Humble tokamak physicist owns generations of cosmological wankers
scottlocklin.wordpress.com
scottlocklin.wordpress.com
https://phys.org/news/2021-03-factoring-gravitomagnetism-dar...
> Current assessments of galactic rotation curves are based upon a framework of Newtonian accounts of gravity, a new paper published in EPJ C, by Gerson Otto Ludwig, suggests that if this is substituted with a general relativity-based model, the need to recourse to dark matter is relieved, replaced by the effects of gravitomagnetism.
> In this paper, Ludwig presents a new model for the rotational curves of galaxies which is in agreement with previous efforts involving general relativity. [...] eliminating the need for dark matter. The theory still needs some development before it is widely accepted, with the author particularly pointing out that the time evolution of galaxies modelled with this framework is a complex problem [...]
That seems absurd, but that's also what the article is saying.
I think the author's, and other's points over the last decade or so has been "Newton neglects too much in the weak field" and that neglected bit isn't insignificant. The surprise here is that sticking that little bit -- which is needed for gravitational waves and in general the finite speed of gravitation interactions to occur -- appears to eliminate the need for dark matter in galaxy rotation curves.
Most surprising is that no one questioned the Newtonian limit being used despite weak field GR and Newton's laws predicting different phenomena.
The real test will be if CMB and other precision observations also work w/o dark matter.
It seems to me that there is a class of concepts in theoretical physics that some people take an almost instinctive dislike to: dark matter, string theory, eternal inflation and the many worlds interpretation of quantum mechanics are prominent ones. I get the impression that there is a correlation; a person who is sceptical about dark matter seems to be much more likely to be sceptical about certain other ideas too, and to have a more favourable view of alternatives like Modified Newtonian Dynamics and Bohmian Mechanics.
It's as though there is a sort of partisanship in opinion about theoretical physics: just as it is often possible to tell what somebody's view is likely to be about a whole slew of political issues if you know their opinion about one or two controversial topics, the same seems to apply to a large extent in theoretical physics too.
I'm fascinated by this because in the early to mid 2000s I would have counted myself as both a dark matter and many worlds sceptic, but I take the opposite view of both now, and I find it difficult to put myself back into my old frame of mind.
My sense is that the divide is somehow related to different attitudes to falsifiability, abstractness and explanatory parsimony.
It’s basically the physicist version of Kevin Malone’s “A mistake plus keleven gets you home by seven."
For me, it feels like we limit the search because we’ve found a pretty good answer. There’s a fashion and if you aren’t fashionable then you can’t get tenure. We should continue to explore the unfashionable edges.
PS I love the idea that it’s similar to clusters of political preferences.
I think I felt that the most likely explanation for the observations was what are now referred to as MACHOs (Massive Compact Halo Objects); massive but dark objects, like black holes, dust clouds etc., and I presumed there must be a lot more of them than thought. The assumption that the anomaly must be due entirely to a new form of matter seemed to me like jumping to conclusions; an unjustified leap.
One thing that began to change my mind was learning more about particle physics. I found it interesting that certain well-understood forms of matter were known to be oblivious to some of the fundamental forces of nature. Hundreds of billions of neutrinos pass through our bodies every second almost completely without trace because they do not feel either the electromagnetic force or the strong nuclear force. I did know about neutrinos when I first formed my views about dark matter, but I initially didn't connect the two.
As I thought more about the ability of some kinds of matter to ignore certain forces, I wondered whether there might be as-yet undiscovered forms of matter even more reluctant to interact than neutrinos are (neutrinos do at least interact via the weak nuclear force, hence how they have been detected). Given that neutrinos don't feel the electromagnetic interaction that dominates our visible world, perhaps there could be undiscovered forces that the matter we are made of is insensitive to as well. This led me to wonder whether there might be a whole zoo of 'shy' particles, perhaps similar to the normal luminous matter we are familiar with, but with their own complex interactions, structures and forces; existing around us, in the same spacetime we inhabit, but unseen. The idea of mutually invisible and non-interacting worlds existing side-by-side was fascinating to me; this is usually the province of speculative fiction, but perhaps there was some analogue to it in the real universe.
I found that this 'dark sector' physics was an active area of research for credible scientists. My understanding is that the latest work tends to indicate that this kind of self-interacting matter can't be the dominant component of dark matter, but the possibility of a role for unseen structure like this remains open, and the idea was so interesting to me that it prompted me to reconsider my views about dark matter.
I knew that before its detection was first confirmed, antimatter had been hypothesised to exist based on consideration of mathematical symmetries in physics. Working through the consequences of the symmetries of nature turned out to be a very productive method for discovering new particles, and the approach culminated in the theory of supersymmetry. In the early to mid 2000s, supersymmetry was well-established among leading particle physicists, and was regarded as the likey future of the field (along with the Higgs boson, it was a central motivation for the construction of the Large Hadron Collider). Supersymmetry predicted that many more particles and forces should exist than predicted by the standard model of paticle physics, and at some point I learned that it was a feature of supersymmetric models that the lightest particle they predict has the properties needed to explain the abundance of dark matter in the universe. This is known as the 'WIMP Miracle' (WIMP standing for Weakly Interacting Massive Particle). That the leading particle physics theory of the time was predicting the existence of a particle that just happened to have the properties needed to explain the observations that had first led dark matter to be postulated struck me as unlikely to be a coincidence.
I understand that supersymmetry has since fallen out of favour to some extent, partly because of disappointment that the LHC hasn't yet found the evidence for it that the particle physics community was hoping for, but I think it would be fair to say that it remains the most credible framework for future research at the moment.
At around the time I first learned of the WIMP Miracle I was also coming to understand more about cosmological inflation. In addition to its predictions being borne-out by measurements of the cosmic microwave background radiation (the Boomerang and WMAP experiments had recently pubished their results), inflation had several other properties that appealed to me, and the theory also happened to predict a mass-energy density for the universe that corresponded very well with the observationally derived abundance of dark matter.
Since then, more recent work to map the distribution of mass in the universe using gravitational lensing; computer simulations that strongly suggest dark matter is required to explain the observed large-scale structure formation in the universe; and more detailed studies of supposed astrophysical evidence like the Bullet Cluster, have all tended to reinforce the conclusion for me that dark matter probably does exist and is an as-yet undetected particle or class of particles.
I'm still a bit heterodox in my view about it though in that I hold out the hope for a significant role for self-interacting dark matter and dark sector forces, because I still think that's potentially the most interesting thing about it.
Not OP, but I think for most educated laymen if you said "15% of the total matter in the universe is unaccounted for and must be something exotic that we so far have been unable to detect" they would respond with "that's fascinating, I'm super excited for the physics community to discover what it is". However, when you flip it around and say, "all the matter that we know about only accounts for 15% of the total matter in the universe, the remaining 85% must be something exotic but we've never been able to detect it" the response tends to be "have you double checked your math?"
It is true. The common feature is, "Sounds like BS to me." The solid answer to them, as to the previous quantum, relativity, and even neutrino skeptics, now satisfied, is evidence, but that stuff has been raggedly thin lately.
This, at the same time that we hear, loudly, "extraordinary claims demand extraordinary evidence".
Thank you, just ordinary evidence will do. The more the better.
I don't think dark matter skeptics are especially enamored with MOND or supercausality; I think they are just rationally suspicious of a one-theory field that prefers to sweep contrary evidence under the rug, while parading the dirty laundry of challengers; and want to see things kept honest.
I have no dog in this fight, but I have seen more than enough echo chambers operating in what is supposed to be science.
Speaking of which: This might be the only useful result, now and forever, of all the $billions spent chasing Tokamak: a generation of physicists not afraid of plasma fluid dynamics. God knows there will never be even one solitary erg of competitive commercial energy production from it.
And, while I'm here: "magnetohydrodynamics" is an extremely limited sub-field of plasma fluid dynamics, involving just the trivial parts. Astrophysicists seem to hate the expression "plasma fluid dynamics", and say "MHD" whenever they feel like they can get away with it. I have interpreted it as shame that they cannot cope with the mathematics that doing real PFD involves anywhere outside of the wholly artificial circumstances where MHD maths work; but I am open to alternatives.
Wait a second there. There are different attitudes about falsifiability? Isn't there a unanimous consensus that something not falisifiable is not science?
Yes.
My understanding is that Karl Popper first proposed falsifiability as a criterion for a scientific theory in part because he found that some supposedly scientific ideas (Freudian psychoanalysis was a particular target of his) failed to make predictions that could be definitively tested: it was a feature of these 'theories' that they could be interpreted in such a way as to accommodate any conceivable observation.
I think you are right that there is a strong consensus among scientists that for a theory to be considered scientific, it must make predictions which can be tested empirically, so that those predictions can be either verified or falsified. The difficulty some scientists have with a strict attitude towards falsifiability though is that many theories that make falsifiable predictions in regimes where the theory can be tested, also make consequential predictions about phenomena that are not readily observable, and might not even be observable in principle.
For instance, observations suggest that the universe is expanding at an accelerating rate; that the pace of expansion increases with distance; and that the expansion is unconstrained by the speed of light. This means that there are vast numbers of galaxies which were once receding from the Earth slowly enough that signals from them (e.g. light that can be imaged by telescopes) could be detected on Earth, but which are now receding from Earth faster than light. Without instruments capable of probing the universe with signals that can exceed the speed of light (something that is currently thought to be impossible), the future of these distant galaxies that have crossed the 'particle horizon' [1] will be forever unknowable to observers on Earth. Predictions about the future evolution, or even existence, of those galaxies are therefore not strictly falsifiable, because it would be impossible to make the necessary observations. Despite this, astronomers have no reason to believe that those galaxies cease to exist, or that the known laws of physics cease to apply to them, once they can no longer be observed by scientific instruments on Earth.
In other words, apparently credible scientific predictions could be made about a large and ever-increasing volume of the universe, but those predictions could never be tested observationally: they would be unfalsifiable. Currently accepted models suggest that, in the distant future, the entire universe except our local group of galaxies will recede into this unobservable domain, and will therefore be inaccessible to any astronomers around at the time.
String theory similarly makes predictions that might never in principle be testable (and thus falsifiable) because doing so is thought to require experiments that would necessarily collapse into black holes were they ever to be attempted.
Proponents of the Many Worlds interpretation of Quantum Mechanics arguably have a similar problem in that it is typically thought to be impossible to interact with the hypothesised other worlds in a way that could confirm or refute their existence. The cosmologist Sean Carroll (who is a proponent of the Many Worlds interpretation) has written a paper on this subject titled 'Beyond Falsifiability' [2], which is discussed on his website [3].
The physicist David Deutsch, who has played an important role in establishing the theoretical basis of quantum computation, has argued that a quantum computer could effectively demonstrate the existence of the other worlds of the Many Worlds interpretation [4], so falsifiability is perhaps less of a problem from that perspective.
[1] https://en.wikipedia.org/wiki/Particle_horizon
[2] https://arxiv.org/abs/1801.05016
[3] https://www.preposterousuniverse.com/blog/2018/01/17/beyond-...
[4] https://thereader.mitpress.mit.edu/the-many-worlds-theory/
Many worlds interpretation rules. Eternal inflation is also a cool idea.
‘Matter’ makes it seem like a specific thing when it was just a made up thing because we didn’t know some other thing. (I like talking in past tense about dark matter)
In general I don’t like the way science is taught as if we already know everything. It would be much better to make more of the things we don’t know. That would be a lot more inspiring.
As it turns out, I'm also working on a fusion device.
It is not necessary for the result to match observations in any but a sampling of galaxies. In particular, they don't need to explain any other phenomena than rotation curves. With those matched, it will be your and your colleagues' responsibility to show why those galaxies lack the dark matter all galaxies are supposed to have, and to explain how your cosmic background curves accommodate the painfully clear lack of sufficient expected dark matter.
The number of galaxies found to have no room for dark matter will only increase.
If your theory accommodates any random amount of dark matter equally well, that removes dark matter from the list of reasons to believe it, and makes having promoted that as a reason a fib.
This 'result' isn't going to hold up.
I'm scientist myself, but lucky enough to have other means to live, so I can do science (and I publish at a good rate) on my own accord.
The model used in this paper is less simplified and correspondingly harder to solve, but note that it is still a simplification. If this paper’s claims are borne out, it is a methodological advance, not a foray into something cosmologists have been ignoring (let alone purposely ignoring).
This article’s failure to present what the actual paper says instead of a hobby horse narrative in no way absolves you from rudely accusing someone of incompetence. If you don’t like fibbing, I’d focus on backing your opinions more solidly (crazy idea: read the introduction of the paper) before you attack others.
If the previous relativistic models failed to produce results different from Newtonian gravity, it was, apparently, because they were wrong. Now, I don't claim to know anything about how to simplify GR solutions, but I know of people whose job it is to be able to simplify GR solutions correctly; and to be able to tell whether they have been simplified correctly. If they are getting that part of their job wrong, what reason do we have to believe they are doing the rest of their job any better?
Even if simplifying the maths is too hard for most of them, some one or other might have thought to run a direct numerical simulation, without any simplification. (Maybe have an undergrad do it? Or, every undergrad?)
I don’t quite follow this objection. Do you mean there are more free parameters in the model than data points so they could have fit anything?
https://backreaction.blogspot.com/2019/10/dark-matter-nightm...
https://backreaction.blogspot.com/2018/07/evidence-for-modif...
https://backreaction.blogspot.com/2018/04/no-that-galaxy-wit...
https://backreaction.blogspot.com/2018/03/modified-gravity-a...
https://backreaction.blogspot.com/2016/10/modified-gravity-v...
As a counter, there's this article which lists a daunting number of phenomena that dark matter explains which competing theories have to also explain, and why that's very difficult: https://medium.com/starts-with-a-bang/only-dark-matter-and-n...
Well, yeah, when your model is a fudge factor that is independently parametrizable at every cubic parsec of space, you'd expect it to be able to explain a ton of things.
It wholly suffices for it (it being GR!) to account for rotation curves, and nothing but rotation curves. It doesn't even need to account for all curves, just a bunch. The more it does, the less DM there is room in the universe for.
It will then be Dark Matter advocates' problem to explain why it exists almost nowhere except in places that needed fudging.
I think it is if considering that she is one of the loudest voices talking negatively about the FCC - I'm personally not convinced either way, but when she is publishing slightly snarky things like this (http://backreaction.blogspot.com/2020/12/well-actually-10-ph...) it just seems like there's a personal vendetta of sorts going on. I'm not saying she's wrong but rather that it just leaves a bad taste in my mouth.
Most /r/physics threads where she is mentioned have fairly good dialectics about her
> Isn’t it obvious the visible stuff is separated from the center of the gravitational pull? But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Indeed, one would expect that modified gravity too should have a path dependence that leads to such a delocalization as is observed in this, and other, cluster collisions.
Indeed since any new dark matter particle would also be a new field, dark matter theories vs modified gravity theories are actually not that different -- both add new fields, the difference is just what kind. I mean OK obviously what kind matters a lot, but the point is that the difference isn't quite as large as it's currently made out to be, and things like the Bullet Cluster that show a matter-vs-gravity distinction aren't the slam-dunk evidence against modified gravity that they're commonly claimed to be.
More significantly, however, and regardless of whether the point about some galaxies behaving as expected is a problem for MOND, the issue here is whether they are a problem for the resolution posited in the paper under discussion.
My guess is that your point here is something like this: MOND did not start as a specific theory, but perhaps something more like a principle, or a question along the lines of "can the anomaly in galaxy rotation be explained without introducing a new type of matter?" (Though by the time I heard of MOND, it took the form of a specific proposal that made specific predictions.)
If this was any other topic than gravity - electrodynamics, for example - the introduction of extra fields, would, I believe, automatically imply additional particles, but I suppose that, without a quantum theory of gravity, this is not necessarily the case here.
So, one can certainly say that modified gravity too could have a path dependence that leads to the delocalization observed in cases like the Bullet Cluster, but MOND still needs a theory which predicts (or at least explains) those observations. This does not go away just because it is facing even greater challenges in getting the early universe right.
None of the above should be construed as a claim that dark matter exists. Also, the more relevant point that I made in the second paragraph of my original post here is independent of any of this, I think.
Like, if you pick a specific interpolation function and value of a_0, and say OK this only applies to gravity (and rewrite the equation appropriately), then I guess this would be a theory of gravity, but it's one we'd know to be wrong, as it's just a simple modfication of Newtonian gravity that doesn't account for relativistic effects at all. It would presumably also violate conservation laws all over the place. The problem then is to come up with a proper theory of gravity that replicates both GR and MOND (and presumably have giant noticeable failures of conservation all over the place). There have been a number of attempts at this.
Or just see Wikipedia: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ov...
> By itself, Milgrom's law is not a complete and self-contained physical theory, but rather an ad hoc empirically motivated variant of one of the several equations that constitute classical mechanics. Its status within a coherent non-relativistic hypothesis of MOND is akin to Kepler's Third Law within Newtonian mechanics; it provides a succinct description of observational facts, but must itself be explained by more fundamental concepts situated within the underlying hypothesis. Several complete classical hypotheses have been proposed (typically along "modified gravity" as opposed to "modified inertia" lines), which generally yield Milgrom's law exactly in situations of high symmetry and otherwise deviate from it slightly. A subset of these non-relativistic hypotheses have been further embedded within relativistic theories, which are capable of making contact with non-classical phenomena (e.g., gravitational lensing) and cosmology.[13] Distinguishing both theoretically and observationally between these alternatives is a subject of current research.
I'm still confused by your statement of the goal: "The problem then is to come up with a proper theory of gravity that replicates both GR and MOND." Surely the problem is to come up with a theory - any theory - that is a) consistent with observations, and b) makes testable predictions?
If that's the point then I agree, but isn't MOND having some difficulty coming up with a single dynamics that works in all cases?
Meanwhile, dark matter is having some difficulty in finding a particle that works at all.
This has been my impression for a while. I won't be surprised if dark matter stops being necessary for galaxy rotation curves but sticks around to explain the CMB spectrum.
That sounds like dark matter with extra steps. If normal matter isn’t producing the field it’s just another unknown phenomenon.
I am just wondering: if GR explains the rotation curve, how can GR still explain the shapiro delay or lensing?
If it accounts for rotation curves, or even just a substantial fraction of rotation curves, that's a box checked: GR and (those) galaxies' rotation turn out consistent after all.
Then it will be DM's problem to explain why DM shows up only where a fudge is needed.
My expectation is that the universe will not cease to mystify for a long time. We can live with the Bullet Cluster losing its (maybe too-) convenient explanation.
https://sciencenode.org/feature/toward-a-realistic-cosmic-ev...
(Note: frame-dragging is another term for gravitomagnetism)
To provide an IT analogy, imagine being tech support for a bunch of alpha-cowboy engineers. They’ll tell you they did all the obvious things, they’ll object to being asked to perform simple diagnostic tests (“does the problem go away if you create a temporary new user account on your system?”), they’ll resent you for not having an immediate solution, and they’ll resent you when you find the basic step they missed. Yes, they’ll accept the solution you offer, but they’ll fight you tooth and nail until it’s unarguably proven that you’re right.
That’s often what cross-domain innovation in science is like.
EDIT: per comment below, this is a much better explanation: https://www.smbc-comics.com/comic/how-math-works
it seems more accurate to say that they retire and the new generation of physics professors are more open to those ideas because they encountered them as students before building a career on some sub-sub field which migh have ended up as a dead end...
I just wonder how many biochemists will need to retire and get Alzheimer's before work on suppressing amyloids is finally abandoned, and something that could possibly work gets a grant.
In the meantime, take your valacyclovir!
While I imagine that does sporadically happen here or there, it's clearly not the usual form of participation :)
I bet it gets more clicks than a moderate tone would.
"Cosmologists hate him! See how he solved dark matter with this one weird trick..."
There are too many academics with huge egos. Too many papers obfuscate simple results behind unnecessarily complex and brittle mathematical models, because huge formulas and proofs make the authors look smart, and impressing the reviewers is a requirement for publication.
Everyone who has a PhD in engineering knows professors who make careers out of solving made up problems with tenuous connection with the real world, and thus highly questionable utility. Maybe I'm old fashioned, but I think engineering is about solving practical problems and building shit.
And then in physics, it's apparently now mainstream to produce theories that aren't testable. Of course, the math has to be very sophisticated and specialized -- dead-end work with simple math wouldn't get funded, much less become an entire field of study.
Which is to say: explaining reality is hard, and all the linear problems have solutions. Today, some find it more convenient to invent another universe that still has open problems, and solve those. To obfuscate, in comes a ton of math with weird notation invented with colleagues who also play this game.
It would be awesome if 3 generations of researchers got scooped by an applied physicist from a different field because nobody bothered to model galaxy rotation curves with general relativity. GR would be too boring, entirely lacking in crazy math.
I find him hilarious but I suppose it's a cultural thing and I understand that lots of people find it degrading.
No idea. But the question that bothers me is: is he right? I mean, is this idea something obvious? I'm not even asking if it's true, just if this is one of the first explanations you should come up with when facing this particular problem.
Because, in that case, it's puzzling.
That's an incredibly small effect. I doubt it will get measured on the ground in a lab.
1 - https://en.wikipedia.org/wiki/Gravitoelectromagnetism#Gravit...
There are other astronomical observations that we still don’t have as good an explanation for (e.g. certain gravitational lensing observations) but this is a step in the direction of being able to dismiss the need for dark matter from our cosmological theories.
If you're a galaxy spinning around, as one does, and you use simple Newtownian physics to calculate the orbits, then there is a problem because at the velocity things are moving, they should be moving apart. And yet they are what appears to be otherwise stable orbits around the galactic center.
So to solve that conundrum, one postulation has been that there is more mass in the galaxy than we can see. Since we can only see things that either emit light, or occlude things that are emitting light, that missing matter has been dubbed "dark."
The paper, on the other hand, says "No, the force holding this thing together is the result of graviomagnetic forces generated by the masses moving around." (not something Newton ever considered, and really only comes into play at very large scales and very large velocities)
If you accept the mathematics and the model in the paper, then it means you don't need dark matter to explain galaxy shape, which would mean that a number of physics Phds work is incorrect in describing what is going on.
The correct equations are mathematically similar to Maxwell’s equations - and plasma physicists are the meanest mofos there is when it comes to E&M. No short cuts there.
So some plasma physicist tried to find what the dark mass of the universe is of the correct set of equations are used. He was surprised that the solution doesn’t exist. When he solved it, he found out that dark matter doesn’t exist.
Hypothetically, a disk that's spinning sufficiently fast, would create the antigravity effect. The only problem is that it would need to spin unrealistically fast.
"In the present article a new model for the rotation curve of galaxies is developed including the effects associated with mass currents...
... It is shown that the inclusion of the gravitomagnetic field clarifies some inconsistencies of the standard potential theory."
> Although filled with controversy, the studies of the galactic rotation via general relativity [16,17,18] reach the same basic conclusion of the present paper. Namely, the dragging effect of a gravitomagnetic component (time-space component of the metric) explains the flat rotation curve at large distances, without the recourse to dark matter. The main problem with the general relativistic models is the very limited number of exact solutions of the Einstein field equations [...]
> The calculated motion in the general relativistic approach corresponds to the free-fall of test particles in the assumed metric, without consistently considering the effect of the large galactic mass density distribution on the metric. Actually, this is the same objection that can be made to the classical approach [...]
> Fluid models, as used in the present paper, consider the mean motion of particles in the Vlasov-like collective gravitational field. The gravitoelectromagnetic weak field approximation to general relativity, used in the present paper, leads to a tractable self-consistent solution of the galactic rotation problem.
Citation [16] in particular is a reference to https://arxiv.org/abs/astro-ph/0610370
> Chua in his 1971 paper identified a theoretical symmetry between the non-linear resistor (voltage vs. current), non-linear capacitor (voltage vs. charge), and non-linear inductor (magnetic flux linkage vs. current). From this symmetry he inferred the characteristics of a fourth fundamental non-linear circuit element, linking magnetic flux and charge, which he called the memristor. In contrast to a linear (or non-linear) resistor the memristor has a dynamic relationship between current and voltage including a memory of past voltages or currents. Other scientists had proposed dynamic memory resistors such as the memistor of Bernard Widrow, but Chua introduced a mathematical generality.
How amazing would it be to identify a quirk allowing us to manipulate gravity using flux and charge! I'm probably misunderstanding this as I'm not even grasping the fields these concepts involve, but, one can dream.
[0] https://en.wikipedia.org/wiki/Memristor#The_memristor_as_a_f...
Obviously, racism exists, I just don't like the hand washy application of "structural racism." i.e. we're all guilty, or it's not me it's society's fault.
If I understand the concept of graviomagnetism as presented in the paper correctly (and there is certainly a big possibility that I do not!) then the motion of mass through spacetime creates a gravity field like the wake of a boat through water. And presumably this is a source of gravity waves when two black holes are orbiting each other.
emtpy space does have energy and energy equals mass
But if moving masses can create moving "magnetic like" gravitational fields, then one could plausibly come up with some history of motion of conventional matter that could create the configuration of gravitational fields that we see in the bullet cluster. Can any such history result in the observed current configuration of conventional matter? I don't know.
Note well: I have only a BS in physics, and have not stayed in the field. Take all opinions with a train of salt. (Yes, I stole that typo from someone here on HN. Don't remember who, but it was worth doing deliberately.)
I thought this was already known to be helpful?
Uuuuummmmmmmmm..
I've had the same idea a few years ago. It's nice because the effect is that the moving matter in a certain orbit causes an increase in the centripetal acceleration (i.e. higher velocity) for orbits further out, and a reduction for orbits further in, which is in the right direction to flatten the rotation curves.
Being a procrastinating amateur and not knowing/being able to invent sophisticated techniques such as the one in the article, I didn't fully go though with the calculations, because after some unsuccessful attempts I convinced myself that the effects involved are way too small to matter.
Consider two equal electric charges a certain distance from each other. If they move perpendicularly to the line between them at constant velocity, there will be an attractive force due to the magnetic field that opposes the electric repulsion. However, the ratio of the magnetic to electric force is the familiar (v/c)^2, at least for non-relativistic speeds, and in any case the magnetic force can never exceed the electric force, because perpendicular distances are preserved in the Lorentz transformation. Hence the most the magnetic force can do is slow down the repulsion between the charges, exactly in the amount consistent with time dilation in the moving frame (where the charges are stationary so there is no magnetic field).
Transposing that to gravitomagnetism, I think that within that approximation the situation is the same. At the very best the gravtomagnetic force can equal the gravitoelectric force when masses move close to the speed of light, making it mathematically impossible to account for the DM mass being greater than the baryonic mass. On top of that, at typical velocities in galaxies the magnetic effect is ~1M times smaller.
Together with the facts that 1. there are other unexplained phenomena that won't be explained by this, and 2. it's trivial in retrospect so probably some truly smart person thought of it already and discarded it a minute later without even thinking it's worth publication, I just abandoned it, always wanting to get back to it eventually just for fun when I have the time (sadly this seems to always mean never).
Fast forward a few years, and I see this article on HN, and really regretted having been lazy with my dismissal, maybe I missed something along with everyone else, and should have really done the calculations. So I found a few hours this weekend, learned julia for the Nth time, and finally did what I should have done years ago... and as expected the gravitomagnetic effects have zero material impact on the rotation curves.
Now, admittedly my calculations are not fully correct, but I think my approximations are reasonable enough that they wouldn't be many orders of magnitude off, validating the intuition about relative strength of electric vs magnetic force. So I'd be very surprised if the results in the article turn out to be right.
I don't think it's fair to attack scientists for not having simulated galaxies with full GR. They realized with much less work than me that it does not matter. At the sizes, masses, and velocities of galaxies, the effects are minuscole, and using Newtonian gravity is sufficient, especially if the phenomenon to explain is so large. It's like in the solar system: GR can explain Mercury's precession, but it wouldn't help if we had orbital anomalies equivalent to effect of 5 solar masses scattered throughout the orbital plane.
Anyway, another thing I've put off forever is making an HN account after a long time reading. So since I saw this story here and it made me finally get that idea out of my system, what better occasion to also create an account and get that out of my system too :)
Something's been bothering me and for whatever reason, the term "Gravitomagnetism" has come up, oddly, several times in the last two days. Oddly because this paper was the start of it. Two days ago I was repairing a brushed DC motor and started thinking about magnetism more generally, which made me realize I've "forgotten" what little fundamental knowledge I used to have about magnetism. My first search was to search the relationship between gravity and electricity[0].
And after a mess of reading, I happened onto a pretty poor set of explanations about what the term "Gravitomagnetism" meant and after a lot of reading, I wasn't entirely clear if that gravitomagnetism was mainstream or crackpot (partly due to fascination reading those Air Force patent articles reducing the amount of time spent actually learning something).
I'm guessing there's an obvious answer and my ignorance of the subject is at fault, but as I read about Dark Matter, it feels like an "excuse". Going simply from Wikipedia[1], "Observational Evidence", the evidence for its existence starts off with something that can be casually paraphrased as "Given our best understanding of how things should work, when we actually measure how things are working, we find the numbers don't work, so 'Dark Matter'."
And as you get further down the evidence list, you start to find things like "why there's so much of it" (except where in the heck is it all?) and why Galaxies are important for observing it. I know my mind is probably polluted with outdated (and, let's face it, probably often bad) information from various reporters covering a subject they know less about than I do (and that's not saying much), but as I read that over, I kept thinking "it really sounds like an idea that is better explained by something else".
Am I completely off, here?
[0] Which led to a bizarre hoax called the Hutchison Effect, several Quara articles of various quality, and all of those weird Air Force patents (due to another questionable 'effect' named after its supposed identifier).
[1] https://en.wikipedia.org/wiki/Dark_matter - no, I'm not expecting this is the best explanation, but it's the best I've got!