The General Relativity picture of curved spacetime, where matter and energy determine how these systems evolve over time, has made successful predictions that no other theory can match, including for the existence and properties of gravitational waves: ripples in spacetime. If quantum theory is right, these ripples must have a particle analogue, as wave-particle duality must apply to all quanta. (LIGO)

Can We Test Gravitational Waves For Wave-Particle Duality?

If quantum gravity is right, these gravitational ripples must be more than waves; they must be particles, too.

Back in February of 2016, LIGO made an announcement that changed our picture of the Universe forever: from more than a billion light-years away, two massive black holes, of 36 and 29 solar masses, had inspiraled and merged. The result of that merger was a single black hole of 62 solar masses, with the remaining 3 solar masses converted into pure energy via Einstein’s E = mc², rippling throughout the Universe in the form of gravitational waves.

Since that time, LIGO has risen into the double digits with the number of detections it’s made, as gravitational waves are now undoubtedly real and teaching us an incredible amount about our Universe. But all of this is still information about our Universe according to our classical theory of gravity: General Relativity. If quantum physics is right, then wave-particle duality is real, even for gravitational waves. Here’s what that means.

This diagram, dating back to Thomas Young’s work in the early 1800s, is one of the oldest pictures that demonstrate both constructive and destructive interference as arising from wave sources originating at two points: A and B. This is a physically identical setup to a double slit experiment. (WIKIMEDIA COMMONS USER SAKURAMBO)