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The Future of Science, and the Universe

In the program for a lecture series at the New York Public Library I saw one vision of the future: Raymond Loewy’s conception of an airliner, as exhibited at the 1939 World’s Fair in New York.1 I was there at the 1939 World’s Fair, but I don’t remember Raymond Loewy’s design. I was very young. What I best remember are the fountains lit up by colored lights. Also, I remember that a dairy company was giving out tiny free ice cream cones. With the Depression still going on, free ice cream was a memorable experience. Whatever predictions of future technology were made at the World’s Fair did not leave much of an impression on me.

It was no great loss. Aside perhaps from the vision of modern superhighways in the General Motors pavilion, the World’s Fair did not score great successes in its predictions of future technology. The illustration of Loewy’s design for an airliner of the future doesn’t look at all like passenger aircraft today. It shows eight engines, and a fuselage resembling a diesel locomotive. I didn’t know it in 1939, but Raymond Loewy had in fact designed diesel locomotives for the Pennsylvania Railroad in the 1930s, giving them a futuristic “streamlined” look without actually paying much attention to principles of aerodynamics. He could get away with this with diesel locomotives, but not with airplanes. But predicting future technology is very difficult even if you don’t ignore the laws of physics. You might better spend your time admiring fountains of colored water.

My subject here is not the future of technology or other applications of science, but the future of science itself. Here we can make a prediction with fair confidence—that sooner or later we shall discover the physical principles that govern all natural phenomena.

We already have a theory that encompasses all the particles out of which we and our surroundings are made, and, except for gravitation, all the forces that act on them. This theory, known as the Standard Model, is expressed in the mathematical formalism of quantum mechanics, which seems to be a universal basis for the laws of physics. But this theory has too many arbitrary elements, like the masses that have to be assigned to the various elementary particles. We also have a theory of gravitation—the General Theory of Relativity—which predated quantum mechanics. This theory can even be interpreted quantum mechanically. The trouble is that our present quantum theory of gravitation only provides approximations whose validity is limited to processes at low energies and large distances. What we do not yet have is a quantum mechanical theory of unlimited validity that encompasses all particles and forces. We are like the plebeians of Rome at the time when the Twelve Tables were still kept secret, not knowing the laws by which we are governed. More specifically, as I will come to later, our ignorance of the final laws of nature makes us uncertain in our predictions of the future of the universe.

There is now a strong suspicion that the final theory will be something like today’s string theories, which you can read about in the popular book by Brian Greene.2 To describe them briefly (which is hardly possible): In a string theory each elementary particle, whether it is an electron or a neutrino or a quark or whatever, is in reality a string, a tiny one-dimensional entity that vibrates as it zips through space, with all the different types of elementary particles corresponding to different modes of vibration of the string. In essentially all string theories it turns out that one of these particle types is the graviton, the massless particle responsible for gravitational forces in the quantum theory of gravitation; so string theory automatically brings gravitation into the picture, along with the other forces.

For some time it seemed that there were five different mathematically consistent string theories, which would be a depressing result, because no one had any insight on why nature would be described by one of these theories rather than one of the other four. To avoid inconsistencies, all of these string theories were formulated in ten space-time dimensions (nine space dimensions plus the dimension of time), which was even more depressing because there is pretty good evidence that we don’t live in ten dimensions. But in recent years there have been growing signs that these five different string theories (as well as one non-string theory in eleven space-time dimensions) are all just different phases of one underlying theory that unfortunately we do not yet know. There are also some reasons to hope that our successful Standard Model of particles and forces in four space-time dimensions—length, width, depth, and time—will be found to be another phase, the one in which we live, of this underlying theory.

It’s a little bit like saying that diamond and graphite are phases of the same material, carbon. If the only thing anyone knew about carbon came from observations of the diamonds in jewelry and the graphite in pencils, it might be very hard to learn that these are phases of the same substance. One could see that there were some things diamond and graphite have in common—the way they interact with neutrons from a nuclear reactor, for instance. But if you didn’t know about the carbon atom, it would be extraordinarily difficult to find a unified theory of diamond and graphite that revealed they were really just different forms of carbon. Of course, this is just an analogy. I am not talking here about different phases of a substance but about different phases of a physical theory, phases characterized by different numbers of dimensions in space and time, among other things. The hope is that these different theories are approximations that can be deduced from the underlying theory under different circumstances, more or less as the properties of diamond and graphite can be derived by approximate calculations from our theory of the carbon atom.

How are we going to find the theory that underlies all these different versions of string theory, plus the theory that describes observed phenomena in our own four-dimensional space-time? The answer is—with difficulty. The time scale for this discovery may be anything from hours to centuries. Tomorrow, when I look at the Los Alamos Web site that I check every morning to see what is new in physics, I may find an article by some previously unknown graduate student, laying it all out. Then again, it may not happen in this century. But I think it will happen, and when it does it will end a certain chapter in the history of science: the search for the fundamental principles that underlie everything.3

It is very likely that the final theory we find in this way will be quite simple, in the sense of being based on only a few fundamental principles. It will probably also be very fragile, in the sense that it will not be possible to make any small change in the theory without its becoming logically inconsistent. We have already gone a great way in this direction. For instance, quantum mechanics, which was developed in the mid-1920s, has survived essentially unchanged to the present day. If you try to think of theories that are similar to quantum mechanics but only a little different, you find they always involve logical absurdities like negative probabilities or causes following effects. When you combine quantum mechanics with relativity, the fragility increases. You find that if you do not construct a relativistic quantum theory carefully, then when you ask a reasonable question, like asking for the rate of a certain reaction, you get an unreasonable answer—the rate is infinite.

Only certain limited classes of theories avoid these nonsensical infinities. This fragility is a good thing, because it goes some way toward telling us why the laws of nature are what they are. We can hope that with its greater fragility, a final theory will not involve any free parameters, like the particle masses in the Standard Model, whose numerical values have to be taken from experiment without our understanding why these numbers are what they are. Fragility also gives our theories much of their beauty, in the same way that a Chopin waltz gains beauty from our sense that no note in it could be changed.

Achieving a final theory will be a great achievement but it will not be entirely satisfactory, because although we will then know why the theory is not slightly different from other theories, we will never know why the theory is not something completely different. For instance, if you are willing to abandon either quantum mechanics or relativity altogether, then you can construct any number of theories that are logically consistent, but that do not describe the real world.

Another limitation: This theory, although I call it a final theory, will not be the end of the road for science. It will not be what is sometimes called a theory of everything, a theory that solves all scientific problems. We already have many scientific problems that will not be illuminated at all. One of them, to take a problem within physics, is to understand the flow of a fluid when it becomes turbulent. This problem has faced us for one hundred years; it still defies solution and may go on resisting solution well after the success of the final theory of elementary particles, because we already understand all we need to know about the fundamental principles governing fluids. We just don’t know how to deal with the complicated kind of fluid flow in which eddies are carried by larger eddies carried by larger eddies and so on, the characteristic feature of turbulence. As with many of the most interesting problems of physics, computers are only of limited help in understanding the turbulence of fluids, because they just tell us what happens in a va-riety of special circumstances, which we could also learn from experiment. What we would really like to understand are the universal properties of strongly developed turbulence under all circumstances.

And, of course, a final theory of physics will not be of much use to our friends in biology. Very likely developments in biology have had and will go on having the largest impact on human culture. One of the greatest moments in the history of human thought was the discovery by Darwin and Wallace in the nineteenth century that no “life force” is needed to explain the evolution of species. Life is not governed by independent fundamental biological laws—it can be described as the effects of physics and chemistry worked out over billions of years of accidents. It is not so long ago that many people’s religious beliefs were based on the argument from design, the argument that the wonderful characteristics of living things could not possibly arise without a divine plan. Lytton Strachey tells how Cardinal Manning came to his faith in just this way. Now that we understand how evolution can occur through the natural selection of random mutations, the argument from design has lost its force for anyone with a reasonable understanding of biological science.

  1. 1

    This article is based on a talk given at the New York Public Library in January 2001, as part of a series of lectures sponsored by the Library and The New York Review of Books, on “Futures: Bright, Dim, and Otherwise.”

  2. 2

    Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (Norton, 1999).

  3. 3

    I have written about this at greater length in Dreams of a Final Theory (Pantheon, 1993).

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