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One of the most famous quantum physics experiments is the double-slit experiment. It’s a simple experiment proving light behaves as a wave and a particle at the same time. You don’t need a fancy set-up to try this at home, as Thomas Young performed a version of the double-slit experiment back in 1801. To be brief, a light source shines on a surface with two slits in front of a wall. The light creates an interference pattern of bright and dark spots since the photons are going through both openings at the same time, acting like a wave. Should one measure the light, in hopes of catching it acting like a wave, light then changes its mind acting like a particle by going through one slit at a time.

If the light “knows” when it’s being measured, John Archibald Wheeler proposed it could be “fooled”.

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’s delayed choice thought experiment utilizing some of the biggest, and most distant, objects in space. Gravitational lensing is when a large object with a strong gravitational field bends light from an object, creating the illusion of there being two. His delayed-choice experiment would use a quasar’s light bent by a galaxy’s gravitational field.

Now Dr. Laurance Doyle at the SETI Institute, and a few of his colleagues, are hoping to turn Wheeler’s thought experiment into reality. They’re scaling it down, and making it a bit more practical. Their plan is to ping Jupiter’s moons Ganymede and Europa with radar when they’re at nearly the same distance from Earth on the other side of the sun. As the radar propagates the 628 million kilometers between us and Jupiter, the beam will spread like a flashlight enabling it to hit both moons at the same time. When the signal bounces back at Earth, Doyle and co. will recombine the beams to see if there’s an interference pattern or not. The interesting part is our sun’s gravity could warp the beams, delaying them and allowing astronomers to see the path the radar took.

What they hope to discover is the nature of time.

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.

One fly in the ointment is how precisely the scientists can measure any time difference. They could set up the experiment so the difference is so large, there will still be an interference pattern. The next time Earth, Jupiter, and its moons are in the correct alignment is 2017. Fingers crossed they’re able to pull this off!

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