# Lightspeed variable say *intellectuels français*

## Vacuum isn't always empty, French boffins argue

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French researchers have proposed a mechanism by which the observed speed of light might not be the constant we think it is: it could, in fact, vary at the attosecond level.

It's not, however, time to reach for the “the old boffins were wrong!” template, because their reasoning is actually elegant and simple: we know that vacuum isn't really vacuum, therefore the condition for a universally-constant lightspeed (that is, the speed of light *in a vacuum*) never actually exists.

The study, published in *European Physical Journal D* and also available on Arxiv here, also takes a shot at explaining three fundamental constants, the permissivity of vacuum, the permeability of vacuum, and the speed of light. Far from “escaping any physical explanation”, the study states, these constants “emerge naturally from the quantum theory”.

Let's tackle the speed of light: James Clerk Maxwell first imagined that light-speed in a vacuum is a constant throughout the universe. However, we've known for some time that if you look at vacuum at a quantum level, it doesn't *stay* a vacuum: pairs of particles and antiparticles (quarks and antiquarks, electron-positron) that arise spontaneously and annihilate each other in vacuum fluctuations.

That “quantum noise” is a spooky property that looks like the Universe is watching us back: it preserves Heisenberg's uncertainty theorem, since even the coldest vacuum of space can't be perfectly described as being empty; and vacuum fluctuations can even interfere with our most sensitive physical instruments.

The study, by Marcel Urban of France's University of Paris-Sud in Orsay, suggests that the energy fluctuations in vacuum could also affect the speed of light, since there's a statistical chance that light will interact with these “virtual particles”. As noted in publisher Wiley's announcement, “The fluctuations of the photon propagation time are estimated to be on the order of 50 attoseconds per square root metre* of crossed vacuum, which might be testable with the help of new ultra-fast lasers.”

The researchers also say their hypothesis could provide a prediction for the number of ephemeral particles in a given amount of space: the “ground state” of the unperturbed vacuum contains “a finite density of charged ephemeral fermions antifermions pairs” in their model. ®

Correction: The original version of this story stated 50 attoseconds per square metre, instead of per square root metre. ®