Heisenberg's Uncertainty Principle Explained

Today I am doing an experiment that demonstrates Heisenberg’s Uncertainty Principle.

So here, I have a green laser, and I am firing it down towards the front of the room through a narrow slit. Now, that slit can be adjusted so it can be made narrower or wider. The laser spot is projected onto a screen behind it.

So what do you think is going to happen to this spot on the screen as I narrow the slit? Well, let’s have a look. You see exactly what you would expect. The spot gets narrower and narrower. The sides are getting cut off by the slit. It makes complete sense.

And if you stop there, you would never realize that Heisenberg’s Uncertainty Principle is at work. But if you keep going, something strange happens. As you make the slit ever narrower, the spot starts to spread out. Isn’t that incredible? You are making the slit narrower, and yet the spot on the wall is getting wider. The narrower you make it, the wider that spot on the wall becomes.

To understand this, we have to look at Heisenberg’s Uncertainty Principle. Heisenberg’s Uncertainty Principle is normally written as delta x, delta p is greater than or equal to h on four pi. So what does this mean? Well, it is about the position and the momentum of a particle.

So x is the position of the particle, and p is its momentum. So delta x is the uncertainty in position, and delta p is the uncertainty in the momentum. Now, if you multiply those two quantities together, they must always be greater than or equal to h on four pi. Now, h is Planck’s Constant.

And that deserves a video all to itself, like this one by 60 Symbols. But for our purposes, it is just a very small number. So in our everyday lives, we don’t come up against this uncertainty relation, because everything is much, much bigger than h. But as we narrowed the slit, we were decreasing delta x for those photons.

So we were getting more and more precise about where the photons were passing through that slit. And at a certain point, you come to this limit so that if you narrow this any further, you are going to break this uncertainty relationship. So what needs to happen is the uncertainty of momentum needs to go up.

I should specify this is uncertainty in momentum in the x direction, in the horizontal direction. So if before photons were going perfectly straight, now they must veer off to the left or to the right to ensure that we don’t break Heisenberg’s uncertainty relation.

And the more you decrease your uncertainty in position, the more narrow you make that slit, the more the uncertainty in momentum has to go up. And so if these photons are going to the left and the right, that is going to produce a much wider beam. It is really, really non-intuitive, but it is the way the world works.

It must be the sun playing tricks with my mind.

What about... this is really going to test you, right? [multiple voices]

I have got to say a big thank you to Professor Walter Lewin at MIT. He inspired me to make this video. And I also have to say a big thanks to the University of Sydney for letting me use their equipment and especially to Tom and Ralph for helping me set this all up.

Oh, and just one more thing. I should point out that this explanation of the experiment is slightly controversial, at least in that Henry from Minute Physics and I have been debating whether it is really that counterintuitive.

I mean, if you see light as a wave, then all light is doing here is diffracting. That is the phenomenon where if a wave passes through the slit, it bends at the corners and radiates out in all directions. And that explains the spreading of the beam.

But that goes to the very nature of light. Is it made of waves or particles? That is something that I would like to explore in the coming weeks. So stay tuned for that.

But Henry has an excellent video about Heisenberg’s Uncertainty Principle, which I think makes it more intuitive and less spooky. So if you want to check that out, click on the annotation. It is a really good video.