Simple Nature - Light and Matter

⊲ Looking up the speed of light in the back of the book, v = 3.0 ×10 8 m/s, we find1 − u v = 0.99999983 ,x / The galaxy M100 in theconstellation Coma Berenices.Under higher magnification, themilky clouds reveal themselves tobe composed of trillions of stars.y / The telescope at MountWilson used by Hubble.i.e., the percentage shift is only 0.000017%.The second example shows that under ordinary earthbound circumstances,Doppler shifts of light are negligible because ordinarythings go so much slower than the speed of light. It’s a differentstory, however, when it comes to stars and galaxies, and this leadsus to a story that has profound implications for our understandingof the origin of the universe.The Big BangAs soon as astronomers began looking at the sky through telescopes,they began noticing certain objects that looked like cloudsin deep space. The fact that they looked the same night after nightmeant that they were beyond the earth’s atmosphere. Not knowingwhat they really were, but wanting to sound official, they calledthem “nebulae,” a Latin word meaning “clouds” but sounding moreimpressive. In the early 20th century, astronomers realized that althoughsome really were clouds of gas (e.g., the middle “star” ofOrion’s sword, which is visibly fuzzy even to the naked eye whenconditions are good), others were what we now call galaxies: virtualisland universes consisting of trillions of stars (for example the AndromedaGalaxy, which is visible as a fuzzy patch through binoculars).Three hundred years after Galileo had resolved the MilkyWay into individual stars through his telescope, astronomers realizedthat the universe is made of galaxies of stars, and the MilkyWay is simply the visible part of the flat disk of our own galaxy,seen from inside.This opened up the scientific study of cosmology, the structureand history of the universe as a whole, a field that had not beenseriously attacked since the days of Newton. Newton had realizedthat if gravity was always attractive, never repulsive, the universewould have a tendency to collapse. His solution to the problem wasto posit a universe that was infinite and uniformly populated withmatter, so that it would have no geometrical center. The gravitationalforces in such a universe would always tend to cancel out bysymmetry, so there would be no collapse. By the 20th century, thebelief in an unchanging and infinite universe had become conventionalwisdom in science, partly as a reaction against the time thathad been wasted trying to find explanations of ancient geologicalphenomena based on catastrophes suggested by biblical events likeNoah’s flood.In the 1920’s astronomer Edwin Hubble began studying theDoppler shifts of the light emitted by galaxies. A former college356 Chapter 6 Waves