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This is a great thread - thanks guys.
I can't get my head quite around the concept of an electric/magnetic field - how does it propagate itself? 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?
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?
What exactly are these things (electric, magnetic and gravity fields)? I know they exist, but what are they?
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