Mark Peaty  wrote:
>This is yet another delayed response; the story of my life really ...
>Jason: "By physically reversible I don't mean we as humans can undo
>that happens, rather physical interactions are time-invertible.  If you
>were shown a recording of any physical interaction on a small scale, an
>elastic collision of particles, the decay of a nucleus, burning of
>hydrogen, it would be impossible for you to tell if that recording were
>being played in reverse or not, since it is always possible for that
>interaction to occur as it does in either direction of time."
>MP: This is only true for 'individual' reactions on the micro scale, but
>even then the 'truth' about the reversibility can only really be
>maintained by hiding the truth about the context. For example, it is
>logically possible for certain atomic nuclei to collide at just the
>right velocities and fusion will occur. In reality however the
>probability of what are normally fission products coming together to
>make a uranium nucleus is so close to zero you are never going to see
>it. [I don't know much about the physics but my casual believe is that
>heavy elements are created through various long and complex 'ratchet'
>accretion pathways in which nuclear isotopes of H or He enter heavier
>nuclei.] Like wise the burning of hydrogen; it seems simple enough and
>yes it is 'reversible', but does the reverse occur? Not where you and I
>can see it.

But according to modern physics, at maximum entropy the probability of any 
reaction should be identical to the probability of its time-inverted one 
(actually, there are a few weak nuclear reactions where you'd need to invert 
charge and parity as well--the laws of fundamental physics are 
CPT-symmetric, but not always T-symmetric). It's thought that the only 
reason some reactions are more likely to happen in the forward direction 
than the backward direction in the real world is because of the low-entropy 
initial boundary conditions of the universe; if we lived in a universe with 
low-entropy final boundary conditions on the big crunch and no constraints 
on the big bang, then the "arrow of time" would be reversed. And the reasons 
for the low-entropy big bang remain fairly mysterious, we may not understand 
it without a complete theory of quantum gravity or TOE (in the physics sense 
of unifying all four forces, not in the list's sense).


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