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If I had a very strong magnet on Earth and a very sensitive compass on Mars (just using planets to illustrate large distance), how long would the compass take to notice if I turned the magnet 180deg? I assume it can't beat the speed of light. Do the outer reaches of the magnetic field move as a rigid body with the turning magnet or do they lag in motion?

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  • $\begingroup$ I don't think the compass would ever notice. The magnetic field of a dipole falls off as $1/r^3$, so at 0.5 AU (75 billion meters), it'd not be perceptible at all. Even from a few meters (say 2 or 3), I doubt your compass would react to the magnet. $\endgroup$
    – Kyle Kanos
    Dec 10, 2013 at 2:44

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The core of your question (whether the magnetic field moves as a rigid body at long distances) has a fairly straight-forward answer: no, it does not. Changes in a magnetic field propagate at the speed of light. This is a fundamental consequence of Maxwell's Equations.

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If I had a very strong magnet on Earth and a very sensitive compass on Mars (just using planets to illustrate large distance), how long would the compass take to notice if I turned the magnet 180deg?

Bad example. People will add all sorts of comments based on this.

I assume it can't beat the speed of light.

Correct. The changes in the field propagate at c. (we call it light speed, but it is not an intrinsic property of light but rather one of the universe).

Do the outer reaches of the magnetic field move as a rigid body with the turning magnet or do they lag in motion?

Nitpick: What is an outer reach of a magnetic field? ;-)
And yes, I expect the magnetic field at a distance to change only after a while (distance divided be speed (c) to propagate there. Much like with other fields.

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