Perhaps the great scientific event of the twentieth century was the revolution in physics symbolized for the public by Albert Einstein (1879-1955). This revolution centered on radical revisions made in the Newtonian world-machine, the mechanistic model of the universe that had been accepted for more than two centuries.
Many nineteenth-century scientists were convinced that light moved in waves and was transmitted through the ether, which supposedly filled outer space. In the 1880s, however, experiments demonstrated that there was no ether. If the ether did exist, then it would itself be moved by the motion of the earth, and a beam of light directed against its current would travel with a velocity less than that of a beam directed with its current. But experiments showed that light traveled at 186,284 miles per second, whether it was moving with or against the hypothetical current.
In 1905 Einstein, who was then twenty-six years old, published a paper asserting that since the speed of light is a constant unaffected by the earth’s motion, it must also be unaffected by all the other bodies in the universe. This unvarying velocity of light is a law of nature, Einstein continued, and other laws of nature are the same for all uniformly moving systems.
This was Einstein’s special theory of relativity, which had many disconcerting corollaries. In particular, it undermined the idea of absolute space and absolute time, and made both space and time relative to the velocity of the system in which they were moving. Thus space and time are not absolutes but are relative to the observer.
Einstein maintained that space and time, therefore, are inseparably linked; time is a “fourth dimension.” An air-traffic controller needs to know the position of an airplane not only in longitude, latitude, and altitude, but also in time, and the total flight path of a plane must be plotted on what Einstein called a four-dimensional space-time continuum—a term he also applied to the universe.
Einstein equated not only space and time but also mass and energy. His famous formula E = mc2 means that the energy in an object is equal to its mass, multiplied by the square of the velocity of light. It means also that a very small object may contain tremendous potential energy. Such an object, for example, can emit radiation for thousands of years or discharge it all in one explosion, as happened with the atom bomb in 1945, when a way was found to unlock the potential energy in uranium.
The problem of mass also involved Einstein in a review of the Newtonian concept of a universe held together by the force of gravity—the attraction of bodies to other bodies over vast distances. Einstein soon concluded that gravity—that is, the weight—of an object had nothing to do with its attraction to other objects. Galileo had demonstrated that both light and heavy bodies fell from the leaning tower in Pisa at the same speed.
Einstein proposed that it would be more useful to extend the concept of the magnetic field, in which certain bodies behaved in a certain pattern, and speak of a gravitational field, in which bodies also behaved in a certain pattern. Einstein did not penetrate the mystery of what holds the universe together, but he did suggest a more convincing way of looking at it. This was the essence of his general theory of relativity (1916), which stated that the laws of nature are the same for all systems, regardless of their state of motion.
One of these laws of nature had been formulated in 1900 by Max Planck (1858-1947), a German physicist, who expressed mathematically the amount of energy emitted in the radiation of heat. He discovered that the amount of energy divided by the frequency of the radiation always yielded the same very tiny number, which scientists call Planck’s constant. The implication of this discovery was that objects emit energy not in an unbroken flow, but in a series of separate minute units, each of which Planck called a quantum.
Quantum physics suggested a basic discontinuity in the universe by assuming that a quantum could appear at two different locations without having traversed the intervening space. Other physicists soon made discoveries reinforcing the idea of continuity, with all physical phenomena behaving like waves. The old dilemma of whether light consisted of particles or of waves was therefore greatly extended.
During the 1920s it became evident that scientists might never be able to answer the ultimate questions about the universe. They could not fully understand the behavior of the electron, the basic component of the atom, because in the act of trying to observe the electron, they created effects that altered its behavior.
Study of the electron led Werner Heisenberg (1901-1976), a German physicist, to propose the principle of indeterminacy or uncertainty, which concludes that the scientist will have to be content with probabilities rather than absolutes. Although the universe is no longer the world machine of Newton, its activity is by no means random, and it can still be expressed in the mathematical language of probabilities.
Indeed, the new sciences of probability theory, molecular biology and biochemistry, and plasma physics, together with the discovery by Murray GellMann (1929– ) of the quark, which is any of three types of elementary particles believed to form the basis for all matter in the universe, have nonetheless led scientists once again to feel that they may be on the verge of a unifying theory by which creation, matter, and even life may at last be explained.
The development of twentieth-century astronomy has been closely linked to that of physics. To the non-scientist, such astronomical concepts as the finite but expanding universe, curved space, and the almost inconceivable distances and quantities of light years and galaxies have made astronomy the most romantic science. A light year is the distance traversed by light in one year, or roughly 5,880,000,000,000 miles.
The Milky Way galaxy, of which our universe is a part, has some 30,000 million stars and nebulae, in the form of a disk with a diameter of about 100,000 light years. To humanize these dimensions, and to make them comprehensible, Western writers produced works of science fiction and created television series and motion picture films that attempted to make sense at the level of popular culture of such abstract concepts as the light year.
One such program, originally to be called Wagon Train to the Stars, became one of the most popular television programs of all time as Star Trek. It posed moral dilemmas in outer space to a generation of young people for whom the customary means of presenting moral issues, whether in church or through literature, seemed ineffective, and for whom the romance of outer space had replaced the romance of the old frontier.
Politicians, too, were caught up in the popular fascination with space, referring to their policies as New Frontiers and using metaphors in their speeches derived from the new language of space travel and space conquest.