Check out this YouTube video on EM waves in free space

[LarryS]

I've always had difficulty grasping why the electric and magnetic fields are in phase in EM waves in a vacuum. Of course, Maxwell's Equations imply that is the case, but I had a hard time intuitively visualizing it. Then I found this short video on YouTube. I would appreciate your opinion regarding this. Does her explanation make sense?

Thanks in advance.

 

[tech99]

Regarding the relative phase of E and B for elliptical polarization, as an elliptical polarised wave can be resolved into two plane waves, each with E and B in phase, I am unable to see at this moment where such a phase shift might arise.

 

[tech99]

As far as I can see, there is no requirement for two constituent plane waves to be spaced pi/2 in phase. As we alter the relative phase, we can rotate the major axis of the ellipse and also reverse the handedness of the radiation. I still don't know how you can obtain a wave where E and B are out of phase, except in a waveguide.

 

[vanhees71]

I've always had difficulty grasping why the electric and magnetic fields are in phase in EM waves in a vacuum. Of course, Maxwell's Equations imply that is the case, but I had a hard time intuitively visualizing it. Then I found this short video on YouTube. I would appreciate your opinion regarding this. Does her explanation make sense?

Thanks in advance.

Well, already in 1 minute are so many misconceptions that I don't watch further. That very misconceptions make it difficult for students to understand, why all the components of a free plane wave are in phase.

There is one and only one field involved here, and that's the electromagnetic field. There is no causal connection between different components of this one field. That's clear, because the split into electric and magnetic field components are frame dependent. What is an electric component in one frame becomes a mixture of magnetic and electric components in another frame and vice versa.

The causal connection is only between the sources (charge-current distribution) and the electromagnetic field as becomes clear from the retarded solution of Maxwell's equations, often called the Jefimenko equations.

 

[LarryS]

You should watch the video beyond the 1 minute mark. The part that is interesting, in my opinion, occurs right after the 3-minute mark. She gives a graphical explanation of the relationship between the curl of one field and the time derivative of the other field. This is nothing new but it is the first intuitive explanation that I have seen.

We know, because of SR, that the EM field/wave is ONE physical thing. Yet it has this duality that we do not see in the other forces. I don't think that Einstein will rollover in his grave if we study this duality.

 

[vanhees71]

Which duality? Take the free spin-2 field, which is an approximate description of gravitation (linearized GR). There you have as well two physical degrees of freedom, as is the case for any massless field with spin $\geq 1/2$.

The explanation after 3min makes indeed much more sense. She only should have realized that it invalidates indeed the wrong idea of one part of the field components causing the others, because the correct explanation considers the entire field, $(\vec{E},\vec{B})$ at one instant of time. So cut out the first few minutes, and it's a pretty good movie! As usual, the math is more clever than we humans are (though we have of course also discovered the math ;-)).

 

[robphy]

Here's a Glowscript version of my electromagnetic wave visualization.
https://www.glowscript.org/#/user/Rob_Salgado/folder/My_Programs/program/robphy-EMwave-2021
which is an update of my
ancient VPython versions
http://www.visualrelativity.com/vpython/EMWave-Maxwell_v277.py
http://www.visualrelativity.com/vpython/EMWave-030216.py

By showing what Ampere and Faraday do
[inspired by Chabay and Sherwood's phrasing as "curly" and anti-"curly" patterns from Electric and Magnetic Interactions ( 0471078476), the precursor to Matter and Interactions ( https://matterandinteractions.org/ ) ],
I am essentially using the same curl argument.

Use your mouse to move the loops.
Use right-click drag or similar to rotate the view.
Use the scroll-wheel or similar to zoom in and out.

My long-time dream has been to render this in terms of the electromagnetic tensor in spacetime,
or at least in terms of the Electric and Magnetic fields as animated spatial differential forms
[with respect to the 4-velocity of this lab frame].

 

[LarryS]

As usual, the math is more clever than we humans