The building blocks of the universe are particles and waves.

Particles, such as neutrons or atoms, come together to create bigger and more complex structures, like molecules. Put together 2 hydrogen atoms and 1 oxygen atom and you end up with a water molecule. Put many water molecules together and you have a glass of water.

Waves however require a bit more abstract thought.

Imagine a stormy sea.

In normal day-to-day life, we say that the ripple and wave in the water is one and the same thing. But in physics, the wave isn’t actually the water moving up and down.

In physics, the wave is the energy that pushes the water molecules to take a ripple shape. This energy makes the water go up and down, until it dissipates and the particles returns to their resting position.

Some waves can only travel in a medium. Sound waves for instance, need air to go from one place to another. But, as some popular movie posters say, “In space, nobody can hear you scream.”  That’s because vacuum can’t transport sound waves.

However, other types of waves don’t need a medium. X-rays, Radio waves, microwaves go through anything – air, vacuum, water, you name it.

The difference between a wave and a particle has some practical implications. A particle exists only at one location at a given time. But a wave is spread out across a lot of space, so it doesn’t have a defined location.

The water droplets in the photo above can be localized into a single position. But the wave itself cannot. The peaks and valleys are not separate waves, but instead they are components of a single wave.

In classical physics, waves and particles were distinct from one another. You are either one or the other.

But classical physics had a hard time figuring out if light is a wave or a particle. Newton said light was a bunch of particles (we now call them photons, but the term didn’t exist in Newton’s time) that come out of a light source.

But many other scientists argued that light was actually a wave.

This conversation lasted for a few hundred years, until one James Maxwell convincingly proved that light behaves like a wave. From this point onward, light was generally treated as a wave.

Despite this, the wave theory of light wasn’t able to explain all of lights properties. It was a better approximation and descriptor than the particle theory of light, but it was nevertheless an incomplete version. Something was missing.

The double slit experiment

In 1905, Albert Einstein, demonstrated the photoelectric effect. Among some of its implications was that in certain conditions light had to behave like particles.  In doing so, he also came up with the basic notions of quantum physics.

So by now, both wave and particle theories of light were proven to be correct. Light was both a wave and a particle at the same time.

Wait, what?

Yes, light is both a wave and a particle simultaneously.

To give you a visual sense of how such a thing can be possible, we will use the most widespread and relevant experiment of quantum physics: the double slit.

In this experiment, physicists use a specialized particle gun that shoots photons towards a screen. But between the screen and the light source you will have a plate with one or two slits.

What we want to do is to light up the screen behind the slit. To do this, you fire a whole bunch of photons through it, machine gun style.

Everything turned out as expected. Most photons concentrated on the part of the screen directly behind the slit, while a few others were spread out across the entire surface.

Now let’s replace the single slit plate with the double slit one.

You will probably expect this result:

Logical enough. After all, the plates block most of the photons, but the ones that manage to pass the two slits will form 2 stripes behind them.

Now let’s see what the real experiment shows us:

This is the point where things get messy. If light was made only of particles, then you would see two stripes, each one behind one of the slits.

Instead, we now have something known as an interference pattern. This pattern takes shape when you have two waves that touch each other. These interference points then move forward and touch the screen, creating those stripes we saw earlier.

Here’s a visualization of it all.

GIF Source

Light being both a particle and a wave simultaneously isn’t something that should happen in classical physics. How can one thing be in two states at once? It’s as if you had a coin that could be simultaneously be both heads and tails.

But this is what all experiments proved, time and again. Thinking of light as both a wave and a particle simultaneously describes all of its properties perfectly. Trying to see it as just a wave or just a particle won’t.

Soon enough though, physicists discovered it wasn’t just photons that could be two things at once. Basically anything that is microscopic in size, such as neutrons, photons and atoms, behaves like both a wave and a particle simultaneously.

Classical physics had reached its limits. So, a new theory was created to explain these weird phenomena.

They called it quantum theory (or quantum physics/mechanics). And the people who wanted to learn more about this new field of physics found even stranger things than the wave-particle duality.

The probability wave, superposition, entanglement

In classical physics, throwing a bowling ball in exactly the same way will make the ball follow the same path 100% of the time.

The ball doesn’t do anything special. It just follows the path and trajectory you set out for it each them you threw it. Its fate was determined the moment you released it from your hand.

This is a critical principle of classical physics called determinism.

Now let’s go back to our two slit experiment. We’ve seen how particles form an interference pattern on the screen if you fire a large amount of them towards the slits.

However, each particle follows a wildly different and unpredictable path, even if the gun points in exactly the same spot.

For this reason, almost no two particles land on exactly the same spot. Indeed, the places where they do land on the screen seem to be almost random.

We say almost random, because the particles seem to have a higher probability of landing on one of the areas corresponding to the five stripes.

However, this isn’t “probability by ignorance” as in the case of the coin toss. This is true probability, meaning that it isn’t affected by any sort of variable. It is absolutely impossible to know beforehand where a particle will land on the screen.

In other words, determinism doesn’t apply to individual particles..

What quantum physicists discovered was that at microscopic levels, the Universe is governed by probability rather than determinism. It was very probable that a particle would land on one of the 5 interference stripes, but you couldn’t be sure that it would do so. It’s almost random.

This went against everything scientists knew about the laws of physics. Einstein, a firm believer in a deterministic Universe, summed up his objections to quantum probability with the phrase “God does not play dice”.

What he meant was that the Universe is ordered and predictable. Everything is interconnected in a long cause and effect chain, and with sufficient information you can predict anything that will happen because nothing is left to chance.

If quantum probability is applied to our everyday lives, then a bowling ball game would look something like this:

Even though you launched the bowling ball on a certain path, it randomly seemed to skip it and go in an entirely different direction and towards a completely different destination.

A particle passes two slits simultaneously

Scientists didn’t easily accept the wave-particle duality of the microscopic Universe. Neither did they like the seemingly random behavior of particles. So they tinkered with their experiment.

They figured that by firing particles in rapid fire, like a machine gun, they ricochet and bounce of each other. This would explain why particles can form wave-like interference pattern and also the probabilistic way they land on a screen.

So instead of firing the particles like a machine gun, they decided to shoot them one by one.

The gun’s firing rate was adjusted, the experiment initiated. All they had to do now was to wait while a few thousand particles landed on the screen.

To their surprise, they got the exact same result:

This proved once again that microscopic particles behave in a probabilistic fashion, rather than a deterministic one.

However, this time around, the real problem was that you still ended up with an interference pattern.

But the stripes only appear if you have two waves that make contact with each other.

And yet the gun fired just one particle at a time.

So the only possible way for a single particle to form an interference pattern was if it could somehow pass through both slits, and then interact with itself on the other side.

On top of all the other issues scientists found with quantum physics, now they had to deal with the fact that a particle could somehow be in two places at the same time.

The weirdness just didn’t seem to end.

The probability wave

By now, you have three strange aspects of quantum physics:

1. Microscopic objects such as neutrons and photons can be both waves and particles at the same time.
2. The microscopic Universe behaves in a probabilistic fashion rather than a deterministic one.
3. A particle can seemingly be in more than one location at the same time.

To better understand all of these 3 aspects, scientists came up with a new concept called a probability wave.

When you propel classical objects forward, such as as bowling balls, you give them a predetermined path to follow.

However, when you fire a particle, it stops having a clearly defined position in space. It also doesn’t have a predefined trajectory and destination.

Instead, the particle is now governed by probability. This means there is x% chance for you to find it in position A, y% chance to find it in position B, z% chance to find it in position C, and so on.

Once you start to add up all the possible positions, you get something like this:

Instead, the particle can occupy multiple possible positions and follow many possible paths on its way to the screen.

Out of all these possible paths and positions, the particle ends up choosing just one position and path, but it is impossible to know beforehand which path/position it will take.

In the sketch above there is just one particle. This is important, so we’ll say it again: there is only one particle. But each transparent shape represents just one possible position where you might find the particle.

As you can see, the particle’s potential positions are clumped up together in certain locations while avoiding others. This corresponds to a wave’s peaks and valleys.

If you were to then imagine this distribution of a particle’s potential positions as a probability wave, it would look something like this:

We will point it out again, but there is only one particle within that probability wave. In fact, you can actually say that the probability wave and the particle area all one and the same thing.

Now that we have a visual representation of how a particle behaves, let’s see how it managed to pass through both slits.

Superposition

Superposition describes a particle’s ability to:

1. Be in multiple states simultaneously, such as both a wave and a particle at the same time.
2. Occupy many potential positions, all at once.

A particle can be both a wave and a particle simultaneously. At the same time, a particle doesn’t have a fixed position, but rather multiple potential positions you might find it in, as we’ve seen in the probability wave section. Put these two concepts together, and you now have the concept of superposition.

The probability wave is just a single particle in a state of superposition. Once the probability wave reaches the screen, its superposition randomly collapses at a single point, with a higher probability to do so in one of the 5 interference stripes.

This still didn’t explain how the particle can somehow pass through both slits simultaneously.

The only thing left to do was to see the particle in action as it passed through both slits simultaneously.

The measurement problem.

Scientists had had enough of quantum shenanigans. This time they did something they should have done all along: put a detector in front of the slits and see the particle in action.

They warmed up the gun and shot the first particle.

Then they fired the second particle.

They just kept firing.

And firing.

And firing.

And firing.

One by one until the pattern on the screen took shape.

A few more particles left.

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