Stephen Hawking by John Gribbin Read Free Book Online Page A

thought up the idea, Einstein then developed a set of equations to describe all this. The task took him ten years. When he had finished, Newton’s famous inverse-square law reemerged from Einstein’s new theory of gravity; but general relativity went far beyond Newton’s theory, because it also offered an all-embracing theory of the whole Universe. The general theory describes all of spacetime and therefore all of space and all of time. (There is a neat way to remember how it works. Matter tells spacetime how to bend; bends in spacetime tell matter how to move. But, the equations also insisted, spacetime itself can also move, in its own fashion.)
    The general theory was completed in 1915 and published in 1916. Among other things, it predicted that beams of light from distant stars, passing close by the Sun, would be bent as they moved through spacetime distorted by the Sun’s mass. This would shift the apparent positions of those stars in the sky—and the shift might actually be seen, and photographed, during a total eclipse, when the Sun’s blinding light is blottedout. Just such an eclipse took place in 1919; the photographs were taken and showed exactly the effect Einstein had predicted. Bent spacetime was real: the general theory of relativity was correct.
    But the equations developed by Einstein to describe the distortion of spacetime by the presence of matter, the very equations that were so triumphantly vindicated by the eclipse observations, contained a baffling feature that even Einstein could not comprehend. The equations insisted that the spacetime in which the material Universe is embedded could not be static. It must be either expanding or contracting.
    Exasperated, Einstein added another term to his equations, for the sole purpose of holding spacetime still. Even at the beginning of the 1920s, he still shared (along with all his contemporaries) the Newtonian idea of a static Universe. But within ten years, observations made by Edwin Hubble with a new and powerful telescope on a mountaintop in California had shown that the Universe is expanding.
    The stars in the sky are not moving farther apart from one another. The individual stars we can see from Earth all belong to a huge system, the Milky Way Galaxy, which contains about a hundred billion stars and is like an island in space. In the 1920s, astronomers discovered with the aid of new telescopes that there are many other galaxies beyond the Milky Way, many of them containing hundreds of billions of stars like our Sun. And it is the galaxies, not individual stars, that are receding from one another, being carried farther apart as the space in which they are embedded expands.
    If anything, this was an even more extraordinary and impressive prediction of the general theory than the bendingof light detectable during an eclipse. The equations had predicted something that even Einstein at first refused to believe but which observations later showed to be correct. The impact on scientists’ perception of the world was shattering. The Universe was not static, after all, but evolving; Einstein later described his attempt to fiddle the equations to hold the Universe still as “the greatest blunder of my life.” Even at the end of the 1920s, the observations and the theory agreed that the Universe is expanding. And if galaxies are getting farther apart, that means that long ago they must have been closer together. How close could they ever have been? What happened in the time when galaxies must have been touching one another and before then?
    The idea that the Universe was born in a super-dense, super-hot fireball known as the Big Bang is now a cornerstone of science, but it took time—over fifty years—for the theory to become developed. Just at the time astronomers were finding evidence for the universal expansion, transforming the scientific image of the Universe at large, their physicist colleagues were developing the quantum