Yes the boss left early. On 7 November 2014 14:37, LizR <[email protected]> wrote:
> On 7 November 2014 13:29, Bruce Kellett <[email protected]> wrote: > >> meekerdb wrote: >> >>> On 11/6/2014 4:08 PM, Bruce Kellett wrote: >>> >>>> meekerdb wrote: >>>> >>>>> >>>>> You seem to overlook that the "expansion" is very likely just >>>>> tautological, i.e. it is nomologically necessary that the AoT points in >>>>> the >>>>> direction of bigger. >>>>> >>>> >>>> No, it points in the direction of higher entropy. >>>> >>> >>> Sure, but the physics is such that entropy must increase in the >>> direction of expansion - the two are linked (that's what I meant by >>> "nomologically necessary"). >>> >> >> I disagree. There is no necessary connection between the expansion and >> the increase in entropy. The total possible entropy might increase with >> expansion, but if we are always a long way below the total possible for a >> given volume, the entropy could increase whether the universe were actually >> expanding or contracting. Anything else and you are necessarily committed >> to a reversal of the arrow of time if the universe begins to re-contract at >> some point. >> >> This may be why the AOT exists, now that we've discovered dark energy. A > recontracting universe may not have one, because the two cancel out, so > anthropically we find ourselves in a U with Dark Energy. (Just a thought.) > > As far as we know the thermodynamic AOT isn't due to fundamental physics. > That is, entropy isn't a fundamental feature of physics (despite that > famous quote from Arthur Eddington) but an emergent one. Below a certain > .level of "coarse graining" it disappears. At the very fine scale (eq > particle) all interactions are reversible and it is impossible to define > entropy (except for bound states - these emerged at an earlier stage of the > universe from a collection of unbound states in which all interactions were > time-symmetric - see below). > > Hence logically you need to connect the thermodynamic AOT to something > that *is* fundamental, or at least more so, to explain why it exists. The > expansion is a possible reason and given that it's THE major feature of the > entire universe that is time-asymmetric, it looks like an obvious > candidate. Plus, even to a bear of little brain like me, the links aren't > particularly obscure, although there are some obscure details involved (but > that's only because we, or at least I, don't know everything about > everything). > > Generically, expansion cools aggregates of particles. It does this by > separating out particles according to velocity - a particle that is moving > faster than average in a region tends to leave it and move to a region > where the average speed is nearer to its own velocity. This effectively > cools the particle, and hence all the particles cool as expansion proceeds. > Also, matter gets less dense, which is also important in generating an AOT > since it allows structures like galaxies to form from an almost uniform > matter background. > > Let's start at the quark soup era. Things are a big vague before that. > > Expansion cools the soup, and eventually collision energies drop enough > for nucleons to form without being blown apart by subsequent collisions. > This is an early (perhaps the earliest) example of how a system that is in > equilibrium, and in which all interactions are time-symmetric, can change > to one in which there is some structure simply by expanding and hence > cooling it. > > Expansion cools the nucleons, until nuclei can form... > Expansion cools the nuclei, until ionised atoms can form... > Expansion cools the atoms, until neutral atoms can form... > > Expansion now allows a more or less uniform gas to clump into larger scale > structures by amplifying any existing inhomogeneities. This allows stars > etc to form, and eventually us, without introducing any new physics; all > the large scale structure is emergent from time-symmetric physics operating > on mass-energy during a non-time-symmetric cosmological expansion. > > (Another way to look at this is that the expansion is producing more > available states for the universe to move into, effectively raising the > entropy ceiling. This means an expanding universe can never reach a state > of equilibrium - this is particularly clear during the BB fireball, which I > would say is very near to equilibrium for a lot of the time.) > > The above sketches how you get the components of the entropy gradient. > Each stage is reversible except for black hole formation (which is another > topic since it may also violate unitarity, and may generally need more > investigation). But if we ignore gravitational collapse, we can definitely > get an AOT from expansion + time-symmetric physics. > > PS as a side issue, note that in gravitational collapse, you effectively > get a mini-big-crunch which illustrates some of the features of time > reversal. In particular, note that in normal time, objects are constrained > to have certain types of pasts - what we can lower entropy. In > gravitational collapse, objects are constrained to have a certain type of > future - it is physically impossible to avoid certain outcomes (at least > assuming GR is correct "all the way down" - which admittedly violates the > BH information paradox...) With the usual caveats, this at least suggests > that time would indeed reverse in a collapsing universe. > > -- 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]. 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