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This question already has an answer here:

In a vacuum, are photons always traveling at 100% light speed? Can photons go any slower in a vacuum? They have no mass which means they can go at 100% light speed, but do they have to?

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marked as duplicate by Bill N, John Rennie visible-light May 10 at 11:07

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  • $\begingroup$ Yes, the speed has a deterministic value, not a probability distribution, if that's what you're asking. $\endgroup$ – Avantgarde May 8 at 21:19
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Photons always do travel in vacuum when measured locally at speed c.

Now you are asking whether they could go slower in vacuum. The answer is only if you measure from far away.

This is the Shapiro effect, and it says that if you measure the speed of light from Earth as it passes next to the Sun, you will get a speed slower then c.

But in vacuum, measured locally you will always get speed c.

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    $\begingroup$ The word weather should be whether. I can't edit because it is only 2 character difference. $\endgroup$ – Bones May 9 at 4:34
  • $\begingroup$ Great point bringing up "local" observations, versus those over long distances. $\endgroup$ – ggcg May 9 at 14:07
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In a flat space vacuum yes, they have to. In a medium, light might appear to move slower, this gives rise to the refractive index of various media. But at the quantum level the time delay is caused by scattering as well as absorption and re-emission of the light by the particles in the material. The macro effect is that it took light longer time to pass through the material.

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