# Stiffness of a magnetic field

1. Feb 19, 2013

### arydberg

What led to relativity is the thought experiment where two people are attached, one to a coil and one to a magnet. It is described at the end of this page .
http://www.physics.ucla.edu/demoweb...cs/special_relativity/special_relativity.html

My question is in the case of the magnet moving toward the loop the magnetic field must move with the magnet. But how "stiff" is this magnetic field. Does it track the magnet perfectly, or does it lag the magnet. If it tracks the field perfectly then something must move faster than light to tell the outer perimeter to keep up with the magnet. This cannot happen. It must lag the magnetic field and in this case is it this lag that is responsible for the electric field.

2. Feb 19, 2013

### ghwellsjr

It doesn't lag. Remember, we're talking about an inertial magnet that has been in constant motion forever. If you want to say that it just started moving a nanosecond ago, then there is a change in the field that will propagate outward at the speed of light.

3. Feb 19, 2013

### Staff: Mentor

I don't think that it makes any sense to talk about the stiffness of the magnetic field. Magnetic fields obey Maxwell's equations. They don't obey Hooke's law.