I once had occasion to testify before the United States Senate Space and Aeronautics Committee on the scientific background of the space program; my talk dealt with the manner in which all substances in the universe are assembled out of neutrons, protons, and electrons as the basic building blocks. After I left the chamber a senior NASA¹ official continued with a summary of the major space science achievements of the last year. Apparently my scholarly presentation had perplexed the senators, although they were anxious to understand the concepts I had presented. However, the NASA official’s relaxed manner reassured them, and someone asked him: “How big is the electron? How much smaller is it than a speck of dust?” The NASA official correctly replied that the size of an electron is to a dust speck as the dust speck is to the entire earth.

The electron is indeed a tiny object. Its diameter is one 10-trillionth of an inch, a million times smaller than can be seen with the best electron microscope. Its weight is correspondingly small; 10,000 trillion trillion electrons make up one ounce. How can we be certain that such a small object exists? No one has ever picked up an electron with a pair of forceps² and said, “Here is one.” The evidence for its existence is all indirect. During the 150 years from the late eighteenth century to the beginning of the twentieth century a great variety of experiments were carried out on the flow of electricity through liquids and gases. The existence of the electron was not proved conclusively by any single one of these experiments. However, the majority of them could be explained most easily if the physicist³ assumed that the electricity was carried by a stream of small particles, each bearing its own electrical charge. Gradually physicists acquired a feeling, bordering on conviction, that the electron actually exists.

The question now was, how large is the electron, and how much electric charge does each electron carry? The clearest answer to this question came from an American physicist, Robert Millikan, who worked on the problem at the University of Chicago in the first decades of the twentieth century. Millikan arranged a device, clever for its simplicity, in which an atomizer⁴ created a mist of very fine droplets of oil just above a small hole in the top of a container. A small number of the droplets fell through the hole and slowly drifted to the bottom of the container. Millikan could see the motions of these droplets very clearly by illuminating them from the side with a strong light so that they appeared as bright spots against a dark background. Millikan discovered that some of these droplets carried a few extra electrons, which had been picked up in the atomizing process. By applying an electrical force to the droplets and studying their motions in response to this force, he could deduce the amount of electric charge carried by the electrons on each droplet. This charge turned out to be exceedingly minute.⁵ As a demonstration of its minuteness, it takes an electric current equivalent to a flow of one million trillion electrons every second to light a 10-watt bulb. All this happened rather recently in the history of science. Millikan’s first accurate measurements were completed in 1914.

The tiny electron, and two sister particles, are the building blocks out of which all matter in the world is constructed. The sister particles to the electron are the proton and the neutron. They were discovered even more recently than the electron; the proton was identified in 1920 and the neutron was first discovered in 1932. These two particles are massive in comparison with the electron—1840 times as heavy—but still inconceivably light by ordinary standards. The three particles combine in an amazingly simple way to form the objects we see and feel. A strong force of attraction binds neutrons and protons together to form a dense, compact body called the nucleus, whose size is somewhat less than one-trillionth of an inch. Electrons are attracted to the nucleus and circle around it as the planets circle around the sun, forming a solar system in miniature.

Together the electrons and the nucleus make up the atom.

The size of a typical atom is one hundred-millionth of an inch. To get a feeling for the smallness of the atom compared to a macroscopic⁶ object, imagine that you can see the individual atoms in a kitchen table, and that each atom is the size of a grain of sand. On this scale of enlargement the table will be 2000 miles long.

The comparison of the atom with a grain of sand implies that the atom is a solid object. Actually, the atom consists largely of empty space. Each of the atoms that makes up the surface of a table consists of a number of electrons orbiting around a nucleus. The electrons form a diffuse shell around the nucleus, marking the outer boundary of the atom. The size of the atom is 10,000 times as great as the size of the nucleus at the center. If the outer shell of electrons in the atom were the size of the Astrodome that covers the Houston baseball stadium, the nucleus would be a ping-pong ball in the center of the stadium. That is the emptiness of the atom.

If most of the atom is empty space, why does a tabletop offer resistance when you push it with your finger? The reason is that the surface of the table consists of a wall of electrons, the electrons belonging to the outermost layer of atoms in the tabletop; the surface of your finger also consists of a wall of electrons; where they meet, strong forces of electrical repulsion prevent the electrons in your fingertip from pushing past the outermost electrons in the top of the table into the empty space within each atom. An atomic projectile⁷ such as a proton, accelerated to high speed in a cyclotron,⁸ could easily pass through these electrons, which are, after all, rather light and unable to hurl back a fast-moving object. But it would take more force than the pressure of the finger can produce to force them aside and penetrate the inner space of the atom.

The concept of the empty atom is a recent development. Isaac Newton described atoms as “solid, massy, hard, impenetrable, moveable particles.” Through the nineteenth century, physicists continued to regard them as small, solid objects. Lord Rutherford, the greatest experimental physicist of his time, once said, “I was brought up to look at the atom as a nice hard fellow, red or grey in color, according to taste.” At the beginning of the twentieth century, J. J. Thomson, a British physicist and one of the pioneers in the investigation of the structure of matter, believed that the atom was a spherical plum pudding of positive electric charge in which negatively charged electrons were embedded like raisins. No one knew that the mass of the atom, and its positive charge, were concentrated in a small, dense nucleus at the center, and that the electrons circled around this nucleus at a considerable distance. But in 1911 Rutherford, acting on a hunch, instructed his assistant, Hans Geiger, and a graduate student named Marsden, to fire a beam of alpha particles⁹ into a bit of thin gold foil. These alpha particles are extremely fast-moving atomic projectiles which should have penetrated the gold foil and emerged from the other side. Most of them did, but Geiger and Marsden found that in a very few cases the alpha particles came out of the foil on the same side they had entered. Rutherford said later, “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Later Geiger told the story of the occasion on which Rutherford saw the meaning of the experiment. He relates: “One day [in 1911] Rutherford, obviously in the best of spirits, came into my room and told me that he now knew what the atom looked like and how to explain the large deflections of the alpha particles.” What had occurred, Rutherford had decided, was that now and then an alpha particle hit a massive object in the foil, which bounced it straight back. He realized that the massive objects must be very small since the alpha particles hit them so rarely. He concluded that most of the mass of the atom is concentrated in a compact body at its center, which he named the nucleus. Rutherford’s discovery opened the door to the nuclear era.

Let us continue with the description of the manner in which the universe is assembled out of its basic particles. Atoms are joined together in groups to form molecules, such as water, which consists of two atoms of hydrogen joined to one atom of oxygen. Large numbers of atoms or molecules cemented together form solid matter. There are a trillion trillion atoms in a cubic inch of an ordinary solid substance, which is roughly the same as the number of grains of sand in all the oceans of the earth.

The earth itself is an especially large collection of atoms bound together in a ball of rock and iron 8000 miles in diameter, weighing six billion trillion tons. It is one of nine planets, which are bound to the sun by the force of gravity. Together the sun and planets form the solar system. The largest of the planets is Jupiter, whose diameter is 86,000 miles; Mercury, the smallest, is 3100 miles across, one-third the size of the earth, and scarcely larger than the moon. All the planets are dwarfed by the sun, whose diameter is one million miles. The weight of the sun is 700 times greater than the combined weight of the nine planets. Like the atom, the solar system consists of a massive central body—the sun—surrounded by small, light objects—the planets—which revolve about it at great distances.

The sun is only one among 200 billion stars that are bound together by gravity into a large cluster of stars called the galaxy. The stars of the galaxy revolve about its center as the planets revolve about the sun. The sun itself participates in this rotating motion, completing one circuit around the galaxy in 250 million years.

The galaxy is flattened by its rotating motion into the shape of a disk, whose thickness is roughly one-fiftieth of its diameter. Most of the stars in the galaxy are in this disk, although some are located outside it. A relatively small, spherical cluster of stars, called the nucleus of the galaxy, bulges out of the disk at the center. The entire structure resembles a double sombrero¹⁰ with the galactic nucleus as the crown and the disk as the brim. The sun is located in the brim of the sombrero about three-fifths of the way out from the center to the edge. When we look into the sky in the direction of the disk we see so many stars that they are not visible as separate points of light, but blend together into a luminous band stretching across the sky. This band is called Milky Way.

The stars within the galaxy are separated from one another by an average distance of about 36 trillion miles. In order to avoid the frequent repetition of such awkwardly large numbers, astronomical distances are usually expressed in units of the light year. A light year is defined as the distance covered in one year by a ray of light, which travels at 186,000 miles per second. The distance turns out to be six trillion miles; hence in these units the average distance between stars in the galaxy is five light years, and the diameter of the galaxy is 100,000 light years.

In spite of the enormous size of our galaxy, its boundaries do not mark the edge of the observable universe. The 200-inch telescope on Palomar Mountain has within its range no less than 100 billion other galaxies, each comparable to our own in size and containing a similar number of stars. The average distance between these galaxies is one million light years. The extent of the visible universe, as it can be seen in the 200-inch telescope, is 15 billion light years.

An analogy will help to clarify the meaning of these enormous distances. Let the sun be the size of an orange; on that scale of sizes the earth is a grain of sand circling in orbit around the sun at a distance of 30 feet; the giant planet Jupiter, 11 times larger than the earth, is a cherry pit revolving at a distance of 200 feet or one city block; Saturn is another cherry pit two blocks from the sun; and Pluto, the outermost planet, is still another sand grain at a distance of ten city blocks from the sun.

On the same scale the average distance between the stars is 2000 miles. The sun’s nearest neighbor, a star called Alpha Centauri, is 1300 miles away. In the space between the sun and its neighbors there is nothing but a thin distribution of hydrogen atoms, forming a vacuum far better than any ever achieved on earth. The galaxy, on this scale, is a cluster of oranges separated by an average distance of 2000 miles, the entire cluster being 20 million miles in diameter.

An orange, a few grains of sand some feet away, and then some cherry pits circling slowly around the orange at a distance of a city block. Two thousand miles away is another orange, perhaps with a few specks of planetary matter circling around it. That is the void of space.