New Year’s Eve is the proper moment to contemplate time. There is nothing logical in this. There is no great discontinuity in anything but our calendar as the clock strikes twelve but the feeling is right. All the better if eyes are bathed, as mine are now, in the ancient light of far away stars.
Tonight as I contemplate time I’ll deviate from the normal genealogical path and return to the mystery of time, that I started to write about in my very first post, Boltzmann’s Grave. Time is, of course, that thing that all family historians deal with yet no one really understands.
Most physics, at its core, is symmetrical with respect to time. That means that you could play a film of a simple physical process backwards and what you would see is something that could actually happen. And not just that but happen with exactly the same likelihood as what you see when you play the film forwards. If everything in the solar system ran in reverse, the sun would rise in the west, which might look odd but there is no basic reason why it shouldn’t. Gravity, the force that rules the motions of the moons and planets, makes no less sense when time is run backwards. From where then does the “arrow of time,” time’s apparent direction come? Why can we seemingly only move one way through time?
For many decades, particle physicists have dealt with three symmetries, that is, ways of flipping things around to get something that is equally plausible as what you started with. One symmetry is time, or T, symmetry. This simply says that playing the film backwards makes no difference. Another is charge, or C, symmetry. Change the electric charge of all the particles, so that positive becomes negative and visa versa and in the end, nothing fundamentally different will occur. More precisely, if you had a special film projector that projected every particle of matter recorded on your film as the equivalent particle of antimatter (and visa versa) everything would look just fine. Nothing odd or unreasonable would be shown on your movie screen.
The final symmetry it parity, or P, symmetry. Think of this as handedness symmetry. If you inverted the film so that everything was shown in mirror image, parity symmetry states that everything should be just as realistic as before.
Physicists once thought that all three symmetries worked for all particles and all their interactions. At the same time it was know that all three symmetries when applied together must result in reactions just like the originals. That is, if you turn all the matter particles into particles of antimatter and visa versa, then take a mirror image and look at the whole thing run backwards, the result will be just as realistic and just as probable as before you started.
Eventually it was discovered that some reactions couldn’t be mirror imaged and still make sense. It turned out though that those interactions also violated C symmetry in such a way that C and P together still worked. Antimatter in a mirror looked like regular matter without a mirror—or so they thought.
Then came one of the strangest results ever in particle physics. Some very few interactions violate the combined “CP” symmetry. If CP symmetry is broken, time symmetry must be broken in a way that compensates. It is as if putting C, P and T together was like adding 1+2+3 with a funny sort of addition that always gives 6 for the full sum but every so often nature says that 1+2 is 4 but whenever that happens 4+3 is 6 and so 1+2+3 is still 6 no matter what.
Breaking the symmetry of time is where this all started. If time isn’t the same backward as it is forward, time suddenly seems to have an arrow. Is this a hint as to why we don’t seem to be able to move backward in time? Why ancestors don’t sometimes come before descendants? Oddly, no one really knows.
Looking out at the stars, time reminds me of a comet hanging in the night sky. Beautiful and serene—seemingly full of motion in one direction and yet showing no direct evidence of that movement as we gaze upon it.
Happy New Year and thanks for reading!