Ray of Enlightenment: Is Light a Wave or a Particle?

Cartoon by Jorge Cham, "Daniel and Jorge Explain the Universe"

Light isn't just a bright patch of sunshine on your windowsill. It's also a metaphor for enlightenment and exploration, a bit paradoxical for a phenomenon that — even after thousands of years of inquiries and endless experiments — scientists still can't quite explain.

In the episode "Is Light a Particle or a Wave?" the hosts of the podcast "Daniel and Jorge Explain the Universe" discuss whether light is a particle or a wave ... or something else altogether. Spoiler alert: Light is not, as co-host and online cartoonist Jorge Cham lightheartedly proposes, "little puppies."

Your eyes tell you a lot about the way light behaves. It travels very fast (about 186,000 miles per second or 300,000 kilometers per second) and blazes through space in laserlike straight lines. But it also bounces, reflects and refracts, and when it interacts with the right medium, like a camera lens, it may curve. We use terms like wavelength and photons to describe its movements, but none of them really encompass light's odd meanderings.

In ancient times, the Greeks used philosophical questions to address light's wide range of behaviors. Perhaps, they wondered, light is actually little bits of stuff that bounces to and fro. In the 1600s, French philosopher Rene Descartes became convinced that light was essentially a wave, one that moved through a mysterious substance called plenum. Isaac Newton thought light was a particle, but he was at a loss for a way to explain many of its properties, like the way it refracted and split in a prism.

"This is back in the day when empirical studies of science weren't the main way to answer questions," says podcast co-host Daniel Whiteson, a physicist who's spent time working on the Large Hadron Collider. People would come up with ideas about how things worked, "... and then argue about them."

In our investigations of light, our hosts point out, it's important to remember some science basics. Waves aren't a thing or a substance, they're a property of a thing, compressing and stretching a particular medium, like an ocean wave rippling across the surface of a lagoon. You can see the waves with your eyes and feel them with your body.

Particles, on the other hand, are not so easy to define. Some people like to think of particles as bits of matter, fragments that are broken down into their smallest and most basic units. Water, for example, is countless particles ... particles that are affected by waves.

So what, then, is light? Is it its own special type of particle, a photon? Or is light really just a wave that's flowing through another medium? Is there some creepy, unknown substance surrounding us that we simply don't perceive or understand?

Whiteson highlights a famous 19th-century double-slit experiment, in which researchers beamed light through two slits and observed the way light struck a screen behind the slits. What they saw was that the streams of light affected each other — like two hands splashing water in the same sink — as if they were waves interfering with one another.

In the 20th century, scientists began their pioneering explorations into particles like neutrons and electrons. Famed physicist Albert Einstein wondered what would happen if you emitted light one photon at a time in the double-slit experiment. What scientists saw dumbfounded them — because the single photons behaved with the same interference pattern that occurred with full-scale beams of light streaming through both slits, introducing elements of quantum mechanics that elevate light's properties to realms we still don't really understand.

Ultimately, say our hosts, you can call light both a particle and wave, and you'd be correct on some level. But as for fully explaining how light works, we're still working on it.