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At the same time, Einstein defined a new entity: a particle of light, the “light quantum,” now called the photon.

“There is no definite information on the length of time elapsing during the process of absorption of a quantum of energy photo-electrically by an electron,” they reported, “and the so-called length of a light quantum—if such a concept has meaning—is equally unknown experimentally.”

Himself a founding father of the early quantum theory and author of the light quantum hypothesis, he sensed that here was a problem that could not be reconciled with classical concepts and imperatively required new ideas and approaches.

Relativity deals with time, space, gravity, the finite speed of light; quantum mechanics with particles, electricity, the action of light.