The time is approaching when Nobel prizes are awarded. This inevitably leads to a lot of speculation and chattering rumor. Last year one publication, I think it was Physics Today, went so far as to publish a list of things various people thought should be recognized. This aspirational list was led, of course, by dark matter. It was even formatted the way prize awards are phrased, saying something like “the prize goes to [blank] for the discovery of dark matter.” This would certainly be a prize-worthy discovery, if made. So far it hasn’t been, and I expect it never will be: blank will remain blank forever. I’d be happy to be proved wrong, as forever is a long time to wait for corroboration of this prediction.

While the laboratory detection of dark matter is a slam-dunk for a Nobel prize, there are plenty of discoveries that drive the missing mass problem that are already worthy of this recognition. The issue is too big for a single prize. Laboratory detection would be the culmination of a search that has been motivated by astronomical observations. The Nobel prize in physics has sometimes been awarded for astronomical discoveries – and should be, for those that impact fundamental physics or motivate entire fields like the search for dark matter – so let’s think about what those might be.

An obvious historical example would be Kepler’s Laws. Kepler predates Nobel by a few centuries, but there is no doubt that his identification of the eponymous laws of planetary motion impacted fundamental physics, being one of the key set of facts that led Newton to his universal law of gravity. Whether Tycho Brahe should also be named as the person who made the observations on which Kepler’s work is based is the sort of question the prize committee has to wrestle with. I would say yes: the prize is for “the person who shall have made the most important discovery or invention within the field of physics.” In this case, the discovery that led to gravity was a set of rules – how the orbits of planets behave – that required both observational work (Brahe’s) and numerical analysis (Kepler’s) to achieve.

One could of course also give a prize to Newton some decades later, though theories are not generally considered discoveries. The line can be hazy. For example, the Nobel Prize in Physics 1921 was awarded to Albert Einstein “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.” The “especially” is reserved for the empirical law, not relativity, though I guess “services to theoretical physics” is doing a lot of work there.

Reading up on that I was mildly surprised to learn that the committee had a hard time finding deserving recipients, initially skipping 1918 and 1921 but awarding those prizes in the subsequent year to Planck and Einstein, respectively. I wonder if they struggled with the definition of discovery: need it be experimental? For many, the answer is yes. A theory by itself, untethered from experimental or observational corroboration, does not a discovery make.

I don’t think they need to skip years any more, as the list of plausible nominees has grown so long that deserving people die waiting to be recognized: the Nobel prize is not awarded posthumously. The story is that this is what happened to both Henrietta Leavitt (who discovered the Cepheid period-luminosity relation) and Edwin Hubble (who used Leavitt’s relation for Cepheids to measure distances to other galaxies, thereby changing the course of cosmology). There is also the issue of what counts as physics. At the time, these were very astronomical discoveries. In retrospect, it is obvious that the impact Hubble had on cosmology counts as physics as well.

The same can be said for the discovery of flat rotation curves. I have made the case before that Vera Rubin and Albert Bosma (and arguably others) deserve the Nobel prize for this discovery. Note that I do not say the discovery of dark matter, because (1) that’s not what they did*, and (2) flat rotation curves are enough. Flat rotation curves are a de facto law of nature. That’s enough, every bit as much as Einstein’s “discovery of the law of the photoelectric effect.” A laboratory detection of dark matter would be another discovery worthy of a Nobel prize, but we already missed out on recognizing Rubin for this one.

Conflating discoveries with their interpretation has precluded recognition of other important astronomical discoveries – discoveries that implicate basic physics regardless of their ultimate interpretation, be it cold dark matter or MOND or something else we have yet to figure out. So, what are some others?

One obvious one is the Tully-Fisher relation. This is another de facto law of nature. Tully has been recognized for his work with the Gruber prize, so it’s not like it hasn’t been recognized. What remains lacking is recognition that this is a fundamental law of physics, at least the baryonic version when flat rotation speeds are measured.

Philip Mannheim pointed out to me that Milgrom deserves the prize for the discovery of the acceleration scale a0. This is a new constant of nature. That’s enough.

Milgrom went further, developing the whole MOND paradigm around this new scale. But that is extra credit material that needn’t be correct. Unfortunately, the controversial nature of MOND, deserved or not, serves to obscure that there is a new constant of nature whose discovery is analogous to Planck’s discovery of his eponymous constant. People argue over whether a0 is a single constant (it is) or whether it evolves over cosmic time (not so far as I can tell). The latter objection could be raised for Planck’s constant or Newton’s constant; these were established when it wasn’t possible to test whether their values might have varied over cosmic time. Now that we can, we do check! and so far, no: h, G, and a0 all appear to be constants of nature, to the extent we are able to perceive.

The above discoveries are all worthy of recognition by a Nobel prize. They are all connected by the radial acceleration relation, which is another worthy observational discovery in its own right. This is one that clearly transgresses the boundaries of physics and astronomy, as the early versions (Sanders 1990, McGaugh 1999, 2004) appeared in the astronomical literature, but more recent ones in the physics literature (McGaugh et al. 2016, Mistele et al. 2024). Sadly, the community seems perpetually stuck looping through the stages of Louis Agassiz‘s progression of responses to scientific discoveries. It shouldn’t be: this is an empirical relation that has long been well established and repeatedly confirmed. It suffers from association with MOND, but no reference to MOND is made in the construction of the observed relation. It’s right there in the data:

The radial acceleration relation as traced by both early (red) and late (cyan) type galaxies via both kinematics and gravitational lensing. The low acceleration behavior maps smoothly onto the Newtonian behavior seen in the solar system at higher accelerations. If Newton’s discovery of the inverse square force law would warrant a Nobel prize, as surely it would had the prize existed in Newton’s time, then so does the discovery of a systematically new behavior.

*Rubin and Bosma both argued, sensibly, that the interpretation of flat rotation curves required dark matter. That’s an interpretation, not a discovery. That rotation curves were flat, over and over again in every galaxy examined, to indefinitely large radii, was the observational discovery.

Stacy McGaugh is an astrophysicist and cosmologist who studies galaxies, dark matter, and theories of modified gravity. He is an expert on low surface brightness galaxies, a class of objects in which the stars are spread thin compared to bright galaxies like our own Milky Way. He demonstrated that these dim galaxies appear to be dark matter dominated, providing unique tests of theories of galaxy formation and modified gravity. Professor McGaugh is currently the chair of the Department of Astronomy at Case Western Reserve University in Cleveland, Ohio, and director of the Warner and Swasey Observatory. Previously he was a member of the faculty at the University of Maryland, having also held research fellowships at Rutgers, the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, and the Institute of Astronomy at the University of Cambridge after earning his Ph.D. from the University of Michigan.

9 thoughts on “Nobel prizes that were, that might have been, and others that have not yet come to pass”

  1. “Milgrom deserves the prize for the discovery of the acceleration scale a0.” Call it “Milgrom’s Constant.” There is a campaign for recognition that is deserving!

  2. I think that at this time it might be better to co-award Milgrom and McGaugh. Everyone who has followed this issue knows Stacy has vested his entirely career defending Migrom’s. And yes, it is sad that we failed to recognize Rubin.

  5. I think you and Milgrom both deserve a Nobel prize, for the RAR and MOND. The prizes are often awarded for discoveries without an explanation – as you say, Rubin and Bosma should have had that for FRCs.

    But we can’t say more than what we know so far – MOND is a great discovery even if it only works across a particular range of scales. We don’t yet know if a0 is a universal constant, and have strong evidence that it isn’t. Not because it changes over time, but because in clusters there’s a parallel constant, 17 times larger. This means that the RAR may in some ways be a larger discovery than MOND, as it’s a more general pattern, even though Milgrom led the way.

    Although it’s very early days, g‡ is being used to represent the larger acceleration scale (symbols don’t always post right, but it looks like a g followed by two plus signs). And you say in the ‘distinct RAR’ paper that there’s a parallel baryonic Tully-Fisher relation, and that the value 2e-9 is arrived at consistently in both ways.

    What no-one seems to have mentioned so far is that 2e-9 is also the starting point for the transition between the inner and outer patterns in galaxies. The transition is also described by an empirical equation you found, related to the RAR in galaxies, with an index containing ‘e’, which I tried out, and it seemed to describe the transition well.

    This means that if there’s a universal acceleration scale, it may be 2e-9, that is, g‡ rather than a0. It’s early days, but what you, Milgrom and others have done should be recognised, regardless of whatever else is involved.

    1. Yes, you can see the parallel RAR in https://tritonstation.com/2024/07/26/the-radial-acceleration-relation-starting-from-high-accelerations/. The scale g++ is what one gets from fitting cluster or BCG data with the same function. The shift in the data is much smaller than implied by the value of g++, which, as you say, is more of a turning point. The amount of data to which this applies is a tiny minority, so in that sense a0 is considerably more fundamental. I am not sure how much to believe the shift in g++ (I am a coauthor of some of that work: I believe in reporting what the data say, so we do, but as an astronomer, I’ve often experienced that being misleading in some way and worry that this may be the case here as it is impossible to measure exactly the same things, just proxies.)

      Most early type galaxies – the red triangles above – don’t exhibit the shift inferred for brightest cluster galaxies (BCGs), so it may be that BCGs are sharing the residual discrepancies of the clusters they’re in rather than it being an intrinsic property. So I suspect the issue is specific to rich clusters.

      Clusters are different from galaxies just as galaxies are different from planets. There’s a set of rules that describe what planets do. Galaxies don’t do that; there’s another set of rules for galaxies. Perhaps there is a different set of rules for clusters. I kinda hope not, and that we’ll discover a simpler reason for why clusters do what they do. That doesn’t detract from the rules for the behavior of individual galaxies.

      1. Hard not to see the hierarchical structure: star systems -> galaxies -> galaxy clusters…

        Hierarchical structures are a manifestation of the emergence of new irreducible/fundamental laws that generate such hierarchical structures. No theory survive unchanged crossing between hierarchical levels. Naive reductionism contradicts objectivity.

        Hard to ignore P.W. Anderson, a man way ahead of his time.

  6. I take the point that it’s far less certain, and that in that field you’ve found small amounts of indirect data can lead to false conclusions. But conceptually the two patterns are similar in quite a few ways, and they start in the same place. So it seems to me that if something actually changes at low accelerations, it happens at g = 2e-9. As we try to understand these things, a general heading under which a lot of interpretations are listed is an actual change of some kind. Whether it’s to matter, inertia, orbits, curvature, other aspects of gravity or whatever else – all of which people have been looking into as possibilities – whatever happens to the nuts and bolts of the universe, if something does, that’s where it happens.

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