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

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.

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 fromClassical 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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