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That leaves the discussion of:

1) How could these been distinguished experimentally, and

2) How precise do tests have to be.

For those that don't know, the Cartan extension to Einstein's GR is in some sense trivial. So much so, that when people are working to combine GR with QM, they actually mean GR with Cartan's extension but don't bother saying it. That is because:

- for GR to handle matter with spin angular momentum, this leads to Cartan's extension

- Cartan's extension (as far as I understand) still uses the same action: the Einstein-Hilbert action, but it considers additional things as physical degrees of freedom to "vary" in the action

Einstein-Cartan theory is still a classical theory, so it should be possible in principle to distinguish from GR with classical experiments. (Although 'falling' neutron interferometer experiments may work well here if the neutrons could be spin polarized or something. Not sure how to calculate magnitudes of effects.)

To start off discussion, I'll point out some things that came up recently that gave me hope such effects could be measured:

1) In natural units, the spin of elementary particles is much larger than their mass. (That is why if any elementary particle was classically considered to be a point particle it would still not be a blackhole, they would be super-extremal. Well, except for the Higgs I guess if that is found.)

2) Spin allignment in ferromagnets can involve so much angular momentum, that it can be seen in classical mechanics experiments (The famous original g = 2 experiments ... Einstein somehow was involved?)

So, any ideas people?

Are there already some experiments in the works?