10
$\begingroup$

Antennas work by accelerating electrons to emit EM radiation. In fact, my understanding is that any accelerating electron will emit EM radiation.

But in relativity, no frame of reference is preferred. Therefore, an electron in an antenna can claim that every other electron in the universe is moving around it.

So does that mean that when an antenna creates a signal, every other electron in the universe sends the same signal back to it?

Improve this question
$\endgroup$
6
  • 2
    $\begingroup$ Antennas work by creating a charge imbalance between the positive, fixed ions in the metal lattice and the electrons. That charge imbalance can persist even when all charges are at rest. We don't even need charges. Spins are perfectly fine magnetic sources of em radiation. $\endgroup$
    – FlatterMann
    May 18 at 1:18
  • 4
    $\begingroup$ Electrons move through typical wires much more slowly than you might imagine: en.wikipedia.org/wiki/Drift_velocity#Numerical_example (micrometers per second!!) $\endgroup$
    – Solomon Slow
    May 18 at 3:06
  • 3
    $\begingroup$ Electric current is often a flow of electrons, but it doesn't have to be. A human body, an ionic conductor, makes a fine VHF antenna. $\endgroup$
    – John Doty
    May 18 at 10:30
  • 1
    $\begingroup$ @SolomonSlow But also 1570 km/s from the same link. It depends on what you mean by "electrons" and "move". Do you want the average speed of a single electron in a wire? Then 1570 km/s. Do you want the average velocity of every electron in a piece of wire under current? Then it is micrometers per second. (electricity: imagine a hurricane. In it, there is an ant waving a fan, adding a gust to it. Electric power is that ant's fan's breeze, the hurricane is baseline electric forces. EM is a very strong force.) $\endgroup$
    – Yakk
    May 18 at 15:05
  • 1
    $\begingroup$ I meant average speed. I meant, in the example from the Wikipedia page—a 1A DC current in a 14ga wire—the magician asks you to "Pick an electron, any electron." So, you pick one, and you write your name on the back of it, and he puts it back in the wire. Then, one second later, he plucks an electron from exactly 23μm further along, and he asks, "Is this your electron?" And by, golly, there's your name written on the back of it. If you're talking about the time it takes for an electron to break its bond with one atom and bond to the one next door, then I guess that could be a different story. $\endgroup$
    – Solomon Slow
    May 18 at 15:25

1 Answer 1

Reset to default
30
$\begingroup$

But in relativity, no frame of reference is preferred.

This isn't exactly correct. The first postulate of special relativity is:

"The laws of physics take the same form in all inertial frames of reference."

Note the key word "inertial" in the statement of the postulate. No inertial frame of reference is preferred over any other inertial frame of reference. However, an accelerating charge's rest frame is not inertial. So the laws of electromagnetism do not follow the usual Maxwell's equations in the charge's frame, which is non-inertial. Therefore you cannot infer that the rest of the charges in the universe are sending the same signal back.

Improve this answer
$\endgroup$
7
  • 4
    $\begingroup$ @MarcDiNino there are several equivalent definitions. My favorite is an operational definition: put a bunch of good accelerometers at rest wrt the reference frame. If they all read zero then the frame is inertial. $\endgroup$
    – Dale
    May 18 at 18:56
  • 3
    $\begingroup$ @MarcDiNino If you sit in a car or train, you can feel with your own body whether the vehicle is moving at a constant velocity relative to the earth (inertial) or whether it is accelerating (non-inertial). $\endgroup$
    – Jannik Pitt
    May 19 at 9:03
  • 2
    $\begingroup$ @MarcDiNino: Generally, "inertia" is the property of being inert, i.e. not doing anything weird. In classical-mechanics, it's when an object-in-motion stays in-motion, because that's when it's not being weird -- acceleration is an exceptional behavior (that has to be motivated by a force). In Special-Relativity, an inertial-frame is a frame in which things are behaving normally -- again, we mean without-acceleration. Then it's the same in General-Relativity, except in GR, we say that gravity isn't a force, so it doesn't cause acceleration, so the inertial-frame is local to gravity. $\endgroup$
    – Nat
    May 19 at 18:21
  • 2
    $\begingroup$ @MarcDiNino: (The other explanations are a lot more standard and are probably easier to understand. Dunno if that explanation was helpful, but wanted to give it a shot.) $\endgroup$
    – Nat
    May 19 at 18:24
  • 2
    $\begingroup$ @MarcDiNino see physics.stackexchange.com/questions/3193/… $\endgroup$
    – leonbloy
    May 20 at 12:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.