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Why does mass increase with velocity, but electric charge does not?

If you know anything about special relativity, you know that a particle's mass increases with its velocity. An electron gets heavier and heavier as it gets accelerated by particle colliders, as does a proton.

Mass-dependent quantities, like momentum or energy, are also dependent on velocity.

By contrast, electric charge doesn't get affected by a particle's velocity at all. At all. Not at low speeds. Not at 0.9999999999 times the speed of light.

The same goes with a quark's 'charge'. An up quark's up-ness doesn't get more uppity. A down quark remains down, no matter how much Prozac you give it. Also, the quark's 'color', like red, green, or blue, doesn't change. It stays invariant with respect to the speed of the quark.

Why should some quantities get increased with speed, but others not?

I don't know the why, but I do know that a concept called relativistic invariance is involved. So if you friend a physicist on Facebook, you can drop that phrase into the conversation. He'll probably think you're pretty hot for doing so.

Electric charge is relativistically invariant. Mass is not.

This has something to do with why it's important that we find the Higgs boson.

You've heard it said that "Magnetism = electricity + special relativity"? Yes? Because, after all, the strength of the magnetic field being proportional to an electric charge's speed, that's pretty odd, right? It's one of the things that tipped off Albert Einstein that maybe physicists' understanding of nature wasn't so complete. So he hummed a little bit and drank a little bit of coffee, and out popped the Special Theory of Relativity.

But if you were a physicist, none of this would make you break a sweat. You'd wave your hands and say, "Face, this is trivial. Trivial! Who doesn't know about relativistic invariance?"

Well, me for one. The older I get, the less I seem to know. *sigh*

"So young, and already so unknown." --Wolfgang Pauli