The Double-Slit Experiment

The Introduction

Ask anyone familiar with the world of quantum mechanics and they’ll tell you that one of the core pieces of knowledge you have to understand above and beyond all else is the double-slit experiment.  The Alpha and Omega, beginning and end, it all points back to this.  The wonderful part is, it’s really not very difficult to grasp, though it may be difficult to accept.  Richard Feynman once said, “Any other situation in quantum mechanics, it turns out, can always be explained by saying, ‘You remember the case of the experiment with the two holes? It’s the same thing.”‘  Indeed if we truly understand the double-slit experiment we can begin to truly understand much of quantum mechanics and indeed delve into the nature of the universe itself.

Instead of jumping into the maths and going at this in an extremely hard core way, instead we’re going to stick with the upper level so to speak.  That doesn’t mean you’re missing out.  The truth is you don’t need to understand the math or Feynman diagrams or any of that to truly understand the nature of this experiment and what possibilities it means to us in our every day existence.

So let us begin with something on a large scale that we’re all familiar with.


Let’s imagine you are standing in front of a large target, and between you and that target is a brick wall.  In the middle of that wall is a small break, a “slit” if you will.  Something like this:

Wall with a slit

Now imagine you fire a gun repeatedly at this “barrier” and you aren’t being too careful about your aim.  Some of the bullets will hit the brick wall and never make it through the slit.  The ones that do make it through the slit will form a pattern on the back wall.  Can you imagine what this pattern will look like?  Obviously it will look like a clump of bullet holes in a shape similar to the slit.  This is what’s called a “clump pattern”.  It would look something like this:

Single Slit Clump Pattern

Now imagine we repeated this same test, but this time instead of a single slit in our brick wall, we had two slits side by side.  Once again we fire hundreds of rounds at the barrier.  It’s fairly easy to imagine that this time we would get two clump patterns, side by side.  And in fact, that’s exactly what we would get.  It would look like this:

Two slit clump pattern


Now before we shrink ourselves down to the world of the small, we need to shift gears and talk about something else still in the realm of the large.  We need to talk about waves.  Waves are essentially energy.  A disturbance typically introduced by adding energy which then travels through a medium from one location to another in a repeating and periodic fashion.  If you dip your finger into the middle of a pool of water you will see a “wave” ripple out.  You introduced a disturbance by pushing the water out of place.  That disturbance then travels outward and repeats until the energy fades.

A wave

Now there are two very important things we need to understand about waves.  The first very important thing we need to understand is that when a wave encounters a barrier like in our first shooting experiment (a wall with a slit in it) the wave will travel through the slit, but will ripple out from the slit as if the slit was it’s origin.  This is due to diffraction.

Diffraction of a wave

The wave on the left side of the barrier in the above image has a straight wave front, but you can imagine it being a wave generated from a single source having spread out to the point where the curvature isn’t even measurable any more.

Now the second very important thing you need to understand about waves is that they can interfere with each other.  Have a look at this diagram:

Wave Interference

When two waves collide with each other where they are in phase the amplitude will increase.  In phase just means that the peaks of one wave line up with the peaks of the second wave, and likewise the valleys line up as well.  This is called constructive interference where the energy of both waves combine to form a more powerful wave. (i.e. the new wave has higher peaks and lower valleys)  On the other hand, when two waves collide and are perfectly out of phase, (meaning the valley of one wave lines up with the peak of the other wave) we have destructive interference.  The energies cancel each other out.

Now if we think of being perfectly in phase as being 0 degrees, and being perfectly out of phase as being 180 degrees, we can imagine the areas between will produce various results depending on where the phasing results.  To better visualize this, think of two fishermen throwing their lures into a pond at the exact same time.

Fishing Interference

If we were to place a piece of film that dissected where these waves intersect along it’s meridian and it recorded the state of these waves we would see what’s known as an interference pattern.  Places where two peaks meet and therefore form higher peaks (denoted as the white peaks in the above image) would form bands along with places where the waves canceled each other out forming voids.  The result would look something like this:

Large Interference Pattern

Now this is not some crazy experiment that requires specialized equipment.  In fact, you can do this right in your very own home.  Take 3 pencil leads out of a mechanical pencil and hold them together in front of a laser pointer.  When positioned correctly you’ll get an interference pattern just like above due to the light waves interfering with each other.

Waves or Particles?

In 1801 Thomas Young performed an experiment similar to the above.  In doing so he proved (or so he thought) that light was indeed a wave.  Later however with the discovery of the photoelectric effect we saw that light can also behave as though it was composed of individual particles.  Thus quantum theory was born.  How is it that light could behave as a particle and show all the properties of particles (photons) in one experiment then show completely different properties and behave as a wave in another experiment?  For quite some time quantum theory lived only in the realm of mathematics and thought experiments.

What offered so much confusion was the fact that even though the light was obviously traveling as a wave and interfering with itself, the light was then absorbed at the screen at discrete points.  In other words, when the light was recorded at the screen it was recorded as individual particles yet it seemed to be traveling as a wave.  One conclusion was that photons (individual discrete particles of light) were just bouncing off of themselves and causing the interference pattern.

In 1909 G.I. Taylor was able to repeat the experiment in such as way that the emission and absorption events were non-overlapping.  In other words, photon were essentially being fired one at a time, so there was no way they could bounce off of each other.  Yet the interference pattern still remained, giving proof that even an individual photon was traveling as a wave.

Hold up… Wait a minute…

Wave-Particle Duality.  At first it may not seem like such a big deal.  You might not realize the implications of this. To do so we need to seriously think about what this means, and to do that we need to bring it up to a scale where we can wrap our heads around it.

So let’s go back to our earlier experiment, where you are shooting a gun through a brick wall with two slits in it.  When each bullet is fired it might hit the brick wall or it might go through either the first or second slit.  This is completely acceptable and understandable to us.

On the other hand imagine being in the middle of a swimming pool with a wall and two slits.  You make a big splash and the waves move up to the wall, that wave bounces off the wall in some places, and other places travels through the first and second slit.  Where a wave travels through both slits, it creates two new phases of itself. Which is like saying two new waves are formed at the slits.  Now those two waves interfere with each other on the other side.  In other words, the wave you create does several things simultaneously.  It doesn’t go through the wall, but it also goes through the first slit, and also through the second slit.  Every possibility all rolled into one.

So now if we combine these two, imagine if our bullets we fire out of the gun travel like waves until they are finally recorded on the target.  That means that when you fire a single bullet it simultaneously hits the brick wall, goes through the first slit, and also goes through the second slit.  Every possibility of where the bullet could have went is encapsulated in that wave.  Every time you fire a bullet it goes everywhere all at once, until it’s finally recorded in one single place on the target.

But where did it really go?

This concept was just as baffling to scientists as it is to you.  They knew that once recorded on the screen each photon of light was a discrete hit.  So if there was this tiny discrete particle they wanted to know where it was REALLY going.  So they set up detectors to tell them just that.  You can imagine this as putting a camera right by the two slits.  This would allow them to see exactly which slit each particle went through as it was fired.

As soon as they did this though something remarkable happened.  The interference pattern disappeared!  Now they had a regular old clump pattern.  Remember this guy?

Two slit clump pattern

Once they knew rather each particle had went through either the first or second slit they no longer had an interference pattern.  The light was no longer traveling as a wave and instead was traveling as we would expect bits of matter to travel, just like our bullets.

Light only traveled as particles (something tangible and real) when we were observing it.  The moment we turn our back and stop looking it travels as a wave.  A wave of possibilities.  A wave function.

Once again let’s slow down and rethink this, because if you haven’t been totally blown away by now then you’re not fully understanding it.  To really simplify it, here’s what we’re saying here.  When it comes to light as it travels through our reality,  if we observe it and actually “learn” where it’s at and what path it’s taking, then it will in fact be a little particle.  A tangible piece of our reality.  On the other hand, if we don’t observe it, if we don’t know what path it’s taking, then it instead isn’t something tangible, instead it becomes simply a wave function of every possibility.  It’s only through the act of us observing that it becomes “real”.

Let’s go back to our bullet example and think of it in a way that better lets us understand it.  Suppose you rig the gun up to fire a bullet without anyone looking.  The gun is fired and then somehow time becomes frozen when it reaches the wall.  What we’re saying here, is that as long as no conscious person is looking at the bullet then it exists at all places all at once.  It’s hit possible position on the wall, it’s went through every possible position of each slit.  If you “could” observe it in that froze moment it would be like billions upon billions of bullets all at once in all possible places with all possible trajectories.


This is what is known as superposition.  A wave function describes every possible state of a particle.  Every possibility all wrapped up into one.  If it were a simple switch it would mean that it was on, but also off, and both off and on all at the same time.  I know that’s a hard thing to wrap your head around, and we’ll delve more into superposition in another post, but for now just try to imagine those bullets.  Billions of possibilities, all existing at the same time, in superposition.  Every single possibility is there all at once.  It’s only once a person (a consciousness) looks to see where it actually is that the wave function (or superposition) collapses.  That is to say one bullet will materialize in one position and all the other possibilities are lost.

Collapsing A Wave Function

That’s worth repeating, so I’ll say it one more time.  It is you, or rather a conscious observer who collapses a wave function and creates our reality.  So when a photon is fired at the wall it exists only as a wave function.  It’s everywhere in every position all at the same time.  Every possibility all at once.  It takes a conscious mind to look at it before it actually pops into existence.  When a consciousness actually observes it, the wave function (which contains all possibilities) collapses and becomes one discrete reality.

If you still haven’t quite wrapped your head around this yet, I’ve found a great video that explains it pretty well and fairly simplistically.  Have a look, it’s definitely worth 5 minutes of your time.

Now you might be thinking, “Ok, that’s really cool… But this is light… We all know that light is kinda “special” in the realm of science.  It has no mass.  Can it really be considered a “thing” when it has no mass?  So all you’ve really shown is that light acts differently than things.”

But that’s not true at all.  Because you’ll note in the video above they mentioned doing this with electrons.  Physicists have indeed done this with electrons, but it goes even further than that.  They’ve also done it with protons and other larger “things”.  The larger the “thing” the more difficult it becomes to perform the experiment.  This is because the wavelength of an object is inversely proportional to it’s momentum, and thus inversely proportional to it’s mass.  That means the more massive your projectile the smaller your slits have to be in order to generate an interference pattern.  With that said, physicsist have currently been able to reproduce this experiment with entire molecules consisting over 800 atoms.  You can read the paper here.  So is there a point where “things” stop behaving in this manner?  The evidence and math says no, that even things on a macroscopic scale like bullets, marbles or even trees and mountains behave in exactly the same way.  That is to say that they are only “things” when they are being observed.  The next time you go to the kitchen to get a drink think about this as your hand reaches for the cabinet door.  The glasses inside that cabinet are not being observed by a consciousness.  So before you open the door and collapse the wave function, they don’t exist at all in our reality.  At least not as a solid object like we would think.  How’s that for weird?

In Conclusion

Hopefully by now you are beginning to understand the heart of quantum mechanics.  There’s a lot more to discuss from here but this truly is the core.  The “reality” of the situation is that there really is no “reality”.  What you see and experience each and every day is really only there because you collapsed the wave functions and created it.  That’s a pretty powerful concept and one that we’ll delve much deeper into as we continue our journey further into the rabbit hole.

Approximately 2500 years ago the Heart Sutra contained the statement, “Form does not differ from emptiness, and emptiness does not differ from form.  Form is emptiness and emptiness is form.  …. No eye, ear, nose, tongue, body or mind.  No form, sound, smell, taste, touch, or mind object.”

2500 years ago Buddhists were able to come to the same core conclusion as quantum physicists today.  The difference is they didn’t have incredible scientific knowledge and technology to help them discover these answer.  Instead they simply looked inside of themselves for the answers.  Quite remarkably they found them.

But the fun doesn’t end here.  Next up on our adventure we’re going to meet a man named Erwin Schrodinger.  I hear he’s fond of cats.


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