• Question: What do the phenomena that differ going forwards and backwards in time, tell us about the nature of time

    Asked by pandabear to Arttu, Ceri, James_M, Monica, Philip on 21 Jun 2011.
    • Photo: James M Monk

      James M Monk answered on 21 Jun 2011:

      This is a good question because such phenomena are needed in the early Universe so that there is some matter left over that does not annihilate with all of the anti-matter. Anti-matter can be thought of as normal matter that is travelling backwards in time, so we need to find a reaction that happens at a slightly different rate for anti-matter to matter. This is called CP violation, and has been observed in a few systems in particle physics, though not enough to explain the matter/anti-matter asymmetry in the Universe.

      The first example found was in the decay of neutral kaons, which are made up of a strange quark and an up quark. A neutral kaon can turn into an anti-kaon (its anti-matter partner) spontaneously through the weak force. The measurable states of a kaon are therefore a mixture of the particle and anti-particle states.

      These two mixed states have different decays, and it turns out that the decay of the anti-kaon happens at a slightly different rate to the kaon. So for example, both the kaon and anti-kaon can decay to a pair of pions, but reversing time by combining two pions to form a kaon it is slightly more likely to make either the anti-kaon or the kaon (I can’t remember which way around it is).

    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 21 Jun 2011:

      The most obvious thing about time is that it seems to be asymmetric. Past is clearly different from the future. This is called the “arrow of time”.

      As James writes, there is a tiny difference in the way particles and antiparticles behave, which we call CP violation. This means that some laws of nature are not exactly the same way going forwards and backwards in time. However, they are very similar, because of another symmetry called CPT which does not seem to be violated. It says that if you could change the signs of all the charges in the universe and look at it through the mirror, these time-reversed laws of nature would be exactly the same as the original ones. Because of this, the laws of nature do not actually seem to have an arrow of time.

      On the other hand, the arrow of time is very clear in thermodynamics. The second law of thermodynamics says that entropy only ever increases. In some sense this just captures our everyday concept of the arrow of time. However, in thermodynamics it follows from the fact that there are many more states with high entropy, and therefore it is just much more likely that we end up these.

      This partly explains the arrow of time, but only if the universe started in a rare low-entropy state. If it had started in a typical state with high entropy, there would be no arrow of time. Therefore the explanation of the arrow of time has to involve the initial conditions of the universe.

      So, the fact that everyday phenomena seem to differ going forwards and backwards not only tells us something about the nature of time, but also that the universe must have somehow began in a state with a very low entropy.

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