Comment on Ask an experimental physicist

Tue, 12 Jun 2012 09:34:33 GMT

The difference I see between photons and your example with billiard balls is that billiard balls have a rest frame. In other words, you can set them up so that they have no preexisting motion relative to you, and any change in their positions is due to your inputs. You can't do this with photons in a vacuum; they are massless, and must always move at c.

Photon-photon scattering is also a rare process in quantum electrodynamics. If you look at the Feynman diagram:

It has four vertices. Each vertex gives the cross-section of the process another factor of the fine structure constant α, which is a small number, about 1/137. A process like electron-electron or electron-positron scattering, on the other hand, has diagrams with only two vertices, so only two factors of α. (Of course, cross-sections also depend on mass, momentum, and so forth, but this gives a very simple heuristic for comparing processes.) The additional factor of α² ~ 0.00005 makes the cross section tiny compared to common QED processes.

If you want to use photons for computing, photonic crystals are your best bet, although the technology is still in early stages of development.

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