In 1905, Einstein published four revolutionary papers in the journal Annalen der Physik, including papers explaining the photoelectric effect and the fundamentals of his special theory of relativity. His work laid the foundation for the field of quantum physics, though he himself had some concerns about the conceptual implications of his own work.
Specifically, Einstein didn't like the fact that quantum physics allows (conceptually at the time, though it has since been demonstrated) that information about a system can be "communicated" instantaneously through quantum entanglement. This violates one of the principles of relativity, which is that no information can travel faster than the speed of light. Einstein's friendly, though heated, debate with colleague Niels Bohr on this issue went on for years, and provided key insights into the developing quantum theory.
In the later years of his life, Einstein focused his work on the attempt to derive a Unified Field Theory (more often called Grand Unified Theory, or GUT, at the time) which would unify the fundamental forces of physics into a single conceptual framework. This attempt was not successful, but it is still the holy grail of physics research, resulting in theories such as string theory, quantum gravity, and loop quantum gravity.
In addition to his work in physics, he also became a prominent figure in popular culture, obtaining a status akin to a modern day rock star. His distinctive appearance may have helped with this, and certainly has made him popular among cartoonists. He wrote a letter to President Franklin Delano Roosevelt in 1939, expressing concern that Germany might be using his own research to develop nuclear weapons. This letter motivated FDR to initiate the Manhattan Project to develop such a weapon for the Allies first.
Einstein later became a vocal supporter of the Zionist movement, although he also expressed concerns about the specifics of the decision to form Israel ... concerns which the continued violence in that region & border dispute shows to have been prescient. He was asked to be Israel's second president, but he chose to decline it.
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