Occasionally I get asked to explain some principle or experiment to a friend or family member, which I love doing. Recently I’ve not been answering though, so I’m getting back into it through the medium of my blog!
Today’s question is on the subject of Young’s double slit experiment. I was forwarded this video, and an accompanying but differently sourced explanation which asks (maybe I exaggerate here) how any of science can be correct when weird shit like this is happening. You can’t explain this stuff, man!
Actually I can.
But let me first rant about the friendly animated KFC Colonel looking character who presents the video. When I first watched it, I was struck by the fact that the artist seemed to have gone to great pains to create a character who looked like a scientist. Note his cheery demeanour. Ok, maybe scientists aren’t all that cheery, but we do all have beards. Ok, we don’t all have beards. But if I had to draw a stereotypical scientist, it would look like that guy.
It turns out that Dr. Quantum is a real guy. Who really was a scientist. He has a real Ph.D. His particular interests are in the physics of consciousness, which would be lovely and interesting if he hadn’t given up on scientific principles like trying to understand things through reasoning, and explaining stuff fully.
Colonel Scientist is not a scientist. Do not trust him to teach you science, because he tells lies. But only later on in the video, after he has drawn you in with some proper real science.
Colonel Scientist presents this experiment, which is a rather famous one, let me tell you, and the results, in an entirely reasonable and accurate way.
A light is shone onto two narrow slits that are positioned closely together, and an interference pattern appears on a screen on the other side of the slits. The interference pattern occurs because the slits create two identical light wave patterns out of the original light wave. These patterns are also diffracted by the slits, meaning they are spread out sideways. Where the two wave patterns meet each other, there are areas where they interfere constructively (they add up to create something positive), and there are areas where they interfere destructively (they cancel each other out). When the light lands on a screen, this pattern of positive and negative destruction is called an interference pattern, and looks like a series of bright and dark spots.
When you Quantum the experiment up, things get weirder, as Colonel Scientist explains.
Instead of a wave, if you fire a particle at the slits, you might expect the particle to go through either one slit or the other. You’d end up with an image of two slits on the screen at the back, and not an interference pattern. What actually happens is that the interference pattern remains!
Wow, says Colonel Scientist, how do you explain that?
To try to explain it, Colonel Scientist describes how sneaky scientists set up a sneaky experiment within the experiment. They put a camera in, just before the slits. That way they would know which slit the particle was actually travelling through. But when they did that, the interference pattern disappeared! It was just like nature said ‘hey, no peeking at my awesome mysteries.’ ‘Isn’t nature awesome?’ says Colonel Scientist.
Presumably, the disappointed scientists gave up and went back to the lab to work on something else. Something that they COULD explain.
This is where Colonel Scientist is a lying git.
Of course we can explain it. In fact, you probably already know a bit about what’s going on, as it has been well analogised by some chap called Schrödinger and his rather unfortunate metaphorical feline companion. This experiment is the physical proof of a quantum mechanical effect
Quantum mechanics is all about possibilities. When things are super tiny, all your measuring equipment is bigger than the thing that you are looking at. If you are looking for a single particle, you can no longer say where it is and where it isn’t. Instead, you have to make a mathematical guess. The best answer you can get will be that it’s possibly mostly here, but it possibly might be a little bit here too. The tricky concept to get your head around is that it is in both places. Until you can properly measure it as being somewhere, it is everywhere. It isn’t a fully formed particle simply hiding somewhere, it is a smear of possibilities. This only changes when some bungling photon comes along and says ‘hey man, you’ve got to decide what you’re gonna do, cos the scientists up there, they want an answer.’
Going back to the real scientist and the metaphorical cat in a box, if the chances are equally likely that the cat is either alive or dead, then quantum mechanically, the cat is both alive and dead at the same time, until you open the box and look at it.
If the chances are equally likely of the particle travelling through one slit or the other, then quantum mechanically, the particle travels through both slits at the same time, until you put a camera in the way.
It was the possibility of travelling through either slit that created the interference pattern. Because it possibly could go through either slit, it did. The diffracted smears of possibilities on the other side of the slit interfered with each other to create the pattern, the same way the diffracted light waves interfered with each other.
When you try to measure which slit the particle travels through, you are forcing the particle to interact with the measuring device. The particle can’t possibly be here, or possibly be there; you are forcing it to be somewhere exact. When you force the particle to act like a particle, it can’t be a wave of possibilities, and you lose your interference pattern.