Is the universe fine-tuned for life? 1
Friday 8 May 2009
FROM RUSSELL STANNARD, EMERITUS PROFESSOR OF PHYSICS AT OPEN UNIVERSITY: "The more the universe seems comprehensible, the more it also seems pointless." Those are the words of Nobel laureate Steven Weinberg in his book The First Three Minutes. He goes on to dismiss human life as "a more-or-less farcical outcome of a chain of accidents."
It is not difficult to appreciate how one might arrive at such a gloomy assessment. Take, for example, the size of the universe. It takes 13.7 billion years for light to reach us from the farthest depths of space, even though it travels at 300,000 kilometers per second. Are we really expected to believe that God designed it as a home for humans? A case of over-design perhaps?
Most places in the universe are hostile to life. The depths of space are incredibly cold. The most prominent objects in the sky, the sun and the other stars, are balls of fire and thus not suitable places to find life. For the great majority of the history of the universe, there was no intelligent life. After the stars have exhausted their fuel, there comes the Heat Death of the universe—an infinity of time when there will again be no life. Hence the view that life is but a fleeting, accidental byproduct of no significance. Or is it?
Suppose you were put in charge of making a universe. You have freedom to choose the laws of nature and the conditions under which your imaginary universe operates. The aim is to produce a universe that is tailor-made for the development of life—the kind of universe a sensible God would have created if it were really intended primarily as a home for life.
Let us assume you start off your universe with a big bang. All the galaxies of stars are to be receding from each other in the aftermath of that great explosion. The first decision is how violent to make your big bang. You might feel, for example, that the actual big bang was somewhat excessive if the aim was simply to produce some life forms. How about something more discreet? It turns out that if you make the violence of your big bang somewhat less—only a little less—then the mutual gravity operating between the galaxies will get such a secure grip that the galaxies will slow down to a halt, and will thereafter be brought together in a big crunch. Moreover, this will happen in less time than the 13.7 billion years it took for us humans to appear on the scene in the actual universe. So, turn the wick down, and you will get no intelligent life.
All right, you might say, I´ll turn the wick up a little. I´ll make my big bang more violent than the actual one. What happens now is that the gases come out of the big bang so fast that they do not have time to collect together to form embryo stars before they are dispersed into the depths of space. Since there are no stars, you get no life. In fact, it turns out that as far as the big bang´s violence is concerned, the window of opportunity is exceedingly narrow. If you are to get life in your universe, the thrust must be just right—and that is what our actual universe has managed to do.
The next point to consider is the force of gravity. How strong will it be in your imaginary universe? If you make it a little weaker than it actually is, you will collect gas together after the big bang. It will squash down, but there will not be enough of it to produce a temperature rise sufficient to light the nuclear fires. No stars, no life.
On the other hand, you must be careful not to have your gravity too strong. If it is, you will get only very massive types of stars. These burn exceedingly fast and last for only 1 million years. For evolution to produce intelligent life on a nearby planet, you must have a steady source of energy for 5,000 million years; you need a medium-sized star like the sun. Indeed, when you come to think of it, the sun is a remarkable phenomenon. After all, what is a star? It is a nuclear bomb going off slowly. Have you any idea how difficult that is to achieve? The amazing thing is that the sun manages this. The secret is the way the force of gravity in the sun conspires to feed the new fuel into the nuclear furnace at the center of the star. It does so at just the right rate for the nuclear fires (governed by the nuclear force, an entirely different force from that of gravity) to consume it at a steady rate extending over a period of 10 billion years.
So, in order for there to be life, the force of gravity—like the thrust of the big bang—must lie within a very narrow range of possible values. And the gravity of the actual universe does just that.
Next, you must turn your attention to the materials from which you wish to build the bodies of living creatures. This is no small matter. After all, what have you got coming from your big bang? The two lightest gases—hydrogen and helium—and precious little besides. And it has to be that way. Remember, we need a violent big bang to stop the universe from collapsing back in on itself prematurely. And because of that violence, only the lightest nuclei could survive the collisions occurring at that time, anything bigger getting smashed up again soon after its formation.
But you cannot make interesting objects like human bodies out of just hydrogen and helium. So the extra nuclei—those that go to making up the 92 different elements found on Earth—must be manufactured somehow after the big bang. That´s where the stars have another important role to play. Not only do they provide a steady source of warmth to energize the processes of evolution, but they also first serve as furnaces for fusing light nuclei into the heavy ones that will later be needed for producing the bodies of the evolving creatures.
But this process is far from straightforward. Perhaps the most important atom in the making of life is carbon. In a sense, it is an especially "sticky" kind of atom, very good at cementing together the large molecules of biological interest. But forming a nucleus of carbon is by no means easy. Essentially, it consists of fusing three helium nuclei together—which is as unlikely as having three moving snooker balls collide simultaneously. It involves something called a "nuclear resonance," and the occurrence of this resonance is so highly fortuitous that its discoverer, one-time atheist Fred Hoyle, was moved to declare that "a commonsense interpretation of the facts suggests that a superintellect has monkeyed with the physics."
So, we have our precious carbon. A collision between some of these carbon nuclei and further helium nuclei yields oxygen—another vital ingredient for life—and so on. Thus, you must be sure to incorporate a fortuitous nuclear resonance in your imaginary universe.
Does this mean that the stage is now set for evolution to take over and convert these raw materials into human beings?
Not so. You have your materials, but where are they? They are in the center of a star at a temperature of about 10 million degrees. Hardly an environment conducive to life. The materials have to be got out. But how?
What happens in the actual universe is that a proportion of the newly synthesized material is ejected by supernova explosions. These occur when massive stars—several times the mass of our sun—run out of fuel. They suddenly collapse in on themselves. But that raises a problem. How can an implosion produce and explosion? This was a conundrum that exercised the minds of astrophysicists for many years.
The mechanism turned out to be the strangest imaginable. The material is blasted out by neutrinos. Neutrinos are famous for hardly ever interacting with anything. One could pass a neutrino through the center of the Earth to Australia 100 billion times before it had a 50:50 chance of hitting anything. Neutrinos are incredibly slippery. How fortunate they were not any more slippery than they are.
There are many other conditions that had to be satisfied in order for there to be intelligent life anywhere within the universe. The sum total of these "coincidences" goes under the name "the anthropic principle."
More tomorrow
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