In a not-quite symmetric world

By Marcus Wilson 13/11/2012

An interesting problem to ask students to think about is this: Write down a definition of ‘left’ (as in the opposite of right). It’s something every adult knows, but how do you define it. There’s little wonder that children take a long time to grasp which is left and which is right.

One might say: Well, if you are in the Southern Hemisphere, face the position of the midday sun and left will be the side of you where the sun sets. But that just shifts the problem to another: Define ‘south’. Then one could resort to physics, and look at the direction that a positive particle is bent under a particularly oriented magnetic field, but that muddies things further – we need to know about positive and negative and also left and right – so we are no better off.

In order to do it, you need some asymmetry in the universe. Fortunately, there are some we can draw from. For a start, the Earth isn’t symmetric – just by saying ‘in New Zealand’ we establish ourselves as being in the southern hemisphere, then we can apply arguments about where the sun sets etc.

There are also  other asymmetries. There is more matter than antimatter, for example. Why? It’s something that the Large Hadron Collider might give some clues about (It wasn’t built just to find the Higgs Boson). There are more subtle ones, concerning CP-violation in particle physics (this actually links back to the matter/anti-matter asymmetry) which suggests that there is a fundamental asymmetry about the universe. But why?

What got me thinking about this was the clockwise rotation of baby Benjamin at the weekend. Lying on the in-laws living room carpet, he seemed to be quite able to rotate clockwise (so that his head swizzled round roughly to where is feet were) but not anti-clockwise. So there is a preferred direction. It could be because one leg is stronger than the other, which is quite possible, but there may be other reasons – for example the ‘grain’ in the carpet might be an issue here.

One thing’s for sure – he’s living in an asymmetric universe.

 Postscript 22 November 2012

This news has just come out: An experimental measurement of time-reversal symmetry breaking. That’s allowed by our current physics understanding – what isn’t allowed is charge-parity-time reversal (CPT) symmetry violation.




0 Responses to “In a not-quite symmetric world”

    • I don’t recall reading that book, but I’m sure I’ve read something of Martin Gardner’s involving an asymmetric alien species that looked like a teapot – when it saw it’s reflection in the mirror it didn’t see itself – it saw a member of another species looking back. Or something like that.

  • I’m not sure that left (or right) could be defined in isolation; the pair are inter-related. Same as north/south, up/down, big/little; it’s all relative…

  • Yes – in a symmetric universe you can’t do it. It’s an arbitrary choice. But we don’t live in a symmetric universe. (The earth doesn’t have a symmetric distribution of land, Benjamin for reasons best known to himself spins clockwise, more people are right-handed than left-handed and there is CP violation in beta decay, to name some of them…)

  • AFAIR faced with the problem of how to tell someone on the other side of the universe what was L and R Martin Gardner ended up using isotopes of molecules that preferentially form with one orientation or the other.