Does magnetic propagation follow the speed of light? E.g. if you had some magnet of incredible strength and attached an iron wire that is one light year long to it, would the other end of the iron wire begin to show a magnetic field in a year, or could it possibly be faster than a year?
In your example, the relevant speed isn't the speed of propagation of disturbances in the magnetic field, but rather the speed of the alignment of iron atoms.
You are really asking "Does magnetization of a wire/metal propagate at the speed of light?"
The answer is no; it propagates at the speed at which each individual iron atom can align its polarity.
If you are asking, "Do changes in the magnetic field propagate at the speed of light?"
The answer is yes; if a giant, huge, powerful magnet appeared one light year away out of nowhere, then it would take exactly one year for magnets on Earth to feel its pull (however small it may be). That is, it would take one year for the "magnetic force" to reach the Earth.
It cannot be faster than a year, because electro-magnetic waves propagate with the speed of light. The magnetization of the wire will take longer, because it depends on the reaction of the iron atoms to the external magnetic field (they align according to the external field).
As a rule of thumb, one can say that "if it is possible to notify a friend far away with this" then we are talking about the transmission of information, and that can be done with light speed at most according to special relativity.
In your case you could set of an alarm clock on the other end of the wire once the magnetic field reaches the end :-)
protected by Qmechanic♦ Jun 15 '14 at 6:14
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