Electromagnetic field disturbance if radiation disappears

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Discussion Overview

The discussion revolves around the hypothetical scenario of an accelerating charge emitting electromagnetic radiation and the implications of a barrier that absorbs this radiation. Participants explore how changes in the electromagnetic field are perceived at a distance, particularly in relation to barriers that may block or alter the propagation of electromagnetic waves.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether a point in space would feel a change in the electromagnetic field if radiation from an accelerating charge is absorbed by a barrier, considering both direct and diffracted paths.
  • Another participant asserts that radiation being absorbed in certain directions is not problematic, as energy once emitted does not return to the source.
  • A participant expresses confusion about how absorbing radiation affects the perception of changes in the electromagnetic field, questioning the role of electromagnetic waves in conveying information about these changes.
  • Some participants argue that if a charge is shielded from radiation, it will not sense changes from the source, suggesting that the barrier's properties influence this interaction.
  • There is a contention regarding the validity of the initial setup, with one participant stating that barriers cannot block electrostatic fields and that changes in the barrier would propagate to distant points.
  • Another participant proposes the idea of superposition of fields generated by both the moving charge and the barrier, questioning whether a perfectly reflective barrier would result in a net field of zero inside it.
  • A later reply clarifies that a perfect conductor would block electric fields, while a barrier with infinite magnetic permeability would block magnetic fields, impacting how a second particle interacts with the first.
  • Participants discuss the implications of finite versus infinite barriers on the diffraction of electromagnetic waves and the resulting interactions with other charges.

Areas of Agreement / Disagreement

Participants express differing views on the effects of barriers on electromagnetic fields and radiation absorption, indicating that multiple competing perspectives remain unresolved throughout the discussion.

Contextual Notes

Participants highlight limitations in the initial assumptions regarding the nature of barriers and their interactions with electromagnetic fields, as well as the conditions under which diffraction occurs.

jorgeha
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Hi everyone!

I've been reading about these topics (Feynman lectures and more on the internet and some books) but I still have a doubt, maybe because I haven't understood the whole of it.

This is my doubt: Think of an imaginary situation in which we have an accelerating charge. The electromagnetic field will change and the charge will emit radiation. As far as I know, a point which is 300,000,000 meters away form the charge won't feel the change until one second in the future, because of c.
Now, what if, as it is seen in the picture, radiation suddenly "disappears" in an angle? Would the point A feel the change, if it is in that area of space without radiation?
(It is possible that electromagnetic waves could reach the point A by diffraction, so I extended the barrier with the purple lines which go to infinity) I also wanted to ask: if electromagnetic waves don't reach that point directly (because of the blue barrier) but they do by diffraction, would the electromagnetic field in that point of space be the same as if they did directly?

I'd be grateful if you could answer me in both situations: with just the blue barrier, and with both the blue and the purple.

Picture: https://ibb.co/foZ1Hv
White lines -> electromagnetic radiation.
Yellow point -> charge.
Blue line -> hypothetical barrier which absorbs radiation 100%
Purple line -> extended hypothetical barrier.

Thank you in advance.
 
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I cannot see how it is a problem for radiation to be absorbed in certain directions. It happens all the time. The source does not care, because once the energy has left, it never comes back. Am I missing something - apologies?
 
tech99 said:
I cannot see how it is a problem for radiation to be absorbed in certain directions. It happens all the time. The source does not care, because once the energy has left, it never comes back. Am I missing something - apologies?

My doubt is, if light "carries" the information about the change of the electromagnetic field, if we absorb every radiation from that source (the yellow charge) in a region of space, will charges in that region feel the change in the electromagnetic field? If yes, then I don't understand the concept of electromagnetic waves carrying information about the change in the field. ((If you know about some insighting books, please let me know). Thanks!
 
As far as I can see, if a charge is shielded from the radiation, then it will not feel a change in the source.
 
I don't think the initial setup is correct, as I don't think there is a barrier that can block an electro-static field. Accelerating the charge should cause a change in the barrier and these changes then propagate towards charge A or point A or whatever that is in your drawing. That's my thoughts at least.
 
Drakkith said:
I don't think the initial setup is correct, as I don't think there is a barrier that can block an electro-static field. Accelerating the charge should cause a change in the barrier and these changes then propagate towards charge A or point A or whatever that is in your drawing. That's my thoughts at least.

So we should now think of the electromagnetic field at a point of space like a superposition of fields: the one generated by the moving charge and the one generated by the "barrier" in reponse to the radiation? If the barrier was something like 100% reflexive (theoretically), would the superposition of both electromagnetic fields, generated by charge and barrier, be 0 in a point inside the barrier?

It is not very intuitive, but it makes sense. Thanks.
 
If the barrier is a perfect conductor and is grounded or a closed surface, then it will block all electric fields. If the material has an infinite magnetic permeability, then it will block all magnetic fields. Let's assume the barrier has these properties for your argument.

For the finite sized shield, electromagnetic waves can diffract around the shield and interact with the second particle. For the infinite shield, the second particle will not feel any fields from the first particle and will not respond to any changes in its motion.
 
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