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All laws of physics are CPT-invariant. Moreover, there is no separation of future and past in thermodynamics. Philosopher Huw Price argued about it in [1] rathe

by _0w8t 4y ago

All laws of physics are CPT-invariant. Moreover, there is no separation of future and past in thermodynamics. Philosopher Huw Price argued about it in [1] rather conveniently. In general it is a deep unresolved question why our personal perception of time with its direction, flow and existence of "now" has no reflections in physical models where time is static, has no future/past asymmetry and there is no preferred time moment like now. [1] - https://prce.hu/w/TAAPch1.html https://prce.hu/w/TAAPch1.html

tsimionescu 4y ago
> All laws of physics are CPT-invariant. That is true as far as we know, but we also thought all laws of nature are P and C and T invariant, and that turned out to be wrong: the weak interaction is not time invariant or, equivalently, not CP invariant (though it is CPT invariant as far as we know) [0]. That is, if you look at two particles with the same parity forwards in time, they will interact differently than if you look at them backwards in time. However, if you look at two particles with the same parity acting forwards in time, and you look at their anti-particles backwards in time and with opposite parity, you will still see the same interaction. Also, I don't understand what you mean when you say "there is no separation of future and past in thermodynamics" - the second law clearly states that entropy increases with time in a closed system. [0] https://en.m.wikipedia.org/wiki/T-symmetry https://en.m.wikipedia.org/wiki/T-symmetry
- _0w8t 4y ago
  Huw Price nicely explains that the normal formulation of the second law of thermodynamic uses circular reasoning about the direction of time. This was already spotted by Boltzmann who even proposed that the physical direction of time was the direction of entropy increase. From Boltzmann point of view if in some part of the Universe all particles would move by a chance as if one sees a movie in reverse then the time there would also reverse the direction. But that is also problematic for many reasons. So nobody really knows why we perceive changes in the world as if coming from a state of low entropy to the state of high entropy. From pure probability reasoning and symmetry of time (even if the latter is not 100% true, it does not matter, since the error will not affect the statistical reasoning) if a system at some moment in a state with a low probability then both in the future and the past it is more likely to find the system in a more probable state with a higher entropy.
  - tsimionescu 4y ago
    I believe one way around this is to posit that there exists a minimum time, T = 0, and that the universe had low entropy at that time. In this case, the symmetry still exists in principle, but it doesn't exist in practice in our own space-time, which thus has a well-defined arrow of time (at a macroscopic level). Of course, it's also possible that we have not yet discovered some other law of nature that is deterministic, but breaks CPT symmetry, that might define an arrow of time at the microscopic level as well. After all, QM still has the measurement problem, where the Born rule I believe also breaks CPT symmetry.
    - _0w8t 4y ago
      The measurement problem of QM arises only when one tries to define an interaction of a classical measurement device and a quantum system. But everything is quantum at the end. So there are no classical devices. In general the appearance of a classical world is unsolved problem in QM. It may well be that answer to the question of why we perceive the world classically will also help to answer the question of why we perceive the direction of time, but since we do not know, one cannot claim the the measurement problem implies the direction of the time. Regarding CPT violation. Even if it was so, it would not imply that one could use it to justify the existence of the time arrow. Asymmetry would imply that one could distinguish the past from future, but it would not explain why what we perceived as the past is the past and not the future. An analogy is a bar that gradually changes from black to white. Surely there is a color gradient. But that alone does not imply that there is a preferred direction from black to white as the choice of the direction is arbitrary.
      - tsimionescu 4y ago
        > The measurement problem of QM arises only when one tries to define an interaction of a classical measurement device and a quantum system. The measurement problem arises when one tries to use QM to predict empirical observations. The concept is fundamental to QM as a physical theory - remove it, and QM can no longer be said to agree with experimental observations (in fact, it contradicts them), so it can no longer be verified to be a physical theory. > But everything is quantum at the end. So there are no classical devices. This remains a supposition. It is possible that the classical world is indeed an emergent phenomenon, as in the many-worlds interpretation for example, but we don't yet have a full, experimentally verifiable account of how that would work. Decoherence has shed some light on aspects of the problem, but it is not enough (it does not account for the choice of observable to measure, the preferred basis problem). > Asymmetry would imply that one could distinguish the past from future, but it would not explain why what we perceived as the past is the past and not the future. Yes, that is a good point. However, moving from the future to the past in SR requires something that can move faster than light (faster than c). Such a particle is in principle possible, but it has negative energy. As such, if such a particle could actually exist, QM would predict that pairs of this particle and regular particles would form all the time in the void, and thus the universe would quickly become pure chaos (this prediction comes from the observation that energy is conserved if in a region of 0 energy you create a particle with energy E and one with energy -E). So, if we can establish that the future and the past are distinguishable, then relativistic quantum mechanics requires that movement is only possible from the past to the future, and this should give us an arrow of time even at the microscopic level.

by _0w8t 4y ago

tsimionescu 4y ago

> All laws of physics are CPT-invariant. That is true as far as we know, but we also thought all laws of nature are P and C and T invariant, and that turned out to be wrong: the weak interaction is not time invariant or, equivalently, not CP invariant (though it is CPT invariant as far as we know) [0]. That is, if you look at two particles with the same parity forwards in time, they will interact differently than if you look at them backwards in time. However, if you look at two particles with the same parity acting forwards in time, and you look at their anti-particles backwards in time and with opposite parity, you will still see the same interaction. Also, I don't understand what you mean when you say "there is no separation of future and past in thermodynamics" - the second law clearly states that entropy increases with time in a closed system. [0] https://en.m.wikipedia.org/wiki/T-symmetry https://en.m.wikipedia.org/wiki/T-symmetry

_0w8t 4y ago
Huw Price nicely explains that the normal formulation of the second law of thermodynamic uses circular reasoning about the direction of time. This was already spotted by Boltzmann who even proposed that the physical direction of time was the direction of entropy increase. From Boltzmann point of view if in some part of the Universe all particles would move by a chance as if one sees a movie in reverse then the time there would also reverse the direction. But that is also problematic for many reasons. So nobody really knows why we perceive changes in the world as if coming from a state of low entropy to the state of high entropy. From pure probability reasoning and symmetry of time (even if the latter is not 100% true, it does not matter, since the error will not affect the statistical reasoning) if a system at some moment in a state with a low probability then both in the future and the past it is more likely to find the system in a more probable state with a higher entropy.
- tsimionescu 4y ago
  I believe one way around this is to posit that there exists a minimum time, T = 0, and that the universe had low entropy at that time. In this case, the symmetry still exists in principle, but it doesn't exist in practice in our own space-time, which thus has a well-defined arrow of time (at a macroscopic level). Of course, it's also possible that we have not yet discovered some other law of nature that is deterministic, but breaks CPT symmetry, that might define an arrow of time at the microscopic level as well. After all, QM still has the measurement problem, where the Born rule I believe also breaks CPT symmetry.
  - _0w8t 4y ago
    The measurement problem of QM arises only when one tries to define an interaction of a classical measurement device and a quantum system. But everything is quantum at the end. So there are no classical devices. In general the appearance of a classical world is unsolved problem in QM. It may well be that answer to the question of why we perceive the world classically will also help to answer the question of why we perceive the direction of time, but since we do not know, one cannot claim the the measurement problem implies the direction of the time. Regarding CPT violation. Even if it was so, it would not imply that one could use it to justify the existence of the time arrow. Asymmetry would imply that one could distinguish the past from future, but it would not explain why what we perceived as the past is the past and not the future. An analogy is a bar that gradually changes from black to white. Surely there is a color gradient. But that alone does not imply that there is a preferred direction from black to white as the choice of the direction is arbitrary.
    - tsimionescu 4y ago
      > The measurement problem of QM arises only when one tries to define an interaction of a classical measurement device and a quantum system. The measurement problem arises when one tries to use QM to predict empirical observations. The concept is fundamental to QM as a physical theory - remove it, and QM can no longer be said to agree with experimental observations (in fact, it contradicts them), so it can no longer be verified to be a physical theory. > But everything is quantum at the end. So there are no classical devices. This remains a supposition. It is possible that the classical world is indeed an emergent phenomenon, as in the many-worlds interpretation for example, but we don't yet have a full, experimentally verifiable account of how that would work. Decoherence has shed some light on aspects of the problem, but it is not enough (it does not account for the choice of observable to measure, the preferred basis problem). > Asymmetry would imply that one could distinguish the past from future, but it would not explain why what we perceived as the past is the past and not the future. Yes, that is a good point. However, moving from the future to the past in SR requires something that can move faster than light (faster than c). Such a particle is in principle possible, but it has negative energy. As such, if such a particle could actually exist, QM would predict that pairs of this particle and regular particles would form all the time in the void, and thus the universe would quickly become pure chaos (this prediction comes from the observation that energy is conserved if in a region of 0 energy you create a particle with energy E and one with energy -E). So, if we can establish that the future and the past are distinguishable, then relativistic quantum mechanics requires that movement is only possible from the past to the future, and this should give us an arrow of time even at the microscopic level.