On Fri, Aug 22, 2014 at 3:47 PM, meekerdb <[email protected]> wrote:
> But entropy is relative constraints, in this case coarse graining. Yes, entropy is the logarithm of the number of microstates that produce the same macrostate times a constant; so the entropy of a square foot of steam is Boltzman's Constant.times the logarithm of the number of ways you could arrange water molecules and still have the same volume pressure and temperature. Ice would have a lower entropy than stem because ice is in a rigid lattice of particles so there are fewer ways to arrange H20 molecules and still have it be ice. There are a lot more ways to arrange H2O molecules and still have it be steam. > > If you take the equation of physics seriously, they are time-symmetric > Yes, but that doesn't mean that the world must be time-symmetric because our world is the way it is because of the laws of physics AND because of initial conditions. As I said before, if things started out (the Big Bang) with the lowest entropy there could be then, whatever the laws of physics are and however they change the state of things, that change must be in the direction of increased entropy because entropy can't get lower than the lowest it can go. > > (assming MWI) > Assuming anything! The known laws of physics are time-symmetric under any quantum interpretation, but our world most certainly is not, so if the laws of physics are not responsible for time's asymmetry there is only one other thing that could be, initial conditions. Things started out in a super low entropy state 13.8 billion years ago and everything has been unwinding toward higher entropy ever since. In a billion trillion years or so things will reach a state of maxim entropy and after that it will be impossible to perform work in the universe. And that will be that. > and entropy never changes. You can't explain entropy's behavior with just the laws of physics alone, in fact it's more general than that; even if Newtonian physics was 100% correct you couldn't predict what entropy or anything else will do with the laws of physics alone, just as important are initial conditions. Physicists started to have a good understanding of entropy about 1880 or so and they had everything they need to have predicted the Big Bang way back then, but unfortunately they did not. > But at the coarse-grained level of description, where entropy increases, > But you can't state what you're trying to prove, we're trying to figure out why entropy increases in one time direction but not the other when the laws of physics work the same in both direction. > > there are fewer past states that could produce the present than there > are future states into which the present can evolve. > I know that, but the question is why there were fewer past states. It's easy to figure out why the laws of physics will cause the entropy tomorrow to be larger than today, there are just VASTLY more high entropy stated than low so things will almost certainly change into one of them. But by using the same reasoning when asked what sort thing evolved into our present state we'd have to say it almost certainly came from one of those very very very very numerous high entropy states. And so if we had nothing but the laws of physics to work with we would state the second law of physics as follows "Things at the present time are in the lowest entropy state there can be and so entropy will be higher in the future and was higher in the past". But this is clearly ridiculous, we must be ignoring something important, and that thing is initial conditions. > > But that the exact same physics applies to a universe that "collapses" > into a very very low entropy states. But if we lived in such a universe, > we'd live our lives and form our memories in the direction of expansion and > we'd say we live in an expanding universe (as we do) and that's why the 2nd > law globally is a tautology. But creature in that universe would have something in common with us, we would both note that the fundamental laws of physics work the same in both time directions but our minds do not, both of us can only remember things from one time direction but not the other, and both of us would note that the universe as a whole doesn't behave the same in both time directions either, it gets larger in one direction and smaller in the other. And although we remember the past and they member the future we both would have to assume initial conditions at one end of the timeline to make sense out of things, the laws of physics alone aren't enough. John K Clark -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
