The upshot is that until a particle strikes a detector, it’s everywhere and nowhere in particular. How this happens is one of the deepest questions. From among these options, definite properties somehow crystallize at the moment of measurement. In quantum theory, a particle has a range of possible locations and speeds. But quantum theory teaches us that precise knowledge of both distance and speed is forbidden. Tunneling time is hard to pin down because reality itself is.Īt the macroscopic scale, how long an object takes to go from A to B is simply the distance divided by the object’s speed. “It was purely theoretical until the measurements were made.” What Time? “How is it possible for to travel faster than light?” Litvinyuk said. Tunneling seems to be incurably, robustly superluminal. In the six decades since Hartman’s paper, no matter how carefully physicists have redefined tunneling time or how precisely they’ve measured it in the lab, they’ve found that quantum tunneling invariably exhibits the Hartman effect. The recent experiments are bringing new attention to an unresolved issue. “What they measure is really the tunneling time,” he said. Luiz Manzoni, a theoretical physicist at Concordia College in Minnesota, also finds the Larmor clock measurement convincing. “The Larmor clock is the best and most intuitive way to measure tunneling time, and the experiment was the first to very nicely measure it,” said Igor Litvinyuk, a physicist at Griffith University in Australia who reported a different measurement of tunneling time in Nature last year. In the most highly praised measurement yet, reported in Nature in July, Steinberg’s group in Toronto used what’s called the Larmor clock method to gauge how long rubidium atoms took to tunnel through a repulsive laser field. None settled the issue.īut the tunneling-time question is making a comeback, fueled by a series of virtuoso experiments that have precisely measured tunneling time in the lab. Physicists eventually derived at least 10 alternative mathematical expressions for tunneling time, each reflecting a different perspective on the tunneling process. “It’s part of the general problem of what is time, and how do we measure time in quantum mechanics, and what is its meaning,” said Eli Pollak, a theoretical physicist at the Weizmann Institute of Science in Israel. The discussion spiraled for decades, in part because the tunneling-time question seemed to scratch at some of the most enigmatic aspects of quantum mechanics. “After the Hartman effect, that’s when people started to worry,” said Steinberg. In short, quantum tunneling seemed to allow faster-than-light travel, a supposed physical impossibility. This means that with a sufficiently thick barrier, particles could hop from one side to the other faster than light traveling the same distance through empty space. Even more astonishing, he calculated that thickening a barrier hardly increases the time it takes for a particle to tunnel across it. When a particle tunnels, the trip takes less time than if the barrier weren’t there. Hartman found that a barrier seemed to act as a shortcut. It wasn’t until 1962 that a semiconductor engineer at Texas Instruments named Thomas Hartman wrote a paper that explicitly embraced the shocking implications of the math. Even earlier stabs might have been made in private, but “when you get an answer you can’t make sense of, you don’t publish it,” noted Aephraim Steinberg, a physicist at the University of Toronto. The first tentative calculation of tunneling time appeared in print in 1932. The trouble was that the answer didn’t make sense.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |