The speed of light, c, in a vacuum is an important physical constant, by definition it is precisely 299,792,458 meters per second. This value c applies not only to the light we see – the colors of the rainbow, but to all electromagnetic radiation, gravitational waves and anything having zero rest mass.
In Einstein’s theory of relativity the speed of light plays the crucial role of a conversion factor between space and time and between mass and energy.
But, here’s a thing. The speed of light is not constant. Only the speed of light in a vacuum is as fast as you can go. Shine a light through a piece of glass, a swimming pool or any other medium and it slows down ever so slightly, it’s why a plunged part way into the surface of a pool appears to be bent.
So, what about the space in between those distant astronomical objects and our earthly telescopes? Couldn’t it be that the supposed vacuum of space is acting as an interstellar medium to lower the speed of light like some cosmic swimming pool? If so, wouldn’t a stick plunged into the pool appear bent as the light is refracted and won’t that affect all our observations about the universe.
Susskind first of all eliminates my concerns about cosmic quantum jitters, the tiny random fluctuations that pervade the vacuum of space. “These are subtle,” he told me, “but whatever they do, they don’t affect the propagation of light through empty space – at least not in the sense of modifying the velocity of light or doing anything to decohere (fuzz out) a light signal.”
He adds that, “The vacuum is a delicate equilibrium of quantum fluctuations and when one speaks of ‘the velocity of light in vacuum,’ one is talking of the propagation of light in that delicate state.”
However, light is affected by thermal fluctuations, protons, atoms, electrons, dark matter, neutrinos and even the pockets of space-time curvature that accompany lumps of matter. Neutrinos are too sparse and too weakly interacting to make any difference. The main effect of atoms, electrons… is to scatter the electromagnetic radiation just as the atmosphere of the earth scatters light.
“But one has to remember that the average density of interstellar particles is incredibly small,” Susskind adds, “The effects of scattering are very weak and not coherent enough to add up to a change in the velocity of light. But they are there and need to be accounted for.”
Dark matter affects light through its gravitational fields. An example is the effects on light when it passes through the time dependent gravitational fields of galactic superclusters as they expand due to the accelerated expansion of the universe. The effect can be to delay the light signal, bend it, and change its frequency. The result distorts the microwave background and make it look as if it has a lumpiness that has nothing to do with the origin of that radiation.
“You are entirely right,” he told me, “there are all sorts of effects on the propagation of light that astronomers and astrophysicists must account for. The point of course is that they (not me) do take these effects into account and correct for them.”
“In a way this work is very heroic but unheralded,” adds Susskin, “An immense amount of extremely brilliant analysis has gone into the detailed corrections that are needed to eliminate these ‘spurious’ effects so that people like me can just say ‘light travels with the speed of light.’
So, there you have it. My concern about cosmic swimming pools and bent sticks does indeed apply, but physicists have taken the deviations into account so that other physicists, such as Susskind, who once proved Stephen Hawking wrong, can battle their way to a better understanding of the universe.