Putting Time in a (Leaky) Bottle

You can tell a lot about a subject by who its muses and mascots are. Neuroscience has philosophers who wax profound about the mind, geology has intrepid explorers and subatomic physics has ... Alice in Wonderland. "Curiouser and curiouser," as Alice said, also describes the subatomic, or quantum, world. With age, this centenarian (quantum physics is 107 years old) has gotten more bizarre. "The surprises keep coming," says physicist David Albert of Columbia University. None is greater than finding loopholes in the hallowed uncertainty principle—and, even more outlandishly, seeing hints that the future may leak into the present.

Since experiments keep proving quantum ideas right, physicists are forced to take them seriously. It isn't easy. They have to admit that a particle can be in two places at once. They have to accept that subatomic systems can become so "entangled" that measuring one affects the other even if the two are light-years apart, which Einstein called "spooky action at a distance." But even as quantum weirdness provides fodder for such drivel as the best-selling book "The Secret," it also fuels debate on subjects as lofty as the nature of reality. Last week a conference at Oxford University explored the idea that every time a subatomic system reaches a decision point—to undergo radioactive decay or not, say—it chooses both possibilities: in this world the particle decays, while in a parallel world it does not. Some physicists buy this "many worlds" interpretation because the alternative is even more unpalatable: that quantum systems choose one possibility or another only when an observer looks. Einstein loathed the idea that reality is created by observers.

New studies suggest, however, that it is possible to measure something without affecting it. The key is doing the experiments, well, gently. Anyone with a vague memory of Physics 101 knows that if you shine a light on what you want to measure, or stick a thermometer in it, you alter it. Taking the temperature of a steak with a cold thermometer, for instance, cools it as heat is transferred from meat to glass. You don't know what the temperature "really" was before you jabbed in the thermometer—a notion enshrined as the uncertainty principle. To circumvent this rule, Israeli physicist Yakir Aharonov got the idea of making "weak measurements," akin to waving your hand over the steak to feel its heat. That's not very precise with meat, but it works with quantum measurements: if you make enough weak measurements, the average comes impressively close to the actual value, experiments are showing. "Weak measurements let you lift the veil of secrecy imposed by the uncertainty principle," says Paul Davies of Arizona State University.

In one use of weak measurements, particles of light (photons) fly toward a screen, one at a time. The screen has two slits. If each photon goes through one slit, they form two bright spots on Venetian blinds beyond the screen. If each photon somehow goes through both slits, however, they form black-and-white stripes when they land on the blinds. Physicists have long known that if a device observes the slits, no zebra pattern forms; it's as if quantum phenomena are too shy to display their magic—one particle going through two slits—when watched. Weak measurements might be able to get around this by being less obtrusive; studies to try are in the works.

In the meantime, experiments have put detectors on the far side of the blinds. If the blinds are open and the detectors peek at the slits, photons fly through only one slit and no zebra stripes form. If the blinds are closed so the detectors cannot see the slits, photons fly through both and form the stripes. Here's the twist: if the blinds open only after photons have passed the slits but before they reach the blinds, the stripes fail to form even though the photons have seemingly done what they must to form stripes—namely, fly through two slits, as they always do when unobserved. The act of observing alters what the photons did earlier, somehow changing things so they passed through one slit and not two. There are "many histories" a photon could have, such as passing through one slit or two, Davies writes in his new book, "Cosmic Jackpot." Making a measurement "chooses which [history] existed."

That interpretation remains speculative, but weak measurements may indeed show that "something that happens now is affected by something that happens in the future," says physicist Jeff Tollaksen of George Mason University. "It suggests that the universe has a destiny—a destiny that is out there and coming back to us from the future." Maybe physicists should replace Alice with a new muse: Trafalmadorians, who in Kurt Vonnegut's "Slaughterhouse-Five" saw past, present and future all at once like a landscape, each moment ever present.

© 2007