Video Article: Solar-System-Sized Experiment to Put Time to the Test

No one could ever accuse Laurance Doyle of thinking small potatoes… The astronomer, based at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, is taking on one of the biggest questions in physics: What is time?

And to learn the answer, all it will take, Doyle says, is an experiment the size of the solar system.

The nature of time is an ancient puzzle, with developments in physics over the past century only heightening the confusion. If you’d asked Einstein, he would have told you that time is another dimension, much like the three dimensions of space. Together they knit together to create a spacetime fabric that pervades the universe. This notion of time as a dynamic, flexible dimension forms the basis of his immensely successful general theory of relativity, which explains how gravity manifests on cosmic scales as matter warps spacetime. On the other hand, however, the equally celebrated theory of quantum mechanics, which governs the nanoscale behavior of atoms and subatomic particles, says that time is unaffected by the presence of matter, serving as an absolute background reference clock against which motion can be measured.

So which one is right? According to Doyle and his colleagues, the answer could be within reach if physicists scale-up a classic test of quantum properties that is often carried out in undergraduate labs. The solution, they claim, could come from a "cosmic interferometer" that bounces radar beams between the planets.

Doyle admits that the proposed experiment is speculative. "Not many quantum physicists are going to use a radio telescope to test whether quantum time is the same as relativistic time," he says. But his imaginative lines of thought have worked in the past. As part of NASA’s Kepler project Doyle, for instance, came up with three of the 12 techniques now used to locate habitable exoplanets hiding amid the void of space.

And in an effort to suss out any ’intelligent’ messages from alien life that may lie hidden in the raft of celestial radio signals that SETI collects, Doyle has also been studying communications between dolphins, humpback whales, monkeys and elephants (see video). It’s all part of what the farm-bred Californian calls his "mix-and-match approach" "to achieving maximum fun."

For his possibly quixotic quest to build a cosmic interferometer, funded with the help of a $25,000 FQXi grant, Doyle is working with teacher and long-term collaborator, David P. Carico, an instructor at the College of the Siskyous in Weed, California, and Gerry Harp, an antenna whiz at SETI. Doyle and Carico have been friends for decades, since grad school, where they began playing "mental ping-pong," with quantum-related cosmic concepts.

The team’s proposed interferometer riffs off the celebrated "double-slit" experiment that is often taught in college physics classes to demonstrate the quantum nature of light. The test shows that light can behave either as a wave or as particle, depending on circumstance. It works like this: Aim a light beam at a barrier with two parallel slits cut out of it. Beyond the barrier there is a screen, which picks up a characteristic pattern—a series of bright and dark fringes—that you would expect to see if light is a wave, passing through both slits and interfering with itself on the other side (see image, below right). So far, things are perfectly reasonable.

Things get odder when you turn down the intensity of the beam, so that the light is spat out one particle, or photon, at a time. In this case, you might expect that these photons would fly through either one slit or the other, destroying any interference pattern. Instead, you still see an interference pattern slowly build up on the screen, one dot of light at time, as though each individual photon is somehow traveling through both slits at the same time.

If that were not strange enough, when you try to track which slit the photon passes through by covering up one of the slits, the interference pattern vanishes. Instead, the photon shoots straight through the open slit, like a bullet through a hole, creating one big blob of light on the opposite side of the screen. It’s a mystery just how the photon ’knows’ whether or not both slits are open and is able to adjust its behavior accordingly. It is as though the light recognizes whether or not the apparatus has been set up to spy upon it.

Changing the Past

In 1979, philosopher-physicist John Wheeler, then at the University of Maryland, described a "delayed-choice" thought experiment that ramped the weirdness up yet another notch. He suggested that the actual point of measurement at which you spy on the path taken by the light could be set up after the light has already passed through the slits but before it arrives at the detector—so the light could not know as it moved through the experiment whether it would be observed or not. Such experiments have since been carried out in quantum laboratories and it turns out that, even then, light could not be fooled. The observer’s later choice of what measurements to make determines whether the photon took one path or two at an earlier point in the test. In other words, the observer seems to have changed what has happened in the past.

Wheeler took this paradoxical notion further. Consider a distant quasar whose light has been bent by the gravitational field of a galaxy so that some of the light streams around one side of the galaxy while the rest travels around the other side. This phenomenon is called "gravitational lensing" and from our viewpoint it looks like there are two light sources but, in truth, it’s only one. Wheeler argued that this observation can be treated as a cosmic delayed-choice experiment. Depending on how the observer samples the streams of light, they could be discerned as waves or photons.

It seems like you might need a fiber-optic cable that is many light years long to carry out this cosmic delayed-choice experiment. But for some time, Doyle and Carico have been tossing around a more practical way of performing the test, using radar rather than light from distant quasars. The team plans to send a radar beam from either the observatory at Goldstone in the Mojave desert or at Arecibo in Puerto Rico out to two of Jupiter’s larger moons, Ganymede and Europa, when they are nearly equidistant from Earth, thereby creating two possible paths for the light to take.

"By the time the radio waves get out there the beam has spread, like a flashlight beam does, so a single beam is wide enough to strike both moons," Carico says. "When the return echo bounces back to earth, you recombine the beams and under the right conditions you can get constructive and destructive interference, which forms a distinctive pattern that tells you things about the nature of that light."

The key difference between the cosmic test and the versions that have already been carried out in quantum labs is that over these large scales, gravity can come into play. "When Jupiter is on the other side of the sun’s gravity, it’ll pass by the sun and the two return beams can travel through warped, curved spacetime," Carico says. Crucially, if this warping forces the beams to be delayed by different amounts depending on the path they take, then in theory, it will be possible to tell which path the light took—and this "which-path knowability" should disrupt the interference pattern, just as closing one slit in the double-slit experiment does. In this case, there should be no pattern.

Engineering Uncertainty

But there’s another quantum twist. According to Heisenberg’s Uncertainty Principle, there’s a limit to how accurately experimenters can measure any time difference. This opens up another interesting possibility. The team could engineer the quantum uncertainty in that time difference to be so large that they would not be able to tell from their readings, with certainty, which path the light took—meaning that the interference pattern will exist.

Will quantum mechanics win out, enabling the interference pattern to show up? Or will the pattern be destroyed—suggesting that quantum effects defer to spacetime warping, and the general relativistic conception of time? Unfortunately, we may have to wait for the answer, since the Earth, sun, Jupiter and its moons next align in the right configuration in 2017.

Even an equivocal result would be welcome, says David Rideout, a physicist and mathematician at the University of California San Diego, who has investigated the possibility of carrying out other space-based tests of quantum mechanics. (See "Testing Reality in Space.") "Despite being speculative, the experiment addresses what is the great challenge of this field," Rideout says, namely "how time can be effectively independent of quantum mechanics but at the same time be so fundamental a dynamic variable in general relativity and gravity."