lew (10/07/82)
Warren Montgomery expressed an understandable concern about the strength of the evidence behind the estimate of the age of the universe. As Mark Twain said,"That's the wonderful thing about science, one gets such wholesale returns of conjecture on such a trifling investment of fact." Closer inspection shows, though, that in this case the investment in carefully made observation and careful reasoning well justifies the returns. Before 1900, no one had any idea that the universe was filled with millions of galaxies. They were not dreamed up or hypothesized, they were discovered. The distance to the nearest stars can be measured by the parallax due to the orbital motion of the earth. This works out to about 200 light years. Hubble was able to measure the 2,000,000 light year distance to Andromeda by the use of the period- luminosity relation among the Cephied variables. These are a group of stars whose absolute brightness is related directly to their period of variation. The relationship was worked out by comparing stars which are physically associated in a globular cluster. The relation was then calibrated using nearby stars. Hubble was able to observe Cepheids in other nearby galaxies and use them to measure the distances to them. Before this, it was not known whether these were indpendent systems like our own galaxy, or local satellite nebulae. As Hubble extended this yardstick he noticed that the redshift of the galactic spectra corellated linearly with their (independently measured) distances. This redshift was then, in turn, used to measure even greater distances. So, Warren, I think we can say that others before us have already had some ideas on this difficult topic. It is well to be skeptical. There is no gain in uncritically echoing every number or parameter you hear mentioned. But don't think that just because it occurs to you to ask, "How DO they know that, anyway?" that you are about to shatter some delicate structure of speculation. You are certainly invited to criticize Hubble's methods. Don't think that others have failed to do so. They have continued to be refined and modified by painstaking work. If your deep yearning for understanding can power you to the nearest library you will find them there expounded in greater detail. Until such time as you have sacrificed a few hours in studying them, do not expect to add to the understanding won by the lifetime efforts of great minds, by suggesting a rocket powered surveying expedition. Lew Mammel, Jr. - BTL Indian Hill