On Thu, Feb 25, 2010 at 12:38 PM, Jason Resch <jasonre...@gmail.com> wrote:
> > > On Feb 25, 2010, at 1:56 AM, Charles <charlesrobertgood...@gmail.com> > wrote: > > On Feb 23, 8:42 pm, Brent Meeker <meeke...@dslextreme.com> wrote: >> >>> I think >>> it's an example of the radiation arrow of time making a time-reversed >>> process impossible - or maybe just vanishingly improbable. Bruce Kellet >>> has written a paper about these problems, see pp 35. >>> >>> http://members.optusnet.com.au/bhkellett/radasymmetry.pdf >>> >> >> I am reading this, and have just come across this passage: >> >> "One possibility that is sometimes raised is that the overall >> expansion of the universe provides >> the local arrow for the direction of time. While cosmology, >> particularly the cosmological initial >> conditions, might be relevant to any final understanding of the arrow >> of time, particularly the >> thermodynamic arrow, it is difficult to see the expansion of the >> universe as being sufficient to >> explain the local asymmetry of every single independent radiation >> event. The basic reason is >> that the expansion of the universe is a cosmological phenomenon; the >> usual understanding of the >> Friedmann-Roberston-Walker solution to Einstein’s equations of General >> Relativity that governs the >> overall evolution of the universe is that, although the fabric of >> spacetime expands on the large scale, >> individual galaxies do not expand, they merely move apart. The >> expansion actually takes place only >> on the scale at which the universe can be seen as homogeneous and >> isotropic. This is the scale of >> galaxies and galactic clusters—only there is the ‘Friedmann dust’ >> model applicable. The model that >> describes the expansion of the universe simply does not apply within >> galaxies, much less within the >> solar system or on the surface of the earth. So the universal >> expansion is simply unable to provide >> an effective arrow of time that is locally available for every >> independent radiation event." >> >> This seems to me to miss a fundamental point, namely that emission and >> absorption events are only local if you ignore what happens to the >> photon beforehand or afterwards. If you trace the trajectory of the >> photon, you will arrive at some other event, and this event in turn is >> linked to a previous / future one. Ultimately all chains of >> trajectories of photons, electrons, quarks and so on connect to either >> the Big Bang or the distant future (timelike infinity, say). If the >> trajectories (or, presumably, waves) are constrained by whatever is at >> either end of their trajectory, as time-symmetry implies, then this >> stops them being local. They are part of a universe-filling web, which >> is "anchored" to whatever boundary conditions obtain on the universe >> as a whole. >> >> Charles >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Everything List" group. >> To post to this group, send email to everything-l...@googlegroups.com. >> To unsubscribe from this group, send email to >> everything-list+unsubscr...@googlegroups.com<everything-list%2bunsubscr...@googlegroups.com> >> . >> For more options, visit this group at >> http://groups.google.com/group/everything-list?hl=en. >> >> > One approach to the problem that I heard regarding the arrow of time > relates to the fact that storing information (either by the brain or in a > DNA molecule in the course of evolution) requires the expendature of energy. > The expendature of energy results in an increase in entropy of the > universe. Thus life evolves and we remember new things in the same > direction of time. > > Jason > Stephen Hawking discusses this idea briefly in "A Brief History of Time", p. 146-147. I'll quote: "It is rather difficult to talk about human memory because we don't know how the brain works in detail. We do, however, know all about how computer memories work. I shall therefore discuss the psychological arrow of time for computers. I think it is reasonable to assume that the arrow for computers is the same as that for humans. If it were not, one could make a killing on the stock exchange by having a computer that would remember tomorrow's prices! "A computer memory is basically a device containing elements that can exist in either of two states. A simple example is an abacus. In its simplest form, this consists of a number of wires; on each wire is a bead that can be put in one of two positions. Before an item is recorded in a computer's memory, the memory is in a disordered state, with equal probabilities for the two possible states. (The abacus beads are scattered randomly on the wires of the abacus.) After the memory interacts with the system to be remembered, it will definitely be in one state or the other, according to the state of the system. (Each abacus bead will be at either the left or the right of the abacus wire.) So the memory has passed from a disordered state to an ordered one. However, in order to make sure that the memory is in the right state, it is necessary to use a certain amount of energy (to move the bead or to power the computer, for example). This energy is dissipated as heat, and increases the amount of disorder in the universe. One can show that this increase in disorder is always greater than the increase in the order of the memory itself. Thus the heat expelled by the computer's cooling fan means that when a a computer records an item in memory, the total amount of disorder in the universe still goes up. The direction of time in which a computer remembers the past is the same as that in which disorder increases. "Our subjective sense of the direction of time, the psychological arrow of time, is therefore determined within our brain by the thermodynamic arrow of time. Just as a computer, we must remember things in the order in which entropy increases. This makes the second law of thermodynamics almost trivial. Disorder increases with time because we measure time in the direction in which disorder increases. You can't have a safer bet than that." -- You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-l...@googlegroups.com. To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/everything-list?hl=en.