# Scharnhorst effect - faster or slower than c?

#### Dmitry67

http://en.wikipedia.org/wiki/Scharnhorst_effect

I am confused.
As wiki states "The time the photon's energy spends as subluminal electron-positron pairs lowers the observed speed of light in a vacuum"

So light travels at c=1, spending some time in a form of subluminal electron-positron pairs with v<1. It makes an effective speed of light <1.

Related Beyond the Standard Model News on Phys.org

#### ofirg

I think the point is that the value of c we measure in vacuum already includes this effect, i.e., The value we measure and qoute is after this "slowing down". Now, when a photon travels between conducting plates, then the "slowing down" is smaller than in vacuum due to the absence of large wavelengths because of the boundry conditions. Therefore the photon's speed will be larger than the what we measure in vacuum.

However, I'm not sure if what they claim is realy true

#### Naty1

Science News, 1990, cited in the Wikipedia says this:

But the predicted increase in speed is exceedingly small and occurs only for light propagating perpendicular to the plates. For parallel plates just 1 micron apart, the change amounts to roughly on part in 1036.

"It's laughably small," Barton says. "The effects are too small by many orders of magnitude to be measured, but appear fascinating as matters of principle."

The results don't call into question anything basic about relativity theory, Barton argues. "All this says is that if you really had infinitely extended, parallel mirrors, then at right angles to these, there is still a maximum speed - in the same way that ordinary relativity says there is a maximum speed called c in empty space."
Seems that, in simple terms, they are saying 'if you slightly change the nature of space, that is it's vacuum energy, the speed of light will change slightly'....
seems reasonable on the surface.

In a separate physics letter cited by Wikipedia, it is concluded by different scientists that no measure of the faster than c velocity of light predicted by the Scharnhorst effect is possible.

#### Dmitry67

It is obvious that you can't measure it: plates (under tension because of Casimir effect) have weight, so light will be travelling in gravitation well, slower than c to external observer.

Still, it is interesting...

Another related question, what's about the magnitude of the same effect in a FALSE (dense) vacuum?

#### cryptist

I don't think they are saying that light will faster than c. They are actually saying that by changing vacuum permittivity and vacuum permeability, it is possible to change the speed of light, c itself!

#### Demystifier

2018 Award
That photon cannot move faster than c, can be derived from two assumptions:
1) Photon mass is zero.
2) Lorentz invariance

In the Scharnhorst effect (and similarly in the effect studied in http://lanl.arxiv.org/abs/hep-ph/0301275 ) photon moves faster than c because it is assumption 2) which is violated. Namely, the plates define a preferred Lorentz frame (with respect to which the plates are at rest), the effect of which is that Lorentz invariance is violated.

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