Are EM signals also analytical backwards in time?

In summary, an antenna radiates radiation even if it's turned off, 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?
  • #1
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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?
 
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  • #2
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.
 
  • #3
berkeman said:
you quench it quickly
But neither instantaneously nor completely. The signal might quickly fall below detectability, but it's never completely gone.
 
  • #4
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?
 
  • #5
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
 
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  • #6
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.
 
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  • #7
This seems like a good time to quench this thread. I wonder if it will still bounce back through time... :wink:
 
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1. What is meant by "EM signals being analytical backwards in time"?

Analytical backwards in time means that the mathematical equations used to describe electromagnetic (EM) signals can also be used to predict the behavior of these signals in the past.

2. Can EM signals truly travel backwards in time?

No, EM signals cannot physically travel backwards in time. The concept of EM signals being analytical backwards in time is purely mathematical and does not have any physical implications.

3. How is this property of EM signals useful in scientific research?

The ability to analyze EM signals backwards in time allows scientists to better understand the behavior of these signals and make more accurate predictions. It also helps in understanding historical data and studying the evolution of EM phenomena.

4. Are there any limitations to this property of EM signals?

Yes, there are limitations to this property. It only applies to certain types of EM signals and in specific conditions. Additionally, it is a theoretical concept and cannot be physically observed or measured.

5. How does this relate to the concept of causality?

The concept of causality states that an effect cannot occur before its cause. While EM signals being analytical backwards in time may seem to challenge this concept, it is important to note that causality is a fundamental principle of the universe and cannot be violated, even in the theoretical realm of mathematics.

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