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.
The big challenge ahead in biology is to understand behavior, which seems to be far more difficult than understanding other aspects of life. I am far from my field of expertise in talking about behavior, but perhaps I can be forgiven for saying that I don’t think that it is an insuperable scientific problem. Very likely this problem will not first be solved for human beings, but for a very well-studied nematode worm, whose full name I can neither spell nor pronounce, but is abbreviated as C. elegans. This worm is one of the classic animals that biologists study, like the mouse and the fruit fly. The worm had its entire nervous system mapped out some years ago, but we don’t understand its behavior because we don’t know the program that governs its behavior. We are in a position like that of an industrial spy, who has bought a personal computer, and can map out the connections of every transistor on its central processing chip and locate every magnetized dot on its hard drive, and has looked over the shoulders of people using the Windows operating system on the computer, but has not been able to figure out how the transistors and magnetized dots make the operating system work. (I don’t know why anyone would want to steal that particular operating system, but we can agree that it’s a very difficult problem.)
This problem has not been solved, but it doesn’t seem insoluble for C. elegans and eventually it will be solved for human beings. I don’t mean that we will necessarily solve the problems about consciousness that bother philosophers. (How can I know that you and I perceive the same thing when we both see the color red?) But I do think that we will have an understanding of behavior, and of whatever aspects of consciousness are necessary to understand behavior. It won’t be a completely predictive theory, like our theory of eclipses, because behavior is too complicated for that, but more like our theory of weather. We can’t always predict the weather, but we know pretty well how the weather works.
Implicit in all this is a conservative assumption, that science will continue indefinitely in the path laid out by Galileo and Newton: the discovery of increasingly comprehensive mathematical laws that will increasingly be shown to account, aside from historical accidents, for all phenomena, biological as well as physical. I can’t be sure that this is correct. Just as medieval Aristotelians could hardly imagine science in the style of Newton, so science in the future may take a turn that we cannot now imagine. But I see not the slightest advance sign of such a change.
The developments in physics that I have described have already illuminated the future of the universe. In fact, unlike the problem of predicting the future of elections or the stock market, it’s much easier to make predictions about the future of the universe than calculations of what must have happened in the past. The universe is expanding and cooling, so when you look back in time you have to consider an era of enormous density and extremely high temperature, densities and temperatures at which our present theories become inapplicable. Without a final, universally applicable theory we cannot penetrate theoretically to the first tiny fraction of a second and understand what happened at the very beginning of the big bang. But looking into the future, we see that the universe will expand, getting less dense and colder, and so it becomes easier and easier to describe—at least for a while.
What I mean by the expansion of the universe needs explanation, because misunderstandings about it keep coming up. When astronomers say the universe is expanding, they don’t mean that space is expanding, although sometimes some of us are guilty of putting it that way. Objects that are bound together, like galaxies and solar systems and tape measures, are not expanding. In fact, if tape measures and other standards of length and everything else were expanding the same way that the universe was expanding, how would you know that anything was expanding? When we say that the universe is expanding, we mean that galaxies that are not bound by gravitation in orbits around each other are rushing away from each other. Our Milky Way galaxy is part of what is called the Local Group of galaxies, containing not only our galaxy and the large spiral galaxy M31 in the constellation Andromeda, but also a number of smaller galaxies, all held together by gravitation. The Local Group is not expanding; in fact, the Andromeda galaxy and our own galaxy are moving more or less toward each other. But any pair of galaxies that are not bound together like the galaxies of the Local Group are rushing away from each other. This is what is meant by saying that the universe is expanding. It is not that our own galaxy is particularly repulsive; instead, the universe is filled with galaxies rushing away from one another, which as far as we can tell fill all space, with no center and no edge.
We don’t really know whether the expansion of the universe will continue forever. It’s possible that even galaxies that are not bound to each other in clusters or small local groups will eventually be drawn back together by the gravitational attraction produced by the energy of the whole universe, including the energy locked up (according to Einstein’s relation E=mc2) in particle masses. Whether or not this will happen depends on how much energy there is on average in each cubic meter of the universe, and on the speed at which the universe is expanding. If there is too little energy per volume, and the gravitational field of the universe is too weak to stop the expansion, the universe goes on expanding forever, getting colder and emptier. If there is too much energy in particle masses per volume, then the expansion stops and reverses, recreating the hot dense conditions of the early universe. The choice is between a big chill and a big crunch.
Surprisingly, astronomical observations over the past few years have indicated that the expansion of the universe is not, in fact, slowing down at all, but rather speeding up. If each galaxy had been moving with a constant speed since the beginning, then the distance away from us that any typical galaxy would have reached by now would be proportional to that speed—the faster they go, the farther they get. If, as had been thought, they have been slowing down under the influence of gravitation, then although faster galaxies would be farther from us than slower ones, they would not be as far as if their distance were proportional to their measured speed, because the speeds away from us that we are measuring are the speeds the galaxies had long ago, when the light we observe was emitted by their stars, and long ago they would have been moving faster than they have since then.
In fact, two large groups of astronomers (the High-Z Supernova Search Team and the Supernova Cosmology Project) have recently found that distant galaxies are even farther from us than if their distances were proportional to the speeds we observe, indicating that they have been speeding up rather than slowing down since the light we now observe left their stars.4 About the cause of this acceleration, the best we can say is that in addition to the energy in the masses of ordinary matter, there may be a “vacuum energy” in space itself.
There have been speculations about a vacuum energy (sometimes called “dark energy”) since Einstein first turned his attention to cosmology,5 and it has long been understood that vacuum energy would produce a kind of antigravity, leading to a repulsive force among the galaxies at great distances from each other, but so far we have no reliable way of predicting the size of this effect, or how it evolves with time. This is a fundamental problem for physics as well as cosmology. One of the drawbacks of our present string theories is that they either make no prediction about the vacuum energy or predict a value much too large to be consistent with observation. Not having a final theory thus stands in the way of predicting the future of the universe as well as its past.
Because we judge the distances of galaxies by the apparent brightness of things they contain, distant galaxies would also look farther away than they actually are if light from these galaxies has been dimmed by intergalactic dust along our line of sight. Fortunately there is a way of distinguishing this dimming from the effect of a vacuum energy. The energy per volume in particle masses was larger at early times than it is now, when the matter of the universe has become less tightly packed, while the vacuum energy per volume is expected to have been (at least roughly) constant; so during very early times mass energy would have dominated over vacuum energy. This means that, despite the presence of vacuum energy, the expansion of the universe at very early times would have been slowing down, not speeding up. Thus, although (as has been observed) moderately distant galaxies would be farther away than they would be if their distance were proportional to their observed speed, an extremely distant galaxy, whose speed we measure as it was at very early times, would be closer to us than it would be if its distance were proportional to its speed, because it would have slowed down for a long while before it began to speed up. Light from such an extremely distant galaxy would therefore seem brighter than it would if the galaxy’s distance were proportional to its speed, an effect that could not be produced by intergalactic dust. Early this year astronomers announced that just this effect had been found, through the study of a supernova that had been observed in 1997 in a galaxy moving away from us at an exceptionally high speed.
If the acceleration of the universe will continue, as it would if it were caused by a constant vacuum energy, then we are definitely in for a big chill, not a big crunch. Also, under reasonable assumptions about the rate of acceleration, we would be surrounded by what is called an event horizon: any galaxy beyond a certain distance would be forever unobservable. Even if we were to set out now in a spaceship traveling at the speed of light, we could never catch up with galaxies beyond the event horizon, because the longer we travel toward them the faster they would be going away from us. Further, as time passes, more and more galaxies leave our event horizon, and become forever out of reach. If you want to explore any part of the universe outside the Local Group (the group of galaxies held together by gravity that I mentioned earlier, including our galaxy and the Andromeda galaxy) then you will have to do it in the next hundred billion years or so. After that it will be too late, and you will never be able to visit any galaxy beyond the Local Group.
It may be that the entire discussion so far has been parochial. Some of the most interesting ideas in modern cosmology involve the possibility that the expanding “big bang universe” that I have been describing is just one episode in a much larger universe in which big bangs go off here and there all the time. It may be that big bangs like our own have happened infinite numbers of times in the past, and will go on happening again and again. We’re very far now from knowing whether this is true. The greatest obstacle, I think, is not a lack of suitable astronomical observations, because it’s hard to see any way that observational astronomy could settle this issue, but rather a lack of a fundamental physical theory. The ideas about multiple big bangs grew out of speculations about certain fields that might appear in a fundamental physical theory, but so far they are only speculations. When we have discovered what I call the final theory, whether it’s a string theory or whatever it is, one of the things we will learn is the answer to the question of whether our big bang accounts for the whole universe. But even if it does not, we will remain forever trapped in our own big bang.
What future is there for us in this ever-expanding universe? In The Time Machine, H.G. Wells tells how the Time Traveler journeys forward 30 million years, to stand on a beach under a sun growing cold, with the sea beginning to freeze and the highest form of life a round thing the size of a football, hopping fitfully about on the beach. We know better now. The sun is what is called a main sequence star, which means that it is getting its energy through nuclear reactions in which hydrogen fuses into helium at the solar core. As the hydrogen gets used up, the sun will at first heat up, not cool down. Our oceans will boil in about three and a half billion years.6
Eventually, the sun will swell into a red giant, with hydrogen no longer available at the core. If you would like to see what the sun will look like then, take a look at the constellation Orion. In one corner of the constellation, there’s a distinctly reddish star, Betelgeuse. Our sun will become a red giant like Betelgeuse in about seven billion years.
We don’t really know whether the Earth will be destroyed then. It may be that it will experience a drag from the expanding atmosphere of the sun, like the atmospheric drag that brings down artificial Earth satellites after a few years, and that this drag will cause the Earth to spiral into the sun. Or it may be that, as the solar wind takes more and more mass from the sun, the Earth’s orbit will expand, and the Earth will escape being drawn into the sun. But with the oceans gone, who will care?
Eventually the sun will become less luminous, turning into a dwarf star, probably what is called a white dwarf, about as big as the Earth is now. Long before then either our species will have disappeared or we will have colonized other parts of the universe, perhaps taking the Earth with us. Right now it doesn’t seem that we’re very active in colonizing even the planets in our own solar system. The human race is not living up to the expectations of science fiction authors.
My guess is that, although expeditions will plant scientific stations on Mars, the asteroids, and the moons of Jupiter and the other outer planets, and perhaps eventually on planets around other stars, we’re not going to be colonizing any of them for a very long time. In part, my argument for this is based on our experience with Antarctica. There are scientific stations in Antarctica, but does anyone think of forming an economically self-sufficient permanent colony there? Yet, compared to Mars, Antarctica is heaven. So as long as we’re not colonizing Antarctica, I can’t conceive of any reason why we would colonize Mars or the moons of Jupiter, let alone planets of a distant star. But we will have a motive for colonizing other planets eventually, at least when the oceans start boiling in three and a half billion years. So the future of humanity may eventually depend on the future of more of the universe than just the solar system.
New stars will continue to form and provide sources of nuclear energy for quite a long while, because there’s lots of interstellar gas and dust in our galaxy and other galaxies that hasn’t yet formed into stars. But star formation will be over in about a trillion years.
At this point, I have to stop using words like billion and trillion, and start to use the common language of science, expressing large numbers as powers of ten. A trillion is 1012, which means that it is ten multiplied by itself twelve times. Or you can think of 1012 as meaning a one followed by twelve zeroes. Generally speaking, the lower the mass of a star, the more slowly it evolves. Energy production in the lowest-mass stars will be over in about 1014 years—100 trillion years. Then the galaxy will contain only brown and white dwarf stars, no longer fueled by nuclear reactions, plus a few neutron stars, which are essentially just large atomic nuclei a few miles in diameter, and black holes.
There will be some rebirth of stars caused by the coalescence of these relics, and so nuclear reactions will now and then start going again. But after a while this too will be pretty well over, because the galaxies will evaporate. Most stars are now held in the gravitational field of their galaxy, and don’t have a chance to escape from the galaxy, just as the moon doesn’t have a chance to escape from the gravitational field of the Earth. But every once in a while, two stars come close enough together so that one of them picks up enough speed to reach es-cape velocity and leave its galaxy. This process is very slow because stars don’t come close to each other very often. But the galaxies will have pretty well evaporated in 1018 years, ending the rebirth of stars.
In the final chapter of my 1993 book The First Three Minutes, I said that the universe “faces a future extinction of endless cold or intolerable heat,” and I concluded that “there is not much of comfort in any of this.” Freeman Dyson, one of the most perceptive and imaginative scientists I know, was moved by this remark to write an article7 with more optimistic conclusions. He acknowledged that there was no hope for us in a big crunch. But he argued that in the case of a big chill (which now seems in store for us)—even though in the deepening cold physical processes would be slower and slower—our descendants could slow down their thoughts even more, so that they would always have an infinite number of thoughts left to think.
Dyson also thought of imaginative slow sources of energy, as for instance cold fusion, i.e., nuclear fusion near or below room temperature. Of course, most scientists are skeptical about the sort of cold fusion that has been in the news in recent years, but Dyson’s version of slow fusion really would happen, if given enough time. The atomic nuclei in molecules or crystals are separated by barriers of electric force, which normally keep them from reacting with each other, but through which in fact they can slowly leak, so that coming into contact they can trigger nuclear reactions. In this way a mass of carbon in a burned-out star will eventually turn to iron, releasing nuclear energy. But this is a process so slow it defies the imagination. Dyson estimated that at low temperatures the fusion of carbon into iron would take about 101500 years.
Dyson may have been too optimistic. There are good reasons to think that the nuclear particles that make up most of the mass of all ordinary matter will decay into lighter particles long before cold fusion can occur. The nuclear particles seem stable under normal circumstances, but it is widely expected that they actually decay,8 with a half-life in the neighborhood of between about 1032 years and 1037 years, much shorter than the time needed for Dyson’s cold fusion.
This is another thing that we can’t predict without a more comprehensive physical theory, but there is a chance that if matter does decay in this way then the decay can be observed. That may seem absurd, but you don’t look for this decay by getting one nuclear particle and waiting 1032 years. Instead, you get more than 1032 particles, which weigh about a hundred tons, and you wait a few years. Well, it’s a little harder than that. But with great big tanks of water in underground mines, like one in Japan, there is a good chance that if nuclear particles decay with a rate at the high end of what we expect, it will be discovered in the next few years.
These speculations about the decay of nuclear particles were fairly new when Dyson wrote his article, so he did not give them much attention. But if they are correct, then after about 1040 years there will be no atomic nuclei left, and hence no atoms or molecules. The only things left in the universe will be radiation and maybe a few electrons, neutrinos, antielectrons, and antineutrinos. The universe will be a very dull place.
That’s not quite true, because the universe will still also contain black holes. It is not that black holes are not made out of nuclear particles. Black holes are made of nuclear particles and electrons, just like ordinary stars, but a black hole is so compact that its gravitational field stops light from escaping from its surface, while any light that escapes from just outside its surface is slowed so much that, to an outside observer, time on the surface of the black hole seems to have stopped. From the point of view of an observer falling into a black hole, nuclear particles will seem to be decaying with the same half-life that we will observe on earth (or hope we will), but from far outside they seem to live much, much longer. But the black hole itself radiates away its energy, so that a black hole with the same mass as the sun will in any case disappear in 1066 years or so. A galaxy-sized black hole lives longer, perhaps 10100 years. Eventually the black holes, too, will all be gone. If the Time Traveler journeyed that far forward in time, he would find no beach, no planets, no stars, no atoms, nothing but “creeping murmur, and the poring dark.”
The view of the future of humanity that I have presented here is not entirely jolly. Putting aside our dismal predictions about the distant future of the universe, we may, in the near term, be able to discover the fundamental laws of nature, but we will never know why they are true. As far as we can tell, these laws will be quite impersonal, not showing any sign of concern for human beings. In our effort to understand human behavior, we have already learned that some of the most precious things in our own lives—the love of a parent for a child, or the love of husbands and wives for each other—originated through natural selection, are governed by chemical signals, and can to some extent be triggered by adding the right hormones to the bloodstream. And although we may learn how we have come to have the values we have, and scientific knowledge will doubtless continue to improve our ability to get the things we value, nothing in science can ever tell us what we ought to value.
This, I suppose, is a rather tragic view of human life. But it did not originate with scientists. It is beautifully expressed, for instance, in Shakespeare’s plays. Prospero could almost have been thinking of the decay of protons or black holes when he described how everything would “dissolve, and, like this insubstantial pageant faded, leave not a rack behind.” The loves of Titania and De-metrius remind us how much accident there is in determining what it is that we will value. And for most of us, as for Shakespeare, none of this cosmological angst is as important as the fact that for each person, the universe will effectively cease to exist in at most about 102 years. As Guiderius sings in Cymbeline, “golden lads and girls all must, as chimney-sweepers, come to dust.” But our tragedy is not like the tragedy of Lear or Othello. Their tragedy is in Shakespeare’s script. Our tragedy is that there is no script.
Or rather, we have to write the script ourselves. We can decide for ourselves which of our inherited values to hold on to, such as loving each other, and which to abandon, like the subordination of women. And there are new values that we can invent. Though aware that there is nothing in the universe that suggests any purpose for humanity, one way that we can find a purpose is to study the universe by the methods of science, without consoling ourselves with fairy tales about its future, or about our own.
November 15, 2001
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.” ↩
Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (Norton, 1999). ↩
I have written about this at greater length in Dreams of a Final Theory (Pantheon, 1993). ↩
The speeds of distant galaxies are measured by the Doppler effect, the increase of the wavelengths of the light from the stars of the galaxy. It is much harder to measure the distance of the galaxies. What is needed is a “standard candle,” some sort of object that can be seen at great distances, and that emits light at a known rate, so that we can judge its distance from its apparent brightness. The discovery that the expansion of the universe is accelerating was made possible by the use of a new standard candle; the thermonuclear explosions of white dwarf stars known as Type 1a supernovas. ↩
Einstein in 1917 introduced a modification in the equations of his 1915 General Theory of Relativity, known as a cosmological constant, which was completely equivalent to attributing to empty space an energy per volume that is the same everywhere and at all times. After the advent of quantum mechanics it was realized that the uncertainty principle does not allow fields to have any constant value even in supposedly empty space, so that fluctuations in these fields would inevitably contribute to a vacuum energy. In some modern theories there is an additional vacuum energy that evolves with the universe; this is sometimes known as quintessence. ↩
The numbers in this article are based on the calculations of many physicists and astronomers, as summarized in two excellent review articles: Freeman J. Dyson, “Time Without End: Physics and Biology in an Open Universe,” Reviews of Modern Physics, Vol. 51, No. 3 (July 1979), pp. 447–460; and Fred C. Adams and Gregory Laughlin, “A Dying Universe: The Long-Term Fate and Evolution of Astrophysical Objects,” Reviews of Modern Physics, Vol. 69, No. 2 (April 1997), pp. 337–372. ↩
Dyson, “Time Without End.” ↩
This is often called proton decay, even though there are neutrons as well as protons inside atomic nuclei, because free neutrons decay quite rapidly into protons, in about ten minutes, while free protons have such long lifetimes that none has ever yet been observed to decay. But neutrons inside ordinary nonradioactive nuclei (like the nuclei of the most common isotopes of most elements) are expected to decay about as slowly as protons, so the discovery of the decay of neutrons inside such nuclei would be just as exciting as the decay of protons. ↩