This book began with a parable about a village that learned how to measure all the way to a far-off mountain. Because of a mistaken assumption—that the vegetation on the mountaintop was the same as that on the valley floor—the inhabitants underestimated the distance, only learning of their mistake after they sent an expedition there.
There is a coda to the story. Much later in their history, the villagers established a scientific outpost on the mountain. They built high towers and learned to concentrate light with tubes outfitted with curved lenses and mirrors. Looking out at the unknown expanse beyond them, they realized that their explorations had barely begun. Their mountain was merely a hillock. What lay on the new horizon was a peak that, magnified many times, was of breathtaking grandeur.
It too was fringed with green, and this time, to avoid fooling themselves, the villagers used the average height of their own vegetation as a standard yardstick. No more mistaking asters for trees. While the mountain they were standing on was a thousand canyon widths from the village, this new mountain appeared to be approximately a thousand times farther still. This place, they knew, would not be visited in their lifetime, and probably not within the lifetime of their people.
One night up in the tower, one of the scientists saw a brilliant light on the horizon. The remote mountain had exploded in flames. Measuring the intensity of the light, the scientist did some calculations. From the distance to the mountain and its apparent size, he had already estimated how big it was. Now he calculated how much light would be produced if the mountain had caught on fire.
The answer didn’t make sense. The flames were so brilliant that they would have to be far more intense than anything resembling ordinary combustion.
When he reported his finding to his colleagues back in the village, they offered several hypotheses. One proposed that some peculiarity of the air may have magnified the light, acting like a natural lens, but few thought that was plausible. More popular was the theory that fire in the distant land burned much hotter, that they had discovered a new kind of energy.
The scientist who had made the observation had a different idea: that this time they had somehow overestimated the distance. If the mountain was really a hundred times closer, the anomaly could be explained away....
SOMETHING LIKE THIS happened here on earth. It was 1963 and Maarten Schmidt, a Mount Palomar astronomer, had just ascertained that a starlike (“quasi-stellar”) object called 3C273 showed a redshift that would put it several billion light-years from earth, as far as some of the most distant galaxies.
Other quasars were soon found to have even more severe redshifts. They appeared to be receding from our part of space at a velocity almost as great as light. The Hubble law put them nearly at the edge of the visible universe. For something so remote to shine so brightly, it would have to be emitting the light of thousands of galaxies, the energy generated, perhaps, by matter pouring into the intense gravitational field of a black hole.
Whatever the cause, accepting the immense distance of these fantastic objects caused all kinds of trouble. The quasar 3C273 (the 273rd entry in the Third Cambridge Catalog of Radio Sources) expels from its core a jet of light that appears to be traveling at several times the speed of light. That of course would be impossible. Astronomers quickly came up with a more palatable explanation: the superluminal motion is probably an illusion. The jet happens to be coming almost straight at us, making it appear to be much faster than it really is.
There is however another possibility: that 3C273 and all the quasars are really very near by. The velocity of the jet would then be much, much smaller. That also would solve another problem. If the quasars are close to us, then we need not conclude that they are so fantastically bright.
For this to be true, redshift would have to be caused by something other than Hubble expansion—maybe by the old “tired light” theory or some other new physics. If so, the entire universe might be vastly smaller, and there may not have been a big bang.
The idea that the redshifts are “noncosmological” is, to say the least, a minority view. Most astronomers are persuaded by a tightening net of circumstantial evidence that quasars really are blinding beacons lying near the edge of what it is possible to see.
One of the strongest arguments involves a weird phenomenon called gravitational lensing. Sometimes astronomers see two quasars, one right next to the other. The doubling, however, is believed to be an illusion. According to the theory of general relativity, gravity can bend light. Something as massive as a galaxy can act like an enormous piece of curved glass, projecting a double image. If all that is true, then the quasar must be behind the galaxy not in front of it, and therefore very far away.
With each step outward, the act of measurement becomes a little more abstruse. With arithmetic and a ruler you can get from the desk to the window, with trigonometry and a transit you can get to the moon and, with a few assumptions, to the nearest stars.
“With increasing distance, our knowledge fades, and fades rapidly,” Hubble once said, in a rare moment of oratorial eloquence. “Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and search among ghostly errors of measurements for landmarks that are scarcely more substantial.”
Establishing the distance of the quasars requires not only the Hubble law, but the entire framework of Einsteinian relativity. Measuring began as a way to gather data to verify theories. Now the measuring stick itself has become one more theory to test.