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Initially it was considered that if em waves travel across vacuum then vacuum must contain certain some mass which does not interact with normal matter but transfers em wave called AETHER they were comparing these waves with normal mechanical waves

Modern concept : It is disturbance in em field which is em wave.

My questions:

  1. If they are disturbances in em field then why don't em waves arise when we move source charge with uniform velocity? Why on acceleration only?

  2. E M field contains energy in the form as in any field energy density is given as: U equal to Half epsilon Electric field square

And energy is equivalent to mass; then these two theories may be considered equivalent (besides some non applicable points)

  1. If this is true then certain modifications like that of stokes can be used after certain required amendments. Or even Michelson Morley experiment could be adapted to sense em field. Edit :

There is extract from principle of relativity original papers by Einstein and minkowski :

"Fresnelian ether cannot be said to behave in any respectable physical fashion,and this led stokes,in1845-46,to construct a more material type of medium, Stokes assumed that viscous motion nensues near the surface of separation of ether and moving matter ,while at sufficiently distant regions they remains wholly undisturbed.He showed how such a viscous ether would explain aberration if all motion in it were differentially ir rotational. "

Can field too become something like viscous near materials

Pranjal Rana
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2 Answers2

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This addresses Q1. 100 years or so ago (post-Michelson-Morley) a popular way to picture the generation of the waves was of field lines originating on charged particles, and of 'kinks' in the field lines arising when the charge was accelerated, these kinks propagating along the lines like waves in a stretched string. It's even possible to get some correct quantitative predictions from this model, and, even in a hand-waving way, it suggests why accelerations are needed for the generation of e-m waves. Note that this is not an (a)ether theory. The idea of field lines goes back to Faraday who, unlike his 'successor', Maxwell, was not an ether theorist.

Philip Wood
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Answering Q1. As is a postulate of relativity, we seem to live in a world where physics is identical in every inertial frame of reference. Hence if a charge is moving with a constant velocity, we can easily change our frame of reference, so the charge is stationary. If the non-accelerating charge were radiating, then in our changed frame of reference, we would observe spontaneous radiation, which we don't. Therefore constant velocity charges can't radiate.

On Q2. This doesn't seem to be a question. Note the energy density in the EM field has a magnetic part too. That is $$ U =\frac{1}{2}\left(\epsilon_0 E^2 +\frac{B^2}{\mu_0} \right) $$

Q3 unclear what you are asking.

CDCM
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