Thursday, December 24, 2015

Is light a wave or a particle?

2015 was the International Year of Light. In May, I came across this video by the Max Planck Society, in which some random people on the street in Munich were asked whether light is a wave or a particle. Most of them answered in German, but here is a rough translation of their replies:
    “Uh, physics. It's been a long time. I guess it’s... a particle. — Particle. — Particle. — A particle. — A particle. — Light is... a particle. — I had physics up to 12th class. We discussed this a whole year. But I still don’t know. — A wave. — A wave? — Is this a trick question? — It’s both! Wave-particle duality. You should know that. — The duality of light. — It acts as both. — It’s hard to quantify what it is. It’s energy. — I am fascinated that nature has paradoxa. That one finds out through physics that not everything can be computed.”
So I thought some explanation is in order:

This is the first time I’ve tried the new green screen. As you can see, it has indeed solved my eye-erasure problem. (And for the experts, I hope you apologize my sloppiness in specifying the U(1) gauge group.)

On that occasion, I also want to wish you all Happy Holidays!

Like what you find on my blog? I want to kindly draw your attention to the donate button in the top right corner :o)


  1. Merry Christmas to you Bee & your crew.

  2. I can answer that without a video. "No." Next question?

  3. Merry Christmas,
    I guess the true light is the (dual) young girls on the photograph.


  4. Carver Mead back in 2001, has, in his "coherent electrodynamics", a somewhat different point of view.

  5. Happy Festivus! Poisson's spot, zone plates, binary optics...light is momentum. If linearly, circularly (spin), or orbitally polarized light is diffracted, what are the spots' polarizations?

    Does a diffracting matter wave experience collisions? Collisional decoherence solves Hundt's paradox. Absent collisions, a beam of resolved chiral molecules racemizes during diffraction as each molecule simultaneously passes through both (all) slits. 2-norbornanone is simple and rigid. D_3-4,7,11-trioxatrishomocubane is a hard case (connectivity).

    "Hund's paradox and the collisional stabilization of chiral molecules,"
    Phys. Rev. Lett. 103, 023202 (2009), arXiv:0811.2140, doi:10.1103/PhysRevLett.103.023202
    "Molecular rotations in matter-wave interferometry,"
    Phys. Rev. A 92(2), 023619 (2015), arXiv:1506.06026, doi:10.1103/PhysRevA.92.023619

  6. Here is what I think. Its not popular. Its a wave, absorbed and emitted in quanta. Anti-bunching experiments as discussed here don't convince me as they seem to miss the point. At any rate whether light is a particle or wave or some weird Hilbert space object is something that should be answered by experiment.

  7. Tom,

    I didn't say the photon is 'some weird Hilbert space object' but that it is described by one. Not the same thing. Besides, I don't think they're all that weird. Best,


  8. Is dark energy a wave function? If dark energy is a wave function, then are there dark energy particles with negative gravitational energy?

  9. David,

    Particles (described by wave-functions) with negative gravitational energy will never give you dark energy. To get something like dark energy, you need something that doesn't dilute the way normal matter does. The effects of dark energy have actually nothing to do with the 'sign' of the energy-density (you shouldn't speak of energy), but are due to the equation of state. Best,


  10. "Fröhliche Weihnacht überall!" Hmm or is it "Fröhliches? It is both no it is neither. That German grammar probably inspired Einstein. ��
    Merry Christmas and happy holiday to you all.

  11. Dear Sabine, why "donate" button? Are you unemployed?

    Happy Holidays to all of you!


  12. Vladimir,

    Luckily, I am no longer unemployed. But my current (2-year) contract is not particularly well paid, which is to say it doesn't cover a full-time position. I don't know why this matters to you though. Writing takes time and effort. Rather than writing for my blog and publishing articles openly accessible I can write for money and then my articles vanish behind paywalls. The donate button is there to give my readers the option to convince me to keep my writing openly accessible. Best,


  13. Sabine,
    I am really shocked and sorry that you almost quit science (from what you wrote on Woit's blog).
    Don't you have a tenure-track position at Nordita with freedom to work on anything you want? or was that position only
    research grant based? Anyhow its sad that someone like you who has worked on so many diverse subjects in theoretical physics (but not on fad subjects) has such a hard time? Arent there equivalent of teaching positions in Europe?
    Anyhow given that I am also on the job market, I know how difficult it is. At any rate you can email your reply, if you don't feel like answering this here.

  14. Shantanu,

    I've tried a couple of times now, but they wouldn't let me go ;) No, the position in Stockholm was a 5-year contract (so are all the assistant professorships at Nordita). The institute does itself not have permanent funding and (as most other institutes) can't offer tenure, even if they wanted to. (The permanent profs at this place have tenure at one of the universities in the vicinity.) Besides this, Sweden still has the 5-year rule that the Germans luckily meanwhile abandoned, according to which everybody employed at a university for longer than 5 years must get a permanent contract. The result of that rule is of course not, as originally intended, that the employees are protected from being held on temporary contracts, but that they instead get booted and replaced after 5 years.

    I have stopped looking for positions in academia because there aren't any in the fields I want to work in, not anywhere close by where my husband and the kids live. I am not willing sacrifice my family life for academia, so I will not push my family through an international move. And in Germany you don't make it on a permanent contract unless you know the right people (which I don't). Consequently I decided to instead apply for research grants. Which was more successful than I thought, but instead of having funds for now 4 different projects at the same time (which I can't all accept, needless to say) I'd much rather just have a job on which I won't have to waste all that time on writing proposals and worrying about paying rent.

  15. "Don't you have a tenure-track position at Nordita with freedom to work on anything you want?"

    By definition, a tenure-track position never includes "freedom to work on anything you want", since it is converted to a tenured position based on an evaluation, and in order to pass said evaluation, it does depend on what one has worked on. (OK, perhaps theoretically one could have the freedom to work on anything one wants, but with no guarantee of tenure, so it would seem to be a better strategy to work on what the evaluators think is good, at least until after the evaluation). Most tenured positions do entail research freedom (though perhaps within a broadly defined area).

    There are few teaching-only positions in Europe. I think this is a good thing, since otherwise those who teach are often regarded as second-class. It is also good to be taught by good researchers. (This doesn't mean that all good researchers are good teachers, though.) Feynman famously declined a position at the IAS (pretty much the most prestigious place to be in physics in the entire world) because it had no teaching obligation. They then offered him a joint appointment with Princeton University so that he could teach as well, but then he told them, in his Far Rockaway accent, to "get lost". :-) In any case, a full-time teaching position would offer little or no time for research, except in one's spare time, but if one is content to do research in one's spare time, then it might be better to get a permanent, well paid, non-demanding, non-academic job as opposed to, well, the opposite in all four respects.

    What is the point of staying in academia if one can't do the research one likes? Sabine's perhaps somewhat unorthodox career move (or lack of one) is, I think, admirable. One needs to have one's priorities straight. It is worth a lot to be able to live with one's family somewhere where one wants to live and have enough money and job security to live comfortably.

  16. Hi Bee,

    I just got an English translation of Gauss's Disquisitiones Arithmeticae. The dedication made me nauseous. I suppose it is a blessing academics don't have to write such tripe today, perhaps the proposal writing process is more dignified. I found the dedication online, here:

    That some man, a Duke or a Prince, takes for granted this kind of stuff makes me want to bring out the guillotine.

    I don't know if makes any modern researcher feel any better though.


  17. Phillip,

    Thanks, that's a good summary :)

  18. Arun,

    Well, it's probably the 19th century version of listing the sponsors in the acknowledgements ;)


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