E = mc2 Energy equals mass times the speed of light squared.

The relativity principle in connection with the basic Maxwellian equations demands that the mass should be a direct measure of the energy contained in a body; light transfers mass. With radium there should be a noticeable diminution of mass. The idea

Come with me and . . . think about the electromagnetic theory of light.

Thus the partial differential equation entered theoretical physics as a handmaid, but has gradually become mistress.

The more success the quantum theory has, the sillier it looks

If you wish to learn from the theoretical physicist anything about the methods which he uses, I would give you the following piece of advice: Don't listen to his words, examine his achievements. For to the discoverer in that field, the constructions of his imagination appear so necessary and so natural that he is apt to treat them not as the creations of his thoughts but as given realities.