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But 20^th century theories of Quantum Electrodynamics indicate that photons interact with the "vacuum polarization" of the vacuum through which they are traveling.<ref>http://www.extremetech.com/extreme/184879-einsteinian-error-the-25-year-old-supernova-that-could-change-the-speed-of-light-forever</ref>  Photons can spontaneously split into electron-positron pairs, which interact with each other gravitationally before recombining.  This causes light to travel about 1 part in 300 million slower than '''c'''.  Since neutrinos don't interact with the electromagnetic force, they are not subject to the vacuum polarization effect, and travel only 1 part in 10²² slower than '''c''', which is faster than photons.

But 20^th century theories of Quantum Electrodynamics indicate that photons interact with the "vacuum polarization" of the vacuum through which they are traveling.<ref>http://www.extremetech.com/extreme/184879-einsteinian-error-the-25-year-old-supernova-that-could-change-the-speed-of-light-forever</ref>  Photons can spontaneously split into electron-positron pairs, which interact with each other gravitationally before recombining.  This causes light to travel about 1 part in 300 million slower than '''c'''.  Since neutrinos don't interact with the electromagnetic force, they are not subject to the vacuum polarization effect, and travel only 1 part in 10²² slower than '''c''', which is faster than photons.

In order to observe this effect, and extremely long travel path for neutrinos and photons is needed.  The supernova SN1987A provided such a path&mdash;it is 186,000 light years from Earth.  (So the supernova actually occurred 186,000 years ago.)  There are theoretical reasons to believe that the flash of light would have been delayed about 3 hours from the neutrino burst, since the photons had to pass through the exploding star, interacting with it, whereas the neutrinos had no such interaction.  But the observation of the light flash was 7.7 hours later than the neutrino burst, indicating that it took 4.7 hours longer to travel to Earth.  This is consistent with the predicted slowing of photons due to vacuum polarization.

In order to observe this effect, an extremely long travel path for neutrinos and photons is needed.  The supernova SN1987A provided such a path&mdash;it is 186,000 light years from Earth.  (So the supernova actually occurred 186,000 years ago.)  There are theoretical reasons to believe that the flash of light would have been delayed about 3 hours from the neutrino burst, since the photons had to pass through the exploding star, interacting with it, whereas the neutrinos had no such interaction.  But the observation of the light flash was 7.7 hours later than the neutrino burst, indicating that it took 4.7 hours longer to travel to Earth.  This is consistent with the predicted slowing of photons due to vacuum polarization.

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