That's not what I took away from this... (Score 5, Interesting)

From what I gathered, the basis of Franson's hypothesis is the idea that a photon can spontaneously split into a matter-antimatter pair (this is also the idea behind Hawking radiation). Unless something crazy happens (again, see: Hawking radiation), that pair will almost instantly recombine, creating a photon with the same size and energy as the original photon.

Franson's idea, as I understand it, is that during the small window between creation and annihilation, the massive particles are under the influence of gravity, which bleeds off energy. When the pair recombines, it results in a reduced velocity of the photon.

Now, as I understand it, reducing the energy of a photon would merely reduce its frequency (red-shifting), not affect its actual velocity.

However, over long distances, the total time required for a photon to travel distance X would thus be slightly more than X/c, based on the proportion of time spent as a pair of massive particles, rather than as a massless photon. From a statistical perspective, this yields an average velocity of slightly less than /c/ (the speed of light in a vaccuum).

This seems reasonable to me, at least at first.

mrsquid0 raises an issue, though: Photons in the visible light range are not sufficiently energetic to create an electron-positron pair. I do not know if the photons in question were in the visible light range or not.

NoNonAlphaCharsHere also raises an important point: the electron-positron pair *cannot* travel at the speed of light. In fact, he/she raises an even better idea than Franson; my reading of Franson's explanation is that gravity is slowing down the particles (gravity field behind the photon), but there's just as much opportunity for gravity to *speed up* the particles (gravity field in front of the photon).

Now, I don't feel like doing all the math for this one little message, so here are the things I would consider before taking this article (and the original paper) at face value:

• This is predicated upon the idea that gravitational fields affect neutrinos less than they do photons and ordinary matter. Do we know this to be true?
• For the slowing down of the e-p pairs bit, there are two opposing forces:
  • I expect that higher-energy (higher-frequency) photons are more likely to generate an e-p pair than lower-energy photons. This means they are more likely to be slowed down
  • When a photon of energy /E/ forms an e-p pair with combined mass /m/, there is E - m*c*c energy unaccounted for. I expect that that energy ends up as kinetic energy, resulting in a velocity v = sqrt(2 (E - m*c*c) / m). Therefore, higher energy photons will have more energy left over and the e-p pair will thus being going faster.
• According to the Wikipedia article on Pair production, the spontaneous formation of a matter-antimatter pair can only occur inside a nucleus (or momentum could not be conserved). However, this necessarily involves the photon traveling through a non-vacuum, which will necessarily slow it down.