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The problem with the definition is that it doesn't specify WHICH entropy never decreases? Of the system that we are observing, of the environment or both at the same time. As already indicated, the definition applies to an isolated system, which is one that exchanges neither mass nor energy with its surroundings, or equivalently it applies to the change in ...


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The second law entropy statement refers to the entropy of an isolated system. If a system can move reversibly between states of different entropy, then it is not an isolated system. But the combination of system with its environment might together be isolated. So then any entropy moving out of the system goes into the environment, and any entropy moving out ...


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There are two ways that the entropy of a closed system can change: By heat flow across the boundary between the system and its surroundings at the boundary temperature $T_B$. This part of the entropy change is given by $\int{\frac{dQ}{T_B}}$, where the integral is carried out along the process path from initial state to final state. This contribution to ...


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Generally, I think what these popular treatments are trying to get at is the Loschmidt paradox. Roughly, this asks the following question: why, if the underlying laws of physics are symmetric, is there any preferred direction of time at all? Why is one direction of time (the one we call the past) different from the other (the one we call the future)? There ...


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Entropic time can only be reversed if all fields, real matter/energy ones and the virtual interaction fields, are momentum reversed. To make this happen for the whole universe would require quite something! And you would surely notice! You would first be aware of the world and then see it. You would hear a reversed thunder stroke and then see a lighting ...


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You are not missing anything, what you said is correct. Physical theories are time symmetric, e.g. Maxwell's equations. There is one exception and that's cosmology, however that's more of a problem for our cosmological models and doesn't show that the arrow of time is not reversible. See also this question Does time symmetry cause the matter/antimatter ...


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It is a convention that we take the direction of increasing time as the direction of increasing entropy. We could reverse the convention. But in either case the thermodynamic arrow of time must align with the perceptual arrow of time because it is not possible to use an observed state in the present (a memory or other record) to infer the details of a state ...


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See here: https://en.wikipedia.org/wiki/G%C3%B6del_metric The Gödel metric is an exact solution of the Einstein field equations in which the stress–energy tensor contains two terms, the first representing the matter density of a homogeneous distribution of swirling dust particles (dust solution), and the second associated with a nonzero cosmological constant ...


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Yes, given a Schwarzschild metric (or any metric satisfying the equivalence principle), you can derive the gravitational redshift or blueshift of a photon traveling from one place to another, and you will find that it is reversible. If you like, you can think of gravitational redshift as caused by gravitational time dilation. For symmetric detector/emitters ...


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