jheimann@BBNCCY.ARPA (06/18/85)
From: John H. Heimann <jheimann@BBNCCY.ARPA> Since we've been talking about cosmic rays and gamma rays, I thought it might be worth pointing out that although photons of particularly high frequency (and therefore energy) are refered to as gamma rays, and those of somewhat lower frequency X-rays, and lower still ultraviolet, etc., the distinctions between types of photons are not well defined in the sense that they aren't based on any intrinsic properties of the photons. After all, a photon is just a photon, and a gamma ray differs from a microwave photon only in its frequency (and hence energy, wavelength, and momentum). The distinctions between gamma rays, X-rays, UV, etc. were made on the basis of how the photons were produced, since the people who discovered the particles didn't at first realize that they were photons. For example, gamma rays were those photons produced by nuclear decay, and X-rays those produced by bombarding metal with accelerated electrons, despite the fact that low energy gamma rays and high energy X-rays of the same frequency are physically identical. John