The Speed of Light May Be Less Constant Than We Thought

There are probably only a few things about physics non-scientists know, but that the speed of light is constant is one hopefully of them. It’s what we all--again, hopefully--learned in grade school and high school textbooks. But new research is saying that that might not be the case. Some scientists are exploring the possibility that the speed of light actually changes due to the nature of the vacuum of space. It might go against what we think we know about the Universe, but that’s just what two papers published in the European Physical Journal D are arguing.

The speed of light, which beams along at about 670,616,629 miles per hour, is the backbone of many cosmological and astronomical theories. It’s the number astronomers use when measuring the size and age of the Universe, the distance between objects, and the qualities of the stuff that makes up everything around us. If the speed of light turns out not to be constant, it could mean that some pretty massive things, like the estimated size of the Universe, are different from what we’ve long thought.

Using different means, the two papers try to derive a the speed of light from the quantum properties of space itself. The key in both papers is that the scientists treat space not as a vacuum, but full of particles.

One paper, led by Marcel Urban from the Université du Paris-Sud, focuses on the cosmic vacuum. According to laws of quantum physics, the science that governs all the tiny things in the Universe, space is full of particles that pop into existence, collide with their counterparts, and promptly pop out of existence. It’s what happens when matter and anti-matter particles collide: they annihilate each other.

This is significant because light interacts with these particles, even during their ultra-brief lifetimes. Photons, the particles that make up light, are actually absorbed and reemitted by these short-lived particles. This means that the energies of these particles, and more importantly the energy they carry, can interact with photons to affect the speed of light. It would be a tiny difference, but one big enough to change a measurement of light speed over a long distance. One example of this is a gamma-ray burst, a short but intense blast of gamma ray radiation typically originating from outside our galaxy.

The second paper, led by Gerd Leuchs and Luis Sánchez-Soto from the Max Planck Institute for the Physics of Light in Erlangen, Germany, uses a different mechanism to explain the changing speed of light. The scientists argue that it’s the number of species of elementary particles existing in the universe that together make up the speed of light.

Leuchs and Sanchez-Soto calculate that there should be somewhere around 100 different sorts of charged particles in the universe; current law governing particle physics, the so-called Standard Model, identifies nine: the electron, muon, tauon, the six kinds of quark, photons, and the W-boson.

The crux of their paper is the claim that all 100 particles they propose exist carry charges. Every particle in a space impedes something traveling through that space. And space in turn, because of the density of the particles that fill it, either resists or supports magnetic fields. In the Universe, the charged particles impede light, and because light waves are both an electric and magnetic wave, how space either resists or supports the magnetic field will affect the speed of light.

Both studies say that light interacts with the particle-antiparticle pairs that permeate all of space. But the evidence Urban and Leuchs and Sanchez-Soto put forth in their papers and the common conclusions they come to aren’t convincing everyone just yet. Their mathematical methods are being called too unconventional by their peers, and their conclusions deemed insufficient enough to warrant further study.

The other issue is the assumption of many more particles beyond the nine explained in the “Standard Model.” And finding these particles is currently out of or beyond the energies currently available by  particle accelerators.

If it turns out light speed does vary, how big of a change can we expect to see in this former cosmological constant? Urban’s team found a difference of about 0.05 femtoseconds for every square meter of space. A femtosecond is a millionth of a billionth of a second. That may be significant for physicists, but for the rest of us, it’s pretty tiny. Still, it’s a pretty neat, if still uncertified, discovery.

More on (the speed of) light:

Travelling Faster Than the Speed of Light is Harder Than it Looks

How Life Harvests Light Even in Total Blackness