I Physical Significance of EM Plane Waves?

[LarryS]

The (covariant) product of the EM Field Tensor with itself is basically the Lagrangian Density for the EM Field. It leads directly to Maxwell's Equations. If there are no charges or currents present, Maxwell's Equations lead directly to an equation of motion for an EM Plane Wave. Now all EM waves are generated by charges/currents somewhere. Does that mean that, hypothetically, if you should encounter an EM Plane Wave in space, then the sources (charges/currents) for that wave must be very far away?

Thanks in advance.

 

[Baluncore]

Does that mean that, hypothetically, if you should encounter an EM Plane Wave in space, then the sources (charges/currents) for that wave must be very far away?

How plane is plane ?
How far is very far ?
What if the signal came from an extended phased array antenna ?

 

[LarryS]

How plane is plane ?
How far is very far ?

I should have said that the shape of the wave becomes a Plane Wave, in the limit as the distance from the source increases.
Never heard of an extended phased array antenna until now. Looks like it could generate an almost perfect plane wave.

 

[Baluncore]

I should have said that the shape of the wave becomes a Plane Wave, in the limit as the distance from the source increases.

You cannot go wrong with a truism.

 

[berkeman]

How plane is plane ?
How far is very far ?

I'm getting dizzy...

 

[sophiecentaur]

How plane is plane ?
How far is very far ?

How deep is the ocean
How high is the sky
And if I ever lost you . . . . . .

 

[tech99]

As Baluncore mentions, a plane wave can also be found in front of some antennas, such as a radiating aperture like a dish. The region where this happens is the Radiation Near Field and extends out to a distance very approximately (diameter^2)/ (2 x lambda), the Rayleigh Distance. So in answer to your question, the accelerating charges are not necessarily far away.