Relativity and quantum mechanics rank among the greatest achievements of 20th-century science, constituting the sum of all fundamental physical knowledge. The former describes the world of the very large, including black holes and the expanding universe. The latter explains the world of the very small, the microscopic realm of atoms and subatomic particles. One man—Albert Einstein—was the undisputed father of the first theory and the godfather of the second.
What was the secret of Einstein’s success? He once said that if a physical theory cannot be explained to a child, it is probably worthless. In other words, he thought in terms of simple physical pictures. His greatness lay in his ability to use such pictures to solve fundamental problems, such as the conflict between Isaac Newton’s theory of mechanics and James Clerk Maxwell’s theory of light.
The Newtonian system was based on common sense—that a second on Earth is the same as a second throughout the solar system. We could synchronize our watches anywhere in the universe because time beats uniformly. Likewise, a foot or a pound on Earth is the same as a foot or a pound in every other location.
Using mental images of clocks and trains, light beams and speeding bicycles, Einstein realized that Newton’s system could not be right because it contradicted Maxwell’s theory of light. Einstein showed that the speed of light must be constant, no matter how fast you move. For that to be true, time must get slower the faster you move. Stranger still, lengths contract and masses must increase as you approach the speed of light. Space and time became relative in his new theory. This pivotal insight overthrew 250 years of Newtonian physics.
Ten years later, Einstein resolved yet another contradiction in physics. According to Newton, gravity traveled instantly throughout the universe. But according to the theory of relativity, nothing can go faster than light. To overcome these incompatible views, Einstein introduced another, even grander theory in which space and time are not empty but are instead like a fabric that can be curved and stretched. This new picture—in which gravity originates from the bending of sheets of space-time—revolutionized cosmology and gave us the most compelling theory of creation, the Big Bang.
Thus, pictures even a child could understand would change the course of history and transform our understanding of the universe.
Special relativity unlocked the secrets of the stars and revealed the fantastic quantities of energy stored deep inside the atom. But the seed of relativity was planted when Einstein was only 16 years old and asked himself a childlike question: What would a beam of light look like if you could race alongside it? According to Newton, you could catch up to any speeding object if you moved quickly enough. If you could catch up to a light wave, Einstein realized, it would look like a wave frozen in time. But even as a teenager, he knew that no one had ever seen a frozen light wave before. In fact, such a wave makes no physical sense.
When Einstein studied Maxwell’s theory of light, he found something that others missed—that the speed of light always appears the same, no matter how quickly you move. Einstein then boldly formulated the principle of special relativity: The speed of light is a constant in all inertial frames (frames that move at constant velocity).
No longer were space and time absolutes, as Newton thought. Space compresses and clocks tick at different speeds throughout the universe.
Time is Relative
Imagine putting one twin on a rocket ship that blasts off at a speed near that of light while the other twin remains back on Earth.Through a telescope, the Earth twin sees that his rocket twin appears younger than himself. When the rocket twin comes back to Earth,the Earth twin has aged more; the rocket twin is much younger.
A Relativity Paradox
From the vantage point of the rocket twin as he took off from Earth, it appeared as if he were at rest and that the Earth moved away from him. Thus, to him it is the Earth twin who has traveled at great speed and become younger while the rocket twin has aged. So who really is younger?
Resolution of the twin paradox
The rocket twin, not the Earth twin, reversed directions during his journey. Since the rocket twin didn’t travel with constant velocity, the two viewpoints are not the same. Hence, you can tell who is younger: the rocket twin.
Special relativity was incomplete because it made no mention of acceleration or gravity. Einstein then made the next key observation: Motion under gravity and motion in an accelerated frame are indistinguishable. Since a light beam will bend in a rocket that is accelerating, a light beam must also bend under gravity.
To show this, Einstein introduced the concept of curved space. In this interpretation, planets move around the sun not because of a gravitational pull but because the sun has warped the space around it, and space itself pushes the planets. Gravity does not pull you into a chair; space pushes on you, creating the feeling of weight. Space-time has been replaced by a fabric that can stretch and bend.
General relativity can describe the extreme warping of space caused by the gravity of a massive dead star—a black hole. When we apply general relativity to the universe as a whole, one solution naturally describes an expanding cosmos that originated in a fiery big bang.
Three of the seminal papers Einstein wrote in his “miracle year” of 1905 probably deserved the Nobel Prize. In one paper, he showed that light has a dual nature—that is, it exhibits both wavelike and particle-like qualities. Einstein’s quantum theory of light is essential to modern electronics, including television, solar cells, lasers, and fiber optics.
He was also the first to give solid justification for the existence of atoms. By analyzing how the random impact of atoms can distort the motion of tiny dust particles, creating a continuous zigzag motion, he showed a practical way to calculate the size of atoms.
Einstein’s 1905 paper on special relativity paved the way for a four-dimensional description of the world. These formulations provide a framework that may ultimately solve his greatest quest: the search for a “theory of everything” that can unify all the laws of nature.
Some fault Einstein for opposing quantum mechanics because he believed that “God does not play dice.” In reality, he did not dispute the undeniable successes of quantum mechanics. Instead, his true goal was to swallow up quantum mechanics with his unified field theory.
A Unified Field?
The greatest legacy of Einstein’s work may be the unified field theory, which would weave all the laws of nature into a single coherent theory. He spent the last 30 years of his life chasing after this theory of everything.
But because little was known about the nuclear force before Einstein’s death, there was a large missing piece to the jigsaw puzzle. Today the leading unifying candidate is string theory.
Relativity and You
Textbooks sometimes state that Einstein’s theories do not affect our everyday life, because we never travel anywhere close to the speed of light. But imagine for a moment what would happen if we could somehow “turn off” relativity
First, our technology would fail. The Global Positioning System (which locates our position on Earth to within 50 feet or less) would malfunction, because the clock on the satellite does not tick at the same speed as Earth clocks. Moreover, since relativity governs the properties of electricity and magnetism, all modern electronics would come to a halt, including generators, computers, radios, and TV.
Without correcting for the effects of relativity, the GPS signals would have errors of several parts per billion, enough to make them useless.