2) Tachyonic neutrinos (not likely) Another possibility that occurred to me (which is probably way off, and has likely already been trivially disproven by some

2) Tachyonic neutrinos (not likely) Another possibility that occurred to me (which is probably way off, and has likely already been trivially disproven by some simple experiment that I'm not thinking of at the moment) is that the "speed of light" that we tend to measure is actually a bit lower than the geometrical "c" that appears in equations (I'm not sure what the state-of-the-art is on measuring that geometrical quantity directly), because for some reason photons are slowed as they move through the vacuum (similar to how we can very easily slow photons by making them move through glass, except this would have to be via interactions with the vacuum or some other form of matter). It could be that the vacuum itself has an index of refraction for photons not quite equal to 1 due to some unknown interaction. Neutrinos interact with almost nothing, so it's conceivable that they even if they have a small amount of mass and travel slower than geometrical "c", they could still end up going faster than photons because they slip through background interactions with ease. Again, this is wild speculation, "c" is used all over the place, and if we had the wrong value for it, that would require a lot of coincidences, that this sort of vacuum index-of-refraction effect also had the side effect of making other measurements involving the speed of light (energy measurements, for instance - I would tend to think a discrepancy would be way more obvious here, involving, as they do, factors of c^2) come out looking correct, and that seems unlikely to me. Then again, so much of what we directly measure is grounded in e/m interactions, so there's a chance (vanishingly small, as it is) that a fifth decimal place discrepancy in photon-in-vacuum-c vs. geometrical-c could escape notice because of the relative difficulty of directly measuring geometrical-c, especially if whatever vacuum effects that slow photon travel had other side effects that dunged up our energy measurements, too.