2015年6月5日 星期五

How black holes, hated by Einstein, entered the mainstream

How black holes, hated by Einstein, entered the mainstream
Margaret Wertheim is the author of “The Pearly Gates of Cyberspace,” a history of Western concepts of space from the Middle Ages to the Internet.
How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved
By Marcia Bartusiak
Yale Univ.
237 pp. $27.50
Who doesn’t love black holes, the most mythical creatures in the physics pantheon? They’ve been posited as the basis of time machines, as gateways to other universes and as the seeds from which baby universes are born. These days you can hardly open a physics magazine without seeing an article about the fabulous things black holes may be able to accomplish. In March, Physics World reported that a theoretician at the Institute for Advanced Studies in Princeton, N.J., has proposed that alien civilizations may be using black holes to create vast particle accelerators whose radiating neutrinos may be detectable here on Earth. He urged astronomers searching for signs of extraterrestrial life to start looking for such signatures.
But physicists haven’t always had a love affair with black holes. Indeed, as Marcia Bartusiak shows in her sparkling new book, at just about every step in the intellectual history of these objects, physicists resisted the concept tooth and nail. When, for example, Stephen Hawking first presented his idea that black holes emit radiation — a notion now widely accepted — he was greeted, Bartusiak tells us, “with total disbelief.” He announced his discovery at a 1974 conference at the Rutherford Laboratory near Oxford, and, at the end of his talk, the chairman of the session bluntly declared, “Sorry Stephen, but this is absolute rubbish.”
One of the delights of this witty book is seeing the many ways physicists historically found to dismiss, deny and disdain black holes. Even Einstein, that most fearless thinker, found them beyond the pale. In 1939 he wrote a paper “attempting to prove they were impossible.”
Most people today associate black holes with Einstein and his general theory of relativity, the centennial of which we celebrate this year: The timing of Bartusiak’s book is no coincidence. But the idea of something like black holes was first proposed in the late 18th century.

Two people came up with them independently. The famous one was Pierre-Simon de Laplace, the great French mathematician who boldly declared that there was no need to evoke God to explain the cosmic system: The mathematical laws of nature were sufficient, he believed. But it was an obscure English genius named John Michell who got there first, in 1783.
Michell lived in the era when light was believed to be a stream of particles. Newton had called them corpuscles. Michell imagined a stream of these particles journeying away from a star out into space. Just as a ball thrown into the air will be pulled back toward the Earth, so he realized that the pull of gravity would tug at the light particles and slow them down. If the gravitational pull was strong enough, the “escape velocity” would exceed the speed of light and the particles would be pulled back completely. “With not one radiant corpuscle escaping from the star,” Bartusiak writes, “it would remain forever invisible.”
Michell and Laplace had done their thinking in a Newtonian framework, but it took general relativity to give “dark stars” a solid theoretical foundation. According to general relativity, space itself has a structure. One can think of this as a vast trampoline, only in four dimensions. (In relativity, time acts like another dimension of space, so there are four dimensions collectively in what is known as space­time). As a trampoline will stretch and warp if you put a bowling ball on it, so, according to Einstein’s theory, a heavy mass such as the sun will warp the fabric of spacetime. If the bowling ball gets dense enough and heavy enough, it will puncture a hole in the trampoline, and this, says general relativity, is what happens when a star gets dense enough — it punctures a hole in spacetime.
What the equations seemed to be saying was that, if a star gets beyond a certain density, its matter will effectively disappear from the universe. Moreover, this dreadful hole or singularity will be hidden within a sphere of unknowingness that we now call the event horizon — or as the French used to call it, a sphère catastrophique. The English physicist Sir Arthur Eddington, who spearheaded the earliest successful attempt to prove a prediction of general relativity, was so appalled by these cosmic holes, he told a meeting of the Royal Astronomical Society in 1935 that “there should be a law of Nature to prevent a star from behaving in this absurd way!”
For fully a half-century, Bartusiak shows us, most physicists believed that black holes were just an artifact of the mathematics and that some physical process would ultimately intervene to prevent them from occurring in the real world. It took the inventions of radio astronomy and X-ray astronomy to produce concrete evidence that what the Russians termed “frozen stars” are not only real, they are a commonplace feature of the cosmos. As Bartusiak notes, “Nearly every fully developed galaxy appears to have a supermassive black hole at its center; it may be that the very existence of a galaxy depends on it.”

One should note here Bartusiak’s use of the word “appears,” for although there is now a vast amount of indirect evidence for black holes — including quasars and pulsars — we still lack direct evidence. That could, however, come soon if physicists detect the gravitational waves that theory predicts they should be emiting, at one of the several vast facilities built for this purpose.
Over the past century black holes have helped to reshape our fundamental understanding of the cosmos. Where the Newtonian universe was static, the Einsteinian universe is a dynamic, evolving and astoundingly active environment. What comes through so beautifully in this book is how theoretical physics is itself a dynamically evolving landscape. In the cosmic arena, black holes gobble up matter voraciously; they collide and explode and shoot out vast jets of energy. In physics departments, intellectual giants collide, tearing into one another’s ideas and shooting out jets of indignation. The stars on show here are both in the heavens and in the halls of academe. On either front it’s hard to think of a more historically contentious or more fun area of scientific inquiry.