5 ms·

> No, I lack information about the state of the coin. And that implies you lack information on the record, which is decisive for predicting successfully its pl

by effie 4y ago

> No, I lack information about the state of the coin. And that implies you lack information on the record, which is decisive for predicting successfully its playback. > But under no circumstances can I ever accurately predict the outcome of flipping the coin again. Because you lack the necessary information, because you assume those experiments are that way. If you set them up so that you don't look at the coin flip result, but in the next coin flip you measure the initial conditions and the flipping process accurately enough, the conclusion would be the opposite - you would accurately predict the new coin flip, and would not be able to predict the playback. > Why? Because there is a fundamental asymmetry between the past and future. Not at all. The asymmetry is due to how you defined constraints on the experiments. > It is possible to make reliable records of the past, ... it is not possible to make reliable records of the future. Record is, by definition, a pattern that has been created in the past, and this is likely to correlate more with the past states than with the future states. This is a property of the concept, not of some universal asymmetry between past and future. We may introduce analogous concept of a future record, which will be created in the future, and which will correlate with the future states. We can talk instead about prediction and retrodiction. And then this is more of an easy/hard thing, not possible/impossible thing, and the distribution of success on this axis depends on details. Predicting Moon's position on sky 2 years into future is easier than retrodicting it 20 000 years into past. None of this explains why your original argument does not apply to past macrostates.

lisper 4y ago
> None of this explains why your original argument does not apply to past macrostates. Yeah, that's gonna be a long row to hoe if I can't even get you to acknowledge that there's a difference between the past and the future independent of the second law itself. Maybe I should just ask: do you think there is any difference between the past and future (independent of the 2nd law)? If so, what is it? But in the meantime: > that implies you lack information on the record This turns on what it means to have information "about" something. All of the data I have first hand access to comes to me through my senses, mainly my eyesight. When I say a coin is heads-up it's because I can see the coin, i.e. there are photons emitted from the coin (presumably) that end up on my retina and create electrical impulses that go to my brain and leave me with the subjective sensation of seeing a heads-up coin. There are all kinds of background assumptions about how light works, how coins work, how my retina works, and how my brain works, but for the most part one chooses to sweep those under the rug and accept as a working hypothesis that the reason I have the subjective sensation of seeing a heads-up coin is that there is in point of actual fact a coin "out there" and that it is, again in point of actual fact, heads-up. But the camera is a very different story. I get direct input from the camera in the same way that I get direct input from the coin. I can see the camera just as I can see the coin. But unlike the coin, the camera's state with respect to what it has recorded is not directly apparent to me. That state is part of the internal state of the camera. It is hidden from me. I cannot directly observe it simply by looking passively at the camera the way I can with the coin while my visual cortex of my brain does the heavy lifting. I cannot even tell that there is hidden internal state based on anything that I can see about the camera while it is recording. Making a prediction about what will happen during "playback" (indeed, even predicting that there will be something interesting to observe at all!) requires a much more complicated model of the world and chain of inference than it took for me to conclude that the coin was heads-up. I introduced multiple cameras so that the structure of the example eliminated all that complexity and everything was symmetric. All the cameras are the same, and my knowledge of them is all the same -- until I start looking at the playback, at which point things start to diverge. When I look at the playback on one camera, I now have direct information about the (previously hidden) state of that camera, but not of any of the others. So here again the question is: why should (direct) information about the state of one camera tell me anything about the state of any of the other cameras? It is because of a very complex chain of inference, part of which includes the hypothesis that there are, in point of actual fact, events which have occurred in the past (and which now cannot be changed) and that there are things in the present (like cameras) whose states correspond to those past events, and for that reason correlate with each other in the present. I've lived a long time and I've engaged in a lot of philosophical discussions, but you are the first person with whom I've ever had to go into that level of detail on such a foundational matter. Not that there's anything wrong with that, you're keeping me on my toes. But my point is just that if you're going to insist that I be excruciatingly precise about something this elementary, it's going to be a very long time before we can come back to talking about thermodynamics.
- o_nate 4y ago
  Is it a fair summary of your rebuttal to say that its not permissible to speak of the probability distribution of past events, because the past just is what it is?
  - lisper 4y ago
    No. Probability is something that arises from incomplete knowledge, not the actual state of affairs (except in quantum mechanics). In a classical universe both the past and future are fully determined by the present. But our knowledge of the present is limited, and that is why we have to introduce probabilities. Because it is possible to have incomplete knowledge of the present (and hence incomplete knowledge of the past), it makes sense to speak of the probability distribution of past events. If I flip a coin but don't look at it, then I can fairly say that the probability of heads (with respect to my knowledge state) is 0.5 despite the fact that the coin definitely, in point of actual fact, either landed heads up or it did not.
    - effie 4y ago
      Probability is useful also when we have complete knowledge of the state of a model involving many particles (e.g. gas), but can't reliably integrate equations of motions. This lack of skill is the same towards the future as towards the past, at least in the context of reversible theories of motion like classical mechanics or evolution governed purely by Schroedinger's equation. Flip a coin, do not look at it, and make a short video of the coin from above when it is at the top position. This video provides us with details of state of the coin. Assuming a fair coin and a strong enough flip, from this state of things we can't predict the resulting face, and also can't predict the starting face on the thumb. This problem is symmetric towards the future and towards the past. The same argument holds for knowledge of one microstate of gas at single time instant. Past and future are equally undetermined, and all probabilities are the same towards the future as towards the past. To get 2nd law, we need to assume the asymmetry in addition, and it's best if it is explicit. E.g a process connecting two equilibrium macrostates, where the two states aren't defined in a symmetric way. For example, as Jaynes does, we define B as the reliable result of autonomous evolution after manual intervention in A. This makes A,B asymmetric, because it is not stated in the assumptions (and usually not considered possible) that also A is the reliable result of autonomous evolution after intervention in B. This asymmetry is what allows us to derive 2nd law. See e.g. this Jaynes' paper, Section IV The 2nd Law. https://github.com/lawrennd/jaynes/blob/main/gibbs-vs-boltzmann-entropies.pdf https://github.com/lawrennd/jaynes/blob/main/gibbs-vs-boltzm... It is interesting that this derives asymmetric 2nd law from symmetric (reversible) underlying microscopic model. The asymmetry comes in exclusively due to the assumption of B being the reliable result of intervention in A.

by effie 4y ago

lisper 4y ago

> None of this explains why your original argument does not apply to past macrostates. Yeah, that's gonna be a long row to hoe if I can't even get you to acknowledge that there's a difference between the past and the future independent of the second law itself. Maybe I should just ask: do you think there is any difference between the past and future (independent of the 2nd law)? If so, what is it? But in the meantime: > that implies you lack information on the record This turns on what it means to have information "about" something. All of the data I have first hand access to comes to me through my senses, mainly my eyesight. When I say a coin is heads-up it's because I can see the coin, i.e. there are photons emitted from the coin (presumably) that end up on my retina and create electrical impulses that go to my brain and leave me with the subjective sensation of seeing a heads-up coin. There are all kinds of background assumptions about how light works, how coins work, how my retina works, and how my brain works, but for the most part one chooses to sweep those under the rug and accept as a working hypothesis that the reason I have the subjective sensation of seeing a heads-up coin is that there is in point of actual fact a coin "out there" and that it is, again in point of actual fact, heads-up. But the camera is a very different story. I get direct input from the camera in the same way that I get direct input from the coin. I can see the camera just as I can see the coin. But unlike the coin, the camera's state with respect to what it has recorded is not directly apparent to me. That state is part of the internal state of the camera. It is hidden from me. I cannot directly observe it simply by looking passively at the camera the way I can with the coin while my visual cortex of my brain does the heavy lifting. I cannot even tell that there is hidden internal state based on anything that I can see about the camera while it is recording. Making a prediction about what will happen during "playback" (indeed, even predicting that there will be something interesting to observe at all!) requires a much more complicated model of the world and chain of inference than it took for me to conclude that the coin was heads-up. I introduced multiple cameras so that the structure of the example eliminated all that complexity and everything was symmetric. All the cameras are the same, and my knowledge of them is all the same -- until I start looking at the playback, at which point things start to diverge. When I look at the playback on one camera, I now have direct information about the (previously hidden) state of that camera, but not of any of the others. So here again the question is: why should (direct) information about the state of one camera tell me anything about the state of any of the other cameras? It is because of a very complex chain of inference, part of which includes the hypothesis that there are, in point of actual fact, events which have occurred in the past (and which now cannot be changed) and that there are things in the present (like cameras) whose states correspond to those past events, and for that reason correlate with each other in the present. I've lived a long time and I've engaged in a lot of philosophical discussions, but you are the first person with whom I've ever had to go into that level of detail on such a foundational matter. Not that there's anything wrong with that, you're keeping me on my toes. But my point is just that if you're going to insist that I be excruciatingly precise about something this elementary, it's going to be a very long time before we can come back to talking about thermodynamics.

o_nate 4y ago
Is it a fair summary of your rebuttal to say that its not permissible to speak of the probability distribution of past events, because the past just is what it is?
- lisper 4y ago
  No. Probability is something that arises from incomplete knowledge, not the actual state of affairs (except in quantum mechanics). In a classical universe both the past and future are fully determined by the present. But our knowledge of the present is limited, and that is why we have to introduce probabilities. Because it is possible to have incomplete knowledge of the present (and hence incomplete knowledge of the past), it makes sense to speak of the probability distribution of past events. If I flip a coin but don't look at it, then I can fairly say that the probability of heads (with respect to my knowledge state) is 0.5 despite the fact that the coin definitely, in point of actual fact, either landed heads up or it did not.
  - effie 4y ago
    Probability is useful also when we have complete knowledge of the state of a model involving many particles (e.g. gas), but can't reliably integrate equations of motions. This lack of skill is the same towards the future as towards the past, at least in the context of reversible theories of motion like classical mechanics or evolution governed purely by Schroedinger's equation. Flip a coin, do not look at it, and make a short video of the coin from above when it is at the top position. This video provides us with details of state of the coin. Assuming a fair coin and a strong enough flip, from this state of things we can't predict the resulting face, and also can't predict the starting face on the thumb. This problem is symmetric towards the future and towards the past. The same argument holds for knowledge of one microstate of gas at single time instant. Past and future are equally undetermined, and all probabilities are the same towards the future as towards the past. To get 2nd law, we need to assume the asymmetry in addition, and it's best if it is explicit. E.g a process connecting two equilibrium macrostates, where the two states aren't defined in a symmetric way. For example, as Jaynes does, we define B as the reliable result of autonomous evolution after manual intervention in A. This makes A,B asymmetric, because it is not stated in the assumptions (and usually not considered possible) that also A is the reliable result of autonomous evolution after intervention in B. This asymmetry is what allows us to derive 2nd law. See e.g. this Jaynes' paper, Section IV The 2nd Law. https://github.com/lawrennd/jaynes/blob/main/gibbs-vs-boltzmann-entropies.pdf https://github.com/lawrennd/jaynes/blob/main/gibbs-vs-boltzm... It is interesting that this derives asymmetric 2nd law from symmetric (reversible) underlying microscopic model. The asymmetry comes in exclusively due to the assumption of B being the reliable result of intervention in A.