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Ok, this may seem obvious(I'm not all that experienced in physics)

Anyway. 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?

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A lot of times when this question is asked, it is with the mentality that light somehow has unique properties that ensure it is the fastest thing traveling ever. As if the universe is watching everything and if it sees something traveling faster than light, it stops it. This is not the case; rather, what is truly happening is that there is simply a speed where causality is violated. Light is just one thing that happens have nothing holding it back (like mass) to keep it from getting there. There are many things that travel at $c$, not just light. – Justin L. Feb 25 '11 at 5:31
What else travels at the speed of light (EM radiation)? Gravity probably does, but that's not conclusively proven. And the two nuclear forces don't travel well. – MSalters Feb 25 '11 at 9:52
@MSalters Heh, actually there aren't many things we have found that don't have mass. But my point was that the speed of light isn't a property of light, but a property of the universe that is determined completely separately from light. – Justin L. Feb 26 '11 at 2:16
@ Justin L. : That's indeed the consensus, and there's no significant evidence to the contrary. But as I pointed out, at the relevant scales (universe/astronomical) our only observations are via some form of EM radiation. That is to say, "c is a property of the universe" is a sound hypothesis, but not an axiom. – MSalters Feb 28 '11 at 9:45
@MSalters: The alternative is "$c$ is a property/speed/number inherent to the photon", which I don't think many people would put too much of a wager on, yet it is exactly what people think when they mention "the speed of light". This discrepancy is a point that not many realize, and one that should be made, I feel. – Justin L. Mar 2 '11 at 7:21

2 Answers

up vote 5 down vote accepted

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.

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I changed to this answer. I think it explains it a bit better. (as in, I understand it a bit more :) ) – Earlz Feb 26 '11 at 1:12
up vote 5 down vote

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 :-)

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