Later Work
After the 1930s, progressively fewer scientists worked on classical unification, due to the continual development of quantum theory and the difficulties encountered in developing a quantum theory of gravity. Einstein continued to work on unified field theories of gravity and electromagnetism, but he became increasingly isolated in this research, which he pursued until his death. Despite the publicity of this work due to Einstein's celebrity status, it never resulted in a resounding success.
Most scientists, though not Einstein, eventually abandoned classical theories. Current mainstream research on unified field theories focuses on the problem of creating quantum gravity and unifying such a theory with the other fundamental theories in physics, which are quantum theories. (Some programs, most notably string theory, attempt to solve both of these problems at once.) With four fundamental forces now identified, gravity remains the one force whose unification proves problematic.
Although new "classical" unified field theories continue to be proposed from time to time, often involving non-traditional elements such as spinors, none has been generally accepted by physicists.
Read more about this topic: Classical Unified Field Theories
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