Physical Significance of EM Plane Waves?

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Discussion Overview

The discussion revolves around the physical significance of electromagnetic (EM) plane waves, particularly in relation to their sources and the conditions under which they can be considered plane waves. Participants explore theoretical implications, practical examples, and the nature of wave propagation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states that the product of the EM Field Tensor leads to Maxwell's Equations, which describe the motion of EM plane waves in the absence of charges or currents.
  • Another participant questions whether encountering an EM plane wave in space implies that its sources must be very far away, prompting further inquiries about the definitions of "plane" and "very far."
  • Some participants discuss the concept that the shape of a wave approaches that of a plane wave as the distance from the source increases.
  • References are made to extended phased array antennas, suggesting they could generate nearly perfect plane waves.
  • One participant mentions the Radiation Near Field and the Rayleigh Distance, indicating that accelerating charges do not necessarily have to be far away from the observer.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the definitions of "plane" and "very far," and there is no consensus on the implications of these terms in relation to EM plane waves and their sources. Multiple competing views remain on the nature of plane waves and their generation.

Contextual Notes

Participants highlight limitations in defining the conditions under which a wave can be considered a plane wave, as well as the implications of source distances, which remain unresolved.

LarryS
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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.
 
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LarryS said:
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 ?
 
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Baluncore said:
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.
 
LarryS said:
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.
 
Baluncore said:
How plane is plane ?
How far is very far ?
I'm getting dizzy...
 
Baluncore said:
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 . . . . . .
 
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.
 
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