Suppose that we have a very, very large cubic box (with the edge much larger than the absolute value of $c$) which is completely empty. Now, suppose that we add in there a small sphere with some charge $q$ in one of the vertices. Will the electric field suddenly appear on the entirety of the box or it will "travel", expanding from the vertex at speed $c$? The same question applies to a magnetic field.

I thought about this while playing with some magnets. Suppose that, at the middle point of the box we have a wall, made of some material. If the magnetic field "travels", will the material change the intensity of the field on the other side? When i was playing with 2 magnets today, i noticed that both magnets would attract each other even when separated by a $3\operatorname{cm}$ thick block of wood. However, in a $0.5 \operatorname{cm}$ thick window glass, the magnets would not attract each other. To sum up, here are the two questions:

Does magnetic/electric field "travel" trough out space?

Does this imply that a material on the way might change the intensity of the field?


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    $\begingroup$ "with the edge much larger than the absolute value of $c$" is a nonsensical statement because edge lengths are not measured in velocity units. $\endgroup$
    – David Z
    Commented Dec 21, 2016 at 3:58
  • $\begingroup$ That's why i said "absolute value". $\endgroup$ Commented Dec 21, 2016 at 12:17
  • $\begingroup$ That doesn't matter. Saying "absolute value" does nothing to address that problem. The absolute value of a velocity is still a velocity. $\endgroup$
    – David Z
    Commented Dec 21, 2016 at 18:13
  • $\begingroup$ I'm not an native english speaker. In my language when we say "absolute value" thats what we mean: the value, just the numeric value. Thank you for noticing it. I think i should have wrote "numerically bigger". $\endgroup$ Commented Dec 21, 2016 at 23:37
  • $\begingroup$ Ah, well the English terminology you want is "numeric value". For example, given a value of "-20 m/s", the numeric value is "-20", the unit is "m/s", and the absolute value is "20 m/s". In any case, none of these are valid measurements of an edge length, so the statement is still nonsensical. $\endgroup$
    – David Z
    Commented Dec 21, 2016 at 23:43

1 Answer 1


The answer is yes on both points.

Electromagnetic radiation travels with speed c in the vacuum. Therefore both components, magnetic and electric travel with this speed.

You cannot perceive it with 'naked eye' it is too fast to see.


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