Essays on Science

“The Physical Worlds” by Henry Eyring

from Reflections of a Scientist, 1983

(Out of print)

My wife and I have a lot that is sixty feet across the front, one hundred feet deep, and extends straight up to the limits of space, so far as I know. Clearly, this qualifies me to speak on the broader aspects of the universe.

We live in a series of six worlds, from the infinitesimally small to the infinitely big. They can be represented by a point surrounded by five circles. This chapter deals with the five physical worlds of the nucleus, atom, living cell, every­ day experience, and stars. (…)

The first, or central, world is the world of the atomic nucleus. The nucleus is where most of the weight of the atom is situated. One hundred thousand atomic nuclei touching each other in a line extend only across one atom, and it takes 100 million atoms to make one inch! Vibrations inside the nuclei of atoms are about a million times more frequent than the vibrations between atoms. 

It is natural to wonder how anything as small as the nucleus can have structure, and even if it does, how we can find out about it. The procedure for finding out is to shoot electrically charged atoms or electrons at nuclei and see how they bounce. This tells us a great deal about the kinds of forces that are acting between the colliding particles.

When a particularly violent collision results in penetra­tion into the nucleus and causes it to fragment, we can watch the tracks left by the fragments in a cloud chamber. In this way we find out that the nucleus is made up of posi­tively charged protons and uncharged neutrons of virtually the same weight.

For twenty-five years scientists accepted parity, the prin­ciple that an atom does not know which end is up and has the same properties in all directions. However, an experiment suggested by Yang and Lee, for which they were given the Nobel Prize, showed otherwise. If we put radioactive cobalt sixty in a magnetic field, its nuclei line up with their south poles pointing toward the north pole of the earth. Every once in a while, one of the cobalt nuclei shoots out an electron. If the nuclei were indeed symmetrical, they would be equally likely to eject the electron through the north pole as through the south pole. However, a Geiger counter, similar to those used to prospect for uranium, reveals that the electrons are shot out preferentially through the nuclear south pole. Thus, the principle of parity must be given up.

Now the scientists have introduced quarks, fractional particles having a charge of one-third. When quarks were proposed in the early 1970s, they were conceived of as being undivided particles, although their charge was fractional.  More recently observations have indicated the existence of one-dimensional conductors called solitons, which have fractional charge and fractional electron number.

By such experiments we come to understand something of the complexity and intricacy of this almost unimaginably tiny world of the nucleus. One hesitates to speculate whether we have even yet found the indivisible building blocks of the physical world.

            The second world is the world of chemistry, made up of atoms and molecules. This, too, is a tiny world. It would take 100 million atoms placed side by side to make an inch. A molecule finishes one of its vibrations in about a ten mil­lion millionth of a second. We call this length of time a jiffy for lack of a better name. The quartz crystal watch you wear on your wrist derives its great accuracy from such rapid vibrations.

An example of the exactness with which the universe works is found in the ammonia molecule. Ammonia consists of a nitrogen atom sitting on three hydrogen atoms. The startling fact is that this umbrella-like molecule turns wrong-side out almost twenty-four thousand million times a second. If you beam radar through a tube containing ammonia, the signal fades just when the radar frequency equals the inversion frequency of the molecules. In this way the ammonia molecules can be used as a clock to break time up into twenty-four thousand millionths of a second with unprecedented accuracy. It is interesting that this ammonia molecule turns wrong-side out just as often whether it is at room temperature or at the lowest temperature obtain­able in the laboratory; whether in Soviet Russia or the United States; and whether this country is Democratic or Republican. Many things we think are important don't seem to affect the ammonia molecule much. The accuracy and order of the universe continue without regard to fashions in hairstyles, clothes, politics, science, or religion. What's true is true. What works, works.

When I went to Berkeley in 1925 to take my Ph.D., no one could explain more than a tenth of the attraction of two hydrogen atoms for each other. We knew that each hydrogen atom was uncharged, being made up of a positive nucleus whose charge was just neutralized by the charge on a negative electron. True, the atoms could distort each other and give about one-tenth of the observed binding energy, but the remaining part of the bond was a mystery.

The complete answer supplied by quantum mechanics was that the electron pair, by keeping out of the way of each other as they circulated around the two nuclei, could spend enough time between the two positive nuclei to glue them together to just the observed amount.

In order for the pair of bonded atoms to exchange part­ners with a similar pair, thus making a new compound, the pairs must approach each other closely. Accordingly, as two pairs of bonded atoms bump into each other, they can only approach and change partners if one or the other pair of electrons are pushed out of the way into an upper state. This collisional work that has to be done so a chemical reaction can take place is called the energy of activation.

Using this picture of the energy of activation accompanying a chemical reaction and building on the labors of my many able predecessors, it was possible for me, in 1935, to write down the general equation for the rate of all chemical reactions. This equation has become an integral part of chemistry, used universally.

The excitement of standing, in imagination, at a pass between two energy valleys representing the initial and final states in a chemical reaction and counting the activated complexes as they make their way over the pass in shatter­ing collisions in which atoms are exchanged between the two colliding molecules is the thrill of a lifetime.

Further, to find that wave mechanics and thermody­namics, which have had so many other triumphs, are the same tools that enable us to quantitatively explain this all pervading aspect of the material world in which we live brings a feeling of awe at the order and exactness of the uni­verse that is never forgotten.

            The third world in which we live is the world of the living cell, the world of biology. Cells vary in size, but typically they are about a micron across, that is, about one ten­ thousandth of a centimeter. An active cell divides into two cells about every twenty minutes.

The word life itself conjures up animation, movement, excitement, and perhaps a little mystery. Discussions of the origin of life disturb some people. Some are particularly dis­turbed by scientists who "tamper with creation" and actu­ally try to start life in a laboratory. It would disturb me more to find that life couldn't be started in a laboratory. If life can't be started somehow in this physical world, then how did I get here? You see, I think I'm alive, although some of you might observe me dozing at my desk and wonder.

The human cell has near its center a nucleus containing forty-six chromosomes. Twenty-three of these come from the father and twenty-three from the mother. The chromo­somes are made up of about a million genes that constitute our inheritance. A gene controls the synthesis of essential molecules, such as enzymes, that build and regulate our unbelievably complicated bodies.

A colleague, Frank Johnson, and I once wrote a paper on evolution and rate theory, "The Critical Complex Theory of Biogenesis." This paper outlines a theory of pre­biological evolution. One of the principal questions addressed is why living things are optically active.

The body is made up of many types of molecules, just as a large building may be made up of many types of brick. Many of these molecules are asymmetrical, and frequently one optical isomer is found to occur in living things to the virtual exclusion of its mirror image. We can understand this selective choice of building blocks if we recognize that the body is built up by molecules that are to be incorporated into the body from the food we eat. This selection is made by a process of fitting of the molecules to the enzyme much as a left hand selects a left-hand glove and rejects a right-hand glove

Muscles and enzymes are made by joining amino acids together into long chains called proteins. There are twenty different amino acids that are joined together in different proportions to form the links in the various types of protein chains. Of these twenty amino acids used by the body, all but one are asymmetric. Further, all the nineteen asymmet­ric amino acids used are like the left-hand glove and are called l-amino acids. In every living thing, the opposite optical isomers, which are called the d-amino acids, if present in the food, are rejected by the enzymes that build proteins, and are eliminated from the body. We therefore call this world we live in an l-amino acid world. The "1" comes from laevo, the Latin word for left; and "d" stands for dextro, or right.

Using absolute rate theory, Johnson and I arrived at a reasonable rate of appearance of these optically active tem­plates, given assumed concentrations of certain chemicals in the primordial "soup."

The interesting thing is that from any given pot of "soup" it is as likely that a d- as an l-type world will start up. We can readily imagine a d-amino acid world. In fact, if we look into a large mirror, the world we see is a d-amino acid world, since every object, including the molecules, is the mirror image of those in the real world. Obviously, every­thing in the d-amino acid world would work exactly as well as everything in our real world, and it is a matter of no obvi­ous consequence which world we happen to have. If there are other worlds that support life, there is no reason for sup­posing that they may not be d-amino acid worlds. If so, such worlds would be completely inhospitable to us, since we could not digest their foods, and marriages between people coming from d and 1 worlds would necessarily be sterile. On the other hand, there is, of course, no reason why people from two such worlds might not converse with each other with complete understanding, and one could not tell the two types of people apart by their appearance.

The fact that in our world every living thing, from the tiniest living cell to man, uses only the l-amino acids along with the d-sugars highlights the unity running through the living world. Everything that grows collects those particular optical isomers needed for food and rejects the opposite iso­mers. Here again, we catch a glimpse of the unity that everywhere characterizes the cosmic design.

The fourth world is the world of everyday. Here, we measure time in seconds or minutes, and distances in feet or miles. This is the world we know most about. 

Sir Isaac Newton discovered the universal law of gravi­tation and developed the laws of mechanics so that his successors have been able to calculate the motion of the planets in their orbits to any desired degree of accuracy.  Astronomers predict exactly when an eclipse will occur. Using this knowledge, men make great preparations, assemble expensive scientific equipment, and move to the ends of the earth when told that an eclipse is imminent.  They get their cameras ready to take pictures and open the shutters at the right moment, and the eclipse begins at the predicted instant. If the eclipse were ever so little off sched­ule, it would make headline news around the world. 

These same laws that enable astronomers to compute eclipses tell precisely, of course, how a satellite goes around in its orbit. A man who makes a good running broad jump only misses becoming an earth satellite by not running fast enough. Thus, if instead of running twenty miles an hour he ran twenty thousand miles an hour, he would find that as he jumped, the earth would curve away beneath him faster than he could fall toward it. Our jumper would settle into an elliptical orbit extending around the earth, except for one thing-the air resistance would slow his speed and set him on fire. To avoid air resistance, the satellite is shot above the atmosphere. This is the only reason for sending it up six hundred miles.

Satellites like Sputnik and the moon have a terrifically frustrating job. The moon has been falling toward the earth and overshooting the mark for billions of years and still has no prospect for a successful hit. Man has not succeeded in sending satellites up to such complete frustration yet. The reason is simple. Even at altitudes of six hundred or a thou­sand miles there is still a trace of hydrogen left. As the satel­lite circles the earth, it bumps into gas. When it has bumped into a weight of gas equal to its mass, its momentum is cut in half, except that by falling closer to the earth it picks up additional speed. Still, it is a remarkable human achieve­ment to launch a satellite and in a small degree become a partner in creation.

All such achievements rely on the fundamental belief that the everyday world is exactly predictable. We may not yet know or completely understand the rules of when earth­ quakes occur, or what causes surprises for the weatherman, but we are sure that such events are not really capricious. The more we learn, the better we understand and the closer are our predictions.

        If we look at the stars, we see the fifth world. Men have probably always looked up and wondered: How far away are the stars? What makes them shine? How long have they been there? Will they exist forever? Some have believed that the stars were gods who controlled their destiny. Others noticed the regularity in the grouping of stars and used their knowledge of stellar movement to help mark the passage of the seasons and fix the times of planting and harvesting.  The early Greeks believed the stars were fixed like nails to the vault of the heavens. Aristotle maintained that celestial objects were permanent, immutable, and perfect. He so convinced the Greeks of this that when a new star appeared in 134 B.C. it was attributed by its discoverer to an omission by his predecessors.

In the Middle Ages, Copernicus showed that the earth was not the center of the solar system. But Aristotle's think­ing continued to dominate astronomy until the 1500s when new stars were discovered and then in the seventeenth century when Galileo used his telescope to discover spots on the sun, demonstrating that the solar complexion was somewhat less than perfect, and to prove that the sky was filled with stars that could not be seen with the naked eye.

Just so you know how old I am, I can remember when astronomers, using ever-larger telescopes, discovered that some of the "stars" thought to be part of the Milky Way were actually other galaxies, each containing billions of stars and lying far beyond the Milky Way's outermost limits.

Even so today, as we look out at the universe, the first impression is one of stupendous size. By using such instru­ments as the huge 200-inch optical telescope on Mount Palomar and newer radio, X-ray, and gamma-ray telescopes, modern-day stargazers have pushed the frontiers of under­standing ever closer to the edges of the universe and into the very cores of the stars. We can see out so far that the light reaching our eyes started on its journey toward us almost 12 billion years ago. You remember how fast light travels: 186,000 miles a second; it goes around the earth, a distance of 24,000 miles, seven and one-half times in a sec­ond. A light-year is the distance that light travels in a year ­about 6 trillion miles. If you multiply 12 billion years by 6 trillion miles, you get a seven followed by twenty-two zeros for the number of miles you can see in any direction you care to look. The known radius of the universe in miles is even bigger than the national debt. It is a very long distance indeed.

When you walk outside on a clear moonless night, all the celestial objects you can see with the naked eye are either planets or stars, or, if you have superman vision, you might be able to see the Andromeda galaxy that looks like a fuzzy star. All the stars you see are part of our own Milky Way galaxy. The closest star, other than our own sun, is Proxima Centauri, four light-years away. At our present rates of space travel, this journey would require one hundred twenty thousand years. Consequently, we seem to be marooned in our solar system, at least for the time being. Present missile travel, which proceeds at a speed about a thousand times as fast as man can run, will need to be speeded up by another factor of a thousand before we can undertake trips beyond our solar system.

Some scientists believe that the universe is the expand­ing remnant of a huge fireball created 20 billion years ago by a giant explosion. The stars and planets are the products of that cataclysmic blast and its aftereffects. In 1929 astronomer Edwin Hubble used shifts in the spectral lines of light coming from distant galaxies to calculate that these islands of stars are moving at tremendous speeds away from the earth-and from each other-like dots painted on the surface of an expanding balloon. Scientists at Bell Labora­tories have even listened with their sensitive radio antenna to radiation interference, which amounts to the hissing echoes of creation: "In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. And God said, Let there be light: and there was light." (Genesis 1:1-3.)

Most cosmologists-scientists who study the structure and evolution of the universe-agree that the biblical account of creation, in imagining an initial void, is uncan­nily close to the truth. I might add, it is as if the one who wrote those words was there, or at least had talked to Someone who was.

Are we the only worshipers in this great cathedral called the universe? Professor Harlow Shapley, emeritus professor of astronomy at Harvard University, has written an interesting book, Of Stars and Men (New York: Washington Square Press, Inc., 1960) in which he estimates that there are 100 million, million, million, million suns in space. Shapley has very conservatively estimated that at least one sun in a thousand should have acquired planets. Most of these satellites are at such distances from their suns that they are either too hot or too cold to support life as we know it. Still others lack life-giving water, while others lack the necessary oxygen. However, Shapley has estimated that of those suns with planets at least one in a thousand has a planet at the right distance for life.

Of those having a planet at the right distance, at least one in a thousand should have a planet large enough to hold an atmosphere, and, finally, that one in a thousand of those having a large enough planet at the right distance should have an atmosphere of the right composition to sup­port life. Thus, he concludes that there should be at the very minimum 100 million planets that could support life, and the number is probably many times more. From the scien­tific point of view, it is hard to doubt that myriads of worlds are suitable for human habitation.

It is accordingly natural to conclude that the universe is filled with intelligent beings and, presumably, always has been. Any unfolding of intelligences on this earth only repeats what has happened previously elsewhere. Even if we believe beings on distant planets have progressed far beyond us, still the barrier to travel posed by interstellar distances seems quite sufficient to explain why mortal space travelers have not visited us.

So we envision a still-expanding universe that began almost 20 billion years ago, extends for 12 billion light-years, and contains 10 billion galaxies-each one an island of hundreds of billions of stars. Actually, we also need to add the dimension of time. We see our nearby sun as it looked a little more than eight minutes ago. We see Proxima Centauri as it was about four years ago, and some of the farther galaxies as they looked billions of years ago. The far­ther out we look, the further we are looking back in time. Some objects we see may no longer exist.

With all that we do know, it is obvious to the serious student that there is a great deal more that we don't know. To the ultimate question-what existed before the big bang-most of modern science is mute. It's as if it were against the rules to ask questions when there isn't any scientific way to approach the answers. It's still a nice question though, isn't it?

From nucleus to galaxies, the universe is complex, orderly, consistent, and very, very interesting. It is also full or energy and motion. There is something peculiar about that.

If you picked up a watch far from human habitation and found it running, you would ask not only "Who made this watch?" but "Who wound it up?" So it is with the universe. The universe is running down. It is a universe of change. People are born and pass from the earth, and stars, too, come into existence and pass away.

The sun is about half hydrogen; the rest of it is composed of other materials. The hydrogen bomb shows us what happens to hydrogen down in the center of the sun where the pressures and temperatures are enormous. Four hydrogen atoms come together to make helium, and, in the process, a little of their weight is changed into energy. This energy falls on the earth as sunlight and makes the plants grow.

Thus, the sun is a giant furnace with a supply of hydrogen for fuel-quite a good supply. But if the sun ever burns out, it is going to be very cold (here). You might like an estimate of the fuel supply on hand. I would say the supply should last at least five billion years, so we don't need to worry about it right away.

This picture of the sun as a furnace with a limited amount of fuel poses interesting problems. The second law of thermodynamics is a formal statement of the familiar fact that if energy is being obtained continuously from some source, then the supply of energy must run out sometime unless it is replenished from an outside source. As with any woodpile, if you keep burning up the hydrogen on the sun, it must ultimately be used up unless it is replenished. There is evidence that in the last billion years the sun's temperature has never varied by the small amount that would make the earth unfit for habitation.

It is a well-known fact of experience that if we set a pot of boiling water on a table in a cool room, the pot cools, and when once cooled it never returns to the boiling point without being reheated. In just the same way the sun is giving off its heat and very gradually growing colder. When the sun ceases to shine, all living things will die; all changes will cease, and the world will reach a deadly, monotonous uni­formity. This state is called the heat death and is a consequence of nature always moving toward more probable states and never in the reverse direction toward less probable states.

An interesting calculation illustrates the complete improbability of a hot sun arising by chance. We suppose that in order to become hot again the sun must accumulate an amount of heat equal to that it gives off during its lifetime. This must be accumulated from its surroundings, which we shall assume in the heat death drops to a temper­ature of 700 degrees centigrade. Then, using the straightfor­ward theory of chemical reactions, we find that a length of time in years equal to at least one with a hundred thousand, billion, billion, billion, billion, billion zeros must elapse before a hot sun has a 50-percent probability of occurring again by chance. This is almost no chance at all! A universe filled with hot suns is no more likely. It is evident that our hot sun, or this universe, did not arise by such a chance fluctuation.

In a very real sense, then, the universe is like a clock that has been wound up. If it is self-winding, it is unique in scientific experience. In a talk before the National Academy of Sciences, I raised the obvious question, "How did the universe get wound up?" No one chose to answer. After the talk, I repeated my question privately to three scientists. President Millikan of the California Institute of Technology said, "I, like you, am a religious man." Professor Van Vleck of the Harvard physics department said, "Of course, one doesn't know." The third man said, "Don't you believe in your religion?" I answered, "Yes, but I wondered about yours."