Extraterrestrial Civilizations (14 page)

The Sun is a huge object compared to the Earth, or even compared to Jupiter. Its diameter is 1,392,000 kilometers (868,000 miles) or 110 times the diameter of the Earth. Its mass is 2 million trillion trillion kilograms, or 324,000 times the mass of the Earth. Nevertheless, it is not remarkable as stars go.

There are stars that are as much as 70 times as massive as the Sun and that shine a billion times as brightly. There are other stars that are only 1/20 the mass of the Sun (and are therefore only 50 times the mass of Jupiter) and that flicker with a light only one-billionth that of the Sun.

Roughly speaking, one must conclude that the Sun is an average star, about equally distant from the extremes of giant size and brilliance on one end of the scale and pygmy size and dimness on the other end of the scale.

If the stars were equally distributed all along the mass scale and if the Sun were really average, then we would assume that there were 150 billion stars in the Galaxy.

As it happens, however, the smaller stars are more numerous than the larger ones, so that it is fair to estimate that the average star is about half the size of the Sun in mass. (There are small stars in which matter is very compressed and which are very dense, but their mass is not unusually high and they do not affect the average.)

If, then, the total mass of the stars in the Galaxy is 150 billion times the mass of the Sun, and the average star is 0.5 times the mass of the Sun, then it follows that there are some 300 billion stars in the Galaxy. This means that for each visible star in the sky, each one a member of the Galaxy, there are 50 million other stars in the Galaxy that we cannot see with our unaided eyes.

Have we now come to an end? Are 300 billion stars all there are in the Universe? To put it another way, is the Galaxy all there is?

Suppose we consider two patches of luminosity in the sky that look like isolated regions of the Milky Way, and that are so far south in the sky as to be invisible to viewers in the North Temperate Zone. They were first described in 1521 by the chronicler accompanying Magellan’s voyage of circumnavigation of the globe—so they are called the Large Magellanic Cloud and the Small Magellanic Cloud.

They were not studied in detail until John Herschel observed them from the astronomic observatory at the Cape of Good Hope in 1834 (the expedition that fueled the Moon Hoax). Like the Milky Way, the Magellanic Clouds turned out to be assemblages of vast numbers of very dim stars, dim because of their distance.

In the first decade of the twentieth century, the American astronomer Henrietta Swan Leavitt (1868–1921) studied certain variable stars in the Magellanic Clouds. By 1912, the use of these variable stars (called Cepheid variables because the first to be discovered was in the constellation Cepheus) made it possible to measure vast distances that could not be estimated in other ways.

The Large Magellanic Cloud turned out to be 170,000 light-years away and the Small Magellanic Cloud 200,000 light-years away. Both are well outside the Galaxy. Each is a galaxy in its own right.

They are not large, however. The Large Magellanic Cloud may include perhaps 10 billion stars and the Small Magellanic Cloud only about 2 billion. Our Galaxy (which we may refer to as the Milky Way Galaxy if we wish to distinguish it from others) is 25 times as large as both Magellanic Clouds put together. We might consider the Magellanic Clouds as satellite galaxies of the Milky Way Galaxy.

Is this all, then?

A certain suspicion arose concerning a faint, fuzzy patch of cloudy matter in the constellation Andromeda; a patch of dim light called the Andromeda Nebula. Even the best telescopes could not make it separate into a conglomeration of dim stars. A natural conclusion was, therefore, that it was a glowing cloud of dust and gas.

Such glowing clouds were indeed known, but they did not glow of themselves. They glowed because there were stars within them. No visible stars could be seen within the Andromeda Nebula. The light from other luminous clouds when analyzed, however, turned out to be completely different from starlight; whereas the light of the Andromeda Nebula was exactly like starlight.

Another alternative, then, was that the Andromeda Nebula was a conglomeration of stars, but one that was even more distant than the Magellanic Clouds, so that the individual stars could not be made out.

When Thomas Wright had first suggested in 1750 that the visible stars were collected into a flat disc, he theorized that there might be other such flat discs of stars at great distances from our own. This idea was taken up by the German philosopher Immanuel Kant (1724–1804) in 1755. Kant spoke of “island universes.”

The notion did not catch on. Indeed, when Laplace developed his notion that the Solar system had formed out of a whirling cloud of dust and gas, he cited the Andromeda Nebula as an example of a cloud slowly whirling and contracting to form a sun and its attendant planets. That was the reason the theory was called the nebular hypothesis.

By the time the twentieth century opened, however, the old notion of Wright and Kant was gathering strength. Occasionally, stars did appear in the Andromeda Nebula, stars that were clearly “novas”; that is, stars that suddenly brightened several magnitudes and then dimmed again. It was as though there were stars in the Andromeda Nebula that were ordinarily too dim to see under any circumstances because of their great distances, but that, upon briefly brightening with explosive violence, became just bright enough to make out.

There are such novas, now and then, among the stars of our own Galaxy, and by comparing their apparent brightness with the brightness of the very dim novas in the Andromeda Nebula, the distance of the Andromeda could be roughly worked out.

By 1917, the argument was settled. A new telescope with a 100-inch mirror had been installed on Mt. Wilson, just northeast of Pasadena, California. It was the largest and best telescope that existed up to that time. The American astronomer Edwin Powell Hubble (1889–1953), using that telescope, was finally able to resolve the outskirts of the Andromeda Nebula into masses of very faint stars.

It was the “Andromeda Galaxy” from that point on.

By the best modern methods of distance determination, it would appear that the Andromeda Galaxy is 2,200,000 light-years distant, eleven times as far away as the Magellanic Clouds. No wonder it was difficult to make out the individual stars.

The Andromeda Galaxy is no dwarf, however. It is perhaps twice as large as the Milky Way Galaxy and may contain up to 600 billion stars.

The Milky Way Galaxy, the Andromeda Galaxy, and the two Magellanic Clouds are bound together gravitationally. They form a “galactic cluster” called the Local Group and are not the only members, either. There are some twenty members altogether. There is one, Maffei I, which is about 3,200,000 light-years away, and it is just about as large as the Milky Way. The remainder are all small galaxies, a couple with less than a million stars apiece.

There may be as many as 1.5 trillion stars in the Local Group altogether, but that isn’t all there are either.

Beyond the Local Group, there are other galaxies, some single, some in small groups, some in gigantic clusters of thousands. Up to a billion galaxies can be detected by modern telescopes, stretching out to distances of a billion light-years.

Even that is not all there is. There is reason to think that, given good enough instruments, we could make observations as far as 12 billion light-years away before reaching an absolute limit beyond which observation is impossible. It may be that there are 100 billion galaxies, therefore, in the observable universe.

Just as the Sun is a star of intermediate size, the Milky Way Galaxy is one of intermediate size. There are galaxies with masses 100 times larger than that of the Milky Way Galaxy, and tiny galaxies with only a hundred-thousandth the mass of the Milky Way Galaxy.

Again, the small objects of a particular class greatly outnumber the large objects, and we might estimate rather roughly that there are on the average 10 billion stars to a galaxy, so that the average galaxy is of the size of the Large Magellanic Cloud.

That would mean that in the observable universe, there are as many as 1,000,000,000,000,000,000,000 (a billion trillion) stars.

This one consideration alone makes it almost certain extraterrestrial intelligence exists. After all, the existence of intelligence is not a zero-probability matter, since
we
exist. And if it is merely a near-zero probability, considering that near-zero probability for each of a billion trillion stars makes it almost certain that somewhere among them intelligence and even technological civilizations exist.

If, for instance, the probability were only one in a billion that near a given star there existed a technological civilization, that would
mean that in the Universe as a whole, a trillion different such civilizations would exist.

Let us move on, though, and see if there is any way we can put actual figures to the estimates; or, at least, the best figures we can.

In doing so, let us concentrate on our own Galaxy. If there are extraterrestrial civilizations in the Universe, those in our own Galaxy are clearly of greatest interest to us since they would be far closer to us than any others. And any figures we arrive at that are of interest in connection with our own Galaxy can always be easily converted into figures of significance for the others.

Begin with a figure that deals with our Galaxy and divide it by 30 and you will have the analogous figure for the average galaxy. Begin with a figure that deals with our Galaxy and multiply it by 3.3 billion and you have the analogous figure for the entire Universe.

We start then with a figure we have already mentioned:

The existence of the stars themselves, in no matter how huge a number, does not guarantee the existence of civilizations, or even of life, if
only
stars exist. The stars supply the necessary energy, but life must develop at a temperature compatible with the existence of the complex organic compounds that are the chemical basis of it.

This means that there must be a planet existing in the neighborhood of the star. On that planet, warmed and, in general, energized by that star, life might conceivably exist.

We must therefore not consider stars, but planetary systems—of which our own Solar system is the only example that we know definitely and in detail.

Unfortunately, we cannot observe the neighborhood of any star other than that of our own Sun with sufficient minuteness to be able to detect, directly, the presence of planets circling them.
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Does this defeat us at the start and make it impossible to come to any further conclusions as to the existence of extraterrestrial intelligence?

Not necessarily. If we can determine how our own Solar system was formed, we might be able to draw conclusions as to the probability of the formation of other planetary systems.

For instance, the first theory of Solar system formation that many astronomers found attractive was Laplace’s nebular hypothesis, which I mentioned earlier in the book. (Actually, something like it had been advanced by Kant in 1755, a half-century before Laplace.)

If the Sun had formed out of the condensation of a spinning cloud of dust and gas (and we can see many such clouds in our Galaxy and in some other galaxies as well), it is reasonable to suppose that other stars formed in the same way.

Since our Sun, as it condensed, could be pictured as spinning faster and faster and losing rings of material from its equatorial region—one ring after another—thus forming the planets, other stars as they formed would do the same.

In that case, every star would have a planetary system.

We could not, however, come to that conclusion on the basis of the nebular hypothesis unless that theory of planetary formation could withstand close examination, and it didn’t.

In 1857, Maxwell (who later worked out the kinetic theory of gases) was interested in reasoning out the constitution of Saturn’s rings. He showed that if the rings were solid structures (as they seemed to be in the telescope) they would be broken up under the influence of Saturn’s gravitational pull. It seemed, therefore, that they must consist of a large aggregate of relatively small particles, so thickly strewn as to seem solid when viewed from a great distance.

Maxwell’s mathematical analysis turned out to be applicable to the ring of dust and gas supposedly shaken loose by the contracting nebula on its way to condensation into the Sun. It turned out that if Maxwell’s mathematics was correct, it was difficult to see how such a ring would condense into a planet. It would at best form an asteroid belt.

An even more serious objection arose out of a consideration of angular momentum, which is the measure of the turning tendency of any isolated body or system of bodies.

Angular momentum depends on two things: the speed of each particle of matter as it rotates about an axis, or revolves about some distant body, or both; and the distance of each particle of matter from the center of rotation. The total angular momentum of an isolated body can’t vary in quantity, no matter what changes take place in the system. That is called the law of conservation of angular momentum. By this law, the velocity of spin must increase to make up for any decrease in distance, and vice versa.

A figure skater demonstrates the principle when she or he begins spinning with the arms outstretched, and then draws those arms in. At this condensation of the human body, so to speak, the rate of spin rapidly increases, and if the arms are then outstretched, it as rapidly slows down again.