# B Is the electromagnetic propagation time asymmetrical?

1. Jan 12, 2017

### calinvass

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 ?

2. Jan 12, 2017

### Staff: Mentor

The phenomenon is time symmetric. Don't forget that H changes sign under time reversal.

3. Jan 12, 2017

### calinvass

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.

4. Jan 12, 2017

### Staff: Mentor

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.

5. Jan 13, 2017

### calinvass

Why doesn't E change direction ?

6. Jan 13, 2017

### Staff: Mentor

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)

7. Jan 13, 2017

### calinvass

Alright, Thanks.
That reminds me of Feynman diagrams.