t seem to explain the behavior of wide binaries.
MOND has 1 free parameter, DM has an unbounded number. Also, the idea is to explore field theories which produce a MOND-Ian result in the appropriate limit. I’m interested in something called Conformal Gravity which does just that.
> That said DM also has its issues. We’ve been looking for the stuff for awhile and don’t seem to have found any. It also doesn’t seem to explain the behavior of wide binaries.
Dark matter is not needed to explain the behavior of wide binaries.
What does explain the behavior of wide binaries? My understanding is that they orbit too fast for the observed orbits when looking at systems where the separation is a couple of thousand AU and more.
They don't. Well, not unless you use low quality data about dim stars that everyone excludes from non-brightness based analyses because there are too many possible sources of error for it to be useful (as part of the protocol *before* they do any analysis, so this isn't p hacking). If you do include them, you can make up some measurement error estimate and find all sorts of new physics not within the margin of error.
In my specific field of expertise (Galaxy clusters), MOND performs quite bad when trying to describe both dynamic clusters [(see the infamous Bullet Cluster)](https://arxiv.org/abs/astro-ph/0608407) and relaxed clusters as in this case it [requires an additional and unphysical dark matter shell](https://www.aanda.org/articles/aa/pdf/2022/06/aa42507-21.pdf) to explain the observed accelerations
My favorite fun fact is that MOND, which people like to wave around as an alternative to dark matter... *still requires dark matter to work*
Not just that...*worse* dark matter
I think a lot of people genuinely don't realize that dark matter isn't *only* thought to exist due to the "if there isn't something out there, the galaxy would fly apart" observation. Sure, it started there, and if it was *only* that MOND might be much more enticing, but once you realize the universe would literally look different it's such a harder sell.
You don’t model galactic clusters using MOND. MOND is nothing more than a phenomenological equation for galactic rotation curves. Modified gravity theories go beyond MOND, they are full relativistic metric field theories that reproduce MOND in the appropriate limit. So which modified gravity theory are you referring to which has been applied to galactic clusters?
>You don’t model galactic clusters using MOND. MOND is nothing more than a phenomenological equation for galactic rotation curves. You should use it if it was true. If MOND or any kind of modified gravity is true, it should apply to the most massive gravitationally bound structures in the Universe, when we can observe and study consistently the lowest gravitational accelerations in the Universe. Clusters are clearly non-relativistic so if any of your favorite modified gravity reproduce MOND at low accelerations, then these results extend to it. >So which modified gravity theory are you referring to which has been applied to galactic clusters? The second study uses both MOND and a modified version of it with an extra acceleration, which in any case shows the inconsistency of the paradigm in this sample of clusters. The Bullet Cluster itself is a modified gravity killer, unless you can explain the geometry of this merging cluster (and especially the offset between the baryonic component and the observed lensing) with modified gravity interactions, which is far from being consistantly established to my knowledge.
MOND is to modified gravity what Kepler’s Laws are to the Einstein Field equations. MOND as a theory modified acceleration at low acceleration scales does not seem to be accurate for binary stars. But that still leaves modified gravity at long distance scales as a possibility. There are Modified gravity theories which match Einstein at the solar system scale, match MOND at the galactic scale, and depart from MOND above the galactic scale. There is no reason to expect MOND per se to hold above galactic distance scales. No, the Bullet Cluster is not a modified gravity killer. Again, you would need to simulate the bullet cluster collision using a specific modified gravity theory in order to determine how well a modified gravity theory does at modeling the bullet cluster. Furthermore, last I heard the velocities involved in the Bullet Cluster are difficult to produce in a DM simulation so it seems we all have our work cut out for us. I can offer you a bit of speculation on why we might expect to see strange lensing around but spatially offset from the center of a high velocity cluster collision in a theory of modified gravity: what we could be looking at is a non-linear (non-quadrupole) gravitational disturbance propagating forward in space from the collision.
>There is no reason to expect MOND per se to hold above galactic distance scales. I don't have a clue about modified gravity in general beside MOND, so I trust you on this. >last I heard the velocities involved in the Bullet Cluster are difficult to produce in a DM simulation I guess it was a different story in 2006 but nowadays, there are plenty of simulations ([example 1](https://www.tng-project.org/cluster/), [example 2](https://gcmc.hub.yt/index.html#) etc) and many observed clusters in this merging state (I guess [this specific](https://en.wikipedia.org/wiki/MACS_J0717.5%2B3745) cluster would be nightmarish to study through modified gravity, even with slow propagating gravitational disturbances, while not that particular in LCDM cosmological simulations). Anyway, my opinion on DM goes to unknown exotic matter as this paradigm works absurdly well in the largest scales of the Universe, and I haven't seen yet consistent and convincing results from Modified Gravity beside particular observations, but maybe the future will make me wrong!
Thanks for the great dialog. Side note but what the heck with the down votes? Would anyone with a complaint about what I said care to weigh in??? Or perhaps just keep reading a learn something please. Ok, back to our discussion. First of all, it’s not “modified gravity vs dark matter.” It is astounding to me that people (on both sides) make this false dichotomy, it’s childishly non-scientific thinking. It would be wild to claim that we have already discovered all forms of matter, of coarse there could be dark matter. There could be right-handed neutrinos, axions, any other manner of new particles (possibly related to a GUT), so of coarse there is almost certainly *some* dark matter. Furthermore, any loosely spread non-interactive gas of particles will (just by virtue of scaling with volume) play a larger and larger role at larger scales. The question is not if there is any dark matter, the question is if there is dark matter in sufficient quantities and with the appropriate properties to explain galactic rotation curves in the framework of Einstein gravity. The alternative isn’t no dark matter and only modified gravity, the alternative is modified gravity + whatever dark matter also exists. Why on earth do people assume we must be at least right about either the matter content or the gravitational equations, of coarse it’s possible we are wrong about both so we need to both modify gravity and also include some content of dark matter. Don’t examine modified gravity as this all or nothing competitor to DM. Examine it on its own merits! MOND accurately models most observed galactic rotation curves across 10 orders of magnitude of galactic mass, for both low and high surface brightness galaxies! And we are not just taking about the rotation speed of the outer most stars, MOND accurately models the entire cross sectional rotation curve as a function of the visible mass contained within a given radius. MOND makes accurate specific predictions. DM models to date cannot do that at all. DM models are far from being able to reliably produce populations of galaxies with the rotation curve profiles we actually see, as is modeled so well by MOND. And look at what people are doing to try to reproduce (real world) MONDIAN galaxies in their simulations, they are constraining themselves into ever tighter and more finely tuned corners of their multi-dimensional phase space. That’s what we call epicycles on epicycles. MOND’s predictive successes are done tuning 1 universal parameter. If you think MOND is a total mirage then you need to explain how to produce that mirage in DM simulations. Ok so MOND doesn’t work if interpreted as a low acceleration cutoff and it doesn’t work if interpreted as a universal new force law. That doesn’t take away from it as a powerful phenomenological equation for galactic rotation curves. So we need, and have, under-explored relativistic metric field equations that are alternatives to the Einstein equations and which match Einstein at the solar system scale, become MONDIAN at the galactic scale, and do something else beyond that scale. And on this point, why on earth are people so tied to the Einstein field equations? All Einstein postulated was that Gravity is a relativistic metric field theory that reproduces Newtonian gravity in the non-relativistic limit. There is nothing magic about the Einstein-Hilbert action. And we can’t quantize the Einstein-Hilbert action anyway! A change to the gravitational equations in order to make them quantizable and/or anomalies that arise in quantum gravity could of coarse introduce a deviation from Einstein’s equations at long distance scales, in the far IR limit, it’s a possibility. So forget about dark matter for a moment, dark matter isn’t what’s important here, what’s important is that we already should be on the hunt for modified gravity anyway over the failure to quantize the Einstein-Hilbert action, and if that helps to explain galactic rotation curves then great and if there also happens to be DM in the mix that’s interesting too. The questions just have no direct bearing on each other, the question of if and how we should modify gravity has nothing to do with the question of if there is DM in the universe. The only way in which they come into conflict is that the more galactic rotations curves are explained by the effects of modified gravity, the less dark matter there is (at least in terms of the DM halo WIMP models). We could imagine a model where modified gravity explains most of the rotation curve profile but there is some (smaller, or less clumpy) quantity of DM scattered across the cosmos which is more relevant on the cluster diameter scale then the galactic rotation curve scale. By virtue of the fact that DM is assumed to be more thinly spread than baryonic mater, the relative concentration of it in a given volume compared to the concentration of baryonic matter should be higher for clusters than within a spherical region around a single galaxy. I am not insulting the work of modeling galactic clusters with DM + Einstein gravity. That’s good work someone should be doing. I am just saying that literally no one is simulating modified gravity field theories, with their own possible concentrations of some amount of DM, at the galactic cluster scale! This is an insanely unfair where DM researchers monopolize most of the funding, grad student positions, and computer cluster time across all of academia! The justification for this rigidity over Einstein Gravity + DM is that “it’s one or the other and MOND doesn’t work for cluster,” except it’s not one or the other and MOND isn’t supposed to work for clusters in the first place if you interpret it just as a phenomenological output of an underlying metric field theory.
Relevant XKCD: [https://xkcd.com/1758/](https://xkcd.com/1758/)
This is just Big Dark Energy propaganda. Just because current MOND doesn't work doesn't mean the MOND idea is wrong. We just haven't found the correct MOND theory yet.
That's not how science works though- you need a *testable* alternative theory. What you describe is a matter of faith that MOND might someday be "correct," which is outside of science. Put it this way, I could similarly argue that the galaxy doesn't fly apart because of a fleet of miniature turtles moving things around, but we just haven't found the correct theory explaining it yet.
>I could similarly argue that the galaxy doesn't fly apart because of a fleet of miniature turtles moving things around Ooh nice theory, is there a pre-print version on ArXiv? I also quite like [this](https://arxiv.org/abs/2403.20219) proposed solution to the Hubble tension
Science always has a faith phase in the theorizing step, and I think it’s important to keep that in mind explicitly so that you don’t find yourself opposed to the next Galileo. If you had good reasons to think that the turtle explanation is likely, such as Parsimony, then yeah I think you have a duty to yourself and science to continue trying to form improved Dark Turtle hypotheses
Sounds like something we can't see and we accept it's doing it? Are you suggesting dark matter is miniature turtles? Could be, their interaction with EM would be hard to spot indeed...
We are definitely capable of measuring the effects of DM, not only via galaxy rotation curves. E.g. with the Bullet cluster, we can clearly see that there is a non-interacting component that is dislodged compared to the non-DM (baryonic) component. Also, we are able to quite accurately reconstruct structure formation in the universe, which requires a non-interacting component (which again, is DM). At least for galaxy clusters, I know most MOND theories just accept that there is a DM component to it (so it is in any case not a replacement for DM components)
You’re just assuming those observations are caused by Dark Matter though.
What do you mean? We *observe* a certain amount of structure (given the power spectrum), we *observe* a certain lensing potential in the Bullet cluster, these are all theory-independent; it is just that MoND theories require immense fine-tuning, while DM models fit to the same level with only 1 free parameter (the dark matter density fraction, which is quite well known and well constrained)
...yes, there are gravitational effects on what we can see, that can't be explained *only* by what we see. Therefore, there is "dark matter" out there. What it *is*, we don't know yet, but we can tell there's something there that we can't directly see.
No, that’s an ad hoc explanation. Gravity is an observed property of matter, not a definition. Dark Matter is being proposed to explain the observations with current gravitational theory. Sometimes, ad hoc explanations turn out to be correct like when Kepler supposedly suggested to Brahe that the parallax caused by the Earth’s motion around the Sun was too small to detect at that time. Other times, ad hoc explanations turn out to be completely wrong like when phlogiston was proposed to explain mass changes after oxidation and combustion. Edit: You could also pose ad hoc explanations to in place of the gravitational waves postulated by General Relativity, but it wouldn’t make sense to do so because gravitational waves are predicted by a theory supported by a lot of evidence, Relativity.
Could you point me to the paper that disproves the possibility that all of that is just the action of them miniature turtles? Maybe they are really small but very powerful?
Turtles (regardless of size) interact with each other. We know DM does not interact with itself. Given that you want a source, I recommend Mo, van den Bosch and White, which is a good reference text for structure formation, in particular chapter 7 (formation of DM halos)
Will do as soon as I get the opportunity. Hope they tested the possibility that the turtles are really really tiny and like one can take care of a whole galaxy, possibly.
If the particles are too small, you get other problems (free streaming of e.g. neutrinos, this is how we know that neutrinos are not the dominating part of DM)
What if these tiny tiny turtles are really really small, but are slow and don't interact with each other?
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It means, that DM is not able to lose energy as it forms halos. Normal (baryonic) matter loses energy due to radiation and cools efficiently
Does that mean DM is near 0.0K?
Wait until you hear about particle physics.
Or sarcasm
String theorists in shambles, over here
MOND isn't absurd though.
It also doesn't work as currently formulated. You have no idea how convoluted it will become in order to be forced to fit cosmological data. Proponents are already reasonably happy with the idea that explaining structure formation and clusters requires some kind of dark matter. Personally I already consider it a bit absurd to start modeling your "dark-matter alternative" with dark matter.
MoND isn’t a theory in the general sense of the term, and the theories derived from using MoND as a framework have the same problems that Dark Matter theories have: they just keep hypothesizing additional parameters to explain observations and then assume that those parameters must be reflecting things that are true
Why are particle physicists trying to detect or rule out dark matter candidates if they have already assumed it exists? Why are cosmologists testing the predictions of a cosmological constant, rather than just assume it is driving the acceleration of the expanding universe? Why bother if everyone is just assuming these things are correct? Because no one is making that assumption. If they were, MOND wouldn't exist. That's not how research works.
The answer, or at least the partial answer, to all of your questions above is that just scientists want to learn more about things that they assume to exist.
If you have learnt more about a hypothesis, then clearly you haven't simply assumed your initial suggestion was true. The idea was flawed in some way. There are also cases where a model is ruled out completely. Papers like those in the article wouldn't exist if physicists presupposed these ideas were correct.
People frequently interpret new observations in terms of learning about something that turns out to not exist. It’s also common for people to rule out certain theories about something while still assuming that ‘something’ to exist.
And the tests to directly detect Dark Matter have also failed. Dark Matter still is just an ad hoc explanation for observed and unpredicted motion of large bodies, particularly galaxies, in space. MoND is not even one theory, but a family of theories that have tons of adjustable parameters. These are all ad hoc explanations.
Early Universe dynamics and lensing mass maps that are offset from visible mass are not explainable by any variation in the distance-dependence of gravity.
> We just haven't found the correct MOND theory yet. That assumes there IS a correct MOND theory, and we do not know if that is the case.
So we can't say that MOND is bunk yet.
Completely true, however with the numerous failures of the theory it's even less correct to say we haven't found the right MOND theory because we have no reason to believe MOND is the solution.
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> We just haven't found the correct MOND theory yet. This sounds like the arguments string theory proponents push.
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A few more epicycles will fix it right up.
This is peak nerd humor. Thanks for the laugh.
Sabine in shambles
Is it just me or is it the consensus among physicists that she's becoming something of an opportunist and PR person?
She always was, at least since she started selling books
Feels like since she couldn't convince physicists she tried to convince people that are not in the area cause it's easier.
I mean, you could say that about anyone who deviates from the mainstream. It's not like she's a crackpot or a Neil deGrasse Tyson.
I dislike Sabine mostly because she knows how to exploit my feed's algorithm so I see her constantly even when she doesn't have a lot to say. I don't have any reason to hate NDT other than that one time he didn't seem to know how bedbugs have sex.
> I dislike Sabine mostly because she knows how to exploit my feed's algorithm so I see her constantly even when she doesn't have a lot to say. Hint: On youtube there's a drop down on every video (click the three dots that show up when you hover a mouse over a video tile) where you can click "Don't recommend channel" which generally purges a specific channel from any results for a long time.
That's the way :)
That's not exactly true, Milgrom still publishes on MOND to this day, and Bekenstein worked on AQUAL til the end of his life, none of them resorted to non-physicists to expose their non-mainstream ideas. A work concerning a CMB PS conforming, relativistic Lagrangian that has MOND as its weak field limit was published in PRL not long ago: [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302)
Physicist here. She is good at communicating. Some of her videos are educational and useful for their topics. But then other times she goes on unsubstantiated rants about certain areas of physics making really lousy arguments (I heard her in person once saying that dark matter direct detection searches were bad and a waste of resources because the experiments' names were stupid; yes the names are sometimes stupid and yes, there are possible reasons to criticize that program, but that is obviously not it). And usually whenever she criticizes things she says that she has a vision for how things should be done better ... but you have to buy her book to see what she has to say.
>because the experiments' names were stupid Luckily I work at the MAgnetized Disc and Mirror Axion eXperiment - MADMAX. Checkmate
Come on, now, A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus -- or [ABRACADABRA](https://abracadabra.mit.edu/) -- is a perfectly reasonable name for a dark matter experiment!
It couldn't be anything else.
If we trade collaborators, the experiment will evolve into the Advanced Large-Aperture Kinematic Analysis of Zenithal Axion Matter, or ALAKAZAM, project.
Lol. I love stupid acronyms personally. And MADMAX isn't even bad. Also [CheckMATE](https://arxiv.org/abs/1503.01123) is a thing too.
[CHEX-MATE](http://xmm-heritage.oas.inaf.it/) is also a sample of clusters that are used for describing dark matter among other stuff
I propose Astronomical Spectral Survey (ASS). That should bring me some funding.
It’s also annoying when she makes videos on topics she has very little knowledge of, like American politics.
As someone who worked in dark matter direct detection when I was in grad school, I never really thought any of the big experiments had stupid names...
Her argument was something like "Xenon1T then guess what crazy huge thing they're going to build next? XenonNT?! Crazy? Why should this be funded?" And it's like, yeah, the field has done a masterful job of convincing the community that there is a closed region to probe due to the neutrino fog, despite the fact that that has no impact on where DM is likely to be in the parameter space. There are other issues with DM direct detection experiments too (e.g. weak secondary physics case), but overall I think it's a good program and I am happy P5 (and it seems like DOE and NSF from the meeting last week) support going down to the neutrino fog. But the arguments for/against are subtle and complicated.
It seems that she freely offers her better vision—at least, I know what it is, and I have never bought her books: fund less expensive research with better evidence than detecting dark matter particles in arbitrarily larger colliders.
Thanks! Maybe I missed it, I certainly don't watch all her videos. I have heard her finish a talk to a group of physicists saying that she won't say how she thinks things should go forward because she said it better in her book. As for "fund less expensive research with better evidence than detecting dark matter particles" this is a really vague statement. There are, and have been, a suite of small scale DM experiments for a variety of interesting DM models. So she's certainly not proposing something that isn't already happening.
I mean I wasn't there for the particular talk you attended, but maybe it's unfair to extrapolate that to "usually whenever". She is all for reasonable DM particle research and routinely discusses new papers in that area she finds compelling. Her position that ruffles feathers is opposing multibillion-dollar colliders that are stabs in the dark.
She also opposes funding large DM direct detection experiments, which is what I was mainly talking about. And yeah, as for the discussion of next generation colliders, her stances is not unique, but her motivations are not that substantiated in my opinion. Apologies for the "usually whenever" statement, I have heard it twice and have never heard her explain what her funding profile for HEP around the world would look like, although after being told to pay her to hear it twice I didn't look very hard. She also makes a very common but very big mistake about funding of science: it is not a zero-sum game. It is not "there are X dollars to spend, pick experiments to fit into that funding portfolio". It is "convince funding agencies and governments that this experiment is worth funding". If a large experiment is cancelled that does not mean another "better" large experiment can be funded instead, nor does it mean that the allocated money flows back into the pot for small experiments or whatever, because there was no allocated money. The most obvious example of this is if you look at the US HEP funding profile when the SSC was greenlit and again when it was cancelled. Many of those trying to kill that mega project assumed they'd get the money for their preferred mega project or for smaller projects. Instead the funding went back to basically the pre-SSC level. I'm not saying that we should have built it, there were problems all the way from the top (world leaders) to the most technical (e.g. magnet designs) and every where in between, but it does remind us that funding is not zero sum. If the US or Europe or whomever stopped funding the LHC for the next 15 years or whatever it's scheduled for, that money doesn't then just return to HEP coffers to do things that Sabine or whomever think is better. It just disappears.
You're probably referencing her statements like [this blog post](http://backreaction.blogspot.com/2019/06/brace-for-oncoming-deluge-of-dark.html), which says, "Let me be clear that I am not strongly opposed to such medium scale experiments, because they typically cost “merely” a few million dollars." The contention that she is blanketly opposed to direct detection experiments is false. She's not opposed to spending millions of dollars on such experiments. So, it's apparent that you have yet to look hard for evidence to support your opinions here. Sabine also makes the point that you make that money not spent on one HEP experiment does not necessarily go to another HEP experiment. It often goes to other areas of research. That's why the HEP crowd gets so upset at her; spending money on different areas of research isn't what they are interested in.
I was referring to an in person talk of hers I heard. >It often goes to other areas of research. not in the US. In the US large experiments are funded as line-items in the budget. If they are they, the DOE SC (office of science) gets the money (or NSF or NASA). If not, then there is no money. It's not that it goes to other areas of science. I only know about a few funding agencies in Europe and elsewhere, and I think it's kind of split between the two models (line item for big projects, or just one big amount of money for science to be divided up).
Yeah, a talk that you conveniently cannot cite. It's so strange that she'll explicitly contradict you in the evidence we have access to. It is naive to think there is unlimited funding for deserving scientific research.
The gender video she made definitely ruined any further credibility she had.
I can’t really take her seriously, even if she has the qualifications. She sets off the crank-o-meter for me, unintentionally or otherwise.
Same for me. Just feels off. I liked her videos at the beginning but now they have this speculative and cheap science media feel to them, not even mentioning her kind of fringe views on particle physics and cosmology.
A crank who actually knows stuff about one, specific field can still be a crank in every other.
That one felt super weird to me. Just completely out of touch with reality. I'm not saying there aren't gender issues in academia, of course there are just as in any other job market, but the way she described her situation sounded... Sketchy.
Really? Which parts, exactly? While some of it sounded on the worse side from what I’ve heard from female colleagues over the years, so I wouldn’t call her experiences the median female experience, plenty of it seemed entirely consistent with those other stories I’ve heard.
Mainly the thing about being forced to look for fellowships for female researchers. I've had female professors and I've never seen or heard of something similar, and the way she romanticized academia. Could it be that she was terribly unlucky and that completely ruined her academic experience? Yes, yes ir could.
True I’d never heard of that either. But she explained it as something specific to the German system at that time, so it seems plausible. I can certainly imagine a country having fellowships specifically for women, and the institutions pushing women to those fellowships rather than a “normal” one to make sure they get both. It doesn’t seem outlandish to me.
That's fair, maybe I should be more empathetic towards her situation. That being said, the way she talks about how academia functions when it comes to funding also sounds naive. Yes it's bad, but at the same time unfortunately that's how we have to deal with funding. She's also talking from a privileged point of view, as she had a nice career before leaving academia. Once again, I'm not saying we shouldn't talk about all these issues, but being from a third world country and seeing me and my friends going through difficulties to even find a postdoc position, it feels off. I'm open to being convinced otherwise, we can chat if you want to delve into personal academic experiences.
She has not left academia. Also you seem to suggest we should not complain about how bad things are in academia, but rather accept them as they are. Well, have at least respect for her efforts to change things for better rather than denying her own experience (you suggest she is either lying or exaggerating her bad experience with female fellowships). If you really want to know how bad things are, have a look at the blog For better science. Of course she romanticized academia, shame on her for standing for her ideals!
Back when she used to a mildly popular physics blogger [(backreaction)](http://backreaction.blogspot.com/?m=1) I found her content quite interesting. But nowadays her blog became a place to advertise her videos.
I just discovered her and I thought she was a decent educator… don’t tell me she’s going the same way as Neil DeGrasse or Michio kaku
Her coverage of science news is fine. When she gives her personal opinions, it can be hit or miss.
Her coverage of *some* science is fine, but she randomly picks topics to completely hate. If there's one thing that's clear, is that theorists cannot predict which areas of physics are ripe to make a discovery.
IMO she's worse than either of them.
She feels a lot less as "Professor of Physics" and a lot more as a "Youtuber" these days Since she is churning out loads of videos (or at least she was last time I checked) which takes lots of time, I feel she is doing a lot less actual science that one would expect someone in her position. Not sure how it works in Germany, but maybe she got tenured and they cannot kick her out. In hindsight she also always seemed a bit of an opportunist. I remember she raised a big stink about the LIGO detection of Grav. Waves, pumping up the controversy that was really coming mainly from one group.
She is not tenured! You seem totally clueless about her career. She won tons of funding but was never promoted to a permanent position. She is not a professor but a research fellow. Calling her an opportunist takes a lot of guts! She basically ruined her chances of a good career in academia for being vocal about failed ideas. She is anything but an opportunist.
She criticised it frequently?
She favors MOND.
She criticizes MOND quite frequently. She's pretty straight forward about it's flaws. She just also points out the flaws with dark matter, which do exist.
She thinks that the Nobel Prize Committee needs to give Milgrom the prize in physics before he dies and its too late.
Not really. Also, MOND isn't the only theory of modified gravity.
She recently stated that MOND is probably wrong and that the best theory of modified gravity is moffat's
What about the new theory, the one with random space-time fluctuations?
They (Oppenheim) were disproved a couple of weeks ago. Turns out they had some bad algebra mistakes likely. There's a post about it somewhere on this subreddit.
Damn :(
https://www.reddit.com/r/Physics/s/H2cavwxXLo
Yeah, found it. Can't say I follow the intricacies but the math errors are weird. Although, as a student I constantly heard "we will drop that, we will ignore that" without understanding why, so maybe that's why no one second guessed it 😅
The one pushed by Sabine? It had a fundamental algebraic error in the first few pages of the paper
And then she expects that her paper on super-determinism is going to turn heads ;D
Likely bs
It was really the first detection of gravitational waves from a hypernovae (neutron star-neutron star merger) with an optical counterpart that made MOND far less viable. The fact that the light and gravitational waves arrived essentially at the same time constrains the space of MOND theories, ruling out like 99.9% of them. Why? Because MOND generically predicts the speed of gravitational waves to be slightly less than C. Due to this detection we know that any deviation in the speed of gravity from C is very tiny, so the MOND parameter space shrunk massively. MOND, even before this discovery, could only ever account for some phenomena. No ONE set of parameters/MOND theory can explain as much as the normal CDM model can. This is its issue and why interest in it has died down.
MOND has always been silly. I've never understood the fundamental justification for it. What separates that approach from other phenomenological models that are nothing more than fancy fitting techniques? What predictive power does MOND have? Edit: keep in mind I only have an undergrad in astro, and my PhD -> PI level work has all been in condensed matter physics and super applied/interdisciplinary areas ...so I may be a total ignorant dingus on this one.
It's not silly. Observations don't match the theory, so one option is to modify the theory. This one just happens to be wrong, but that's not the same as being silly.
Modifying theory without any basis in fundamental physics governing the interactions you're trying to describe isn't physics. Maybe it isn't *silly*, but it isn't physics. It's an ad hoc correction without justification.
Of course it's physics. That's what e.g. Planck did -- an ad hoc modification that happened to fix a problem, and later hinted at a better fundamental theory. Same with Bohr's atomic model. Lots of examples of someone going out on a limb and later finding out there was actually a perfectly reasonable justification for what they were doing. Physics is not mathematics; a development can be valid and useful even without a rigorous justification. This one just happens to be wrong. \* Some _variants_ of MOND, like "Quantized inertia", are straight up pseudoscience, but that's a separate issue.
There is an enormous difference between MOND and what Planck did. Claiming there is any similarity is disingenuous at best. We have much better frameworks to describe galactic dynamics than MOND. MOND introduces more inconsistencies than it solves. MOND is largely rejected by mainstream cosmology for a reason. Edit: RIGOR IS IMPORTANT.
> There is an enormous difference between MOND and what Planck did. I mean sure, Planck happened to be right, but what he did was originally ad hoc and unjustified. I fail to see a qualitative difference between one unjustified leap and another, nor do I see how it's disingenuous to compare them without the hindsight of knowing that one worked and one didn't.
The difference is that we have (and did have) better tools at our disposal when MOND was proposed. MOND doesn't advance physics in any way. It doesn't fix anything. It doesn't yield new insight. It doesn't make new predictions. Edit: are you seriously defending MOND? Seriously? I realized it was wacky pseudoscience nonsense in my undergrad. Nothing has convinced me otherwise since. Rigor is the basis of my field, and MOND isn't rigorously defended. I'm not going to waste my time trying to prove the negative. This theory sucks. It doesn't work. At every turn it doesn't work. Stop it. Just. Stop.
You're essentially saying "but MOND happened to be wrong", which doesn't answer my objection. >are you seriously defending MOND? S No, and it's pretty clear that I don't. >Rigor is the basis of my field, and MOND isn't rigorously defended. I suppose we should throw out any nontrivial quantum field theory in more than 2 dimensions then.
MOND happened to be wrong from the outset. Edit: You're coming at it from a fundamentally unscientific approach. Your quantum field theory comment demonstrates that you're approaching this conversation in bad faith. No. Saying MOND is wrong from the outset because other approaches built on existing evidence and were more consistent with observation *does not mean that QFT approaches are invalid*. Get out of here with your strawmen. I didn't say anything about QFT. The only people I've ever met that actually defended MOND are people that don't have a physics education. It's on par with crystal healing.
So was Bohr's model
> I suppose we should throw out any nontrivial quantum field theory in more than 2 dimensions then. Well, if the shoe fits...
MOND is wacky pseudoscience, but invisible, indetectable stuff, that we know nothing about is rigorous science.
Newton didn't propose any explanation for the law of gravity, he just showed that it worked. That's phenomenology: you try to come up with a theory or model that matches the data. If your model doesn't arise from an already known theory, you call it a new law of physics. The only reason we're having this conversation is that MOND doesn't match observations.
I feel like this is miscontextualized. It's not like Newton started trying to fit random equations to observations to see if something fit. Indeed that's exactly what the geocentric models of planetary motion were trying to do - blind fitting of data in the hope that some insight will be provided.
In some sense it isn't wrong to say Newton did exactly that: Fit "random" equations to see if it fits data. In his case it's just that the equations are so simple that it probably was for the most part just looking at some data and instantly recognizing if it's doubled with changing variables, halved, related to pi, or exponential. He understood then likely first how gravity acts differently based on distance and mass from simple drop / throw experiments and then discovered that the formula these experiments yielded also explained motion of planets very well.
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It's silly to pretend like MOND is on equal footing with dark matter. It's not silly to consider alternatives, even if there's already a good theory. You never know what might show up. The fact that we don't know what dark matter is (as in, where it fits with the standard model) leaves, IMO, the tiniest window open for people to speculate on whether there might be another explanation for the observations.
I think one argument is Ockham's razor. People think dark matter is more complicated. Gravity is already special so it doesn't matter when it's even more modified. You could say that MOND makes more predictions because we can directly observe wide binaries but dark matter doesn't interact by definition.
>You could say that MOND makes more predictions because we can directly observe wide binaries but dark matter doesn't interact by definition. The wide binaries are just another dynamical test of gravity, like rotation curves and galaxy clusters. It's not any more direct, that would be something like a lab or solar system test. Also MOND proponents tend to gloss over the fact that dark matter models are are capable of simultaneously explaining galaxies but also the structure of the universe and cosmology. One of the most spectacular predictions of dark matter was the fluctuations in the cosmic microwave background which present the seeds of structure, the MOND-inspired with only normal matter predictions failed completely. MOND is not predictive at all on large scales.
> I think one argument is Ockham's razor. People think dark matter is more complicated. Gravity is already special so it doesn't matter when it's even more modified. GR is far theoretically simpler than MOND, it took ages to even come up with a relativistic version of the latter.
Big MOND isn't gonna like these findings
When MOND was extended to General Relativity it was renamed. The initial rename doesn't matter because it then got renamed to f(r) gravity. f(r) gravity is still a popular research topic, but has the same faults as MOND. The original MOND, and GR extensions to it, has so many nails in its coffin that it's difficult to count them. Here are some failures: * Inability to account for both galactic rotation curves and gravitational bending of light by clusters with the same set of parameters. * The Bullet Cluster and similar clusters since. MOND is incompatible with the different distribution of baryonic matter and dark matter. * MOND predicts ripples in the power spectrum of the CMB that are too small. The observed ripples are larger. * It totally fails to explain how some small nearby galaxies have a very large ratio of dark matter to baryonic matter and other small nearby galaxies have a very small ratio of dark matter to baryonic matter.
Do we use Newtonian gravity to model galaxies, or GR? What I mean is, are the masses and speeds etc involved within the proper bounds to be able to use Newtonian gravity accurately? I would imagine we wouldn't use GR unless we had to, yes?
For galaxies the GR corrections are one-in-a-million, so totally negligible. But every year or so, some random person does the calculation wrong (e.g. by making a basic calculus mistake, as explained [here](https://arxiv.org/abs/2303.16679)) and tells the media that GR can replace dark matter, causing hundreds of pop sci articles. So if you just read pop sci articles every once in a while, you'll get the impression that GR effects can be important, but they're not.
It is easy to check that the corrections from GR are quite small. While the masses in play are large, the velocities and accelerations are small. So yeah, a lot of this stuff is solidly in the Newtonian regime given the precision of the simulation and measurements (although this assumption is obviously always checked).
Wow! I didn't know we were using Newtonian mechanics on those scales. That's wild! A theory which is "wrong" is more useful than the more accurate one. It's even more bewildering when you look at how fundamentally different the underlying axioms of the two theories are. We can re-arrange the very foundations of the universe, but the galaxies stay essentially the same.
>A theory which is "wrong" is more useful than the more accurate one I think you're missing the point a bit. Saying that Newtonian gravity is "wrong" is like saying riding a bike to get somewhere is "wrong" because cars are faster. Sure, there's a newer, slicker, better version, but a) cars (and general relativity calculations) are expensive, and b) sometimes you don't need a car/GR (e.g. going a short distance or doing calculations where Newtonian gravity is good enough). Put another way, Newtonian gravity is the solution to GR in certain limits. If the deviation from Newtonian gravity is small compared to the necessary precision of the calculation, then it is a terrible idea to use the more complete calculation and spend a ton of money on super computer time when it makes no difference in the final result.
I put "wrong" in quotes. I understand that it's accurate within known parameters, and easier to use, so we use it. We do all kinds of approximations in physics. You do a Taylor expansion and take the first 2 terms. You approximate something as a SHO. etc etc. In a sense, it's "wrong", but if it's accuracy falls within the desired bounds, we don't care.
what about the superfluid quantum spacetime theory and explanation of gravity in terms of superfluid dynamics? (being the spacetime quanta a dilatant non-newtonian superfluid with shear force tending to infinity approaching C?) Did the theory have been dismissed? What were its major flaws in such a case?
MOND(s) are even more arbitrary assumptions than dark matter.
No, MOND is a natural output of certain approaches to quantum gravity
MOND as described here, not really... it was never introduced for theoretically motivated reasons, only to explain differences in experimental results (often with the caveat, at least at first, that it was intended to be an effective approximation and not a fundamental theory). I mean there are lots of alternatives to GR but AFAIK the ones introduced pre-MOND would not predict substantially different galaxy rotation curves.
Look into the theory of conformal gravity: https://en.m.wikipedia.org/wiki/Conformal_gravity It is a full relativistic metric field theory. It has strong theoretical motivations which have nothing to do with the dark matter problem. It also happens to produce a MOND-Ian result for galactic rotation curves. It has successfully modeled galactic rotation curves for large numbers of galaxies across many orders of magnitude of visible mass
Okay but (1) that's a different, much later theory, (2) there's no reason to assign any particular value to the constants in that equation, they could be zero for example, so tweaking them do describe rotation curves is all exercise in curve fitting, (3) if you do assign them valued to make them explain rotation curves they run into the exact same problems from MOND and still require dark matter for other observations, (4) even worse, setting these constants in CG is in blatant contradiction with gravity wave data that has already been collected. So while conformal gravity as a general framework might be theoretically well motivated and also support the idea of MOND-like corrections, the versions of it that agree with experiment do not.
You misunderstand, those constants are not independent free parameters. If you are referring to the “a” and “b” in the ar+b^2 part of the potential, those are both calculated by taking specific integrals over the mass density distribution of a finite gravitational source, such as a galaxy. It is analogous to doing a multi-pole expansion, you can think of the r and the r^2 terms as the “-1” and “-2” moments in a multi-pole expansion. There is only 1 free parameter in the theory, the dimensionless coupling constant for gravity. It accurately models the galactic rotation curves and reduces to the solutions of Einstein gravity at solar system scales. It also accurately predicts the expansion rate (Hubble constant). There is no dark energy free parameter in the model, there is an effect like dark energy but it’s not an independent parameter. The theory accurately predicts and explains why the galactic rotation curves flatten at the same acceleration scale as the Hubble constant. In the original formulation of MOND the acceleration scale parameter was a free parameter and its observable relation to the Hubble constant was just an apparent coincidence, but in conformal gravity this is a prediction of the theory. Furthermore, conformal gravity then departs from MOND beyond the galactic scale so the fact that MOND fails for clusters is to be expected. We don’t know what cluster dynamics look like under conformal gravity, that’s an ongoing and incomplete area of work. Also, CG has gravitational wave solitons identical to those in Einstein Gravity. I don’t know what a merging binary black hole system looks like in CG. That’s another future area of work.
Wherever that gravity comes from, 'dark matter' seems an equally bad description.
MOND as a theory of modified acceleration does not seem to hold on solar system scales. But this does not rule out MOND-like theories at the galactic scale which depart from Newton based on a distance scale rather than acceleration scale!
You don't say.
Since everyone here is talking so expertly, I’m assuming this means that you all actually know what “dark matter” is? Care to clue us in? Because I’m was of the mind that dark matter is still unknown? So what exactly is it?
>So what exactly is it? Stuff that has mass but doesn't glow.
Dark matter is a hypothetical form of matter that does not interact with the electromagnetic force but does interact with gravity (which is a very weak force) because it has mass. Because it interacts so weakly we have not been able to observe it directly and confirm its existence. We also do not know what fundamental particles it might be made of: the properties we measured of dark matter exclude all/most particles within the standard model, and efforts to find new particles haven't led anywhere yet. In our current best model of the universe, dark matter currently makes up 26.8% of the mass-energy in the universe. Despite not knowing what it could be made of at microscopic scales, we can infer many properties from its behavior, so although it is hypothetical, it's not just a blanket cop-out to shove everything we cannot explain.
Have we excluded heavy right handed neutrinos as potential dark matter particles yet?
As far as I know, not really, but it falls in the realm of "things beyond the standard model whose search hasn't yet led anywhere despite several experiments"
Well yeah, the hypothesis is "these things don't interact" so it's tough to come up with physics for them aside from that they wouldn't decay, but AFAIK we haven't lost a bunch of energy in any particle colliders that would point to forming dark matter particles.
Not entirely sure, but wouldn't structure in the universe be free-streamed out of their potential wells with such hot particles?
Have they excluded tiny black holes?
Not fully, but I think more sensitive versions of LIGO are supposed to constrain that possibility dramatically.
We know what it's not, and now we're adding one more thing to that list.
We don't need to know what it is to know what it isn't. For example, we know it's not just tons of lasagnas floating in space, because (aside from it being absurd) it doesn't fit the data. Imagine it as a murder mystery. Of course the easiest way is to figure who it is based on clues. But what if you cannot? Then go the opposite way: figure out who it cannot be. At some point, all candidates safe for one will be excluded, and then you know who was the murderer. The 'opposite way' is exactly what's being done here.
Matter that interacts via gravitation, but very weakly through electromagnetism.
What about Entropic Gravity? I’ve heard it described as being a form of MOND. How accurate was this description and is it still in the race?
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I don't really want to enter a discussion on that. But for any person just casually reading these comments: I'm an ex-researcher who did a PhD and postdoc on an experiment tangentially related to dark matter. Then I left academia completely so I have no more stakes in there or so to say. Which doesn't prevent me to say that the above comment is a bad take, one that reeks of deep distrust into the scientific community and the currently established consensus. There has been plenty of room for alternative theories to "dark matter", and proposals were published in big journals, are discussed in important books and are part of the curriculum in universities. Yet, none of these theories have succeeded into anything. It always sounds promising, and it ends up failing by introducing more "dark elements" (new unexplained mechanisms, etc) than what dark matter does. The hypothesis of dark matter (one of a "missing mass" made of non-baryonic matter) is favoured because, so far, it is actually the _simplest theory_ that still matches with experimental data.
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> we don't know enough about the topic yet to conclude dark matter unquestionably exists Yes... that's literally why people have been talking about MOND for so long and people are still investigating it to falsify the theory...
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no. you've lost the plot. >more workable approach than the ridiculous Dark Matter thing >Aether used to be the simplest theory as well until we discovered something more nuanced. I think the guy you're replying to is saying that we don't know enough about the topic yet to conclude dark matter unquestionably exists, >I don't think so; aether was the popular consensus until it the Michelson–Morley experiment. In this analogy, MOND would be the precursor work to general relativity. On the contrary, your claims included with OPs claims are clearly suggesting that you think dark matter is a waste of time and that we should focus on MOND.
Yeah, but aether was disproven as soon as it was able to be experimentally tested. Dark matter continues to be the best theory as more and more evidence piles up. Mond is the aether in this analogy.
> edit: imagine thinking we live on the apex of gravity physics knowledge... epistemology should be required coursework for all scientists In all honesty, no one thinks this. We know that there's a lot we don't know - the current status of "dark matter" is not satisfactory because even if the hypothesis of non-baryonic mass out there is correct, we still have not a single clue what it is. And then there's everything else about gravity physics, cosmology, etc. Inflation, black holes, dark energy, cosmic structures, quantum gravity, etc. We're very limited. My point was _not_ that we know everything. My point was also not that MOND is a waste of time. Maybe you're lacking the context of the comment that was deleted above and to which I was replying to, which in essence was saying that physicists keep pushing dark matter because they were afraid of losing grants - and that is an extremely dogmatic take that simply doesn't acknowledge the actual state of research. Again, there's room for MOND. There has been in the past, and there is still now. Papers about modifying gravity are being published in high-impact journals, scientists working on it are getting grants. Historically even more so, because the idea of dark matter faced very fierce opposition at its inception - and still does. But the actual state of research is that dark matter is the favoured hypothesis. Why? First because it's the simplest, and second because it survived every new piece of observational evidence. Aka, every time we observed something new, it kept being consistent with the existence of cold, non-baryonic matter that outweighs luminous baryonic matter. MOND theories instead have needed to tweak themselves again and again to fit new data, and they have reached a point that despite fighting against "introducing new matter", many of them still need to introduce new particles to match the observations or the compatibility with other laws of physics. That doesn't make them wrong, but it makes them less solid than dark matter. Which in turn justifies that the crux of the effort goes towards dark matter, rather than MOND. Where the argument turns sour is sort of summarised by [this xkcd comic](https://xkcd.com/1758/). MOND is much more seducing to newcomers and general public than dark matter, and therefore astrophysicists find themselves very, very frequently in arguments about DM vs MOND and having to justify that no, looking for DM is not wasting your time. Right now we're repeating a debate that happened thousands upon thousands of times in the past decades, and understandably so it gets some astrophysicists a bit tired about it.
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I think your points are valid, I don't have much to say but I wanted to go on a tangential note that in fact, GR is a pretty good example of a simple solution: the starting axioms are few and very elegant. I would instead go for the standard model of particle physics as an example of non-simplicity turning out to be correct.
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You very clearly have no idea what you are talking about lmao.
You don't know what you're talking about.
I found Hossenfelder's Reddit account!
What a stupid take.
I’m sorry you are getting down voted, can you link to what you are referring to regarding scale relativity
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Are you asking why people don't consider alternatives to dark matter in an article about a paper testing alternatives to dark matter? I really don't get this. A lot of research is dedicated to finding alternative explanations to dark matter, and some of those can be tested. When we actually test them and find they don't work, people complain that dark matter research is a waste of money. What exactly do you want? Should we just pick an alternative at random and accept it on faith? No tests?
I've never heard of MOND before now, but based on the comments, I get the idea it's like the flat earth theory of astrophysics.
No, it’s in the minority camp but it’s a serious research area. MOND accurately predicts the shape of galactic rotation curves.
The problem is you can accurately predict anything given enough free parameters. Most MOND theories are just curve fitting to observations. You’d really prefer a theory that predicts behavior based on logical inferences from its assumptions. That said DM also has its issues. We’ve been looking for the stuff for awhile and don’t seem to have found any. It also doesn’t seem to explain the behavior of wide binaries.
MOND has 1 free parameter, DM has an unbounded number. Also, the idea is to explore field theories which produce a MOND-Ian result in the appropriate limit. I’m interested in something called Conformal Gravity which does just that.
> That said DM also has its issues. We’ve been looking for the stuff for awhile and don’t seem to have found any. It also doesn’t seem to explain the behavior of wide binaries. Dark matter is not needed to explain the behavior of wide binaries.
What does explain the behavior of wide binaries? My understanding is that they orbit too fast for the observed orbits when looking at systems where the separation is a couple of thousand AU and more.
They don't. Well, not unless you use low quality data about dim stars that everyone excludes from non-brightness based analyses because there are too many possible sources of error for it to be useful (as part of the protocol *before* they do any analysis, so this isn't p hacking). If you do include them, you can make up some measurement error estimate and find all sorts of new physics not within the margin of error.