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Is Action&Reaction Strictly Synchronous

  1. Aug 3, 2009 #1
    I have got a problem:
    It's well known that two electric charges have either attractive or repulsive forces on each other.
    And Einstein told us that there is no message that can travel at a speed beyond c.
    Besides I was told that the actions and reactions between the two charges are based on the exchange of some kind of particles.

    So, here comes the question.Let's assume the following condition:
    There is one fixed charge(+)(named A),and put another neutral point(B) somewhere at a distance.If we charge B(-), it must send out a message to the A. This may be a wave. And the point B itself soon got an attractive force,which was always believed given by A.How was this force formed?My assumptions are:
    1. This force was formed once B sent out the message or wave.
    2. This force was formed after A received the message.A received the message, and sent something back, so that when B received, B got a force.

    If we choose the former, it's hard to answer how B had been "informed" the quantity of A---as no message can travel beyond c---so that the message can act on B as a force with respect to the Coulomb Law.
    If we choose the latter(preferable as I think), it is possible to make a machine, which can charge and discharge object B in ultra-high frequency, so that before the message is returned, B had already been discharged and no reaction force is formed---therefore it became possible to make a UFO?
  2. jcsd
  3. Aug 3, 2009 #2

    Jonathan Scott

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    Gold Member

    Firstly, you cannot create or destroy charge; you can only move it from one place to another.

    According to the classical theory of electromagnetism, which is compatible with special relativity, a charge at rest or in uniform motion has an associated field, which effectively contains energy and momentum that can be transferred to another charge acted on by that field. If the source charge is moved in a non-uniform way, that causes changes to propagate within its field at c (in a vacuum, or slower in some other materials), associated with flows of energy and momentum. Rapid changes may cause some of the energy to be radiated away in the form of electromagnetic waves. Overall, electromagnetic energy and momentum are locally conserved everywhere.

    In quantum theory, it is less easy to say exactly where energy and momentum are located and whether they are really continuously conserved, or only on average. However, the general effect is still the same.
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