6 ms·

> By intervention I mean some external agent causing the system to leave the initial equilibrium macrostate OK, that's what I thought. > We need additional as

by lisper 4y ago

> By intervention I mean some external agent causing the system to leave the initial equilibrium macrostate OK, that's what I thought. > We need additional assumption about the non-equilibrium process that makes the argument invalid for the past. Ah. No, you don't need any additional assumptions. All you need is to observe that the present constrains the past differently from the future. There are fewer pasts compatible with a given present than there are futures compatible with that same present. You can't just say that there are many high-entropy microstates, and so you are as likely to encounter one in the past as you are in the future, because any candidate past state has to be able to evolve into the present state. And if the present state is a low-entropy state, the precursor for that is much more likely to be an even lower-entropy state. Yes, there are high-entropy microstates that will evolve into low-entropy ones, but the fraction of those states chosen from among all high-entropy microstates is indistinguishable from zero, and so the odds of a high-entropy microstate in general being the precursor of a present low-entropy state is likewise indistinguishable from zero. By way of very stark contrast, the successor states of a low-entropy state (or any state for that matter) is much more likely to be a high entropy state than a low-entropy one for the exact same reason: the present constrains the past more rigidly than it does the future.

effie 4y ago
> you don't need any additional assumptions What you wrote after that is such an assumption. You can't derive asymmetry from nothing, logic or mechanics. The only way to have it, is to assume it in addition to those.
- lisper 4y ago
  > What you wrote after that is such an assumption. Not quite. There is an underlying assumption, but that's not it. I can demonstrate the asymmetry between the past and future. For example: Suppose I flip a fair coin, and I make a video of the coin flip. Let's say the coin comes up heads. I can predict with near 100% certainty (modulo shenanigans) that if I watch the video I will see a coin being flipped and coming up heads. By way of very stark contrast, I cannot predict the outcome of actually flipping the coin again with better than 50% odds. I claim that this asymmetry in my prophetic abilities (100% ability to predict what will be seen on the video vs 50% for re-flipping the actual coin) is a result of the asymmetry between the past and future. Note that this is not a circular argument; this demonstration would work equally well in a universe where it is possible to unscramble an egg. All it requires is for there be some experiment whose outcome I can record but not predict, even with arbitrarily advanced technology. The existence of such an experiment is an assumption, but it is a very weak assumption, much weaker than the second law itself. Indeed, it is a necessary assumption for doing thermodynamics at all, indeed, for doing anything. If I can reliably predict the outcome of any experiment, that means I know everything that is going to happen before it happens, including my own actions. So yes, I have to assume that I cannot be omniscient. Or maybe a better word here would be "hope" because being omniscient would render my entire existence meaningless.
  - effie 4y ago
    > I claim that this asymmetry in my prophetic abilities (100% ability to predict what will be seen on the video vs 50% for re-flipping the actual coin) is a result of the asymmetry between the past and future. No, this and other similar memory-induced asymmetries have much more prosaic reason. It is more successful to predict systems we have enough knowledge and control of (video playback) than those we don't (coin flip). Predicting and retrodicting systems we understand and have memory of is successful, and predicting and retrodicting systems we don't understand and don't have memory of is not. This has nothing to do with universal asymmetry of past vs future. Your memory of what has happened (e.g. some system had lower entropy in past) does not explain why your original argument does not apply now to retrodiction of past macrostates; the memory only contradicts the result of that argument. > If I can reliably predict the outcome of any experiment, that means I know everything that is going to happen before it happens, including my own actions. That would in no way prevent validity of 2nd law.
    - lisper 4y ago
      > It is more successful to predict systems we have enough knowledge and control of (video playback) than those we don't (coin flip). Nope. Consider this variation on the experiment: I flip a coin, I make a video recording, but I do not look at the coin, I only reveal it to the video camera. Now I cannot predict the result of watching the video despite the fact that my "knowledge and control" of my video recorder is exactly the same as before. And, BTW, my ability to make a reliable prediction in the first case is in no way contingent on my knowledge of how video recorders work, nor is it contingent on my having control of the camera. I could be completely ignorant of the inner workings of video recorders, and the camera could be completely outside of my control, and I could still make an accurate prediction, but only if I saw the coin after if was flipped. > That would in no way prevent validity of 2nd law. I never said otherwise, but that is neither here nor there.
      - effie 4y ago
        When you do not look at the coin result that gets recorded, your knowledge of the state of the recorder (the record inside) is not "exactly the same as before". You lack information on the state of the recorder that you would have if you looked. Yes, knowledge of details of how the recorder works is indeed largely irrelevant, but that was obvious. The point was that being successful with predictions or retrodictions of some system is correlated with having enough information on its state and the laws it obeys. This difference of success for predicting playback vs predicting result of a future experiment sensitive to unknown conditions is due to sufficient information vs insufficient information. I don't see anything in your responses that shows some universal asymmetry between past and future that would explain why your original argument does not work for past macrostates, but does work for future ones.

by lisper 4y ago

effie 4y ago

> you don't need any additional assumptions What you wrote after that is such an assumption. You can't derive asymmetry from nothing, logic or mechanics. The only way to have it, is to assume it in addition to those.

lisper 4y ago
> What you wrote after that is such an assumption. Not quite. There is an underlying assumption, but that's not it. I can demonstrate the asymmetry between the past and future. For example: Suppose I flip a fair coin, and I make a video of the coin flip. Let's say the coin comes up heads. I can predict with near 100% certainty (modulo shenanigans) that if I watch the video I will see a coin being flipped and coming up heads. By way of very stark contrast, I cannot predict the outcome of actually flipping the coin again with better than 50% odds. I claim that this asymmetry in my prophetic abilities (100% ability to predict what will be seen on the video vs 50% for re-flipping the actual coin) is a result of the asymmetry between the past and future. Note that this is not a circular argument; this demonstration would work equally well in a universe where it is possible to unscramble an egg. All it requires is for there be some experiment whose outcome I can record but not predict, even with arbitrarily advanced technology. The existence of such an experiment is an assumption, but it is a very weak assumption, much weaker than the second law itself. Indeed, it is a necessary assumption for doing thermodynamics at all, indeed, for doing anything. If I can reliably predict the outcome of any experiment, that means I know everything that is going to happen before it happens, including my own actions. So yes, I have to assume that I cannot be omniscient. Or maybe a better word here would be "hope" because being omniscient would render my entire existence meaningless.
- effie 4y ago
  > I claim that this asymmetry in my prophetic abilities (100% ability to predict what will be seen on the video vs 50% for re-flipping the actual coin) is a result of the asymmetry between the past and future. No, this and other similar memory-induced asymmetries have much more prosaic reason. It is more successful to predict systems we have enough knowledge and control of (video playback) than those we don't (coin flip). Predicting and retrodicting systems we understand and have memory of is successful, and predicting and retrodicting systems we don't understand and don't have memory of is not. This has nothing to do with universal asymmetry of past vs future. Your memory of what has happened (e.g. some system had lower entropy in past) does not explain why your original argument does not apply now to retrodiction of past macrostates; the memory only contradicts the result of that argument. > If I can reliably predict the outcome of any experiment, that means I know everything that is going to happen before it happens, including my own actions. That would in no way prevent validity of 2nd law.
  - lisper 4y ago
    > It is more successful to predict systems we have enough knowledge and control of (video playback) than those we don't (coin flip). Nope. Consider this variation on the experiment: I flip a coin, I make a video recording, but I do not look at the coin, I only reveal it to the video camera. Now I cannot predict the result of watching the video despite the fact that my "knowledge and control" of my video recorder is exactly the same as before. And, BTW, my ability to make a reliable prediction in the first case is in no way contingent on my knowledge of how video recorders work, nor is it contingent on my having control of the camera. I could be completely ignorant of the inner workings of video recorders, and the camera could be completely outside of my control, and I could still make an accurate prediction, but only if I saw the coin after if was flipped. > That would in no way prevent validity of 2nd law. I never said otherwise, but that is neither here nor there.
    - effie 4y ago
      When you do not look at the coin result that gets recorded, your knowledge of the state of the recorder (the record inside) is not "exactly the same as before". You lack information on the state of the recorder that you would have if you looked. Yes, knowledge of details of how the recorder works is indeed largely irrelevant, but that was obvious. The point was that being successful with predictions or retrodictions of some system is correlated with having enough information on its state and the laws it obeys. This difference of success for predicting playback vs predicting result of a future experiment sensitive to unknown conditions is due to sufficient information vs insufficient information. I don't see anything in your responses that shows some universal asymmetry between past and future that would explain why your original argument does not work for past macrostates, but does work for future ones.