- Shopping Bag ( 0 items )
In this engrossing book, John and Mary Gribbin relate the developments in twentieth-century astronomy that have led to this shattering realization. They begin their account in the 1920s, ...
In this engrossing book, John and Mary Gribbin relate the developments in twentieth-century astronomy that have led to this shattering realization. They begin their account in the 1920s, when astronomers discovered that the oldest stars are chiefly composed of the primordial elements hydrogen and helium, produced in the birth of the universe in a Big Bang. They then describe the seminal work of the 1950s and 1960s, which unlocked the secret of how elements are "cooked" by nuclear fusion inside stars. The heart of the story is their discussion of supernovae, only recently understood as great stellar explosions in which the resulting ash is spread far and wide through the cosmos, forming new generations of stars, planets, and people. Focusing on the relationship between the universe and the Earth, the authors eloquently explain how the physical structure of the universe has produced conditions ideal for life.
Life and the Universe
This book explains the relationship between life and the Universe, from the Big Bang to the arrival of the molecules of life on the surface of the Earth. It is a complete and self-consistent story, describing our cosmic origins from stardust. But it is not necessarily the whole story of life and the Universe, and before delving into the details, I want to describe briefly some of the more intriguing current ideas that may, if they are proved correct, take us beyond the story so far. The caveat is that "intriguing" doesn't necessarily mean "correct." But science progresses by making reasonable speculations, then testing those speculations to see how well they stand up. And in a book which claims to offer the best available scientific evidence for our own origins, it would be derelict not to make it clear just how science arrives at these profound conclusions.
One particular speculation about the relationship between life and the Universe has the merit of being relevant to my story in addition to highlighting the scientific method at work. It is actually a rather old idea that has recently been revived and improved in the light of present astronomical knowledge. It's especially interesting because it shows how scientific ideas can go in and out of fashion--and then back in again--as new discoveries are made and opinions change. In fact, as is so often the case in science, the first person to put the idea forward was way ahead of his time. In 1871, William Thomson (who later became Lord Kelvin) pondered the mystery of the origin of life on Earth in his presidential address to the British Association for the Advancement of Science. He made an analogy with the way life appears on a newly formed volcanic island, saying that "we do not hesitate to assume that seed has been wafted to it through the air, or floated to it on raft--we must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instant no life existed upon this Earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with natural vegetation."
Thomson's comments are especially interesting as an echo of their times -- they came just a dozen years after Charles Darwin and Alfred Wallace published the theory of evolution by natural selection, a key point of which was the way life forms appeared on isolated islands and evolved there into new species. The meteoric reference also echoes Thomson's own interest in how the Sun stayed hot--in a step toward his idea that the Sun could release heat by contracting slowly under its own weight, Thomson had investigated theoretically the possibility that it might be kept hot by a continual rain of meteoric debris falling onto its surface. But Thomson seldom gets any credit for his ideas about the origin of life on Earth, and although he deserves a mention, it is probably fair that his theories should be relegated to a footnote in scientific history, since he never developed them fully (or at all!) and left them as pure speculation.
The story really begins in 1907, with a suggestion made by the Swedish chemist Svante Arrhenius. Arrhenius was a good enough chemist to have won the Nobel Prize in 1903 for his work on electrolysis, and the breadth of his scientific thought is reflected in the fact that he was one of the first people, in 1905, to express concern about the prospect of global warming caused by a buildup of carbon dioxide in the atmosphere (the greenhouse effect) as a result of burning fossil fuels. His interest in the workings of the atmosphere of our planet led directly to his speculations about the origin of life on Earth, after he realized that it would be possible for microorganisms (things like bacteria) to be carried high into the atmosphere, where they might escape into space and be pushed outward from the Solar System by the pressure of the Sun's radiation. It is known that some forms of such microorganisms can remain dormant for long periods of time in hostile environments (particularly under arid conditions) and then be brought back to active life when their essential requirements (particularly water) are once again available. Perhaps, he reasoned, they could even cross the desert of interstellar space in this dormant state, reviving when they fell upon another Earth-like planet.
But why should this be a one-way process? If living spores from Earth could escape into space in this way, Arrhenius pointed out, then spores from other planets, orbiting round other stars, could also escape into space; life on Earth might therefore have descended from such interstellar travelers which entered the atmosphere of the Earth when our planet was young. This hypothesis for the origin of life on Earth was called "panspermia," meaning "life everywhere," and it fitted in rather neatly with the image of the Universe people had at the beginning of the twentieth century. Arrhenius knew nothing of Thomson's speculation, and in any case he offered a properly worked-out hypothesis, trying to explain not only how life could have got from rocks in space onto a planet but how it could have got off a planet and into space. He deserves the pride of place that he is usually given in the story of panspermia.
At that time, what we now know as the Milky Way Galaxy was thought to be the entire Universe. Astronomers already knew that individual stars were born, lived, and died within the Milky Way, but it was thought that the "Universe" itself was essentially eternal and unchanging--as an analogy you might think of an ancient wood that has existed since time immemorial, even though every tree in the wood has been replaced by new trees very many times. The key feature of this picture of the Universe is that there was no origin, so the question of how the Universe began did not have to be addressed. On the other hand, there was clearly a question about how life on Earth had originated, since radioactive dating techniques had begun to put a date on the age of the Earth by the time Arrhenius was pondering this puzzle. But by moving the origin of life off the Earth and out into what was then thought to be an eternal Universe, Arrhenius "solved" the puzzle by removing it from consideration altogether. If the Universe was eternal and essentially unchanging, even though generations of stars ran through their life cycles within the Universe, then it seemed reasonable to argue that life had always existed in the Universe and had spread from old planets to new ones as part of the cycle of the generations. And in an infinitely old Universe, even if life did have to emerge by chance processes, there would be infinite time available to do the job, and then infinitely more time for life to spread from its planet of origin to populate the entire Universe. This was entirely sensible reasoning, given what was known about the Universe in the first decade of the twentieth century.
Even so, it was not taken very seriously, and as our understanding of the stars, the Milky Way, and the Universe at large developed over the next half century (a story elaborated on in the heart of this book), most of the people who thought about the problem of the origin of life worried about how to make complex organic molecules from simple chemicals such as methane and ammonia under the conditions that were thought to have existed on the early Earth--as we shall see, it was not until the late 1960s that radio astronomy began to reveal the richness of interstellar chemistry.
It was also in the 1960s that interest in the panspermia idea stirred once again--though this actually happened before the discovery of complex organic molecules in space. Part of the impetus for this interest came from balloon flights that carried unmanned instrument packages to great heights in the stratosphere and showed that microorganisms do indeed float around in the upper atmosphere. But the key calculations were carried out by the American astronomer Carl Sagan, when he collaborated with the Russian Iosef Shklovskii on the epic book Intelligent Life in the Universe, first published in 1966 (but still well worth reading). Instead of simply speculating about the fate of such microorganisms, Sagan actually calculated the effect of solar radiation on particles of different sizes (something Arrhenius could not do, of course, because in the early 1900s there was insufficient information about the Sun and the interplanetary environment).
Because gravity pulls particles in toward the Sun, and the radiation pressure pushing them outward is rather feeble, it turns out that only very small particles can be blown away from the orbit of the Earth--microbes less than about half a millionth of a meter in diameter. This is intriguing on two counts: first, because there are living microorganisms just that size; and second, because this is also just the size of the particles of dust that have now been detected in interstellar clouds. Such a bacterial particle departing from the Earth would pass the orbit of Mars in a few weeks, Jupiter in a few months, escape from the Solar System in a few years, and perhaps mingle with an interstellar cloud within a million years. Although the original panspermia idea envisaged the microorganisms drifting down into the atmospheres of newly formed planets, a more modern interpretation would see them as becoming part of the material out of which new planetary systems form. But there is a snag, which Sagan was quick to point out.
As soon as the microorganisms leave the Earth's atmosphere, they are exposed to ultraviolet (UV) radiation from the Sun and also to particles, such as protons and electrons, that form part of the solar wind (solar cosmic rays). Even the most resistant bacteria around on Earth today would be killed by the UV radiation within a day of leaving the Earth, and even if there existed an organism that was impervious to solar UV, it would be killed by the cosmic rays before it got out of the Solar System.
There's another problem, at least in the original version of this idea. If microorganisms about 0.5 millionths of a meter in size are blown outward from the orbit of the Earth, certainly nothing in this size range could have fallen and landed on the young Earth, even if it had escaped from a similar planet somewhere across the Universe. This led Sagan and Shklovskii to discuss the possibility of the seeds of life arriving on planets farther out from their parent star--planets like Jupiter or Saturn. This rather begs the question of how life originated on Earth; but in any case this problem doesn't really arise if we envisage the microorganisms as becoming integrated into interstellar clouds from which new planetary systems form, because they would be carried down onto young planets by cometary impacts, part of the natural process of planet formation that I describe in Chapter 9.
By the early 1970s, Sagan was convinced that the panspermia idea would not work, because the environment of space is too hazardous for the kind of living things that could escape from the Earth today. But at about the same time, Francis Crick, an eminent British biologist, was becoming convinced that the astronomical and geological evidence allowed too little time for life to have evolved from scratch on the Earth itself--there is clear geologic evidence that life existed on Earth less than 600 million years after the planet formed, and biologists such as Crick see no way it could have emerged from a mixture of simple chemicals in that time. While the astronomer was rejecting panspermia on biological grounds, the biologist was about to begin promoting it on astronomical grounds. Together with the American Leslie Orgel, Crick developed a variation on the theme, which he called "directed panspermia," arguing that the Earth had been deliberately seeded with life, in the form of microorganisms (essentially bacteria) carried across space inside an alien spacecraft safely shielded from cosmic radiation.
This would not necessarily have to be interpreted as a deliberate attempt to seed the Earth--we very nearly have the technology today to send small unmanned probes out in more or less random directions, carrying bacteria to be dumped on any planet they encounter. It may seem an inelegant way for life on Earth to have got started, but it was one step better, perhaps, than the proposal put forward by the astronomer Tommy Gold, who suggested, only slightly tongue in cheek, that all of life on Earth might have descended from the organic debris left behind on the planet by some aliens who stopped by for a picnic!
Since the 1970s, though, the pendulum has swung again, and a new variation on the panspermia theme has suggested how, in spite of the radiation problem, microorganisms could, after all, escape by natural means from a planet like the Earth and cross interstellar space to infect other planets with life. Jeff Secker, at Washington State University, working with Paul Wesson and James Lepock, at the University of Waterloo, in Canada, took another look at the problem of the survivability of interstellar spores and this time took account of the way a star like the Sun changes as it ages and becomes what is known as a red giant. Their first step was to imagine the living (or dormant) microorganisms being protected by being embedded in grains of dust. This only partially solves the radiation problem today and also makes the particles heavier, so it is harder for them to be blown out of the Solar System. When the Sun becomes a red giant, the intensity of ultraviolet radiation from its surface will be much less, but its overall brightness will increase (thereby increasing the pressure of radiation pushing tiny grains outward from Earth orbit) and the strength of the solar wind of material blowing away into space will be much greater--as we shall see, red giants eject a lot of material into space, and life-bearing material from a planet in orbit around the red giant could well be included in this. Once again, it is easy to see how the living material could end up in interstellar clouds from which new planetary systems form. And it doesn't even have to be living (or dormant) material. In 1996 Secker and his colleagues pointed out something that everyone else seems to have missed: even what they call "inactivated" biological material (in the form of fragments of molecules such as DNA, the remains of once-living material broken apart by cosmic rays) could, if introduced to a suitable planet, "enhance the chance that life will evolve there, and could possibly explain the (apparently) rapid evolution of early life on Earth." Once again, though, like everyone who has promoted panspermia in its various guises, Secker and his colleagues are thinking in terms of the biological material falling onto a preexisting planet and have missed the point that it is much more simple for the material to mingle with the stuff from which planets form in the first place.
From their point of view, the good news is that since a star like the Sun becomes a red giant only after 10 billion years or so of its existence as a stable star, much the same state that we see it in today, there is ample time for life to evolve on a planet orbiting around such a star. The first time this happened, it could indeed have taken much more than 600 million years for life to emerge. Then, panspermia does the rest. The bad news is that if such a long time span is needed for life to emerge once, it must have happened on a planet formed a very long time before the formation of the Solar System. The Solar System formed about 4.5 billion years ago, and this argument would seem to suggest that the whole process of the emergence of life, followed by panspermia occurring after the parent star had become a red giant, would have taken much longer. Even if we imagine that the star involved was a little more massive than the Sun (which would mean that it ran through its life cycle a little more quickly than the Sun), we still have to allow billions of years for it to become a red giant, because the whole point of the argument is that you need those billions of years for the first living things to evolve. It is no good invoking, say, a star with three times the mass of the Sun, which runs through its life cycle in just 500 million years, because if life can evolve on a planet around such a star in that short a span of time, it could have done so on Earth in the first few hundred million years of our own planet's existence.
So the "red giant panspermia" argument pushes back the formation of the system in which life first emerged beyond 10 billion years, uncomfortably close to the best estimates of the age of the Universe and allowing very little time for the first planetary systems to form after the Big Bang. That far back in time, the stars had had little opportunity to synthesize the heavier elements, and it is a matter of conjecture whether there would have been enough of the right stuff around on the earliest planets to provide the raw materials of life as we know it. And it has to be life as we know it, because the whole argument is that we are descended directly from those first living things.
For my money, the bottom line is that panspermia could work, but that, for reasons spelled out in this book, neither "natural" nor "directed" panspermia is needed to explain the presence of life on Earth. Both ideas seem more contrived than the suggestion that the young Earth was seeded with complex organic molecules that arose through natural chemical processes going on in the interstellar cloud from which the Solar System formed--an idea promoted by Sagan in the late 1970s, in work carried out with Christopher Chyba. And if I were going to speculate further, I would wager that the complex chemistry of interstellar clouds could have gone all the way to producing genuine living molecules, rather than introduce another cumbersome step into the calculation by venturing a guess that those molecules evolved on other planets and then got ejected into space to mingle with those interstellar clouds. In the late 1990s, laboratory experiments in which the kinds of molecules found in interstellar clouds today were dosed with ultraviolet light led to the formation of a slew of organic molecules which themselves react further to produce amino acids and other biochemical molecules. Dump that lot on the Earth when it was about 600 million years old, and the difficulties which so worried Francis Crick in the 1970s disappear. As the astronomer David Buhl has put it, "the predominance of organic species [in interstellar clouds] and their similarity to the products obtained in the [laboratory] synthesis of amino-acids in the study of the origin of life suggests a very close parallel between interstellar clouds and pre-biotic chemistry."
Even though I do not think panspermia is a likely explanation of our own origins, there is no doubt that in the near future we will have the capability to seed other planets with life, which raises intriguing ethical questions but goes beyond the scope of the present book. The message I hope you get from my brief account of the history of panspermia is how much progress has been achieved in the past century. In a way, the original panspermia idea was a counsel of despair. Because nobody knew how life could have originated, the speculation was that it had always existed and simply spread from place to place in the Universe. We still do not know exactly how life began--nobody had yet seen a mixture of chemicals come to life in a test tube. But, unlike Arrhenius, we do know precisely what mixture of chemicals is required for the existence of life as we know it. And we know exactly where those chemicals came from: they are the natural by-products of the processes of star formation and evolution. That is the story I am going to tell you now, beginning with the basics of what life itself is all about.
Excerpted from Stardust by John R. Gribbin Copyright © 2001 by John R. Gribbin.
Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
My agent complained about this, since apart from the first book I wrote with Douglas Orgill (The Sixth Winter) none of these novels were commercially successful. But a strange thing happened. Through the discipline of learning how to plot stories, build up suspense, and get inside believable characters, I found that my nonfiction writing was getting better. Even my agent had to agree. By writing novels, I became a much better science writer, even if the novels didn't succeed on their own terms.
It started out the other way around -- The Sixth Winter grew out of my interest in the nonfiction developments in the scientific study of climate change, and another collaboration with Douglas Orgill, Brother Esau, out of my interest in human evolution. Both appeared in the early 1980s, just before I set to work on what is still my best-known book, In Search of Schrödinger's Cat. Writing about science fact that reads like fiction (quantum physics) clearly benefited from the novel-writing! The circle from fact to fiction and back again was completed when I returned to the theme of human evolution in Being Human (written with my wife, Mary), and the storytelling aspect of writing fiction was particularly useful in a string of scientific biographies that I got involved with, including Richard Feynman: A Life in Science (also written with Mary).
Emboldened by this, I went back to writing fiction. I wrote my very best novel (The Sixth Winter included) and had a good publication deal lined up. Then, the publisher went bankrupt. I still have the novel, if anyone would like to take a chance on publishing it, but I've promised my agent not to write any more unless and until it sees the light of day. Even so, for those with eyes to see (as they say), my latest book, The Birth of Time, clearly bears the stamp of a writer who has been at least once around the fiction-writing block. Since this book deals (in part) with my own scientific work, it involved some personal storytelling which I could never have managed so well without that background.
So when people ask me who the biggest influences on my career have been, the answer has to be, first, Isaac Asimov and Arthur C. Clarke (for their fiction, not their nonfiction), and then Douglas Orgill and David Compton, for teaching me how to do it. And I've not given up. Maybe I'll try a movie script....
John Gribbin is a visiting fellow in astronomy at the University of Sussex and the author of many popular science books, including In Search of Schrödinger's Cat, Almost Everyone's Guide to Science, and Q Is for Quantum. He is a fellow of the Royal Society of Literature (a unique honor for a science writer).