Depending on how light is measured, it can either be particle-like, lighting up a camera pixel, for example, or wavelike, interfering with other waves like ripples on the surface of water. It’s one of the many oddities of quantum mechanics. Before light is measured, quantum theory suggests, it is in a particle-wave limbo, neither purely one nor the other.
Physicists have tested this idea by performing “delayed-choice” experiments in the lab, in which researchers send light into a device and randomly choose whether or not to flip a switch that seems to retroactively change the light’s behavior (SN: 5/30/15, p. 9). In one configuration, the light travels down two paths at once and acts like a wave, interfering with itself. In the other, the light acts like a particle, taking a single path. That choice of configuration can be made even after the light has already traveled through the device but before being measured, revealing that light remains in quantum limbo until it is finally detected.
For the first time, physicist Paolo Villoresi of the University of Padua in Italy and colleagues took the technique into space. The researchers sent light through a lab apparatus and up to a satellite equipped with reflectors, which bounced the light back down to the device. While the light was in transit, the scientists used a random number generator to determine whether to configure their apparatus so that the light would behave like a particle or a wave. The light performed as expected, verifying that quantum mechanics holds even over the round trip into space and back.