Nicola answers your science questions – Young’s double slit experiment

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.


About nicolajrolfe

Dr. Nicola J. Rolfe is currently looking for her next opportunity to make a big splash in East Kent industry.
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7 Responses to Nicola answers your science questions – Young’s double slit experiment

  1. Martyn Boyle says:

    Excellent explanation. In as much as it can be possible to explain that you are tooking at the result of something being theoretically in 2 places at once whilst not actually being in 2 places at once because you are looking at it. But until you look at it it could be in 2 places at once which gives you the result you are looking at. But if you look for the cause of the effect, the effect ceases to be. My brain aches I think I’ll have a lie down.

  2. nicolajrolfe says:

    Nailed it, Martyn.

  3. nicolajrolfe says:

    Carrie asked ” So it’s not because someone (the camera) is watching (obviuosly). It’s because the camera is a physical thing in the way?” And I replied (on facebook) Not quite, it IS because the camera is watching. The camera isn’t physically in the way, but you can’t think of the camera as being some passive thing in the system, snapping away in the distance. The presence of the camera affects the thing it is looking at, in the quantum world. When you look at an object, you see it because the light bounces off of it and into your eyes. When the camera, or any measuring device, looks at the particle, the particle has to send a signal to the measuring device to let it know it’s there. As soon as you know where it is, it can’t be anywhere else. In order for the freaky quantum effects to happen, you need the particle to be in both places at once.

  4. nancy says:

    So if you are not observing it when it reverts to a wave, how do you know it is a wave if its not being observed, but you must be observing it as you know it is a wave, and why doesn’t it display as a particle when it knows you are observing. I could go around and around.

    • nicolajrolfe says:

      Thanks for the question. The answer is that you aren’t observing IT directly, but you are observing the EFFECTS of it. We know it is acting as a wave because of the interference pattern, which is something that only waves can do. If you went to measure it directly, you would measure a single particle because there is no longer the possibility that it could be anywhere. That’s the key thing, if there is a possibility that it could be in more than one place (i.e. either of our slits) is IS in both slits (i.e. a wave of possibilities). If you tell it that it can only be in your handy measuring device, it will only be there (i.e. a particle). Hope that helps!

  5. Martyn says:

    So does this go some way to explaining an argument I listened to a while back that said something along the lines of. “Reality doesn’t exist until it is observed or measured”.

    I mean, at the moment I have no idea where you are, you could be at home or you could be on the moon for all I know. And the same works the other way around. I could be at work or I could be dancing the tango with a chimp. I’ll let you guess which one.

    The question is, is anything real until it is seen?

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