{"id":3222,"date":"2008-12-12T10:50:20","date_gmt":"2008-12-12T10:50:20","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2008\/12\/12\/reflections-on-quantum-reflect\/"},"modified":"2008-12-12T10:50:20","modified_gmt":"2008-12-12T10:50:20","slug":"reflections-on-quantum-reflect","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2008\/12\/12\/reflections-on-quantum-reflect\/","title":{"rendered":"Reflections on Quantum Reflection"},"content":{"rendered":"

I subscribe to Scientific American<\/cite>, 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<\/a>:<\/p>\n

\n

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.<\/p>\n

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.”<\/p>\n<\/blockquote>\n

Pretty surprising, no? The physics isn’t the surprising part, though, at least not for me.<\/p>\n

<\/p>\n

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<\/a> 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.<\/p>\n

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.<\/p>\n

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<\/a>. It’s also the basis for the only single-author experimental Phys. Rev.<\/cite> Letter<\/a> that I can remember seeing.<\/p>\n

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<\/cite>, because it’s been around a long time.<\/p>\n

Now, to be fair to the authors, the paper that this is based on<\/a> 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:<\/p>\n

\"i-b6dde6ab05caaf99ef13b6646a3f6c0d-reflection.jpg\"<\/p>\n

(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.)<\/p>\n

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<\/cite> sort of article.<\/p>\n

It’s not a stunning new development, though.<\/p>\n","protected":false},"excerpt":{"rendered":"

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… Continue reading Reflections on Quantum Reflection<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-3222","post","type-post","status-publish","format-standard","hentry","category-physics","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3222","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=3222"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3222\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=3222"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=3222"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=3222"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}