"In light of knowledge attained, the happy achievement seems almost a matter of course, and any intelligent student can grasp it without too much trouble. But the years of anxious searching in the dark, with their intense longing, their alterations of confidence and exhaustion and the final emergence into the light -- only those who have experienced it can understand it."

Einstein's theories sprang from a ground of ideas prepared by decades of experiments. One of the most striking, in retrospect, was done in Cleveland, Ohio, by Albert Michelson and Edward Morley in 1887. Their apparatus, shown above, was a massive stone block with mirrors and crisscrossing light beams, giving an accurate measurement of any change in the velocity of light. Michelson and Morley expected to see their light beams shifted by the swift motion of the earth in space. To their surprise, they could not detect any change. It is debatable whether Einstein paid heed to this particular experiment, but his work provided an explanation of the unexpected result through a new analysis of space and time.

As noted on the previous page, when Einstein used his equations to study the motion of a body, they pointed him to a startling insight about the body's mass and energy.

Conversion of energy into mass.

The deep connection Einstein discovered between energy and mass is expressed in the equation E=mc² . Here E represents energy, m represents mass, and c² is a very large number, the square of the speed of light. Full confirmation was slow in coming. In Paris in 1933, Irène and Frédéric Joliot-Curie took a photograph showing the conversion of energy into mass. A quantum of light, invisible here, carries energy up from beneath. In the middle it changes into mass -- two freshly created particles which curve away from each other.

Click here for Einstein's voice explaining the formula

Meanwhile in Cambridge, England, the reverse process was seen: the conversion of mass into pure energy. With their apparatus John Cockcroft and E.T.S. Walton broke apart an atom. The fragments had slightly less mass in total than the original atom, but they flew apart with great energy. (In 2005, the centennial of Einstein’s great year, a team made the most accurate test yet of his equation. They measured the tiny change in mass of radioactive atoms before and after the atoms emitted gamma-rays. And they measured the energy of the rays. The missing mass times c² equalled the energy of the rays to within 4 hundred-thousandths of one percent.)