The Revolutionary Experiment that Remains One of Physics’ Greatest Mysteries

By: Saatvik Sanjay

Image by Bhushan Sadani via Unsplash 

The double-slit experiment is famous for changing the way we understand light — but what is it and how has it become one of the greatest unsolved mysteries of our time?

In the year 1801, British polymath Thomas Young came up with the idea for his now-famous double-slit experiment. But English society wasn’t ready to believe their beloved Newton was wrong. So his experiment remained tucked away… until it resurfaced as a paradox in modern quantum mechanics.

What is the experiment?

Young originally started as a physician, dissecting ox eyes to test how they focus from different distances. He was also fascinated by languages and did a dissertation on the human voice. His interest in the eyes and vocals of humans eventually led to an interest in light and sound.

He read Newton’s Opticks (a book created by Sir Newton about light) at 17 and admired his work. Later on, he found some problems with Newton’s theory of light as particles. For example, it had trouble explaining why different colors of light refract to different degrees. He felt that light, much like sound, was a wave. As a result, light “waves” like sound waves should also exhibit some interference which can be seen. To understand interference we must first understand the parts of a wave. The peak of a wave is called a crest and the bottom dip is known as a trough.

When two crests or two troughs come together, they amplify each other resulting in constructive interference. Conversely, when a crest and a trough come together they cancel each other out, resulting in destructive interference.

Young knew that when two sound waves cross, they interfere and produce a beat. He believed that light was like sound and could also display interference.

Originally, he didn’t use two slits at all. Instead, he went with a single thin card that could slip the light beam in half. He covered his window with paper and cut out a single hole that would allow a thin beam to pass through. After getting split by the card, the two beams would interfere, creating alternating bands of light and dark on the opposing wall. This was the first definitive proof of light as a wave and not as a particle as Newton thought.

Or so it may seem-

In reality, light is both a particle AND a wave

Subsequent research in quantum mechanics has shown that light has properties of both waves and particles. But how is this possible?

How is light both a particle and a wave?

At the macroscopic level, light behaves much like a wave which we saw in Young’s experiment. However, things start getting a bit strange when we scale it down to the subatomic level.

Light is made of subatomic particles called photons. As you might expect, they exhibit the behaviors of a particle. But how?

To understand this, let’s first imagine we’re shooting tiny grains of rice through the slits and onto a sensor. The grains should fall around the same area more or less, with some spread out. Now replace these grains of rice with photons and cover one of the slits. We observe that the photons do indeed cluster together after passing through the slit, much like a particle.

Now for the strange part-

When we open both the slits and fire photons through it, we already know that they form band-like interferences. But what if we fire them one by one so there’s no chance for them to interfere? They should still behave like particles, right?

Wrong! At first, it seems like they are randomly scattered, but over time the interference patterns start emerging. Each photon contributes to the wave-like pattern even though they were launched individually and there was no chance for them to interfere with one another. It seems as though each photon “knows” that there are two slits and somehow splits up and passes through both of them, rejoining to form the interference on the sensor.

To test this out, scientists placed a detector at the slits to check which slit each photon passes through. They found that 50% of the photons went through one slit and 50% of them went through the other. Nothing too strange.

But when we take a look at the sensor, we see something different. The pattern is the same as the one in the first scenario — where we shot the photons through one slit. In other words, the pattern formed is that of a particle. Huh, that's odd. Maybe it has something to do with the detector?

What if we turn off the detector but still keep it in place? Nothing else changes — same slits, same photons but this time the detector is off. Surely the same particle-like pattern should be formed right?

This is when things turn from strange to super, super weird.

When we pass the photons through the slits with the detector off, it once again starts to form a wave-like interference pattern! It’s like the photons know when you’re watching them and act as particles when you are and as waves when you’re not.

How could this possibly be? Well this is still up to question!

Works Cited:

• This month in Physics history. (n.d.). American Physical Society. https://www.aps.org/archives/publications/apsnews/200805/physicshistory.cfm

• Zhang, H., Yang, C., Yu, Y., Zhou, Y., Quan, L., Dong, S., & Luo, J. (2020). Origami-tessellation-based triboelectric nanogenerator for energy harvesting with application in road pavement. Nano Energy, 78, 105177. https://doi.org/10.1016/j.nanoen.2020.105177