einstein

his one-sentence drunken postcard to him. In September, he wrote yet another letter to Habicht, this one trying to entice him to come work at the
patent office. Einstein’s reputation as a lone wolf was somewhat artificial. “Perhaps it would be possible to smuggle you in among the patent
slaves,” he said. “You probably would find it relatively pleasant. Would you actually be ready and willing to come? Keep in mind that besides the
eight hours of work, each day also has eight hours for fooling around, and then there’s also Sunday. I would love to have you here.”
As with his letter six months earlier, Einstein went on to reveal quite casually a momentous scientific breakthrough, one that would be expressed
by the most famous equation in all of science:
One more consequence of the electrodynamics paper has also crossed my mind. Namely, the relativity principle, together with Maxwell’s
equations, requires that mass be a direct measure of the energy contained in a body. Light carries mass with it. With the case of radium there
should be a noticeable reduction of mass. The thought is amusing and seductive; but for all I know, the good Lord might be laughing at the
whole matter and might have been leading me up the garden path. 81
Einstein developed the idea with a beautiful simplicity. The paper that the Annalen der Physik received from him on September 27, 1905, “Does
the Inertia of a Body Depend on Its Energy Content?,” involved only three steps that filled merely three pages. Referring back to his special relativity
paper, he declared, “The results of an electrodynamic investigation recently published by me in this journal lead to a very interesting conclusion,
which will be derived here.” 82
Once again, he was deducing a theory from principles and postulates, not trying to explain the empirical data that experimental physicists
studying cathode rays had begun to gather about the relation of mass to the velocity of particles. Coupling Maxwell’s theory with the relativity theory,
he began (not surprisingly) with a thought experiment. He calculated the properties of two light pulses emitted in opposite directions by a body at
rest. He then calculated the properties of these light pulses when observed from a moving frame of reference. From this he came up with equations
regarding the relationship between speed and mass.
The result was an elegant conclusion: mass and energy are different manifestations of the same thing. There is a fundamental interchangeability
between the two. As he put it in his paper, “The mass of a body is a measure of its energy content.”
The formula he used to describe this relationship was also strikingly simple: “If a body emits the energy L in the form of radiation, its mass
decreases by L/V 2 .” Or, to express the same equation in a different manner:L=mV 2 . Einstein used the letter L to represent energy until 1912,
when he crossed it out in a manuscript and replaced it with the more common E. He also used V to represent the velocity of light, before changing
to the more common c. So, using the letters that soon became standard, Einstein had come up with his memorable equation:
E=mc 2
Energy equals mass times the square of the speed of light. The speed of light, of course, is huge. Squared it is almost inconceivably bigger. That is
why a tiny amount of matter, if converted completely into energy, has an enormous punch. A kilogram of mass would convert into approximately 25
billion kilowatt hours of electricity. More vividly: the energy in the mass of one raisin could supply most of New York City’s energy needs for a day. 83
As usual, Einstein ended by proposing experimental ways to confirm the theory he had just derived. “Perhaps it will prove possible,” he wrote,“to
test this theory using bodies whose energy content is variable to a high degree, e.g., salts of radium.”