Why the past is so different from the future, even though the laws of physics are time symmetric?
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Answer:
The standard model of particle physics is almost symmetric concerning the direction of time, but not quite: there are very small violations of time-reversal symmetry, first detected in neutral kaon oscillations.
This was the violation of CP reversal symmetry, but if the PCT theorem is true, and the standard model does satisfy the conditions for the PCT theorem to be true, then CP violation implies T violation as well.
However, it is not thought that this explains the arrow of time or our subjective and objective experience of it. The effect is too small, and it concerns very short-lived particles.
Cosmology, on the other hand, is not time-reversal invariant: the universe is expanding.
Further, travel into the past is not possible, and seems to result in theoretical paradoxes if it were a possibility.
There is no widely accepted basic explanation for this. Vague ideas are having to do with some generalization of the second law of thermodynamics somehow determining the arrow of time. There are other vague ideas, too.
But there is no detailed or accepted basic explanation for why it is that time goes in only one direction. It's just an observed fact.
It may have been built into the structure of the universe by some sort of initial condition, such as Penrose's Weyl curvature hypothesis, or by some other mechanism.
Answer:
Sure, Irfatnoor. All the best!
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Just do NOT call me "Thou lady", or madam, please.
Anything but those two.