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.