"Of all the communities available to us there is not one I would want to devote myself to, except for the society of the true searchers, which has very few living members at any time."

• Max Planck found the first hints of the quantum theory in 1900.
• H.A. Lorentz: "He meant more than all the others I have met on life's journey."
• Erwin Schrödinger and Louis de Broglie developed a quantum theory that appealed to Einstein. He said de Broglie had "lifted a corner of the great veil." But it was soon found that this theory was mathematically equivalent to the Heisenberg theory, which Einstein distrusted.
• Max Born, another pioneer of the quantum theory, was a friend of Einstein for many years.

In 1916 Einstein devised an improved fundamental statistical theory of heat, embracing the quantum of energy. His theory predicted that as light passed through a substance it could stimulate the emission of more light. This effect is at the heart of the modern laser. This theory was further developed by the Indian physicist S.N. Bose. He sent a draft paper to Einstein, who was inspired to develop a still more general approach. The terms stimulation and cooperative phenomena, used in laser physics, could describe the discovery process as well.

LASER: Light Amplification by Stimulated Emission of Radiation.

The Danish physicist Niels Bohr showed in 1913 how the quantum idea could explain the actions of electrons inside atoms.

By the 1920s most physicists had realized that their familiar mechanics, developed over centuries by Newton and many others, could not fully describe the world of atoms. Physics had to be rebuilt to take into account the fundamental discreteness of energy that was first pointed out by Planck and Einstein. Einstein himself contributed a number of key ideas to the developing quantum theory. But through the early 1920s much in quantum theory remained obscure.

Beginning in 1925 a bold new quantum theory emerged, the creation of a whole generation of theoretical physicists from many nations. Soon scientists were vigorously debating how to interpret the new quantum mechanics. Einstein took an active part in these discussions. Heisenberg, Bohr, and other creators of the theory insisted that it left no meaningful way open to discuss certain details of an atom's behavior. For example, one could never predict the precise moment when an atom would emit a quantum of light. Einstein could not accept this lack of certainty; and he raised one objection after another. At the Solvay Conferences of 1927 and 1930 the debate between Bohr and Einstein went on day and night, neither man conceding defeat.

The German physicist Werner Heisenberg. His 1925 quantum equations opened the way to a complete description of atomic mechanics. See our big online exhibit Werner Heisenberg and the Uncertainty Principle.

"Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us closer to the secret of the 'Old One.' I, at any rate, am convinced that He is not playing at dice."

Einstein and Bohr

By the mid 1930s, Einstein had accepted quantum mechanics as a consistent theory for the statistics of the behavior of atoms. He recognized that it was "the most successful physical theory of our time." This theory, which he had helped to create, could explain nearly all the physical phenomena of the everyday world. Eventually the applications would include transistors, lasers, a new chemistry, and more. Yet Einstein could not accept quantum mechanics as a completed theory, for its mathematics did not describe individual events. Einstein felt that a more basic theory, one that could completely describe how each individual atom behaved, might yet be found. By following the approach of his own general theory of relativity, he hoped to dig deeper than quantum mechanics. The search for a deeper theory was to occupy much of the rest of his life.