Transcript: What Popularizers of Quantum Mechanics Don’t Want You to Know by Ron Garret

I’ll then show you how that story can’t possibly be true, because if that story were true, it would lead to a violation of relativity, in particular, it would lead to faster-than-light communication. And it doesn’t do this in the usual way that most people think that it leads to faster-than-light communication, it does it in a more subtle way that really hasn’t gotten a lot of attention. So you physicists in the room, bear with me.

Then, I’m going to walk you through some of the actual underlying mathematics of quantum mechanics, in a way that is accessible to anyone who knows – can do basic algebra and knows what algorithm is.

And finally, tell a new story based on our understanding of what the underlying mathematics actually says about what’s really going on and hopefully we’ll achieve enlightenment at the end of that.

Two-Slit Experiment

So, is there anybody here who has not heard of the two-slit experiment? All right, good. I will just blast through this very quickly. So, this is — you have a laser that shines through two-slits, and you get an interference pattern that shows that light is a wave and can interfere with itself like any other wave.

And there are two strange things about this. If you look at the results of this experiment with very low intensity light, what you find is, and this isn’t showing up very well, but this top image here shows just some dots scattered randomly. And then dots get denser and denser and denser until down here at the bottom you have a dense enough pattern of dots that you can start to see this interference pattern start to emerge. And this is an actual photograph of laser light going through a single slit and going through two slits. And you can see this interference pattern here, this is an actual photograph of the same experiment, this particular one happened to be done with electrons but the underlying physics is the same.

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And the thing to notice here is that the total amount of light that you get in this pattern when there are two slits is brighter than the overall amount of light that you get with one slit, which is what you would expect. But that there are some places here where you have these dark bands that were bright up here when you only had one slit. And this is the interesting part that you want to kind of focus your attention on because what this means is that there’s a spot here where light was shining and then you open up an extra path for light to get to the screen and that spot goes dark. And that is the manifestation of interference.

But the strange thing about it is that this is not a continuous phenomenon, it’s an accumulation of all these particles. Now, I can actually — I used to think that this was a fairly subtle experiment that you need a specialized equipment to conduct this experiment. It turns out it’s not true, you can actually do this experiment yourself. These are some pencil leads that I’ve taped together with scotch tape and this is an ordinary laser pointer. And if I pass these, and there’s just some very narrow gaps between these leads, so you can actually see this happen if I pass the leads in front of this pointer, you can see the light start to spread out. And if you’re close enough, you can actually see the interference bands. I don’t think you can see it in the back. But if you’re interested, after the talk, come up, I’ll give you a closer look at it. You can actually see the interference pattern.

The point here is that this is not a subtle phenomenon and it’s not something that you need expensive equipment to reproduce. This is an everyday experience for modern humans, at least.

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Okay, so this is not yet intractably weird, because there are all kinds of explanations that we can postulate about how this might be happening. So, for example, photons and electrons might be real particles that have real locations and velocities like our intuitions about particles that might be pushed around by some kind of underlying wave. And — oh, I forgot to mention — the location where these particles accumulate is random, there’s no known way to predict other than statistically where these particles are going to end up on the screen.

So, the randomness might just be due to some underlying real physical property that we just don’t know how to measure. But it turns out we can eliminate this possibility as well. And the way we do that is by trying to track the path of a particle and ask with — on its way to the screen, on its way to producing this interference pattern, which of these two slits did it go through? And we can do that. We can add detectors to the slits and we can measure which of the two slits a particle went through.

But it turns out that when we do that the interference pattern goes away and the phenomenon that we were trying to get a better grip on has changed. And it turns out that this is an inherent feature of quantum mechanics, that any modification that we make to this experiment that allows us to determine, even in principle, which of these slits this particle went through destroys the interference. This is the famous wave particle duality, any modification that allows us to determine even in principle — yeah?

[Audience: Inaudible]

So, I’ve been asked to ask the people on the VC to mute their mics, did I get that right? Okay.

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