was so sufficiently cooled that nuclear reactions on a universe-wide scale, on a cosmic scale, ceased, and the gross nuclear structure of the world was frozen out as, in fact, what we see it to be today, which is three-quarters hydrogen and a quarter helium. The early universe was very simple and made only very simple things. It made only the two simplest chemical elements, hydrogen and helium, and those two elements have a very boring chemistry. There is nothing very much that you can do with them.
The chemistry of life actually requires about thirty elements, of which possibly the most important is carbon. We call ourselves, when we think about it, carbon-based life. The reason for that is that carbon is the basis of those very long-chain molecules, and the chemical properties of carbon seem to be necessary for living entities. But the early universe has no carbon at all. So, where did carbon come from?
As the universe began to get a bit clumpy and lumpy as gravity began to condense things, stars and galaxies began to form. Then, as the stars formed, the matter inside the stars began to heat up, and nuclear reactions began again, no longer on a cosmic scale, no longer universe-wide, but in the interior nuclear furnaces of the stars. It is here in the interior nuclear furnaces of the stars that all heavy elements— there are ninety of them altogether and about thirty of them necessary for life— were made beyond hydrogen and helium.
One of the great triumphs of the second half of the twentieth century in astrophysics was working out how those elements were made by the nuclear reactions inside the stars. One of the persons who played an absolutely leading role in that was a senior colleague of mine in Cambridge called Fred Hoyle. Fred was with Willy Fowler from Caltech— they were thinking together about how these things might happen, and they were absolutely stuck at the start.
The first element they really wanted to make was carbon, and they could not for the life of them see how to make it. They had helium nuclei— we call them alpha particles— around. To make carbon, you have to take three alpha particles and make them stick together. That turns helium-4 into carbon-12, and that is a very, very difficult thing to do. The only way to do it is to get two of them and make them stick together. First of all, that makes beryllium, and then you hope that beryllium stays around for a bit. A third alpha particle comes wandering along and eventually sticks on and makes carbon-12. Unfortunately, it does not work in a straightforward way, because beryllium is unstable and does not oblige you by staying around to acquire that extra alpha particle.
So, Fred and Willy really could not figure out how to do it. On the other hand, there they were, carbon-based life, thinking about these things: It must be possible to make carbon. And then Fred had a very good idea. He realized it would be possible to make some carbon out of even this very transient beryllium if there was an enhancement— what, in the trade, we call a resonance— present in carbon that would produce an enhanced effect. That is, it would make things go much, much more quickly than you would expect. However, you not only had to have a resonance but also had to have it at the right place; it had to be at the right energy for this process to be possible. If it were anywhere else, it would not affect the rate at which things happen.
So, Fred is convinced that there must be a resonance in carbon at precisely this energy and writes down what the energy is. The next thing, he goes to the nuclear data tables to see if carbon had such resonance, and it is not in the nuclear data tables. Fred is so sure it must be there that he rings up his friends the Laurences, who are very clever experimentalists at Caltech, and says to them, “You look. You missed the resonancy in carbon-12, and I’ll tell you exactly where to look for it. Look at this energy, and you’ll find it.” And they did— a very staggering scientific achievement. It was a very, very great thing, but the point is this: That resonance would not be there at that absolutely unique and vital energy if the laws of nuclear physics were in the smallest degree different from what they actually are in our universe.
When Fred saw that and realized that— Fred has always been powerfully inclined toward atheism— he said, in a Yorkshire accent, which, I am afraid, is beyond my powers to imitate, “The universe is a put-up job.” In other words, this cannot be just a haphazard accident. There must be something lying behind this. And, of course, Fred does not like the word God; he said there must be some capital-I Intelligence behind what is going on in the world. So, there we are; we are all creatures of stardust. Every atom of our bodies was once inside a star, and that is possible because the laws of nuclear physics are what they are and not anything else.
Let me give you just one more example of fine-tuning. This is the most exacting example of all. It is possible to think of there being a sort of energy present in the universe, which is associated simply with space itself, and that energy these days is usually called dark energy. It used to be called the cosmological constant, but it has come to be called dark energy, because just recently astronomers believe they have measured the presence of this dark energy. In fact, it is driving the expansion of the universe.
What is striking about that expansion is that this energy is very, very small, compared to what you would expect its natural value to be. You can figure out— now, I won’t go into the details— what you would expect the natural value of this energy to be. If you’re in the trade, it is due to vacuum effects and things of that nature, but it turns out that the observed dark energy— if the observations are correct— is ten to the minus-120 times the natural expected value (10-120); that’s one over one followed by 120 zeroes.
Even if you’re not a mathematician, I am sure that you can see that is a very small number indeed. If that number were not actually as small as it is, we would not be here to be astonished at it, because anything bigger than that would have blown the universe apart so quickly that no interesting things could have happened. You would have become too diluted for anything as interesting as life to be possible.
So, there are all these sorts of fine-tunings present in the world. All scientists would agree about those facts. Where the disagreements come, of course, is in answering the meta-question: What do we make of that? What do we think about the remarkable character of the world, the specific character of the world? Was Fred right to think that the universe is, indeed, a put-up job and that there is some sort of Intelligence behind it all?
I am sure you all know that these considerations about the fine-tuning of the universe are called the anthropic principle— not meaning that the world is tuned to produce literally Homo sapiens, but anthropoi, meaning beings of our self-conscious complexity. I have a friend who thinks about these things and has written, I think, the best book about the anthropic principle, Universes. His name is John Leslie. He’s an interesting chap because he does his philosophy by telling stories, which is very nice. He’s a parabolic philosopher. That is very nice for chaps like me who are not trained in philosophy, because everybody can appreciate a story, and he tells the following story:
You’re about to be executed. You are tied to the stake, your eyes are bandaged, and the rifles of ten highly trained marksmen are leveled at your chest. The officer gives the order to fire, the shots ring out, and you find that you have survived. So, what do you do? Do you just walk away and say, “Gee, that was a close one”? I don’t think so. So remarkable an occurrence demands some sort of explanation, and Leslie suggests that there are really only two rational explanations for your good fortune.
One is this: Maybe there are many, many, many executions taking place today. Even the best of marksmen occasionally miss, and you happen to be in the one where they all miss. There have to be an awful lot of executions taking place today for that to be a workable explanation, but if there are enough, then it is a rational possibility. There is, of course, another possible explanation: Maybe there is only one execution scheduled for today, namely yours, but more was going on in that event than you are aware of. The marksmen are on your side, and they missed by design.
You see how that charming story translates into thinking about the anthropic fine-tuning— the special character of the universe
