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Electromagnetism Paradox

  1. Feb 22, 2009 #1
    Correct me if I am wrong, but, if electromagnetism is relative then in a frame where charges have relative motion between the observer, the observer would detect electromagnetic radiation being emitted from the charge, and that to an observer moving alongside the charge at the same speed, no electromagnetic radiation would be detected.

    So, if what is above is correct then this paradox comes up in my mind. Lets say we have two observers, each is inertial with one standing on the ground and the other moving by in his rocket. Inside the rocket is a van de graaf generator with a strong electrostatic charge and a bomb that is triggered by EM radiation. To the observer on the ground, the charge emits radiation as the rocket ship begins its flight, setting off the bomb and destroying the ship, while the observer in the rocket detects no radiation and believes himself to be safe from the bomb exploding.

    Can someone clear this up for me?
  2. jcsd
  3. Feb 22, 2009 #2


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    Inertial frames move with constant velocity relative to each other. A charge moving with constant velocity in an inertial frame does not radiate. Accelerating charges radiate. A charge that is accelerating in one inertial frame will accelerate in all inertial frames.

    Incidentally, a frame is not a single observer. It's often described as a lattice of rods and clocks, which is not totally necessary, but it does effectively convey the global nature of a frame. You can use any inertial frame to work out what any observer observes.
  4. Feb 22, 2009 #3


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    Are you talking Special Relativity or General Relativity?

    In SR a uniformly moving charge doesn't "radiate". You can however work out very nicely the magnetic field of a moving charge by simply finding the electrostatic field as seen in the frame where it is stationary and then transforming to the frame where it is moving.

    It is when a charge is accelerated that you get emitted electromagnetic waves. For example an electron moving in "circles" in a uniform B field will actually spiral in toward the center of rotation as they slowly loose momentum from the emitted betatron radiation.
  5. Feb 22, 2009 #4
    Oh thx for the reply atyy, a while back before I read about relative EM, I had asked my teacher if only accelerated charges radiated, if not, Earth's motion would make it impossible for an electrostatic charge to exist on Earth's surface, was my argument. But my teacher was pretty crazy and got totally sidetracted without answering my question lol.
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