# Is the speed of light in a vacuum constant?

The news site is titled, “Speed of Light May Not Be Constant, Physicists Say.” It talks about how the speed of light in a vacuum is not constant and that estimates of the size of the universe might be off.

Would this also affect the estimates of the age of the universe, or would the differences of the speed of light would be so minute that it would not cause a drastic change?

• As the article says (in a massive understatement), “Some scientists are a bit skeptical, though.” Jul 12 '19 at 20:51
• There is plenty of evidence that it is, and no evidence that it is not. Jul 12 '19 at 20:53
• Possible duplicates: physics.stackexchange.com/q/2230/2451 and links therein. Jul 12 '19 at 21:11

Even if these theories are correct, it's a tiny effect- $$\sim 5\times 10^{-17}~{\rm s\over \sqrt{m}}$$ of fluctuation according to one of the papers. For the diameter of the observable universe ($$\sim9\times 10^{10}~\rm ly$$), that comes out to about $$400~\rm km$$, or $$5\times10^{-22}$$ times smaller. This is much, much less than the margin of error we already have for the radius of the observable universe, and so would not affect estimates of the universe's size or age at all.

As per our currently accepted models, SM, and GR, the speed of light is always c when measured locally, in vacuum.

Now the article states two things:

1. The vacuum has fluctuations, that is, virtual particles (pairs of virtual particles and anti particles) are popping in and out of existence. As the photons travel through the vacuum fluctuations, the photons scatter off these virtual particles (actually it says that the photons are absorbed and re-emitted) and these interactions of the photons with these virtual particles slows the wavefront. Now photons do not interact with virtual particles. Virtual particles cannot absorb or re-emit photons.

2. The other says that there are currently 9 elementary particles in the SM with EM charge, but they assume that in reality there might be 100 particles with EM charge. These particles EM charge could change the permettivity (electric fields) and permeability (magnetic fields) of space, and since light is an EM wave, this could change the speed of light in vacuum.