Let’s say you have a cycle where you start with a particle B and it’s antiparticle B*. You have a cyclical decay process whereby B -> C -> B* -> D -> B and the time reverse of this decay process B* -> C -> B -> D -> B*. You start with B and B* in exact 50/50 equilibrium. If the decay rates are different, however, after time, you will favor either B or B* over the other. You know the arrow of time, so long as you look at time scales less than the period of the longer oscillation.

How is this any different from ergodic recurrance, other than the fact that we typically can’t measure time scales large enough to see the gas recompress into the left half of the piston? And given the short times in the early universe, couldn’t this have had something to do with picking out the direction of time in the early universe?

]]>er, NO.

WTF!!!!

But I’m at least arguing that such a scenario is not possible, so I don’t have to defend my position so much as yours – but frankly, if you believe in determinism – then any way is ok. :–)

]]>You know how a backwards entropy human would think? – impressive.

Well, no, perhaps it’s not impressive, a backwards in time human would no doubt perceive the high entropy state as the past (Which is a big deal, no?)

However, I don’t believe such a backwards in time human is possible, since I don’t believe the world is deterministic.

]]>Sean is correct if Nature is deterministic, since in a deterministic theory we could just reverse all microscopic dynamics and have the entropy decreasing from an initial high-entopy state, then this state would be the past

eg see http://www.scholarpedia.org/article/Time%27s_arrow_and_Boltzmann%27s_entropy

However, if the universe is non-deterministic then the only possible global behaviour is increasing entropy or equilibrium at max entropy – except in ultra improbable scenarios (unlikely in exponential googolplexes of universes).

However, even in this non-deterministic universe we have a low-entropy initial state (otherwise we would just have noise everywhere today) – BUT this low entropy initial state just explains why we haven’t reached heat death yet, IT IS NOT THE REASON EGGS DON’T UNBREAK – eggs can hardly ever ever unbreak anywhere in any universe because Nature is not deterministic, not because the initial state was a low entropy one.

(local decreases in entropy are of course possible – otherwise life wouldn’t exist)

]]>So indeed all these things are very well understood but get confused by different groups/people — the SLAC headline of the discovery of the quantum arrow of time totally confuses the issue, and Sean’s claims that the thermodynamic arrow of time is a great mystery also totally confuses the issue. ]]>

And, in fact, I would think these results fit very nicely with the concept that the macroscopic arrow of time (via entropy and irreversibility) arise from microscopic statistical processes.

]]>“….This new measurement in the B meson system — indeed, the entire phenomenon of T violation — has absolutely nothing to do with that arrow of time…..”

I agree up to a point. The new work is evidence for a different arrow of time, that’s certainly true. But T violation may well underpin all other arrows. T violation is associated with a time-asymmetric dynamical law (the weak interaction). All the other arrows, including the thermodynamic arrow, are associated with time-symmetric dymanical laws. Their asymmetry arises from time-asymmetric boundary conditions – starting from a low entropy state, for example. I published some work last year on this very topic:

Found. Phys. 41, 1569-1596 (2011) http://dx.doi.org/10.1007/s10701-011-9568-x (http://arxiv.org/abs/0911.4528)

The way I like to think of the situation is like this. Imagine a tree loosing its leaves in autumn. The position on the ground where the leaves fall depends on the direction of the wind. If you see the leaves lying on only one side of the tree, you can say which direction the wind was blowing. The pattern of fallen leaves is evidence of the direction of the wind. But this evidence doesn’t make the wind blow in any direction! This is like the thermodynamical arrow – increasing entropy is evidence of the direction of time evolution. But it doesn’t tell you why the universe evolves in one particular direction.

The T violation work I am doing is really embryonic at this stage. But it does show that T violation can induce large scale effects via interference. T violation may give an explanation of why the universe appears to continue to move in one direction. This is like explaining why the wind blows continuously in one direction.

So T violation may well underpin the thermodynamic arrow.

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