Monday, 9 December 2013

What colour is an electron?

What colour is a beam of blue light?
Not long ago I facetiously commented on Facebook that electrons were pink. The next day, an X-ray crystallographer asked me 'As someone with a physics background, what colour would you say an electron is?' I almost fell off my chair. But once I started to think about it, it's a really interesting question - and one that might be worth first approaching by asking another apparently silly question.

What colour is a beam of blue light? The answer certainly doesn't have to be blue. Before I explain why, let's put relativity out of bounds. Once you start moving, colours are moveable feasts - think blue/red shift. But I'm envisaging a much simpler situation. I show you a beam of blue light and ask you what colour it is. I can guarantee you would not answer 'blue'.

To avoid distraction, what I will do is shine the blue light down a cylinder with a black interior, turn the lights off in the room and open a door on the side of the cylinder so you can see the light passing through. What would you see? Nothing. Because in one sense you can't see light. Obviously this sounds bonkers. Light is all we do see. But the point is that when we see that we see light we mean something totally different to seeing, say, a postbox. When I say I see a postbox, what I mean is that the light from, say, the sun, hits the postbox, is re-emitted by the box towards my eyes, and I see the box. So 'seeing' usually means detecting the light that hits an object and comes back towards us. This just doesn't happen with light. Light passes right through a beam of light - it doesn't reflect off it. So we don't see a light beam sideways on.

You may at this point be thinking, 'but what about laser beams and spotlight beams and such? You see those sideways on.' And as the picture above demonstrates, you do. But only because the beam is hitting something in its path - dust, water vapour or smoke, for instance - and some of the photons are being scattered off their path towards your eyes. Otherwise there would be nothing to see.

So bearing this in mind, let's go back to 'What colour is an electron?' My initial thought in response to the crystallographer was that it doesn't have a colour as you could either consider it a dimensionless point or a spread out, fuzzy quantum collection of probabilities, and in both cases the concept of colour is meaningless. But he had something different in mind.

He said that physicists usually think of electrons as blue (perhaps as a result of the ńĆerenkov radiation given off by electrons in nuclear power plants), and tend to think of positively charged things as red, which is the opposite of the convention he used. He was just talking about arbitrary conventions. But now that I come back to the concept given the insight from 'What colour is a beam of blue light?' I think that maybe there is a real answer.

When we see a postbox as red, what is happening is that the box is absorbing photons from the sun with a range of energies corresponding to the whole visual spectrum (and beyond). Much of the energy from the photons simply goes into increasing the energy of the atoms in the box (effectively warming it up a little), but some of the energy is re-emitted as photons, preferentially in the red range, so we see the box as red. 'What colour is a postbox?' really means 'What energy range of photons are re-emitted by the box?'

Specifically, the particles responsible for that re-emitting are electrons. When light hits an object, the electrons around the atoms in the object absorb the light energy, jumping up one or more levels. The light that comes back off the object, enabling us to see it, is the result of those electrons dropping back down in energy, releasing a new photon or photons. So arguably the colour of the electron is the colour of the light it re-emits. This varies depending on the electron's state - so  you could argue that the real answer to 'What colour is an electron?' is not 'It doesn't have a colour,' but rather 'An electron is a bit like a chameleon. It has different colours depending on the state and situation it is in.'

There's nothing like a silly question to get the brain in action.

Image from Wikipedia

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