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TrickyDicky
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What is more fundamental and why, the postulated time symmetry of QM tie evolution or the time asymmetry of the CPT theorem?
Wouldn't this suggest counterintitively that QM is more fundamental than QFT? or does it rather point to some discrepancy between the two since after all QFT from which the CPT symmetry is derived is based on QM with unitary time evolution.atyy said:The time reversibility of QM is more fundamental - it is basically guaranteed by unitary evolution of the wave function. The time asymmetry of CPT is more a matter of definition as to what we mean by time reversibility. It is not so much different from the fact that in classical physics, to reverse time, one has to also reverse the direction of velocity.
TrickyDicky said:Wouldn't this suggest counterintitively that QM is more fundamental than QFT? or does it rather point to some discrepancy between the two since after all QFT from which the CPT symmetry is derived is based on QM with unitary time evolution.
That was my point when saying "after all QFT from which the CPT symmetry is derived is based on QM with unitary time evolution".atyy said:By QM I included QFT. Unitary time evolution holds in QFT also.
TrickyDicky said:That was my point when saying "after all QFT from which the CPT symmetry is derived is based on QM with unitary time evolution".
So Carroll's argument is that the important symmetry is the whole CPT and there's no point splitting it into CP violations and T-violation?
I see. That makes sense.atyy said:No, he just means that for the purposes of the second law arrow of time, unitary time evolution guarantees that the second law is not fundamental within the quantum mechanical framework.
TrickyDicky said:I see. That makes sense.
Is that in general seen like a problem for QM or for the second law? I mean, is the second law as fundamental as it used to be(I'm thinking of the famous Eddington quote on the second law)?
Sorry, I'm not following what you mean by this.atyy said:For a long time the second law has not been fundamental.
TrickyDicky said:Sorry, I'm not following what you mean by this.
Ok, thanks.In classical mechanics and in quantum mechanics, the dynamics are deterministic in a way that given full knowledge of the state at anyone time, the entire past and future are known. In contrast, the second law tells us that the future is more uncertain than the past. So the second law and classical and quantum mechanics are in contradiction if we consider both to be fundamental. The majority point of view has been to take the classical and quantum dynamics as fundamental, and consider the second law to be emergent or an accident of the initial conditions.
(Here we ignore the Copenhagen interpretation, in which a definite or irreversible macroscopic outcome is fundamental.)
TrickyDicky said:So on this particular point you think the majority pov differs from the Copenhagen pov that highlights the contradiction, right?
Is the time parameter in the time evolution operator required to be continuous? It would seem that for interacting systems is not required.atyy said:No, it just means that all the statements hold only for the part of quantum mechanics in which the time evolution is completely governed by unitary time evolution.
TrickyDicky said:Is the time parameter in the time evolution operator required to be continuous? It would seem that for interacting systems is not required.
Time symmetry in quantum physics refers to the idea that the fundamental laws of physics should remain the same regardless of the direction of time. This means that the behavior of particles and systems should be the same whether time is moving forward or backward.
Time symmetry is important in quantum physics because it allows us to make predictions about the behavior of particles and systems in the past and future. It also helps us understand the underlying principles of the universe and how it operates.
Time symmetry is closely related to the concept of causality, which states that every event has a cause and every cause has an effect. In quantum physics, time symmetry means that the cause and effect of a particle's behavior should be the same regardless of the direction of time.
There is strong evidence for time symmetry in quantum physics based on experimental observations and mathematical models. However, it is still an area of ongoing research and there is still much to be discovered and understood about the fundamental laws of the universe.
While time symmetry is a fundamental principle in quantum physics, there are some scenarios where it may not apply. For example, in extreme conditions such as near a black hole or during the early stages of the universe, time symmetry may break down. Additionally, certain interpretations of quantum mechanics, such as the Copenhagen interpretation, do not necessarily require time symmetry to hold.