I think there are several really critical aspects in the development of physics. These include the origin of the universe, the establishment of a heliocentric world, the origin of quantum mechanics, the laws of relativity, and spectroscopy. There are others, but these are certainly momentous, and they are all intimately involved with light.
Most scientists now believe that the universe began in a giant explosion called the Big Bang. All the matter of the universe was in a lump smaller than a golf ball. It then expanded as a giant plasma. As the Bible says, the universe began with light. Calculations show that as a result of 15-billion years of expansion, the radiation of that initial explosion has gradually cooled to 3 K, three degrees above absolute zero. That is the theory.
About 20 years ago my friend John Mather headed a team from NASA to measure this. It was called the COBE DIRBE experiment. Radiometric and spectrometric devices were put in a satellite and kept at a temperature below 3 K with liquid helium. Sure enough, they found that the background radiation looks like a 3 K blackbody radiation.
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