Back in the late 1970s, physicist Johan Wheeler tossed around a thought experiment in which he asked what would happen if tests allowed researchers to change parameters after a photon was fired, but before it had reached a sensor for testing—would it somehow alter its behavior mid-course? He also considered the possibilities as light from a distant quasar made its way through space, being lensed by gravity. Was it possible that the light could somehow choose to behave as a wave or a particle depending on what scientists here on Earth did in trying to measure it? In this new effort, the team in Italy set out to demonstrate the ideas that Wheeler had proposed—but instead of measuring light from a quasar, they measured light bounced from a satellite back to Earth.

The experiment consisted of shooting a laser beam at a beam splitter, which aimed the beam at a satellite traveling in low Earth orbit, which reflected it back to Earth. But as the light traveled back to Earth, the researchers had time to make a choice whether or not to activate a second beam splitter as the light was en route. Thus, they could test whether the light was able to sense what they were doing and respond accordingly. The team reports that the light behaved just as Wheeler had predicted—demonstrating either particle-like or wave-like behavior, depending on the behavior of those studying it.