The Day We Found the Universe - Marcia Bartusiak [78]
What de Sitter assumed was that the universe contained no matter. He discovered that Einstein's equation could be solved if he imagined that the universe was both stable and empty. On the face of it, this seemed like a ludicrous assumption, but de Sitter wondered if cosmic densities were so low that the universe could be considered essentially barren. By making this conjecture he was able to construct a model of space-time in which “the frequency of light-vibrations diminishes.” That is, light waves get longer (more red) with increasing distance from their source. The unique properties of space-time that arose in his solution demanded it. Einstein was not up on the latest astronomical news, but de Sitter was. In fact, he would soon be director of the Leiden Observatory, in the Netherlands. He was very aware that V. M. Slipher, at the Lowell Observatory, had recently discovered some spiral nebulae seemingly racing away from the Milky Way—and at very high velocities as measured by their redshifts. De Sitter was one of the few at the time who was sure that the spiral nebulae being sighted by astronomers in ever greater numbers were probably “amongst the most distant objects we know,” indisputably located beyond the Milky Way. And he surmised that their tendency to display appreciable redshifts could be proof of his model. In his paper, he suggested that the nebulae might only appear to be moving outward because their light waves were getting longer and longer (hence redder and redder) as the light traveled toward Earth. This set up the illusion of movement.
On the other hand, there was another way to interpret the effects in de Sitter's universe: Any bit of matter dropped inside its space-time would immediately fly off. That was another possible reason for the red-shifts Slipher was noticing. Eddington liked to say that “Einstein's universe contains matter but no motion and de Sitter's contains motion but no matter.”
Before the publication of his bizarre yet fascinating solution, de Sitter exchanged a number of letters with Einstein arguing over its details. Einstein was clearly flummoxed by de Sitter's quirky take on the universe. It “does not make sense to me,” he wrote. Where was the “world material” in his cosmos, where were the stars? It didn't seem based in reality. In Einstein's eyes, de Sitter's solution was physically impossible. The properties of space could not be determined, he believed, without the presence of matter.
Albert Einstein and Willem de Sitter working out a problem
at the Mount Wilson Observatory's Pasadena headquarters in 1932
(Associated Press)
De Sitter was certainly making a huge assumption by considering a cosmic density so low that the universe could be regarded as devoid of matter. But what was exciting about his model was that it was testable. If distances to the spiral nebulae could be measured precisely, then astronomers would be able to see if the redshifts truly increased “systematically,” as de Sitter noted in his paper. That is, the more distant a spiral, the larger its redshift. But in 1917 carrying out such rigorous measurements was a pipe dream. At that time astronomers were still not settled on what a spiral was, much less able to figure out its exact distance.
Besides, few astronomers were paying attention to Einstein's theory as yet. World War I had kept Einstein's work from being widely circulated outside Germany, and when astronomers did hear of it, they weren't quite sure what to make of its unconventional and perplexing view of gravity. George Hale, like many astronomers at the time who were trained to observe rather than to tinker with mathematical equations, said he feared “it will always remain beyond my grasp.” All of that changed, though, once the findings of a British solar-eclipse expedition in 1919 transformed the name of Einstein, the former Swiss patent clerk, into