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EM Wave creation in different frames of reference

  1. Oct 24, 2015 #1
    OK, I'm sure I'm just not thinking about this the right way, so please point out my simple mistake.

    Imagine a charged particle and 2 observers.

    Observer S is stationary relative to the charged particle, so sees no magnetic field from it.

    Observer A moves back and forth in front of the particle, accelerating, then decelerating over and over, back and forth.

    Would not Observer A see the charged particle as accelerating and decelerating, from their point of view/frame of reference?
    Would not the acceleration of a charged particle from Observers A perspective create a EM wave which would propagate outwards?

    Now from Observer S's perspective, there is just a charged particle and the other Observer moving around, no EM wave is created.

    What am I missing??
    Thanks for your time!
  2. jcsd
  3. Oct 25, 2015 #2


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    Staff: Mentor

    Because S and the particle are moving back and forth (as opposed to inertially, in a straight line with constant speed) they know that they are being accelerated at each turnaround. S and A are not equivalent observers; if they're both holding a mass on a spring, S's spring is alternately tensing and relaxing while A's is not. Thus, they both know to do the calculations for an accelerated charge I stead of one at rest.

    (Be aware, however, that you still will find a problem if you try calculating the exact speed of the electromagnetic radiation relative to the two observers using classical physics. That's why we need special relativity)
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