A good working definition of quantum mechanics is that things are the exact opposite of what you thought they were. Empty space is full, particles are waves, and cats can be both alive and dead at the same time. Recently a group of physicists studied another quantum head spinner. You might innocently think that when a particle rolls across a tabletop and reaches the edge, it will fall off. Sorry. In fact, a quantum particle under the right conditions stays on the table and rolls back.
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.”
In both cases, the explanation lies in the wave nature of particles, which in turn reflects the fact that a quantum particle generally has an ambiguous location. The wave describes the range of locations where it could be found. This wave behaves much like ordinary waves such as sound. Whenever any wave encounters a barrier that is not absolutely rigid, some of the wave will penetrate into the barrier, albeit with diminishing intensity. If the barrier is not too thick, the wave can reemerge on the other side. That is analogous to tunneling.
For antitunneling, the analogy is that whenever any wave encounters any abrupt change of conditions—even ones more favorable to its propagation—some of it will reflect back. Something similar happens when a scuba diver looks up and sees the sea surface acting as a mirror. To be sufficiently abrupt, the distance over which conditions change must be shorter than the wavelength (which for a particle is related to momentum). If the change is too gradual, the wave will simply go along, and the particle will act like a soccer ball after all.
Garrido and his colleagues undertook a numerical analysis to rule out the possibility that the phenomenon was an artifact of idealized assumptions. They also calculated how long a particle will tend to roll around the table before going over the edge; it gets longer the higher the table is. David Griffiths of Reed College, author of a widely used introductory quantum mechanics textbook (the second edition of which gives a version of antitunneling as a student exercise), calls it “a very sweet paradox.” Physicist Frank Wilczek of the Massachusetts Institute of Technology says, “It’s a solid analysis, and it points out an interesting phenomenon I hadn’t been consciously aware of.”
Antitunneling might have applications for building laboratory particle traps, describing nuclear decay or exploring the foundations of quantum mechanics, but its main appeal is to remind physicists how a nearly century-old theory has lost none of its capacity to surprise.
Note: This article was originally published with the title, "Quantum Brinkmanship".
Already a Digital subscriber? Sign-in Now
If your institution has site license access, enter here.
See what we're tweeting about
23 Comments
Add CommentVery cool!
Reply | Report Abuse | Link to thishow will this affect philosophy and how we think of the universe, incredible
Reply | Report Abuse | Link to thisWhy would I automatically assume that a particle rolled to the edge of a table would "fall off?" From the particle's "point of view" what, exactly, would constitute "off" or "fall?" Looking upon all things as residing within our very limited perceptual framework is hardly "innocent." At best it is arrogant, at worst, ignorant. Quantum mechanics IS strange and bizarre, but it isn't precisely the "opposite" of every thing one, in his or her narrow perception, assumes to be the absolute truth. The cat is neither both dead and alive at the same time, nor alive and dead a the same time. We can't even be sure there even IS a cat in the box, or what color it is...this amazing cat is dead and alive and present and not present AND every possible color... male and female.... so the reality is very much stranger than a simple 'black is white' dichotomy. Opposite would be easy! Sesame Street plays 'one of these things doesn't belong here.' The same game with QM is much more complicated.... and, of course, simpler at the same time. Interesting article though... poor start... but interesting.
Reply | Report Abuse | Link to thisThanks DeCarte.
Reply | Report Abuse | Link to thisI am very disappointed in sci-am lately. First the "Nature breaks the second law of thermo" article adn now this!?
Reply | Report Abuse | Link to thisThey are putting too much "weirdness" into quantum mechanics so that laypeople will never understand it.
I feel like sci-am makes philosophy/English grads read physics textbooks and write something interesting about very simple concepts.
The thing about a particle and a ball is that a particle has the wave/particle duality.
In this case the particle acts more like a wave of light. Where some will penetrate and go through the barrier(ie a piece of glass) and some will reflect (ie a piece of glass/mirror);
This article is reading way too much into a very elementary evaluation of Schrodinger equation.
Dear mrsaturn42,
Reply | Report Abuse | Link to thisDon't you know the new paradigm yet? Magazines and television no longer share knowledge, they entrap readers/viewers just long enough for the advertisements to work. No one respects truth or rigorous exposition anymore, it is all about the advertising dollar. Like Pogo said long ago,
"We have met the enemy, and he is us."
buddha_dust@yahoo.com
The analogy of a ball rolling off a cliff to a particle encountering a drop in potential energy is not right at all. As a ball moves off a cliff there is no discontinuity in the energy it has, it has to actually fall in order to gain any energy. In contrast, to demonstrate quantum tunneling, you need an abrupt or discontinuous spatial change in energy. Immediately after encountering the barrier/anti-barrier the energy of the particle must change. A better analogy might be to consider a ball rolling onto a moving walkway (like those in airports). As soon as the ball hits the walkway, it immediately gains kinetic energy. (This isn't quite right either, but the idea is better). David Griffith's exercise mentions this exact fallacy in reasoning, so I'm not sure why the article is incorrect.
Reply | Report Abuse | Link to thisWhoah! Now that is pretty cool isnt it.
Reply | Report Abuse | Link to thisJess
http://www.privacy.mx.tc
Particles are particles. Objects are objects.
Reply | Report Abuse | Link to thisA particle is a theoretical concept (albeit an extremely useful one) in physics, it is supposed to be infinitely tiny and massless.
What happens to an object of a given mass in real life could be entirely different (if not exactly the opposite) to what happens to a particle in the quantum arena. They are incompatible.
Where comes the weirdness? Where is the surprise?
(btt1943@yahoo.com)
Amazing!
Reply | Report Abuse | Link to thisChrisJones: Well, the beginning of the article was meant to be tongue-in-cheek. Quantum theory is very rich and in general can't be considered the opposite of classical physics; it *subsumes* classical physics. But in many cases, including tunneling and antitunneling, quantum behavior *is* the opposite of classical behavior. The gradient of the potential in antitunneling would, in classical physics, pull the ball off the table.
Reply | Report Abuse | Link to thismrsaturn42: To paraphrase Feynman, if you don't find wave-particle duality astounding, then you don't really understand it. (Incidentally, where did Sci Am report that "Nature breaks the second law of thermo"?)
amoebadrew: Yes, the classical analogy is imperfect, as all analogies are, but that does not affect the analysis of the quantum problem. I should point out that the cliff and table analogies are not original to me but appear in the technical paper on which the article is based.
ChrisJones: Well, the beginning of the article was meant to be tongue-in-cheek. Quantum theory is very rich and in general can't be considered the opposite of classical physics; it *subsumes* classical physics. But in many cases, including tunneling and antitunneling, quantum behavior *is* the opposite of classical behavior. The gradient of the potential in antitunneling would, in classical physics, pull the ball off the table.
Reply | Report Abuse | Link to thismrsaturn42: To paraphrase Feynman, if you don't find wave-particle duality astounding, then you don't really understand it. (Incidentally, where did Sci Am report that "Nature breaks the second law of thermo"?)
amoebadrew: Yes, the classical analogy is imperfect, as all analogies are, but that does not affect the analysis of the quantum problem. I should point out that the cliff and table analogies are not original to me but appear in the technical paper on which the article is based.
It is indeed pretty cool but not very weird, not at all, if you're used solving Schrodinger equations.
Reply | Report Abuse | Link to thisThe trouble with giving examples of quantum effects using our sphere of reference is that at the sub-atomic scale we must forget gravity, so sorry : no rolling balls please! Most descriptions of quantum phenomenon are confusing : The only decent account I have understood explains them in terms of probability, which is at least logical. But time running backwards still defies the imagination?....More explanations please.
Reply | Report Abuse | Link to thisThe description of an electron which Vasily Yanchilin gives in his book The Quantum Theory of Gravitation may help to understand the phenomenon: He sees the electron as innumerous times appearing and disappearing discontinuously within a sphere with dimensions of the Heisenberg uncertainty relation. For simplicity I call such an appearing an iet so that the word electron can be reserved for the whole. When a photon hits an iet the sphere shrinks to almost a point, after which expansion resumes. Has it grown enough then the sphere will reach beyond the area where the electron is supposed to be according Newtonian mechanics. Part of the sphere can be beyond the top of the slope and an iet may occur over the top on the other side. When this iet is hit there then shrinking occurs and you have the electron on the other side of the slope. Note that quantum mechanics allows measurable uncertainty only to extremely small objects and the electron as taken example serves here as a model.
Reply | Report Abuse | Link to thisI see this as very interesting. Another version of this is seen in radio waves; particles of a much longer wavelength than the quantum ones we are discussing in this article.
Reply | Report Abuse | Link to thisYour antennae use the same principle to 'trap' a radio wave (or should I call it a radio particle). The antennae’s length is cut to an even integral length of or division of length of the wave that it is capturing and the energy of that wave 'rings' when it enters the antennae. The energy is then tapped and sent down a waveguide that is known as a coaxial cable to your television.
The table in this case is the antennae. The edge is the ends of the antennae.
The quantum world is only weird if you try to explain it using classical terms. Classical physics is only an average of what comes out of particle physics. Things only seem weird because they behave in a counter intuitive way. This intuition that we have is a result of the human observation from a classical physics point of view; the world looks entirely different to an astronaut on the ISS than is does to the guy shovelling the snow from his driveway.
Terrible article. Reads like something out of a tabloid. Does the writer have even a cursory understanding of QM? I feel there was probably more to this research and the writer has just picked up on a minor point that has been know since the conception of QM. Its one of the most basic implications of the theory.
Reply | Report Abuse | Link to thisWOW... as someone who has no experience whatsoever with QM or Physics in general, I found this article rather interesting, although I must admit dumbfounding. But reading the comments has to be the best part, its almost like watching an episode of the TV sitcom The Big Bang Theory...amusing yet not entirely understood. Good job Scholars!! Keep up the good work!!
Reply | Report Abuse | Link to thisMy perception of the problem the author is running into is that he has encountered the same issue anyone explaining quantum mechanics does. How do you explain color to a blind person? With out a frame of reference humans can't process information.
Reply | Report Abuse | Link to thisThis is like the example of Schroedingers cat. Schroedinger never said the cat is alive and dead at the same time. That would be stupid. What he said is that without direct observation it is impossible to know the status of the cat and with quantum mechanics the scale is so small that direct observation is impossible. With quantum mechanics we (people that know what they are doing in this arena, which does not actually include me) have to work with probabilities and implied phenomenon. Most people seem to try to take the analogy literally instead of as a symbolic construct.
The tools that work with one scale are meaningless at another scale. The actions of one person in Siberia may have little to no corelation to the impact of the human population of North America upon global oil production. The difference of scale tends to make the analogy meaningless. If the person is representing the quantum and all humans in North America represent the ball, do we expect the same behaviour? Emotionally yes, but logically no.
Do you measure the distance to Proxima Centauri in light years or nanometers? When it gets to the end of the day do most of us really care? Only if we see a direct impact on ourselves.
Xsjado: If you doubt my account of this work, why not read the original paper? If you are doing to make such sweeping condemnations, it might help if you backed them up with some specifics. Antitunneling may be "one of the most basic implications of the theory" but that makes it no less interesting.
Reply | Report Abuse | Link to thisbucketofsquid: Schr�dinger's main point was that the theory predicts that macroscopic objects can be in superpositions, not only microscopic ones. He *did* say the cat could be both alive and dead, and that its status is not merely a matter of observation. In the conventional interpretation, quantum uncertainty is not a matter of observational limitations; it is a genuine property of the world. (I did fudge this a bit in the article so as to leave open the possibility of a Bohmian or other realist interpretation.)
George
Thanks George for this account. Anti-tunneling makes me think of those cartoon characters that while running beyond the edge of a cliff, only just manage to grasp a tuft of grass and come back on safe ground. A logical conclusion could then be that quantum particles are not like little spheres but have some 'extended' structure that allows them to make it back. Wave-particle duality is not the only plausible explanation and there are many other unexplored explanations.
Reply | Report Abuse | Link to thisI agree with mrsaturn42 that putting too much weirdness into quantum mechanics is not helpful for a thorough understanding of counter-classical aspects of QM. Neither is the fact of referring too much to Feynmanian (true or paraphrased) quotes that nobody understands quantum mechanics. Such quotes should only be used as consolation (quantums of solace;-)) if discouragement shows up when trying to understand QM, not when someone has some relevant view on the subject. I think we should use more positively oriented Feynmanian quotes, like in his Quantum Lectures Epilogue: 'I hope ... that you will find someday that, after all, it isn't as horrible as it looks', or like eyeopeners as in his QED Lectures: 'All we do is draw little arrows on a piece of paper - that's all'. With such quotes it is possible to develop intuitive approaches towards Quantum Physics. In my experience, this makes quantum phenomena easier to explain to children than classical orbital motion of planets, for example.
So lemme get this straight...if I walk into an antitunneling bar, drink Jack Daniels all night and start dancing alone with my shirt untucked and my fly at half-mast, I'll not only be acknowledged as a marvelous solo performer, but I will begin to attract beautiful creatures of the opposite sex...the toilet paper stuck to my shoe merely acting as a flag to summon my legion of admirers.
Reply | Report Abuse | Link to thisScience is the BALLS, man!
"They also calculated how long a particle will tend to roll around the table before going over the edge; it gets longer the higher the table is. " -- Is this a gravitational effect?
Reply | Report Abuse | Link to this