Yet it was his first paper, written in March, that was perhaps his most revolutionary.
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In it, Einstein argued that light is not a wave, as most physicists previously thought, but instead a stream of tiny packets of energy that have since come to be known as photons. This helped explain the photoelectric effect (the emission of electrons by certain substances when subjected to light or radiation).
The theory won Einstein the Nobel Prize in 1921 and helped lay the foundation for quantum theory, which states that physics cannot make definite predictions. It can only predict the probability that things will turn out one way or another.
The quantum theory, with its statistical description of nature at the subatomic scale, has turned out to be right.
However, Einstein came to reject the unpredictability of quantum mechanics, famously saying, "God does not play dice with the universe." Instead he saw it as a mere path to a deeper and more complete description of the universe.
"He couldn't accept that so deeply woven into the fabric of the cosmos was an element of uncertainty," said Brian Greene, a physics and mathematics professor at Columbia University in New York. "He hoped the probabilistic framework of quantum mechanics was merely an intermediary point physicists reached in their study. But that doesn't seem to be the case," said Greene, who wrote the best-selling The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory.
Holton, the Harvard physicist, says it is a mistake to look at Einstein's papers individually.
"They all show the same motivation," Holton said. "In the very first lines [Einstein says] there is something wrong with the way we understand nature that there are microscopic bodies big enough to see, and a submicroscopic world of atoms and molecules, which we can't see. Einstein said, 'No, there must be just one kind of physics. God would not have made two kinds of physics.'"
Einstein became convinced that one unified theory could explain the order of the universe.
"His way of approaching physics was to find connections between things which had been viewed as separate," Greene said. "His ultimate goal was to find a connection between all of nature's forces."
Einstein never succeeded in his search for a theory of everything. But many people consider string theorists such as Greene to be Einstein's natural successors.
String theory is a physical model that says that the fundamental building blocks of the universe are vibrating filaments of energy within every particle.
"We're certainly carrying on a program that Einstein initiated," Greene said. "Whether we are on the right track, I don't know. But if it is correct, then it would be the kind of theory that Einstein spent 30 years searching for but never found. That would be quite a wonderful thing."
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