The Science Behind Final Theory
Einstein's Physics
In 1905 Albert Einstein laid the foundations of modern physics. In a paper entitled Zur Elektrodynamik bewegter Körper (On the Electrodynamics of Moving Particles) Einstein introduced the special theory of relativity, which overthrew all the conventional notions of 19th-century physics by showing the malleability of space and time. The theory asserts that measurements of length, mass and duration depend on the observer's frame of reference; to a man standing on a subway platform, for example, the length of a train car speeding past will appear slightly shorter than it does to a man sitting inside the train.
At the time when Einstein's paper appeared, no scientist had ever observed this kind of relativistic effect — the difference in length measurements wouldn't be noticeable unless the train were moving thousands of miles per second — but later experiments proved Einstein right. One of the consequences of his special theory of relativity was the famous equation that relates the energy content of an object to its mass:
E = mc2
Because c, the speed of light, is an enormous number, the formula implies that even small bits of mass contain tremendous amounts of energy. Four decades later, this discovery led to the invention of the atom bomb.
In 1915 Einstein astounded the physics community again by proposing the general theory of relativity, which incorporated gravity into his new concept of space and time. He showed that a massive object (say, the sun) warps the spacetime around it, bending the paths of any other objects (say, the planets) moving nearby. The general theory of relativity is elegantly summarized in Einstein's field equation:
Gµν = 8πGTµν
(Gµν is an array of numbers describing the curvature of spacetime, Tµν is a similar array describing the energy content of spacetime, and G is Newton's gravitational constant.)
During the same miraculous year that Einstein introduced the special theory of relativity, he also paved the way for quantum theory, the second great advance of 20th-century physics. In another 1905 paper he proposed a novel explanation for the photoelectric effect, a phenomenon that had baffled previous researchers. When scientists exposed certain materials to ultraviolet light, they detected the emission of electrons. Einstein claimed that the material was absorbing discrete particles of light — which he called quanta — that were energetic enough to dislodge electrons from the substance.
Because physicists at the time were accustomed to thinking of light as an electromagnetic wave, many resisted the idea that light could also act as a particle. But some scientists realized that this concept could explain many other odd features of the atomic world, and in the 1920s they created a whole new branch of physics called quantum mechanics. And Einstein soon discovered that he didn't like their approach at all.