First I would like to say thankyou to Professor Chemla and to everyone else who was involved in inviting me here. Thankyou for your hospitality, and for giving me the chance to meet and talk with your students and faculty. I am sorry that my visit is so brief that I met only a few of you.
I am a physicist by trade, but this is supposed to be a lecture for the whole campus, so I won't talk about physics. I will talk about astronomy and biology, subjects which I have always been interested in although I am not an expert. Everybody knows roughly what astronomy and biology are about. Astronomy is about the universe and biology is about life. I am interested in seeing how the universe and life fit together. Since I am not an expert in either sub'ect, I can say things that the experts might find outrageous. Don't be surprised if some of the things I say turn out to be wrong. In science it is better to be wrong than to be vague. Often we find the right way only after we tried all the wrong ways first. That is why it is fun to be a scientist. You don't need to be afraid of being wrong.
I decided to talk about three questions we all can understand, even if we can't answer them. Here are my three questions. Why is the universe friendly to the evolution of life? How did life begin? And where should we be looking for evidence? I will talk first about a general framework of ideas into which the search for life in the universe can fit. Then I will talk about the origins of life, a subject about which we are all equally ignorant. Then I will end with a bet about where we might find it.
Let us look first at the astronomical universe. We know that the universe is friendly to the evolution of life. Otherwise we would not be here to discuss it. But we don't know why the universe is so friendly. To make it possible for life to evolve either on the earth or anywhere else, two things were essential. There had to be violent events with very short time-scales, supernova explosions to bring the chemical elements essential to life out of the insides of stars where they are made, collisions between little planets crashing together to form big planets. And there had to be long quiet periods in which slowly evolving life was sheltered from violence. Time in our universe has always had two faces, the quick violent face and the slow gentle face, the face of the destroyer and the face of the preserver. How did these two faces of time come to coexist in a single universe?
In the old days, people who thought about the natural history of the earth were divided into catastrophists and uniformitarians. Catastrophists believed that time molds the earth by means of sudden catastrophes, the most famous example being Noah's flood. Uniformitarians believed that time works slowly, molding the earth by long-continued action of the same processes that we observe at work today. We now know that both sides in the debate were right. Time loves to sit quietly for millions of years, and then to pounce suddenly in a single hour of fury. Nothing is permanent, but the illusion of permanence can last for a long time. We see in the channeled scablands of Washington State the traces of a flood more violent than Noah's, when a huge lake formed by the melting Canadian ice-cap broke through an ice-dam and destroyed everything in its path as it roared down to the sea. And we see, a few hundred miles to the north in Canada, the Burgess shale, where delicate fossils of our ancient ancestors have been marvelously preserved in rocks that are five hundred million years old.
When we move from the earth to the heavens, the contrast between the quiet and the stormy faces of time becomes even starker. In our quiet little corner of the universe, the sun and moon and planets serenely ride in their orbits, the stars shine steadily for billions of years. Elsewhere, in places remote from us in space or time, sudden cataclysms fill the sky with violence as heavenly bodies are born or die. The birth of our moon was such a cataclysm, more than four billion years ago, when the earth was young. Another planet, smaller than the earth but larger than the moon, collided obliquely with the earth. In a few minutes the incoming planet was torn apart, half of it plunged into the earth, the other half formed a ring of orbiting debris which condensed to make our moon. Whatever structures existed on the earth before the collision were totally obliterated. The heat of impact made the earth white-hot, with an atmosphere of vaporized rock. Afterwards the earth cooled down and became the planet that we know. No object of comparable size has collided with it in more recent times. Later impacts were smaller and did only superficial damage. But we see, far away across the universe, cataclysms vastly more violent than the collisions of planets. We see supernova explosions in which the core of a massive star collapses in a fraction of a second and the envelope of the star is ejected into space. We know that such an explosion must have occurred in our neighborhood shortly before the earth and the sun were born, to produce the mixture of heavy atoms that we see on earth today. And we see other cataclysms even more abrupt and energetic than supernova explosions. We call these events gamma-ray bursts, but nobody knows how or why they happen. The universe is full of cataclysms. Luckily for us, they are separated by immense distances and by eons of time. During the intervals between cataclysms, time shows her quiet face and life can survive and prosper.
The existence of life depends crucially on the fact that time has these two faces, the quiet and the violent, cleanly separated from each other. The violent face created the stuff that we are made of. The quiet face sustains us and allows us to evolve. We need to understand the reasons why these faces exist. There are two main reasons, one superficial and one fundamental. The superficial reason is that the universe is extravagantly large. Distances are so great that accidental collisions hardly ever happen. This is why the sun with its family of planets can run like a perfect clock, with orbits undisturbed by close encounters with alien sta,rs, for billions of years. We are protected from cataclysms by the sheer size of the interstellar spaces.
The more fundamental reason for the two faces of time is the fact that the universe is dominated by the force of gravitation. Gravitational energy is quantitatively the largest reserve of energy, and qualitatively the least disordered. It carries zero disorder and zero heat. Because of its superior quality, gravitational energy can change easily and irreversibly into other forms of energy. Other forms of energy are associated with disorder and heat, but Gravitational energy is cool. Gravity is cool in both senses of the word. This is why gravitational energy can drive turbines in a hydroelectric power-station with almost a hundred percent efficiency, while other kinds of power-station, powered by coal or oil or natural gas or uranium, struggle to reach fifty percent. The gravitational energy of the ice in the high Canadian ice-cap during the last ice age waited quietly for a hundred thousand years, until the thaw and the break in the ice-dam released it. As soon as it was released, it changed into the turbulent energy of the flood that excavated a trillion tons of rock and created the scablands. Similarly, the gravitational energy of the planet that collided with the earth stayed cool for millions of years, and then suddenly changed into heat at the moment of impact when the moon was born. All over the universe, when conditions are right for gravitational energy to be released, it can change instantly into heat and radiation, and a cataclysm results. The two faces of time are a consequence of the two faces of gravitation. Gravitation is the ordering principle that holds our earth together as a stage for us to walk on, and gravitation is the ultimate reservoir of energy that can smash our world to pieces.
The best popular account of the science that explains how the universe
can be friendly to life is a book,
Creation of the Universe, by
the Chinese astronomers Fang Li Zhi and Li Shu Xian. The book was
translated into English and published by World Scientific Publishing
Company of Singapore in 1989. Fang Li Zhi is the famous dissident
astronomer now living in exile in the United States. I particularly
recommend Chapter 6, with the title
How Order was born of
Chaos. This tells the same story that I am telling you today, but
with more detail and more depth.
Now I come to the second question, how life might have actually begun, either here or on Mars or anywhere else. There are two strongly divergent views of the state of affairs when life began. One view is the RNA world, which says that life began with replicating molecules. RNA, ribonucleic acid, is a big molecule, a modified form of DNA, more flexible but equally capable of replicating itself. The RNA world has molecules like RNA coming together by chance, replicating themselves and organizing the replication of other molecules to make the first living creatures. The other view is the garbage-bag world, which says that life began with little bags of garbage, membranes made of oily scum or other common chemicals that like to form membranes, enclosing volumes of dirty water containing miscellaneous garbage. The RNA world is the orthodox theory, the party line of the molecular biology community, accepted by the majority of expert biochemists and geneticists. The experts love RNA because it is marvelous stuff to do experiments with. You can put RNA into a test-tube and watch it replicate and evolve. The experts believe in the RNA world even more strongly since Thomas Cech discovered in 1986 that RNA can not only replicate itself but can also act as an enzyme to catalyze reactions between other molecules. RNA enzymes are called ribozymes and are an important part of the genetic machinery in all modern cells. The RNA world is a beautiful scene, with busy little ribozymes cooperating to organize the beginnings of life.
The garbage-bag world is not so elegant and not so widely accepted. It was originally proposed by the Russian biologist Oparin in the nineteen-twenties. Unfortunately Oparin later fell into ill repute, because he was a prominent academician at the end of his life during the bad times when Lysenko was destroying Russian genetics. Oparin was a friend of Lysenko, and did not lift a finger to save the geneticists when Lysenko was denouncing them to Stalin and making sure that they were imprisoned or killed. The prevalent opinion among western biologists was that, since Oparin was a bad guy, his theory must have been bad too. Fortunately, bad guys sometimes produce good theories. One of the main proponents of the garbage-bag theory today is Doron Lancet, a chemist at the Weizmann Institute in Israel. I myself prefer it, partly because I like to be in the minority, and partly because I find it chemically more plausible. The idea of the garbage-bag world is that a random collection of molecules in a bag may occasionally contain catalysts that cause synthesis of other molecules that act as catalysts to synthesize other molecules, and so on. Very rarely a collection of molecules may arise that contains enough catalysts to reproduce the whole population as time goes on. The reproduction does not irieed to be precise. It is enough if the catalysts are maintained in a rough statistical fashion. The population of molecules in the bag is reproducing itself without any exact replication. While this is happening, the bag may be growing by accretion of fresh garbage from outside, and the bag may occasionally be broken into two bags when it is thrown around by turbulent motions. The critical question is then, what is the probability that a daughter bag produced from the splitting of a bag with a self-reproducing population of molecules will itself contain a self-reproducing population? When this probability is greater than one half, a parent produces on the average more than one functional daughter, a divergent chain reaction can occur, the bags containing self-reproducing populations will multiply, and life of a sort has begun.
The life that begins in this way is the garbage-bag world. It is a world of little proto-cells that only metabolize and reproduce themselves statistically. The molecules that they contain do not replicate themselves exactly. Reproduction is not the same thing as replication. Cells can reproduce but only molecules can replicate. The Darwinian process of evolution by natural selection does not require exact replication to be effective. Darwin had never heard of DNA or RNA or exact replication when he developed his theory of evolution. Statistical reproduction is a good enough basis for natural selection. As soon as the garbage-bag world begins with crudely reproducing proto-cells, natural selection will operate to improve the quality of the catalysts and the accuracy of the reproduction. It would not be surprising if a million years of selection would produce proto-cells with many of the chemical refinements that we see in modern cells.
Life is not one thing but two, metabolism and replication, and the two things are logically separable. Metabolism is the normal chemical activity of a living cell. Replication is the precise copying of a gene. There are accordingly two logical possibilities for life's origins. Either life began only once, with the functions of replication and metabolism already present and linked together from the beginning, or life began twice, with two separate kinds of creatures, one kind arising first, capable of metabolism without exact replication, the other kind coming much later, capable of replication without metabolism. If life began once, the beginning was something like the RNA world. If life began twice, the first beginning was the garbage-bag world, with creatures containing all kinds of molecules. These garbage- bag creatures might have existed independently for a long time, perhaps as long as one or two billion years, eating and growing and gradually evolving a more and more efficient metabolic apparatus. The second beginning might have been with replicating parasites made of RNA, arriving later and preying upon the garbage-bag creatures. The parasites could use the products of the garbage-bag metabolism as life-support to help them achieve their own replication.
Another feature of the universe that has been essential to the evolution of life is symbiosis. Symbiosis is the coming together of two creatures, after they have been detached from each other and have evolved along separate paths for a long time, so as to form a combined creature with behavior not seen in the separate components. Symbiosis is a familiar concept both in biology and astronomy. In biology, almost all higher plants and animals make use of symbiotic bacteria to perform many of their functions. Nitrogen-fixing bacteria in the roots of soybean plants and cellulose-digesting bacteria in the stomachs of cows are two well- known examples. Lynn Margulis collected the evidence to prove that symbiosis played an even more fundamental role in the evolution of modern cells from bacteria. She proved that the mitochondria and chloroplasts that are essential components of modern cells were once independent free-living creatures. They first invaded the ancestor of the modern cell from the outside and then became adapted to living inside. The combined cell then learned to coordinate the activities of its component parts, so that it acquired a complexity of structure and function that neither component could have evolved separately. In this way symbiosis allows evolution to proceed in giant steps. A symbiotic creature can jump from simple to complicated structures much more rapidly than a creature evolving by the normal processes of step-by-step mutation. I am suggesting that the earlier jump from garbage-bag creatures to cells with a modern genetic apparatus also came about by symbiosis. The nucleic acid creatures, originating as parasites within the garbage-bag creatures, gradually learned to cooperate with their hosts. The garbage-bag creatures learned to tolerate the parasites and to exploit their capacity for exact replication. Together, the two components of the symbiosis created a modern cell that was so efficient, both in metabolism and in replication, that it wiped out all earlier forms of life.
Symbiosis is also a dominant factor in the evolution of the non-living universe. Symbiosis is as frequent in the sky as it is in biology. Astronomers are accustomed to talking about symbiotic stars. The basic reason why symbiosis is important in astronomy is again the double mode of action of gravity. When gravity acts upon a uniform distribution of matter occupying a large volume of space, the first effect of gravity is to concentrate the matter into lumps separated by voids. The separated lumps then differentiate and evolve separately along different evolutionary histories. They become distinct types of object. But then, after a period of separate existence, gravity acts in a second way to bring lumps together and bind them into pairs. The binding into pairs is a sporadic process depending on chance encounters. It usually takes a long time for a given lump to be bound into a pair. But the universe has plenty of time. After a few billion years, a large fraction of objects of all sizes become bound in symbiotic systems, either in pairs or in clusters. Once they are bound together by gravity, dissipative processes of various kinds tend to bring them closer together. As they come closer together, they interact with one another more strongly and the effects of symbiosis become more striking.
Examples of astronomical evolution caused by symbiosis are to be seen wherever one looks in the sky. On the largest scale, symbiotic pairs and clusters of galaxies are very common. When galaxies come into close contact,, their internal evolution is often profoundly modified. A common sign of symbiotic activity is an active galactic nucleus. An active nucleus is seen in the sky as an intensely bright source of light at the center of a galaxy. The various varieties of active galactic nuclei commonly arise from the symbiotic effects of other galaxies nearby. The probable cause of the intense light is gas falling into a black hole at the center of one galaxy as a result of the gravitational perturbations caused by another galaxy. It happens frequently that big galaxies swallow small galaxies. Nuclei of swallowed galaxies are observed inside the swallower, like mouse-bones in the stomach of a snake. This form of symbiosis is known as galactic cannibalism.
But this talk is supposed to be about life, and I must not digress further into astronomy. Let me come back to the subject of life. From the point of view of life, the most important example of astronomical symbiosis is the symbiosis of the earth and the sun. The whole system of sun and planets and satellites, the system which we call the Solar System, is a typical example of astronomical symbiosis. At the beginning, when the Solar System was formed, the sun and the earth were born with totally d]fferent chemical compositions and physical properties. The sun was made mainly of hydrogen and helium, the earth was made of heavier elements such as oxygen and silicon and iron. The sun was physically simple, a sphere of gas heated at the center by the burning of hydrogen into helium and shining steadily for billions of years. The earth was physically complicated, partly liquid and partly solid, its surface frequently transformed by phase-transitions of many kinds. The symbiosis of these two contrasting worlds made life possible. The earth provided chemical and environmental diversity for life to explore. The sun provided physical stability, a steady input of energy on which life could rely. The combination of the earth's variability with the sun's constancy provided the conditions in which life could evolve and prosper.
In addition to the sun and the planets and their satellites, the solar system also contains a large number of asteroids and comets, smaller objects gravitationally bound to the sun but not sharing in the orderly motions of the planets. The asteroids and comets are an important part of the symbiosis that binds the system together. Since they have disordered motions, they occasionally collide with planets and produce catastrophic disturbances of the local environment. Traces of these impacts are visible on the surface of the earth, and even more visible on the moon. A few years ago we could see the huge scars on the face of Jupiter caused by the impact of comet Schumacher-Levy. Impacts large enough to affect the whole earth and cause extinctions of species on a global scale occur about once in a hundred million years. The random obliteration of ecologies by major impacts has been a part of the history of life on earth since the beginning. It is likely that these catastrophes drove evolution forward by destroying species that were too well adapted to static environments, making room for species that were adaptable to harsher and more rapidly changing conditions. Without the occasional impact catastrophe to reward adaptability, it is unlikely that our own species would have emerged. We are among the most adaptable of species, offspring of a symbiosis in which sun, planets, asteroids and comets all played an essential part.
The most famous experiment exploring the origin of life was done by Stanley Miller in Berkeley in 1953. Miller filled a flask with a reducing atmosphere composed of methane, ammonia, hydrogen and water, passed electric sparks through it and collected the reaction products. He found a thin soup containing a remarkably high fraction of interesting organic compounds. He also tried the experiment in an oxidizing atmosphere, and in a neutral atmosphere composed of nitrogen, carbon dioxide and water. Other people have repeated the experiment with many variations. The results are always the same. With a reducing atmosphere you get plenty of interesting organic chemicals. With an oxidizing or neutral atmosphere you do not.
We now know that the earth's reducing atmosphere, if it ever existed, had disappeared by the time the heavy meteoritic bombardment of the earth ceased, about 3.8 billion years ago. So Miller's beguiling picture of the origin of life, in a warm little sunlit pond full of dissolved chemicals under a reducing atmosphere, has been discredited. Recently, a new beguiling picture has come to take its place. The new picture has life originating in a hot, deep, dark little hole on the ocean floor. Four experimental discoveries came in rapid success on to make the new picture seem plausible. First, the discovery of abundant life existing today around vents on the mid-ocean ridges, several kilometers below the surface, where hot water emerging from deep below is discharged into the ocean. The water entering the ocean is saturated with hydrogen sulphide and metallic sulphides, so that it provides a reducing environment independent of the atmosphere above. Second, the discovery that bacterial life exists today in strata of rock deep underground, in places where contact with surface life is unlikely. In some cases, the deep underground bacteria do not belong to any previously known species. Third, the discovery of strikingly life-like phenomena observed in the laboratory, when hot water saturated with soluble iron sulphides is discharged into a cold water environment. The sulphides precipitate from the water as membranes and form gelatinous bubbles. The bubbles look like possible precursors of living cells. The membrane surfaces adsorb organic molecules from solution, and the metal sulphide complexes catalyze a variety of chemical reactions on the surfaces. Fourth, the discovery that many ancient lineages of bacteria are thermophilic, that is to say, specialized to live and grow in hot environments. Many of them are found today in hot springs, often in places where the water temperature is close to boiling.
These four lines of evidence, from ocean ridges, from deep oil-well
drilling, from labora- tory experiments and from genetic analysis,
combine to make the picture of life originating in a hot deep
environment credible. Since we know almost nothing about the origin of
life, we have no basis for declaring any possible habitat for life to
be likely or unlikely until we have explored it. The picture of life
beginning in a deep hot crevice in the earth is purely speculative and
in no sense proved. It has an important corollary. If it is true, it
implies that the origin of life was largely independent of conditions
on the surface of the planet. And this in turn implies that life might
have originated as easily on Mars as on the earth. Thomas Gold is an
astronomer friend of mine who is constantly pouring out new ideas. Not
all of his ideas are crazy. He recently wrote a book called
Deep Hot Biosphere. He postulates a deep, hot blosphere still
existing in the crust of the earth. He presents evidence that the deep
biosphere may contain as much biomass as the surface biosphere with
which we are familiar. He remarks,
If in fact such life originated
at depth in the earth, there are at least ten other planetary bodies
in our solar system that would have had a similar chance for
originating microbial life. I don't know which ten objects
Gold had in mind. Certainly Mars, Europa, Titan and Triton would be on
the list. Europa is a moon of Jupiter, Titan is a moon of Saturn,
Triton is moon of Neptune. All of them are cold on the surface, but
warm underneath where life might be hiding.
There is a possible analogy between the origin of life and the origin
of elaborate body- plans in higher organisms. Half a billion years
ago, after life had existed for about three billion years, there was a
sudden efflorescence of elaborate body-plans. The efflorescence is
known as the
Cambrian explosion, and produced in a geologically
short time all the major body-plans from which modern higher organisms
evolved. In the Burgess shale in Canada, the fossils of these newly
evolved creatures are marvelously preserved. Stephen Gould describes
them in his book,
Wonderful Life. Something must have happened
shortly before the Cambrian epoch to make the genetic programming of
elaborate body-plans possible. What might have happened was the
indirect development, the system by which an
embryo sets aside a, package of cells that are destined to grow into
an adult, the body-plan of the adult having no connection with the
body-plan of the embryo. The advantage of this system is that the
embryo provides life-support to the adult during the vulnerable stages
of its early growth, while the adult is free to evolve elaborate and
fine-tuned structures unconstrained by existing structures of the
embryo. Three California paleontologists, Davidson, Peterson and
Cameron, have collected evidence that the great majority of existing
body-plans arose from indirect development. This fact was overlooked
until recently because the two best-known body-plans, the vertebrate
and the arthropod, are exceptions to the rule, Humans and lobsters
grow by direct development from embryo to adult. The vertebrates and
arthropods, the two most successful groups of animals, probably began
like the others with indirect development, but later evolved a
short-cut system of direct development with the adult body-plan
growing directly from the embryo. If the system of indirect
development came first, it means that multicellular organisms evolved
by a two-step process. The first step was the evolution of embryonic
forms of limited complexity, lacking the genetic machinery to program
specialized structures. The second step was the evolution of adult
forms with the modern system of genetic controls, and with
life-support provided by the embryo. I am proposing that the early
evolution of life followed the same two-step pattern as the evolution
of higher organisms. First came the embryonic stage of life, cells
with functioning metabolism but without any genetic apparatus, unable
to evolve beyond a primitive level. Second came the adult stage, cells
with genetic machinery allowing the evolution of far more fine-tuned
metabolic pathways, and again with life-support provided by the first
stage while the second stage evolved.
To me, one of the most attractive features of the garbage-bag theory of the origin of life is that it shows life following the same pattern, a major step in evolution divided into two separate 'umps, at three crucial periods of its history. First, the period of origins about four billion years ago, when the two jumps were metabolism and replication. Second, the evolution of modern cells according to Margulis, about two billion years ago, when the two jumps were parasitic invasion and symbiosis. And, third, the evolution of higher organisms, about half a billion years ago, when the two jumps were the embryo and the package of cells that grew into an adult. In each of the three revolutions, the first stage relied on crude and simple modes of inheritance, and the second stage jumped to new levels of sophistication in the translation of genetic language into anatomical structure.
About a hundred and fifty million years after the Cambrian explosion, life made one of its greatest jumps when it moved from the ocean onto land. To survive on land it had to invent lungs, and weight-carrying bones, and a dry impermeable skin that could prevent loss of water. The first animals that came ashore did not yet have impermeable skins. They were the amphibians, animals like frogs that hatch from eggs in the water and live only part of their lives on land. They desiccate and die if they sit too long in sunshine. It took another fifty million years for the descendants of the amphibians to become reptiles fully adapted to living on land. The reptiles with their impermeable skins spread all over the earth and made it their home. The liberation of life from the ocean made possible all the later inventions that make the land beautiful, fur and feathers and forests and flowers.
For the last part of this talk, I move from the past to the future. When I look at things from the point of view of an astronomer,, the four hundred million years that elapsed since life escaped from the ocean until now is a short time. That was perhaps only the first jump in another revolution that needs two 'umps to be complete. The first jump was from the ocean onto the land. The second jump will be from the land into space. The revolution will only be complete when life has escaped from this planet and made the universe its home. We are beginning the second 'ump now with our exploring of the planets and our quick trips to the moon. But for a long time, so long as we depend on spacecraft and spacesults to stay alive in space, we shall be amphibians. We can survive in space for a limited time, and we must return to our home planet to breathe air under an open sky. This amphibian phase may last for a few hundreds of years, while the life that we carry with us away from earth is still confined to artificial habitats.
After that, perhaps sooner than most people expect, we shall breed plants and animals tfiat do not need to be confined but are adapted to living wild in space. The jump from breathing air to living in a vacuum is no greater than the jump from breathing water to breathing air. Plants and animals will need some genetic engineering to be at home in a vacuum. Plants will need new organs of photosynthesis that produce liquid or solid peroxides instead of oxygen gas. Animals will need new organs of respiration to take in oxygen in the form of peroxides instead of from air. Instead of lungs, animals would have an organ like a liver that dissociates peroxides slowly into molecular oxygen and feeds the oxygen into the blood. Both plants and animals will need stronger skin to hold internal pressure and prevent their blood from boiling. The vapor pressure of water at blood temperature is quite small, so the skin will not need to be thick to hold it. In cold places far from the sun, animals will need thicker layers of fur and plants will need thicker layers of bark to provide thermal insulation. This will be a challenge for plant and animal breeders, but with a mastery of the techniques of genetic engineering they should be able to do it. When they have done it, life will have moved again from the age of amphibians to the age of reptiles. The second jump of the new revolution will be complete, and life will be on its way to the next phase of evolution. Life adapted to vacuum will evolve to create new ecologies on all the worlds where sunlight and the chemical elements essential to life are to be found.
I am not an astronomer myself, but I like to talk with astronomers. I see the universe through the eyes of the astronomers. Everywhere I look, I see beauty. But everything we see, except our own planet, is dead. And one planet with life is more beautiful than a whole universe dead. The universe would be far more beautiful and meaningful if it were full of life, if life were spread out over those millions of worlds. Somehow life must find a way to spread and make a home for itself in every corner of the universe, just as it made a home for itself in every corner of this planet. Perhaps our job is to be the midwives, to help the living universe to be born. I believe in space-travel, not as an end in itself, but as a means to bring the universe to life.
Human beings did not have to exist for life to escape from the earth. There might have been other midwives, other species that developed intelligence and tools to give a start to space-travel. It might have been possible for life to spread over the universe even without intelligent midwives. Pieces of rock are occasionally blasted away from Mars by a comet or asteroid impact and arrive on Earth without being destroyed. We have picked up more than a, dozen Mars-rocks in Antarctica and other places on Earth. If Mars had possessed living inhabitants, some Martian creatures might have survived the voyage and made their home on Earth. Perhaps we are their descendants. Organisms from Earth might occasionally arrive in the same way on Mars. No place in the universe is totally isolated from its neighbors. The universe has plenty of time. The universe could have waited long enough to see life moving out from Earth by the natural processes of comet impacts and meteorite infall. If we take responsibility for spreading life through the universe only speeding up the natural processes.
Today the rockets that we use to get from earth into space are absurdly expensive. The public believes that space-travel will always be too expensive for ordinary people. But this need not always be so. Space-travel need not always be a spectator sport, with a small elite of stars paid for by the millions who stay on the ground and watch the show on television. To make space-travel cheap in the future, we need public highways into space. To be accessible to the general public, space-travel must be about a hundred times cheaper than it is to-day. The cost of launching payload into space should be reduced from ten thousand dollars a pound to a hundred dollars a pound. This sounds hard to do, but it may be possible if we develop radically new methods of propulsion, leaving the source of energy on the ground so that each spacecraft does not need to carry its own fuel. The source of energy for launching payloads from the ground into space should be like a public highway, serving anyone who arrives at the site with a spacecraft and is willing to pay the toll.
There are several possible ways of building a public highway into space. A laser beam pointing from the ground up into the sky can be a public highway. Two years ago in Princeton we saw a film of the first flight of Leik Myrabo's Lightcraft Technology Demonstrator, a little toy model of a laser-propelled spacecraft. Myrabo is a professor at Rensellaer Polytechnic Institute in Troy, New York State. His model rose three feet into the air, not so high as the first flight of the Wright brothers in December 1903. More recent Lightcraft flights flew higher, but the first flight was the decisive step. The first flight demonstrated that laser propulsion is possible, 'ust as the 1903 flight demonstrated that human heavier-than-air flight was possible. The sound-track accompanying the film of the Lightcraft made a noise like a machine-gun, as the laser fired high-intensity pulses at a rate of ten per second. The vehicle has a little fish-shaped body with a blunt nose and a shiny reflecting dish around its waist. It weighs two ounces and has a diameter of six inches. Each laser pulse is focussed by the dish and heats the air at the focus to a high temperature, causing a shock-wave that propels the model upward. The average power in the beam is ten kilowatts. Myrabo borrowed the laser from the United States Air Force at White Sands, New Mexico. Last year Myrabo was back in Princeton with a film of the Lightcraft flying 75 feet up a laser beam. It flies stably up to 75 feet. To fly higher he will need a better laser. A simple calculation shows that if it takes a ten kilowatt laser to lift a two-ounce space-craft off the ground, it will take a one gigawatt laser to lift a five-ton spacecraft. The five-ton spacecraft could fly all the way up the laser-beam into space. Using laser-heated water as propellant, a spacecraft with a take-off weight of five tons should be able to carry two tons of propellant and a useful payload of at least a ton, enough for a couple of passengers and their baggage. A one gigawatt laser will be expensive, but it is not absurd. We already have hundred-megawatt lasers. The laser is the easy part of a laser-propulsion system. The hard part is the engine. We don't yet know how to design the engine for a full-scale system.
It took seventy years to go from the Wright brothers' Flyer One of 1903 to the modern air-transport system with huge numbers of commercial aircraft flying routinely all over the world. Perhaps it will take another seventy years to go from Myrabo's Lightcraft Technology Demonstrator to a laser-propelled public highway system with huge numbers of spacecraft traveling routinely to destinations in orbit and on the moon and beyond. At each destination there must be a massive infrastructure comparable with a major airport and including the associated industries and hotels. All this will take a long time, measured on a human time- scale, but a short time, measured on an astronomical timescale.
Building public highways is difficult because they become cheap only when they are constantly used. To keep a public highway into space busy, we need a hundred thousand launches per year, roughly one every five minutes. The first users of the highway will be big corporations and governments who can afford to pay for its initial cost. After it has been built, it will be available for ordinary people, for private groups who go into space at their own expense, like the Pilgrim Fathers who rented a second-hand boat and sailed froi-n England to America in 1620. The ordinary people will not only take themselves and their families into space. They will take plants and animals too. By that time, we will have plants and animals genetically engineered to live wild in strange places. People will spread life with them wherever they go.
Cheap space-travel will sooner or later be developed. There is no law of physics that decrees that space-travel must always be expensive. So far as the laws of physics are con- cerned, if one measures the cost of moving into space by the amount of energy required, the cost of launching a person from Earth into space should be no greater than the cost of a commercial flight from New York to Tokyo. But to bring the cost of space-launches down to the cost of commercial air travel requires a huge volume of traffic. Space-travel will only be cheap when millions of people can use it. And millions of people can only use it when there are cities in the sky to which travelers can go. The growth of space-habitats and the decline of costs will be a slow process. Much can be done in a hundred years, but not enough to have a major impact on the social problems of Earth. The expansion of life into space will not come in time to solve the problems of our grandchildren. When it finally comes, it will be a mixed blessing for humanity. Only one thing is certain. Once life escapes from this little planet into the universe, there will be no stopping it. It will keep on moving and changing. It will go on its way, with or without our help. Like good midwives, we should step aside and watch it grow.
Finally, this talk has a punch-line, and here it is. I come now at last to the third of my three questions. Where should we be looking for evidence of life? Dreams of a possible future have practical consequences for the present. Things that for us are in the speculative future may have happened in the past somewhere else. When we begin a serious search for life existing elsewhere in the universe, we should keep in mind the possibility that life elsewhere is already adapted to living in vacuum. For life adapted to living in vacuum, planets are not the most likely habitat. Life would be more likely to be found on small bodies such as asteroids or comets, places where gravity is weaker and moving from one world to another is easier. For life adapted to vacuum, a planet would be a death-trap. A planet for them would be like a deep well full of water for a human child. A swarm of small ob'ects like the Kulper Belt, the ring of icy worlds that we see 'ust outside the orbit of Neptune, provides far more habitable surface area than all the planets together. It would be a friendlier place for vacuum-life to flourish. We see many stars like Beta Pictoris with clouds of debris orbiting around them. These stars have much more real estate in their Kulper Belts than we have in ours. When we are searching for evidence of alien life, we should not look only at planets. Planets may be the most likely places for life to begin, but they are not the most likely places for life to be found in the sky. In searching for evidence of life, as in all branches of astronomy, you have the best chance of making important discoveries if you look where other people are not looking. I am not saying that we should give up looking for evidence of life on Mars, only that we should look in other places too, Europa and the Kuiper Belt and giant molecular clouds. Wherever there is water and carbon and nitrogen and starlight, life might already be teeming. I am willing to bet even money that when the first alien life is found it will not be on a planet. This is a bet which I will be happy either to win or to lose.