Double Slit Experiment

At the start of the 19th century, there were scientific debates on the substance of light. Many argued that light behaved as individual particles moving through space, whereas others believed it to be composed of waves (much like sound). In 1803, scholar Thomas Young proposed an experiment to the Royal Society. He planned to end the argument in scientific discovery.

To understand his experiment (which would become known as the famous Double-Slit Experiment), we must first look into a phenomenon known as wave interference.

The image above details a quick demonstration done by the YouTuber Veritasium. He pushes two identical bobbers up and down in a lake to create two sets of waves. As the waves collide, a pattern emerges. In the places where two crests hit, they constructively interfere and a larger crest is born. Similarly, when two troughs hit, they create a larger trough. However, when a crest meets a trough, the two waves “cancel out” or destructively interfere and form a flat spot on the water. You can clearly see these places of destructive interference as flat lines among the wave patterns in the image.

Now, imagine a single wave, but put a wall in front of it to block any motion on the other side. If we open up two holes or “slits” in the wall to allow the wave to pass at each point, then we get an effect similar to what Derek did. Two waves will form on the other side of the wall, as seen in the image below.

This is the setup of the Double-Slit Experiment! If light behaves in a wavelike manner, those interference patterns would emerge behind the screen. However, if light consists of particles, then the light would appear in two distinct lines behind each screen.

Let's look at a simulation of this experiment... light is behaving as a wave!

Cool, right? Well this is just the beginning. If you just wanted a classical understanding of the Double-Slit Experiment, I suggest you stop now, because the next few passages will make you question everything. I must warn you, we are approaching the threshold of quantum physics. It will NOT make sense to you, that is because it doesn’t make sense to anybody. You won’t understand it, I don’t understand it, a PhD physicist doesn’t understand it. I will describe what happens, but I can only try to describe why it happens. As we begin to look at the hidden absurdity of our world, pay attention to the words of Richard Feynman (a pioneer on the subject and arguably one of the greatest physicists of all time): "If you think you understand quantum mechanics, you don't understand quantum mechanics."

Set up that previous experiment in your mind once again, but this time, instead of a blast of light, we are going to fire one single photon through the slits at a time. What do you think will happen? Well, it doesn’t seem likely that the wave pattern will emerge, because the particle can only go through one slit. Guess what? Nothing changes. That’s right. When we fire ONE single photon at the screen, the pattern we see is the result of two waves traveling through the slits.

How is this possible? Does the photon split into two halves and travel through each slit? Nope. I’ll spare the details, but the photon cannot split, it must stay as a whole.

As with much of our universe, the answer lies in mathematics. Fully understanding this math takes years of university-level education, but that shouldn’t stop you from getting a basic sense of it. Physicists use wave functions, denoted as the symbol Ψ (psi), to describe this phenomenon. Instead of giving a definite position, wave functions use probabilities to show where a particle could be.

This is the key. Earlier, we learned that light behaves as a wave, but that doesn’t make it like waves on the ocean (or sound). Rather than being physical waves, the photons act as wave functions, each photon is a wave function, so it can go through both slits at once and give the interference pattern.

However, there’s always another thought experiment to run! Imagine a special high-tech device that can detect if a single photon passes by it. We’re going to purchase two of the photon detectors, and place one in front of each slit. Again, let’s fire an individual photon at the wall. What happens?

Well, nothing in the actual experimental setup has changed (even with the detectors), so it’s safe to assume that we will see the wave interference just like last time. Except, we don’t. The light is fired, travels towards the screen, and then one detector goes off while the other doesn’t. The photon no longer travels through “both” slits and we see a single band of light behind that slit; there's no longer a wave interference band!

Physically, it makes no sense. How can the act of putting a detector (that doesn’t directly interfere with the light in any way) completely change the results of our experiment? But mathematically, things seem to find a way to work. If a wave function represents probabilities of a particle's location, then measuring the exact location of a particle should collapse the wave function—the function shows the particle at one, single location. Think of it as going from a bell curve, to one single column having 100% of the proportion. Therefore, light can also act as a particle.

So, this modernized Double-Slit experiment demonstrates the wave-particle duality of light!

Still confused? The problem is that you are likely trying to imagine the situation in a physical, visible way. The Copenhagen Interpretation can ease this confusion. Developed in the 1920s, the Copenhagen Interpretation makes it clear that the wave function does NOT describe what is really happening, it is just what COULD happen.

When we saw each photon going through both slits at once, it existed in something called superposition, meaning, it exists in “two quantum states at once.” With that being said, the Copenhagen Interpretation states that before a quantum system is measured, it exists in a superposition of every possible location. So the photon isn’t really at a location in a physical sense until you measure it!

And this interpretation seems to hold true in our own lives. You don’t physically see light being superpositioned, but that’s because by looking at it, you are in a sense “measuring” it!

You never can predict exactly what the result of the wave function will be for a photon, though. We can only estimate probabilities, so randomness is truly baked into nature. In conclusion, the Copenhagen Interpretation states that these wave functions are not actually what is happening, but just mathematical tools to predict the outcome! This attitude has been jokingly summarized by the statement, “shut up and calculate.”

Congratulations! If you’ve made it this far (into what I hope isn’t too boring of a lecture), then you should hopefully be utterly confused by quantum mechanics like a true physicist!

There is one last thing I’d like to discuss. This is my personal favorite part of the double-slit experiment, and I’ve gotta admit, I barely understand it! First, turn our double-slit experiment into a triple-slit experiment. The photon is now superpositioned between all three slits.

Now, there is a story (which is unfortunately not based on true events), that said a young Richard Feynman heard this simple point described in a college lecture. He raised his hand, and asked the professor “What would happen if there were four slits?” The professor told him that the photon would then be superpositioned between four slits. A few seconds later, Feynman’s hand went into the air again. “What if there were five slits?” The professor, a little annoyed, answered “Well, then it would be superpositioned between all five slits, and so forth.” Feynman then asked “What if there was a wall with an infinite number of slits, and then we took this wall design, and layered an infinite number of them on top of each other?”

This question changes everything! Following the previous logic, the light should be superpositioned between every single path of travel possible, and that’s true! Technically, one could shine a laser across the room, and there is a path of travel where the light goes to Neptune, around Saturn, and then touches the Moon before hitting the wall on the other side of the room! This is where that wave interference finally applies to our everyday world. The path of travel that we see light take, is simply a result of all other possible paths canceling out and one “winner” being left.