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This is true! I oversimplified a bit, because it's much easier to concretely discuss that scenario and its consequences, and it's more important from a philosop

This is true! I oversimplified a bit, because it's much easier to concretely discuss that scenario and its consequences, and it's more important from a philosophical perspective. Just like it's much easier to catch ice cubes forming on film by rewinding a tape than to run the camera waiting for spontaneous entropy decrease. Nonetheless, if the entropy of the whole universe were to consistently decrease for a sufficient time, even along a different route than time reversal, you should gradually start to notice that it's easier to form predictions or "memories" of the future than to remember the past, due to Landauer's principle. Before long, you should go back to considering the future to be the state of higher entropy.

PaulDavisThe1st 6y ago
I think you're priviledging microstates. There are plenty of entropy decreases that are possible without the ice cube reforming. They're (extremely) unlikely of course, but somewhat less unlikely than the new, singular microstate you're asserting is somehow "less of a coincidence". It's vastly more likely (for example) that 1% of the particle velocities reverse than all of them doing so, and that could (depending ...) still lead to a decrease in the entropy of the system (but not a reformed ice cube). I don't see how a long term entropy decrease that does not follow the time reversal route could be easier to form predictions in. There are many possible higher entropy states for the water than the water sitting in the glass, and if the universe enjoyed random entropy decreases, I see no reason why it would tend to pick certain macrostates over others.
- jbay808 6y ago
  Ah, I think we're just miscommunicating. I'm not saying that local time-reversal is more likely to happen than other kinds of entropy decrease; I'm not saying that a system is more likely to retrace its past than to enter a different state of low-entropy. Spontaneous local entropy decreases happen all the time, of course (but are overpowered by entropy increases), and the majority of those won't be exact reversals. And I'm not saying that locking yourself in a refrigerator will allow you to predict the future! I'm mainly picking on time reversal because it's easier to concretely communicate and reason about as an example of how a system behaves under entropy decrease, and is philosophically important because depending on one's definition of the sign of 't', you could actually view our universe as undergoing entropy decrease right now. But redefining the arrow of time won't let you form memories of the future -- you'll have memories of the lower entropy universe, regardless of which way you call "the future". The deeper point is, that's not a coincidence and doesn't depend on entropy decreases specifically being tied to time reversal. If we lived in a universe where the second law were somehow different and entropy "statistically always" decreased with time the way it "statistically always" increases now, then... well, it's hard to reason about such a universe because either it has very different laws than our own, or it's just our own universe with exactly that arbitrary t <--> -t transformation. But in most logically-consistent interpretations of that scenario, that universe's version of Landauer's principle would also flip the arrow of time perceived by its local inhabitants, and they'd end up with only memories of their "future". If I'm not quite making my point clear, try asking yourself why, if the laws of the universe are time reversible, why can't you remember the future the way you can remember the past? This gets into the mechanics of how memory works (as a general concept, not human memory specifically; it's easier to think about computer memory). (Edit): Why isn't this a trivial point? Well, if you imagine a universe composed of a chain of of "linked states" that goes from (low entropy) - (high entropy) - (low entropy), you'd find that any inhabitants of that universe would perceive a universe with directional time that progresses in the entropy gradient, even though that universe has no consistent directional time.
  - PaulDavisThe1st 6y ago
    >why can't you remember the future the way you can remember the past this is getting a bit meta, maybe even off-topic. But I think this is fairly simple to explain: you can't remember states you haven't been in. Generalized memory implies some record of a state that has occured - states that have not occured cannot be remembered. The problem is that memory of types that we are familiar with (human, written, computer) all involve macrostates, not microstates. And increasing entropy (aka "more probable things are more likely to happen") mean that there is asymmetry at the macrostate level, and thus in memory. A memory system that only recorded microstates would, I suggest, have no concept of time, and a much reduced notion of causality. There may be some statistical patterns that could be observed and could perhaps be "strong" enough to infer that "after state N we frequently end up in state P", but the sheer number of states would likely interfere with this. There's also the timescale problem. A memory system that operates on the timescales of typical human experience will notice relatively constant change in much of the world, as macrostates come and go. But a memory system that operates at, say, geological timescales won't record many macrostate changes at all, and will tend to indicate that almost nothing happens in the world. All those macrostates ("tables", "chairs", "houses", "books") that came and went without being noticed form no part of this system's memory of the world. Of course, there are processes still taking place (new macrostates made up of even more vast microstates), but these going to be even more directional/asymmetric. The one part of this view that leaves me a little confused is that at very short timescales, the unchanging nature of many macrostates is echoed in relatively unchanging microstates for most solids. The piece of metal that makes your <whatever> isn't changing macrostates at any appreciable pace (which is why its eventual wearing out forms an asymmetric experience of time for us), but it also isn't changing microstates in any notable way either. I find this confusing.
    - deleted 6y ago
      [deleted]
    - raattgift 6y ago
      > You can't remember states you haven't been in If only memory worked that way! False memories are commonplace. Where exactly did you put your keys? Even if you give the right location, is that a true memory of the state you and the keys were in when you separated, or is it a retrodiction ("I probably put them in their usual storage place")? > A memory system that operates on the timescales of ... You mean one that measures parts of the world periodically? ISTR we discussed this some exp(10^120) years ago[1] but I forget whether we reached any conclusions. - -- [1] Dyson, Kleban & Susskind, 2002, https://arxiv.org/abs/hep-th/0208013 https://arxiv.org/abs/hep-th/0208013 eqn (5.2).

PaulDavisThe1st 6y ago

I think you're priviledging microstates. There are plenty of entropy decreases that are possible without the ice cube reforming. They're (extremely) unlikely of course, but somewhat less unlikely than the new, singular microstate you're asserting is somehow "less of a coincidence". It's vastly more likely (for example) that 1% of the particle velocities reverse than all of them doing so, and that could (depending ...) still lead to a decrease in the entropy of the system (but not a reformed ice cube). I don't see how a long term entropy decrease that does not follow the time reversal route could be easier to form predictions in. There are many possible higher entropy states for the water than the water sitting in the glass, and if the universe enjoyed random entropy decreases, I see no reason why it would tend to pick certain macrostates over others.

jbay808 6y ago
Ah, I think we're just miscommunicating. I'm not saying that local time-reversal is more likely to happen than other kinds of entropy decrease; I'm not saying that a system is more likely to retrace its past than to enter a different state of low-entropy. Spontaneous local entropy decreases happen all the time, of course (but are overpowered by entropy increases), and the majority of those won't be exact reversals. And I'm not saying that locking yourself in a refrigerator will allow you to predict the future! I'm mainly picking on time reversal because it's easier to concretely communicate and reason about as an example of how a system behaves under entropy decrease, and is philosophically important because depending on one's definition of the sign of 't', you could actually view our universe as undergoing entropy decrease right now. But redefining the arrow of time won't let you form memories of the future -- you'll have memories of the lower entropy universe, regardless of which way you call "the future". The deeper point is, that's not a coincidence and doesn't depend on entropy decreases specifically being tied to time reversal. If we lived in a universe where the second law were somehow different and entropy "statistically always" decreased with time the way it "statistically always" increases now, then... well, it's hard to reason about such a universe because either it has very different laws than our own, or it's just our own universe with exactly that arbitrary t <--> -t transformation. But in most logically-consistent interpretations of that scenario, that universe's version of Landauer's principle would also flip the arrow of time perceived by its local inhabitants, and they'd end up with only memories of their "future". If I'm not quite making my point clear, try asking yourself why, if the laws of the universe are time reversible, why can't you remember the future the way you can remember the past? This gets into the mechanics of how memory works (as a general concept, not human memory specifically; it's easier to think about computer memory). (Edit): Why isn't this a trivial point? Well, if you imagine a universe composed of a chain of of "linked states" that goes from (low entropy) - (high entropy) - (low entropy), you'd find that any inhabitants of that universe would perceive a universe with directional time that progresses in the entropy gradient, even though that universe has no consistent directional time.
- PaulDavisThe1st 6y ago
  >why can't you remember the future the way you can remember the past this is getting a bit meta, maybe even off-topic. But I think this is fairly simple to explain: you can't remember states you haven't been in. Generalized memory implies some record of a state that has occured - states that have not occured cannot be remembered. The problem is that memory of types that we are familiar with (human, written, computer) all involve macrostates, not microstates. And increasing entropy (aka "more probable things are more likely to happen") mean that there is asymmetry at the macrostate level, and thus in memory. A memory system that only recorded microstates would, I suggest, have no concept of time, and a much reduced notion of causality. There may be some statistical patterns that could be observed and could perhaps be "strong" enough to infer that "after state N we frequently end up in state P", but the sheer number of states would likely interfere with this. There's also the timescale problem. A memory system that operates on the timescales of typical human experience will notice relatively constant change in much of the world, as macrostates come and go. But a memory system that operates at, say, geological timescales won't record many macrostate changes at all, and will tend to indicate that almost nothing happens in the world. All those macrostates ("tables", "chairs", "houses", "books") that came and went without being noticed form no part of this system's memory of the world. Of course, there are processes still taking place (new macrostates made up of even more vast microstates), but these going to be even more directional/asymmetric. The one part of this view that leaves me a little confused is that at very short timescales, the unchanging nature of many macrostates is echoed in relatively unchanging microstates for most solids. The piece of metal that makes your <whatever> isn't changing macrostates at any appreciable pace (which is why its eventual wearing out forms an asymmetric experience of time for us), but it also isn't changing microstates in any notable way either. I find this confusing.
  - deleted 6y ago
    [deleted]
  - raattgift 6y ago
    > You can't remember states you haven't been in If only memory worked that way! False memories are commonplace. Where exactly did you put your keys? Even if you give the right location, is that a true memory of the state you and the keys were in when you separated, or is it a retrodiction ("I probably put them in their usual storage place")? > A memory system that operates on the timescales of ... You mean one that measures parts of the world periodically? ISTR we discussed this some exp(10^120) years ago[1] but I forget whether we reached any conclusions. - -- [1] Dyson, Kleban & Susskind, 2002, https://arxiv.org/abs/hep-th/0208013 https://arxiv.org/abs/hep-th/0208013 eqn (5.2).