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> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measureme

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is wrong. no other field would invent invisible stuff to explain that the model is still right. (well, maybe finance)

fuhrysteve 10y ago
> I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. I have always had the same inclination. It seems like a classic candidate for using Occam's razor. Would love to hear someone who knows more about physics weigh in with a more nuanced viewpoint.
- deleted 10y ago
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- pja 10y ago
  (I have a physics degree, but it’s very rusty at this point) Particle physics has a long history of noticing gaps in the experimental data that don’t match up with current theories and positing that maybe a new particle would solve the problem. This goes right back to things like the neutrino (which was posited to preserve momentum in certain particle interactions IIRC) and even the neutron. Sometimes the suggestion pans out, sometimes it doesn’t. You can see this kind of thing in action - when there was that anomaly in the CERN data a year or two ago there were a flurry of papers suggesting possible particles, none of which panned out because the anomaly turned out to be statistical noise. The various particulate explanations for the observed discrepancy between the spiral galaxy rotation curves and the observed distribution of matter are very similar: Take a gap in the data & speculate about what kind of particle would fill the hole. When you combine the fact that, historically, speculating about whether new particles could explain anomalies in the observed data has been very productive for C20th physics with the reality that physicists are also loath to toss aside general relativity (which has been spectacularly successful) just because of one observed anomaly that has yet to be explained, it’s entirely unsurprising that physicists are keen on new particles as explanations for the galactic rotation curves. At the same time, every physicist will happily admit that there’s a great big hole in the standard model - i.e. that it doesn’t include gravity at all - and that any unifying theory might also explain away the galaxy rotation curves in a way that doesn’t require new particles. Indeed, if a unifying theory could pull off that trick it would be evidence that the theory has a good chance of being true. Sadly, to date, no one has managed to come up with such a unification, and we have no real idea what such a theory would look like beyond the obvious: that it should explain the observed facts & match QFT & GR in the regimes where those theories are accurate.
  - pka 10y ago
    My impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.
    - pja 10y ago
      No, there is no quantum theory of gravity & QFT doesn’t include gravity at all. The best you can do is add gravitational effects as an after-thought, but the things you might analyse with the full QFT are generally far too small & short lived for gravitational effects to matter anyway.
      - raattgift 10y ago
        > No, there is no quantum theory of gravity Here's a couple of examples of quantum theories of gravity. http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-talks/FrankSaueressig13.pdf http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-tal... http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html The extent to which these are complete, consistent, natural (in the fine-tuning sense), and so forth is debatable but these are certainly existing examples of quantum theories of gravity, and indeed the first is a perfectly reasonable Effective Field Theory that people work in regularly. > QFT doesn’t include gravity at all I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity). > the things you might analyse with the full QFT Atoms and molecules have gravitational fields; when you send one through a double slit, which way does their gravitational field go? http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.html http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.... A quantum theory of gravity is needed to answer that. Assemble a huge number of particles in superposition with 1/(sqrt 2) (|M @ a> + |M @ b>), with a & b separated. A quantum theory of gravity is needed to describe the gravitational influence of M on a small test object (General Relativity's answer is just wrong :( ).
- roywiggins 10y ago
  Adding in a single extra particle is a lot simpler than rejiggering the entirety of our theories of gravitation. It's also a lot easier to test, just go out and find the particle. This worked for neutrinos, antimatter, the Higgs boson... The Higgs boson took a long time to be discovered, but people thought it would be there because the alternative was the Standard Model being wrong. And so they looked, and there it was. Perfectly compatible with the Standard Model.
- mikeash 10y ago
  If you're looking at the sky and there appears to be a lot more stuff there than just the bits that emit light, wouldn't Occam's Razor suggest that the presence of more stuff is the most natural explanation, rather than rejiggering the fundamental laws of physics?
  - roryisok 10y ago
    It's more like you're looking at the sky and what you can see doesn't weigh as much as it should. Does Occam's Razor suggest that there's invisible stuff? or that you're weighing stuff wrong?
    - mikeash 10y ago
      Except it's not, because the "missing" mass is not distributed in the same way as the stuff that emits light. Some places have a lot more "missing" than others.
- olavk 10y ago
  Occam's Razor would only apply if the theories made the same prediction with and without the dark matter. You cannot use Occam's Razor to explain away a discrepancy between theory and observation.
- dahart 10y ago
  What does Occam's razor say about a contradiction? If two things that are observably exactly the same behave differently, then what should one conclude is the simplest explanation? If you look in a telescope for a year, and notice that an apparently free-floating star out in the galaxy with no neighbors is taking a sharply curved path and everything else in the sky is going basically straight, you have two choices: 1) either something you can't see yet is there and acting on the star, 2) conservation of momentum, and the law of equal and opposite reactions need to be thrown out because they're wrong, and we need to start from scratch with physics. Which of those two would Occam say is more likely?
- cozzyd 10y ago
  If it was just galactic rotation curves that would be explained by adding a (mostly) non-interacting dark matter particle, then MOND would probably have many more proponents. It's harder to understand how to replace non-interacting particles in explaining the cosmic-microwave-background power spectrum, galaxy clusters, and structure formation. Maybe it's possible to make a complicated theory do those things, but at that point, I'm not sure it wins on simplicity anymore.
snowwrestler 10y ago
Ok, but physics has had a long and fruitful history of inventing invisible stuff when theory and results disagree. I mean, have you ever seen an atom? A proton? A neutrino? Scientific theories are generally not just amalgamations of tweaks. General relativity is internally consistent, explains so much, and just got a huge boost last year with the first detection of gravity waves... which BTW are invisible stuff that was invented based on theory. It is not lightly cast aside.
- contravariant 10y ago
  It's also worth pointing out that several similar 'holes' in theories have turned out to be correct. Things like missing elements in the periodic table, and fundamental particles like the Ω− baryon or the Higgs boson, where all things that seemed like holes in a theory, but were later discovered to exist.
  - Cyph0n 10y ago
    And how many "holes" proved to be incorrect? I think that using the history of past true positives is a weak argument for dark matter.
    - contravariant 10y ago
      Obviously it's not sufficient evidence for dark matter. But I'm pretty sure that more often than not predictions of unobserved phenomena based on theories with great predictive power and mathematical elegance, turned out to be correct. Although sometimes, like with the neutrino, things turn out to be slightly different than expected. In my opinion Verlinde's theory is a better candidate as a replacement of dark matter, since it doesn't require changing the theory of gravity as drastically as MOND does. That said, his theory does require a combination of general relativity with quantum physics, which is tricky. Compared to both just adding a bit of dark (not necessarily invisible) matter is a lot simpler.
    - publicfig 10y ago
      I'm not sure it's as much an argument for dark matter and more an argument against the idea that if you must create a force or type of matter (or matter itself) in order to make a model work, then the flaw is in the model. By showing examples of this being a useful method in the past, it helps discredit the idea that the model has to be wrong, as opposed to proving idea that the model has to be right which are two different arguments entirely.
baldfat 10y ago
The reason why is so much of the theory is right that knowing how to fix the correct pieces to include the new is the difficult part. As a Theologian/Philosopher Degree holder I love these debates. I think there might be more in the Philosophy side then people care to accept. > "But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need?" https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy https://science.nasa.gov/astrophysics/focus-areas/what-is-da...
d3ckard 10y ago
Same feelings here. I genuinely do not understand how adding unobservable objects would be the right approach instead of just fixing the equations.
- m1el 10y ago
  You don't comprehend the magnitude of "just fixing the equations". As other commenters said, physics has a history of "adding unobservable objects", and then finding a way to observe them.
  - mitchty 10y ago
    We already observe it via its gravitational effects on galaxies. Additionally, good luck getting "fixed equations" to agree with the reams of data.
    - jack9 10y ago
      Isn't that what MOND is doing, to some degree?
      - mitchty 10y ago
        From what I have looked into, it breaks down with observed things on the fringes, aka black holes and supercluster type stuff. This is the "silver bullet" if you will for a lot of these theories: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster It would be great if "just" modifying the equations works. But it has an uphill climb against observed data.
- cozzyd 10y ago
  Adding a particle is "just" fixing another equation (the standard model Lagrangian).
- misnome 10y ago
  Because every other observation for hundreds of years is explained perfectly by "The equations", so you need to "Fix" them in a way that doesn't change any of the existing answers. And yes, as Cozzyd points out, matter is actually part of these equations. The simplest changes involve adding an extra term to the equation with such-and-such properties that disappear in the normal cases - that is dark matter.
jerf 10y ago
One of the mental problems that I think people get in this area is that you can't help but mentally imagine that there's just these few points of data that we need to hit, and so we have a lot of freedom in how to construct formulae that match the observation, so why are scientists being so obstinate about the possibility that MOND may be true? But that's not the case. We have petabytes of data from space (if not more). Your theory needs to match all of them, give or take noise. The common example of the Bullet Cluster [1] comes up a lot, but it's just one particularly exciting example of the problem. I've seen people create variants of MOND that try to address the Bullet Cluster problem, but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass. Which I suppose isn't necessarily out of the question given that we're intrinsically discussing a non-linearity in gravity, but it's still a bit strange. By the time you work out what MOND is going to need to look like in order to explain everything we see in space, it ends up losing its "Oh, it's just one more simple term on gravity" nature and starts looking pretty weird itself. It's constrained by the real data to be at least as weird as dark matter itself, because the data itself is weird. Given the history physics has had with "calling its shots" and identifying the necessity of particles years or decades before they are detected, dark matter is something that I consider at least not out of the question. I also am sympathetic to MOND solutions as well. After all, "the math looked awfully weird by the time we finished working out the theory" also has a long and illustrious history in physics too! What's true is true, regardless of how we feel about how beautiful it is. We also have a Standard Model of particle physics that, whatever its known flaws may be, doesn't immediately obviously have a hole for dark matter in it, so perhaps the "we've called our shots before" argument doesn't apply so well here. There is something to be said for not trying to push a new particle in there. But if MOND is the answer, you're likely to find the final result to have non-trivial weirdness in it, too. [1]: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...
- blueprint 10y ago
  > but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass That's honestly not so "weird" at all. There are TONS of examples of objects and substances having that kind of behavior in nature. If I crash my car into something, the metal "remembers" what happened to it. If I run electricity through an inductor, the magnetic field "remembers" that electricity was being run through that space and doesn't just disappear the moment the electricity is turned off. Momentum is the same thing. Humans remember what happened to them because consciousness records everything that happens to us. What prevents gravity from being something similar? Gravity itself may mean exactly the same thing as space having a kind of memory and operating fundamentally by the law of cause and effect. Things may seem weird only in the absence of such a clear principle.
michaelbuckbee 10y ago
There used to be this theory of Phlogiston, which was a material substance of heat (if you put a lit match under a brick, phlogiston would move from the match to the brick.) Today of course, this sounds ridiculous. But my physics teacher was always keen to point out that the people involved weren't stupid and that it was an important theory because it was a good starting mental model of how things actually worked and led to our better understanding today. I wonder if dark matter is similar. It's not flat wrong, it's the best explanation that we have so far and in it will lead us into another breakthrough of understanding. 1 - https://en.wikipedia.org/wiki/Phlogiston_theory https://en.wikipedia.org/wiki/Phlogiston_theory
- mikeash 10y ago
  Phlogiston is an absolutely reasonable theory given what was known at the time. It's a really interesting example. Phlogiston is basically negative oxygen. It's very similar to how our traditional notion of electrical current moves in the opposite direction from the actual electrons, because people had no idea whether positive or negative charge was the one that moved around, and they just arbitrarily assigned the positive charges as the ones that moved. It doesn't quite work that way, because charges can be positive and negative, whereas oxygen can only be positive or zero. But until you're able to do some pretty careful measurements, or create a good vacuum, it's hard to tell. It's a great analogy. Phlogiston theorized that combustion involved an undetectable (with the technology of the day) substance. That substance was investigated over time and the theory was refined until eventually people figured out that the sign was inverted. If you think of "the opposite of oxygen" as still being a pretty solid theory, not absurd, it comes out looking pretty good.
  - saalweachter 10y ago
    Oxygen is also really weird from a certain point of view. Your fuel is exhausted by gaining something. Your fuel may even still be there, but you can't keep burning it because there is too much of an extra thing, which is also necessary for burning.
- badosu 10y ago
jcranmer 10y ago
I've also been in the same boat, preferring the intellectual idea of modified Newtonian dynamics, primarily based on the fact that the only real checks we have on gravity are based on relatively short distances cosmologically. AIUI, assuming that gravity tends to 1/r at galactic distances "solves" the dark matter problem, for galactic rotation at least. The problem is that there are other phenomena that involve invoking dark matter that aren't explained by MOND. In short, changing gravity sounds appealing, but the math doesn't quite work out, just as xkcd commented about it (see https://xkcd.com/1758/ https://xkcd.com/1758/ ).
- Avshalom 10y ago
  Also assuming gravity tends to 1/r at galactic distances makes no god damned geometric sense and would require like extra dimensions with funny topology for gravity to be traveling though.
  - effie 10y ago
    > would require like extra dimensions with funny topology for gravity to be traveling though. Why? I think I can use simple force formula such as F=k/r without any idea of extra dimensions or topology.
    - Avshalom 10y ago
      Geometrically if a force radiates uniformly it either A)has infinite density B)drops off as 1/r^2 We know that gravity drops off as 1/r^2 for all cases except galaxy rotation curves so for it become 1/r for just this specific scale requires that from gravity's perspective the surface area of a sphere usually but doesn't always increase as r^2. Which in turn means it doesn't move through the 3 spacial dimensions that everything else does.
      - jcranmer 10y ago
        The electromagnetic force (specifically, the Coulomb interaction) is 1/r^2. The strong force is actually effectively constant at long ranges, since it becomes energetically favorable to create an antiquark/quark pair. The residual strong force is exp(-r)/r. The weak nuclear force is exp(-r)/r^2 (or exp(-r)/r? I can't really tell with the links on hand--I just know for sure there's an exp(-r) term in it). Gravity being 1/r wouldn't be a surprise given the other known forces.
mikeash 10y ago
Remember the discovery of Neptune? I mean, not directly, since everyone from that time is dead, but.... That's a nice example of a prediction being wrong, inventing an as-yet-unobserved chunk of matter to fix it, and that invention then being correct. Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relativity. So it's not always the right answer. If you want examples involving entirely new classes of matter, I believe neutrons and neutrinos would fit the bill. Both were invented to explain discrepancies in theories well before they were directly observed. Remember that coming up with a new type of matter is a form of fixing the model. The model is, essentially, "what does the universe actually look like?" If adding a new type of matter gets you closer, then that's fine. If changing the equations works better, then that's fine too. Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right. It is a comprehensive theory that allows for observations and predictions. No, nobody knows just what it actually is, but there's a great deal of evidence that there is a lot of "stuff" in the universe which interacts gravitationally but not in other ways. I see this sentiment a lot, and I'm not sure that dark matter skeptics understand just how much supporting evidence there is for dark matter, or just how difficult it is to "fix Newton." People have been trying real hard for a long time, and nobody has been able to come up with anything like a coherent modification of gravitation that explains the observations.
- corey_moncure 10y ago
  >particles invented before being directly observed as support of dark matter hypothesis Correct me if I'm wrong, which is quite likely, but isn't it one of the qualities ascribed to dark matter that it doesn't interact with 'ordinary' matter or photons in any way other than gravity, and therefore can't be directly observed? Or to turn the question around, is the dark matter hypothesis actually falsifiable?
  - mikeash 10y ago
    It obviously doesn't interact with ordinary matter very much, or else it would have been observed non-gravitationally by now. It could interact a little bit, though. Consider neutrinos: they almost don't interact with ordinary matter at all, but they can be observed with enough patience and care, and they are definitely real. Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but it's an example of something that would interact weakly. And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it. Or if someone finally comes up with a change to the known laws of physics which explains the observations better than dark matter does, that would do it. Remember, nothing is truly "directly observed." All we can do to observe stuff is to see how it affects other stuff. Whether it's with photons or with gravity, it's all ultimately indirect.
yk 10y ago
Newtonian physics can predict fixed curves, you just have to assume that there is additional matter you don't see through a telescope, aka 'dark' matter. And as it turns out, if you then look at the cosmic microwave background, you see that cosmology works a lot better if you have an additional uncharged field, that has a similar mass density as the missing matter in galaxy. When you then look at the large scale structure of galaxies, then it turns out that it looks like there is an additional component to galaxy formation, that again seems to be uncharged but otherwise looks a lot like normal matter. Now, the picture is only mostly as neat now than it was a few years ago, since there is now quite a bit strain with particle physics, but still everywhere in astrophysics it looks like there is some component missing. If that component is dark-matter or a modification of gravity or a remnant of something else is a question about which story journalists want to tell. The thing is, nature does not have to adhere to how we name the left or right hand side of an equation, and that is on a phenomenological level really the difference between mond and dark matter, you either write the term on the gravity side or on the matter side of Newton's law. It is not that physicists invent an invisible component to make a model work, physicists look at the universe and find hints of a missing matter component everywhere. It is just that journalists love to claim that scientists are puzzled.
dahart 10y ago
> no other field would invent invisible stuff to explain that the model is still right. This is a great line! But, it's not fair. The inventing of invisible stuff is an attempt to fix the model. The current model is known to be wrong, because the experimental data isn't matching the model. We have evidence of motion we can't explain... ok, something must be causing it. Right? Black holes and gravity are invisible things predicted by physics.
sandgiant 10y ago
Remember "dark matter" is just a placeholder name for a number of observational anomalies. All of which can be accurately explained by introducing a single extra particle that interacts weakly with photons. So what you call "invisible stuff" is not necessarily that outrageous. A lot of new particles have already been proposed for other reasons, and some of them fit the requirements quite well (SUSY, sterile neutrinos etc.). That said, I believe the main reason that many physicists prefer some kind of dark matter "particle" is that it's extremely difficult to modify general relativity (GR) without breaking the theory. GR provides such accurate predictions that modifying it just the slightest is almost guaranteed to break something somewhere else (e.g. at some other scale), which forces you to introduce nasty (if you ask me) scale dependent effects. This does not mean that we shouldn't modify GR, it just means that it's very difficult to make it work. And physicists tend to like simple theories that work (for reasons such as https://en.wikipedia.org/wiki/Occam's_razor https://en.wikipedia.org/wiki/Occam's_razor).
dhimes 10y ago
Well, it seemed to work for neutrinos.
throw_away_777 10y ago
The models of the universe with and without dark are fundamentally different, it is not inventing "invisible stuff to explain that the model is still right". The nature and composition of fundamental particles are an integral part of models of the universe. There is also a long track record of predicting the existence of particles and then later discovering these particles, for example the Higgs boson. Of course far more particles have been predicted and not (yet) discovered, such as theories of super-symmetry, but these predictions usually don't have the same type of consensus that the Higgs or dark matter has. While dark matter is invisible in the classical sense, it is observable by its gravitational effects. We have also observed what looks like clumps of dark matter, which modified theories of gravity have great difficulty explaining. With that said, until someone actually experimentally observes dark matter it is still a theory and could well be wrong. But this is true of all competing theories - there is no experimental evidence for theories other than dark matter.
manigandham 10y ago
> but the prediction is off by several orders of magnitude, the model is wrong. no other field would invent invisible stuff to explain that the model is still right And yet physics has done this continuously with many discoveries happening in theory first before being verified by reality. In fact, this is also part of making a model that works. There's no rule that models must be bound by what you only know right now.
pharrington 10y ago
This isn't right. Dark matter is a shorthand of saying "our current theory is at least incomplete, there is probably an entirely new class of thing (as in, dark matter is known to be a misnomer), and maybe (with a ridiculously high degree of uncertainty) a separate theory is craftable that precisely explains both what we're currently calling "dark matter" and all other other observed gravitational phenomena. The problem with MOND is that it straight-up isn't compatible with relativity. The core of relativity is that the laws of physics from all reference frames are the same. Sure, you can make a MOND that gels with alot of the math resulting from general relativity, but saying "MOND is correct at the large scale and Newtonian mechanics is correct at smaller scales" is gibberish if you actually want to keep relativity itself.
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- fader 10y ago
  The article does mention this: > In 2004, Bekenstein proposed his TeVeS, or Relativistic Gravitational Theory for MOND. Since then, several different relativistic MOND formulations have been put forth, including one by me, called Bimetric MOND, or BIMOND. > So, no, incorporating MOND into Einsteinian physics is no longer a challenge. I hear this statement still made, but only from people who parrot others, who themselves are not abreast with the developments of the last 10 years. There are several relativistic versions of MOND. What remains a challenge is demonstrating that MOND can account for the mass anomalies in cosmology. I'm not a physicist and definitely not qualified to evaluate those claims, but it doesn't seem to be a concern to Milgrom.
  - pharrington 10y ago
    Just to clarify, that was specifically what that part of my comment was about. The fact that ctrl+f'ing "reference" finds nothing in that article is rather telling. addendum: If you're going to throw away relativity, step up, stop being a coward: straight up throw away relativity and say Einstein was wrong. Again, you need to come up with some very compelling evidence and a seriously strong theory, but don't weasel around the fact that you're saying that just as Newtonian mechanics accurately models a subset of relativity's domain, relativity accurately models a subset of your theory. Present that new theory or acknowledge you're a mad man that accepts variable physics; don't do neither.
nscalf 10y ago
I'll never forget my banking and financial markets class while getting an economics undergrad degree. The teacher said something close to "I know when you're reading the Wall Street Journal, it seems like just about everything we're teaching now doesn't work, but I promise it's real. The world is just weird right now." This was during the early days of Quantitative Easing, the Federal Reserve and many other nations (notably from the lesson I'm thinking of, Japan's National Bank) were flooding money into the market, so QE wasn't working properly. There is a serious problem with economics on the larger scales, and I've yet to hear anyone talk about it.
- mcguire 10y ago
  Debunking Economics by Steve Keen. The book is pretty horrible, but his arguments seem reasonable and interesting.
zeven7 10y ago
Part of the problem is that fixing Newton is more than just adjusting the equation; it requires making up a whole new constant, which to some is worse than making up a whole new class of matter. Constants don't have explanations; they're just like saying, "I don't know why it works out this way, it just does." It's better to have a reason that explains why something happens than to throw up your hands and say "let's just add a new constant to make things work out."
- jack9 10y ago
  A constant is a lot more constrained (existing in a specific context) than a class of matter that can't be observed, which is used as a hand wave to any measurement that is unexpected because not only is it not observed, it's not fully understood/constrained.
- dnautics 10y ago
  That's not terribly convincing argument. Effectively MOND means adding a single free parameter to the model and dark matter means adding a entire continuous distribution to the model.
- wnoise 10y ago
  A new class of matter has a whole bunch of constants -- mass, interaction coefficients, mixing angles, etc.
pdonis 10y ago
> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." The problem with this is that we already know Newtonian physics is wrong. It makes incorrect predictions for planetary orbits in the solar system (the original wrong prediction was the motion of Mercury, but the perihelion shift that is not predicted by Newtonian physics but is predicted by general relativity has now been verified, AFAIK, for all the inner planets). So the way to "fix Newton" is to adopt general relativity instead. And once you adopt general relativity, you can't just arbitrarily change the equation for "acceleration due to gravity" the way MOND does. That equation is not a free parameter in GR; it's derived from the Einstein Field Equation. So you would have to change the Einstein Field Equation, and that would change all of the predictions of GR, not just the prediction for galaxy rotation curves. What I've just stated is the standard objection to MOND among physicists. Milgrom mentions relativistic formulations of MOND and says that this issue is "no longer a problem", but I think that's much too optimistic.
danielhooper 10y ago
I used to feel pretty similar, but then I realized that the people working on these problems are even more skeptical than I am. That's why they work on this stuff. If there is a name for this specific kind of bias, let me know!
jug 10y ago
But isn't the problem that we have pretty-damning-but-still-not-direct evidence for dark matter? If you choose to rather adjust Newtonian physics, we have to go all in and the new problem becomes not only how to "fix" it but also how to explain away the clumping of the Cosmic Microwave Background that is thought to be due to dark matter, the graviational lensing in the Bullet Cluster, the large-scale "web" structure of galaxies in the Universe. Structures and effects that actually fit a dark matter universe pretty well. But yes, maybe there _is_ something else going on, and as a software developer sometimes working on the most terrible things of all: combinations of bugs interacting, and where you keep hunting down a futile rabbit hole assuming a single bug. Maybe this is a combination of bugs in our theories interacting. It could be that this is part flaws in the Newtonian model, part about faults in some other model? That might explain why we've been bashing our heads on the dark matter wall for so long now.
AKifer 10y ago
Your intuition is just right, the way theorists are adding the dark matter/dark energy factor just to make their models fit with observations is the equivalent of 19st century methodology leading to the consideration of space filled with ether. We are just living in a pre-Eisntein like era regarding this issue.

fuhrysteve 10y ago

> I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. I have always had the same inclination. It seems like a classic candidate for using Occam's razor. Would love to hear someone who knows more about physics weigh in with a more nuanced viewpoint.

deleted 10y ago
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pja 10y ago
(I have a physics degree, but it’s very rusty at this point) Particle physics has a long history of noticing gaps in the experimental data that don’t match up with current theories and positing that maybe a new particle would solve the problem. This goes right back to things like the neutrino (which was posited to preserve momentum in certain particle interactions IIRC) and even the neutron. Sometimes the suggestion pans out, sometimes it doesn’t. You can see this kind of thing in action - when there was that anomaly in the CERN data a year or two ago there were a flurry of papers suggesting possible particles, none of which panned out because the anomaly turned out to be statistical noise. The various particulate explanations for the observed discrepancy between the spiral galaxy rotation curves and the observed distribution of matter are very similar: Take a gap in the data & speculate about what kind of particle would fill the hole. When you combine the fact that, historically, speculating about whether new particles could explain anomalies in the observed data has been very productive for C20th physics with the reality that physicists are also loath to toss aside general relativity (which has been spectacularly successful) just because of one observed anomaly that has yet to be explained, it’s entirely unsurprising that physicists are keen on new particles as explanations for the galactic rotation curves. At the same time, every physicist will happily admit that there’s a great big hole in the standard model - i.e. that it doesn’t include gravity at all - and that any unifying theory might also explain away the galaxy rotation curves in a way that doesn’t require new particles. Indeed, if a unifying theory could pull off that trick it would be evidence that the theory has a good chance of being true. Sadly, to date, no one has managed to come up with such a unification, and we have no real idea what such a theory would look like beyond the obvious: that it should explain the observed facts & match QFT & GR in the regimes where those theories are accurate.
- pka 10y ago
  My impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.
  - pja 10y ago
    No, there is no quantum theory of gravity & QFT doesn’t include gravity at all. The best you can do is add gravitational effects as an after-thought, but the things you might analyse with the full QFT are generally far too small & short lived for gravitational effects to matter anyway.
    - raattgift 10y ago
      > No, there is no quantum theory of gravity Here's a couple of examples of quantum theories of gravity. http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-talks/FrankSaueressig13.pdf http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-tal... http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html The extent to which these are complete, consistent, natural (in the fine-tuning sense), and so forth is debatable but these are certainly existing examples of quantum theories of gravity, and indeed the first is a perfectly reasonable Effective Field Theory that people work in regularly. > QFT doesn’t include gravity at all I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity). > the things you might analyse with the full QFT Atoms and molecules have gravitational fields; when you send one through a double slit, which way does their gravitational field go? http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.html http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34.... A quantum theory of gravity is needed to answer that. Assemble a huge number of particles in superposition with 1/(sqrt 2) (|M @ a> + |M @ b>), with a & b separated. A quantum theory of gravity is needed to describe the gravitational influence of M on a small test object (General Relativity's answer is just wrong :( ).
roywiggins 10y ago
Adding in a single extra particle is a lot simpler than rejiggering the entirety of our theories of gravitation. It's also a lot easier to test, just go out and find the particle. This worked for neutrinos, antimatter, the Higgs boson... The Higgs boson took a long time to be discovered, but people thought it would be there because the alternative was the Standard Model being wrong. And so they looked, and there it was. Perfectly compatible with the Standard Model.
mikeash 10y ago
If you're looking at the sky and there appears to be a lot more stuff there than just the bits that emit light, wouldn't Occam's Razor suggest that the presence of more stuff is the most natural explanation, rather than rejiggering the fundamental laws of physics?
- roryisok 10y ago
  It's more like you're looking at the sky and what you can see doesn't weigh as much as it should. Does Occam's Razor suggest that there's invisible stuff? or that you're weighing stuff wrong?
  - mikeash 10y ago
    Except it's not, because the "missing" mass is not distributed in the same way as the stuff that emits light. Some places have a lot more "missing" than others.
olavk 10y ago
Occam's Razor would only apply if the theories made the same prediction with and without the dark matter. You cannot use Occam's Razor to explain away a discrepancy between theory and observation.
dahart 10y ago
What does Occam's razor say about a contradiction? If two things that are observably exactly the same behave differently, then what should one conclude is the simplest explanation? If you look in a telescope for a year, and notice that an apparently free-floating star out in the galaxy with no neighbors is taking a sharply curved path and everything else in the sky is going basically straight, you have two choices: 1) either something you can't see yet is there and acting on the star, 2) conservation of momentum, and the law of equal and opposite reactions need to be thrown out because they're wrong, and we need to start from scratch with physics. Which of those two would Occam say is more likely?
cozzyd 10y ago
If it was just galactic rotation curves that would be explained by adding a (mostly) non-interacting dark matter particle, then MOND would probably have many more proponents. It's harder to understand how to replace non-interacting particles in explaining the cosmic-microwave-background power spectrum, galaxy clusters, and structure formation. Maybe it's possible to make a complicated theory do those things, but at that point, I'm not sure it wins on simplicity anymore.

snowwrestler 10y ago

Ok, but physics has had a long and fruitful history of inventing invisible stuff when theory and results disagree. I mean, have you ever seen an atom? A proton? A neutrino? Scientific theories are generally not just amalgamations of tweaks. General relativity is internally consistent, explains so much, and just got a huge boost last year with the first detection of gravity waves... which BTW are invisible stuff that was invented based on theory. It is not lightly cast aside.

contravariant 10y ago
It's also worth pointing out that several similar 'holes' in theories have turned out to be correct. Things like missing elements in the periodic table, and fundamental particles like the Ω− baryon or the Higgs boson, where all things that seemed like holes in a theory, but were later discovered to exist.
- Cyph0n 10y ago
  And how many "holes" proved to be incorrect? I think that using the history of past true positives is a weak argument for dark matter.
  - contravariant 10y ago
    Obviously it's not sufficient evidence for dark matter. But I'm pretty sure that more often than not predictions of unobserved phenomena based on theories with great predictive power and mathematical elegance, turned out to be correct. Although sometimes, like with the neutrino, things turn out to be slightly different than expected. In my opinion Verlinde's theory is a better candidate as a replacement of dark matter, since it doesn't require changing the theory of gravity as drastically as MOND does. That said, his theory does require a combination of general relativity with quantum physics, which is tricky. Compared to both just adding a bit of dark (not necessarily invisible) matter is a lot simpler.
  - publicfig 10y ago
    I'm not sure it's as much an argument for dark matter and more an argument against the idea that if you must create a force or type of matter (or matter itself) in order to make a model work, then the flaw is in the model. By showing examples of this being a useful method in the past, it helps discredit the idea that the model has to be wrong, as opposed to proving idea that the model has to be right which are two different arguments entirely.

baldfat 10y ago

The reason why is so much of the theory is right that knowing how to fix the correct pieces to include the new is the difficult part. As a Theologian/Philosopher Degree holder I love these debates. I think there might be more in the Philosophy side then people care to accept. > "But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need?" https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy https://science.nasa.gov/astrophysics/focus-areas/what-is-da...

d3ckard 10y ago

Same feelings here. I genuinely do not understand how adding unobservable objects would be the right approach instead of just fixing the equations.

m1el 10y ago
You don't comprehend the magnitude of "just fixing the equations". As other commenters said, physics has a history of "adding unobservable objects", and then finding a way to observe them.
- mitchty 10y ago
  We already observe it via its gravitational effects on galaxies. Additionally, good luck getting "fixed equations" to agree with the reams of data.
  - jack9 10y ago
    Isn't that what MOND is doing, to some degree?
    - mitchty 10y ago
      From what I have looked into, it breaks down with observed things on the fringes, aka black holes and supercluster type stuff. This is the "silver bullet" if you will for a lot of these theories: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster It would be great if "just" modifying the equations works. But it has an uphill climb against observed data.
cozzyd 10y ago
Adding a particle is "just" fixing another equation (the standard model Lagrangian).
misnome 10y ago
Because every other observation for hundreds of years is explained perfectly by "The equations", so you need to "Fix" them in a way that doesn't change any of the existing answers. And yes, as Cozzyd points out, matter is actually part of these equations. The simplest changes involve adding an extra term to the equation with such-and-such properties that disappear in the normal cases - that is dark matter.

jerf 10y ago

One of the mental problems that I think people get in this area is that you can't help but mentally imagine that there's just these few points of data that we need to hit, and so we have a lot of freedom in how to construct formulae that match the observation, so why are scientists being so obstinate about the possibility that MOND may be true? But that's not the case. We have petabytes of data from space (if not more). Your theory needs to match all of them, give or take noise. The common example of the Bullet Cluster [1] comes up a lot, but it's just one particularly exciting example of the problem. I've seen people create variants of MOND that try to address the Bullet Cluster problem, but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass. Which I suppose isn't necessarily out of the question given that we're intrinsically discussing a non-linearity in gravity, but it's still a bit strange. By the time you work out what MOND is going to need to look like in order to explain everything we see in space, it ends up losing its "Oh, it's just one more simple term on gravity" nature and starts looking pretty weird itself. It's constrained by the real data to be at least as weird as dark matter itself, because the data itself is weird. Given the history physics has had with "calling its shots" and identifying the necessity of particles years or decades before they are detected, dark matter is something that I consider at least not out of the question. I also am sympathetic to MOND solutions as well. After all, "the math looked awfully weird by the time we finished working out the theory" also has a long and illustrious history in physics too! What's true is true, regardless of how we feel about how beautiful it is. We also have a Standard Model of particle physics that, whatever its known flaws may be, doesn't immediately obviously have a hole for dark matter in it, so perhaps the "we've called our shots before" argument doesn't apply so well here. There is something to be said for not trying to push a new particle in there. But if MOND is the answer, you're likely to find the final result to have non-trivial weirdness in it, too. [1]: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...

blueprint 10y ago
> but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass That's honestly not so "weird" at all. There are TONS of examples of objects and substances having that kind of behavior in nature. If I crash my car into something, the metal "remembers" what happened to it. If I run electricity through an inductor, the magnetic field "remembers" that electricity was being run through that space and doesn't just disappear the moment the electricity is turned off. Momentum is the same thing. Humans remember what happened to them because consciousness records everything that happens to us. What prevents gravity from being something similar? Gravity itself may mean exactly the same thing as space having a kind of memory and operating fundamentally by the law of cause and effect. Things may seem weird only in the absence of such a clear principle.

michaelbuckbee 10y ago

There used to be this theory of Phlogiston, which was a material substance of heat (if you put a lit match under a brick, phlogiston would move from the match to the brick.) Today of course, this sounds ridiculous. But my physics teacher was always keen to point out that the people involved weren't stupid and that it was an important theory because it was a good starting mental model of how things actually worked and led to our better understanding today. I wonder if dark matter is similar. It's not flat wrong, it's the best explanation that we have so far and in it will lead us into another breakthrough of understanding. 1 - https://en.wikipedia.org/wiki/Phlogiston_theory https://en.wikipedia.org/wiki/Phlogiston_theory

mikeash 10y ago
Phlogiston is an absolutely reasonable theory given what was known at the time. It's a really interesting example. Phlogiston is basically negative oxygen. It's very similar to how our traditional notion of electrical current moves in the opposite direction from the actual electrons, because people had no idea whether positive or negative charge was the one that moved around, and they just arbitrarily assigned the positive charges as the ones that moved. It doesn't quite work that way, because charges can be positive and negative, whereas oxygen can only be positive or zero. But until you're able to do some pretty careful measurements, or create a good vacuum, it's hard to tell. It's a great analogy. Phlogiston theorized that combustion involved an undetectable (with the technology of the day) substance. That substance was investigated over time and the theory was refined until eventually people figured out that the sign was inverted. If you think of "the opposite of oxygen" as still being a pretty solid theory, not absurd, it comes out looking pretty good.
- saalweachter 10y ago
  Oxygen is also really weird from a certain point of view. Your fuel is exhausted by gaining something. Your fuel may even still be there, but you can't keep burning it because there is too much of an extra thing, which is also necessary for burning.
badosu 10y ago

jcranmer 10y ago

I've also been in the same boat, preferring the intellectual idea of modified Newtonian dynamics, primarily based on the fact that the only real checks we have on gravity are based on relatively short distances cosmologically. AIUI, assuming that gravity tends to 1/r at galactic distances "solves" the dark matter problem, for galactic rotation at least. The problem is that there are other phenomena that involve invoking dark matter that aren't explained by MOND. In short, changing gravity sounds appealing, but the math doesn't quite work out, just as xkcd commented about it (see https://xkcd.com/1758/ https://xkcd.com/1758/ ).

Avshalom 10y ago
Also assuming gravity tends to 1/r at galactic distances makes no god damned geometric sense and would require like extra dimensions with funny topology for gravity to be traveling though.
- effie 10y ago
  > would require like extra dimensions with funny topology for gravity to be traveling though. Why? I think I can use simple force formula such as F=k/r without any idea of extra dimensions or topology.
  - Avshalom 10y ago
    Geometrically if a force radiates uniformly it either A)has infinite density B)drops off as 1/r^2 We know that gravity drops off as 1/r^2 for all cases except galaxy rotation curves so for it become 1/r for just this specific scale requires that from gravity's perspective the surface area of a sphere usually but doesn't always increase as r^2. Which in turn means it doesn't move through the 3 spacial dimensions that everything else does.
    - jcranmer 10y ago
      The electromagnetic force (specifically, the Coulomb interaction) is 1/r^2. The strong force is actually effectively constant at long ranges, since it becomes energetically favorable to create an antiquark/quark pair. The residual strong force is exp(-r)/r. The weak nuclear force is exp(-r)/r^2 (or exp(-r)/r? I can't really tell with the links on hand--I just know for sure there's an exp(-r) term in it). Gravity being 1/r wouldn't be a surprise given the other known forces.

mikeash 10y ago

Remember the discovery of Neptune? I mean, not directly, since everyone from that time is dead, but.... That's a nice example of a prediction being wrong, inventing an as-yet-unobserved chunk of matter to fix it, and that invention then being correct. Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relativity. So it's not always the right answer. If you want examples involving entirely new classes of matter, I believe neutrons and neutrinos would fit the bill. Both were invented to explain discrepancies in theories well before they were directly observed. Remember that coming up with a new type of matter is a form of fixing the model. The model is, essentially, "what does the universe actually look like?" If adding a new type of matter gets you closer, then that's fine. If changing the equations works better, then that's fine too. Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right. It is a comprehensive theory that allows for observations and predictions. No, nobody knows just what it actually is, but there's a great deal of evidence that there is a lot of "stuff" in the universe which interacts gravitationally but not in other ways. I see this sentiment a lot, and I'm not sure that dark matter skeptics understand just how much supporting evidence there is for dark matter, or just how difficult it is to "fix Newton." People have been trying real hard for a long time, and nobody has been able to come up with anything like a coherent modification of gravitation that explains the observations.

corey_moncure 10y ago
>particles invented before being directly observed as support of dark matter hypothesis Correct me if I'm wrong, which is quite likely, but isn't it one of the qualities ascribed to dark matter that it doesn't interact with 'ordinary' matter or photons in any way other than gravity, and therefore can't be directly observed? Or to turn the question around, is the dark matter hypothesis actually falsifiable?
- mikeash 10y ago
  It obviously doesn't interact with ordinary matter very much, or else it would have been observed non-gravitationally by now. It could interact a little bit, though. Consider neutrinos: they almost don't interact with ordinary matter at all, but they can be observed with enough patience and care, and they are definitely real. Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but it's an example of something that would interact weakly. And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it. Or if someone finally comes up with a change to the known laws of physics which explains the observations better than dark matter does, that would do it. Remember, nothing is truly "directly observed." All we can do to observe stuff is to see how it affects other stuff. Whether it's with photons or with gravity, it's all ultimately indirect.

yk 10y ago

Newtonian physics can predict fixed curves, you just have to assume that there is additional matter you don't see through a telescope, aka 'dark' matter. And as it turns out, if you then look at the cosmic microwave background, you see that cosmology works a lot better if you have an additional uncharged field, that has a similar mass density as the missing matter in galaxy. When you then look at the large scale structure of galaxies, then it turns out that it looks like there is an additional component to galaxy formation, that again seems to be uncharged but otherwise looks a lot like normal matter. Now, the picture is only mostly as neat now than it was a few years ago, since there is now quite a bit strain with particle physics, but still everywhere in astrophysics it looks like there is some component missing. If that component is dark-matter or a modification of gravity or a remnant of something else is a question about which story journalists want to tell. The thing is, nature does not have to adhere to how we name the left or right hand side of an equation, and that is on a phenomenological level really the difference between mond and dark matter, you either write the term on the gravity side or on the matter side of Newton's law. It is not that physicists invent an invisible component to make a model work, physicists look at the universe and find hints of a missing matter component everywhere. It is just that journalists love to claim that scientists are puzzled.

dahart 10y ago

> no other field would invent invisible stuff to explain that the model is still right. This is a great line! But, it's not fair. The inventing of invisible stuff is an attempt to fix the model. The current model is known to be wrong, because the experimental data isn't matching the model. We have evidence of motion we can't explain... ok, something must be causing it. Right? Black holes and gravity are invisible things predicted by physics.

sandgiant 10y ago

Remember "dark matter" is just a placeholder name for a number of observational anomalies. All of which can be accurately explained by introducing a single extra particle that interacts weakly with photons. So what you call "invisible stuff" is not necessarily that outrageous. A lot of new particles have already been proposed for other reasons, and some of them fit the requirements quite well (SUSY, sterile neutrinos etc.). That said, I believe the main reason that many physicists prefer some kind of dark matter "particle" is that it's extremely difficult to modify general relativity (GR) without breaking the theory. GR provides such accurate predictions that modifying it just the slightest is almost guaranteed to break something somewhere else (e.g. at some other scale), which forces you to introduce nasty (if you ask me) scale dependent effects. This does not mean that we shouldn't modify GR, it just means that it's very difficult to make it work. And physicists tend to like simple theories that work (for reasons such as https://en.wikipedia.org/wiki/Occam's_razor https://en.wikipedia.org/wiki/Occam's_razor).

dhimes 10y ago

Well, it seemed to work for neutrinos.

throw_away_777 10y ago

The models of the universe with and without dark are fundamentally different, it is not inventing "invisible stuff to explain that the model is still right". The nature and composition of fundamental particles are an integral part of models of the universe. There is also a long track record of predicting the existence of particles and then later discovering these particles, for example the Higgs boson. Of course far more particles have been predicted and not (yet) discovered, such as theories of super-symmetry, but these predictions usually don't have the same type of consensus that the Higgs or dark matter has. While dark matter is invisible in the classical sense, it is observable by its gravitational effects. We have also observed what looks like clumps of dark matter, which modified theories of gravity have great difficulty explaining. With that said, until someone actually experimentally observes dark matter it is still a theory and could well be wrong. But this is true of all competing theories - there is no experimental evidence for theories other than dark matter.

manigandham 10y ago

> but the prediction is off by several orders of magnitude, the model is wrong. no other field would invent invisible stuff to explain that the model is still right And yet physics has done this continuously with many discoveries happening in theory first before being verified by reality. In fact, this is also part of making a model that works. There's no rule that models must be bound by what you only know right now.

pharrington 10y ago

This isn't right. Dark matter is a shorthand of saying "our current theory is at least incomplete, there is probably an entirely new class of thing (as in, dark matter is known to be a misnomer), and maybe (with a ridiculously high degree of uncertainty) a separate theory is craftable that precisely explains both what we're currently calling "dark matter" and all other other observed gravitational phenomena. The problem with MOND is that it straight-up isn't compatible with relativity. The core of relativity is that the laws of physics from all reference frames are the same. Sure, you can make a MOND that gels with alot of the math resulting from general relativity, but saying "MOND is correct at the large scale and Newtonian mechanics is correct at smaller scales" is gibberish if you actually want to keep relativity itself.

deleted 10y ago
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fader 10y ago
The article does mention this: > In 2004, Bekenstein proposed his TeVeS, or Relativistic Gravitational Theory for MOND. Since then, several different relativistic MOND formulations have been put forth, including one by me, called Bimetric MOND, or BIMOND. > So, no, incorporating MOND into Einsteinian physics is no longer a challenge. I hear this statement still made, but only from people who parrot others, who themselves are not abreast with the developments of the last 10 years. There are several relativistic versions of MOND. What remains a challenge is demonstrating that MOND can account for the mass anomalies in cosmology. I'm not a physicist and definitely not qualified to evaluate those claims, but it doesn't seem to be a concern to Milgrom.
- pharrington 10y ago
  Just to clarify, that was specifically what that part of my comment was about. The fact that ctrl+f'ing "reference" finds nothing in that article is rather telling. addendum: If you're going to throw away relativity, step up, stop being a coward: straight up throw away relativity and say Einstein was wrong. Again, you need to come up with some very compelling evidence and a seriously strong theory, but don't weasel around the fact that you're saying that just as Newtonian mechanics accurately models a subset of relativity's domain, relativity accurately models a subset of your theory. Present that new theory or acknowledge you're a mad man that accepts variable physics; don't do neither.

nscalf 10y ago

I'll never forget my banking and financial markets class while getting an economics undergrad degree. The teacher said something close to "I know when you're reading the Wall Street Journal, it seems like just about everything we're teaching now doesn't work, but I promise it's real. The world is just weird right now." This was during the early days of Quantitative Easing, the Federal Reserve and many other nations (notably from the lesson I'm thinking of, Japan's National Bank) were flooding money into the market, so QE wasn't working properly. There is a serious problem with economics on the larger scales, and I've yet to hear anyone talk about it.

mcguire 10y ago
Debunking Economics by Steve Keen. The book is pretty horrible, but his arguments seem reasonable and interesting.

zeven7 10y ago

Part of the problem is that fixing Newton is more than just adjusting the equation; it requires making up a whole new constant, which to some is worse than making up a whole new class of matter. Constants don't have explanations; they're just like saying, "I don't know why it works out this way, it just does." It's better to have a reason that explains why something happens than to throw up your hands and say "let's just add a new constant to make things work out."

jack9 10y ago
A constant is a lot more constrained (existing in a specific context) than a class of matter that can't be observed, which is used as a hand wave to any measurement that is unexpected because not only is it not observed, it's not fully understood/constrained.
dnautics 10y ago
That's not terribly convincing argument. Effectively MOND means adding a single free parameter to the model and dark matter means adding a entire continuous distribution to the model.
wnoise 10y ago
A new class of matter has a whole bunch of constants -- mass, interaction coefficients, mixing angles, etc.

pdonis 10y ago

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." The problem with this is that we already know Newtonian physics is wrong. It makes incorrect predictions for planetary orbits in the solar system (the original wrong prediction was the motion of Mercury, but the perihelion shift that is not predicted by Newtonian physics but is predicted by general relativity has now been verified, AFAIK, for all the inner planets). So the way to "fix Newton" is to adopt general relativity instead. And once you adopt general relativity, you can't just arbitrarily change the equation for "acceleration due to gravity" the way MOND does. That equation is not a free parameter in GR; it's derived from the Einstein Field Equation. So you would have to change the Einstein Field Equation, and that would change all of the predictions of GR, not just the prediction for galaxy rotation curves. What I've just stated is the standard objection to MOND among physicists. Milgrom mentions relativistic formulations of MOND and says that this issue is "no longer a problem", but I think that's much too optimistic.

danielhooper 10y ago

I used to feel pretty similar, but then I realized that the people working on these problems are even more skeptical than I am. That's why they work on this stuff. If there is a name for this specific kind of bias, let me know!

jug 10y ago

But isn't the problem that we have pretty-damning-but-still-not-direct evidence for dark matter? If you choose to rather adjust Newtonian physics, we have to go all in and the new problem becomes not only how to "fix" it but also how to explain away the clumping of the Cosmic Microwave Background that is thought to be due to dark matter, the graviational lensing in the Bullet Cluster, the large-scale "web" structure of galaxies in the Universe. Structures and effects that actually fit a dark matter universe pretty well. But yes, maybe there _is_ something else going on, and as a software developer sometimes working on the most terrible things of all: combinations of bugs interacting, and where you keep hunting down a futile rabbit hole assuming a single bug. Maybe this is a combination of bugs in our theories interacting. It could be that this is part flaws in the Newtonian model, part about faults in some other model? That might explain why we've been bashing our heads on the dark matter wall for so long now.

AKifer 10y ago

Your intuition is just right, the way theorists are adding the dark matter/dark energy factor just to make their models fit with observations is the equivalent of 19st century methodology leading to the consideration of space filled with ether. We are just living in a pre-Eisntein like era regarding this issue.