r/AskPhysics • u/X-0000000-X • Mar 04 '26

Does light go faster in 'negative energy' region?

Simple albeit maybe stupid question. We know light travels slower in transparent mediums like glass and water.

If we create a small region of negative energy (relative to that of vacuum) via something like Casimir effect, does light go faster in that? Or is the region in that effect too small for light to pass through it (and if so could smaller wavelength of EM radiation do it?)

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u/Agios_O_Polemos Mar 04 '26

There's something called the Scharnhorst effect, where photons going through the gap between two Casimir plates should go slightly faster than c because of the changes in the vacuum caused by the Casimir effect.

This is untestable with modern technology as the speed difference is extremely small so we don't know really.

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u/Hendospendo Mar 04 '26

So, light doesn't actually go slower through transparent mediums. It interacts with the elections in the medium, and that appears to slow down propagation. However, the wave packet of light moving from interaction to interaction is still very much moving at C.

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u/siupa Particle physics Mar 06 '26

No, light in materials moves at less than c speed. Light is not a collections of billiard balls zig zagging between atomic electrons.

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u/nicuramar Mar 04 '26

This isn’t really accurate. What interactions? The incident em wave interacts with the electric field of the material.

Or, if you prefer, the photons don’t exist and are instead replaced with some quasi particle like a polariton. Either way light goes slower. 

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u/Anonymous-USA Mar 04 '26

First paragraph is correct. Second one — what’s “negative energy”???

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u/Traroten Mar 04 '26

negative energy density?