Stephen Hawking (16 page)

It all began with the discovery of quasars in 1963. The quasar story actually began on the last day of 1960. During the 1950s, astronomers using telescopes sensitive to radio waves rather than visible light had identified many objects in the Universe that produce a lot of radio noise. Some of these objects were also visible as bright galaxies and were known as radio galaxies, but others had not yet been identified with any known visible object. Then, at the end of 1960, the American astronomer Allan Sandage reported that one of the radio sources (known as 3C 48) discovered during a survey carried out by radio astronomers in Cambridge, England could be identified not with a distant galaxy but with what seemed to be a bright star. More of these radio “stars” were identified over the next few years, but nobody could explain how they produced the radio noise. Then, in 1963, Maarten Schmidt, working at the Mount Palomar Observatory in California, explained why another of these objects, known as 3C 273, had a very unusual spectrum.

All stars (and other hot objects) reveal their composition by the nature of the light they emit. Each kind of atom, such as hydrogen, helium, or oxygen, absorbs or emits energy only at very precise wavelengths, because of the quantum effects mentioned in
Chapter 2
. So when light from a star or galaxy is spread out, using a prism, into a spectrum, we see that the
spectrum is crossed by a series of dark and bright lines at different wavelengths, corresponding to the presence of atoms of different elements in the atmosphere of the star (or in the stars that make up the galaxy). These spectral lines are as characteristic as fingerprints, and for a particular type of atom they are always produced at the same distinctive wavelengths.

Astronomers already knew, though, that these spectral lines are shifted a little bit toward the red end of the spectrum in the light from galaxies outside the Milky Way. This famous “redshift” is caused by the expansion of the Universe, which stretches space, and therefore stretches the wavelength of light
en route
to us from a distant galaxy. Indeed, it was the discovery of the redshift that told astronomers the Universe must be expanding, just as Einstein's equations had predicted, but Einstein himself had at first refused to believe it.

The fact that light from 3C 273 was redshifted—the discovery Maarten Schmidt made—was not a surprise; but the size of the shift, nearly 16 percent toward the red end of the spectrum, astonished astronomers in 1963. Typical redshifts for galaxies are much less than this, about 1 percent, or 0.01. With the realization that such large redshifts were possible, other radio “stars” were re-examined, and it turned out that they all showed similar—or even larger—shifts. 3C 48, for example, has a redshift of 0.368 (nearly 37 percent), more than twice that of 3C 273, and the record redshift now stands above 4 (in other words, the light from the most distant quasars known is stretched to more than four times its original wavelength).

In the expanding Universe, redshift is a measure of distance (the farther light has to travel on its way to us, the more it will be stretched by Universal expansion). So these objects were
not stars at all, but something previously unknown—objects that looked like stars but were far away, in most cases farther away than the known galaxies. They soon became known as quasi-stellar objects, or “quasars.”

In order to be visible at all at the huge distances implied by their redshifts, quasars must produce prodigious amounts of energy. A typical quasar shines with the brightness of three hundred billion stars like the Sun, three times as bright as our whole Milky Way Galaxy. Having sought in vain to find any alternative means to explain the power of quasars, astronomers were reluctantly forced to consider the possibility that they might be black holes. We now know that each quasar is a black hole containing at least a hundred million times as much mass as our Sun, contained within a volume of space with about the same diameter as our Solar System. (This is just the kind of large, low-density black hole described in
Chapter 5
.) Each one actually lies at the heart of an ordinary galaxy and feeds off the stellar material of the galaxy itself. Ever-improving telescope technology has enabled us, in many cases, to photograph the surrounding galaxy itself, faint alongside the quasar.

Although a hundred million solar masses is large by everyday standards, this still represents only one-tenth of 1 percent of the mass of the parent galaxy in which a quasar lurks. When such an object swallows matter, as much as half the mass of the matter can be converted into energy, in line with Einstein's famous equation
E = mc
²
. As we saw in
Chapter 5
, the factor
c
²
is so huge that this corresponds to a vast amount of energy. This process of energy production is so efficient that, even if only about 10 percent of the infalling mass is actually converted into energy, a quasar can shine as brightly as
three hundred billion Suns, bright enough to be seen across the vast reaches of intergalactic space, if it is swallowing just one or two solar masses of material every year. The material forms a great, hot, swirling disc around the black hole itself. This disc is where the energy that produces the radio noise, and the visible light, comes from, even though the hole itself, as the name implies, is black. And with a hundred billion stars to eat, even if a quasar only dines off 1 percent of the mass of the parent galaxy, it can shine that brightly for a billion years.

The existence of quasars shows that large, low-density black holes really do exist. In 1967, just four years after the redshift of 3C 273 was measured, the Cambridge radio astronomers achieved another breakthrough with the discovery of the rapidly varying radio sources that became known as “pulsars.” And although pulsars are not themselves black holes, they opened the eyes of most astronomers to the possibility that super-dense, compact black holes might also really exist, just as the general theory of relativity predicted.

The first pulsars were discovered by a research student, Jocelyn Bell, while testing a new radio telescope. The astonishing thing about these radio sources is that they flick on and off several times a second (some of them several
hundred
times a second) with exquisite precision. This is so much like an artificial signal, a kind of cosmic metronome, that, only half-jokingly, the first pulsars discovered were labeled “LGM 1” and “LGM 2”—the initials “LGM” stood for “Little Green Man.” As more of them were discovered, though, it
became clear that there were far too many to be explained as interstellar traffic beacons set up by some super-civilization, and the accepted name became pulsar, from a contraction of “pulsating radio source” and because the name chimed with quasar.

But what natural phenomenon could produce such regular, rapid pulses of radio noise? There were only two possibilities. The pulses had to signal either the rotation or the vibration of a very compact star. Anything bigger than a white dwarf would certainly rotate or vibrate too slowly to explain the speed of the known pulsars, and rotating white dwarfs were soon ruled out—a simple calculation showed that a white dwarf rotating that fast would break apart.

For a short time early in 1968, it seemed that vibrations of a white dwarf, literally pulsing in and out, might explain the variations in the radio noise from pulsars. But it was fairly straightforward to calculate the maximum rate at which a white dwarf could pulsate without breaking apart. Indeed, one of us (J.G.) did exactly that as part of the work for his Ph.D. The answer was disappointing (for him) but conclusive: white dwarfs simply cannot pulsate at the required speed, which meant that the stars involved in the pulsar phenomenon must be even more compact, and denser, than white dwarfs.

They must, in short, be neutron stars, predicted by theory but never previously discovered. Within months of the announcement of the discovery of pulsars, it was established that these objects are actually rotating neutron stars, definitely within our Galaxy, producing beams of radio noise that sweep past us like the flashing beams of a lighthouse. They are created by supernovas, explosions of giant stars. And, as theorists
were well aware from the outset, the same theory that predicted the existence of neutron stars, a prediction which had been largely ignored for thirty-odd years, also predicted that if you added just a little more mass to a neutron star (or by having a little more debris left over from a supernova explosion), you would create a collapsar.

It is no coincidence that John Wheeler coined the term “black hole” in this connection the year following the discovery of pulsars, for the realization that pulsars must be neutron stars triggered an explosion of interest in the even more exotic predictions of the general theory of relativity. That explosion had already been primed, however, by yet another discovery made using radio telescopes, which had confirmed the reality of the Big Bang itself.

When the Universe was more compressed, it was hotter, just as the air in a bicycle pump gets hot when it is compressed. The Big Bang was a fireball of radiation in which matter initially played an insignificant role. But as the Universe expanded and cooled, the radiation faded away, and matter, in the form of stars and galaxies, came to dominate the scene.

All this was known to astronomers in the 1940s and 1950s. George Gamow and his colleagues even carried out a rough calculation of what temperature this leftover radiation would have cooled to by now. In 1948 they came up with a figure of about 5 K (
minus
268°C). By 1952 Gamow was inclined to think that it might be rather higher, and in his book,
The Creation of the Universe
, he said that the temperature ought
to be somewhere below 50 K. But 5 K or 50 K, it was still a very low temperature, and in the 1950s nobody seriously contemplated the possibility of trying to detect this echo of creation, a cold background sea of radiation filling the entire Universe and left over from the Big Bang.

In the early 1960s, though, the possibility of actually measuring the strength of this background radiation, and thereby testing the Big Bang model, occurred to a few astronomers. One way of understanding how and why the radiation has cooled is in terms of redshift. Radiation that filled the Universe in the Big Bang still fills the Universe; but because space has stretched since then, the waves making up that radiation have had to stretch accordingly in order to fill the space available. This means that energy that started out in the form of X-rays and gamma rays would now be in the form of microwaves, with wavelengths of around 1 millimeter or so. These are just the kind of radio waves used in some communications links and in radar. With the technology developed for radar and radio communications, and the associated rapid development of radio astronomy, researchers in both the Soviet Union and the United States saw that the background radiation predicted by the Big Bang model might be detectable, and they set about designing and building radio telescopes to do the job.

But they started just too late. The American team, based at Princeton University, was headed by Robert Dicke, who had worked in radar during the Second World War. In the early 1960s he gave a team of young researchers the task of building a microwave background detector using an updated version of equipment he had helped to design during the war.
By 1965, things were progressing nicely when Dicke received a phone call from a young researcher at Bell Laboratories, just 30 miles away from Princeton. The caller, Arno Penzias, wanted Dicke's advice about some puzzling radio interference that Penzias and his colleague Robert Wilson had been getting on their radio telescope at Bell Labs since the middle of 1964.

Penzias and Wilson had, in fact, been using an antenna designed for use with the early communications satellites, modified to operate as a radio telescope. They found that, wherever they pointed the telescope in the sky, they seemed to be getting a signal corresponding to microwave radiation with a temperature of just under 3 K. After trying everything they could to sort out what was wrong with the telescope (including cleaning pigeon droppings off the antenna, in case that was what was causing the interference), they gave up and called Dicke, an expert on microwaves, to ask if he had any idea what was going on.