Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Relativity and magnetism.

  1. Jun 17, 2012 #1
    i apologise if the following statements may sound a little unscientific, or appear to beat about the bush.

    Now if two positive charges are travelling together side by side with a certain velocity wrt to me, i will witness a magnetic attraction between them. But if i am travelling together with these charges; i.e. they are stationary wrt to me, i will not witness any magnetic force.

    So whether you witness any magnetic force or not depends on your reference frame.

    Light itself is an electromagnetic wave. But according to special relativity, the speed of light is independent of your reference frame.

    So the existence of a magnetic force between 2 charges is dependent on your reference frame, but the speed of an electromagnetic wave is completely independent? Although both seem to be manifestations of the electric force.

    This gets even more confusing when you realise the only way to explain the magnetic force between both positive charges is to use special relativity and lorentz contraction.

    So its kinda like using the invariance of one electromagnetic property to demonstrate the variance of another electromagnetic property.
  2. jcsd
  3. Jun 17, 2012 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Classical (as well as quantum) electrodynamics is a Poincare-invariant theory and thus there can never be a contradiction with electrodynamics and relativity. Of course, you have to take the right perspective.

    Here it's important to note that the distinction between electric and magnetic field is indeed observer dependent. The right covariant object is the antisymmetric Faraday tensor, describing the electromagnetic field as one entity.
  4. Jun 17, 2012 #3


    User Avatar
    Science Advisor

    It is correct that the interpretation of effects is frame-dependent, but the effect isn't. If you start with charges at rest + you as an observer at rest what you will observe is an electric field i.e. an electric force (no magnetic field i.e. no magnetic force). For a moving observer (w.r.t. the charges) the electromagnetic field has to be transformed i.e. you will find a magnetic field as well; therefore you can interpret the effect as a mixture of electric force plus Lorentz force.

    However the effect itself isn't frame dependent, i.e. the spacetime points defining the trajectories depending on the proper times of the charges are invariant objects (they become frame dependent only if you introduce frames i.e. coordinates).
  5. Jun 17, 2012 #4
    The distinction between electric and magnetic fields is artificial, which is why a true treatment of EM theory in special relativity can avoid splitting the EM field (as expressed through the Faraday bivector) into electric and magnetic parts.

    There is one electromagnetic field (in vacuum, at least), and it acts on currents, whether those currents be time-directed (stationary charges) or space-directed (what we would conventionally call a current).
  6. Jun 17, 2012 #5


    User Avatar

    Staff: Mentor

    Yes. And the more you think about it, the odder this is. This oddity can be described in completely classical terms (you just did, in the quoted text), so predates SR. In fact, Einstein uses it in the introduction of his 1905 paper ("On the electrodynamics of moving bodies") as an example of the problem that he was solving.

    Almost right.... Replace the bolded words above with "electromagnetic field" and you'd have it. Mathematically, it's as vanhees71 says: the Faraday tensor describing the electromagnetic field is a single frame-invariant object that describes the electrical and magnetic fields that an observer at a given point with a given velocity will experience.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook