I subscribe to Scientific American, but I’m usually several weeks behind on reading it, so it was only Thursday that I noticed this surprising article about particles bouncing back from attractive forces:
This effect is the converse of the well-known (if no less astounding) phenomenon of quantum tunneling. If you kick a soccer ball up a hill too slowly, it will come back down. But if you kick a quantum particle up a hill at the same speed, it can make it up and over. The particle will have “tunneled” across (although no actual tunnel is involved). This process explains how particles can escape atomic nuclei, causing radioactive alpha decay. And it is the basis of many electronic devices.
In tunneling, the particle can do something the ball never does. Conversely, the particle might not do something the ball always does. If you kick a soccer ball toward the edge of a cliff, it will always fall off. But if you kick a particle toward the edge, it can bounce back to you. The particle is like one of those little toy robots that senses the edge of a table or staircase and reverses course, except that the particle has no internal mechanism to pull off its stunt. It naturally does the exact opposite of what the forces acting on it would indicate. The researchers behind the analysis–Pedro L. Garrido of the University of Granada in Spain, Jani Lukkarinen of the University of Helsinki, and Sheldon Goldstein and Roderich Tumulka, both at Rutgers University–call this phenomenon “antitunneling.”
Pretty surprising, no? The physics isn’t the surprising part, though, at least not for me.
The surprising part is finding this presented as something new. I’ve known about it for at least ten years– the theoretical model we used in the spin-polarized collisions paper from my thesis work used exactly this effect. The ionizing collisions in xenon are so strong that we could model it by assuming that any colliding pair getting closer together than about 50 Bohr radii would ionize with 100% probability. The only thing limiting the low-temperature collision rate was the reflection off the attractive long-range interaction potential– exactly the effect that’s talked about here.
In fact, I’ve used exactly this sort of situation as an exam problem, in a sophomore level modern physics class. I didn’t expect them to do all the boundary-matching stuff– they just had to recognize that they could plug numbers into a formula– but the problem was to find the reflection coefficient for particles hitting a square potential well.
While “antitunneling” is a new name for it, I’ve always know it as “quantum tunneling,” which, in fact, is notable enough to have its own Wikipedia article. It’s also the basis for the only single-author experimental Phys. Rev. Letter that I can remember seeing.
Somebody really dropped the ball on this one. I’m not sure how this wound up as a breaking news sort of article in Scientific American, because it’s been around a long time.
Now, to be fair to the authors, the paper that this is based on looks like a nice pedagogical sort of paper about the phenomenon. They’ve got some nice numerical simulations of a wavepacket bouncing off a soft attractive potential:
(Those are snapshots at several different times (not equally spaced, alas), with time running from the top of the left column down, and then the top of the right column down.)
I’ll have to look a little more closely at the article– a quick look at it suggests it might be useful as a basis for an assignment in a modern physics class. I’m not sure where it’s been submitted, if it’s been submitted, but it looks like a good American Journal of Physics sort of article.
It’s not a stunning new development, though.
A discontinuous boundary with impedence allows reflection. Transparent glass is visible in air. Impedence-match the boundary (tapered hydraulic segments, anti-reflection coatings, immerse in a matching medium) and reflections are supressed (e.g., stealth coatings have the impedence of free space and are lossy). For all that, a sufficiently birefringent solid immersed in refractive index-matching liquid is still visible.
The mechanical analog is a freeway suddenly widening to more lanes. Above a certain vehicle density-velocity it is a choke point. Southern California specializes in maximized traffic mismanagement. The worse it gets the more political control is enabled and the higher the revenues for “fixing” it (making it even worse).
Physics folklore tells of a meeting between Aharonov and Feynman in which Aharonov challenged Feynman to solve several quantum “paradoxes” not long after Aharonov had completed his Ph.D. thesis. Many of the paradoxes can be found in his recent book with Rohlich. “Antitunneling” does not appear in the book, but it is in the lecture notes on which they were based and I was always led to believe that it was one of the puzzles that he challenged Feynman to solve. I’m not sure if this is the first discovery of antitunnelling, but it is at least an early one.
Slow news month?
Seriously, unless there’s some great new application, or something puzzling but commonplace that someone just attributed to this, this is… well, obvious, even to an engineer.
The sort of quantum/wave tunneling/reflection I find particularly interesting is “frustrated total internal reflection.” For example, light at 45 degrees (past the critical angle) reflects off the ending surface of glass with index 1.5. But now, bring another diagonal very close (wavelength scale) and some of that light goes on through as if the gap wasn’t there.
One thing I’m curious about is the phase of the exiting light – is it the same as if there had been solid glass, is it set back by the amount of the gap, or what? Also, many have perplexed over energy relations in symmetrical beamsplitters (either silvered or using FTIR) used as recombiners. If made symmetrical (like metal film or frustrated gap diagonal in solid glass, no preferred surface) the BS can’t show favor in phase changes dependent on which face the light entered. If you do the simplistic interference for the outputs of two input beams of entering amplitudes both A0, it looks like e.g. you can cheat for more energy by adding 0.7 A0 + 0.7 A0 out of both sides (try it.) Something has to give, but how does it work? Are the phase relations still preserved and the energy “just isn’t there” – that seems inelegant. Tunneling concepts show an answer?
If you are still confused about tunneling anywhere on the planet===> http://www.physicsworks.ca
You’re welcome (if your are not a physics professor!)
Hell you even get reflection of light from glass, while most goes through……
This isn’t even quantum really, its what waves do.
I am sure that I remember this being in my Part 1 Physics course at Cambridge fifty years ago, but I no longer have the notes to hand.
Quantum Mechanics being too mathematical for me, I went off and made a career in something more conventional, so I am unlikely to have come across it in the intervening years except from reading Scientific American.
Tom Radford
Dear Prof.Orzel, I am from China, in your blog, I have known that the researchers, including Pedro L. Garrido etc. have put forward the “antitunneling”, can you tell me in which Journal, does these researchers called “antitunneling” firstly, thanks.