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 >anything >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). Jesse _________________________________________________________________ Don’t miss your chance to WIN 10 hours of private jet travel from Microsoft® Office Live http://clk.atdmt.com/MRT/go/mcrssaub0540002499mrt/direct/01/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---
