Are EM signals also analytical backwards in time?

Click For Summary

Discussion Overview

The discussion revolves around the nature of electromagnetic (EM) signals, particularly focusing on their behavior when an antenna is turned on and off. Participants explore the implications of time asymmetry in electrodynamics, the analytical properties of EM fields, and the effects of circuit dynamics on signal propagation. The scope includes theoretical considerations and conceptual clarifications related to electromagnetic radiation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that when an antenna is turned off, the radiation does not disappear instantly but rather dies out smoothly, implying a time-asymmetric behavior in electrodynamics.
  • Another participant questions the assumption that radiation is never completely gone, indicating that the way an antenna's transmission waveform turns off depends on the drive circuit, particularly in pulsed radar applications.
  • Some participants argue that while signals may quickly fall below detectability, they are never completely gone, raising concerns about the implications of this for theoretical models.
  • One participant introduces the idea that the discussion may be limited by unrealistic approximations and emphasizes the importance of boundary conditions in understanding the phenomena.
  • Another participant humorously suggests that the thread may be futile, questioning the value of the discussion if it merely reiterates that unrealistic approximations lead to unrealistic predictions.

Areas of Agreement / Disagreement

Participants express differing views on the nature of EM signals and their behavior when an antenna is turned off. There is no consensus on whether the radiation is ever completely gone or the implications of time asymmetry in this context.

Contextual Notes

Limitations include potential misunderstandings of the physical behavior of EM signals, the dependence on specific circuit configurations, and unresolved questions regarding the analytical properties of EM fields.

greypilgrim
Messages
581
Reaction score
44
Hi.
If I turn on an antenna, it starts sending out radiation. If I turn it off again, the radiation doesn't instantly disappear but dies out smoothly (exponentially?). But this also means the radiation is never completely gone.

This looks time-asymmetric, which is weird for electrodynamics. It would also mean that the EM field needs to be described by a non-analytic function at the time it's turned on. Do they really occur in nature?

Another way out might be looking closer at the moment the antenna is turned on. No switch is instantaneous, bringing the contacts closer together will already have some impact on the antenna circuit, so the radiation might also be analytical backwards in time. But this would also mean that it was already there when the copper of the circuit was still in the mountains, which is also weird.

Or is there another way out of this?
 
Physics news on Phys.org
greypilgrim said:
If I turn on an antenna, it starts sending out radiation. If I turn it off again, the radiation doesn't instantly disappear but dies out smoothly (exponentially?). But this also means the radiation is never completely gone.
Why do you think that? How the antenna TX waveform turns off is a function of the drive circuit. If you are using pulsed radar, for example, it would be a bad thing to let an output tank circuit ring down; you quench it quickly on purpose.
 
berkeman said:
you quench it quickly
But neither instantaneously nor completely. The signal might quickly fall below detectability, but it's never completely gone.
 
greypilgrim said:
But neither instantaneously nor completely. The signal might quickly fall below detectability, but it's never completely gone.
Sure it is. What is your level of experience with E&M and antennas? What is your experience with E&M noise?
 
greypilgrim said:
But neither instantaneously nor completely. The signal might quickly fall below detectability, but it's never completely gone.
And the rubber ball bouncing on the table continues forever. These are all vestiges of the mathematical fiction. In the real world
1 Their are no isolated systems
2 Quantum Mechanics exists
 
  • Like
Likes   Reactions: russ_watters
What exactly is the point of this thread? If it's "an unrealistic approximation can lead to unrealistic predictions", I think we all agree. But that idea is sterile - it doesn't lead to any insights beyond "don't make unrealistic approximations".

If the thread goes beyond that, a) we should let this go, and b) two words: boundary conditions.
 
  • Like
Likes   Reactions: russ_watters and DaveE
This seems like a good time to quench this thread. I wonder if it will still bounce back through time... :wink:
 
  • Haha
  • Like
Likes   Reactions: Klystron, davenn and hutchphd

Similar threads

Graduate From Maxwell's equations to EM waves
  • · Replies 9 ·
Replies
9
Views
9K
Graduate EM waves, longitudinal EM propagation?
  • · Replies 15 ·
Replies
15
Views
3K
Undergrad Estimation of E-field strength at a distance from dipole antenna
  • · Replies 4 ·
Replies
4
Views
3K
Simple timed controller
  • · Replies 5 ·
Replies
5
Views
663
Graduate Understanding the physics of antennas
  • · Replies 3 ·
Replies
3
Views
4K
Undergrad An attempt to understand the associated variable conventions in circuits
  • · Replies 1 ·
Replies
1
Views
2K
Signal Speed Below the Speed of Light for Circuit Abstraction
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Entropy Real? End of Time? - Veritasium Video
  • · Replies 14 ·
Replies
14
Views
7K
Graduate Project Progress Report and possible Radio Astronomy Technique?
  • · Replies 9 ·
Replies
9
Views
2K
Undergrad Can time run backwards in an accelerating frame?
  • · Replies 125 ·
5
Replies
125
Views
8K