There is good reason to believe that we are in a universe permeated with life, in which life arises, given enough time, wherever the conditions exist that make it possible. How many such places are there? Arthur Eddington, the great British physicist, gave us a formula: one hundred billion stars make a galaxy, and one hundred billion galaxies make a universe. The lowest estimate I have ever seen of the fraction of them that might possess a planet that could support life is one percent. That means one billion such places in our home galaxy, the Milky Way; and with about one billion such galaxies within reach of our telescopes, the already observed universe should contain at least one billion billion -- 1018 -- places that can support life
So we can take this to be a universe that breeds life; and yet, were any one of a considerable number of physical properties of our universe other than it is -- some of those properties basic, others seeming trivial, almost accidental -- that life, that now appears to be so prevalent, would become impossible, here or anywhere.
I can only sample that story here and, to give this account a little structure, I shall climb the scale of states of organization of matter, from small to great.
But first, a preliminary question: How is it that we have a universe of matter at all?
Our universe is made of four kinds of so-called elementary particles: neutrons, protons, electrons, and photons, which are particles of radiation. (I disregard neutrinos, since they do not interact with other matter; also the host of other particles that appear transiently in the course of high‑energy nuclear interactions.) The only important qualification one need make to such a simple statement is that the first three particles exist also as antiparticles, the particles constituting matter, the anti-particles anti-matter. When matter comes into contact with anti-matter they mutually annihilate each other, and their masses are instantly turned into radiation according to Einstein’s famous equation, E = mc2, in which E is the energy of the radiation, m is the annihilated mass, and c is the speed of light.
The positive and negative electric charges that divide particles from anti-particles are perfectly symmetrical. So the most reasonable expectation is that exactly equal numbers of both particles and anti-particles entered the Big Bang, the cosmic explosion in which our universe is thought to have begun. In that case, however, in the enormous compression of material at the Big Bang, there must have occurred a tremendous storm of mutual annihilation, ending with the conversion of all the particles and anti-particles into radiation. We should have come out of the Big Bang with a universe containing only radiation.
Fortunately for us, it seems that a tiny mistake was made. In 1965, Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in New Jersey discovered a new microwave radiation that fills the universe, coming equally from all directions, wherever one may be. It is by far the dominant radiation in the universe; billions of years of starlight have added to it only negligibly. It is commonly agreed that this is the residue remaining from that gigantic firestorm of mutual annihilation in the Big Bang.
It turns out that there are about one billion photons of that radiation for every proton in the universe. Hence it is thought that what went into the Big Bang were not exactly equal numbers of particles and anti-particles, but that for every billion anti-particles there were one billion and one particles, so that when all the mutual annihilation had happened, there remained over that one particle per billion, and that now contitutes all the matter in the universe -- all the galaxies, the stars and planets, and of course all life.
I should like now to raise two problems to do with protons and electrons, one involving their masses, the other their electric charge.
Every atom has a nucleus composed of protons and neutrons, except the smallest one, hydrogen, which has only one proton as its nucleus. Electrons orbit these nuclei at distances relatively greater than separate our sun from its planets. Both protons and neutrons have masses almost two thousand times the mass of an electron -- 1840 times when I last looked -- so virtually the whole mass of an atom is in its nucleus. Hence the atom is hardly disturbed at all by the motions of its electrons, and an atom can hold its position in a molecule, and molecules their positions in larger structures. Only that circumstance permits molecules to hold their shapes, and solids to exist.
If on the contrary the protons and neutrons were closer in mass to the electrons, whether light or heavy, then the motions of the electrons would be reflected in reciprocal motions by the others. All structures composed of such atoms would be fluid; in such a universe nothing would stay put. There could not be the fitting together of molecular shapes that permits not only crystals to form, but living organisms.
And now, electric charge: How does it come about that elementary particles so altogether different otherwise as the proton and electron possess the same numerical charge? How is it that the proton is exactly as plus-charged as the electron is minus-charged?
It may help to accept this as a legitimate scientific question to know that in 1959 two of our most distinguished astrophysicists, Lyttleton and Bondi, proposed that in fact the proton and electron differ in charge by the almost infinitesimal amount, 2 x 10 -18e -- two billion billionths e, in which e is the already tiny charge on either the proton or electron. The reason they made that proposal is that, given that nearly infinitesimal difference in charge, all the matter in the universe would be charged, and in the same sense, plus or minus. Since like charges repel one another, all the matter in the universe would repel all the other matter, and so the universe would expand, just as it is believed to do. The trouble with that idea is that yes, the universe would expand, but -- short of extraordinary special dispensations - it would not do anything else. Even so small a difference in electric charge would be enough to overwhelm the forces of gravitation that bring matter together; and so we should have no planets, no stars, no galaxies -- and, worst of all, no physicists.
No need to worry, however. Shortly after Lyttleton and Bondi’s proposal, John King and his group at the Massachusetts Institute of Technology began to test experimentally whether the proton and electron differ in charge, and found that the charges appear to be wholly identical. That is an extraordinary fact, and not made easier to understand by the present belief that, though the electron is a single, apparently indivisible particle, the proton is made up of three quarks, to of them with charges of +2/3 e, and one with a charge of -1/3 e.
To summarize, if the proton and neutron did not have enormously greater mass than the electron, all matter would be fluid; and if the proton and electron did not possess exactly the same electric charge, no matter would aggregate. These are primary conditions for the existence of life in the universe.
Now, to leave the elementary particles and go on to atoms, to elements. Of the 92 natural elements, ninety-nine percent of the living matter we know is composed of just four: hydrogen (H), oxygen (O), nitrogen (N), and carbon (C). That is bound to be true wherever life exists in the universe, for only those four elements possess the unique properties upon which life depends.
Their unique position in chemistry can be stated in a sentence: They -- in the order given -- are the lightest elements that achieve stable electronic configurations (i.e., those mimicking the inert gases) by gaining respectively one, two, three, and four electrons. Gaining electrons, in the sense of sharing them with other atoms, is the mechanism of forming chemical bonds, hence molecules. The lightest elements make not only the tightest bonds, hence the most stable molecules, but introduce a unique property crucial for life: of all the natural elements, only oxygen, nitrogen and carbon regularly form double and triple bonds with one another, so saturating all their tendencies to combine further.
Now, professors sometimes tell their students foolish things, which the students carefully learn and reproduce on exams and eventually teach the next generation. When chemistry professors teach the periodic system of elements, one has those horizontal periods of the elements and the professors say, “If you go down vertically, the elements repeat their same properties.” That is utter nonsense, as any kid with a chemistry set would know. For under oxygen comes sulfur. Try breathing sulfur somethime. Under nitrogen comes phosphorus. There is not any phosphorus in that kid’s chemistry set. It is too dangerous; it bursts into flame spontaneously on exposure to air. And under carbon comes silicon.
If that chemistry professor were talking sense, there are two molecules that should have very similar properties: carbon dioxide (CO2) and silicon dioxide (SiO2). Well, in carbon dioxide the central carbon is tied to both of the oxygen atoms by double bonds O=C=O. Those double bonds completely saturate the combining tendencies of all three atoms, hence CO2 is a happy, independent molecule. It goes off in the air as a gas, and dissolves in all the waters of the Earth, and those are the places from which living organisms extract their carbon.
But silicon cannot form a double bond, hence in silicon dioxide the central silicon is tied to the two oxygens only by single bonds, leaving four half‑formed bonds -- four unpaired electrons -- two on the silicon and one on each oxygen, ready to pair with any other available lone electrons. But where can one find them? Obviously on neighboring silicone dioxide molecules, so each molecule binds to the next, and that to the next, and on and on until you end up with a rock -- for example quartz, which is just silicone dioxide molecules bound to one another to form a great super-molecule. The reason quartz is so hard is that to break it one must break numerous chemical bonds. And that is why, though silicon is 135 times as plentiful as carbon in the Earth’s surface, it makes rocks, and to make living organisms one must turn to carbon. I could make a parallel argument for oxygen and nitrogen.
These four elements, Hydrogen, carbon, oxygen and nitrogen, also provide an example of the astonishing togetherness of our universe. They make up the “organic” molecules that constitute living organisms on a planet, and the nuclei of these same elements interact to generate the light of its star. Then the organisms on the planet come to depend wholly on that starlight, as they must if life is to persist. So it is that all life on the Earth runs on sunlight. I do not need spiritual enlightenment to know that I am one with the universe -- that is just good physics.
Now let’s go up a step, to molecules. By far the most important molecule for living organisms is water. I think we can feel sure that if there is no liquid water, there is no life, anywhere in the universe. Water also happens to be the strangest molecule in all chemistry; and its strangest property is that ice floats. If ice did not float, I doubt that life would exist in the universe.
Virtually everything contracts on cooling. That is how we make thermometers: a bit of red-dyed alcohol, mercury if you can afford it, put in a capillary tube contracts on cooling, and you read the temperature. Everything does this. So does water, down to four degrees centigrade. But between four and zero degrees centigrade, where it freezes, it expands, so rapidly that the ice that forms is less dense than liquid water. The complete hydrogen bonding among the water molecules in ice holds them more widely spaced than in liquid water, so ice floats.
Nothing else does that. But what if water behaved like virtually everything else, and continued to contract on cooling? Then the increasingly dense water would constantly be sinking to the bottom, and freezing would begin at the bottom, , not as now at the top, and would end by freezing the water solidly. A really large mass of ice takes forever to melt, even at higher temperatures.
In my region of the United States, New England, the fishermen wait all winter for the ponds to freeze over. That is the best time to go fishing. They take their fishing equipment in one hand, a bottle of whiskey in the other, and cut themselves a round hole in the ice. Up to that point the fish were getting along fine. These creatures live through the winter with no trouble, and as soon as the warm weather comes, that skin of ice on the surface melts and with that everything is free again. If ice did not float, it is hard to see how any life could survive a cold spell. On any planet in the universe, if a freeze occured even once in many millions of years, that would probably be enough to block the rise of life, and to kill any life that had arisen.
And now another step up, to stars. The first generation of stars began as hydrogen, and lived by fusing it to helium. A hydrogen atom is composed of a proton as nucleus and one electron moving about it; but at temperatures of about five million degrees they are driven apart, and one is dealing with naked protons, hydrogen nuclei. Now four such protons, each of mass 1, begin to fuse to a helium nucleus of about mass 4, but in this process a very small amount of mass is lost -- four protons have a slightly larger mass than a helium nucleus -- and this tiny loss of mass is converted into radiation according to Einstein’s equation, E=mc2. Even so small a loss of mass yields a huge amount of radiation, and that flood of radiation pours out in the interior of what had been a collapsing mass of gas and stops its further collapse, stabilizing it, and is also the source of starlight.
Eventually, though, this process runs every star short of hydrogen. With that, it generates less energy and so begins to collapse again, and as it collapses it heats up some more. When the temperature in its deep interior reaches about one hundred million degrees, the helium nuclei begin to fuse. Two helium nuclei, each of mass 4, fuse to make beryllium, of mass 8, a nucleus so unstable as to disintegrate within 10-16 second (ten million billionths of a second).Yet in these enormous masses of material and at such high temperatures there are always a few beryllium nuclei, and here and there one of them adds another helium: 8 and 4 make 12, the mass of carbon. That is how carbon comes into the universe. Then a carbon nucleus can add another helium: 12 plus 4 make 16, the mass of oxygen, and that is how oxygen enters the universe. Also carbon, even at somewhat lower temperatures, can add hydrogens, and carbon-12 plus two hydrogens make 14, the mass of nitrogen. That is how nitrogen enters the universe.
These new processes, together with its heating by collapse, have by now puffed up our star to enormous size. It has become a Red Giant, a dying star. In its dying, it has made the elements of which life is composed. It is a moving realization that stars must die before organisms can live.
These Red Giants are in a delicate condition, and by distillation and in such stellar catastrophes as flares, novas, and supernovas they spew their substance out to become part of the great masses of gases and dust that fill all interstellar space. Over eons of time, great masses of those gases and dust are drawn together by their mutual gravitation to form new generations of stars. But such latecomers, unlike the first generation of stars made wholly of hydrogen and helium, contain also carbon, nitrogen, and oxygen. And we know that our Sun is such a later-generation star because we are here, because the Earth is one of those planets in the universe that supports life.
Finally, we have a cosmic principle: To have such a universe as this requires an extraordinary balance between two great cosmic forces: that of dispersion (expansion), powered by the Big Bang, and that of aggregation, powered by gravitation. If the forces of expansion were dominant, that would yield an isotropically dispersed universe lacking local clusters, galaxies or planetary systems; all the matter would be flying apart, and there would be no large solid bodies, hence no place for life. If, on the contrary, gravitation were dominant, the initial expansion produced by the Big Bang would have slowed up and come to an end, followed by a universal collapse, perhaps in preparation for the next Big Bang. There would be no time for life to arise, or it would be quickly destroyed.
We live in a universe in which it has just lately been realized that those two forces are in exact balance, so that the universe as a whole is expanding wherever one looks, everything very distant is going away from us, but locally there are so-called local groups and clusters, where whole clusters of galaxies are held together by gravitation. Our own relatively small cluster contains, in addition to the Milky Way, the Andromeda galaxy (M31). It is very much like our galaxy, but a little smaller, and there is also a still smaller galaxy, all part of our local group. Most of you have probably heard that we measure the expansion of the universe by the so-called red shift. The further one looks out into space, the redder the light is, compared to the same sources on earth. That is interpeted as an expression of the Doppler Effect, and taken to mean that the more distant an astronomical body, the faster it is receding from us. But the first such color shift ever to be discovered, by the astronomer Slipher back in 1912, was not a red shift by a blue shift. He was looking at our sister galaxy, Andromeda, and observed a blue shift because, far from receding, the Andromeda galaxy is coming toward us at about 125 miles per second. It is just this exact balance between the steady expansion of the universe as a whole and its stability locally that affords both enormous reaches of time and countless sites for the development of life.
I have here only sampled briefly an argument that extends much further. The nub of that argument is that our universe possesses a remarkably detailed constellation of properties, and as it happens, it is just that constellation that breeds life. It takes no great intelligence or imagination to conceive of other universes, indeed any number of them, each of which might be perfectly good, stable universes, but lifeless.
How did it happen that, with what seem to be so many other options, our universe came out just as it did? From our own self‑centered point of view, that is the best way to make a universe: But what I want to know is, how did the universe find that out?
It may be objected that the question would not arise if we were not here to ask it. Yet here we are, and strangely insistent on asking that kind of question. Perhaps that indeed is the answer: That this is a life‑breeding universe precisely in order eventually to bring forth creatures that ask and attempt to answer such questions, so that through them the universe can come not only to be, but to be known; indeed can come to know itself. That leads me to my other great problem, that of consciousness.
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