Sciencey Wiencey: The Weeping Angels – Quantum Locking

Guest contributor Caleb Howells investigates more Who science.

In 2007, Steven Moffat struck gold with one of the most critically acclaimed episodes ever, and also one of the most popular monsters. The Weeping Angels were a very simple, but very clever idea. As long as you can see them, they’re just statues, but as soon as you look away, they can move. It’s a creepy idea, and one that I could relate to as a child. Lying in bed at night, I would keep staring at whatever was scaring me on that occasion, certain that it would do something if I stopped looking at it.

I don’t know how exactly Steven got the idea, but he chose to use quantum mechanics to explain how they worked. They are “quantum locked” only in their statue form when someone’s observing them.

Before I continue, I’d just like to give a brief explanation as to how quantum mechanics work. There are various different aspects other than the one we’ll be looking at, and they’re all fascinating, but irrelevant. The Weeping Angels only use one concept, and that is that of the true state of electrons (sort of).

In school, you would have been taught that electrons orbit round the nucleus of an atom in much the same way as planets orbit a star. This is not the case. They don’t move in such boring, linear ways. They are essentially all around the atom at once. They don’t “decide” where exactly to be until someone looks at them. As I said, that’s just one aspect of quantum mechanics and there are plenty of others.

However, objects only behave like this on a very small scale. Large scale objects appear to have their own, different laws of physics, which is why there’s so much talk about trying to come up with a unified theory.

So, as I said, large scale objects don’t behave like that, so how can the Weeping Angels? Well, here’s something which I’m sure you’ll agree is very, very interesting. Scientists have found that the closer something is to absolute zero (-273.15 degrees C), the more it behaves like it does on a quantum scale.

To clarify: Heat is, in actuality, the vibration of atoms or molecules. The more they vibrate, the hotter the substance measures. Absolute zero is the temperature at which there is no movement whatsoever. The substance has no energy at all. Scientists haven’t been able to make anything this cold, but they’ve come very, very close.

Now, as I said, things tend to behave in a more quantumy way when they’re really, really close to absolute zero. However, as I mentioned earlier, there are many different aspects of how things behave on a quantum scale. The whole ‘doesn’t decide where it is until you look at it’ thing is only one aspect. Quantum mechanics is very complicated (and due to how new a science it is, if you go into too much detail you’re likely to be out of date to one degree or another after not very long), and if there really was a creature that behaved in a quantum manner then doors wouldn’t be able to stop it – it could just teleport to the other side.

So this is really a very messy explanation for how the Weeping Angels could work. You can only cheat the laws of physics so much. But, because there’s still interesting science to tell you about, let’s just assume for a moment that you could make a statue-sized object be at virtually absolute zero. It would, obviously, be very cold! So naturally, it would make everything else around it utterly freezing as well. How could we get around that?

Well, first of all, let me explain what ‘cold’ really is. Heat is the vibration of particles, as I stated earlier. If something is really hot, its particles are vibrating a lot (the Kelvin temperature scale is directly based on this fact – hence it has no minus numbers, because 0 Kelvin is the absolute absence of energy). So if something is really cold, then its particles aren’t vibrating nearly as much. So what is it, exactly, that you feel?

You don’t feel the kinetic energy of its particles. No, what you feel is the kinetic energy of your particles being taken away. This is because everything wants to be at the same temperature. It’s very similar to diffusion in water. If you put two liquids in a cup, they won’t stay apart. They will diffuse together and be completely evenly spread out. And it’s the same with temperature. So the cold thing that you’re touching will try to take away your heat until both you and it are at an equal temperature.

This explains why some things feel colder than others, even if they’re actually not. For example, imagine you had a wooden spoon and a metal spoon. If you put them in the fridge for a while, they’ll end up the same temperature. But then when you touch them, the metal spoon will feel considerably colder than the wooden one. Why? It’s not colder. So why does it feel colder?

It’s because metal is much faster at taking away your heat than wood is. The wooden spoon is equally cold, but it takes much longer for it to take away the kinetic energy from your particles, so it doesn’t feel as cold.

Now imagine you made a Weeping Angel out of a material that was virtually perfect at not absorbing energy. It wouldn’t make anything else cold. So, that solves our problem.

Of course, unless it was truly perfect, it would eventually get warm, and then it wouldn’t have its quantum abilities.

Unfortunately, the Weeping Angels are not conceivably possible. At least, not in a genuine quantum physics way, as the show states. It may well be possible in some other, more conventionally biological way (such as receptors that detect if something’s looking at them – whatever way that would work – and then the outer cells are sent signals which tell them to become ridged), but in terms of being “quantum locked”… no. That wouldn’t work.