The arrow of time
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...b10010b...
Member # 869
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written Monday, July 18 2005 15:34
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Seems to me that people are complicating the situation more than it needs to be. They're right about one thing, though; it's all about what's going on in the brain. The first step is to think of memories as objects. Okay, they're not quite the same kind of physical objects as we're normally used to dealing with, and each memory isn't literally confined to a specific physical location in the brain that overlaps with no other memories. But the concept of an object is itself fuzzy enough that we can think of memories as objects anyway, in the sense that they are pieces of information stored in the brain. Now, if a memory of event E exists at time t, then at time t we can remember event E. If no memory of event E exists at time t, then at time t we cannot remember event E. This much is obvious. So the question we're really asking is this: why does the formation of memories only occur in one temporal direction? The easy answer, of course, is to say that the biochemical mechanisms the brain employs to form memories cause a net increase in entropy, which is why the psychological arrow of time aligns so neatly with the thermodynamic arrow. This is necessarily true as far as it goes, but it feels like a bit of a non-answer. More as I think of it. It seems likely that anything that carries on from this line of argument is going to require a fairly rigorous idea of the physical differences between "past" and "future", since using the psychological differences between past and future to justify the psychological differences between past and future would be hopelessly circular. Saying that the past has happened already and the future hasn't happened yet won't do, because (as is my understanding) in a physical sense "has happened already" and "hasn't happened yet" aren't really very solid concepts for most things -- there are only a fairly small number of differences between the pastward and futureward directions which make temporal direction non-arbitrary. [ Monday, July 18, 2005 15:45: 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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? Man, ? Amazing
Member # 5755
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written Monday, July 18 2005 16:59
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Stop teasing us Thur and just write out the darn equation already! :P *this message sponsored by supporters of my bubble theory* Posts: 4114 | Registered: Monday, April 25 2005 07:00 |
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...b10010b...
Member # 869
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written Monday, July 18 2005 17:48
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Given the laws of physics, there can only be one temporal direction in which entropy tends to increase throughout the universe; entropy can't increase and then decrease again as one continues along the same temporal direction. Conveniently, time is asymmetrical in at least one other way too; it has an endpoint (the Big Bang). You can't just go on forever in either direction through time; you're either moving toward the Big Bang or away from it. Now, at the Big Bang the universe is essentially in a state of minimal entropy; when the whole universe is compressed into a single point, there aren't really many different states it can be in. Therefore, entropy has to increase from the Big Bang onwards, defining the thermodynamic arrow of time. (If both ends of the universe were equivalent, they'd both have to be at maximum entropy and thus life couldn't arise, so we can eliminate that possibility from consideration based on our friend the anthropic principle. If time is cyclical, things get REALLY ugly and I don't want to think about the psychological implications.) So now we at least have a good basis on which to define the direction time moves in. We're still left with the problem of why the brain records memories in the same direction as the thermodynamic arrow of time. I'm not entirely sure whether the fundamental reasons for it are physical, computational or evolutionary. I'll make another attempt when I have a better understanding of the question. [ Monday, July 18, 2005 17:58: 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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Nuke and Pave
Member # 24
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written Monday, July 18 2005 18:49
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That's an interesting explanation. Does it mean that laws of thermodynamics make expansion of universe irreversible, regardless of mass? Or is the answer that if universe was massive enough to go from Big Bang to Big Crunch, laws of physics would have been different and we wouldn't have the law that enthropy must increase? -------------------- Be careful with a word, as you would with a sword, For it too has the power to kill. However well placed word, unlike a well placed sword, Can also have the power to heal. Posts: 2649 | Registered: Wednesday, October 3 2001 07:00 |
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...b10010b...
Member # 869
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written Monday, July 18 2005 23:16
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quote:Good question. In fact, your question is so good that it's given headaches to serious theoretical physicists. The Big Bang and Big Crunch aren't completely symmetrical events, so a universal contraction doesn't necessarily violate thermodynamics (in the same way that the formation of a black hole doesn't). But yeah, strange and wonderful things happen when you apply thermodynamics to the universe as a whole, especially when it's expanding or contracting, and I'm not really qualified to talk about them. -------------------- 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 Tuesday, July 19 2005 01:37
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Here is a simple idea of why we cannot percieve the future directly. The past in our perception is fixed. We can remember most of it in a general outline. Those who can remember almost every moment of their past tend to be overwhelmed mentally and have problems focusing on the importance of events. One event becomes almost as important as the next. A classic case of this occurring comes from a book called The Mind of A Mnemonist by A.R. Luria. Our mind is designed to forget things to place emphasis on what is important. Assuming there is more than one possible future and the future is not finite. The brain purposefully blocks out what it does not consider important on a subconscious level about the future. It would be too much information for our brains to see both the past and all possible futures at the same time without becoming insane. Thus we are aware of a limited number of possible futures. -------------------- 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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Infiltrator
Member # 4637
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written Tuesday, July 19 2005 02:10
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Is there any physics equation where the time line can be reversed? I don't think so. Is time linked to entropy? I don't think so (i'll explain ahead). If Big Bang existed, then we have to agree that was a point in time where entropy was a minimum. But that doesn't mean that time didn't exist or stoped. You could stare to that dot in space for 5 seconds, and that's elapsed time. Although there's lot of equations that use time, time is an abstract notion, not a physical event, so it can't be changed or manipulated, because time, in a certain way, doesn't exist, it's just an abstract notion. (I don't know if I'm being clear about what I mean). quote:Although it's true that there is a tendency for entropy, entropy doesn't quite equate to disorder. I know its definition and I know that what i'm saying seems to go directly agains't that definition, but even disorder leads to order (complex, organized and ordered systems (like life beings) were formed and better organized through time, for example). Also, scientists think that the Universe can come from Big Ban to Big Ban: that is, although time is unidirectional, that doesn't mean entropy must be unidirectional as well, linked to time. So no, time doesn't need to be reversed to reverse entropy (or, to be more correct, reverse entropy's original effect). Linking this with the fact that time doesn't physically exist (it's an abstraction), there's no way we could have future memmories. Of course, some could argue about prophets and their future memmories... ;) Some can just play with probabilities, others can be charlatans, others can be guided by God, who can defy His own Laws (or use them in ways we can even understand), etc. But in a scientific point of view, there can't be future memmories, because they are an impossibility, defying our today's logic. [ Tuesday, July 19, 2005 02:13: Message edited by: Overwhelming ] -------------------- Visit the Blades of Avernum Center and the Beta Testing Center -------------- "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ." Colossians 2:6-9 Posts: 483 | Registered: Tuesday, June 29 2004 07:00 |
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Triad Mage
Member # 7
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written Tuesday, July 19 2005 02:49
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Nostradamus remembered the future! After all, he did predict the Great Fire of London and 9/11. -------------------- "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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...b10010b...
Member # 869
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written Tuesday, July 19 2005 03:45
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quote:Actually, pretty much all the laws of physics are symmetrical with respect to time, at least locally. That's the problem. A photon travelling forwards in time in one direction is physically indistinguishable from a photon travelling backwards in time in the other direction. An electron travelling forwards in time is indistinguishable from a positron travelling backwards in time. quote:Time doesn't really have much meaning at a singularity. (And besides, even if it did, you can't observe a singularity, so your choice of phrasing is unfortunate.) quote:You're not, but perhaps I'm in no position to hold that against you. quote:I know quite well what entropy is, hence my reference to the number of states a system can have in my previous post. quote:Your conclusion does not follow from your premises. If time doesn't exist, in what way is the future different from the past? [ Tuesday, July 19, 2005 03:47: 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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Electric Sheep One
Member # 3431
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written Tuesday, July 19 2005 06:36
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I'm afraid I don't understand much from the proposed answers in this thread, but Zeviz and Thuryl have raised what are considered the basic issues. As Zeviz argues, it could well be that our equations are missing something yet; in fact, that's what I think myself. If only I knew what it was. As Thuryl indicates, there are usually said to be three arrows of time, although it is really only the psychological one that interests me (why we can't remember tomorrow). The thermodynamic and cosmological arrows are the other two. The thermodynamic arrow is expressed in the Second Law, that the entropy of an isolated system cannot spontaneously decrease. Entropy, though, is an awfully dodgy concept. On the one hand you can measure it, by burning things and keeping track of temperatures. The fact that this quantity tends to increase is pure empirical observation, with no explanation. The theory of statistical mechanics provides an alternative definition of entropy, in terms of numbers of available states. Although this entropy seems to agree with the empirical one in the simple cases where it can be unambiguously computed, the number of available states is not really well defined, in general, and often seem to turn on psychological issues. As Thuryl observes, this gets circular. Most people assume that the psychological and thermodynamic arrows are one and the same; but neither is understood. The cosmological arrow of time is the one that is understood, but it is hard to see how it relates to the other two. The universe started from a state of zero volume (not from a point: all the points there would be were there). It isn't clear cosmologically that this would have to be a state of low entropy; rather, the very earliest universe is thought to have been incredibly hot. -------------------- 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 # 73
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written Tuesday, July 19 2005 07:17
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t Overwhelming As far as my minimal understanding goes, according to the current understanding of physics, time is no more an abstract notion than length, width, or height. It is simply another dimension. This is of course according to Einsteinian physics. I know next to nothing about quantum physics, so I may be completely off-base. (Note that everything I know about Einsteinian physics comes from this page which someone linked to a while back, so don't hold me to anything I've said in this post.) -------------------- The Lyceum - The Headquarters of the Blades designing community The Louvre - The Blades of Avernum graphics database Alexandria - The Blades of Exile Scenario database BoE Webring - Self explanatory Polaris - Free porn here Odd Todd - Fun for the unemployed (and everyone else too) Famous Last Words - A local pop-punk band They Might Be Giants - Four websites for one of the greatest bands in existance -------------------- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Posts: 2957 | Registered: Thursday, October 4 2001 07:00 |
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Post Navel Trauma ^_^
Member # 67
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written Tuesday, July 19 2005 09:59
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If time went the other way, we'd be wondering why we could remember the past but not the future. -------------------- 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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Triad Mage
Member # 7
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written Tuesday, July 19 2005 10:56
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That is a profound statement. -------------------- "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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Electric Sheep One
Member # 3431
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written Tuesday, July 19 2005 11:11
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I suppose it is, but I'm afraid physics pretty much kicked itself free of semantics a long time ago, and observations like that don't really get acknowledged as advances any more. The main question is why time goes one way. If we could settle that, the question of which way it goes would be less profound, but still real, even though whichever way it went would determine our terms 'past' and 'future'. For whichever way it does go, one can still ask, why not the other way? Here's a question most people think is related to the real arrow of time. How can you tell when a film is running backwards? -------------------- 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 Tuesday, July 19 2005 11:14
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If time ran the other way, there'd be a whole lot of energy becoming matter. Could you imagine a fire "unburning" a log? Posts: 2242 | Registered: Saturday, April 10 2004 07:00 |
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? Man, ? Amazing
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written Tuesday, July 19 2005 11:14
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the music sounds funny. *this message sponsored by terse* Posts: 4114 | Registered: Monday, April 25 2005 07:00 |
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Nuke and Pave
Member # 24
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written Tuesday, July 19 2005 11:23
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quote:This question is much easier: water doesn't normally go up the waterfall, pool ball doesn't normally jump out of its hole and rush back towards a place where it was hit, etc. :) I guess the ball example is the law of inertia, combined with laws of thermodynamics. (Ball will not move unless it's pushed. It can be pushed back by air it has displaced and accelerated by absorbing energy previously lost to friction, but that would violate laws of thermodynamics.) EDIT: It looks like most examples I can think of go back to laws of thermodynamics. So I guess our psychological arrow of time is the same as thermodynamic arrow. (Which kind of makes sence, because our brains run on physical processes, governed by laws of physics, including laws of thermodynamics, etc.) [ Tuesday, July 19, 2005 11:26: Message edited by: Zeviz ] -------------------- Be careful with a word, as you would with a sword, For it too has the power to kill. However well placed word, unlike a well placed sword, Can also have the power to heal. Posts: 2649 | Registered: Wednesday, October 3 2001 07:00 |
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Electric Sheep One
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written Tuesday, July 19 2005 11:41
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The technical point is that for every initial state after which a log burns into ash and smoke, or a stream plunges into foam, there is another initial state after which ash and gas assemble into a log, and turbulent spray launches up a cliff and flows smoothly away uphill. Among the states we encounter, these alternative initial states are evidently incredibly rarer than the first one. But among possible states, they are equally common. So what is it that selects typical states? Fire and water are both good examples for this question, but unfortunately are very complicated. It's hard to see just what is the issue of principle involved, under the ferocious complexity. I'm trying to identify the simplest system that might exhibit an arrow of time; if I succeed I'll publish a paper. -------------------- 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 Tuesday, July 19 2005 12:20
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How about a baseball careening from over the fence that's hit by a batter directly into a pitcher's "throwing" arm? (Disregarding the bat rising from the ground into the batter's hands. I think this is a little easier than a car becoming uncrashed.) [ Tuesday, July 19, 2005 12:21: Message edited by: Drew ] Posts: 2242 | Registered: Saturday, April 10 2004 07:00 |
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Shaper
Member # 32
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written Tuesday, July 19 2005 12:22
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Why not just dropping a ball? Why have all these situations be so complex? [ Tuesday, July 19, 2005 12:24: Message edited by: Lt. Sullust ] -------------------- Lt. Sullust Cogito Ergo Sum Polaris Posts: 2462 | Registered: Wednesday, October 3 2001 07:00 |
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Shock Trooper
Member # 6102
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written Tuesday, July 19 2005 13:12
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If there are so many finite realities, wouldn't that cause some kind of time paradox? I believe that with time, there are different realities associated with it. Like most of you in this thread have already inputted, for every action is a reaction, with multiple possibilities adjacent with it. It's always the "what ifs" in life that cause such realities to exist, yet we never know the "what ifs" of other alternate realities. Do we really see life as it is or what we think it is? That is the question that I believe that the individual believes he/she perceives life as. -------------------- "Truly, if there is evil in this world, it lies in the heart of mankind." -Edward D. Morrison Posts: 220 | Registered: Monday, July 11 2005 07:00 |
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Law Bringer
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written Tuesday, July 19 2005 13:54
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Overwhelming: Entropy is true in a closed system. Earth isn't a closed system, so you can't even say that lifeforms becoming more organized violate the concept of entropy as increasing disorder. (Although I can't see what makes a human any more or less disordered than an amoeba, really). —Alorael, who can think of some very interesting takes on time travel if quantum-level uncertainty does refute determinism. If you go back in time, there's no guarantee that the past you end up in is the past you remember. Then the future is even less likely to be the present you left. Not really physically relevant, but it makes good sci-fi. Posts: 14579 | Registered: Saturday, December 1 2001 08:00 |
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E Equals MC What!!!!
Member # 5491
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written Tuesday, July 19 2005 16:45
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quote:This reminds me of a question that has been bugging me for a while. Earth may not be a closed system, but surely the Universe is. The arising of something as incredibly complex as life and it's subsequent evolution into it's current unbelievably varied state contradicts the 2nd Law of Thermodynamics, surely? Or could you say that the Universe right now is less ordered overall (despite the inclusion of Earth) than it was a second after the Big Bang? Actually, that question doesn't bug me. The one that I wonder about is how does mainstream science answer it? -------------------- Sex is easier than love. Posts: 1861 | Registered: Friday, February 11 2005 08:00 |
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...b10010b...
Member # 869
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written Tuesday, July 19 2005 19:57
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quote:A thought: maybe it's just not possible (at least for a human brain) to store information in a form in which it can be usefully retrieved without relying on processes that increase entropy. Maybe it's overly optimistic to think this is actually true, but if it were it'd solve the problem rather neatly. To test it, though, we'd need a better understanding of the physicochemical processes by which memories are stored and retrieved. Considering that consciousness theory is a philosophical quagmire at the moment, don't hold your breath on an answer from those quarters. -------------------- My BoE Page Bandwagons are fun! Roots Hunted! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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...b10010b...
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written Tuesday, July 19 2005 20:02
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Apologies for the double post. quote:Firstly, try not to fall into the trap of equating entropy with the popular notion of "disorder". Secondly, the smallest amount of entropy a system can theoretically have is zero, so no matter how highly "ordered" a system is at any given point in time, its existence can't actually reduce the universe's entropy; it can just contribute to it by a smaller amount. And the universe is a big, big place. So one planet remaining in a state of relatively low entropy isn't going to have a huge effect on the entropy of the universe as a whole. By analogy, think of throwing an ice cube into the ocean. No matter how cold the ice cube is, it can't be colder than absolute zero, and the ocean is so big that one ice cube won't make a discernable difference to its average temperature. Likewise, there's a minimal amount of entropy that any system can have; no matter how "ordered" you make one planet, it's just not possible to keep its entropy low enough to balance out the trend in the rest of the universe. It's also worth noting that the universe as a whole isn't anywhere near maximum entropy yet; that will only happen billions of years down the track, after all the stars in the universe have burned out. [ Tuesday, July 19, 2005 20:08: Message edited by: Thuryl ] -------------------- My BoE Page Bandwagons are fun! Roots Hunted! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |