Re: How does the exchange of virtual photons cause attraction?

Hi Andy,

I think the problem with this analogy is that you really have to imagine a virtual basketball, and recognize that it has different properties than a real basketball. A real basketball moves in one direction and carries momentum in that direction. When you catch a basketball coming from your left, of course it must carry momentum to push you to your right. A virtual basketball could come at you from the left but carry momentum in either direction.

A real basketball has a certain mass (about 600 grams), but a virtual basketball can have any mass---including a negative mass! So, you see, real world analogies are very difficult to apply to quantum particles. George Gamow makes an attempt in his book "Mr Tompkins in Wonderland", where he imagines playing pool on a quantum pool table---the balls behave very differently than real balls would.

The root of the problem is this: modern physics---especially quantum physics---often starts with equations. In classical physics you can often look at a diagram or a model of a system, figure out how it will behave, and write down equations to describe it. In quantum physics, usually you have to write down the equation first, and the equation tells you how to draw the diagram, or how the model will behave. In the case of "virtual particles", we started with a bunch of exact, correct quantum equations---usually the Dirac Equation or the Klein-Gordon equation. Solving the equations tells us, for example, that like charges repel and opposites attract. Richard Feynman was the first person to notice that the equation looks like the equation for particle exchange, so he (and others) came up with the "virtual particle" description that we use today. (That's a useful line of scientific reasoning: it's worth noting how closely the equation for ocean waves "looks like" the equation for a vibrating violin string, or a beam of light; or how the equation for a discharging capacitor looks like the draining of a water tank.) The description is a bit weird---that's why you have to put up with the changing masses, and the backwards momenta---but that's the only way we have thought of. In spite of all the weirdness, most physicists find Feynman's description easier to use than Dirac's; that is why we talk about virtual particles, rather than talking about things like gamma matrices and the LSZ formula.

Hope this helps. I've answered several MadSci questions about virtual particles, for example this one. Cheers,

-Ben