Slower than the Speed of Light

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.

I asked theoretical physicist Leonard Susskind, author of The Black Hole War, recently reviewed in Science Books to explain this apparent anomaly.

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.

8 thoughts on “Slower than the Speed of Light”

  1. There is an idea about the speed of light which does not seem to have entered into scientific thinking, which comes from the way information is transferred.

    If one tries to send information, it travels in “packets” which have a maximum propagation speed in any direction of the average speed of the “packet carrier wave.” This would hold true whether the carrier wave be made up of Pony Express Riders, Sound Waves in Water, or Electromagnetic Waves in what ever constitutes the “vacuum” of space…

    The speed of light is the carrier wave  velocity in any direction, in our universe; therefore, it seems logical to suspect it to be an AVERAGE value of the velocity in any direction of the motions of the entities “carrying” the information.

    Taking off from this idea, has led to a new “Theory of Everything,” which everyone is invited to help develop or refute.  The URL is

  2. I’m not sure I understand what you mean by average speed. Light has an upper velocity, a speed limit as it were, which is c in a vacuum, it can go slower but no faster, what do you think you’re averaging?

  3. Actually, I may not be averaging anything. If the carriers of the information packets exist, and there is no good reason not to think they exist, if they have varying velocities in a given direction, then the speed of light would be the average of those varying velocities.

    It may well be that in the unimaginable eons that these carriers (I think they are tiny oscillators) have existed, they have pretty well “done the averaging” themselves so that they have a constant rotation with a tangential velocity of “c” –or tangential velocities varying very closely around “c.”– and a quite constant torque of “H/c.”  They apparently may even even have a constant average “size” of (h/c)^0.5.

  4. From the short description given, I’m afraid that I can’t take the idea too seriously. It implies tha a pictire formed from light wave patterns would have independent action possibilities.

    How do we get living, self-aware creatures to exist within a holgram?  An interesting challenge in robotics and artificial intelligence to say the least.  DLS

  5. It’s just as bizarre an idea that sentient beings composed almost entirely of empty space can exist and ponder such things? It does, however, beggar questions regarding the nature of consciousness if “we” are projections from a distant horizon. But, it also might solve such anomalies as double slit interference, spooky action at a distance etc.

  6. I agree that the idea that beings made up mostly of “nothing” can contemplate such things is bizarre.

    However, I’m just crackpot enough to consider that perhaps beings existing in a universe made up of a substance consisting of oscillators, rather than empty space, might be able to answer the various questions. Also, such a substance might account for the phenomena that you mention….

     However, of course, we are still stuck with “Where did the oscillators come from…??”

     For more of Dean Sinclair’s ideas check

  7. Of course, all the theorizing in the world cannot *yet* answer that ultimate question even if we bow to Heisenberg (on uncertainty) and Hawking (on time).

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