The Story of Astronomy (25 page)

If we could build a spaceship to take us close to a black hole we would no doubt learn much more about these objects than we know at present. Of course, this is not a scenario within our current means of technology, and it is hard to envisage how it ever could be. Nevertheless, by studying black holes with the tools available to us we can still deduce much about these mysterious objects,
even if we cannot get physically close to one of them. So, with such knowledge as we already possess, let us go on an imaginary journey to a black hole instead. We shall be visiting a single black hole—in other words, one with no orbiting companions. As we approach the black hole there will be changes in the space and time around it, although we may not be able to detect them due to relativity. So long as our spaceship keeps at a safe distance there is no immediate danger to us. We can orbit around the black hole just as we can orbit around the Earth.

Imagine now that we dispatch an astronaut in a space pod to make a closer approach to the black hole and to try to get inside it. To help us track his movements the astronaut sends us a pulse every second. As he approaches the event horizon, the boundary of the black hole where escape velocity is equivalent to light speed, we will notice that the interval between the pulses becomes longer and longer. This is because the space around the black hole is distorted so that time passes at a slower rate. We observe that the space pod and the astronaut are subjected to what are called tidal forces by the gravity of the black hole. This means that the part of the space pod nearer to the center of the black hole is subjected to a greater force than the part that is further away. The astronaut reaches
a point where these forces impose a great stress on his pod. However, it is still not too late for him to return to the main spacecraft, provided he is still outside the event horizon, and can fire his rockets to travel at almost the speed of light. If he continues on his journey, then he and his space pod will be drawn inexorably into the black hole.

According to theoreticians the astronaut in the space pod will perceive events quite differently from those outside, again due to relativity. He is in his own space–time zone, distorted by the strong gravitational field which makes time pass relatively more slowly. Once the pod actually enters the black hole there is no way for the astronaut to communicate his findings to those outside and there is also no way for him to get out again into the universe he has left behind.

English mathematical physicist Roger Penrose (b. 1931) and English theoretical physicist Stephen Hawking (b. 1942) have shown that there is a singularity (a point at which density and temperature go to infinity) at the center of the black hole, but we know nothing else about the space inside it. Space and time are stretched at the boundary of the hole such that the space inside is infinite. If the black hole is very massive then, although the gravitational field is stronger, the tidal forces are weaker. We would like to think that by entering into the
black hole we could enter a new universe, similar to our own with stars and galaxies and habitable planets. And as we shall see later, there are even some who believe our own universe itself could in fact be located within a black hole. Science fiction writers have suggested that the black hole could be a wormhole—a connection between two very distant parts of the universe that would provide an ideal way of crossing the great distances between the stars. However, this idea remains in the realms of science fiction and is not a view held by astronomers.

In the late 1950s astronomers at Cambridge in England were putting together a catalog of all the radio sources in the sky. Sometimes it was possible to follow up the radio source with an optical sighting, but the two are not always compatible and the radio emitters are not normally bright optical stars. In 1960 the American astronomer Allan Sandage (b. 1926) used the 5-meter (200-inch) Palomar telescope to study the star known as 3C 48 in the Cambridge catalog. The star had some unusual features. Its spectrum showed strange emission lines that could not be identified from comparison with those of known elements. In 1962 another radio star called 3C 273 was discovered. It was unusual in that a long luminous jet was clearly visible in its optical image. Astronomers
were puzzled by the strange emission lines that it also showed. In 1963, however, the Dutch astronomer Maarten Schmidt (b. 1929), working at the California Institute of Technology, was able to demonstrate that the unidentified lines in the spectra were in fact the well-known hydrogen lines. They had simply not been recognized due to the very high degree of redshift they were exhibiting. The objects were given the name “quasars,” an abbreviation for “quasi-stellar objects.” Furthermore, they did not originate in our own galaxy; judging from their redshift they were traveling away from us at an unprecedented speed and they were therefore an unbelievable distance away. In fact it is no exaggeration to say that they were the most distant objects ever observed in the sky. The quasar 3C 273 was estimated to be at a distance of two billion light years.

The most distant galaxies studied before the discovery of quasars were not, after all, the end of the universe. The quasars were located in galaxies much further away, and because we were able to detect them from such a great distance it followed that they must have a very powerful source of energy. Once the first quasars had been identified, astronomers began to look for more objects with very large redshifts. They found that there were thousands of quasars visible with the most powerful telescopes. Redshifts of over 90 percent have been measured,
compared with measurements of only 2 or 3 percent for the galaxies. It indicates that these quasars are many times further away. In fact, the most distant quasars are estimated to be 13 billion light years away. This means that we are observing them close to the time of the Big Bang and the origins of the universe. When the quasars came under close scrutiny it was discovered that they were located in galaxies, showing that the most distant galaxies were much further away than was originally thought.

When quasars came to be studied more closely it was found that their brightness was not constant, but the variations from the norm were usually very short-lived. Sometimes the quasars could flare up to be as bright as 100 galaxies—another indication that they must have a very compact but powerful source of energy to be so highly visible at such a great distance. There is only one power supply that could satisfy such a voracious appetite for energy. The evidence suggests that a quasar must contain a very massive black hole at its center. The quasars are objects so powerful that they consume stars. Millions of stars must have been swallowed up to create the massive black holes that power the quasars, and when they devour a new star the energy released is so great that we see the flare of its death throes from the Earth. With the quasars we seem to have reached the very limits of the universe, but we have been wrong about this so many times that
nobody can be sure. There was a time when the edge of the universe was thought to be the edge of the Earth, then it was thought to be the furthest part of the solar system, then it became the Milky Way. In the early 20th century the limit was assumed to be the most distant galaxies, now we think the quasars are at the end of the universe—but we cannot even say this with too much confidence.

Using the Spitzer Space Telescope infrared spectrograph instrument, scientists found a wealth of dust grains in a quasar called PG 2112+059 located at the center of a galaxy eight billion light years away. The grains, which include sapphires, rubies, peridot and periclase (naturally occurring in marble), are not normally found in galaxies without quasars, suggesting they might have been freshly formed in the quasar's winds.

These findings are another clue in an ongoing cosmic mystery: where did all the dust in our young universe come from? Dust is crucial for efficient star formation as it allows the giant clouds where stars are born to cool quickly and collapse into new stars. Once a star has formed, dust is also needed to produce planets. Theorists had predicted that winds from quasars growing in the centers of distant galaxies might be a source of this dust. While
the environment close to a quasar is too hot for large molecules like dust grains to survive, dust has been found in the cooler, outer regions. Astronomers now have evidence that dust is created in these outer winds.

The universe is often described as a four-dimensional space–time continuum, consisting of the well-known three spatial dimensions that we experience (length, width and depth), plus another dimension—the time dimension—that has a special significance in Einstein's special relativity theory. Ignoring the time dimension for the moment, there is some evidence that the three spatial dimensions of the universe do not extend to infinity; the universe may be confined in a closed space in the same way that the sphere of the Earth is enclosed by its surface.

Imagine a two-dimensional being at the North Pole. We have drawn a set of concentric circles—lines of latitude—for him or her about the pole. These circles get larger and larger as they progress south, eventually reaching a maximum width at the equator. From there, the circles grow smaller and smaller until they reach the South Pole, at exactly the opposite point from the equator as the North Pole. If our two-dimensional traveler heads south from the North Pole he will eventually reach the South Pole, and if he still keeps going in the same direction he will
eventually come back again to the very point he started from at the North Pole.

Now imagine we stop the Earth's rotation and delete the latitude and longitude lines so that the North and South Poles do not hold any privileged position on the sphere. A voyage around the world can start from any chosen point on the surface and the traveler, provided he can keep on a straight course, can travel round the world and arrive back at the point from where he started. The traveler can claim that his starting and finishing point is the center of the world. We, as three-dimensional beings, can see that the claim is not valid; any point on the surface of the sphere can claim to be at the center. We can see the whole sphere and we know that although the traveler's world does have a center, it is not on the surface of the Earth. The center does not have a latitude or longitude. It is in a third dimension at the center of a sphere. If our two-dimensional being is clever enough, he may be able to deduce that his space is curved. The traveler defines a straight line as the shortest distance between two points; it is what we know as a great circle. He knows that in a flat space the angles of a triangle add up to 180 degrees or two right angles. He draws a very large triangle using great circles for the sides and discovers that the angles add up to more than 180 degrees. If he takes two points on the equator, separated by 90 degrees of longitude,
and joins them both to the pole, then he can create an equilateral triangle whose angles add up to three right angles. He would also find, if he were to draw a very large circle, that its circumference is less than the value given by the familiar formula 2π
r
. The radius of the equator, for example, is the same as the radius of the Earth from our viewpoint, but for our two-dimensional being the radius would be a line from the North Pole to the equator and the circumference of the circle would not be multiplied by the radius, it would be four times the radius. In theory, the traveler could use this result to calculate the size of his spherical world.

Thus the surface of the Earth can be thought of as a two-dimensional world needing a third dimension to explain it. The universe, excluding the time dimension, is a three-dimensional world, but if it is confined within a boundary then it needs a fourth dimension to explain where the boundary lies. We can define a three-dimensional “spherical” space as the boundary of a four-dimensional super-sphere. We are not capable of envisaging where the center of this super-sphere lies, but mathematicians can deduce many of its properties. The circle of radius
r
, for example, can be represented by the equation
x
²
+
y
²
=
r
²
, the sphere with the same radius can be written as
x
²
+
y
²
+
z
²
=
r
²
and the four-dimensional “super-sphere” of radius
r
can be expressed by the equation
x
²
+
y
²
+
z
²
+
w
²
=
r
²
where the four axes, (
x, y, z, w
) are all at right angles to each other. From this the mathematicians can work out all the properties of the super-sphere. If we cut it with the plane “
w
= 0,” for example, then we find the three-dimensional section is the sphere
x
²
+
y
²
+
z
²
=
r
²
. We can work out all the properties of the super-sphere such as volumes, areas, distances and angles.

Now we are ready to send out two intrepid observers from our planet to the most distant quasars. We will remain safely on Earth. When our observers arrive at their destination they, and we, will measure the angles of the triangle we have formed. If the angles add up to more than two right angles then this shows that the universe is closed. If we know the distances then we could make a measure of the curvature of the universe and we could calculate how far our two distant observers would need to travel to get back again to the point from where they had started. If the angles of our triangle add up to exactly two right angles then the universe is perfectly flat and it extends to infinity in all directions. If the angles add up to more than two right angles then the universe is divergent; the two-dimensional analogy is the surface of a saddle where the perimeter of a circle, for example, is greater than 2π times its radius.