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After becoming extremely bored while studying for an Afrikaans exam, I started thinking about virtual particles. So, can light (photons) interact with virtual particles (even though they only exist for a very short time)? And can these virtual particles "slow down" light in the same way as atoms delay light when travelling through a medium?

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Yes, light can interact with "virtual particles". It can also interact with itself via virtual particle interactions (see Delbruck Scattering), although I believe direct observation of this effect is currently outside of our experimental capability.

Edit: Just realised I didn't address the second part. When a photon propagates, the propagation receives contributions from its splitting into an electron-positron pair which recombine etc. These processes contribute corrections to the photon propagator, but they do so in such a way that the propagator pole remains in the same place, which in turn means that the propagation speed is unaffected.

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Direct observation is within experimental capability. See "Measurement of the Magnetically Induced QED Birefringence of the Vacuum and an Improved Laboratory Search for Axions" by Siu Au Lee, William M Fairbank Jr, and Walter Toki (Colorado State Univ) - Fermilab proposal, 1995 (that was a while ago!) –  DarenW Oct 31 '12 at 18:15
    
@DarenW Thanks, wasn't aware of that! –  twistor59 Nov 1 '12 at 7:31
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