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Thoughts on Photons and Neutrinos

October 26, 2011

Disclaimer: I am a computer scientist, not a physicist. These are merely my thoughts on the topic, and I know that I may well be wrong. Feel free to mock me comment.

Neutrinos were recently observed apparently going faster than the speed of light. The first reaction everyone had: there must be a measurement error. Let’s stop for a minute and consider the possibility that the measurement was correct. Perhaps neutrinos do travel faster than the speed of light. More precisely, perhaps they travel faster than photons.

Suppose you’re experimentally measuring the average speed of photons and neutrinos going through empty space. Maybe you’ll want to measure this speed along a great distance to reduce experimental error. Except there is a slight issue right there: there is no such thing as empty space in the real world. Even the void between galaxies contains some nonzero density of particles. Photons are liable to interact with the particles found in space. The probability is small, especially in the vacuum of space, but not zero (and it grows with the distance your photons travel!). Such interactions could slow a photon down. We already know that light travels much slower in water than in air, which causes refraction.

We also know that the energy of different wavelengths of light refracts differently. This is because photons of different wavelengths have different energies, and travel at different speeds when interacting with matter. The same potentially happens with neutrinos, but according to Wikipedia:

1.6 to 3.4 eV: the photon energy of visible light.


the combined mass of the three neutrino varieties must be less than 0.3 eV

So neutrinos have much less energy than the photons of visible light. The kind of light we’d probably measure the speed of. Thus, they are probably less likely to interact with particles, in general, than visible light photons. As such, their speed would be less affected (less slowed down?) by the stuff that floats around in space. This would make neutrinos able to, on average, travel closer to the speed of light the absolute speed limit of the universe, also known as c. Perhaps this is the answer to our problem. The constant c is not the speed of photons. It is not the speed of light, per-se. It’s a property of the universe. Perhaps neutrinos, on average, travel slightly closer to c than visible light photons do… Without either breaking this absolute speed limit.


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