In today’s blog we talk to Dr. Moti Milgrom, Isidor Rabi Professor Emeritus of Physics at the Weizmann Institute, in Rehovot, Israel, and author of Modified Newtonian Dynamics (MOND).
David: Hello Moti. I find dark matter’s ability to distribute itself such that MOND works, to be an enormous fine-tuning issue. I don’t, however, hear dark matter proponents concerned with any fine-tuning issues. Am I confused?
Moti: DM proponents as you call them do not make a homogeneous lot. There certainly are some of them who take the Malta-yok attitude, like those who refused to look through the telescope at Galileo’s time because they already knew the answers and did not need to check the facts. But I am sure most of them are concerned with this `fine tuning’ to one or another degree. But admitting this in writing is another matter, which is why perhaps you do not hear many write this headache for DM.
David: While dark matter may be made compatible with flat rotation curves and the Tully-Fisher relation, MOND actually predicts or explains them. Do proponents of both MOND and dark matter agree with this?
Moti: They certainly should appreciate this important distinction between how MOND and DM account for the host of galactic phenomena. Indeed, DM theories (and there are many such now) strive to explain these phenomena after the fact by invoking complicated and many-knobbed processes. And even so they do not quite succeed; and even so there remain many observations that are in conflicts with the results of DM simulations. This is what has given rise to a proliferation of DM models trying to fix the shortcomings of existing ones. I don’t think I can name one definite successful prediction made by DM in the context of galactic dynamics. MOND on the other hand has made many unavoidable predictions, which were subsequently vindicated. The way MOND accounts for galactic dynamics is similar to Newtonian dynamics implying, without any leeway, all of Kepler’s laws in the solar system and other planetary systems. What DM is trying to do (the best it could do) would be analogous to trying to explain Kepler’s laws after the fact as resulting from some complicated processes governing the way planetary systems formed and evolved over time, not as strict, inevitable laws.
David: The a0 constant in MOND was chosen for empirical reasons but it seems to hold deeper significance. Can you comment on this?
Moti: Physical theories -- at least all the ones we now know and use -- have limited validity domains; they are good descriptions of only a limited set of phenomena. When one considers extending a theory so as to cover a wider range of phenomena, one first has to define the boundary within which he thinks the existing theory is valid to a high accuracy. Such boundaries are usually defined or marked by some new physical constant that the new, extended theory introduces. For example, quantum theory brought to light the fact that the old, classical mechanics of Newton is only valid when the angular momentum (more technically, the action) of the system at hand is much larger than a certain constant – the Planck constant, h. Below this value is where the extension is needed. For the departure from the old physics brought about by relativity (the special and the general) the role of boundary constant is played by the `speed of light’, c. MOND hinges on departure from standard dynamics for small accelerations; so, it had to define the validity domain of the latter by introducing an acceleration constant, a0. This constant is built into the extended theory (MOND in this case) and it appears in many of the predictions that this theory has made (just as h appears in all quantum predictions, or c in results of the theory of relativity). Comparison of these predictions with data, and their validation, yields the value of the constant. As a first crucial test, all these different determinations of the constant, using different predictions, should, of course, give consistent values. This has indeed been the case for the MOND constant, a0. Once its value is established, you can ask whether it has any special significance, for example whether it can connect in some way with other known constants related to other phenomena. And if so, perhaps this points to some hidden, previously unsuspected connections between these different theories.
In the case of the MOND constant, it was realized, right from the start, that the deduced value of a0 (which incidentally is approximately 1 Angstrom per second per second) is near some accelerations relevant to cosmology. For example, the one defining the accelerated expansion of the Universe (attributed to `dark energy’) . a0 is also near in value to the product of the speed of light and the Hubble constant (the present rate of expansion of the Universe). These numerical `coincidences’ may point to deep connections between the overall state of the Universe (cosmology) with what dictates the dynamics within systems, such as galaxies, that are very small on cosmic scales. It may, for example, hint at the origin of inertia being related to cosmology. There are various ideas of how such influences can be effected, but I will not detail them here. But if they indeed occur this would be an insight of enormous magnitude.
David: Outsiders may think that scientists are especially well equipped to spot bad ideas, and to identify and promote, good ones. How reasonable is that picture?
Moti: I suppose you mean judging ideas related to science, not, for example, to economy or politics, where I don’t think they are better than other intelligent people. So yes, naturally, by and large, scientists are better equipped to judge scientific ideas, by training and by natural knack. But in the context of the MOND vs DM clash, it is good to remember that there are definitely many important examples where scientists, as individuals, or as whole communities, turned out to be wrong, sometimes very wrong. At the individual level, examples are known of great physicists who judged wrongly (Einstein on quantum mechanics, black holes, and cosmic expansion, Lorentz on special relativity, Mach on the reality of atoms, etc.). There are also quite a few examples known of young physicists (you asked about scientists, but my experience is from physics) who came up with major breakthroughs, but were discouraged by their great mentors from pursuing them (e.g., Bohr by Rutherford on the former’s theory of the atomic model). We also know of historical examples where most of the science community had held the wrong opinions. The most famous perhaps is that of the contemporaries of Kepler and Galileo who stuck to the Ptolemaic world picture even when the correct alternative was laid before them. The adherence of Maxwell and a host of 19thcentury great physicists to the existence of the Aether is another example; but in this case there was no competing alternatives to dismiss. And, of course, I feel that the reception MOND has had from much of the community will prove to be another example.
So, yes, physicists (scientists in general) can be very wrong sometimes. But one should by no means use this fact to argue that they should not be listened to on science matters. Those examples I brought up are, by far, more the exceptions then the rule that scientists can judge about scientific matters much better than others. So when should we listen to scientists and when shouldn’t we? It is like knowing when to take the advice of your doctor on medical matters, and when not to.
David: Thank you Professor!