In Quantum Physics, Even Humans Act As Waves

Ethan SiegelSenior ContributorStarts With A BangContributor GroupScienceThe Universe is out there, waiting for you to discover it.

“Is it a wave or is it a particle?” Never has such a simple question had such a complicated answer as in the quantum realm. The answer, perhaps frighteningly, depends on how you ask the question. Pass a beam of light through two slits, and it acts like a wave. Fire that same beam of light into a conducting plate of metal, and it acts like a particle. Under appropriate conditions, we can measure either wave-like or particle-like behavior for photons — the fundamental quantum of light — confirming the dual, and very weird, nature of reality.

This dual nature of reality isn’t just restricted to light, either, but has been observed to apply to all quantum particles: electrons, protons, neutrons, even significantly large collections of atoms. In fact, if we can define it, we can quantify just how “wave-like” a particle or set of particles is. Even an entire human being, under the right conditions, can act like a quantum wave. (Although, good luck with measuring that.) Here’s the science behind what that all means

The debate over whether light behaves as a wave or a particle goes all the way back to the 17th century, when two titanic figures in physics history took opposite sides on the issue. On the one hand, Isaac Newton put forth a “corpuscular” theory of light, where it behaved the same way that particles did: moving in straight lines (rays) and refracting, reflecting, and carrying momentum just as any other kind of material would. Newton was able to predict many phenomena this way, and could explain how white light was composed of many other colors.

On the other hand, Christiaan Huygens favored the wave theory of light, noting features like interference and diffraction, which are inherently wave-like. Huygens’ work on waves couldn’t explain some of the phenomena that Newton’s corpuscular theory could, and vice versa. Things started to get more interesting in the early 1800s, however, as novel experiments began to truly reveal the ways in which light was intrinsically wave-like.