The world forever changed back in February of 2016, when the LIGO collaboration made arguably the biggest scientific announcement of the 21st century, altering our view 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.
Directly detecting those waves, first achieved in September of 2015 and announced six months later, was the catalyzing event. At last, no one could doubt the physical reality of gravitational waves, including the facts of: Since that time, LIGO has been joined by additional gravitational wave detectors such as Virgo, has detected several hundred confirmed gravitational wave events, and has witnessed neutron star mergers as well as black hole mergers, spanning nearly a factor of ~100 in the mass ranges of their detected objects. Gravitational waves are now undoubtedly real, and the process of observing them is teaching us an incredible amount about our Universe.
But all of this information still only probes the predictions that arise from our classical theory of gravity: general relativity. If quantum physics is right, then wave-particle duality must be real, even for gravitational waves. Here’s what that means, and how we might someday go about putting it to the ultimate, experimental test.
This diagram, dating back to Thomas Young’s work in the early 1800s, is one of the oldest pictures that demonstrates 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, even though it applies just as well to water waves propagated through a tank. The locations marked C, D, E, and F correspond to 100% destructive interference.
It’s no stretch to claim that wave-particle duality is one of the strangest quantum phenomena ever uncovered. The independent ideas of waves and particles started out simply enough: matter was made of particles, things like atoms and their constituents, and radiation was made of waves. You could tell something was a particle because those things would exhibit behaviors like: Similarly, you could tell if something was a wave because it would exhibit wave-like phenomena, such as diffraction, interference, and a superposition of states, where waves could interfere both with other waves and also with themselves.
Newton got this one wrong about light, thinking it was made of particles, but others such as Huygens (his contemporary) and then the early-1800s scientists like Young, Fresnel, and Arago showed definitively that light exhibited properties that couldn’t be explained without considering it as a wave. Perhaps the most famous wave-like phenomenon of all, interference, appears when you pass light through a double slit. The pattern that shows up on a background screen shows that the light interferes both constructively (leading to bright spots) and destructively (leading to dark spots).
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