The arrow of time
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Shaper
Member # 5450
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written Wednesday, July 20 2005 00:21
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quote:My simple answer that may of already been said: We don't remember tomorrow because it has not happened yet. Its the same as 2 hours away: We don't know what is going to happen. -------------------- I'll put a Spring in your step. Polaris Posts: 2396 | Registered: Saturday, January 29 2005 08:00 |
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...b10010b...
Member # 869
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written Wednesday, July 20 2005 01:07
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The problem with your answer, which has already been described in this thread, is that in physics it's not obvious how you define what hasn't happened yet without making reference to what we remember. "We don't remember it because it's in the past, and it's in the past because we don't remember it" is obviously a circular argument. [ Wednesday, July 20, 2005 01:08: Message edited by: Thuryl ] -------------------- My BoE Page Bandwagons are fun! Roots Hunted! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Agent
Member # 2210
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written Wednesday, July 20 2005 01:25
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If we want to make things even more interesting, we can add another twist with physics-- the idea that there are many worlds-- if we make a prediction about what may happen, it will happen, but it will not happen in a place which we can experience the event unfolding. -------------------- Wasting your time and mine looking for a good laugh. Star Bright, Star Light, Oh I Wish I May, I Wish Might, Wish For One Star Tonight. Posts: 1084 | Registered: Thursday, November 7 2002 08:00 |
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By Committee
Member # 4233
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written Wednesday, July 20 2005 03:47
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If we could get the reversal of time down, there'd be all kinds of useful technological advances from it. For example, imagine if we could find a way to collect spent or "wasted" energy, like heat - we wouldn't have to worry about a power source ever again! :) EDIT: Maybe I went off the deep end on that one. [ Wednesday, July 20, 2005 03:48: Message edited by: Drew ] Posts: 2242 | Registered: Saturday, April 10 2004 07:00 |
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Electric Sheep One
Member # 3431
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written Wednesday, July 20 2005 04:31
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Thuryl's idea that human memory requires increasing entropy is probably true; but all it does is unify the psychological and thermodynamic arrows, which most people already think are unified (for reasons just like this). The thermodynamics of information processing, as opposed to information theory per se, is a fascinating but underdeveloped subject. The main result at this point is due to one Rolf Landauer, a giant in this under-recognized field. He showed, not that remembering increases entropy, but that forgetting does. It also costs energy. Resetting one bit to zero requires, on average, at least one unit of thermal energy (i.e. k_B T, Boltzmann's constant times the ambient absolute temperature). (Actually, I don't think it's an exact inequality like Heisenberg, but an "order-of" figure.) In conventional computing architecture, this minimal input energy is converted to heat. Since heating is a major practical problem in current computers, there has been some interest in reversible computing, in which no information is ever discarded, so no heat (or entropy) is generated. This uses more memory, though, and since I don't think that current heat generation is anywhere close to Landauer's minimum, I don't think reversible computing is going to show up at Intel anytime soon. It is, however, a requirement for quantum computation; so there has been an academic revival of interest in it for this purpose. -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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Electric Sheep One
Member # 3431
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written Wednesday, July 20 2005 04:34
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The home run is certainly a good example of something that would let you tell whether the film is running forwards or backwards. Unfortunately, though, I'm pretty sure that the source of the time asymmetry here is the human brains of the batter and pitcher, which are at least as complicated as fire and water. So I'm still looking for a simple system with an arrow of time. -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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Shaper
Member # 32
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written Wednesday, July 20 2005 04:36
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Well if we truly wish to see time flow, all we need to do is have a four dimensional being 'lift' us out of our 3-d space and shift or relation to the axes... -------------------- Lt. Sullust Cogito Ergo Sum Polaris Posts: 2462 | Registered: Wednesday, October 3 2001 07:00 |
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By Committee
Member # 4233
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written Wednesday, July 20 2005 04:43
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quote:Indeed, it would require infinite memory. I suppose this would be our friend the Turing Machine? [ Wednesday, July 20, 2005 05:03: Message edited by: Drew ] Posts: 2242 | Registered: Saturday, April 10 2004 07:00 |
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Electric Sheep One
Member # 3431
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written Wednesday, July 20 2005 04:50
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Spring's answer, that the future hasn't happened yet, deserves comment, because it's the obvious answer, and the reason that physicists are not happy with it is old (for physicists) but important. Where is tomorrow before it happens, and where is yesterday when it is gone? Questions like this sound simple, but are very subtle, so the only hope for humans to make any progress on them is to be very careful in deciding on their essential meaning, and not get distracted by connotations of their wording. For instance, 'where' is just a metaphor. It is conceivable that God keeps yesterdays in boxes somewhere, but when we ask our question we are not really insisting that the answer has to be about spatial locations. What the question is really about, as far as physicists are concerned, is determinism. Are the past and the future implicit in the present? Could the instantaneous state of the universe, at this moment, have arisen from only one prior sequence of states? Is there one unique sequence of states that must follow the present one? Is the entire history of the universe, in fact, predestined and unalterable? Common sense may well answer these questions with 'Hell, no!'; but physics basically assumes their answer is Yes, for the equations of natural law as we now know them have this implication. Could we but know the exact state of the universe at any moment, the equations would fix the entire past and future. In practice, of course, we can't come remotely close to knowing the exact state of the universe at any moment; so we are not going to be able to predict everything. But the point is that this could be done in principle, and so the whole future and the whole past are implicit in any present instant. This means that, effectively, the future does already exist. We just haven't gotten to it yet. But we are advancing steadily towards it, at a rate of one second per second. :) -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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Electric Sheep One
Member # 3431
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written Wednesday, July 20 2005 04:56
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Reversible computing doesn't necessarily take infinite memory, just more. You just need to add one bit that starts at 0 for every time that you would normally plan to reset a bit. Then instead of resetting, you swap. If you want to be clever you may be able to reduce the number of extra bits needed, but this is the most you might need. -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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By Committee
Member # 4233
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written Wednesday, July 20 2005 05:09
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Given that in this framework the future is theoretically immutable, what would be the benefit to anyone in knowing about it beforehand, so to speak? Posts: 2242 | Registered: Saturday, April 10 2004 07:00 |
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...b10010b...
Member # 869
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written Wednesday, July 20 2005 05:22
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Changing unpleasant surprises into unpleasant anticipation. -------------------- My BoE Page Bandwagons are fun! Roots Hunted! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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By Committee
Member # 4233
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written Wednesday, July 20 2005 05:36
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Of course, we wouldn't have a choice whether we end up being surprised or anticipating with dread. :) The illusion of free will is one I will choose to cling to, I think. Posts: 2242 | Registered: Saturday, April 10 2004 07:00 |
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...b10010b...
Member # 869
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written Wednesday, July 20 2005 05:39
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Well, if it is in fact an illusion, you don't really have a choice in the matter. :P Congratulations in advance, by the way. -------------------- My BoE Page Bandwagons are fun! Roots Hunted! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Master
Member # 4614
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written Wednesday, July 20 2005 09:33
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For now, I'll be content with the assumption that time is the movement from low entropy to high entropy as caused by the Big Bang. (For those of you who think I am deserting my Chritian ideals, I'll say that I believe that the Big Bang happened - God was the one that made it happen.) But we still have our time - our seconds, our hours, our years - and people age and die, to use a classic analogy, an egg splatters but according to what we've seen cannot unsplatter, and so on. Something must be driving this all to go in that way, or, if I will, driving entropy up. Some say dark energy and the like of that, but until things like that are discovered, factualized, and experimented with, or conception of the arrow of time will remain a puzzle. At least from our place in our three-dementional universe. -------------------- -ben4808 For those who love to spam: CSM Forums RIFQ Posts: 3360 | Registered: Friday, June 25 2004 07:00 |
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Triad Mage
Member # 7
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written Wednesday, July 20 2005 09:36
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Christianity doesn't need to mean that you don't listen to science. -------------------- "At times discretion should be thrown aside, and with the foolish we should play the fool." - Menander ==== Drakefyre's Demesne - Happy Happy Joy Joy desperance.net - We're Everywhere ==== You can take my Mac when you pry my cold, dead fingers off the mouse! Posts: 9436 | Registered: Wednesday, September 19 2001 07:00 |
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Master
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written Wednesday, July 20 2005 09:41
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Right, and I am listening to science. But why is the universe the way it is now? Why are you and me here? Why is there a universe in the first place? Someone had to make it. I'm not refuting that many scientific theorys are utter bogus because of God and Christianity. I'm just saying that we observe things the way we do because that how God intended them to be. -------------------- -ben4808 For those who love to spam: CSM Forums RIFQ Posts: 3360 | Registered: Friday, June 25 2004 07:00 |
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Triad Mage
Member # 7
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written Wednesday, July 20 2005 09:45
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And I'm saying that science and religion are not mutually exclusive. It's the same thing :P -------------------- "At times discretion should be thrown aside, and with the foolish we should play the fool." - Menander ==== Drakefyre's Demesne - Happy Happy Joy Joy desperance.net - We're Everywhere ==== You can take my Mac when you pry my cold, dead fingers off the mouse! Posts: 9436 | Registered: Wednesday, September 19 2001 07:00 |
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Post Navel Trauma ^_^
Member # 67
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written Wednesday, July 20 2005 11:02
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Sorry if this has been said: I think the distinction between the log and the pile of ashes is that if you perturb the log slightly, it will burn to something that counts as a pile of ashes, but if you perturb the pile of ashes and reverse time, it won't go back to being a log. Simple system with arrow of time: To particles in the plane, moving in some direction at some speed, starting close together. -------------------- Barcoorah: I even did it to a big dorset ram. desperance.net - Don't follow this link Posts: 1798 | Registered: Thursday, October 4 2001 07:00 |
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Electric Sheep One
Member # 3431
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written Wednesday, July 20 2005 11:06
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By 'system' I mean a set of equations (in particular, a Hamiltonian operator), not a particular state. It's easy to find states that evolve in ways that sort of look entropic, but that actually has little if anything to do with the problem. The issue is what makes such states more common than their time-reversed images. I don't think that ignition is technically a perturbation, but it does seem a relatively small change, compared to what it would take to make a pile of ashes into a log again. But this is really just restating the problem of the arrow of time: why is it easy to burn things, and hard to achieve the time-reversed process? [ Wednesday, July 20, 2005 11:10: Message edited by: Student of Trinity ] -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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Post Navel Trauma ^_^
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written Wednesday, July 20 2005 11:37
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Sorry, I didn't make myself clear. And perturbation was probably the wrong word. I'll try again. Suppose you have a log that you've just started to burn. If you move the atoms a little bit (and/or change their velocities a little bit), then what you get is still a burning log, and it will still turn into a pile of ashes. If you have a pile of ashes that came from burning a log, then running time backwards will give you a log. But if you move the atoms in your pile of ashes a bit first, then you will end up with a pile of ashes that won't "unburn" to a log. And now I've said all that, I can't quite remember what, if anything, my point was. Oh well. -------------------- Barcoorah: I even did it to a big dorset ram. desperance.net - Don't follow this link Posts: 1798 | Registered: Thursday, October 4 2001 07:00 |
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Electric Sheep One
Member # 3431
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written Wednesday, July 20 2005 11:58
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If it's already burning, jiggling the atoms a bit would be a perturbation. It's just the ignition that I think must be non-perturbative -- but this is irrelevant pedantry. You're probably onto something, here, Khoth; but it's still not clear to me. Firstly we need to have a closed system, since when a log actually burns, the end products are not just ashes, but also gases and heat that are released into the environment. So we should put the whole thing in a sealed tank, and talk about perturbing the air and electromagnetic field as well as the ashes and log. For every initial state with a log that evolves into a final state with a pile of ash, there is a time reversed image of that final state. (Basically, just reverse the velocity of every particle, and there is the image of your state under time reversal.) This time reversed image constitutes an initial state from which an ash pile would indeed 'unburn' into a log. So among all possible states of the stuff in our tank, 'ash piles that are about to unburn' are exactly as common as 'logs that are about to burn'. This set of 'unburning ash pile' states is surely a minute fraction of the size (phase space volume) of the set of states that a human would recognize as 'ash pile states'. In this sense, ash piles that are about to unburn are miraculously rare among ash piles. But we are still left with the puzzle of why 'logs that are about to burn', which are exactly as rare, among all possibilities, as 'ashes that are about unburn', should be no more than moderately less common, among typically encountered states, than ordinary 'ash piles'. Which, alas, is disappointingly similar to the problem we started with. [ Wednesday, July 20, 2005 12:16: Message edited by: Student of Trinity ] -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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...b10010b...
Member # 869
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written Wednesday, July 20 2005 18:33
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Can't we just chalk it all up to the anthropic principle? Given that we've already established that fundamental processes of thought inherently increase entropy, it seems reasonable to suggest that conscious life could only exist in a universe in which entropy has a strong tendency to increase. So there may be possible universes in which piles of ash tend to unburn into logs more often than logs burn into piles of ash, but they don't contain anyone to observe them. -------------------- My BoE Page Bandwagons are fun! Roots Hunted! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Shock Trooper
Member # 3980
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written Wednesday, July 20 2005 19:11
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The solution of the equation of motion is not determined without boundary conditions, so while the microscopic equations may be symmetric under time reversal, the solutions are not necessarily so unless the boundary conditions are. So SoT's question seems to be: Why are the boundary conditions not symmetric in time? 1. How do you know they are not? Maybe they are but we just live in a period when what we perceive as the arrow of time has one direction. 2. What do you mean by "Why"? Are you looking a) for some more abstract or more general principle? There may be several possible ones to invent if we are only sufficiently creative with sufficient time at our disposal. b) for some possible "cause"? In order to find that out, we would have to be able to imagine a world where this cause would not have happened - which we cannot because we just lack any experience without arrow of time. No hope. What you may want to imagine is some time "before" the big bang. Do you want to believe that the same equations of motions were applicable then? My point is that we do not know how far the reversability in time of our equations extends. At the big bang there may have been a lot more interactions than those we know today and whether time reversal symmetry was borken by any of them we just cannot know. Just take that and try to live on in this world if you can - as I wish. Posts: 311 | Registered: Friday, February 13 2004 08:00 |
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Electric Sheep One
Member # 3431
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written Thursday, July 21 2005 04:38
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Yes, you could frame the question as, "Why is the set of typical initial conditions not invariant under time reversal?". It is conceivable -- though I wouldn't bet a nickel -- that other eras or regions have different arrows of time. But this doesn't make the arrow of time we observe any less interesting. The preponderance of probability against the 'unburning ash' states, in our observations, is far too overwhelming to write off our arrow of time as a fluctuation. I do want an explanation, but for starters I'll settle for anything that sheds some more light. I don't think lack of experience with alternatives is an insurmountable barrier. Where the mind's eye is blind we can feel our way with math. The anthropic principle is a cop-out, of course. It only makes sense if you postulate an enormous number of universes, so that the occurrence of our peculiar one among them ceases to surprise; and that doesn't exactly seem like economy of premisses. Moreover, we don't understand how consciousness depends on an arrow of time. We're just conjecturing that it relies on processes that are subject to the thermodynamic arrow of time. So I don't see that we can reduce the thermodynamic arrow to the psychological one in the first place. -------------------- It is not enough to discover how things seem to seem. We must discover how things really seem. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |