Word: light
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Dates: during 1940-1949
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Meanwhile, he will look for evidence that the "red shift" does not indicate speed but is due to some other effect, such as light getting "tired." Hubble does not expect such evidence, but will welcome it if he finds it. Tired light, he thinks, would be a discovery quite as sensational as the exploding universe...
Curved Space. Hubble also intends to count the nebulae that can be photographed up to the billion-light-year range of the 200-inch. Behind this humdrum-sounding chore lies the eerie, brain-staggering problem of the curvature of space. Mathematical physicists believe (from Einstein's ubiquitous Relativity) that space is curved back upon itself, in a four-dimensional way, by the gravitational effect of the matter it contains. The curvature is too slight to be detected on earth or even in the enormous sphere, 500 million light-years in radius, penetrated by the 100-inch telescope. But theory...
Spheres in the realm of the nebulae may behave in the same way. When Hubble looks out into a billion-light-year sphere of space with the 200-inch's doubled range of penetration, he may not find eight times as many nebulae as in the 500-million-light-year sphere of the 100-inch. Since the nebulae are apparently pretty evenly distributed, will this mean that the larger sphere has actually less than eight times the volumes of the smaller one? If so, perhaps space is really curved-and Hubble will be the first...
...space is really curved, then the universe must be "finite," of limited (though perhaps expanding) size'. One "model of the universe" (Einstein's) gives the "circumference of space" (the path which a beam of light would cover as it circles around finite space and back to its starting place) as about 300 billion lightyears...
...nebula-dotted universe is finite, what lies beyond it? Robertson does not know. Perhaps, he admits, there may exist, far off in some medium that is thinner than space, still other universes. But each, presumably, is sealed in its own bubble of space, the light from its stars circulating endlessly, never escaping to reach our astronomers' telescopes. Scientists, briskly dusting their hands of other universes, say that if they exist, they must be penetrated by "nonphysical means...