phinds said:
I am NOT a fan of ANY of the "multiverse" type ideas.
This is not actually a multiverse theory in any sense. It is a theory that the particles of dark matter are more or less in 1-to-1 correspondence with the particles of the standard model - there would be dark quarks and dark leptons, interacting via dark gauge bosons. But the particles of this dark standard model would only interact with the particles of 'our' standard model via gravity. So the 'mirror world' here does not refer to another universe. The visible 'world' and the mirror 'world' are two types of matter dwelling in the same space-time.
Here is the problem I have with a mirror explanation of dark matter. Dark matter is there to explain the orbital speeds of stars towards the outer edge of galaxies. The explanation is that there is a large halo of dark matter, more massive than all the visible stars of the galaxy, and extending beyond it, and it's the gravitational effect of the mass in the halo which alters the orbital profile of galaxies. But this requires that the halo mostly consist of particles that don't interact with each other, and are spread homogeneously throughout the halo.
On the other hand, if the physics of dark matter is the same as the physics of the standard model, then the dark matter should behave like visible ('baryonic') matter, and cluster into stars. The dark matter should therefore form a disk, like the visible galaxy, though possibly tilted at a different angle. But then its gravitational effects wouldn't give you the galactic rotation curve that you are trying to explain! (It occurs to me that not all visible galaxies are disks, some are 'elliptical', and so you might think that a dark galaxy which formed an ellipsoid of dark stars, rather than a disk of dark stars, would be closer to the halo concept. But presumably it's still not close enough to explain the observations - as I have never heard anyone advocate for such a model.)
So just to be clear, the problem is that if mirror matter is just a copy of normal matter in how it interacts with itself, it won't produce the homogeneous halo that is needed to explain the effects of dark matter. By comparison, the commonest supersymmetric explanations for dark matter worked because the dark matter was supposed to be made of stable supersymmetric particles that had no significant interactions, so they had nothing to do but form halos. The failure of supersymmetry to show up at LHC means those theories are not likely to be correct, but at least they could have worked.
If I ask how a mirror matter sector of physics could give you a dark matter halo, I say OK, maybe you could get that if the parameters are different. In other words, the standard model says, there are quarks, leptons, gauge bosons, and their couplings have the following values, and that implies atoms and all the rest of what we see. The mirror standard model could have the same kinds of particles, but with different couplings, and maybe that could give you a physics where, say, the 'mirror electron' has the stability and inertness you want in a halo particle, e.g. because the mirror electromagnetic coupling is much much weaker than the electromagnetic coupling of the visible world. In this case, even though mirror matter would have a similar roster of particles to visible matter, the physics would be quite different because the masses and interactions have different strengths in the mirror world.
Marilyn67 said:
what is inexplicable is this constant 10-second difference since 30 years, and that the standard model doesn't explain !
This will sound like a word game, but I believe it leads to a valid point: there is a difference here between 'inexplicable' and 'unexplained'.
In particle physics, ever since the 1970s, it has actually been very easy to invent possible new physics. What happened at the start of the 1970s, is that the mathematical understanding of the possibilities of quantum field theory was greatly improved. This made it possible to figure out what is apparently the correct theory of particles (at least at low energies), the standard model. But having figured out how to calculate with gauge theories and so on, we can now also modify the standard model in unlimited ways - what if there's this new force, those new particles, these extra symmetries - and obtain possible theories whose predictions can then be calculated.
'Unexplained' means we don't know the explanation, but 'inexplicable' somehow implies (in English idiom) that we don't even see any way to explain it, we don't know how it could be true at all. And I think is not true, I think there are a variety of theories of the neutron discrepancy. This theory in which neutrons can turn into mirror neutrons and disappear from detection, is just one such possibility. There must be simpler theories which use the same mechanism - neutron becomes something else - but which don't require a whole mirror sector, just a few new types of particle.
In fact, there's an even 'simpler' theory of the neutron discrepancy. There are two experiments, neutrons in a beam and neutrons in a bottle. The experiments measure the rate at which neutrons turn into protons, I think by counting the protons. But protons can also just be lost, i.e. escape without being detected. If one experiment loses protons at a different rate to the other experiment, then the proton count will be different, not because of new physics, but just because one experiment is harder to control than the other experiment. I think most people actually expect that some side effect of the experimental details, rather than new physics, is the reason for the discrepancy.
((One final note, these are not subjects I have studied as a specialist, and there could be errors of technical detail in what I just wrote, but hopefully the gist of it is correct.))
