Sunday, November 07, 2004

A Paradox

In the (albeit belated) spirit of Halloween, I shall unleash a physical trick-or-treat on my poor unsuspecting readers. Score one for American cultural imperialism. Mwahahahaha!

Now we all know that an electron and an anti-electron can annihilate each other to liberate a blinding flash of light (ok I exaggerate). Can it produce just a single photon of light? No it can't, because we can always find a frame of reference in which the electron and anti-electron have zero total momentum. But a photon knows no rest - it has non-zero momentum in any frame of reference. So this process can never occur, right? But why can't I just write down a momentum A for the electron, B for the anti-electron and C for the photon, where C = A + B? Then, at least in this frame of reference, this “reaction” can occur, right? So where's the catch?


Anonymous Anonymous said...

the catch is that there are not enough degrees of freedom to statisfy consevration laws if there's only 1 photon. This is a lame trick question...

Mon Nov 08, 04:00:00 AM GMT+8  

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