Small wonder, or why quantum physics will blow your mind!

Many people think physics is boring. I'd like to show how, when you look closely, and I mean REALLY closely at the tiny particles we're all made of, they behave in ways so amazing, it'll change the way you think of the world.

You may remember the name 'Isaac Newton'. Newton created a set of laws that explain how and why things move. Things like tennis balls for instance. Tennis balls obey 'Newtonian' laws of physics.

The way very small things move, like electrons, is called quantum mechanics. We'll see how, at this size, Newton's laws don't apply.

Bear with me, I'd like to tell you about something called the double slit experiment.

It's just a load of balls

Picture a tennis court. A tennis ball firing machine at one end and a white wall at the other.

If we removed the net from our tennis court and replaced it with a brick wall, our tennis balls won't get very far.

Lets make a gap in the brick wall, slightly wider than a tennis ball. Now when we fire the balls towards the wall, some of them get through the gap and pile up at the other end of the court. As you would expect, most of the balls pile up in the middle, opposite the gap in the wall.

If we now make two gaps in the wall, some of the balls go through one gap, some of them go through the other and some bounce off. We end up with two piles of balls. So far so good. Now, let's do the same thing, but use electrons instead of tennis balls.

Particle physics

If we fire electrons through a narrow slit in a tiny wall, some of them go through and end up hitting a screen at the other end of our 'quantum tennis court'. Just like the tennis balls, we see most of them piling up roughly in the middle, opposite the gap.

Now lets have two gaps like we did in our brick wall. These are the double slits which give this experiment its name. What would we expect to see happen? Two bands of electrons hitting the screen? Well, here's where it gets strange. We actually see many vertical bands across the screen.

This happens because the electrons  are behaving like waves.

Wave goodbye to common sense

Imagine a tank of water. A wave in the water passes through two gaps in a wall and we see the interference pattern this produces, just like dropping two pebbles in a pond.

Waves have peaks and troughs, rising and falling, like on the sea. If two waves meet, sometimes one peak meets another, whilst a trough meets another trough. These waves are 'in phase'.

However, sometimes a peak will meet a trough, canceling each other out. These waves are 'out of phase'.

If a wave passes through two gaps in a wall, two waves will come out the other side. These two waves will interfere with each other and produce an 'interference pattern'.

Incredibly, this is what our electrons are doing. they are behaving like waves, producing an interference pattern.

This seems strange. Maybe they are bouncing off each other as lots of them pass through the slits together. To find out, let's fire electrons through our slits again, but this time only one at once. That way, how could they bounce off another one?

Now it gets really strange. When we fire them one at once, we still see an interference pattern! It might sound crazy, but the only explanation is that the electrons are traveling through both slits at once!

They know we're watching

If they are, let's try and see it happening. This time we will put a tiny detector - like a little camera - pointing at the slits. We should be able to see which slit an electron passes through - one, the other, or both!

What we find is quite amazing - difficult to believe, but true. When we look at the slits we don't get an interference pattern on the screen. When we stop looking, the interference pattern comes back!!

The electrons seem to know they are being watched and behave differently!

Studying quantum mechanics, looking closely at the way particles like electrons behave as waves, has shown us behaviour that defies all explanation! Quantum physics shows us that what we believe to be real can change depending on whether we are watching or not.

You still think physics is boring?