Are EM signals also analytical backwards in time?

[greypilgrim]

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?

 

[berkeman]

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.

 

[greypilgrim]

you quench it quickly

But neither instantaneously nor completely. The signal might quickly fall below detectability, but it's never completely gone.

 

[berkeman]

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?

 

[hutchphd]

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

 

[Vanadium 50]

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.

 

[berkeman]

This seems like a good time to quench this thread. I wonder if it will still bounce back through time...