[peter303]

The neutrinos arrived 3 hours before the photons. Thats 1/500,000,000 difference.

Plenty of things could have slowed the photons.

[analog31]

That raises an interesting thought: Does interstellar space have a non-unity refractive index? If there's any kind of matter out there, then I suppose the answer has to be yes. But I wonder if it's quantifiable.