ORIGIN OF LIFE ON EARTH

How did life originate on Earth? There are various theories, most of which fall into the four classes: special creation theories, spontaneous creation theories, panspermia theories, and biochemical theories.

Special creation theories

The belief that life originated as a supernatural event is the metaphysical theory of special creation. It has numerous mythic variations. Most recorded myths distinguish between nonliving and living things. Often the nonliving world comes first, the living world follows, and creation is thus a twofold act. Catastrophe theories of the eighteenth and nineteenth centuries elaborated on such beliefs and proposed that many acts of creation had occurred in the past. After a catastrophe had destroyed the terrestrial environment, newly created life arose in more evolved forms, and evolution occurred supernaturally, not naturally. Organic life even in its rudest forms was thought to be composed of substances fundamentally different from those of nonliving things. To this day we speak of organic and inorganic chemistry, although this distinction is now a matter of convenience only, and organic chemistry deals mostly with the numerous compounds containing carbon atoms. It came as a shock when the chemist Friedrich Wöhler in 1828 first made urea (a simple organic substance) from inorganic chemicals. Subsequent developments showed that chemicals are interchangeable between inorganic and organic things, thereby unifying the living and nonliving worlds at the atomic and molecular levels.

OUR GALAXY

Milky Way

Our Galaxy, an enormous system of clouds of glowing gas and 100 billion stars, is also known as the Milky Way. Light takes 100 000 years to cross the Galaxy from side to side, and the center of the Galaxy lies in the constellation of Sagittarius, obscured from view by clouds of dusty gas that drift among the stars. Far from the center of the Galaxy is our own star the Sun.

The disk and halo

The Galaxy consists of two basic components: disk and halo (see Figures 6.1 and 6.2). The Milky Way is actually our panoramic view of the disk that has a diameter of about 100 000 light years and a thickness of about one-twentieth, or less, of the diameter. The disk is composed of stars and interstellar gas, and contains over half the visible mass of the Galaxy. The gas amounts to one-tenth of the matter in the disk, and the dust amounts to about 1 percent or more of the mass of the gas. The disk of stars, gas, and dust rotates about the center, or nucleus, of the Galaxy like a giant carousel.

STATIC NEWTONIAN UNIVERSE

Until the 20th century everybody believed that the universe is naturally static: not expanding and not contracting. Even Albert Einstein, after the discovery of general relativity, continued to hold this belief for several years.

In the late 17th century, belief in a static order remained unshaken when Newton advanced the theory of universal gravity. In response to a question in a letter from the young clergyman Richard Bentley (Chapter 3), Newton wrote in reply that in an infinite universe it would be impossible for all matter to fall together and form a single large mass, but “some of it would convene into one mass and some into another, so as to make an infinite number of great masses, scattered at great distances from one to another throughout all that infinite space.”

The Newtonian theory of universal gravity, in which all bodies attract one another, reinforced the growing belief that the universe must be edgeless and therefore infinite. For if the universe were finite and bounded by a cosmic edge, it would have a center of gravity, and the attraction between its parts would cause it, said Newton, to “fall down into the middle of the whole space, and there compose one great spherical mass.” This argument led him finally to abandon the finite Stoic cosmos in favor of the infinite Atomist universe.

STATIC UNIVERSES

Before the twentieth century most Europeans and people of European descent believed the universe was created only a few thousand years ago, or at most a few hundred thousand years. Some people in the 18th and 19th centuries, more radical in outlook, thought the static Newtonian universe was in a steady state – everything remaining eternally unchanged – and the stars would shine endlessly. The realization in the late 19th century that stars have finite energy resources brought to an end the idea of a perpetually unchanging cosmos.

Static Einstein universe

In 1917, Einstein contrived an ingenious static universe using his recently developed theory of general relativity. In this universe, as in all universes we discuss, all places are alike and matter is distributed with uniform density.

Space and time in the new theory of general relativity had at last been awakened from the dead and become active participants in the world at large. Einstein, believing the universe to be static, tranquilized spacetime with a counteracting agent. In his 1917 paper, “Cosmological considerations on the general theory of relativity,” he wrote, “I shall conduct the reader over the road I have myself traveled, rather a rough and winding road, because otherwise I cannot hope that he will take much interest in the result at the end of the journey. The conclusion that I shall arrive at is that the field equations of gravitation that I have championed hitherto still need a slight modification.”

After Newton, astronomical advances in observation and theory were at first slow. Better telescopes had yet to be developed, photography and spectroscopy introduced into astronomy, and the chemical compositions and radial velocities of stars and nebulae determined. The puzzling nature of the nebulae had yet to be resolved, nebulae in the Galaxy to be distinguished from extragalactic nebulae, distance indicators to be found and calibrated, globular clusters to be identified as systems of stars lying in and on the outskirts of the Galaxy, and the confusing obscuration of starlight caused by interstellar absorption to be recognized. All this would be accomplished and accompanied by continual debate over controversial issues from the time of Newton to the time of Einstein during the eighteenth, nineteenth, and early twentieth centuries.

THE DISTANT STARS

Light travel time

We look out from Earth and see the Sun, planets, and stars at great distances (see Figure 5.1). The Sun, our nearest star, is at distance 150 million kilometers or 93 million miles. Kilometers and miles, suitable units for measuring distances on the Earth's surface, are much too small for the measurement of astronomical distances (see Table 5.1).

Almost all information from outer space comes to us in the form of light and other kinds of radiation that travel at the speed 300 000 kilometers per second (see Table 5.2). Light from the Sun takes 500 seconds to reach the Earth, and we see the Sun as it was 500 seconds ago. We say the Sun is at distance 500 light seconds. The time taken by light to travel from a distant body is called the light travel time. Light travel time is an attractive way of measuring large distances and has the advantage that we know immediately how far we look back into the past when referring to a distant body. A star 10 light years away (almost 100 trillion kilometers) is seen now as it was 10 years ago. Always, when looking out in space, we look back in time.

THE LOCATION PRINCIPLE

The Greeks developed the “two-sphere” universe that endured for 2000 years and consisted of a spherical Earth surrounded by a distant spherical surface (the sphere of stars) studded with celestial points of light. This geocentric picture was finally overthrown by the Copernican revolution in the sixteenth century and replaced by the heliocentric picture with the Sun at the center of the cosmos. The sphere of stars remained intact. But revolutions, once begun, do not readily stop, and by the seventeenth century the heliocentric picture had also been overthrown. Out of the turmoil of the revolution emerged an infinite and centerless universe that ever since has had a checkered history. In the eighteenth century the idea arose of a hierarchical universe of many centers, and in the nineteenth came the idea of a one-island universe – the Galaxy – in which the Sun had central location. Once again, in the twentieth century, we have the centerless universe.

EUCLIDEAN GEOMETRY

In the ancient Delta civilizations, geometry was the art of land measurement, and indispensable in the construction of such mammoth works as the Great Pyramid of Giza and Stonehenge. Geometry at first consisted of trial-by-error and rule-of-thumb methods. According to the sacred Rhind Papyrus, the Egyptians of 1800 bc used for π, the ratio of the circumference and the diameter of a circle, the value (16/9)2 =3.1605, as compared with its more exact value 3.1416. The Babylonians of 2000 bc and the Chinese of 300 bc used the rule that the circumference of a circle is three times its diameter, and this value for π is found in Hebraic scripture. The Greeks, in their thorough fashion, developed geometry into a science that climaxed in the axiomatic and definitive treatment presented by Euclid at the Museum in Alexandria in the third century bc.

Axioms

The axiomatic method starts with a set of self-consistent propositions (called postulates or axioms), which are often the simplest and most obvious truths, and examines their logical consequences. Suppose that we wish to persuade someone that statement S is true. We might try to show that this statement follows logically from another statement R that the person already accepts. But if the person is unconvinced of the truth of R, then we must try to show that R follows logically from yet another statement Q.

PRINCIPLE OF EQUIVALENCE

Gravitational and inertial forces produce effects that are indistinguishable – this is the principle of equivalence. It serves as an essential stepping-stone to the theory of general relativity, and makes a basic connection between motion and gravity. It leads to a second stepping-stone: the realization that geometry and gravity have much in common. Then, in an inspired leap across the gulf of non-Euclidean geometry, we enter a country into which comparatively few explorers have ventured. No person entering the third millenium may claim to have a liberal education who has not glimpsed, however briefly, the universe of general relativity.

An inertial force, such as centrifugal force, exists when a body is accelerated. We recall from Newtonian theory that when a body is in free fall, and hence moves freely in space under the influence of gravity, it follows a path of such a kind that the sum of the inertial and gravitational forces is zero. With items of knowledge such as these, sufficient to land men on the Moon, we have made our first step toward the theory of general relativity.

NEW IDEAS FOR OLD

Old Ideas

Newtonian space and time were public property, which all observers shared in common. Its intervals of space and intervals of time separating events were absolute. They were the same for everybody. One person in an apple orchard would see an apple fall from a tree and take 1 second to drop 5 meters. Another person in motion relative to the tree also would see it drop 5 meters in 1 second, no matter how fast that person moved. Now things have changed. The old Newtonian universe, with its ideas on the fixity of intervals of space and time, is no longer the universe in which we live.

Space-and-time diagrams, displaying events and world lines, were used in the Middle Ages, and there is nothing particularly frightening or difficult about them. Until the beginning of this century they were a convenient graphical way of representing things in motion. Then came the theory of special relativity, and diagrams of this kind acquired a new physical meaning.

New ideas

The theory of special relativity emerged toward the end of the nineteenth century and was brought into final form in 1905 by the genius of Albert Einstein. It has withstood countless tests and is now in everyday use by physicists. Yet even nowadays, when we pause to reflect, the theory is as astonishing as when it first emerged.

THE UNIVERSE IN A NUTSHELL

Reflecting walls

We look out in space and back in time and the things seen at large distances are similar to things that existed in this part of the universe long ago. The scenery billions of light years away, as we see it, is the same as the scenery here billions of years ago. With a time machine that could travel back into the past we would have less need of large telescopes that strain to reach the limits of the observable universe.

This argument prompts the following thought. Things are very much the same everywhere at the same time, why not then confine our attention to a single region, concentrate on its history and ignore the rest of the universe? The history of what happens in this single region is the same as the history of what happens everywhere.

But this argument has an apparent drawback. Any chosen sample region is influenced by other regions near and far, how then can we afford to ignore the affect of these other regions? Light, for instance, travels great distances and influences what happens in the sample region. If we are to pay undivided attention to a single region, ignoring all other regions, we must in some way allow for their influence.

THE GREAT RIDDLE

An inferno of stars

There is a simple and important experiment in cosmology that almost everybody can perform. It consists of gazing at the night sky and noting its state of darkness. When we ask, why is the sky dark at night? (Figure 24.1) the natural response is the Sun is shining on the other side of the Earth and starlight is weaker than sunlight. It takes an unusual mind to realize that the relative weakness of starlight is of cosmological significance, and such a person was the astronomer Johannes Kepler, imperial mathematician to the emperor of the Holy Roman Empire.

In a forest (Figure 24.2), a line of sight in any horizontal direction must eventually intercept a tree trunk, and the distant view consists of a background of trees. Similarly, on looking away from Earth at night, we see a “forest” of stars (Figure 24.3). If the stars stretch away endlessly, a line of sight must eventually intercept the surface of a distant star. The distant view of the universe should consist of a continuous background of bright stars with no separating dark gaps.