What we don't understand about the speed of light

Last June, researchers from the Hong Kong University of Science and Technology published the results of an experiment that proved that light does not move faster than light—specifically, that single photons can't move faster than the official speed of light under certain conditions.

Today, Skulls in the Stars—the nom de Internet of a UNC Charlotte physics professor—has a really great blog post up about this paper. It's very much worth a read. After all, this was basically a test to double check something we were already pretty sure was true. And what's the benefit to proving something you already knew?

A big part of why I'm recommending this post is because Skulls in the Stars does a good job of explaining some tangly optical physics in a way that is quite clear and should make good sense even if you don't have a deep background in this stuff. If you follow along, you'll come away with a good idea of why this particular study matters, and with a deeper understanding of the speed of light itself.

Let’s talk about how we measure the speed of an object first. If we’re looking at the motion of a rigid object, like a speeding car or a thrown baseball, the speed can be determined simply by measuring how much time it takes for an object to travel a distance. The speed is simply the distance divided by the time

There’s a small subtlety to this definition: cars and baseballs are extended objects! To accurately measure an object’s speed, we have to be consistent in how we define its position. For a car driving down the road, for instance, we should do all measurements of its position from a fixed position, such as the front bumper, to measure the speed.

But what do we do when the object doesn’t have a fixed position on it? For example, what is the best way to measure the speed of a hurled bucketful of water?

Image: Tap's eye view of water falling into bucket., a Creative Commons Attribution Share-Alike (2.0) image from kamathln's photostream


  1. Interestingly, quite recently, the speed-of-light ceiling that Einstein’s theory of relativity enforces has been challenged by researchers studying neutrino’s. But we’ll have to wait for the experiment to be reproduced before anyone can make claims. http://www.wired.com/wiredscience/2011/09/neutrinos-faster-than-light/

    1. “We don’t allow faster-than-light neutrinos in here,” says the bartender.  A neutrino walks into a bar.

  2. Great article; thanks for the link.

    One thing I continue to be puzzled by re: the special theory of relativity is that the invariance of the speed of light in all reference frames is an assumption of the theory, not an outcome.  Isn’t that just assuming what you’re trying to explain?  “The speed of light *seems* to be the same no matter how you measure it… So let’s just assume that’s the case!”

    I bring this up not as some crackpot doubting the theory, but as a layperson trying to wrap his head around one aspect of it.  Though the recent neutrino experiment puts the above assumption to the test, it seems likely there’s an anomaly in the experiment, since so many other circumstances and experiments (like the one linked to above) have verified the invariance of the velocity of light.

    But long before those test results appeared, I’ve been baffled by this aspect of the special theory.  At what point is there an *explanation* for the invariance of the speed of light for all inertial frames?  If anyone out there has a link to a good article covering this topic, I’d love to see it.

    1. Sure, maybe you start by assuming the speed of light is invariant. Then you figure out “assuming this, what interesting consequences can we derive?” — in order for the speed of light to be invariant in all reference frames, some pretty unintuitive stuff has to also be true, like time dilation, otherwise the math doesn’t make any sense.

      So then you go out and test the consequences. Turns out time dilation happens in real life! So does frame dragging, and all sorts of other side effects. Also, as far as we can measure it the speed of light really is invariant. Once it starts looking like everything hangs together well, you stop and say “hey looks like we have a new model for the way things work.”

      At what point is there an explanation for the invariance of the speed of light? You could say “because all these other effects we measured, the speed of light has to be invariant,” but that’s a circular definition.

      The real answer is that we will probably never actually know why, fundamentally. Science doesn’t really answer “why is the world like this,” only “what is the world like?” You can answer a few levels of “why” — “why is the sky blue?” Rayleigh scattering. “Why is there Rayleigh scattering?” The interaction of electromagnetism and particles, see, here’s some neat math. “Why does math predict the world?” …I don’t know, it just works, but it’s been working really well for hundreds of years.

      The fundamental why questions, those are for metaphysicians and theologians. Science was never meant to answer these questions, and I don’t think it can. 

      1. You were doing really well – right up to the point you suggested that if Science couldn’t address a question that a theologian could?

        Thanks very much, but I prefer my explanations to come from folks that understand the concepts of evidence and logic, rather than those that choose to have beliefs without evidence or it spite of it!

  3. In his QED lectures that he gave at New Zealand and UCLA, Richard Feynman said that the speed of light, c,  was an average, i.e., some photons went faster than c, some slower. 

    1. Did he really say ‘average’? He was being loose with his language, then.

      Under the feynman interpretation, the photon samples all possible paths (long and short) and then may interfere with itself when you calculate the probability of arriving somewhere. Those photons who sampled the long path might, by a peculiar understanding, be thought of as moving slower than those who sample the short path.

      Of course, the probability density is only for one photon. Once you observe it, that photon (and all others) picked one path and did so at the speed of light. Nevertheless, the REALLY WEIRD THING about the feynman interpretation isn’t that a single photon samples all paths, but that it samples all paths REGARDLESS OF DISTANCE. Feynman introduced a second, stranger interpretation of advanced and retarded potentials to try to explain this. Basically, when you see a star, the photon reaches forward in time from the star and *backwards* in time from your eye.

      ‘Average’ isn’t a bad word, but it’s an ‘average’ for one photon. that’s quantum weirdness for you! If if you look it up, Ill bet he said, “a sort of an average”.

    2. Yes, how else to explain light entering a crystal, some diffracted and sent off at varying angles, and therefore longer trajectories, but all emerging at the same “time” on the other side?  Kinda weird… but yes… c is an average.

  4. Yeah there’s a difference between the phase velocity and the group velocity. Normally these experiments:

    a) toy around with the difference to prove they’re not the same
    b) mess around with the shape of the pulse, amplifying the front and attenuating the back. this creates a superluminal group velocity.

    my understanding of the experiment is that photons within the wave are always just moving at ‘c’. The significance of this finding would be to shut up people who run experiments like a) and b), above.

Comments are closed.