Monday, 7 May 2007

Is the Universe Fine Tuned? by Steven Weinberg

Now, it doesn't settle the matter for me to say that we cannot see the hand of a designer in what we know about the fundamental principles of science. It might be that, although these principles do not refer explicitly to life, much less human life, they are nevertheless craftily designed to bring it about.

Some physicists have argued that certain constants of nature have values that seem to have been mysteriously fine-tuned to just the values that allow for the possibility of life, in a way that could only be explained by the intervention of a designer with some special concern for life. I am not impressed with these supposed instances of fine-tuning. For instance, one of the most frequently quoted examples of fine-tuning has to do with the energy of a certain excited state of the carbon nucleus. The build-up in stars of elements necessary for life, like carbon and oxygen, depends on the carbon nucleus having an excited state at an energy within a narrow range, where in fact just such a state is found. The reason that it has to have this energy is to provide a way for carbon nuclei be formed in stars in collisions of helium nuclei with unstable beryllium nuclei, which is a necessary step in the build-up of all elements heavier than helium. But recent calculations show that, as has been long expected, without any fine tuning of the constants of nature one would in any case expect the carbon nucleus to have a state like an unstable molecule, consisting of a helium nucleus and a beryllium nucleus, which would naturally have an energy close to the values necessary for the synthesis of carbon and heavier elements.This excited state of the carbon nucleus is 7.65 million electron volts (MeV) above the energy of the carbon nucleus in its normal state, the state of lowest energy. Calculations [M. Livio, D. Hollowell,...

There is one constant whose value does seem remarkably well adjusted in our favor. It is the energy density of empty space, also known as the cosmological constant. It could have any value, but from first principles one would guess that this constant should be very large - much too large to allow matter to clump together in the early universe, which is the first step in forming galaxies and stars and planets and people. It's too early to tell if this is a real problem, or if there is some fundamental principle that explains why the cosmological constant must be this small.

But even if there is no such principle, recent developments in cosmology offer the possibility of an explanation why the measured values of the cosmological constant and other physical constants are favorable for the appearance of intelligent life. Sidney Coleman has shown how quantum mechanical effects can lead to a picture of the wave function of the universe in which the wave function is the sum of many different terms, each term corresponding to a big (or little) bang in which what we call the constants of nature take all possible values. Also, as you have heard here from Alan Guth, in the `chaotic inflation' theories of Andre Linde and others our big bang is supposed to be just one episode in a much larger universe in which big bangs go off all the time, each with different values of the fundamental constants.

In any such picture, in which the universe contains many parts with different values for what we call the constants of nature, there would be no difficulty in understanding why these constants take values favorable to intelligent life. There would be a vast number of big bangs in which the constants of nature take values unfavorable for life, and much fewer where life is possible. You don't have to invoke a benevolent designer to explain why we are in one of the parts of the universe where life is possible. In all the other parts of the universe there is no one to raise the question.

To conclude that the constants of nature have been fine-tuned by a benevolent designer is like saying “Isn't it wonderful that God put us here on earth, where there's water and air and the surface gravity and temperature are so comfortable, rather than some horrid place, like Mercury or Pluto.” Where else in the solar system but on earth could we have evolved?

Reasoning like this is called “anthropic.” Sometimes it just amounts to an assertion that the laws of nature are what they are so that we can exist, without further explanation. This seems to me to be little more than mystical mumbo-jumbo. On the other hand, if there really is a large number of worlds in which some constant takes different values, then the anthropic explanation of why in our world it takes values favorable for life is just common sense, like explaining why we live on the earth rather than Mercury or Pluto. The actual value of the cosmological constant, recently measured by observations of the motion of distant supernovas, is about what you would expect from this sort of argument; it is just about small enough to prevent it from interfering with the formation of galaxies and so on. But we don't yet know enough about physics to tell whether there are different parts of the universe in which what are usually called the constants of physics really do take different values. This is not a hopeless question; we will be able to answer it when we know more about the quantum theory of gravitation than we do now.

No comments: