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Originally Posted by zen_tom
i.e. Say an electro-magnet is turned on to make a magnetic field. The field may stretch out for say 5", but does it stretch out at the speed of light, or is it instantaneous?
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It goes out at the speed of light. It doesn't stop at a few inches though. The range is essentially infinite, although of course the field gets weaker the farther out you go.
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What about gravity (another force-field type effect) how does that propagate? If a planet suddenly appeared next to the moon, would it take time before its gravitational effects were noticable by us, or would they take time to reach us at a distance?
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The answer to this is the same. Any disturbances travel outward at the speed of light. To be more correct, small disturbances in a very strong gravitational fields will actually travel at speeds less than and equal to the speed of light. The gravitational waves that appear to move more slowly are actually being scattered off of the spacetime curvature.
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What exactly are these things (electric, magnetic and gravity fields)? I know they exist, but what are they?
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Are you asking for a definition? They represent how strongly an object placed at a particular point will be affected by electric, magnetic, or gravitational forces. Say you place an uncharged particle of mass m at some point and measure the force on it (F). The (Newtonian) gravitational field is then defined as g=F/m. You make a similar construction for electric and magnetic forces.
The original point of this was that g is independent of which m you choose. Once you know g, you know all there is to know about how all massive bodies will be accelerated. Although this seems like mere mathematical convenience, the field concept eventually took on a reality of its own. Its 'reality' is required for locality, energy and momentum conservation, and so on.