Is the electromagnetic propagation time asymmetrical?

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Discussion Overview

The discussion centers on the asymmetry of electromagnetic wave propagation in relation to time symmetry, exploring whether the propagation direction of electric (E) and magnetic (H) fields is time asymmetric or symmetric. The scope includes theoretical considerations and implications of time reversal in electromagnetic theory.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant suggests that the propagation direction of electromagnetic fields is determined by the cross product E x H and questions whether this phenomenon is time asymmetric.
  • Another participant argues that the phenomenon is time symmetric, noting that the magnetic field H changes sign under time reversal.
  • A different viewpoint indicates that reversing the arrow of time would lead to the propagation direction being given by H x E, yet the waves would propagate similarly, suggesting a simulation of macroscopic time asymmetry using time-symmetric laws.
  • Further clarification is provided that under time reversal, the direction of energy flow remains defined by E x H, but the sign of H changes, thus altering the direction of energy propagation.
  • A participant questions why the electric field E does not change direction under time reversal.
  • Another participant explains that while charge remains unchanged under time reversal, current does change, which influences the signs of the fields according to Maxwell's equations.
  • A participant references Feynman diagrams in relation to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the time symmetry of electromagnetic propagation, with some arguing for time symmetry and others suggesting time asymmetry based on the behavior of E and H fields under time reversal. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference specific laws and principles, such as Maxwell's equations and Coulomb's law, which may imply certain assumptions about the behavior of fields under time reversal that are not fully explored in the discussion.

calinvass
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For a point in space if we have the values of E and H fields at a certain instant of time, we can tell the direction of the propagation of these values, which is the direction of the cross product vector E x H. Is this phenomenon time asymmetric or not ?
 
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The phenomenon is time symmetric. Don't forget that H changes sign under time reversal.
 
If we change the arrow of time the propagation direction will be given by H x E, but basically the waves will propagate the same way. Is that what you meant? A world where S=H x E would be indistinguishable our world.
However, this doesn't mean that if we know the values of the fields at an instant, we can't predict the future propagation of the wave.

It seems that the apparent arrow of time at a macroscopic level can be simulated using time symmetrical laws.
 
calinvass said:
If we change the arrow of time the propagation direction will be given by H x E, but basically the waves will propagate the same way. Is that what you meant?
No, under time reversal the direction of energy flow is still given by E x H, but H changes sign so the direction of energy propagation changes sign.
 
Why doesn't E change direction ?
 
Charge does not change sign under time reversal, but current does. Then Maxwell's equations dictate the sign of the fields from that (consider Coulomb's law and the Biot Savart law respectively to see it in the simplest cases)
 
Alright, Thanks.
That reminds me of Feynman diagrams.
 

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