Originally published 9 August 1993
Einstein said: “The most incomprehensible thing about the world is that it is comprehensible.”
And when you think about it, it is indeed remarkable that a mass of nerve fibers the size of a softball can comprehend the cosmos — a vast geography of galaxies with a 15 billion year history. With the evolution of the human brain, the universe became self-conscious.
Of course, some softball-sized masses of nerve fibers are more remarkable than others. Einstein proposed “laws of nature” so remote from ordinary experience that a generation passed before experiments could be devised to test them decisively.
For example, in the special and general theories of relativity Einstein proposed an idea of time that ran counter to experience and common sense. He suggested that time passes at different speeds for observers who are moving relative to one another. In such a world, a twin could go on a fast journey and return younger than the sibling she left behind.
Einstein also predicted that gravity effects the passage of time (or more precisely, that mass affects space-time). A clock runs more quickly on the top floor of a building, where gravity is slightly weaker, than in the basement, which is closer to the center of the Earth. If gravity is strong enough — at the edge of a black hole — time stands still.
Stands still? Yes, literally! There are places in the universe — near collapsed massive stars, for example — where clocks would cease ticking and people cease to age. Except that neither clocks nor people could survive intact in such intense gravitational fields.
In other words, time is not what we thought it was, something that ticks away with absolute regularity, without beginning or end, and without regard to place or observer.
How did Einstein come up with his revolutionary notions of time? By applying his knowledge of science and mathematics, assisted by a formidable gift for fancy.
Einstein’s genius was being able to imagine what no one had imagined before.
Alan Lightman has written a delightful little novel that could serve as a preparation for the study of relativity. It is called Einstein’s Dreams, and purports to recount dreams Einstein had in 1905, the year of publication of his special theory of relativity. Each chapter of the novel describes a world in which time is different from what we have believed it to be. In one of Lightman’s worlds, time is cyclic; the same events happen over and over. In another, time comes to an end at a precisely appointed moment. And so on.
It is one thing for a novelist to dream fanciful kinds of time; it is something else to propose that a fancy is true. This is precisely what Einstein did in his special and general theories of relativity.
For a long time scientists believed Einstein’s theory because it was mathematically beautiful, in spite of scant experimental verification. Now, many different kinds of observational tests have confirmed the theory.
One recent confirmation of general relativity is so elegant, so unexpected, as to take my breath away. Bear with me in what follows, the conclusion is exciting.
At the end of a star’s life, gravity causes the star to collapse upon itself. Stars rather more massive than the sun are squeezed so strongly by gravity that even atoms are collapsed; electrons are squeezed into protons to create neutrons. The collapsed star becomes an object about as big as Rhode Island, spinning extremely rapidly for the same reason ice skaters spin more quickly when they draw their arms closer to the axis of spin. Radiation from the spinning star will be emitted in a beam and observed in pulses, like light from a rotating lighthouse lamp.
These collapsed stars are called neutron stars or pulsars. The first pulsar was observed in 1967. Today, thousands of them are known. Their pulses of radiation are as exact as the best atomic clocks on Earth.
One pulsar, called PSR 1913+16, is part of a binary system — two stars locked in a whirling dance. Einstein predicts that two masses revolving about each other will emit gravitational waves, in the same way that oscillating electric charges emit radio waves. We do not have the technology to detect these gravitational waves, but they should carry energy away from the rotating star system and the flash rate of the pulsar should slow down.
The orbits and masses of the linked objects that make up PSR 1913+16 can be calculated with extreme precision from subtle variations in the pulsar frequency, the so-called Doppler effect. The rate of energy loss can be calculated from Einstein’s theory. Astronomer Joseph Taylor and his colleagues have been observing the binary pulsar for 20 years. The slowdown is exactly as predicted by Einstein’s theory. The agreement is breathtakingly precise.
A collapsed star thousands of light-years away flashes precisely as predicted by equations dreamed up by Einstein long before pulsars were discovered. A cosmic clock ticks in accordance with Einstein’s dream. Nothing in nature is more incomprehensible than that this should be true.
Astronomers Joseph Taylor and Russell Hulse were awarded the 1993 Nobel Prize in Physics for their work on pulsar PSR 1913+16. In 2016, the first observations of gravitational waves were made by the LIGO and Virgo collaborations, confirming yet another of Einstein’s fancies. ‑Ed.
