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Speed of light is in general $c/n$ where $n$ is a refractive index. But for example introducing two parallel plates with very small spatial separation will perturb the energy density of vacuum reducing it in between the plates, thus effectively lowering $n$. So the rate of induction in this part of space would increase giving larger than $c$ light speed.

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    $\begingroup$ Why do you assume that the refractive index of the vacuum is related to "the energy density"? $\endgroup$ – probably_someone May 7 at 13:31
  • $\begingroup$ That's not really the point, but the more mass energy the harder it is for light to pass through $\endgroup$ – Leo Kovacic May 7 at 14:22
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    $\begingroup$ Even if your intuition were correct in normal materials (which in general it isn't - for example, both olive oil and ethanol are less dense than water, but have a higher refractive index), why would it automatically extend to the properties of the vacuum? $\endgroup$ – probably_someone May 7 at 14:26
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    $\begingroup$ That's not really how scientific reasoning works. In order to extrapolate past the end of limited data (which is what you're doing here, when you take a not-even-right-most-of-the-time rule of thumb for the high-density regime and try to apply it to the zero-density regime), you need to justify why the relationship shown by your data should extend to regions you haven't measured yet. The default assumption will always be that the relationship isn't the same outside your dataset. $\endgroup$ – probably_someone May 7 at 16:10
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    $\begingroup$ Otherwise, every newlywed could say, "I had zero wives yesterday, and I have one wife today. Extrapolating, I'll have 14 wives in two weeks. Why would I not?" Of course, in this example we know for a fact that the relationship between number of wives and time changes outside his dataset. But what if you're making the same kind of mistake here? "Why not?" isn't enough. (also, see physics.stackexchange.com/questions/44850/…, where it's demonstrated that there's not really a correlation between refractive index and density.) $\endgroup$ – probably_someone May 7 at 16:14
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This question has even been publicly studied ( since such research is highly classified) there is not too much to read on the subject and it's hard to find. See https://en.m.wikipedia.org/wiki/Scharnhorst_effect

Basically since Casmir plates lower the density of " Dirac sea particles" photons spend less time interacting with them thus spending less time as subliminal products of those interactions effectively moving at a velocity larger than c

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You can find a related question on stackexchange here

Related to the Casimir effect , see this link.

A hand waiving, intuitive answer would be, if you change the average energy density in a region of space, you change the curvature, so you change the geodesics, but this is far from a decent answer.

I don't think there is an easy answer to this question, but I like it. Therefore, I propose to actually perform the experiment. Measure the speed of light in a direction parallel to the Casimir plates, inside a Casimir cavity.

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    $\begingroup$ This does not seem like an answer. I think it's more like an extended comment. $\endgroup$ – StephenG May 7 at 16:08
  • $\begingroup$ It might be the only answer right now and that is it's unknown I guess. If you can cancel out some modes of quantum fields simply by using two plates there ought to be more technical ways of doing it so to extend the ' Casmir vacuum ' volume. I don't think that current theories can calculate this, so is there any such experiment? $\endgroup$ – Leo Kovacic May 7 at 16:14
  • $\begingroup$ In any case, I would discard any assumption of FTL signaling, since it would enter in conflict with causality. But I think this would be an experiment worth considering. $\endgroup$ – Cristian Dumitrescu May 7 at 16:18
  • $\begingroup$ I am not aware of any previous experiment of this type. $\endgroup$ – Cristian Dumitrescu May 7 at 16:19
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    $\begingroup$ Changing the geodesics is not the same thing as changing the speed. $\endgroup$ – probably_someone May 7 at 16:24

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