Zitterbewegung!

One of the few sad things about the recent American domination of physics (says the American physicist) is that new physical phenomena are now mostly given boring, prosaic American English names. Don’t get me wrong, I like being able to pronounce and interpret new phenomena, but when the pre-WWII era of European dominance faded away, we lost those awesome German names. Like, for example, zitterbewegung, a word that just demands an exclamation point: Zitterbewegung!

The phenomenon it describes, ironically, comes out of work by the great English physicist and odd duck Paul Dirac. Dirac’s eponymous equation for describing the behavior of relativistic equations predicted a number of new phenomena. The most famous of these was the existence of antimatter, which shows up as negative-energy solutions to the Dirac equation. Another consequence of those negative-energy states is the phenomenon given the awesome name zitterbewegung which, while it sounds to American ears like a dramatic exclamation actually translates to the relatively anticlimactic “trembling motion.”

The motion in question is a rapid oscillation of an electron at relativistic speeds around it average position. The jitter in question takes place at exceptionally high frequencies– 1021 Hz, give or take– and so has never been observed directly with electrons, as there’s just no practical way to make such a measurement. The phenomenon has broken into the news recently, though, because Rainer Blatt’s group in Innsbruck has come up with a way to observe an analogous motion in a trapped ion.

If you put a single ion in the right sort of trap, the equation describing its motion in the trap has the same mathematical form as the Dirac equation for an electron at relativistic speeds, though with a much lower characteristic velocity. This means that there should be a corresponding jitter of the ion about the center of the trap, that takes place at much lower frequencies, and can be observed by using lasers to manipulate and detect the state of the trapped ion.

This is another variant of the use of trapped ions to simulate electron behavior, though in this case, they use many repeated measurements of individual ions, rather than a large ensemble of neutral atoms. Like other analogues of high-energy phenomena, a failure to observe zitterbewegung in this system wouldn’t’ve ruled out the real phenomenon in relativistic electrons, nor does this measurement absolutely confirm that relativistic electrons undergo zitterbewegung. It is, however, reassuring to see that at least in the lower-energy case the mathematical prediction does reflect reality, and isn’t some pathological failure of the Dirac Equation.

There’s also an interesting side note to be made here relating to SciCurious’s open letter demanding links from media outlets covering science news. I first read about this via a press release and a Physics World news item about the research, neither of which included a citation of the actual paper (free arxiv version). So it’s not just the mass media who are at fault– dedicated science organizations also are prone to the reference-that-isn’t-a-reference.

Whose fault is it? Nature‘s. Their own news story, released at the same time as the press release and the Physics World piece, contains the correct citation, because they also publish the journal, and know in advance what the proper citation will be (which depends on the page number). They probably know this information far enough in advance that they could give it to the relevant science news outlets, but they don’t, most likely because most people reading news articles don’t care.

Anyway, that’s the latest from the world of AMO simulations of phenomena from other fields of physics. This continues to be a really cool area of science, and incidentally, provides an excuse to say “zitterbewegung” a lot.

Zitterbewegung!

Comments

  1. #1 Brett
    January 9, 2010

    This sort of gives the impression that we don’t know whether zitterbewegung is real or not. It’s true that it’s never been observed directly, as motion, but its physical effects are easy to pick up. Without zitterbewegung, there would be no Darwin term in the electron Hamiltonian, which would lead to all sorts of readily observable effects (such as a huge Lamb shift).

    The physical origin of the zitterbewegung is, I think, fascinating. On time scales less than h/2mc^2, the electron energy is so uncertain that one can’t really know whether a specific particle is an electron or the absence of a positron. Interference between the wave functions for the two possibilities produces oscillations at frequency 2mc^2/h, which are the zitterbewegung.

  2. #2 Chad Orzel
    January 9, 2010

    This sort of gives the impression that we don’t know whether zitterbewegung is real or not. It’s true that it’s never been observed directly, as motion, but its physical effects are easy to pick up. Without zitterbewegung, there would be no Darwin term in the electron Hamiltonian, which would lead to all sorts of readily observable effects (such as a huge Lamb shift).

    You’re right, that was imprecise on my part. Zitterbewegung can be inferred from some other effects, but it has not been directly observed.

  3. #3 Jonathan Vos Post
    January 9, 2010

    Thus ends the zitterbewegung zugzwang.

  4. #4 Arrow
    January 9, 2010

    I believe Zitterbewegung is a tantalizing clue to the next level of fundamental physics. In this I subscribe to Hestenes interpretation although I see it as just the first step which needs to be taken.

    Here is a great essey by Hestenes which explains his zitterbewegung interpretation of QM:
    http://www.fqxi.org/community/forum/topic/339

  5. #5 Otto
    January 10, 2010

    “They probably know this information far enough in advance that they could give it to the relevant science news outlets, but they don’t, most likely because most people reading news articles don’t care.”

    Are you certain? From the NPG site, “Our press releases are very short (around 200-word) summaries of selected papers; we advise and encourage journalists to look at the full papers which we make available ahead of publication via a password-protected website.”

    Even if the versions provided aren’t paginated, which strikes me as unlikely, they’re surely typeset and would carry a DOI.

  6. #6 ChicagoMolly
    January 11, 2010

    As a matter of complete irrelevance, zitterbewegung looks as if it should be translated to jitterbugging.

  7. #7 Hamish Johnston
    January 11, 2010

    I can’t believe that I’m about to support Nature regarding part of its news embargo policy — but here goes.

    Nature does give the citation and DOI of embargoed papers ahead of time to journalists (and it was cited in the physicsworld.com news article, to the right of the text under “Restricted links”).

    As for the embargo policy itself – I spotted this paper on the arXiv in Oct, but we sat on it (at the request of the authors) until last week. On one hand you could argue that Nature’s peer review process has given the story added weight — on the other hand it was old news to some in the physics community when we and others finally published it.

    Cheers
    Hamish

  8. #8 mmanu
    August 12, 2010

    i totally agree with arrow, the zitterbewegung is the first act of the imminent scientific revolution, especially within the crystal-clear mathematical framework developed by hestenes, now followed by so many others.

    concerning the experimental, direct, evidence of the phenomenon, hestenes did the calculations of the gouanère’s 2005 channelling experiment (cited in the fqxi essay above), relating it to his zitter model (see ‘zitter in QM’ http://geocalc.clas.asu.edu/html/GAinQM.html or ‘reading the electron clock’ for a more dedicated article, or ‘hunting for snarks in QM’ for a more didactical one).

    today i found another “experimental” evidence for zitter, with the Quantum simulation of Gerritsma et al. : http://www.nature.com/news/2010/100106/full/news.2010.2.html
    & http://arxiv.org/abs/0909.0674 … Quantum simulation is really good news !

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