A Philosophical Approach to MOND is a new book by David Merritt. This is a major development in the both the science of cosmology and astrophysics, on the one hand, and the philosophy and history of science on the other. It should be required reading for anyone interested in any of these topics.
For many years, David Merritt was a professor of astrophysics who specialized in gravitational dynamics, leading a number of breakthroughs in the effects of supermassive black holes in galaxies on the orbits of stars around them. He has since transitioned to the philosophy of science. This may not sound like a great leap, but it is: these are different scholarly fields, each with their own traditions, culture, and required background education. Changing fields like this is a bit like switching boats mid-stream: even a strong swimmer may flounder in the attempt given the many boulders academic disciplines traditionally place in the stream of knowledge to mark their territory. Merritt has managed the feat with remarkable grace, devouring the background reading and coming up to speed in a different discipline to the point of a lucid fluency.
For the most part, practicing scientists have little interaction with philosophers and historians of science. Worse, we tend to have little patience for them. The baseline presumption of many physical scientists is that we know what we’re doing; there is nothing the philosophers can teach us. In the daily practice of what Kuhn called normal science, this is close to true. When instead we are faced with potential paradigm shifts, the philosophy of science is critical, and the absence of training in it on the part of many scientists becomes glaring.
In my experience, most scientists seem to have heard of Popper and Kuhn. If that. Physical scientists will almost always pay lip service to Popper’s ideal of falsifiablity, and that’s pretty much the extent of it. Living up to applying that ideal is another matter. If an idea that is near and dear to their hearts and careers is under threat, the knee-jerk response is more commonly “let’s not get carried away!”
There is more to the philosophy of science than that. The philosophers of science have invested lots of effort in considering both how science works in practice (e.g., Kuhn) and how it should work (Popper, Lakatos, …) The practice and the ideal of science are not always the same thing.
The debate about dark matter and MOND hinges on the philosophy of science in a profound way. I do not think it is possible to make real progress out of our current intellectual morass without a deep examination of what science is and what it should be.
Merritt takes us through the methodology of scientific research programs, spelling out what we’ve learned from past experience (the history of science) and from careful consideration of how science should work (its philosophical basis). For example, all scientists agree that it is important for a scientific theory to have predictive power. But we are disturbingly fuzzy on what that means. I frequently hear my colleagues say things like “my theory predicts that” in reference to some observation, when in fact no such prediction was made in advance. What they usually mean is that it fits well with the theory. This is sometimes true – they could have predicted the observation in advance if they had considered that particular case. But sometimes it is retroactive fitting more than prediction – consistency, perhaps, but it could have gone a number of other ways equally well. Worse, it is sometimes a post facto assertion that is simply false: not only was the prediction not made in advance, but the observation was genuinely surprising at the time it was made. Only in retrospect is it “correctly” “predicted.”
The philosophers have considered these situations. One thing I appreciate is Merritt’s review of the various takes philosophers have on what counts as a prediction. I wish I had known these things when I wrote the recent review in which I took a very restrictive definition to avoid the foible above. The philosophers provide better definitions, of which more than one can be usefully applicable. I’m not going to go through them here: you should read Merritt’s book, and those of the philosophers he cites.
From this philosophical basis, Merritt makes a systematic, dare I say, scientific, analysis of the basic tenets of MOND and MONDian theories, and how they fare with regard to their predictions and observational tests. Along the way, he also considers the same material in the light of the dark matter paradigm. Of comparable import to confirmed predictions are surprising observations: if a new theory predicts that the sun will rise in the morning, that isn’t either new or surprising. If instead a theory expects one thing but another is observed, that is surprising, and it counts against that theory even if it can be adjusted to accommodate the new fact. I have seen this happen over and over with dark matter: surprising observations (e.g., the absence of cusps in dark matter halos, the small numbers of dwarf galaxies, downsizing in which big galaxies appear to form earliest) are at first ignored, doubted, debated, then partially explained with some mental gymnastics until it is Known and of course, we knew it all along. Merritt explicitly points out examples of this creeping determinism, in which scientists come to believe they predicted something they merely rationalized post-facto (hence the preeminence of genuinely a priori predictions that can’t be fudged).
Merritt’s book is also replete with examples of scientists failing to take alternatives seriously. This is natural: we have invested an enormous amount of time developing physical science to the point we have now reached; there is an enormous amount of background material that cannot simply be ignored or discarded. All too often, we are confronted with crackpot ideas that do exactly this. This makes us reluctant to consider ideas that sound crazy on first blush, and most of us will rightly display considerable irritation when asked to do so. For reasons both valid and not, MOND skirts this bondary. I certainly didn’t take it seriously myself, nor really considered it at all, until its predictions came true in my own data. It was so far below my radar that at first I did not even recognize that this is what had happened. But I did know I was surprised; what I was seeing did not make sense in terms of dark matter. So, from this perspective, I can see why other scientists are quick to dismiss it. I did so myself, initially. I was wrong to do so, and so are they.
A common failure mode is to ignore MOND entirely: despite dozens of confirmed predictions, it simply remains off the radar for many scientists. They seem never to have given it a chance, so they simply don’t pay attention when it gets something right. This is pure ignorance, which is not a strong foundation from which to render a scientific judgement.
Another common reaction is to acknowledge then dismiss. Merritt provides many examples where eminent scientists do exactly this with a construction like: “MOND correctly predicted X but…” where X is a single item, as if this is the only thing that [they are aware that] it does. Put this way, it is easy to dismiss – a common refrain I hear is “MOND fits rotation curves but nothing else.” This is a long-debunked falsehood that is asserted and repeated until it achieves the status of common knowledge within the echo chamber of scientists who refuse to think outside the dark matter box.
This is where the philosophy of science is crucial to finding our way forward. Merritt’s book illuminates how this is done. If you are reading these words, you owe it to yourself to read his book.
26 thoughts on “A Philosophical Approach to MOND”
Thanks – already ploughing through it, and wondered about your opinion
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Seconded. I’m just a third of the way through it and it is really the best book of its kind I have read. Chapters 1 and 2 should be required reading for all scientists, too many of whom only pay lip service to Popper.
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Having just completed reading it, a thought struck me. At the end of Chapter 8, Merritt speculates on the numerical coincidence of 2πa₀ and cH₀. It occured to me that this should be testable. If we go back to the rotation curve data in Figure 4.2, it looks like a change of 10% in δV/V should be detectable, which would correspond to a = 1.46a₀ and a corresponding change in H. This would be eqivalent to a lookback time of about 32% of the age of the Universe or about 4.4 bn years. Has anyone looked at rotation curve data for moderately distant galaxies? Could new radio telescopes like the Square Kilometer Array have sufficent resolution and sensitivity to make 21 cm measurements of these sources, or would they be better measured with an optical telescope?
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Yes, people have looked into this, thinking that maybe a0 evolves with H(z). My reading of the data is that there is no clear evidence of evolution in the baryonic Tully-Fisher relation – and hence a0 – out to redshift 1.5 or so. It ought to be detectable by then if it evolves. Another numerical coincidence is with the value of the cosmological constant that drives the accelerated expansion of the universe. So it is certainly striking that these acceleration scales seem connected.
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Hi Stacy, I have the book on my Kindle and it is next in my queue. I’m reading the new Peebles book now. Relating to your thoughts above about new ideas and how scientists react to them, are there any terms upon which I could get an audience with you to discuss a new idea? I know all the standard reasons why the odds are so low that an outsider could come up with a new important idea, but what if someone really does have that idea? How will it ever get through to the science community? I don’t know if you saw my tweets but I could probably afford to fund some directed research by a grad/postgrad. I don’t know if that kind of thing is done. I guess it would sort of be like a modest grant where the direction of the research is specified by the grantor. The topic I would like to have scientifically investigated is a first phase assessment of a galaxy-local cosmology. Setting aside all the details of how (we can go into it later) briefly the idea is that an SMBH can serve as a giant recycling furnace and disgorge matter-energy it ingested. (this is probably via the polar jets that somehow breach the event horizon — which requires some more patience for me to explain how that is possible – but you can listen to Dr. Natarajan on Strogatz’s podcast say that she and Rees decided apriori to fit SMBH into LCDM). Anyway, with a galaxy-local cosmology — we would need to blend Hoyle/Lemaitre/Guth and others to re-envision the “Big Bang” as intermittent galaxy-local mini-bangs, with inflation of the ejecta, and outward galaxy-local expansion. So, here is something that might hook you — with this idea galaxy local expansions are in opposition to each other — and it would be perfectly sensible that we would measure various Hubble rates. There is so much more I have, and I think I can take the existing science – meaning the observations and math – and rewrite the overall narrative to one that makes far more sense. I can also trace through the decision points where physics and cosmology chose the wrong narrative that led to the situation we are in now. Would love to dialogue, discuss, talk — on your terms. All I would ask is some psychological safety and to hear me out. Could probably explain the narrative in one hour. It is actually very simple narrative-wise and that may be a factor in why it has been so difficult to convince scientists. Best, Mark
Hi Stacy, Here is a post I made on a Discord channel tonight.
Physics Historians (if you happen to find this Discord channel) : There is some irony given what is now known in your time to be fact, but at my present time the physicists are focused on relativity for observers in different reference frames in the same spacetime construct. That is the hardest way to do science on fundamentals. It is hard enough working in the Riemannian geometry. It is worsened by using a pure Riemannian geometry when spacetime is actually a physical construct and isn’t smooth near the Planck scale where it gets absolutely chunky with Planck sphere electrinos and positrinos. It seems we need to back up to the period 1870-1930 and straighten things out regarding the aether, which all physicists believe in today but they won’t call it aether for some unknown reason. Maybe its pride, but even then pride is holding them back. They need to get over themselves and I say that with tough love. They are doing fine work in the hardest possible way. But there is a much easier way. I’ve been trying to communicate the path forward, but their nonsense detection circuit AI has not been properly trained and keeps trippin’ out. Anyway – here is another try: Map One is the actual foundation: Absolute 3D Galilean/Cartesian/Euclidean space and time. Nature is a trickster so the lowest level of emergence is a construct – a neutral aether composed of Planck sphere electrinos and positrinos and that aether contracts as local energy increases and expands as local energy decreases. Thus we describe this spacetime construct with Map Two, which is Riemannian. Except there is that little oopsie about pure mathematical continuous geometry vs. a geometry that is very real and discrete at the Planck scale. I just listened to Dr. Stacy McGaugh on a podcast where he said it will take a century, 100 years, to resolve the current conundrums in physics and cosmology. There are people on this channel who can change that to less than 1 year.
Milgrom’s and your papers are refreshingly clear in stating predictions, tests, and conclusions. Nevertheless, I have one topic that I do not understand, and which might also interest others. Milgrom (and others) often speculates that a_0 is due to the cosmological constant Lambda. This is very convincing, as it avoids introducing a_0 as separate constant of nature. On the one hand, MOND means that gravity at low accelerations is *stronger* than Newtonian gravity. On the other hand, Lambda is usually said to *reduce* Newtonian gravity (it is “a repelling force”). What is the solution to this confusion?
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One way to think about how such a thing can happen is with a comparison to QCD: inside a proton, the force is very strong, and outside the proton the force is weak. If gravity on scales of galaxies was to have features similar to QCD, inside a galaxy you would have stronger force compared to Newtonian gravity (and you could infer missing mass) and outside galaxies, you would have weaker gravity compared to Newtonian (and you could infer Lambda).
Click to access s10052-019-7393-0.pdf
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More on Deur’s work on gravity can be found at: http://dispatchesfromturtleisland.blogspot.com/p/deurs-work-on-gravity-and-related.html I think he is the researcher closest to the truth on dark matter, dark energy, and quantum gravity of anyone currently doing work in the field even though he is an “outsider” (his day job is in QCD physics).
Itamar, but MOND seems different from QCD. MOND is stronger than Newtonian Gravity far from the galaxy. Inside the galaxy MOND is essentially equal to Newtonian gravity. So QCD does not solve the problem I mentioned.
Gianni. The connection to QCD is by analogy only and arises mostly from the fact that in both QCD and quantum gravity, the force carrying mediator has non-negligible self-interactions. Deur’s insight is that MOND effects are significantly a function of the extent to which the distribution of matter is non-spherical. This also explains why MOND doesn’t hold into the cluster regime (where it underestimates dark matter phenomena).
1. I meant that it is possible to imagine a modified gravity theory that will result in enhanced gravity inside structures and suppressed gravity outside structures (for which an intuitive example would be if gravity for galaxies has similarities to QCD for protons)
2. MOND would be the limiting case of such a theory inside structures, to describe cosmology you need to full theory
Yes, I can imagine such a thing. The numerical coincidence of a0 and accelerated cosmic expansion is striking. It may be a chance bit of numerology, but it is closer to spot on than the WIMP miracle was, and that launched a billion dollar industry in DM detection. The trick, as stated, is the switch in signs: Lambda is effectively repulsive, while MOND increases the net force of attraction.
If I knew how to do this, I would say so. I don’t. The analogy to QCD is intriguing; one might imagine a that the effect arises as we delineate expanding space from that of bound galaxies. But the mechanism by which this happens is unclear.
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Lee Smolin has proposed an alternative, loop quantum gravity approach, to recovering MOND. https://arxiv.org/abs/1610.01968
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also Smolin deduces MOND from a positive cosmological constant. He explicitely says so in his text. The argument is simple and convincing, and is essentailly the same as that of Verlinde, and the same as the suggestion by Milgrom himself, as he writes in his comment on Verlinde on arxiv. But why does a positive Lambda increase gravitational acceleration? MOND produces a higher attraction than Newtonian gravity: Milgrom states this in all his papers. But intuition seems to say that a positive Lambda decreases Newtonian acceleration. After all, a positive Lambda drives things apart, thus reduces the relative acceleration. Somehow I am missing something. It seems that Lambda drives things apart, but MOND’s a_0 drives things together. Where is my mistake?
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You make no mistake, as stated: Lambda is repulsive while MOND is a net additional attraction. But it isn’t like Lambda is being invoked directly to do MONDian things; these theories all try to connect both phenomena to the properties of the vacuum. So you can, at least in principle, get different manifestations resulting from said properties – as Verlinde might put it, both MOND and Lambda are emergent consequences of the same vacuum physics.
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Sabine Hossenfelder posted her own review (see http://backreaction.blogspot.com/2020/04/book-review-philosophical-approach-to.html).
In that review, she talks about the CMB third peak: “Having said that, Merritt clearly points out that MOND (or its relativistic generalizations) has certain problems, notably the third peak of the CMB is a headache.”
I thought that you can’t really use MOND to calculate values related to this third peak, so perhaps what she’s really referring to are these “relativistic generalizations”. As far as I know, they have all been ruled out, or never been tested, so my stance was that MOND itself doesn’t have a problem.
Whether I’m right or wrong, do you think Merritt covered this particular topic properly in his book?
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The history of the CMB & MOND is a story I’ve been meaning to tell here, but it has been too long and daunting to take up. Certainly can’t go through it all in a reply here. In very brief, MOND is a non-relativistic theory, so it is mute about the CMB peaks. One needs a relativisitic theory that encompasses both GR and MOND. That’s hard; the first serious candidate (TeVeS) does not do the right thing for the CMB peaks. Lots of cosmologists have argued that it would be impossible. Just the other day, Skordis claims to have done it: https://arxiv.org/abs/2007.00082. Whether this new theory proves right in the long run is less important than the fact that it can be done at all: the impossible-for-anything-but-cold-dark-matter argument cosmologists have made for the last 17 years is wrong.
I sure do hope you get a chance and the energy to tell the “CMB and MOND” story. I would really like to read that. As Merrit makes clear – you personally are to be congratulated on your use of MOND to get a very good _prediction_ of the CMB 2nd peak. And – the way I read it – a better prediction for the 3rd peak than LCDM – fig 6.5 of the Merrit book. Hope you get the chance to write it.
I do hope to. Have for a while. There is just so much to that story.
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To follow up on my earlier comments, there is a new paper that talks about acceleration to very high energies within the jet of Centaurus A. (https://www.nature.com/articles/s41586-020-2354-1) This is (in my admittedly biased assessment) a confirmation of my prediction that cosmology is actually galaxy local and that jets are the source of inflation in the universe. If this is true then expansion is also galaxy local and in opposition to each other and we would expect Hubble’s ‘number’ to vary depending on the path a photon took through the intervening galaxies and where each galaxy was in its recycling life-cycle. I have a question for everyone here — is there any history of research into galaxy local cosmologies? I haven’t come across any. I can see the decision history for the last 150 years that may have influenced scientists away from looking at galaxy local cosmologies, but if you were to clear your mind for a moment and think hmm — these SMBH consume an emormous amout of matter-energy and there are these jets that look like they are coming from the SMBH that carry an enormous amout of matter-energy — then isn’t it logical to wonder if these are connected? I mean even if you have heard about event horizons, wouldn’t you question if it were possible to breach the event horizon at the poles or for the event horizon to take on some different shape that made it possible to breach the event horizon? And don’t we already have similar models at different scales for hot cores exploding as jets? Nature loves to repeat itself.
Thank you Stacy for allowing me to comment here, even if you are not at the point of responding or engaging. I know there is huge professional risk for engaging with outsiders ideas and that 99.999999% of those ideas are worthless. Hopefully mine is in the 0.000001% that has value. p.s., I have the whole remainder of the narrative worked out as well and there are so many touch points where it makes sense throughout cosmology and physics. Also, as I mentioned earlier, I am willing to consider several months of funding to professional physicists or cosmologists to help me with the ideas.
I think you exaggerate by saying 99.999999% of these ideas are worthless. I think 99.999999% of these ideas are wrong. But I do not think they are worthless. It is a common fallacy to think that wrong ideas are worthless. There is plenty of serious research that indicates that when people ask questions to determine information, they rate the answer “no” lower than the answer “yes”, even if the answer “no” provides more information. You have an idea, it is wrong, you learn something, you go back to step 1. At least, that is how I have done research for 40 years, and it worked for me – for the first 35 years, anyway. In physics, it seems to be different, and I find this very disturbing.
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In case it wasn’t clear, the research paradigm that I am criticising is “You have an idea. It is wrong. You learn nothing. You do not go back to step 1.”
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Many of the points in the book were made earlier by Merritt in an article. My main criticism of that article (and, hence, also a major criticism of the book, which in the meantime I have read) is that he attacks a strawman version of ΛCDM.
Alas, some points are too long to make in a comment box, so I direct readers to my long article on MOND vs. &LambdaCDM (published in the latest version of The Observatory), a major part of which is a point-by-point rebuttal of Merritt’s arguments. I hope that I have provided enough historical context and references to convince the reader that Merritt’s arguments against ΛCDM are mostly wrong.
My main motivation for writing a critique of the article is that I think that MOND has something to offer (though I’m not sure what). MOND phenomenology needs to be investigated more and needs a convincing explanation (whether via something like MOND or something else). However, strawman attacks hurt the cause much more than they help it. I have actually met people who were interested in MOND, maybe even had done some work on it, then moved on, turned off by the over-the-top rhetoric.
In another post, Stacy writes: “These things were known by the mid-90s. Nowadays, people seem to think Type Ia SN discovered Lambda, when really they were just icing on a cake that was already baked.”
Indeed. First, there were always people who considered a non-zero Λ, but observations weren’t good enough to determine it, so many set it to 0 just as a practical matter. Second, by the middle of the 1990s the evidence was pretty conclusive that Λ is greater than 0 and even that the Universe is accelerating. (That is an inference, not a measurement. Accelerated expansion has not been measured. Supernova cosmology does not measure an acceleration; that is an inference.)
A good (or bad, depending on one’s point of view) example of such a claim that supernova cosmology somehow discovered Λ is that made by Merritt in the book and article mentioned above. As I note in my article, this is one of many things he gets wrong. Just because he likes MOND shouldn’t be a reason to put him on a pedestal. In the big picture, people will get interested in MOND only on the basis of high-quality arguments, not strawman attacks.
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