THEORY OF RELATIVITY

August 27, 2008 at 2:31 am (SCIENCE) ()

Einstein Overview

He discovered, just by thinking about it, the essential structure of the cosmos. The scientific touchstones of our age—the bomb, space travel, electronics—all bear his fingerprints. We may as well join him in 1905, when he was a Patent Office clerk in Zurich—not the revered white-haloed icon of a thousand photographs but a confident 26-year-old with wavy black hair and cocky wide eyes. That year, in his spare time, he produced three different world—shattering papers for a single volume (now priceless) of the premier journal Annalen der Physik.They were “blazing rockets which in the dark of the night suddenly cast a brief but powerful illumination over an immense unknown region,” as the physicist Louie de Broglie said. One offered the revolutionary view that light comes in particles as much as waves—setting the stage for generations of deep tension between granularity and smoothness in physicists’ view of energy and matter. Another calculated the size of molecules and incidentally proved their very reality; many scientists, as the century began, still doubted that atoms existed. And the third—well, as Einstein said in a letter to a friend, it “modifies the theory of space and time.” Ah, yes. Relativity.

The time had come. The Newtonian world-view was already fraying at the edges. The 19th century had pressed its understanding of space and time to the very limit. Everyone believed in the ether, that mysterious background substance of the whole universe though which light waves supposedly traveled, but where was the experimental evidence for it? Nowhere, as Einstein realized. He found it more productive to think in terms of utterly abstract frames of reference—because these could move along with a moving observer. Meanwhile, a few imaginative people were already speaking of time in terms of a fourth dimension—H. G. Wells, for example, in his time-obsessed science fiction. Humanity was standing on a brink, ready to see something new. It was Einstein who saw it. Space and time were not apples and oranges, but mates—joined, homologous, inseparable. “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows,” said Herman Minkowski, a teacher of Einstein’s and one of relativity’s first champions, “and only a kind of union of the two will preserve an independent reality.” Well, we all know that now. “Spacetime,” we knowingly call it. Likewise energy and matter: two faces of one creature. E = mc2, as Einstein memorably announced.

All this was shocking and revolutionary and yet strangely attractive, to the public as well as scientists. The speed of light; the shifting perspective of the observer—heady fare. A solar eclipse in 1919 gave the English astronomer Arthur Eddington the opportunity to prove a key prediction of relativity, that starlight would swerve measurably as it passed through the heavy gravity of the sun, a dimple in the very fabric of the universe. Newspapers and popular magazines went wild. More than 100 books on relativity appeared within a year. Einstein claimed to be the only person in his circle not trying to win a $5,000 Scientific American prize for the best 3,000-word summary (“I don’t believe I could do it”).

The very name relativity fueled the fervor, for accidental and wholly unscientific reasons. In this new age, recovering from a horrible war, looking everywhere for originality and novelty and modernity, people could see that absolutism was no good. Everything had to be looked at relative to everything else. Everything—for humanity’s field of vision was expanding rapidly outward, to planets, stars, galaxies. Einstein had conjured the whole business, it seemed. He did not invent the “thought experiment,” but he raised it to high art: imagine twins, wearing identical watches; one stays home, while the other rides in a spaceship near the speed of light . . . Little wonder that, from 1919 on, Einstein was the world’s most famous scientist.

In his native Germany he became a target for hatred. As a Jew, a liberal, a humanist, an internationalist, he attracted the enmity of nationalists and anti-Semites, abetted by a few jealous German physicists—an all-too-vigorous faction that Einstein called, while it was still possible to find this amusing, “the Antirelativity Theory Company Ltd.” His was now a powerful voice, widely heard, always attended to, especially after he moved to the United States. He used it to promote Zionism, pacifism and, in his secret 1939 letter to Franklin D. Roosevelt, the construction of a uranium bomb.

Meanwhile, like any demigod, he accreted bits of legend. That he flunked math in school (not true). That he opened a book and found an uncashed $1,500 check he had left as a bookmark (maybe—he was absent-minded about everyday affairs). That he was careless about socks, collars, slippers . . . that he couldn’t work out the correct change for the bus . . . that he couldn’t even remember his address, 112 Mercer Street in Princeton, where he finally settled, conferring an aura on the town, the university and his Institute for Advanced Study.

He died there in 1955. He had never accepted the strangest paradoxes of quantum mechanics, the theory of atomic and subatomic phenomena that he had done so much to create. He found “intolerable,” he said, its abandonment of strict causality in describing what particles do and when they do it. (“In that case I would rather be a cobbler, or even an employee in a gaming-house, than a physicist.”) He never achieved what he considered a complete, unified field theory. Indeed, for some years he had watched the burgeoning of physics, its establishment as the most powerful and expensive branch of the sciences, from a slight remove. He had lived, he said, “in that solitude which is painful in youth, but delicious in the years of maturity.”

And after the rest of Albert Einstein had been cremated, his brain remained, soaking for decades in a jar of formaldehyde belonging to Dr. Thomas Harvey, the Princeton Hospital pathologist. No one had bothered to dissect the brains of Freud, Stravinsky or Joyce, but in the 1980’s bits of Einsteinian gray matter were making the rounds of certain neuropsychologists, who thus learned . . . absolutely nothing. It was just a brain—the brain that dreamed a plastic fourth dimension, that banished the ether, that released the pins binding us to absolute space and time, that refused to believe God played dice, that finally declared itself “satisfied with the mystery of life’s eternity and with a knowledge, a sense, of the marvelous structure of existence.”

In embracing Einstein our century took leave of a prior universe and an erstwhile God. The new versions were not so rigid and deterministic as the old Newtonian world. Einstein’s God was no clockmaker, but he was the embodiment of reason in nature—”subtle but not malicious.” This God did not control our actions or even sit in judgment on them. (“Einstein, stop telling God what to do,” Niels Bohr finally retorted.) This God seemed rather kindly and absent-minded, as a matter of fact. Physics was freer, and we, too, are freer, in the Einstein universe. Which is where we live.

Einstein’s Theory of Relativity

Special Relativity proposed that distance and time are not absolute. The ticking rate of a clock depends on the motion of the observer of that clock; likewise for the length of a “yard stick.” Published in 1915, General Relativity proposed that gravity, as well as motion, can affect the intervals of time and of space. The key idea of General Relativity, called the Equivalence Principle, is that gravity pulling in one direction is completely equivalent to an acceleration in the opposite direction. (A car accelerating forwards feels just like sideways gravity pushing you back against your seat. An elevator accelerating upwards feels just like gravity pushing you into the floor. If gravity is equivalent to acceleration, and if motion affects measurements of time and space (as shown in Special Relativity), then it follows that gravity does so as well. In particular, the gravity of any mass, such as our sun, has the effect of warping the space and time around it. For example, the angles of a triangle no longer add up to 180 degrees and clocks tick more slowly the closer they are to a gravitational mass like the sun. Many of the predictions of General Relativity, such as the bending of starlight by gravity and a tiny shift in the orbit of the planet Mercury, have been quantitatively confirmed by experiment. Two of the strangest predictions, impossible ever to completely confirm, are the existence of black holes and the effect of gravity on the universe as a whole (cosmology).


Black Holes according to Einstein

A black hole is a region of space whose attractive gravitational force is so intense that no matter, light, or communication of any kind can escape. A black hole would thus appear black from the outside. (However, gas around a black hole can be very bright.) It is believed that black holes form from the collapse of stars. As long as they are emitting heat and light into space, stars are able to support themselves against their own inward gravity with the outward pressure generated by heat from nuclear reactions in their deep interiors. Every star, however, must eventually exhaust its nuclear fuel. When it does so, its unbalanced self gravitational attraction causes it to collapse. According to theory, if a burned- out star has a mass larger than about three times the mass of our sun, no amount of additional pressure can stave off total gravitational collapse. The star collapses to form a black hole For a non rotating collapsed star, the size of the resulting black hole is proportional to the mass of the parent star; a black hole with a mass three times that of our sun would have a diameter of about 10 miles. The possibility that stars could collapse to form black holes was first theoretically “discovered” in l939 by J. Robert Oppenheimer and H. Snyder, who were manipulating the equations of Einstein’s General Relativity. The first black hole believed to be discovered in the physical world, as opposed to the mathematical world of pencil and paper, was Cygnus X-1, about 7000 light years from earth. (A light year, the distance light travels in a year, is about six trillion miles.) Cygnus X-1 was found in 1970. Since then, a dozen excellent black hole candidates have been identified. Many astronomers and astrophysicists believe that massive black holes, with sizes up to ten million times that of our sun, inhabit the centers of energetic galaxies and quasars and are responsible for their enormous energy release. Ironically, Einstein himself did not believe in the existence of black holes, even though they were predicted by his theory. Beginning in 1917, Einstein and others applied General Relativity to the structure and evolution of the universe as a whole. The leading cosmological theory, called the Big Bang theory, was formulated in 1922 by the Russian mathematician and meteorologist Alexander Friedmann. Friedmann began with Einstein’s equations of General Relativity and found a solution to those equations in which the universe began in a state of extremely high density and temperature (the so- called Big Bang) and then expanded in time, thinning out and cooling as it did so. One of the most stunning successes of the Big Bang theory is the prediction that the universe is approximately ten billion years old, a result obtained from the rate at which distant galaxies are flying away from each other. This prediction accords with the age of the universe as obtained from very local methods, such as the dating of radioactive rocks on the earth. According to the Big Bang theory, the universe may keep expanding forever, if its inward gravity is not sufficiently strong to counterbalance the outward motion of galaxies, or it may reach a maximum point of expansion and then start collapsing, growing denser and denser, gradually disrupting galaxies, stars, planets, people, and eventually even individual atoms. Which of these two fates awaits our universe can be determined by measuring the density of matter versus the rate of expansion. Much of modern cosmology, including the construction of giant new telescopes such as the new Keck telescope in Hawaii, has been an attempt to measure these two numbers with better and better accuracy. With the present accuracy of measurement, the numbers suggest that our universe will keep expanding forever, growing colder and colder, thinner and thinner. General relativity may be the biggest leap of the scientific imagination in history. Unlike many previous scientific breakthroughs, such as the principle of natural selection, or the discovery of the physical existence of atoms, General Relativity had little foundation upon the theories or experiments of the time. No one except Einstein was thinking of gravity as equivalent to acceleration, as a geometrical phenomenon, as a bending of time and space. Although it is impossible to know, many physicists believe that without Einstein, it could have been another few decades or more before another physicist worked out the concepts and mathematics of General Relativity.


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