LIFE IN THE QUANTUM UNIVERSE Steven Weinberg
We comprehend the Universe and our place in it. But there are limits to what we can explain at present. Will research at the boundaries of science reveal a special role for intelligent life?
STEVEN WEINBERG was educated at Cornell University, the Niels Bohr Institute in Copenhagen and Princeton University and has received honorary doctoral degrees from a dozen other universities. His work has spanned a wide range of topics in elementary particle physics and cosmology, including the unification of the electromagnetic with the weak nuclear force, for which he shared the 1979 Nobel Prize for Physics. Weinberg has won numerous other prizes and awards, including in 1991 the National Medal of science. He is a member of both the National Academy of Sciences and Britain�s Royal Society, as well as of many other academies and honorary societies. This year he is president of the Philosophical Society of Texas. Since 1982 he has been a member of the physics and astronomy departments of the University of Texas at Austin. His latest book is Dreams of a Final Theory: The Search for the Fundamental Laws of Nature.
In Walt Whitman's often quoted poem "When I Heard the Learn'd Astronomer," the poet tells how, being shown the astronomer's charts and diagrams, he became tired and sick and wandered off by himself to look up "in perfect silence at the stars." Generations of scientists have been annoyed by these lines. The sense of beauty and wonder does not become atrophied through the work of science, as Whitman implies. The night sky is as beautiful as ever, to astronomers as well as to poets. And as we understand more and more about nature, the scientist's sense of wonder has not diminished but has rather become sharper, more narrowly focused on the mysteries that still remain.
The nearby stars that Whitman could see without a telescope are now not so mysterious. Massive computer codes simulate the nuclear reactions at the stars' cores and follow the flow of energy by convection and radiation to their visible surfaces, explaining both their present appearance and how they have evolved. The observation in 1987 of gamma rays and neutrinos from the supernova in the Large Magellanic Cloud provided dramatic confirmation of the theory of stellar structure and evolution. These theories are themselves beautiful to us, and knowing why Betelgeuse is red may even add to the pleasure of looking at the winter sky.
But there are plenty of mysteries left, many of them discussed by other authors in this issue. Of what kind of matter are galaxies and galactic clusters made? How did the stars, planets and galaxies form? How widespread in the universe are habitats suitable for life? How did the earth's oceans and atmosphere form? How did life start? What are the relations of cause and effect between the evolution of life and the terrestrial environment in which it has occurred? How large is the role of chance in the origin of the human species? How does the brain think? How do human institutions respond to environmental and technological change?
We may be very far from the solution of some of these problems. Still, we can guess what kinds of solutions they will have, in a way that was not possible when Scientific American was founded 150 years ago. New ideas and insights will be needed, which we can expect to find within the boundaries of science as we know it.
Then there are mysteries at the outer boundaries of our science, matters that we cannot hope to explain in terms of what we already know. When we explain anything we observe, it is in terms of scientific principles that are themselves explained in terms of deeper principles. Following this chain of explanations, we are led at last to laws of nature that cannot be explained within the boundaries of contemporary science. And in dealing with life and many other aspects of nature, our explanations have a historical component. Some historical facts are accidents that can never be explained, except perhaps statistically: we can never explain precisely why life on the earth takes the form it does, although we can hope to show that some forms are more likely than others. We can explain a great deal, even where history plays a role, in terms of the conditions with which the universe began, as well as the laws of nature. But how do we explain the initial conditions? A further complex of puzzles overhangs the laws of nature and the initial conditions. It concerns the dual role of intelligent life - as part of the universe we seek to explain, and as the explainer.
The laws of nature as we currently understand them allow us to trace the observed expansion of the universe back to what would be a true beginning, a moment when the universe was infinitely hot and dense. some 10 to 20 billion years ago. We do not have enough confidence in the applicability of these laws at extreme temperatures and densities to be sure that there really was such a moment, much less to work out all the initial conditions, if there were any. For the present, we cannot do better than to describe the initial conditions of the universe at a time about 10-12 second after the nominal moment of infinite temperature.
The temperature of the universe had dropped by then to about 1015 degrees, cool enough for us to apply our physical theories. At these temperatures the universe would have been filled with a gas consisting of all the types of particles known to high- energy nuclear physics, together with their antiparticles, continually being annihilated and created in their collisions. As the universe continued to expand and cool, creation became slower than annihilation, and almost all the particles and antiparticles disappeared. If there had not been a small excess of electrons over antielectrons, and quarks over antiquarks, then ordinary particles like electrons and quarks would be virtually absent in the universe today. It is this early excess of matter over antimatter, estimated as one part in about 1010, that survived to form light atomic nuclei three minutes later, then after a million years to form atoms and later to be cooked to heavier elements in stars, ultimately to provide the material out of which life would arise. The one part in 1010 excess of matter over antimatter is one of the key initial conditions that determined the future development of the universe.
In addition, there may exist other types of particles, not yet observed in our laboratories, that interact more weakly with one another than do quarks and electrons and that therefore would have annihilated relatively slowly. Large numbers of these exotic particles would have been left over from the early universe, forming the "dark matter" that now apparently makes up much of the mass of the universe.
Finally, although it is generally assumed that when the universe was 10-12 second old its contents were pretty nearly the same everywhere, small inhomogeneities must have existed that triggered the formation, millions of years later, of the first galaxies and stars. We cannot directly observe any inhomogeneities at times earlier than about a million years after the beginning, when the universe first became transparent. Astronomers are currently engaged in mapping minute variations in the intensity of the cosmic microwave radiation background that was emitted at that time, using them to infer the primordial distribution of matter. This information can in turn be used to deduce the initial inhomogeneities at 10-12 second after the beginning.
From the austere viewpoint of fundamental physics, the history of the universe is just an illustrative example of the laws of nature. At the deepest level to which we have been able to trace our explanations, those laws take the form of quantum field theories. When quantum mechanics is applied to a field such as the electromagnetic field, it is found that the energy and momentum of the field come in bundles, or quanta, that are observed in the laboratory as particles. The modern Standard Model posits an electromagnetic field, whose quanta are photons; an electron field, whose quanta are electrons and antielectrons; and a number of other fields whose quanta are particles called leptons and antileptons. There are various quark fields whose quanta are quarks and antiquarks, and there are 11 other fields whose quanta are the particles that transmit the weak and strong forces that act on the elementary particles.
The Standard Model is certainly not the final law of nature. Even in its simplest form it contains a number of arbitrary features. Some 18 numerical parameters exist whose values have to be taken from experiment, and the multiplicity of types of quarks and leptons is unexplained. Also, one aspect of the model is still uncertain: we are not sure of the details of the mechanism that gives masses to the quarks, electrons and other particles. This is the puzzle that was to have been solved by the now canceled Superconducting Super Collider. We hope it will be unraveled by the Large Hadron Collider being planned at CERN near Geneva. Finally, the model is incomplete; it does not include gravitation. We have a good field theory of gravitation, the General Theory of Relativity, but the quantum version of this theory breaks down at very high energies.
It is possible that all these problems will find their solution in a new kind of theory known as string theory. The point particles of quantum field theory are reinterpreted in string theory as tiny, extended one-dimensional objects called strings. These strings can exist in various modes of vibration, each mode appearing in the laboratory as a different type of particle. String theory not only provides a quantum description of gravitation that makes sense at all energies; one of the modes of vibration of a string would appear as a particle with the properties of the graviton, the quantum of the gravitational field, so string theory even offers an explanation of why gravitation exists. Further, there are versions of string theory that predict something like the menu of fields incorporated in the Standard Model.
But string theory has had no successes yet in explaining or predicting any of the numerical parameters of the Standard Model. Moreover, strings are much too small for us to detect directly the stringy nature of elementary particles; a string is smaller relative to an atomic nucleus than is a nucleus relative to a mountain. The intellectual investment now being made in string theory without the slightest encouragement from experiment is unprecedented in the history of science. Yet for now, it offers our best hope for a deeper understanding of the laws of nature.
The present gaps in our knowledge of the laws of nature stand in the way of explaining the initial conditions of the universe, at 10-12 second after the nominal beginning, in terms of the history of the universe at earlier times. Calculations in the past few years have made it seem likely that the tiny excess of quarks and electrons over antiquarks and antielectrons at this time was produced a little earlier, at a temperature of about 1016 degrees. At that moment the universe went through a phase transition, something like the freezing of water, in which the known elementary particles for the first time acquired mass. But we cannot explain why the excess produced in this way should be one part in 1010, or calculate its precise value, until we understand the details of the mass- producing mechanism.
The other initial condition, the degree of inhomogeneity in the early universe, may trace back to even earlier times. In our quantum field theories of elementary particles, including the simplest version of the Standard Model, several fields pervade the universe, taking non-zero values even in supposedly empty space. In the present state of the universe, these fields have reached equilibrium values, which minimize the energy density of the vacuum. This vacuum energy density, also known as the cosmological constant, can be measured through the gravitational field that it produces. It is apparently very small.
In some modern theories of the early universe, however, there was a very early time when these fields had not yet reached their equilibrium values, so that the vacuum would have had an enormous energy density. This energy would have produced a rapid expansion of the universe, known as inflation. Tiny inhomogeneities that would have been produced by quantum fluctuations before this inflation would have been magnified in the expansion and could have produced the much larger inhomogeneities that millions of years later triggered the formation of galaxies. It has even been conjectured that the inflation that began the expansion of the visible universe did not occur throughout the cosmos. It may instead have been just one local episode in an eternal succession of local inflations that occur at random throughout an infinite universe. If this is true, then the problem of initial conditions disappears; there was no initial moment.
In this picture, our local expansion may have begun with some special ingredients or inhomogeneities, but like the forms of life on the earth, these could be understood only in a statistical sense. Unfortunately, at the time of inflation gravitation was so strong that quantum gravitational effects were important. So these ideas will remain speculative until we understand the quantum theory of gravitation - perhaps in terms of something like a string theory.
The experience of the past 150 years has shown that life is subject to the same laws of nature as is inanimate matter. Nor is there any evidence of a grand design in the origin or evolution of life. There are well-known problems in the description of consciousness in terms of the working of the brain. They arise because we each have special knowledge of our own consciousness that does not come to us from the senses. In principle, no obstacle stands in the way of explaining the behavior of other people in terms of neurology and physiology and, ultimately, in terms of physics and history. When we have succeeded in this endeavor, we should find that part of the explanation is a program of neural activity that we will recognize as corresponding to our own consciousness.
But as much as we would like to take a unified view of nature, we keep encountering a stubborn duality in the role of intelligent life in the universe, as both subject and student. We see this even at the deepest level of modern physics. In quantum mechanics the state of any system is described by a mathematical object known as the wave function. According to the interpretation of quantum mechanics worked out in Copenhagen in the early 1930s, the rules for calculating the wave function are of a very different character from the principles used to interpret it. On one hand, there is the Schr�dinger equation, which describes in a perfectly deterministic way how the wave function of any system changes with time. Then, quite separate, there is a set of principles that tells how to use the wave function to calculate the probabilities of various possible outcomes when someone makes a measurement.
The Copenhagen interpretation holds that when we measure any quantity, such as position or momentum, we are intervening in a way that causes an unpredictable change in the wave function, resulting in a wave function for which the measured quantity has some definite value, in a manner that cannot be described by the deterministic Schr�dinger equation. For instance, before a measurement the wave function of a spinning electron is generally a sum of terms corresponding to different directions of the electron's spin; in such a state the electron cannot be said to be spinning in any particular direction. If we measure whether the electron is spinning clockwise or counterclockwise around some axis, however, we some how change the electron's wave function so that it is definitely spinning one way or the other. Measurement is thus regarded as something intrinsically different from anything else in nature. And although opinions differ, it is hard to identify anything special that qualifies some process to be called a measurement, except its effect on a conscious mind.
Among physicists and philosophers one finds at least four different reactions to the Copenhagen interpretation. The first is simply to accept it as it stands. This attitude is mostly limited to those who are attracted to the old, dualistic worldview that puts life and consciousness on a different footing from the rest of nature. The second attitude is to accept the rules of the Copenhagen interpretation for practical purposes, without worrying about their ultimate interpretation. This attitude is by far the most common among working physicists. The third approach is to try to avoid these problems by changing quantum mechanics in some way. So far no such attempt has found much acceptance among physicists.
The final approach is to take the Schr�dinger equation seriously, to give up the dualism of the Copenhagen interpretation and to try to explain its successful rules through a description of measurers and their apparatus in terms of the same deterministic evolution of the wave function that governs everything else. When we measure some quantity (like the direction of an electron's spin), we put the system in an environment (for instance, a magnetic field) where its energy (or momentum) has a strong dependence on the value of the measured quantity. According to the Schr�dinger equation, the different terms in the wave function that correspond to different energies will oscillate at rates proportional to these energies.
A measurement thus makes the terms of the wave function that correspond to different values of a measured quantity, such as an electron spin, oscillate rapidly at different rates, so they cannot interfere with one another in any future measurement, just as the signals from radio stations broadcasting at widely spaced frequencies do not interfere. In this way, a measurement causes the history of the universe for practical purposes to diverge into different noninterfering tracks, one for each possible value of the measured quantity.
Yet how do we explain the Copenhagen rules for calculating the probabilities for these different "worldtracks" in a world governed by the completely deterministic Schr�dinger equation? Progress has recently been made on this problem, but it is not yet definitely solved. (For what it is worth, I prefer this last approach, although the second has much to recommend it.)
It is also difficult to avoid talking about living observers when we ask why our physical principles are what they are. Modem quantum field theory and string theory can be understood as answers to the problem of reconciling quantum mechanics and special relativity in such a way that experiments are guaranteed to give sensible results. We require that the results of our dynamical calculations must satisfy conditions known to field theorists as unitarity, positivity and cluster decomposition. Roughly speaking, these conditions require that probabilities always add up to 100 percent, that they are always positive and that those observed in distant experiments are not related.
This is not so easy. If we try to write down some dynamical equations that will automatically give results consistent with some of these conditions, we usually find that the results violate the other conditions. It seems that any relativistic quantum theory that satisfies all these conditions must appear at sufficiently low energy like a quantum field theory. That is presumably why nature at accessible energies is so well described by the quantum field theory known as the Standard Model.
Also, so far as we can tell, the only mathematically consistent relativistic quantum theories that satisfy these conditions at all energies and that involve gravitation are string theories. Further, the student of string theory who asks why one makes this or that mathematical assumption is told that otherwise one would violate physical principles like unitarity and positivity. But why are these the correct conditions to impose on the results of all imaginable experiments if the laws of nature allow the possibility of a universe that contains no living beings to carry out experiments?
This question does not intrude on much of the actual work of theoretical physics, but it becomes urgent when we seek to apply quantum mechanics to the whole universe. At present, we do not understand even in principle how to calculate or interpret the wave function of the universe, and we cannot resolve these problems by requiring that all experiments should give sensible results, because by definition there is no observer outside the universe who can experiment on it.
These mysteries are heightened when we reflect how surprising it is that the laws of nature and the initial conditions of the universe should allow for the existence of beings who could observe it. Life as we know it would be impossible if any one of several physical quantities had slightly different values. The best known of these quantities is the energy of one of the excited states of the carbon 12 nucleus. There is an essential step in the chain of nuclear reactions that build up heavy elements in stars. In this step, two helium nuclei join together to form the unstable nucleus of beryllium 8, which sometimes before fissioning absorbs another helium nucleus, forming carbon 12 in this excited state. The carbon 12 nucleus then emits a photon and decays into the stable state of lowest energy. In subsequent nuclear reactions carbon is built up into oxygen and nitrogen and the other heavy elements necessary for life. But the capture of helium by beryllium 8 is a resonant process, whose reaction rate is a sharply peaked function of the energies of the nuclei involved. If the energy of the excited state of carbon 12 were just a little higher, the rate of its formation would be much less, so that almost all the beryllium 8 nuclei would fission into helium nuclei before carbon could be formed. The universe would then consist almost entirely of hydrogen and helium, without the ingredients for life.
Opinions differ as to the degree to which the constants of nature must be fine-tuned to make life necessary. There are independent reasons to expect an excited state of carbon 12 near the resonant energy. But one constant does seem to require an incredible fine-tuning: it is the vacuum energy, or cosmological constant, mentioned in connection with inflationary cosmologies.
Although we cannot calculate this quantity, we can calculate some contributions to it (such as the energy of quantum fluctuations in the gravitational field that have wavelengths no shorter than about 10-33 centimeter). These contributions come out about 120 orders of magnitude larger than the maximum value allowed by our observations of the present rate of cosmic expansion. If the various contributions to the vacuum energy did not nearly cancel, then, depending on the value of the total vacuum energy, the universe either would go through a complete cycle of expansion and contraction before life could arise or would expand so rapidly that no galaxies or stars could form.
Thus, the existence of life of any kind seems to require a cancellation between different contributions to the vacuum energy, accurate to about 120 decimal places. It is possible that this cancellation will be explained in terms of some future theory. So far, in string theory as well as in quantum field theory, the vacuum energy involves arbitrary constants, which must be carefully adjusted to make the total vacuum energy small enough for life to be possible.
All these problems can be solved without supposing that life or consciousness plays any special role in the fundamental laws of nature or initial conditions. It may be that what we now call the constants of nature actually vary from one part of the universe to another. (Here "different parts of the universe" could be understood in various senses. The phrase could, for example, refer to different local expansions arising from episodes of inflation in which the fields pervading the universe took different values or else to the different quantum-mechanical worldtracks that arise in some versions of quantum cosmology.) If this is the case, then it would not be surprising to find that life is possible in some parts of the universe, though perhaps not in most. Naturally, any living beings who evolve to the point where they can measure the constants of nature will always find that these constants have values that allow life to exist. The constants have other values in other parts of the universe, but there is no one there to measure them. (This is one version of what is sometimes called the anthropic principle.) Still, this presumption would not indicate any special role for life in the fundamental laws, any more than the fact that the sun has a planet on which life is possible indicates that life played a role in the origin of the solar system. The fundamental laws would be those that describe the distribution of values of the constants of nature between different parts of the universe, and in these laws life would play no special role.
If the content of science is ultimately impersonal, its conduct is part of human culture, and not the least interesting part. Some philosophers and sociologists have gone so far as to claim that scientific principles are, in whole or in part, social constructions, like the rules of contract law or contract bridge. Most working scientists find this "social constructivist" point of view inconsistent with their own experience. Still, there is no doubt that the social context of science has become increasingly important to scientists, as we need to ask society to provide us with more and more expensive tools: accelerators, space vehicles, neutron sources, genome projects and so on.
It does not help that some politicians and journalists assume the public is interested only in those aspects of science that promise immediate practical benefits to technology or medicine. Some work on the most interesting problems of biological or physical science does have obvious practical value, but some does not, especially research that addresses problems lying at the boundaries of scientific knowledge. To earn society's support, we have to make true what we often claim: that today's basic scientific research is part of the culture of our times.
Whatever barriers now exist to communication between scientists and the public, they are not impermeable. Isaac Newton's Principia could at first be understood only by a handful of Europeans. Then the news that we and our universe are governed by precise, knowable laws did eventually diffuse throughout the civilized world. The theory of evolution was strenuously opposed at first; now creationists are an increasingly isolated minority. Today's research at the boundaries of science explores environments of energy and time and distance far removed from those of everyday life and often can be described only in esoteric mathematical language. But in the long run, what we learn about why the world is the way it is will become part of everyone's intellectual heritage.