4/6/2023 0 Comments Physics science experimentsWhen the pulses first leave the interferometer, they have different polarizations. Of course, the physicists must choose which thing happens. ![]() The apparatus can either undo the time shift so that the two pulses overlap and interfere like waves or double it so that no interference is possible. Once the pulses return, the experimenters run them back through the interferometer. ![]() The difference in path lengths splits the single pulse into two, separated in time by 3.5 nanoseconds, which the telescope then shoots skyward. Instead, they send a photon through a Mach-Zehnder interferometer on Earth that has paths of very different lengths. At such distances, physicists cannot make light take two parallel paths, Villoresi notes, as the spreading beams would overlap and merge. ![]() Now, a team led by Francesco Vedovato and Paolo Villoresi of the University of Padua in Italy has performed a version of the experiment using the 1.5-meter telescope at the Matera Laser Ranging Observatory in southern Italy to bounce photons off satellites thousands of kilometers away. Quantum theory avoids the issue by assuming that, until measured, the photon remains both a particle and a wave. That assertion suggests, weirdly, that a decision in the present determines an event in the past: whether the photon split like a wave or took one path like a particle. Wheeler realized that experimenters could even wait to remove the second beam splitter until after the photon had passed the first beam splitter. As the paths cross where the second beam splitter would have been, the detectors click with equal probabilities regardless of the paths' lengths. Instead, the first beam splitter sends the photon down one path or the other, like a particle. Remove the second beam splitter and interference becomes impossible. Remove the second beam splitter and, like a particle, the photon must take one path or the other and is equally likely to hit either detector. Wave or particle?Ī photon ordinarily takes both paths through a Mach-Zehnder interferometer, and wavelike interference can then shunt it toward one detector or the other. Which detector is triggered depends on the difference in the two paths' lengths, as expected for interfering waves. A second beam splitter then recombines the waves, which interfere with each other to shunt the photon toward either one of a pair of detectors. Using a mirrorlike "beam splitter," the interferometer splits the entering photon's quantum wave in half and sends the two waves along different paths, like people walking opposite ways around the block. Wheeler imagined sending photons one at a time through a so-called Mach-Zehnder interferometer, which accentuates light's wave nature. In the late 1970s, famed theoretician John Archibald Wheeler realized that experimenters could even delay the choice until the photon had made its way almost completely through an apparatus configured to emphasize one property or the other, thus proving that the photon's behavior isn't predetermined. "It's a very nice experiment that demonstrates their ability to do quantum physics in space."Ī photon can act like a bulletlike particle or rippling wave-but not both at once-depending on how experimenters decide to measure it. But the new work shows that a photon's nature remains undefined even over thousands of kilometers, says Philippe Grangier, a physicist at the Institute of Optics in Palaiseau, France, who collaborated on an earlier test. Other researchers have demonstrated the same counterintuitive effect in the laboratory. ![]() Such delayed-choice experiments might someday probe the fuzzy frontier between quantum theory and relativity, researchers say. Now, by bouncing photons off satellites, a team has confirmed that an observer can make that decision even after a photon has made its way almost completely through the experiment-seemingly well past the point at which it would become either a wave or a particle. Physicists have long known that a quantum of light, or photon, will behave like a particle or a wave depending on how they measure it. An odd space experiment has confirmed that, as quantum mechanics says, reality is what you choose it to be.
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