Does Everything Make Quantum Jumps? Clever New Experiment Could Find Out (Patreon)

[This is a transcript with links to references.]

I got a lot of questions this week about an article which said that physicists have designed “a Way to Detect Quantum Behaviour in Large Objects, Like Us”. Quantum Behaviour like what? Going through two doors at the same time? Tunnelling through a wall? Separating my grin from my face? I had a look at the paper.

First of all, this is a really neat paper. It’s about what I consider without exaggeration the most important question in physics today, that is, why do we not observe quantum effects in everyday life. Like why aren’t cats ever dead and alive for real.

I often hear people say that quantum mechanics is a theory only for small things and just doesn’t apply to large things like us. But that isn’t true, it’s really a theory for all things. We know from our experience that large things like you and I don’t seem to be able to do quantum things. At least I haven’t tunnelled through any walls recently. But the thing is that the mathematics of quantum mechanics doesn’t have any such distinction.

This is why Schrödinger was going on about this cat which is both dead and alive. His point was to say that this is one of the consequences of the mathematics of quantum mechanics. Yet, somehow macroscopic objects don’t seem to display this quantum behaviour.

 Now since Schrödinger’s days, physicists have solved part of this riddle. They have figured out why it’s basically impossible to *measure* the quantum properties of large things like cats. It’s because of a process called “decoherence” that has the effect of spreading quantum links into the environment until you can no longer tell they exist.

But what this decoherence does it that it converts a cat which is in this funny dead and alive state into a cat that dead with 50% probability and alive with 50% probability. This is no longer a quantum state, because now you have normal probabilities. But it’s not what we observe either. So physicists solved the question why we don’t observe quantum properties of large objects, but they haven’t figured out why objects always exist with probability either 0 or 1.

One way to explain this is that quantum behaviour indeed goes away for macroscopic objects. If that was so, this would be a really really big deal. Because as I said, quantum mechanics itself says that quantum behaviour does not go away for big things. The trouble is it’s very difficult to measure this.

The authors of the new paper now put forward a very clever idea. They propose to track the path of an object to see whether it makes quantum jumps. You see the key feature of quantum mechanics is that it’s fundamentally random. So quantum objects sometimes change direction or location for no particular reason. The question is then whether large objects still have this kind of unpredictable change in their behaviour.

The trouble is that well if you track the path of an object it seems you need to measure it and that will disturb the object and that would look like a change but it wasn’t a quantum jump it was something that you caused. The cool thing about the paper is that they’ve figured out a way to measure the path of an object without ever looking at it.

This may sounds somewhat unintuitive, just maybe, but the idea is really fairly simple.

Suppose you have a big box with a marble inside. There’s no light in the box, so you don’t know where the marble is. You want to know where the marble is and whether it’s doing quantum jumps. But you don’t want to shine any light on the marble because that could oh so slightly move it. What you do instead is that you look only at one side of the box. And if you do *Not* find the marble there, you know it’s on the other side, even though you never looked at the marble.

This is called an interaction-free measurement. The researchers now devise a protocol in which you do this repeatedly by updating which part of the box you measure to gather information about where the marble is. And importantly they show that this kind of measurement does not depend on the mass of the object. So it should work for heavy objects. In principle, as heavy as you and I. And this is where the headline came from, it’s that their protocol for inferring quantum properties of an object doesn’t depend on the mass of the object.

But as usual the devil is in the details. Because while their protocol is in principle independent of the mass, in practice making the measurement becomes more difficult with mass. This is because, as you might have noticed, heavier objects also tend to be larger. And the larger they are, the more particles they are made of. And the more particles they’re made of, the noisier they are. There’s also the issue that I suspect it won’t be possible to tell apart a truly random from a possibly chaotic motion.

Still, I find this a very intriguing idea, and I can’t wait for the experiment to show that if I just hit my head at a wall often enough, I will indeed eventually tunnel through.