Even single photons must obey Einstein


One of the basic tenents of Special Relativity is that the speed of light (a self-propagating electro-magnetic wave) is a universal constant for all observers. That’s what makes all the neat reference frame trickery work. It’s why it’s a fundamental property of relativity that there is no absolute reference frame and simultaneity is relative.

The thing about this idea though is that it didn’t come from a theoretical background. It came by observation. No matter how carefully scientists in the 19th century tried to measure a change in the speed of light in space, they couldn’t find one. Most people thought the experiment was flawed. Einstein, along with Lorentz and a few others, imagined instead that they needed to create a model that baked that result in from the beginning.

But then along came the idea of wave/particle duality and conceptual nightmare that has ensued for our macroscopically prejudiced minds. The idea that it wasn’t actually a wave that was moving at the speed of light but an actual particle just made the whole situation bizarre. Given the constants of electricity and magnetism it’s straightforward to see the speed of light – but putting a particle into the mix, and getting rid of the wave nature to imagine just a point particle, and the idea that this particle’s motion is a constant for all just hurts our heads.

Some physicists recently decided to double check this idea. (This is how science works and awards are won.) We know that light moving in a medium moves more slowly than light in a vacuum. The electrons in the matter interact with the light to change the propagation characteristics of the wave and effectively retard its motion. But what about light in a vacuum – and what about a single photon? Could a single photon, a particle, be induced to go faster than the speed of light? Unlike the situation where we see the wave-nature, there’s no obvious reason that a massless particle shouldn’t be able to go just as fast as it wants.

“To address this question, [Professor Shengwang Du from The Hong Kong University of Science and Technology] and his coauthors’ demonstration required not only producing single photons, but separating the optical precursor, which is the wave-like propagation at the front of an optical pulse, from the rest of the photon. Previous experiments based on macroscopic electromagnetic wave propagation (involving lots of photons) have shown that the optical precursor is the fastest part in the propagation of an optical pulse. But this study is the first to experimentally show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon wave packet.

[…]“In the slow light (with a group velocity slower than c) case, the central part of the main wave packet follows the group velocity,” Du explained. “When the medium density increases (with more atoms), the slow group velocity decreases. In the fast light or superluminal (with a group velocity faster than c or negative group velocity) case, the main wave packet seems to get ‘confused’ and does not follow the group velocity. …We are sure that the main wave packet cannot travel faster than the precursor, which travels at c.”

The results agree with previous studies that have analyzed single photons whose precursor and main wave form have not been separated, which have reported an oscillatory structure. The interference of the precursor and the slightly delayed main waveform can explain this structure.”

Full details here.

It’s a more subtle problem than I lay out above but it’s manageable and the experiment verifies that even for a particle, the speed of light is an absolute. Why? It’s not really clear. The requirement is easy to bake into an equation, but the physics still escapes us. Of course the duality of particle and wave nature escapes us too.

But no matter. What the experiment shows is that Einstein is right again. His theory posits from the beginning that in 3-space it is impossible to state pretty much anything about the world we live in with certainty. Things are relative. One person’s yes can be another person’s no and they’re both correct.

Makes trying to makes sense of scholastic and reformation strands of theology a real challenge.

The Author

Episcopal bishop, dad, astronomer, erstwhile dancer...


  1. ruidh says

    I thought Feynman said that the quantum amplitudes for a photon traveling (in a vacuum) faster than the speed of light are canceled out by those for it traveling less than the speed of light.

  2. That sounds right Ruidh – or at least it sounds like something Feynman would argue given his ideas about coherence. But that’s a theoretical argument to constrain the photon’s velocity to be in agreement with relativistic principles. In my thinking this is an experiment – which in the lingua franca of Physics these days is the ultimate referee. No theory needed here. In fact it’s the theory which must now be agreement with the experiment. (Which it is, so yay Mr. Feynman!)

  3. Paul Martin says

    It was always my understanding that the photon was a particle and a wave simultaneously. That’s probably based on what makes sense to me, rather than on any profound understanding of either theory or experiment. An observation on my part (such as a double slit experiment) may reveal something about the photon, but I never understood how it would affect the nature of the photon. After all, how does the photon know that I’m watching? (If a photon encounters a double slit experiment in the forest while no one is watching . . . )

    There is probably a reason I call myself an engineer rather than a physicist.

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