Built on Facts

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…

Comments

  1. #1 Bee
    July 30, 2011

    I had a prof who was opposed to the nomenclature ‘elementary particles.’ He insisted on calling them ‘elementary things.’

  2. #2 Adrian Morgan
    July 30, 2011

    Speaking of sensationalist headlines, the ABC News in Science headline was: Experiment shows time travel impossible.

  3. #3 phayes
    July 31, 2011

    “But it’s murky waters…”

    Unnecessarily so, as usual:

    “Thus the talk about photons is usually done inconsistently; almost everything said in the literature about photons should be taken with a grain of salt.”

    http://www.mat.univie.ac.at/~neum/physfaq/topics/photon
    http://www.mat.univie.ac.at/~neum/physfaq/topics/complementarity

    Or as Ballentine’s QM textbook warns at the beginning of its EM chapter:

    “In some respects the quantized EM field behaves as a system of bosons, although these particle-like excitations, called photons, play a less fundamental role than is suggested by some informal accounts of the subject.”

  4. #4 Annonymous
    July 31, 2011

    If names of electrons or photons cannot be substituted for
    free variables in the language of physics then they hardly
    qualify even as “things”.

    It is probably best to interpret talk of electrons or photons as non-representational done primarily for heuristic
    or suggestive value and not about anything.

    The representational parts of the language of physics would
    then be theoretical physics which is about mathematical
    entities and the basic part of ordinary language which are
    abut ordinary things.

    If talk of electrons and photons is fictitious and not
    representational then inconsistencies in this talk are
    not important. In some of the stories of Sherlock Holmes
    Watson’s first name is given as John and in other stories
    it is given as James. We do not worry about the John-James
    duality because we treat the stories as non- representational. They are not about anything or anybody.
    We just read them for entertainment.

    Talk of electrons and photons is not about anything. It is
    done just for intuitive guidance. There is no reason why it has to make sense. It is just useful nonsense.

  5. #5 complex field
    August 1, 2011

    “Some of those experiments showed ‘superluminal’ behaviour, in which photons travelled faster than the speed of light in a given medium.”

    Isn’t that the basis for Cerenkov Radiation, which results from photons travelling faster that the speed of light in the medium, without exceeding c?

  6. #6 Matt Springer
    August 1, 2011

    Good point, complex field. I parsed that statement as “traveled faster than the vacuum speed of light”, but they might mean Cerenkov-type effects as well (though those result from massive particles moving faster than c over the refractive index).

  7. #7 complex field
    August 5, 2011

    oops. my bad about photons vs. massive particles.

  8. #8 derya sonay
    August 12, 2011

    thank for you, yerliyeşilçam derya sonay izmir.
    If talk of electrons and photons is fictitious and not
    representational then inconsistencies in this talk are
    not important. In some of the stories of Sherlock Holmes
    Watson’s first name is given as John and in other stories
    it is given as James. We do not worry about the John-James
    duality because we treat the stories as non- representational. They are not about anything or anybody.
    yukardakie vatandaşıda belirtiği gibibler bende anyınsoruyu sarocaktımki birde bakdımea ge göreyemi benden öcelbireler yaptım ve cevvbalrı hala alamık, siteyakıştrıamadim ama hale belikrizo cevah işçi şimdiden teşekkürler.

  9. #9 SureArrow
    August 24, 2011

    Here’s a fact for you! This “experiment” comes out, after earlier this year, the Government of China (who controls Hong Kong and it’s projects) Banned all stories for television that involved time travel. Sounds almost unreal! See the link below and read the news yourself or Google “China Bans Time Travel”! Time for some dot connecting!

    Hmmm…what’s going on here? Have they been working a their own “Manhattan Project”? Time for a little dis-information campaign to alter the thinking of the masses. Also, thanks to our lame and lazy western “reporters”, that info was not part of the story! I blame that on just dumb journalistic stupidity.

    Read the April 14th article yourself: http://business.blogs.cnn.com/2011/04/14/china-bans-time-travel-for-television/

  10. #10 sesli chat
    September 2, 2011

    Elektronlar ve fotonlar Talk, bir şey hakkında değildir. It is .
    done just for intuitive guidance. sezgisel bir rehberlik için yapılır. There is no reason why it has to make sense. Neden mantıklı hiçbir neden yoktur. It is just useful nonsense. Bu sadece yararlı bir saçmalık.