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Politics : Formerly About Advanced Micro Devices -- Ignore unavailable to you. Want to Upgrade?

To: J_F_Shepard who wrote (586865)	9/22/2010 9:51:29 AM
From: longnshort	3 Recommendations Read Replies (1) \| Respond to of 1578743

This is one of the strange aspects of quantum mechanics. Sunlight, composed of photons of various energies, has MOMENTUM but no MASS. That is, it does not "weigh" anything in the usual sense that we use the term "weigh" because the earth's gravitational field has almost no effect upon photons. They are not attracted by the force of gravity. When we "weigh" some object that has a certain mass, we measure the force that the earth exerts upon that mass. When our scale, or balance, comes to equilibrium the force of gravity is just balanced by the extension/compression of a spring or by the "weights" we place in the other pan of the balance. Of course electronic balances operate on a somewhat different principle of piezo-electrical effects, but the principle is the same. The mass comes to rest, and hence has a "rest mass". Since photons, at least from the Sun, always travel at the speed of light they can have no "rest mass". However, they can and do exert a force, e.g. solar "winds". Ali Khounsary, Ph.D. Argonne National Laboratory