Science Forums 2

[CraigD]

Originally Posted by logy

the question asked was how come EM waves are not affected by EM force fields, and yes, EM waves don't have a constant charge, but they do have an alternating electical/magnetic field and as a result some of the time they do exist as a static electrical field or as a magnetic field, so why are they not affected by by a magnet?

I think I’m wandering onto semantically difficult ground, but I’d explain this by noting that magnetic fields aren’t affected by magnetic fields, rather changes in the position and velocity of charged particles result in differences in their associated magnetic fields. Magnetic field are really just descriptions of how a hypothetical, arbitrarily low mass and change test body’s momentum would change at points within them.

Originally Posted by logy

the answer IMHO lies in the fact that they are only fields and the interaction between two charged particles relies on they mass, distance and charge according to the formula
force = (KxQ1xQ2)/R^2
acceleration = (force x distance)/mass
…

I think what you’re saying here is that particles with zero invariant (“rest”) mass () can’t be meaningfully considered to be subject to force or acceleration.

A problem with this limitation is that such particles (eg: photons) behave in many ways as if they have non-zero masses when their speed is, as it always is, the speed of light in vacuum (c). They appear to be subject to gravity, and to transfer momentum (mass x velocity) when they collide with other particles.

Though formal purists cringe at it, an easy solution to this problem is to consider photons and other such particles to have “relativistic” (dialated) mass , where is the photon’s frequency, and is Planck’s constant.

One of the reasons purists cringe at this is that it implies some arithmetic absurdities. For example, the usual equation for mass dilation is . being defined for a body with speed implies a nonsensical .

However, if you’re careful not to use it inappropriately, the concept of photons with non-zero relativistic masses is useful and convenient.

Justification and objections to this approach can be found in many texts and websites, including the wikipedia articles “mass in special relativity” and “photon”.