Here AND there; or, brain addling for beginners

This post is going to addle your brain. I make no apologies for this, for it is in the name of science, and knowledge, and enlightenment. Tying your brain in knots is good mental exercise and provides me with a little light entertainment.

You see, I understand this. I do. Well, when I say I understand it, what I mean is that I understand my lack of understanding. But explaining it to others is incredibly difficult, because it requires the suspension of every day common sense, and a lot of people find it very, very difficult to do that. Plus, it requires a new language of the faintly absurd.

Here goes.

Electrons and the double-slit experiment

Electrons are particles. They have mass (1/1836th of a proton) and charge (negative, 1/2 spin). When they are fired out of an electron gun, they arrive one by one as points of light on a screen. The screen is coated with a phosphor that will produce a small burst of light when an electron strikes it. (Incidentally, this is how old-fashioned TVs used to work.)

When electrons are fired at a screen in this way, but a barrier with a double slit cut into it is put in the way, you would expect the pattern on the screen to look like this, right?


Common sense tells you to expect this result.

This is where the strangeness of the quantum world begins. The pattern doesn’t look like that.  It looks like this:


This is the pattern produced by the electrons travelling through the double-slit to the screen.

If you’re familiar with the properties of waves, you’ll recognise this as an interference pattern. When two waves, such as water waves, meet they either reinforce each other, or cancel each other out. What you end up with is something like the above: an interference pattern.

This means that the electrons are somehow interfering with each other as if they were waves. Doesn’t it? Well, no.

Slowing things down

The experiment was slowed right down. One electron at a time was fired from the gun; the point of light was recorded; a short time passed to allow things to settle down; and another electron was fired. And so on.

This is where it becomes really strange: the interference pattern built up gradually. From individual electrons. Whatever is causing the interference pattern does not involve two or more electrons interacting with each other. So what is happening here?

Is the electron passing through both slits at once? In fact, there isn’t any way to know this. We have no idea how the electron is travelling from gun to screen; only that it is, and what the pattern looks like when it gets there.

This is where you need to be able to step out of the ordinary, everyday world of large objects like trees, spoons and elephants, and dive into the quantum world. Suspend your disbelief; leave behind your prejudices and preconceptions (actually, this is good advice for life in general); and open your mind to the world of the natural.

Somehow, each electron is interfering with itself as it passes through the slit(s). As if it were a wave. We need to be very clear: electrons are NOT waves; they are particles. But somehow on their travels from the electron gun to the screen they are behaving like waves. They have wave-like properties.

As each electron travels from the gun to the screen, it spreads out into what I like to call a “probability wave”. It occupies every conceivable path from A to B simultaneously, only “deciding” where it is when it reaches its destination. As the probability wave passes through the double slit, two new waves are formed, which interfere with one another just as any other wave would.

This is why each individual electron produces a random interference pattern. It is impossible to predict exactly where the electron will land on the screen; it could land in any one of an infinite number of places. The overall pattern, however, is predictable as the stripes above.


Stretch your mind. Think about this experiment from start to finish: is there any way – any way at all – in which the interference pattern could be produced from single electrons if they behave in an everyday way? (There isn’t, but think about it anyway because once you’ve convinced yourself, it’s easier to accept the crazy.)

Then think a little more about probability waves. It’s mind bending, and completely awesome.


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