Quanta in a Nutshell (part 2)

Waving Particles

Why duality is like an elephant: an overview of the double-slit experiment and its implications

Sidharth Jain
Published in
7 min readJun 23, 2019


Last time, I told you about the “Uncertainty Principle”, and why we can’t simultaneously find both the position and momentum of a particle. That bit was all weird and confusing but there’s one question we didn’t solve: Are electrons really particles or waves?

Are particles waves?
Are waves actually particles?

Actually, that’s a whole bunch of questions, so let's start from the beginning.

What does an electron look like? Is it a particle, or a wave?

You can’t actually say what an electron “looks” like, because “looking” means light bouncing off the object and coming into your eyes, and electrons are too small for light to show you clearly.

As for what electrons behave like, scientists too were in the same dilemma. And that was when, in the early 19th century, Thomas Young conducted his famous “Double Slit Experiment”.

Arguably the most famous experiment in Science, the “Double Slit Experiment” demonstrates, with unparalleled strangeness, that electrons, traditionally thought to be particles, behave like waves. To learn more, let’s take a look at three similar experiments.

Trust me, this is as weird as it gets.

Let’s start off, by imagining a wall, with two slits in it. Now imagine firing paint balls at the wall. Obviously, most paint balls will hit the wall, but some of them will travel through the slits. Now, if there’s another wall behind the first, the balls that have traveled through the slits, will hit it, leaving spots on the place of impact.

Now, what would you see on the second wall?

You’ll see two strips of paint, roughly the same shape as that of the slits, as shown in the image.

All makes sense? So far, so good.

Now, let’s do the same experiment with light. Again, imagine shining a light on the wall with the same two slits.

We know that light is a wave — so just like water-waves, it must be having peaks and troughs. Here, the blue lines show the peaks. Now, as the wave passes through the slits, it splits into two new waves, each one spreading out from one of the slits. These two waves interfere with each other. Peaks, cancelling out troughs and making new bigger peaks when two of them collide. The place with bigger peaks, give the brighter light when they meet at the wall behind.

When the light meets the second wall, the reinforced waves, with bigger peaks, form bright bars and what you end up seeing is a stripy pattern, called an interference pattern.

Now, let’s dive into the Quantum realm. Imagine firing electrons at our wall with the two slits — but this time, block one of the slits.

You’ll find that some of the electrons, will pass through the open slit and will strike the wall behind. Just like the paint balls, they form a rectangular strip of roughly the size of the slit.

Now, open the second slit as well. You’d expect two rectangular strips on the second wall — but no! What you actually see is something very strange.

The spots where the electron hit the wall replicate the interference pattern. This indicates that electrons actually are nothing but waves.

Don’t believe me? Here are images from the real double slit experiment performed with electrons. See for yourself.

Image: Dr. Tonomura and Belsazar, CC BY-SA 3.0

The more electrons you fire, the more interference you get — just like having more rapid ripples in the water.

Naturally, Thomas Young was surprised by his experiment’s results. So was everyone else. Why would electrons form an interference pattern if, say, balls of paint don’t?

One possibility might be that electrons somehow interfere with each other: like a crowd gone mad, they don’t arrive in the same places they would if they were alone.

To test this, scientists redid the experiment sending only one electron through the slit at a time. But no luck: even then, the series of single electrons still formed an interference pattern. Surely, they couldn’t interfere with each other if one started after the other was done?

The scientists weren’t out of ideas yet. Perhaps each electron somehow splits, passes through both slits at once, interferes with itself, and then recombines to meet the second screen as a single, localised particle?

To find out, they placed a detector by the slits, to see which slit the electron passes through. And that’s the really weird bit.

When they placed the detector between the walls, leaving the rest of the experiment the same, the interference pattern vanished. The normal, particle-made, paint-ball pattern appeared in its place.

Somehow, it seems the act of measurement made the electrons behave like little paint balls. It is as if they knew they were being watched, and decided not to be waves but particles.

So then, are electrons waves or particles? How do they switch between the two states? Or perhaps, we’re asking the wrong questions altogether.

Three blind men once decided to observe an “elephant” that had come to town. Being blind, they went to where the elephant was standing and began using their hands to feel different parts of its body.

The first person felt the elephant’s ear. “Oh!” he exclaimed, “the elephant is like a sort of large fan!”

“No,” said the second person, whose hand had fallen on the elephant’s smooth body. “Can’t you feel the elephant? It’s like a big, tough wall.”

“What are you two saying?” the third person exclaimed. Holding the tail in his hands, he proclaimed: “The elephant is just a thick piece of rope!”

With that, the three people began a fierce argument, each calling the others fools and insisting their own view was right. They never figured out what exactly an elephant was.

What does Young’s double-slit experiment tell us? It suggests that the things we call “particles”, like electrons, somehow have the combined characteristics of particles and waves.

Remember those wave equations from Part One? These equations for particles describe their behavior very well, but we don’t know whether they’re a real thing or not. Whenever we try to solve the equation, all we get is a point particle, not a wave.

Perhaps, like the blind men and the elephant, we only can only see one aspect at a time, when in reality electrons and other particles are all those aspects and more.

That’s the famous wave-particle duality of quantum mechanics for you.

The Double-Slit experiment has other implications as well. It means electrons are not always present in one place but can be in many places at once. And, it also means the act of observing, or measuring, a quantum system, has a profound effect on the system itself.

When psychologists are conducting experiments, they take great lengths to make things seem natural.

That’s because people behave differently in different scenarios. In a study about empathy, for example, participants could be more likely to help a stranger than they would in real life. We’re all self-conscious, and aware when we’re being judged. (Incidentally, the same thing once happened with self-conscious cars as well).

So, psychologists put a lot of effort into taking account of these effects. They’re always aware that their act of measuring affects what they’re measuring.

With the double-slit experiment, physicists realized they have a similar issue as well — the Measurement Problem. The act of observing, or measuring, a quantum system, has a profound effect on the system itself. Of course, the reasons are different. With people, it’s because we’re self-aware and afraid of being judged. With particles, it’s…well nobody knows. Yet.

Duality is a fundamental property on which the entire Quantum Mechanics is based. And is the one factor that differentiates it from Classical Mechanics.

Despite this weirdness and major holes like the measurement problem in it, Quantum Mechanics has been one of the most successful theories in theoretical physics and has been till date, the most beneficial to us.

Even if it implies particles can act “self-conscious” too.

Quanta in a Nutshell: This article is second of a four-part series on quantum mechanics. Up next: superposition, walking through walls, and a cat that’s both dead and alive.

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Sidharth Jain

Science student, in love with space, robots, rockets, and cars!