Posted by Our department here at Texas A&M has a student chapter of the Optical Society of America, and each week a student or professor gives a talk about something interesting while the rest of us eat pizza. I’ve been working on and off on a talk I’m going to give, tentatively titled “Just what the @#$% is a photon anyway?”. The more I dig into the subject, the more I start to think that (like the rubber-sheet analogy in GR) the “particles of light” view that tends to be the common impression tends to cause more confusion than enlightenment. I have some good company here – E.T. Jaynes wrote a famous paper expressing his own problems with the concept, and I have to say he’s pretty convincing.
The BBC article percolating aroud the web reenforces my suspicions. “Time travel: Light speed results cast fresh doubts” Its intro sentence:
Physicists have confirmed the ultimate speed limit for the packets of light called photons – making time travel even less likely than thought.
Honestly the article isn’t that bad. I’m used to much worse in the popular press. It does get across the point relatively intact. The headline is a little sensationalist – as I told an emailer, really it should be something like “Scientists measure speed of a light photon in rubidium vapor, turns out to at travel speed of light”. It is, after all, just one (important) measurement in one (very interesting) physical system. It doesn’t prove that the result holds in all times and places, indeed no experiment can. The paper is here, if you’re curious. It’s a elegant experiment and I congratulate the authors on a fine job.
Here’s the sketchy BBC paragraphs that take the “photon as particle” view too literally and run into trouble:
While the limit in vacuum is a fixed number – some 300,000km per second – the speed of light can vary widely in different materials.
These differences explain everything from why a straw looks bent in a glass of water to experiments in cold gases of atoms in which light’s speed is actively manipulated.
Some of those experiments showed “superluminal” behaviour, in which photons travelled faster than the speed of light in a given medium.
It remained, however, to determine whether or not individual photons could exceed the vacuum limit.
The last sentence directly contradicts the one before it, because in so-called superluminal experiments nothing actually propagates faster than the speed of light. The pulse of light looks like it exceeds c, but only because the material has been “pre-prepared” in such a way as to amplify the leading edge of the pulse which makes the pulse peak appear to shift forward.
Now, Shengwang Du and colleagues at the Hong Kong University of Science and Technology have measured what is known as an optical precursor.
Like the wind that moves ahead of a speeding train, optical precursors are the waves that precede photons in a material; before now, such optical precursors have never been directly observed for single photons.
By passing pairs of photons through a vapour of atoms held at just 100 millionths of a degree above absolute zero – the Universe’s ultimate low-temperature limit – the team showed that the optical precursor and the photon that caused it are indeed limited to the vacuum speed of light.
“By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon,” said Professor Du.
“The waves the preced photons in a material?” Oh dear. Light is light. Waves are made of photons, and individual photons (contra their particle-like popular image) express wave-like behavior. But however you look at it, light never travels faster than c according to both classical E&M and modern QED.
I’d also quibble a tiny bit with professor Du’s quote. He has shown that that particular precursor in that particular material travels at c. Of course we would all be stunned if any material turned out to be an exception, but as careful scientists we shouldn’t state that any single experiment proves a universal truth.
Still, not a bad article. Now if I can figure out exactly what photons are, I’ll let you know. But it’s murky waters…
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