1
$\begingroup$

If suppose there is some charge which is not under influence of any other thing. Let us for observation surround this charge with circles of pointers pointing in the direction of its field. If I move this charge a little bit the pointers will change their direction corresponding to new field generated at that point in space. My question is when I move the charge, will "All" the pointers also move at the same instant? Or the pointers will realise the change depending on how far away they are from the charge? Is the transfer of information instantaneous in this case? If not, why? Can there be any other possible scenario where transfer of information is instantaneous (except for the case of entanglement)?

$\endgroup$
  • $\begingroup$ Why should the pointers move instantaneously? $\endgroup$ – Jon Custer Jul 8 '16 at 11:33
  • $\begingroup$ @JonCuster partly because intuition. And there is otherwise no reason I can find which is causing the delay. $\endgroup$ – user282856 Jul 8 '16 at 11:37
  • $\begingroup$ Aside from the fact that the field is mediated by photon exchange that propagated at the speed of light? All covered well in standard E&M textbooks. $\endgroup$ – Jon Custer Jul 8 '16 at 11:57
  • $\begingroup$ @JonCuster you can guess I don't about this. Could you tell me what topic the question can be set under? What should I study about to get my answer? $\endgroup$ – user282856 Jul 8 '16 at 12:01
2
$\begingroup$

There is a name for machines which move charges around to cause the field to vary: wireless transmitters. As is well-known the disturbance propagates at $c$.

No: if special relativity is correct (and it is extremely well tested) there is no way of transmitting information 'instantaneously', nor in fact is there any well-defined notion of what it would mean to do so, since whether two distinct events occur simultaneously or not depends on who you ask. Indeed, because of this observer-dependence of simultaneity, if such a thing were possible it is immediately possible for someone to send information into their own past. Entanglement is not an exception to this rule.


As an aside, note that if it were possible to do this, then there is essentially no limit to the amount of money a financial market trader would pay for something that would do it. Yet no one has ever constructed such a system to do it: this should be a huge hint that none is possible.

$\endgroup$
2
$\begingroup$

In classical electrodynamics, perturbations in the electromagnetic field will propagate at the speed of light. Thus, the electric field at distance $r$ from the charge will be disturbed at time $t=\frac{r}{c}$ after the charge is first moved.

For a justification/derivation of this, you can look up Green's function of the EM wave equation - this is a function of space and time that describes the response of the EM field to an instantaneous, point-like disturbance.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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