Laurance Doyle at the SETI Institute likes to think big. Really big. When the late legendary physicist John Archibald Wheeler suggested there was a way to test a key aspect of quantum physics on a cosmic scale, Doyle and his partners at the SETI Institute in California figured out a way to do it.
Wheeler was half serious when he imagined the thought experiment, and when Doyle later told him that he had devised a way to do it by bouncing radar off the moons of other planets in the solar system, Doyle said Wheeler paused for several seconds before skeptically telling him, "Good luck!"
The big idea, as Doyle told me via Skype, is to test the true nature of time itself, and whether it abides by the rules of quantum physics, or by the rules of Einstein's general theory of relativity.
Recently Doyle and his colleagues received a grant from the Foundational Questions Institute (FQXi) to carry out their tests in early 2017 when Jupiter and its moons align with the sun.
If this sounds arcane, the outcome of his experiment could have strong implications for physicists trying to work out a grand unified theory of everything (GUT).
Many believe that in order to arrive at a GUT, quantum physics and general relativity need to be fundamentally reconciled. Which is another way of saying, general relativity needs to be broken down and reworked in quantum terms.
But what if general relativity is actually the more fundamental theory? Doyle's experiment would provide an answer by testing time at scales where both quantum physics and general relativity must have an effect.
Steve Ashley at FQXi explains in detail.
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. 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.
As bizarre as this is, when you try to track which slit the photon passes through by covering up one of the slits, Ashley writes, 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."
Somehow 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."
Where Wheeler enters the picture is with his suggestion that you try and fool the photon.
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 then scaled his idea up to the size of the universe. Find a distant quasar whose light is split by a gravitational "lens", he said, bent by the gravitational field of a galaxy or black hole so that some of the light streams around one side of the mass while the rest travels around the other side. From the perspective of Earth, it looks like there are two light sources, but in fact there is only one. As Ashley writes, "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."
Doyle and his team scaled the idea down to the size of the solar system. They plan to send a radar beam from either their 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 on their return journey.
When Jupiter is on the other side of the sun’s gravitational field, "it'll pass by the sun and the two return beams can travel through warped, curved spacetime," according to Doyle's colleague David P. Carico who teaches at the College of the Siskyous in Weed, California.
If the bending of space-time forces the beams to be delayed by different amounts depending on the path they take, then in theory, Ashley writes, "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."
And if so, then time --even in a quantum experiment--would appear to be fundamentally more subject to general relativity than quantum physics.
As part of a report on his project for the FQXi Community Podcast, I interviewed Professor Doyle on Skype to discuss the experiment in more detail.
You can listen to my segment of the FQXi podcast with Doyle here.
You can listen to the complete FQXi podcast here. (It's also accessible on iTunes.)
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