Paul Davies tells Chris Smith about electrons and photons, and he’s even thrown in Schrödinger’s cat for good measure…
Chris – One of the things we did when I was doing physics at school was to shine light waves through slits. And you could see that they form patterns of rings. And if you put the light waves through two slits, you see an overlapping pattern of rings where you get light patches and dark patches. And are you saying that matter particles, because I’m comfortable how a light wave would do that, are you saying then if you did the same experiment with electrons, particles, I’d similarly see a pattern of rings then, and light and dark patches?
Paul – That’s absolutely right. What you don’t see is light and dark patches. What you can do is count the electrons where they arrive on an image screen, and they build up a pattern in a speckled sort of way. Each electron hits at a certain place. And when you have enough speckles, you see a pattern just like you do with light. We call it an interference pattern because the waves going through one slit interfere with those going through the other slit. Now, that seems straightforward if we just think in terms of waves. But now, what happens if you file just one electron at a time?
Chris – Because that experiment’s been done, hasn’t it? We’ve actually got to the stage where you can say, well, I’ll send one electron through at a time, because the hypothesis would be if it’s a particle, it can only be in one place at a time. If it’s a wave, it can go through both slits. So if I fire just one, I should just see one spot in one place.
Paul – Right. You’re exactly right. And you can do the experiment, and you do fire them one at a time, and you do get a spot in one place. And so a single spot doesn’t tell you anything. But when you’ve got a million spots one at a time, that accumulated pattern shows the interference fringes. And you might think, well, why can’t I just station some little device near the slits and figure out which slit it’s going through? Because surely it can only go through one and not both. And if you do that, the pattern goes away.
Chris – So the electron knows you’re watching?
Paul – Yes, sort of. It does seem very creepy. And you can do it with photons as well. And with photons, it’s even more spectacular because you can send a photon through a slit system like that and then through a crystal that makes the one photon split into two and one of these photons can go to the image screen where you’re going to record the pattern, and the other can go off to Albania or somewhere. And you can do a measurement on the Albanian photon that tells you which slit the photon went through. And when you do that, when you choose to make that measurement, you don’t see the pattern.
Chris – So hang on a minute. I shine light through a slit, and I put a crystal behind that splits the original photon into two. So I’ve got two new photons, one that goes to the screen that I’m watching, one that goes to a detector. If I don’t have the detector turned on, I get a nice pattern as though there was no detector there, no crystal there, like the normal experiment. The minute I turn the detector on, it goes away.
Paul – Well, you have to interpret this very carefully. It’s not the minute you turn the detector on. It’s when you have recorded that information, when you go back and sit down with your accomplice in the lab saying, show me the results of your counting on the image screen, you see that there is a correlation and that on the occasions when you’ve got the information about which slit the photon went through, it doesn’t contribute to the pattern and vice versa. And it’s even more creepy than that because there’s something called the quantum eraser experiment, where what you do is you get the information, and potentially it’s available to you, but then you erase it before you can inspect it. And under those circumstances, the interference pattern remains. These experiments have been done. This is not just some mystical mumbo-jumbo I’m talking about. This is real physics, and not only real physics, but physics which is now used in technology, because all of this stuff that we’re talking about and often a lot of it goes under the term entanglement, because a photon over here is entangled with a photon over there. This entanglement is the basis of a lot of what I call quantum 2.0, things like quantum computing and teleportation and the quantum internet, and quantum cryptography. It uses this entanglement as a resource to be exploited so that you can monetise the weirdness of quantum physics, which initially, of course, was only investigated just to try and settle arguments about what is really going on in the quantum realm. It seems so weird.
Chris – But Einstein was sceptical, wasn’t he? He dubbed this spooky action at a distance. It was the photon knowing that you’re measuring its counterpart and therefore erasing the information, the pattern. He was uncomfortable with that.
Paul – Einstein didn’t like quantum mechanics right from the outset. “God does not play dice with the universe” was a famous quote because what we’re talking about here is results which are probabilistic. I mentioned that earlier. These are statistical results, and he never liked the idea that nature is not fully determined down at the microscopic level. So he dreamt up an experiment in 1935 – which we would now call entanglement – with two photons or two electrons, doesn’t matter, that fly apart, and then you have two physicists, you know, Alice and Bob, they’re usually called, and they can perform measurements independently on their respective photons. But the key thing is they can change their minds at the last minute. So, you know, Alice and Bob might say, well, we’ll measure the momentum of each particle, and we’ll compare or measure the position or something like that, and we’ll compare. But Alice might say, “No, I’ve changed my mind, and I’m going to measure something else”. And Einstein, he was of the opinion that only made sense if the particles already possessed definite values of those quantities. But according to Niels Bohr, particles do not possess properties in advance of you measuring them. It’s not fair to say that when you do the measurement, you’re simply uncovering what already exists. Somehow, you bring it into being, and Einstein hated that idea, and he called it spooky action at a distance. But the final point about this sort of Alice and Bob and opposite sides of the lab is that those experiments have been done in the 1980s, and Einstein got it wrong. It is, in fact, the case that the properties of the particles, what you choose to measure, are brought into existence by the act of measurement, and they’re not already there from the get-go when the particles fly apart from the common centre. So somehow the experiment or the measurement is bringing into being the reality of these separated particles. It seems a bit like telepathy, and some people mistakenly think you can send information faster than light that way. You can’t do that, but there is a correlation between these separated events that cannot be explained by saying that these particles had really existed in well-defined states prior to being measured.
Schrödinger’s cat was another way of illustrating the weirdness of quantum mechanics by saying, well, it’s all very well down at the level of atoms and photons and so on, but at what point, when you consider larger and larger objects, do these quantum effects go away? And Schrödinger had this idea of incarcerating a cat in a box. I should say, for cat lovers, this is a thought experiment only, I’m told that Schrödinger did have a cat when he was living in Oxford called Milton; and the plan was you put the cat in the box and there’s a radioactive source, a file of cyanide, and a 50% chance that after one minute the source has decayed and triggered the hammer that smashes cyanide and kills the cat. But if you seal the box, if you consider it a sort of totally isolated system, you have to conclude the cat is both alive and dead at the same time with a 50% probability, it’s in a hybrid, it’s an amalgam of alternative realities. You open the box and see, and then the quantum state collapses into either one or the other. But in the absence of the observation, it’s an amalgam of both. And that just is a way of illustrating that surely somewhere between atom and cat, this theory breaks down. And yet many of my colleagues think it doesn’t break down. They think it applies to the universe as a whole, and that the only way of making sense of Schrödinger’s cat is to suppose that there are two universes, one with a live cat and one with a dead cat.