Quantum Particles Take the Road Most Traveled

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By Kelly Dickerson

 

For the first time ever, physicists have mapped the path that particles are most likely to take when moving from one quantum state to another.

In physics, a concept called the “path of least action” describes the trajectory that an object is most likely to follow, similar to the familiar concept of the “path of least resistance.” For example, a tossed football follows a parabolic arc through the air instead of spinning off in crazy loops or zigzags. That’s because a parabola path requires fewer “actions” than a looped or zigzag path.

However, physicists didn’t know whether quantum particles, like electrons, neutrinos or photons, follow the same rule. Many of the classic rules of physics don’t seem to apply to these tiny particles. Instead, they are governed by the weird rules of quantum mechanics that even Einstein called “spooky.”

Quantum particles can exist in states where they are in multiple places at once — a phenomenon called superposition. A mathematical equation called a wave function describes the many possible locations where a quantum particle might simultaneously exist. But as soon as someone tries to measure the location or the velocity of one of these particles, its wave function collapses and the particle will appear in only one spot, falling back under the laws of conventional physics.

This makes studying quantum particles extremely difficult, because the moment scientists start probing around, the particles’ quantum states collapse. However, physicists have developed a way to isolate the wacky quantum world and peer into it in a noninvasive way; this allows them to map the path that particles are most likely to take when changing from one state to another.

“It’s a great breakthrough in terms of being able to monitor quantum systems,”Andrew Jordan, a physicist at the University of Rochester, who worked on the original theory, told Live Science. “We’re just scratching the surface of the kinds of physics permitted here.”

Jordan developed the theory, and brought the idea to experimental physicists at the University of California, Berkeley and Washington University in St. Louis who helped design an experiment to test it. Kater Murch, a professor of physics at Washington University, sketched out possible paths that the particles might take, then polled the research team to see which path they thought the experiment would most likely reveal.

3 COMMENTS

  1. Thank you for the preprint, Michael.

    Articles like this make physicists like me cry. I’ll just pick on three major gripes.

    a tossed football follows a parabolic arc through the air instead of spinning off in crazy loops or zigzags. That’s because a parabola path requires fewer “actions” than a looped or zigzag path.

    You can tell Dickerson was trying to guess why the principle has that name. Action is a quantity you can calculate (although we almost never bother doing so, because proving what it takes to make it “stationary” – technically, it need not be minimal – does not require such a calculation). However, it’s not something you can count, as Dickerson implies. It’s a continuous quantity that usually has units, and it’s not unique because if you like you can, say, double it. For example, Snell’s law is a consequence of a least action principle; the action is usually taken as the time light takes to traverse a path from A to B. And the usual action for a moving ball has the units of angular momentum, but you shouldn’t read too much into that fact until you get to quantum material.

    physicists didn’t know whether quantum particles, like electrons, neutrinos or photons, follow the same rule. Many of the classic rules of physics don’t seem to apply to these tiny particles. Instead, they are governed by the weird rules of quantum mechanics that even Einstein called “spooky.”

    The idea that quantum mechanics is magic seems to come up whenever journalists discuss it, but the only people who think so turn out to also know little about classical mechanics. Almost every aspect of quantum mechanics has a classical analogue. What Einstein called spooky was entanglement, which admittedly doesn’t have a classical analogue, but it has no bearing on the question of stationary action. Feynman argued that the reason classical systems have stationary actions is because, although quantum mechanics allows the action to be nonstationary with nonzero probability, the contributions these possibilities make to the probability amplitude cancel. (They’re complex numbers that “point in different directions”.) If you can neglect relativity, the exact calculation that recovers the classical action from this approach was done decades ago. It’s now taught to fourth year undergraduates.

    Quantum particles can exist in states where they are in multiple places at once — a phenomenon called superposition.

    If you’re one of those people who cares about the “interpretation” of quantum mechanics, you will care a great deal whether superposed particles “are in multiple places at once” or “have no definite place, because a location is an eigenvalue and their state isn’t an eigenket, but upon measurement of location will have a specific location with a probability given by such and such a formula”. I’m not going to wade in to “who’s right”, but I will say that students are taught the latter immediately in most universities, and personally I think it would make more sense to both students and the ordinary public.

  2. Jordan developed the theory, and brought the idea to experimental physicists at the University of California, Berkeley and Washington University in St. Louis who helped design an experiment to test it.
    Blockquote

    And I thought she was just silicon and make up … shows you just can’t tell.

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