Electromagnetic radiation phenomena exhibit a temporal asymmetry: we observe radiation coherently diverging from a radiating source, such the light emitted by a star, but we do not observe radiation coherently converging into a source, unless we delicately set up such a system. What can explain this asymmetry? And how is the asymmetry related to the causal asymmetry, on the one hand, and the thermodynamic asymmetry, on the other?
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the issue is discussed as Arrow of time. Relevant: H.Reichenbach, The Direction of Time as well as Savitt (editor), Time's Arrows Today– Mauro ALLEGRANZAJul 25 at 10:06
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1The 2nd law of thermodynamics essentially means that energy tends to dispersion. That is physics. Causality is on another plane: metaphysics. None can be explained, or if so, only if we were gods.– RodolfoAPJul 25 at 10:14
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There are more ways for energy to radiate outward than to converge inward. As energy radiates away, things cool.– Scott RoweJul 25 at 10:18
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@ScottRowe "there are more ways"?? In what proportion? Who says so? References?– RodolfoAPJul 25 at 10:20
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1@RodolfoAP stand outside and look at people. How many can you see? How many are looking at you? Asymmetry?– Scott RoweJul 25 at 10:26
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Electromagnetic radiation phenomena exhibit a temporal asymmetry: we observe radiation coherently diverging from a radiating source, such the light emitted by a star, but we do not observe radiation coherently converging into a source, unless we delicately set up such a system. What can explain this asymmetry?
What causality reall means is that light has to he emitted before it is observed, which ultimately had to do with the finite speed of interactions (including the electromagnetic one.) The light wave propagating away from the source means that more and more observers are able to see it (if observation could be instantaneous, as was believed in pre-relativistic physics, some problems with causality would indeed arise.)
That the wave us diverging from the source is secondary, and has to do more with geometry than causality. Indeed, the observer sees a wave as moving towards them, but the observer could be positioned anywhere in respect to the source, without the knowledge of the latter: the source us sending waves in all directions, but the observer looks from a specific direction. If the source and the observer were communicating via a waveguide, like an optical fiber, the observer would see the wave moving towards them, and calling one point the source and the other observer would be a matter of terminology (but only after the emitted wave reached the observer.)
And how is the asymmetry related to the causal asymmetry, on the one hand, and the thermodynamic asymmetry, on the other?
The situation described above is actually symmetric in respect to time reversal. Indeed, we can think of a converging wave, retracing exactly the evolution of the diverging one. This wouldn't violate any physics laws, and such situations can be engineered.
On the other hand, the laws of thermodynamics are inherently asymmetric in respect to the time reversal. There's a number of paradoxes related to this, arising when trying to understand thermodynamics microscopically: like the entropy should be decreasing, if we reversed the velocities of all the molecules, but this never really happens. This has to do with the statistical/approximate nature of the thermodynamic laws. E.g., a sand castle presents a very limited set of configurations of sand particles, in comparison to the all their possible arrangements on the beach. This is why we often see a sand castle destroyed by waves, but to see one erected by waves is very unlikely in our lifetime or during the existence of the mankind or perhaps even during existence of the universe.
