Magnetism and Relativity

Two positively charged particles(without wires) moving parallel, with same velocity, in same direction. Do their magnetic fields affect each other. (in this case I am not talking about their recoil, because their are same charge). I am just wondering, because they are moving parallel, their speed according to each other is 0. So their magnetic fields should not affect each other?
- Girts (age 20)
Latvia

Nice question. In fact, their magnetic fields do affect each other, making a force. How can that be, you properly ask, since in the right frame they're both at rest and there's no magnetic force? However, even in that frame there's an electrostatic force. The name of the force partially changes in the different reference frames but not its reality. (The magnitude also changes somewhat in the relativistic transformations.)

There are wonderful discussions of these issues in Purcell's Electricity and Magnetism book.

Mike W.

(published on 04/26/2013)