Against dark matter: globular clusters

One of the best 'sign-post' papers I ever read, which convinced me which way to go, was by Scarpa et al. (2006) (see reference below). There have been more conclusive ones published since, but this was the one I happened to read first.

To make dark matter fit general relativity to the oberved galaxy rotations you have to assume that it stays spread out in a halo around the galaxy, and therefore does not have structure on small scales. Scarpa et al. looked at globular clusters which are small areas within the Milky Way, where the stars are arranged slightly more densely than in surrounding areas. They found that the globular clusters behaved like little galaxies: whenever their internal accelerations dropped below a critical acceleration, a0, their dynamics became non-Newtonian. Their crucial point was that you can't use dark matter to explain the anomalous dynamics of tiny globular clusters since to fit it to galaxies you've already specified it must spread out: you can't have it both ways.

Scarpa et al. also pointed out that the external acceleration on the globular clusters due to the galaxy was larger than a0, but the anomalous behaviour still occured when the internal accelerations dropped below a0. I'm happy to say that this points away from MoND, and towards MiHsC which relies on the mutual accelerations of nearby matter.

I still have this glorious paper and I wrote on my copy: “Brilliant stuff! Tells me which way to go :)”. It seems to have been ignored by most of the astrophysical community, but it shows that the dark matter idea is unworkable.

Scarpa, R., G. Marconi and R. Gilmozzi, 2006. Globular clusters as a test for gravity in the weak acceleration regime. Arxiv: 0601581v1.

McCulloch, M.E., 2012. Testing quantised inertia (MiHsC) on galactic scales. A&SS, 342, 575. Arxiv: 1207.7007.

Beyond the new horizon

In 1935, Einstein, Podolsky and Rosen published a paper in which they imagined a photon which emits an electron and positron which zoom off in opposite directions as two entangled particles. Quantum mechanics says they do not have definite spin. If you then measure the spin of one of them when they are far apart, and it happens to be spin up, then the other particle must be spin down. Since the two particles suddenly have these definite spins, information must have travelled instantaneously between them over a potentially huge distance: violating special relativity. This unnerved Einstein because he, by then, supported the idea of "local realism". Local means that information cannot travel faster than light and realism says that the Moon is there even when you can't see it. The young Einstein did not agree with realism. His special relativity came from saying "if you can't observe it, it doesn't exist", quantum mechanics relies on this too, but the later Einstein and others argued that the required spin information is always there hidden inside the entangled particles (as a hidden, dark, variable) and is only revealed upon measurement.

However, in a 1964 paper John Bell identified a measurable experimental difference that would occur depending on whether local realism (the hidden variable) was true or not. If you measure the spin of the two particles at random angles, then if you happen to measure them parallel you will get anticorrelated results, and you measure perpendicularly you will get no correlation, but at intermediate angles, if local realism was true there would be a linear dependence of correlation on the angle, if not there would be a cosine dependence. In the 1970s and 1980s Freedman and Clauser and Alain Aspect and others measured this, and found a cosine dependence. So it looks like the common sense thing: local realism, does not work. It is true that all of the experiments that have been done so far have had loopholes in them that might allow them to still be explained by local realism, but they have all had different loopholes, and so you would need a different version of local realism to explain each case, which seems unlikely. A loophole-less experiment would be conclusive, but may not be possible.

As Heisenberg always said "How fortunate that we have found a paradox. Now we have some chance of making progress!". These experiments support a philosophy that has always seemed to work, and was used by Berkeley, Mach and the younger Einstein, that if things can't be seen "in principle" then you have to assume they do not exist. I also like this philosophy. In my recent paper (EPL, 101, 59001, arxiv preprint: 1302.2775) I argued that the inertial mass of an object is caused by Unruh radiation and that, for example, when an object accelerates to the right, a Rindler horizon forms to its left because objects behind the horizon can never hope to catch up so their information is lost to the object. This means (to simplify) that Unruh waves longer than the distance to this new Rindler horizon can never be seen by the object and therefore, with this philosophy, don't exist as far as it is concerned. As a result of this, there is less Unruh radiation to the left of the object, less radiation pressure on it from the left and this produces a net force towards the left that opposes the acceleration. This works neatly as an model for inertia (see 1302.2775). All this relies on the principle that if something cannot be seen in principle, it dissapears, or: undergoes total existence failure (a amusingly over-complicated term used by Douglas Adams).

Einstein once said to Abraham Pais (who did not believe in realism): "Do you really think that the Moon doesn't exist when you are not looking at it?". My answer would be that it is there if you don't look, but if it is impossible for you to look (a fundamental horizon gets between you) then it would not be there, or more importantly: its effects would not be. I think that this can be tested, more on this later..

References:

Einstein, A.; Podolsky, B.; and Rosen, N., 1935. Can Quantum-Mechanical Description of Physical Reality Be Considered Complete. Phys. Rev., 47, 777-780.

Bell, J.S., 1964. Physics 1, 195-200.

McCulloch, M.E., 2013. Inertia from an asymmetric Casimir effect. EPL, 101, 59001. http://arxiv.org/abs/1302.2775

Against Dark Matter - If the shoe fits..

Well done to the people behind the Alpha Magnetic Spectrometer (AMS) on the Space Station that has just reported some results. It is always good to collect data in new regimes. Whatever they find, they will find something new. They do not seem to have anything significant to say about where their extra positrons are coming from though.

The AMS was designed to search for dark matter: a terrible theory in my view, because it is not falsifiable and it looks to me like an attempt to support a theory (general relativity), that was devised before we knew about galaxy rotations, by adding invisible matter ad hoc to galaxies to force the galactic mass and rotation data to fit general relativity (dark matter is so flexible that you can fit many models to the data). There was a similar argument in Galileo's time. Aristotle had said heavier objects fall faster, but Galileo noticed in a hailstorm that big and small hailstones fell together and later on he showed, using balls and inclined planes, why this must be. His contemporaries, to play safe and support Aristotle, said "Aha! The big ones must have fallen from a greater height". This could have been right, but the clue was in the ad hoc way they had to set up the initial height of the various sizes of hail: there was no reason for it. There is also no reason why dark matter should be in a diffuse halo around a galaxy.

If dark matter is too vague to be falsified, all one can do is suggest less arbitrary alternatives.

Milgrom's empirical MoND (Modified Newtonian Dynamics) is a little less arbitrary. MoNDians suggest that we tweak either gravity or inertia at low accelerations (so there are two varieties). They do not have a specific mechanism in mind, but at least they have less wriggle room than dark matter. MoND has a formula (well, a few choices), but it does still have one fitting parameter: a0. By varying a0 MoND can be made to fit galaxy rotation curves very well, but is a bit off with clusters.

Then there is MiHsC, which has a very specific physical model of inertia. MiHsC predicts galaxy rotations and galaxy cluster dynamics within the uncertainty in the observations, with comparable accuracy to MoND, and without any adjustable parameters at all, see: http://arxiv.org/abs/1207.7007 (Journal: Astro. & Space Sci., 342, 2, 575-578). See also the figure in my blog entry here.

An analogy: You go into a shoe shop. The owner proudly produces his revered Einstein Shoe (nice curves, no sock needed). He measures your foot, concludes with regret that your foot is too big for the shoe and suggests chopping most of your foot off. That's the dark matter way. A MoND shoe would fit with some adjustment at the cobblers. A MiHsC shoe would fit with no adjustments, but it's an unknown brand..