5

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?

10
  • the issue is discussed as Arrow of time. Relevant: H.Reichenbach, The Direction of Time as well as Savitt (editor), Time's Arrows Today Jul 25, 2023 at 10:06
  • 1
    The 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.
    – RodolfoAP
    Jul 25, 2023 at 10:14
  • There are more ways for energy to radiate outward than to converge inward. As energy radiates away, things cool.
    – Scott Rowe
    Jul 25, 2023 at 10:18
  • @ScottRowe "there are more ways"?? In what proportion? Who says so? References?
    – RodolfoAP
    Jul 25, 2023 at 10:20
  • 1
    @RodolfoAP stand outside and look at people. How many can you see? How many are looking at you? Asymmetry?
    – Scott Rowe
    Jul 25, 2023 at 10:26

2 Answers 2

3

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.

0

The asymmetry is in what we call the state of the system (i.e. the situation at say some time) and not in the laws of physics (although time reversal symmetry is broken in the Standard Model of particle physics but that is not the important part here).

If you would have a uniform distribution of light filling the space there would be no sources. We experience certain things as light sources because we are in a state that has a high concentration of energy at special points.

We don't really understand why the initial state of the universe was as it was (or if there even was such a thing as an initial state). We understand a little better how things got to what things are like now from some time after the supposed "big bang" when light started to travel more freely through the universe (which we are seeing now as the Cosmic Microwave Background).

In our theories it would also be just as consistent to start with a uniform distribution of light as our initial state (and then basically nothing that interesting would happen after at least on time scales of the life-time of our universe). That universe would be as causal as ours is. (Where I use causal as the technical term that speaks about the vanishing of commutators of space-like separated observables).

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .