The Bohr–Einstein debate over the ‘‘weighing of energy’’ and the validity of the time–energy uncertainty relation is reexamined in the context of gravitation theories that do not respect the equivalence principle. Bohr’s use of the equivalence principle is shown to be sufficient, but not necessary, to establish the validity of this uncertainty relation in Einstein’s ‘‘weighing-of-energy’’ gedanken experiment. The uncertainty relation is shown to hold in any energy-conserving theory of gravity, and so a failure of the equivalence principle does not engender a failure of quantum mechanics. The relationship between the gravitational redshift and the equivalence principle is reviewed.
The paper begins with a brief account of classical theory of radiation and its failure to explain black body radiation.It is followed by the introduction of Quantum Theory of radiation by Max Planck.The dual nature of matter by de Broglie has been extensively dealt with.The uncertainty principle of Heisenberg has been critically discussed by many eminent authors who finally concluded it as one of the unavoidable mysteries of nature.As a picture speaks thousand words and as face is the index of the mind, the author has added pictures of eminent physicists at appropriate places.The subject matter of the paper will form a good text for any graduate students of Physics.