Fast, Messy and Magnetic
'We can detect these wiggles; we call it light.
Patrick Kennedy-Hunt | 6 March 2016

This will be the last article I ever write for INK, and consequently the last piece of writing I will publish for a long time; possibly forever. It seems fitting, therefore, to attempt to convey something of the magic of physics to an audience who, by and large, will have no experience of the discipline. Of course you will have studied the subject at GCSE and possibly throughout the sixth form, but without further reading it would be easy to think of physics as simply about forces and some confusing business about a chap called Flemming with a penchant for left hands. 

Consequently, my aim here is to explain two boring, everyday observations in a novel and remarkable way. Although I cannot even hope to begin to give you a flavour of what physics is really about, and if you do not understand every word then the fault can only lie with me, but perhaps you will begin to understand that physics is so much more than what our exam based education system would have you believe...

Thermodynamics is all about heat and work (the transfer of energy); fairly dull surely? Let me convince you otherwise. When you look at the universe around you, everything seems to be going forwards; cause leads to effect. But why? Why, when I drop a vase, does it shatter to a million pieces, but never do a million pieces spontaneously form a vase? We can understand this as coming from a concept called entropy: disorder generally increases in any closed system. But we can also understand this in terms of statistical mechanics; the laws of physics (by which I mean the stuff of Newton) are perfectly symmetrical in time. Show a physicist a video of some molecules moving and he will not be able to say if it is being played forwards or backwards. To an atom then, time forwards and backwards is indistinguishable. All that we observe in thermodynamics is that it is astronomically more likely for disorder to increase, just because there are so many more ways to arrange a million pieces of glass to look like a mess than to look like a vase. This understanding lets us realise that, from Newton’s point of view, nothing stops all the molecules in a ball (which are in continuous random motion) from moving upwards and our ball flying up spontaneously; it is just incredibly unlikely, as the random motion of billions of molecules goes in billions of directions. Then what determines the direction of time? A better discussion of that can only come with an understanding of undergraduate (and beyond) physics.

A particularly nice area of physics which I will use to explain my second observation is known as special relativity. I cannot go into too much detail here, but basically as you go really fast (like... really fast) a few strange things happen. Firstly if you are in a spaceship travelling at great speeds, then everything appears shorter. You would measure the radius of the earth as a fraction of its “real” (rest frame) value. Likewise you would see a clock on earth appear to go slowly. Weirdly, a person on earth would also see a space ship’s clock go slowly and the ship appear shorter than it really is. This is an incredibly powerful, almost magical, set of ideas, and also happens to be relatively easy to understand: Mermin’s “It’s about time” is a fascinating read for those interested.

Perhaps the area of physics most abused by our GCSE education system is electrodynamics: you learn about motors and magnets and all that, but why would you want to know this? As it turns out, all matter is made of atoms, and atoms have, within them, charged particles called electrons. Now obviously when you have a charged electron this will exert a force on other charged particles. But because of relativity, the fact that you have moved an electron cannot be conveyed faster than the speed of light. So by wiggling our electron, we set a mass of ripples spreading out from our wiggling electron through space. Right near the wiggling electron you get a massive mess of forces. But go out a little way and all the short range effects no longer matter and we have just the long range effect of a wiggling charge. This long range effect of a wiggling electron propagates through space at the fastest speed possible. We can detect these little wiggles; we call it light.

Here is another rather nice way to think about this; what happens if the electron moves really fast? Near light speed fast? Well imagine you are in a wire with another wire nearby carrying an identical, very fast, group of electrons at the same speed. You travel at the speed of the electrons. Now it looks a lot like the wires, containing positive ions, are just moving backwards and you are stationary. The problem comes from relativity; it means that the positive ions in the wire appear to occupy less space (they are all squeezed together), so you have a higher density of positive charge than of negative charge on the other wire. As a result, you, as electrons, are attracted towards the other wire. We call this force of attraction (or repulsion for opposite currents) magnetism. And you don’t even need to point the fingers of your left hand in weird ways...

Original image by Elizabeth Timoshchenko.

James Routledge 2016