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]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. 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