Heisenberg's Uncertainty Principle

If you remember, I had written an article about 4 months ago about the Double Slit Experiment, possibly one of the first and most influential experiments ever performed within the quantum physics world.

Well, I had lied to you.

Ok, not really. I just told you part of the experiment, and conveniently left out the rest. The reason was because that experiment is already a bit of a challenge to explain without any significant visual aids, and so I thought that I could explain the second half of the experiment in a separate article.

And this is the article in question.

So, after long a wait, here is the explanation to the 'Double Slit' Experiment.

Before I formally start, I would like to say that this article will likely make very little sense unless you read part one, so click here before reading on.

Alright. So when reading the prior article, everything makes some amount of sense, until you reach the end, where I snuck in this:

"This time, you place an 'observing machine' by the blackboard, and conduct the experiment. There are two lines formed on the blackboard, just like with the spitballs [instead of the interference pattern]."

Which I then went on to not explain at all.

The explanation is one of the most incredible, non-intuitive things I have ever come across in physics: The Heisenberg Uncertainty Principle.

Now, the Heisenberg Uncertainty Principle relies on some very basic concepts, so I will quickly explain them to you.

The most significant concept that you are required to grasp is that everything can behave as either a particle or a wave. And I'm not just talking about things at levels smaller than an atom. I'm talking about absolutely everything.

But our 'wavelength' (the distance between the peaks/valleys of a wave) is hardly measurable, because it is so very small.

See, if a wave has a high momentum (described as momentum = mass x velocity) then the wavelength will be really short. Since we are all super heavy and we have a really large mass, then our momentum is going to be really large, and so our wavelength will be so small it's almost impossible to measure.

However, once we get to a small enough level, masses become super small, and momentums decrease. At this point, wavelength can be measured, and the wave properties of small bits of matter are more easily recognised.

Now here’s the thing.

Heisenberg’s uncertainty principle was (in some way) born out of this experiment, since it provides a perfect explanation as to what on earth is happening.

When looking at the double slit experiment, we see patterns that would be perfectly predicted by assuming that the matter behaves as waves. However, once we start to measure which slit the matter travels through using a camera-esque device, we see particle like behaviour.

Particles, however, are not sentient. So how are they able to know when we are looking at which slit they are going through?

The answer lies in the Heisenberg Uncertainty principle.

Heisenberg’s uncertainty principle states that we cannot know both the exact momentum and position of a particle, which at first glance seems ludicrous. Why would it be impossible to know those two things at the same time? Surely, if we were to measure perfectly accurately, we would get perfectly accurate results, right?

Wrong.

Let’s go back to the experiment for a second. As I said before, a wave has momentum, and this momentum is measured by looking at the distance between the valleys or peaks of the wave. When looking at the pattern resulted from the experiment, we can fairly easily deduce the wavelength of the waves that created it. In short, we have a very accurate measure of momentum.

However, waves do not have any single, measurable position. So, the momentum of what is going through the slit is known to us, but the position is not.

What if we start to try to measure the position of the matter going through the slit, as we did with our camera. Particles have a definite location in space, or a definite position. With this measurement, we know the position to a fairly high accuracy. But once we do this, the matter begins to act differently, as aforementioned.

We no longer get an interference pattern, instead, we get two straight lines, patterns that could only be drawn by particles. With the two straight lines, we have no way to measure the momentum of the matter.

As can be seen, the behaviour of the matter changes according to how me measure it, and which properties we chose to measure. This, is the reason as to why the double slit experiment has the results it has.

This may all be very weird to you, and that’s because it’s all very non-intuitive. Things don't change behaviour according to how we measure it, why would there be a reason as to why we can't reach absolute accuracy in all our measurements?

Well, we don’t experience Heisenberg’s uncertainty principle in everyday life, it only applies to the world of the very small. The rules and laws that govern this world, the quantum world, are all very strange and very warped and very different from our everyday experiences.

And that's all! As you can see, this is an especially long article, but that's mostly because this is a slightly harder concept to explain. Nevertheless, I hope you did learn something!

Quantum physics, illusion or reality? - Alaistair I. M. Rae

http://1.bp.blogspot.com/-u5VXCATuFJg/UkyxYTedX7I/AAAAAAAAAUg/vH2-deKdg7E/s1600/Heisenberg.png

https://www.youtube.com/watch?v=lWfxsSeR9lQ