6 The double-slit experiment

And here we are, the granddaddy of all quantum weirdness, the infamous double slit experiment. To understand this experiment, we first need to see how particles or little balls of matter act. If we randomly shoot a small object, say, a marble at the screen, we see a pattern on the back wall where they went through the slit and hit. Now, if we add a second slit, we would expect to see a second band duplicated to the right.

[METALLIC CLINKING]

Now, let's look at waves. The waves hit the slit and radiate out, striking the back wall with the most intensity directly in line with the slit. The line of brightness on the back screen shows that intensity. This is similar to the line the marbles make.

But, when we add the second slit, something different happens. If the top of one wave meets the bottom of another wave, they cancel each other out. So now, there is an interference pattern on the back wall. Places where the two tops meet are the highest intensity, the bright lines, and where they cancel, there is nothing.

So when we throw things, that is matter, through two slits, we get this, two bands of hits. And with waves, we get an interference pattern of many bands. Good so far. Now let's go quantum.

An the electron is a tiny, tiny bit of matter, like a tiny marble. Let's fire a stream through one slit. It behaves just like the marble, a single band. So if we shoot these tiny bits through two slits, we should get, like the marbles, two bands.

What? An interference pattern. We fired electrons, tiny bits of matter, through. But we get a pattern like waves, not like little marbles. How? How could pieces of matter create an interference pattern like a wave? It doesn't make sense.

But, physicists are clever. They thought maybe those little balls are bouncing off each other and creating that pattern, so they decide to shoot electrons through one at a time. There is no way they could interfere with each other, but after an hour of this, the same interference pattern is seen to emerge.

The conclusion is inescapable. The single electron leaves as a particle, becomes a wave of potentials, goes through both slits, and interferes with itself to hit the wall like a particle. But mathematically, it's even stranger. It goes through both slits and it goes through neither. And it goes through just one and it goes through just the other.

All of these possibilities are in superposition with each other, but physicists were completely baffled by this, so they decided to peek and see which slit it actually goes through. They put a measuring device by one slit to see which one it went through and let it fly.

But the quantum world is far more mysterious than they could have imagined. When they observed, the electron went back to behaving like a little marble. It produced a pattern of two bands, not an interference pattern of many. The very act of measuring or observing which slit it went through meant it only went through one, not both.

The electron decided to act differently, as though it was aware it was being watched. And it was here that physicists stepped forever into the strange never world of quantum events. What is matter, marbles or waves? And waves of what?

And what does an observer have to do with any of this? The observer collapsed the wave function simply by observing.