Space, Time, and One Very Good Year

On September 27, 1905, Annalen der Physik received a three-page paper by Albert Einstein: "Does the Inertia of a Body Depend Upon Its Energy Content?" It contained the most famous equation in science. E = mc2. Mass and energy are equivalent. The coda to his special relativity paper, closing the argument with a startling conclusion: matter itself is frozen energy.

Einstein was 26, a patent examiner in Bern. He published four papers that year, any one career-making. The photoelectric effect paper earned the Nobel Prize. Brownian motion helped prove atoms exist. Special relativity resolved contradictions between Newton and Maxwell. The September paper drew the implication: E = mc2.

Special relativity rests on two postulates: the laws of physics are the same for all observers at constant velocity, and the speed of light is constant regardless of the source. The consequences are vertiginous. Time passes differently at different speeds. Objects in motion contract. Space and time are aspects of a single four-dimensional continuum.

a light clock diagram illustrating time dilation, a key consequence of special relativity. source: wikimedia commons

The theory is sometimes described as saying everything is relative. Nearly the opposite. What it establishes is that certain quantities -- the speed of light, the laws of physics -- are absolutely invariant. Relativity is a consequence of deeper invariance.

GPS satellites move fast enough that their clocks run measurably differently from ground clocks. Without relativistic corrections, positioning would drift kilometers per day. Einstein published four papers from a rented apartment while working a day job. He looked at two equations that disagreed and concluded the universe was stranger than imagined. The strangeness has been useful ever since.