or the most part, our universe is made of four kinds of elementary particles: neutrons, protons, electrons, and particles of radiation called photons. (I leave out neutrinos, which interact only negligibly with matter, and also the hundreds of particles that come out of high-energy nuclear reactions.) The first three-protons, electrons, and neutrons exist not only as particles but as antiparticles. The particles constitute matter; the antiparticles antimatter. If one looks at objects far out in the universe, one cannot be sure whether they are made of matter or antimatter, for all our information arrives via radiation, and photons do not differentiate. They are, as we say, their own antiparticles.

Why do we have a universe of matter at all? In 1952 I was giving the Vanuxem Lectures at Princeton University on the origins of life and biochemical evolution. Albert Einstein, whom I had come to know, was walking with me before the first lecture and asked, "Why do you think the natural amino acids are all left-handed?" As you know, all amino acids except the simplest, glycine, exist in two geometries that are mirror images of each other-like right and left hands. However, all the natural amino acids happen to be left-handed. Einstein went on to say, "I have wondered for years how the electron came out to be negative. Negative and positive are perfectly symmetrical principles in physics, so why is the electron negative?" All I could think of was: the negative electron won in the fight. I said, "That is exactly what I think of those left-handed amino acidsthey won in the fight." But he was talking about a different fight-the fight between matter and antimatter. As he said, these types of matter are perfectly symmetrical. Thus, the neatest idea of what went into the big bang at the start of the known universe were equal amounts of matter and antimatter.

In the fantastic compression of the

initial stages of the big bang, there must have been a wild fire storm. Whenever a particle of matter contacts its antimatter partner, mutual annihilation results and the masses of both particles are converted to radiation. Thus, at the end of the big bang there should have been a universe of radiation with neither matter nor antimatter. In fact, Arno Penzias and Robert Wilson of Bell Laboratories discovered a background of microwave radiation filling the universe that comes equally from all directions and is thought to be the residue of the fire storm in the big bang. The radiation is identical with the radiation that would come off a black body, say a piece of black iron, at the very cold temperature of 2.8 degrees above absolute zero, or

approximately -270 degrees centigrade.

One now realizes there are roughly a billion times as many photons of that residual radiation moving around in the universe as there are particles with mass. So we have to modify our neat idea to include a little discrepancy, a little mistake if you will: for every billion parts of antimatter involved in the big bang there were one billion and one parts of matter. Thus, when the fire storm of mutual annihilation had exhausted itself, one part in one billion of matter was left over. This residue constitutes the matter of our universe, that is, the galaxies and stars and planets and us. This little one part per billion mistake is the first element in my story.

Now how is it that we find ourselves in a universe well supplied with protons and electrons as well as neutrons? The reason is that free neutrons-neutrons outside of atomic nuclei and outside of highly dense neutron stars-disintegrate with a half life of 10.6 minutes into an electron, a proton, and radiation. If you start with a collection of free neutrons, ten minutes later half are still neutrons, but the other half is everything else you need to make a universe like ours.

Why does the reaction go in that direction? Only because a neutron is a tiny bit more massive than a proton plus an electron. Any such reaction has to go in the direction of lower mass. But the loss of mass in this case is less than one part in a thousand-in fact, eight parts in ten thousand. But what if the reaction went the other way? If it did, we would be in a universe of neutrons. The neutrons would have long since mopped up all the protons and electrons, and we would not have the chemical elements, molecules, new radiation, or, of course, life. Another small but vital discrepancy.

nuclei of all atoms are made of protons and neutrons, which are heavy particles each almost two thousand times the mass of an electron. The result is that almost the entire mass of an atom is concentrated in a nucleus that

holds its position no matter what the electrons roaming around the periphery are doing. This fact is very important because it is the reason anything stays put in the universe. What would our universe be like if the nuclear particles and the electrons were somewhat closer together in mass? The motions of any one particle would produce reciprocal motions in the others; they would revolve around each other, and all matter would be fluid, none would be solid. Could indeed such atoms form stable bonds? You would not have molecules whose shapes you could draw with great confidence. This fact is critical because the shape of a molecule-the way one molecule fits into another-means everything in living organisms.

Here is another extraordinary circumstance. Although there is an enormous difference in mass between the proton and electron-one thousand eight hundred forty times the magnitude of their electric charge is apparently identical. Why is it that the proton and the electron, which are so unlike in every other regard, have the same numerical charge?

Is this a legitimate scientific question? In 1959 two of the world's most distinguished astrophysicists, R.A. Lyttleton and Herman Bondi, published a long paper in the Proceedings of the Royal Society of London in which they proposed that the proton and the electron differ in charge by the almost infinitesimal amount 2 x 10-18e, where e is the tiny charge on either particle. One's first thought is who gives a damn about two billion billionths, but Lyttleton and Bondi explained that this tiny difference would result in a net charge on all particles, and thus there would be a net repulsion between all matter in the uni

verse. Their hypothesis would account for the observed expansion of the universe. The only trouble I have with this idea is that the universe would do nothing but expand. Such a tiny difference in charge is enough to completely overwhelm the force of gravity that brings matter together, and so there would be no galaxies, no stars, no planets, and, worst of all, no physicists.

Before the ink was dry on that paper, John King and his group at the Massachusetts Institute of Technology were searching for a measurable difference in charge. By now they have shown that any difference has to be less than 10-20e. However, the growing consensus for the existence of quarks, which have fractional charge, has not made the equivalence of charge on the electron and proton any easier to understand. The electron is an indivisible

unitary particle-an electron is an electron is an electron-whereas a proton consists of three quarks, two up and one down. It is a little strange that the sum of the quark charges is exactly equal to the charge of an electron.

et us move up a step in organization to the elements. Of the 92 natural elements, 99 per cent of living matter is made of just four: hydrogen, oxygen, nitrogen, and carbon. I think it has to be this way wherever life arises in the universe because those four elements have unique properties critical to the existence of life. There are no other elements like them in the periodic table. Although I studied chemistry a long time ago, I suspect some of the same silly things are still being said. We were told that if you move vertically down a column of elements in the peri

odic table, those elements repeat properties. Well, any kid with a chemistry set knows better. Under oxygen is sulfur; try breathing sulfur sometime. Under nitrogen is phosphorus; there isn't any phosphorus in that kid's chemistry set because it is too dangerous: it bursts spontaneously into flames when exposed to air. Under carbon is silicon; there is about 130 times as much silicon in the crust of the earth as carbon. Then why are we made of carbon?

A strange attribute critical to the properties of these four elements is that carbon, nitrogen, and oxygen are the only elements that form real double and triple chemical bonds. What is the importance of this for life? Well, just compare two molecules that, based on the positions of their central atoms in the periodic table, should be very much alike: carbon dioxide and silicon dioxide. Carbon dioxide is a symmetrical molecule in which the carbon atom is tied to two adjacent oxygen atoms by double bonds. Those multiple bonds completely saturate the combining tendencies of all three atoms, and carbon dioxide can float off into the air as a perfectly happy and independent molecule and dissolve in the waters of the earth. Those are the places where living organisms find their carbon.

Silicon dioxide cannot form a double bond. Thus each silicon atom is tied to each oxygen with a single bond, leaving four half-formed bonds, or lone electrons, two on the silicon and one on each of the oxygens. These electrons are just dying to combine with something, but with what? Each silicon dioxide molecule combines with its neighbors until an enormous supermolecule has formed-in fact, a rock. The reason quartz is so hard to break is that you have to break a lot of chemical bonds. That is why silicon is fine for making rocks, whereas carbon is fine for making living organisms. One can make similar arguments for oxygen and nitrogen.

Now we move up another step and examine molecular organization. The most important molecule, by far, in living organisms is water. But water is the strangest molecule in the whole of chemistry, and its strangest property is that ice floats. If ice did not float, I doubt there would be life. Everything contracts on cooling, including water down to 4 degrees centigrade. However, between 4 degrees and the freezing point at 0 degrees, water expands so rapidly that ice is less dense than liquid

water, and it floats. If water shrank as it cooled like everything else, colder water would be heavier and would keep sinking. Freezing would begin not at the top of the lake or ocean but at the bottom, and, in the end, the body of water would freeze solid, a disaster for underwater life. Where I live the best time to go fishing is in the winter. You take your fishing equipment in one hand and a bottle of whiskey in the other and cut yourself a hole in the ice. Up to that point the fish were having a ball, getting along fine down there. Another problem that would arise if large bodies of water froze solid is that a big chunk of ice takes forever to melt. With a relatively thin skin of ice on top, the first warm weather melts it, spring arrives, and everything is happy again.

row I take a big jump to the stars. It

Nis as easy for a camel to go through

the eye of a needle as for a star to enter the kingdom of heaven. The needle's eye in this case is the first step in the fusion of hydrogen to helium. Every main-sequence star lives by fusing hydrogen to helium. A physicist at Oak Ridge during the Manhattan project who became an administrator and then an Episcopal priest was once quoted in the New Yorker as having said, "God must love hydrogen bombs because He made so many of them in the form of stars." The man should have known better, both as a physicist and a priest, because you can make stars out of hydrogen but you cannot make hydrogen bombs out of hydrogen. You have to use the rare, heavy isotopes of hydrogen in bombs. A mixture, say fifty-fifty of deuterium and tritium, is needed because the conversion of ordinary hydrogen to deuterium is perhaps the slowest reaction known. It takes a hundred billion years, which is the only reason stars last so long. They are not hydrogen bombs, although once you get to deuterium even a star could explode. As a result, stars

last a long time, and life has a chance to start evolving at those with suitable planets.

Why is the conversion of hydrogen to deuterium so slow? The nuclei of normal hydrogen are simply positively charged protons, and even at the temperatures of main-sequence stars, say around five million degrees, the collision of two protons will most likely result in their just bouncing off each other. The rare event that has to occur if such a collision is to generate deuterium is for one of the protons to disintegrate and change to a neutron as it collides with the other proton. That is an improbable event. But mainsequence stars have lots of time and just keep slowly turning sets of four hydrogen nuclei into pairs of deuterium nuclei and then into helium nuclei. The slight

loss of mass in the reaction is turned into radiation, which is our sunlight.

How do you get carbon? The first thought is just to keep adding protons. This will not work because if one proton is added to helium, the result is a mass-five isotope, and there is no atomic nucleus with mass five. What is the path around this barrier? Well, the only alternative is to fuse helium nuclei, but that reaction requires a very much higher temperature, say a hundred million degrees, which is only achieved when the star begins to die as a red giant. When the core of a red giant gets that hot, the helium nuclei begin to fuse.

From this point on it should just be simple arithmetic, but there is another barrier. When two helium nuclei fuse, the result is a mass-eight isotope of beryllium, which is one of the most unstable atomic nuclei to exist, disintegrating in 10-16 second. Fortunately again, there just happens to be an excited state of the carbon-12 nucleus whose energy is equal to the energy in a beryllium-8 nucleus plus a helium-4 nucleus plus the kinetic energy at the temperature at which these nuclei can collide. This wild coincidence is a fortunate energy resonance that turns a very improbable reaction into a very efficient one. So beryllium-8 fuses with helium-4 to make carbon-12. The important point is that there are multiple barriers in the synthesis of the elements, but each of these barriers is overcome in a very ingenious way.

Once carbon is formed in a red giant, two protons can be added to the carbon-12 nucleus to give mass 14, which brings nitrogen into the universe. Add helium-4 to the carbon-12 and you have mass 16, which brings oxygen into the universe. The story goes on and on this way, but eventually such stars grow unstable and explode, sending their material off into space. Finally, suns and planets such as ours grow out of this material.

Now just think! Life, wherever it arises in the universe, has to invent a way to keep going, and that way must depend on the energy given off by a nearby star. As we know, life on the earth runs on sunlight through the process of photosynthesis. How do we get our sunlight? We get it from the various reactions of the elements that constitute life itself. The first way is to fuse hydrogen to helium-the proton-proton chain. The second way uses a catalytic process the carbon-nitrogen-oxygen cycle-which starts by fusing carbon with 2 protons to yield nitrogen-14, then picks up 2 more protons to give