Whether all physical phenomena will ultimately be described as merely manifestations of the properties of space and time remains today still an open question. So far only the force of gravity has been simply and convincingly merged with spacetime.
General relativity is the least tested theory of nature that is widely accepted. Its three famous original predictions—a subtlety in the motion of Mercury, the deflection of light by the Sun, and the gravitational red shift—have been verified, to varying degrees of accuracy. The last of these, the gravitational red shift, although small on and near Earth, does have one practical implication of enormous importance. Were it not taken into account, your GPS unit would be useless, taking you to Newark when you wanted to go to Hoboken.
Later implications of the theory, such as the existence of black holes and ripples in spacetime (gravitational radiation), have also been established. Yet tests of general relativity do not rival, in number or in precision, tests of other established theories such as special relativity, quantum mechanics, or the pre-twentieth-century classical theories. Nevertheless, the beauty and economy of general-relativity theory coupled with its tests to date have been sufficient to convince most scientists of its correctness. We are rather like the worm on the surface of the Earth who has discovered tiny but significant discrepancies between Euclidean geometry and observation, and on this basis asserts that the Earth is a sphere. A pragmatic worm could declare: “Since a sphere is nothing I can visualize anyway, and since these discrepancies you report are entirely too small to be important, I think your theory is irrelevant. The discrepancies could probably be explained in some simpler way anyhow.” To which the scientist-worm might reply: “The discrepancies, small or not, are nevertheless present. Since the sphere explains them, and since the sphere is such a magnificently beautiful and simple figure, I choose to believe in it.”
The sphere of the worm is the universe of humankind. So minute is the average curvature of our space that we are normally unaware of it. In our everyday world, and downward into the submicroscopic world, the manifestations of general relativity are too small to impinge directly on our senses or to alter the theory of quantum mechanics. (A caution, though: Extrapolation to distances shorter than any measured so far is dangerous, for new surprises, including a possible significant role of spacetime curvature, may await us there). But over the enormous distances of intergalactic space, the cumulative effects of space curvature become large—just as to the worm traveling an appreciable fraction of the distance around the Earth, Earth’s curvature would become significant. For this reason, the most interesting speculations on the consequences of general relativity have been in the cosmological domain. Cosmologists have proposed various “models” of the universe. According to the most widely accepted model—the “big bang” model—the universe is continously expanding away from a highly condensed state that existed some thirteen billion years ago. According to another model, never very popular but convincing to some, the universe is only apparently expanding, and is in a “steady state” in which new matter is continually coming into existence. Each of these models is consistent with general relativity and with astronomical observation. On top of whatever uncertainty remains about the history of our universe is the engrossing question: Do we live in only one among many universes?
It has been a century since Einstein proposed the general theory of relativity, a century in which, one by one, experiments have built a structure of support for the theory, Yet questions remain: Is the universe closed or open? Is it finite or infinite? Is the universe we inhabit all there is or is our universe one of many? What is the long-term fate of our universe? Is the amount of matter (and energy) in the universe truly a constant? Is there a connection between the world of the very large and the world of the very small (can quantum mechanics and general relativity be united)? Scientists have dared to grapple with these questions—as questions of science—over the past century, and answers may appear within decades, or perhaps a century—or perhaps never. When the “true” picture of the universe emerges, it will be a truth of simplicity, convenience, consistency, and successful predictions, just as is the truth of every other accepted theory of physical science.
2 Wheeler, who died in 2008 at the age of 96, missed out on the big spacetime news of 2016, the detection of gravitational waves. These waves came from the merger of two very distant black holes, entities that had been named and studied by Wheeler.