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Message: <blockquote data-quote="freyar" data-source="post: 6867684" data-attributes="member: 40227">You're right, this wouldn't lead to "tired light" or slow down the speed of light.  Basically, as a single photon travels along, it constantly excites (and de-excites) virtual electrons but in such a way that the wave speed of light is unaffected (and specifically is still independent of frequency).  The reason for this is that the interaction between the photon and the electrons still respects special relativity.  What it does effect, however, is how strongly photons and electrons interact, in other words, the value of the electric charge.  You might have heard something to the effect that an electron is surrounded by a cloud of virtual electrons & anti-electrons that collectively "screen" the electron's charge.  A photon hitting an electron with more energy gets closer to the electron, so it is screened less and effectively sees a larger electron charge.This same kind of effect means that two photons can also "hit" each other.  Most of the time, two photons will just pass each other by, but sometimes they will excite the same virtual electrons and can both bounce off that virtual electron.I'm not an experimentalist, so I don't keep track of every result like this terribly carefully.  The following is my understanding, but I might have missed something: Two photons coming together and creating an electron/anti-electron pair has been observed in a lab starting in the late 1990s AFAIK.  The caveat is that the only way we have to produce energetic enough photons on earth is in high energy collisions of other things, so the photons involved are themselves fairly virtual (it's worth mentioning that virtual is not a binary descriptor but a continuous thing --- every particle is in reality a little virtual).  On the other hand, this process appears to happen fairly commonly in astrophysics since there are lots of things in space that create high energy photons.I don't believe two photons bouncing off each other in the vacuum has been observed yet, and in fact the predicted rate for this to happen is below current experimental sensitivity --- if we'd seen it by now, it would mean there's something we don't understand going on.But it's worth mentioning that both of these types of events are embedded and inevitable in the Standard Model of particle physics AND any way we might know how to modify particle physics consistent with things we've already measured.  If they didn't happen, it would mean we'd literally have to scrap almost a century of particle physics theory which has been otherwise remarkably successful.  What I'm trying to say is that there are different degrees of "still just in theory."  There's stuff like this, which pretty much has to happen given what we know already; there is a concept like dark matter, which we don't know a lot about but can be extremely confident exists from what we know; there is something like string theory or another approach to quantum gravity, which we can't directly access experimentally for the time being but is a logical extension of what we know and have observed (and can be tested mathematically); and there is something like the exotic matter needed for wormholes and warp drives, which doesn't really naturally come up for any other reason and can easily violate basic physical principles if you're not really careful about it.</blockquote>

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