Today's (7/24/2012) New York Times Science section (on the top of page D3) has an article dissecting why certain Pioneer spacecraft have been slowing down more than expected (about 300 miles per year). After many years and research dollars Slava G. Turyshev has determined that asymmetric heat radiation (photons in the electromagnetic spectrum) from these space probes is the culprit. Or to quote the article, "The idea that light, of which heat radiation is one form, can carry momentum and thus propulsive force is implicit in the the basic equations of electromagnetism. A comet's tail, blown by sunlight is one example. [L]ight can be thought of as little particles ... called photons -- that carry energy and momentum."
Now photons, according to any physics book you might read, have no mass. But the definition of momentum is "mass times velocity" (see: Wikipedia Entry). Sooo, if photons have no mass, how can they have momentum? And how are photons (light) bent by gravity as they pass by large stars? And how can black holes have so much gravity that light (photons) cannot escape?
Somehow, physicists are allowed to have it both ways (kinda like many politicians) ... but I, for one, am still perplexed.
