For tens of thousands of years our ancestors understood the world through myths, and the pace of change was glacial. The rise of scientific understanding transformed the world within a few centuries. Why? Physicist David Deutsch proposes a subtle answer
Tuesday, October 27, 2009
Monday, April 27, 2009
Colors
[condensed from Chapter 3, The Embodied Mind, of “Philosophy In The Flesh”, by George Lakoff and Mark Johnson, published by Basic Books, 1999]
We are neural beings. Each human brain has 100 billion neurons and 100 trillion synaptic connections. It is common in the brain for information to be passed from one dense ensemble of neurons to another via a relatively sparse set of connections.
To take a concrete example, each human eye has 100 million light-sensing cells, but only about 1 million fibers leading to the brain. Each incoming image must therefore be reduced in complexity by a factor of 100. That is, information in each fiber constitutes a “categorization” of the information from about 100 cells. Neural categorization of this sort exists throughout the brain, up through the highest level of categories that we can be aware of. When we see trees, we see them as trees, not just as individual objects distinct from one another. The same with rocks, houses, windows, doors, and so on.
Since we are neural beings, our categories are formed through our embodiment. What that means is that the categories we form are part of our experience. What we call concepts are neural structures that allow us to mentally characterize our categories and reason about them.
What could be simpler or more obvious than color concepts? The sky is blue, fresh grass is green, blood is red, the sun and moon are yellow. We see color, and yet it is false, as false as another thing we see, the moving sun rising past the edge of the stationary earth.
Our experience of color is created by a combination of four factors: wavelengths of reflected light, lighting conditions, and two aspects of our bodies: (1) the three kinds of color cones in our retinas which absorb light of long, medium, and short wavelengths, and (2) the complex neural circuitry connected to those cones.
However, one physical property of the surface of an object matters for color: its reflectance, that is, the relative percentages of high-, medium-, and low-frequency light that it reflects. That is a constant. But the actual wavelengths of light reflected by an object are not a constant.
The wavelengths of light coming from an object depend on the nature of the light illuminating it: tungsten or fluorescent, daylight on a sunny or on a cloudy day, the light of dawn or dusk. Under different conditions, the wavelengths of light coming from an object will differ considerably, yet its color will be relatively constant, it will look pretty much the same. Color, then is not just the perception of wavelength; color constancy depends on the brain´s ability to compensate for variations in the light source.
Another crucial thing to bear in mind is that light is not colored. Visible light is electromagnetic radiation, like radio waves, vibrating within a certain frequency range, and only when such radiation impinges on our retinas are we able to see. We see a particular color when the surrounding conditions are right, when radiation in a certain range impinges on our retina and when our color cones absorb the radiation, producing an electric signal that is appropriately processed by the neural circuitry of our brains. The qualitative experience that this produces in us is what we call “color”.
Colors as we see them, say, the red of blood or the blue of the sky, are not out there in the blood or the sky. Indeed, the sky is not even an object. It has no surface for the color to be in. Without a physical surface, the sky does not have a surface reflectance to be detected as color. The sky is blue because the atmosphere transmits only a certain range of wavelengths of incoming light from the sun, and of the wavelengths it does transmit, it scatters some more than others. Thus, the sky is blue for a very different reason than a painting of the sky is blue. What we perceive as blue does not characterize a single “thing” in the world, neither “blueness” nor wavelength reflectance.
Color concepts are “interactional”; they arise from the interactions of our bodies, our brains, the reflective properties of objects, and electromagnetic radiation. Colors are not objective; there is in the grass or the sky no greenness or blueness independent of retinas, color cones, neural circuitry, and brains. Nor are colors purely subjective; they are neither a figment of our imaginations nor spontaneous creations of our brains.
Evolution has worked with physical limitations: only certain wavelengths of light get through the atmosphere, only certain chemicals react to short, medium, and long wavelengths, and so on. We have the color concepts we do because the physical limitations constraining evolution gave evolutionary advantages to beings with a color system that enabled them to function well in cucial aspects. We have evolved within these limitations to have the color systems we have, and they allow us to function well in the world.
Wednesday, January 7, 2009
The Persistence of Memory
From Carl Sagan’s “Cosmos, The Book”, Chapter 11, (Ballantyne Books paperback edition Nov.1985, pages 227-231).
“…..The information stored in the DNA double helix of a whale or a human or of any other beast or vegetable on earth is written in a language of four letters – the four different kinds of nucleotides, the molecular components that make up DNA. How many bits of information are contained on the hereditary material of various life forms? How many yes/no answers to the various biological questions are written in the language of life? A virus needs about 10,000 bits – roughly equivalent to the amount of information on a full page. But the viral information is simple, exceeding compact, extraordinarily efficient. Reading it requires very close attention. These are the instructions it needs to infect some other organism and to reproduce itself – the only things that viruses are any good at. A bacterium uses roughly a million bits of information – which is about 100 printed pages. Bacteria have a lot more to do than viruses. Unlike the viruses, they are not thoroughgoing parasites. Bacteria have to make a living. And a free-swimming one-celled amoeba is much more sophisticated; with about four hundred million bits in its DNA, it would require some eighty 500-page volumes to make another amoeba.
A whale or a human being needs something like five billion bits. These bits of information in our encyclopaedia of life – in the nucleus of each of our cells – if written out in, say, English, would fill a thousand volumes. Every one of your hundred trillion cells contains a complete library of instructions on how to make every part of you. Every cell in your body arises by successive cell divisions from a single cell, a fertilized egg generated by your parents. Every time that cell divided, in the many embryological steps that went into making you, the original set of genetic instructions was duplicated with great fidelity. So your liver cells have some unemployed knowledge about how to make your bone cells, and vice-versa. The genetic library contains everything your body knows how to do on its own. The ancient information is written in exhaustive, careful redundant detail – how to laugh, how to sneeze, how to walk, how to recognize patterns, how to reproduce, how to digest an apple.
Eating an apple is an immensely complicated process. In fact, if I had to synthesize my own enzymes, if I consciously had to remember and direct all the chemical steps required to get energy out of food, I would probably starve. But even bacteria do anaerobic glycolysis, which is why apples rot: lunchtime for the microbes. They and we and all creatures in between possess many similar genetic instructions. Our separate gene libraries have many pages in common, another reminder of our common evolutionary heritage. Our technology can duplicate only a tiny fraction of the intricate biochemistry that our bodies effortlessly perform: we have only just begun to study these processes. Evolution, however, has had billions of years of practice. DNA knows.
But suppose what you had to do was so complicated that even several billion bits was insufficient. Suppose the environment was changing so fast that the pre-coded genetic encyclopaedia, which served perfectly well before, was no longer entirely adequate. Then even a gene library of 1,000 volumes would not be enough. That is why we have brains.
Like all our organs, the brain has evolved, increasing in complexity and information content, over millions of years. Its structure reflects all the stages through which it has passed. This brain evolved from the inside out. Deep inside is the oldest part, the brainstem, which conducts the basic biological functions, including the rhythms of life – heartbeat and respiration. According to a provocative insight by Paul Maclean, the higher functions of the brain evolved in three successive stages. Capping the brainstem is the R-complex, the seat of aggression, ritual, territoriality and social hierarchy, which evolved hundreds of millions years ago in our reptilian ancestors. Deep inside the skull of every one of us there is something like the brain of a crocodile. Surrounding the R-complex is the limbic system or mammalian brain, which evolved tens of millions of years ago in ancestors who were mammals but not yet primates. It is a major source of our moods and emotions, of our concern and care for the young.
And finally, on the outside, living in uneasy truce with the more primitive brains beneath, is the cerebral cortex, which evolved millions of years ago in our primate ancestors. The cerebral cortex, where matter is transformed into consciousness, is the point of embarkation for all our cosmic voyages. Comprising more than two thirds of the brain mass, it is the realm of both intuition and critical analysis. It is here that we have ideas and inspirations, here that we read and write, here that we do mathematics and compose music. The cortex regulates our conscious lives. It is the distinction of our species, the seat of our humanity. Civilization is the product of the cerebral cortex.
The language of the brain is not the DNA language of the genes. Rather, what we know is encoded in cells called neurons – microscopic electrochemical switching elements, typically a few hundredths of a millimeter across. Each of us has perhaps a hundred billion neurons, comparable to the number of stars in the Milky Way Galaxy. Many neurons have thousands of connections to their neighbors. There is something like a hundred trillion such connections in the human cerebral cortex.”
“…..The information content of the human brain expressed in bits is probably comparable to the total number of connections among the neurons – about a hundred trillion bits. If written out in English, say, that information would fill some twenty million volumes, as many as in the world’s largest libraries. The equivalent of twenty million books is inside the heads of every one of us. The brain is a very big place in a very small space. Most of the books in the brain are in the cerebral cortex. Down in the basement are the functions our remote ancestors mainly depended on – aggression, child-rearing, fear, sex, the willingness to follow leaders blindly. Of the higher brain functions – reading, writing, speaking – seem to be localized in particular places in the cerebral cortex. Memories, on the other hand, are stored redundantly in many locales…….”
“…..The brain does much more than recollect. It compares, synthesizes, analyzes, generates abstractions. We must figure out much more than our genes can know. That is why the brain library is some ten thousand times larger than the gene library. Our passion for learning, evident in the behavior of every toddler, is the tool for our survival. Emotions and ritualized behavior patterns are built deeply into us. They are part of our humanity. But they are not characteristically human. Many other animals have feelings. What distinguishes our species is thought. The cerebral cortex is a liberation. We need no longer be trapped in the genetically inherited behavior patterns of lizards and baboons. We are, each of us, largely responsible for what gets put into our brains, for what, as adults, we wind up caring for and knowing about. No longer at the mercy of the reptile brain, we can change ourselves.”
“…..The information stored in the DNA double helix of a whale or a human or of any other beast or vegetable on earth is written in a language of four letters – the four different kinds of nucleotides, the molecular components that make up DNA. How many bits of information are contained on the hereditary material of various life forms? How many yes/no answers to the various biological questions are written in the language of life? A virus needs about 10,000 bits – roughly equivalent to the amount of information on a full page. But the viral information is simple, exceeding compact, extraordinarily efficient. Reading it requires very close attention. These are the instructions it needs to infect some other organism and to reproduce itself – the only things that viruses are any good at. A bacterium uses roughly a million bits of information – which is about 100 printed pages. Bacteria have a lot more to do than viruses. Unlike the viruses, they are not thoroughgoing parasites. Bacteria have to make a living. And a free-swimming one-celled amoeba is much more sophisticated; with about four hundred million bits in its DNA, it would require some eighty 500-page volumes to make another amoeba.
A whale or a human being needs something like five billion bits. These bits of information in our encyclopaedia of life – in the nucleus of each of our cells – if written out in, say, English, would fill a thousand volumes. Every one of your hundred trillion cells contains a complete library of instructions on how to make every part of you. Every cell in your body arises by successive cell divisions from a single cell, a fertilized egg generated by your parents. Every time that cell divided, in the many embryological steps that went into making you, the original set of genetic instructions was duplicated with great fidelity. So your liver cells have some unemployed knowledge about how to make your bone cells, and vice-versa. The genetic library contains everything your body knows how to do on its own. The ancient information is written in exhaustive, careful redundant detail – how to laugh, how to sneeze, how to walk, how to recognize patterns, how to reproduce, how to digest an apple.
Eating an apple is an immensely complicated process. In fact, if I had to synthesize my own enzymes, if I consciously had to remember and direct all the chemical steps required to get energy out of food, I would probably starve. But even bacteria do anaerobic glycolysis, which is why apples rot: lunchtime for the microbes. They and we and all creatures in between possess many similar genetic instructions. Our separate gene libraries have many pages in common, another reminder of our common evolutionary heritage. Our technology can duplicate only a tiny fraction of the intricate biochemistry that our bodies effortlessly perform: we have only just begun to study these processes. Evolution, however, has had billions of years of practice. DNA knows.
But suppose what you had to do was so complicated that even several billion bits was insufficient. Suppose the environment was changing so fast that the pre-coded genetic encyclopaedia, which served perfectly well before, was no longer entirely adequate. Then even a gene library of 1,000 volumes would not be enough. That is why we have brains.
Like all our organs, the brain has evolved, increasing in complexity and information content, over millions of years. Its structure reflects all the stages through which it has passed. This brain evolved from the inside out. Deep inside is the oldest part, the brainstem, which conducts the basic biological functions, including the rhythms of life – heartbeat and respiration. According to a provocative insight by Paul Maclean, the higher functions of the brain evolved in three successive stages. Capping the brainstem is the R-complex, the seat of aggression, ritual, territoriality and social hierarchy, which evolved hundreds of millions years ago in our reptilian ancestors. Deep inside the skull of every one of us there is something like the brain of a crocodile. Surrounding the R-complex is the limbic system or mammalian brain, which evolved tens of millions of years ago in ancestors who were mammals but not yet primates. It is a major source of our moods and emotions, of our concern and care for the young.
And finally, on the outside, living in uneasy truce with the more primitive brains beneath, is the cerebral cortex, which evolved millions of years ago in our primate ancestors. The cerebral cortex, where matter is transformed into consciousness, is the point of embarkation for all our cosmic voyages. Comprising more than two thirds of the brain mass, it is the realm of both intuition and critical analysis. It is here that we have ideas and inspirations, here that we read and write, here that we do mathematics and compose music. The cortex regulates our conscious lives. It is the distinction of our species, the seat of our humanity. Civilization is the product of the cerebral cortex.
The language of the brain is not the DNA language of the genes. Rather, what we know is encoded in cells called neurons – microscopic electrochemical switching elements, typically a few hundredths of a millimeter across. Each of us has perhaps a hundred billion neurons, comparable to the number of stars in the Milky Way Galaxy. Many neurons have thousands of connections to their neighbors. There is something like a hundred trillion such connections in the human cerebral cortex.”
“…..The information content of the human brain expressed in bits is probably comparable to the total number of connections among the neurons – about a hundred trillion bits. If written out in English, say, that information would fill some twenty million volumes, as many as in the world’s largest libraries. The equivalent of twenty million books is inside the heads of every one of us. The brain is a very big place in a very small space. Most of the books in the brain are in the cerebral cortex. Down in the basement are the functions our remote ancestors mainly depended on – aggression, child-rearing, fear, sex, the willingness to follow leaders blindly. Of the higher brain functions – reading, writing, speaking – seem to be localized in particular places in the cerebral cortex. Memories, on the other hand, are stored redundantly in many locales…….”
“…..The brain does much more than recollect. It compares, synthesizes, analyzes, generates abstractions. We must figure out much more than our genes can know. That is why the brain library is some ten thousand times larger than the gene library. Our passion for learning, evident in the behavior of every toddler, is the tool for our survival. Emotions and ritualized behavior patterns are built deeply into us. They are part of our humanity. But they are not characteristically human. Many other animals have feelings. What distinguishes our species is thought. The cerebral cortex is a liberation. We need no longer be trapped in the genetically inherited behavior patterns of lizards and baboons. We are, each of us, largely responsible for what gets put into our brains, for what, as adults, we wind up caring for and knowing about. No longer at the mercy of the reptile brain, we can change ourselves.”
The Natural Sciences
From Edward O. Wilson’s “Consilience: The Unity of Knowledge”, chapter 4, pp.45-48
“….Without the instruments and accumulated knowledge of the natural sciences – physics, chemistry and biology – humans are trapped in a cognitive prison. They are like intelligent fish born in a deep, shadowed pool. Wondering and restless, longing to reach out, they think about the world outside. They invent ingenious speculations and myths about the origin of the confining waters, of the sun and the sky and the stars above, and the meaning of their own existence. But they are wrong, always wrong, because the world is too remote from ordinary experience to be merely imagined……
With instrumental science humanity has escaped confinement and prodigiously extended its grasp of physical reality. Once we were nearly blind, now we can see – literally. Visible light, we have learned, is not the sole illuminating energy of the universe, as pre-scientific common sense decreed. It is instead an infinitesimal sliver of electromagnetic radiation, comprising wavelengths of 400 to 700 nanometers (billionths of a meter), within a spectrum that ranges from gamma waves trillions of times shorter to radio waves trillions of times longer. Radiation over this span, in wildly varying amounts, continually rain down on our bodies. But without instruments we were oblivious to its existence. Because the human retina is rigged to report only 400 to 700 nanometers, the unaided brain concludes that only visible light exists.
Many kinds of animals know better. They live in a different visual world, oblivious to part of the human visible spectrum, sensitive to some wavelengths outside it. Below 400 nanometers, butterflies find flowers and pinpoint pollen and nectar sources by the pattern of ultraviolet rays reflected off the petals. Where we see a plain yellow or white blossom, they see spots and concentric circles in light and dark. The patterns have evolved in plants to guide insect pollinators to the anthers and nectar pools.
With the aid of appropriate instruments we can now see the world with butterfly eyes.
Scientists have entered the visual world of animals and beyond because they understand the electromagnetic spectrum. They can translate any wavelength into visible light and audible sound and generate most of the spectrum from diverse energy sources. By manipulating selected segments of the electromagnetic spectrum they peer downward to the trajectories of sub-atomic particles and outward to star birth in distant galaxies whose incoming light dates back to near the beginning of the universe. They (more accurately we, since scientific knowledge is universally available) can visualize matter across thirty seven orders of magnitude. The largest galactic cluster is larger than the smallest known particle by a factor of one with about thirty-seven zeroes following it.
I mean no disrespect when I say that pre-scientific people, regardless of their innate genius, could never guess the physical reality beyond the tiny sphere attainable by unaided common sense……and notwithstanding the emotional satisfaction it gives, mysticism, the strongest pre-scientific probe into the unknown, has yielded zero. No shaman’s spell or fast upon a sacred mountain can summon the electromagnetic spectrum. Prophets of the great religions were kept unaware of its existence, not because of a secretive god but because they lacked the hard-won knowledge of physics………
All our other senses have been expanded by science. Once we were deaf; now we can hear everything. The human auditory range is 20 to 20,000 Hz, or cycles of air compression per second. Above that range, flying bats broadcast ultrasonic pulses into the night air and listen to echoes to locate moths and other insects on the wing. Many of their potential prey listen with ears tuned to the same frequencies as the bats……..
We have even uncovered basic senses entirely outside the human repertory. Where humans detect electricity only indirectly by a tingling of skin or flash of light, the electric fishes of Africa or South America, a medley of freshwater eels, catfish, and elephant-nosed fishes, live in a galvanic world. They generate charged fields around their bodies with trunk muscle tissue that has been modified by evolution into organic batteries……They also communicate with one another by means of coded electrical bursts. Zoologists, using generators and detectors, can join the conversation. They are able to talk as through a fish’s skin.
From these and countless other examples can be drawn an informal rule of biological evolution important to the understanding of the human condition: if an organic sensor can be imagined that picks up any signal from the environment, there exists a species somewhere that possesses it. The bountiful powers of life expressed in such diversity raise a question about the incapacity of the unaided human senses. Why can’t our species, the supposed summum bonum of Creation, do as much as all the animals combined, and more? Why were we brought into the world physically handicapped?
Evolutionary biology offers a simple answer. Natural selection……prepares organisms only for necessities…..It follows that each species lives in its own sensory world.…….Natural selection, in short, does not anticipate future needs. But this principle, while explaining so much so well, presents a difficulty. If the principle is universally true, how did natural selection prepare the mind for civilization before civilization existed? That is the great mystery of human evolution: how to account for calculus and Mozart.
“….Without the instruments and accumulated knowledge of the natural sciences – physics, chemistry and biology – humans are trapped in a cognitive prison. They are like intelligent fish born in a deep, shadowed pool. Wondering and restless, longing to reach out, they think about the world outside. They invent ingenious speculations and myths about the origin of the confining waters, of the sun and the sky and the stars above, and the meaning of their own existence. But they are wrong, always wrong, because the world is too remote from ordinary experience to be merely imagined……
With instrumental science humanity has escaped confinement and prodigiously extended its grasp of physical reality. Once we were nearly blind, now we can see – literally. Visible light, we have learned, is not the sole illuminating energy of the universe, as pre-scientific common sense decreed. It is instead an infinitesimal sliver of electromagnetic radiation, comprising wavelengths of 400 to 700 nanometers (billionths of a meter), within a spectrum that ranges from gamma waves trillions of times shorter to radio waves trillions of times longer. Radiation over this span, in wildly varying amounts, continually rain down on our bodies. But without instruments we were oblivious to its existence. Because the human retina is rigged to report only 400 to 700 nanometers, the unaided brain concludes that only visible light exists.
Many kinds of animals know better. They live in a different visual world, oblivious to part of the human visible spectrum, sensitive to some wavelengths outside it. Below 400 nanometers, butterflies find flowers and pinpoint pollen and nectar sources by the pattern of ultraviolet rays reflected off the petals. Where we see a plain yellow or white blossom, they see spots and concentric circles in light and dark. The patterns have evolved in plants to guide insect pollinators to the anthers and nectar pools.
With the aid of appropriate instruments we can now see the world with butterfly eyes.
Scientists have entered the visual world of animals and beyond because they understand the electromagnetic spectrum. They can translate any wavelength into visible light and audible sound and generate most of the spectrum from diverse energy sources. By manipulating selected segments of the electromagnetic spectrum they peer downward to the trajectories of sub-atomic particles and outward to star birth in distant galaxies whose incoming light dates back to near the beginning of the universe. They (more accurately we, since scientific knowledge is universally available) can visualize matter across thirty seven orders of magnitude. The largest galactic cluster is larger than the smallest known particle by a factor of one with about thirty-seven zeroes following it.
I mean no disrespect when I say that pre-scientific people, regardless of their innate genius, could never guess the physical reality beyond the tiny sphere attainable by unaided common sense……and notwithstanding the emotional satisfaction it gives, mysticism, the strongest pre-scientific probe into the unknown, has yielded zero. No shaman’s spell or fast upon a sacred mountain can summon the electromagnetic spectrum. Prophets of the great religions were kept unaware of its existence, not because of a secretive god but because they lacked the hard-won knowledge of physics………
All our other senses have been expanded by science. Once we were deaf; now we can hear everything. The human auditory range is 20 to 20,000 Hz, or cycles of air compression per second. Above that range, flying bats broadcast ultrasonic pulses into the night air and listen to echoes to locate moths and other insects on the wing. Many of their potential prey listen with ears tuned to the same frequencies as the bats……..
We have even uncovered basic senses entirely outside the human repertory. Where humans detect electricity only indirectly by a tingling of skin or flash of light, the electric fishes of Africa or South America, a medley of freshwater eels, catfish, and elephant-nosed fishes, live in a galvanic world. They generate charged fields around their bodies with trunk muscle tissue that has been modified by evolution into organic batteries……They also communicate with one another by means of coded electrical bursts. Zoologists, using generators and detectors, can join the conversation. They are able to talk as through a fish’s skin.
From these and countless other examples can be drawn an informal rule of biological evolution important to the understanding of the human condition: if an organic sensor can be imagined that picks up any signal from the environment, there exists a species somewhere that possesses it. The bountiful powers of life expressed in such diversity raise a question about the incapacity of the unaided human senses. Why can’t our species, the supposed summum bonum of Creation, do as much as all the animals combined, and more? Why were we brought into the world physically handicapped?
Evolutionary biology offers a simple answer. Natural selection……prepares organisms only for necessities…..It follows that each species lives in its own sensory world.…….Natural selection, in short, does not anticipate future needs. But this principle, while explaining so much so well, presents a difficulty. If the principle is universally true, how did natural selection prepare the mind for civilization before civilization existed? That is the great mystery of human evolution: how to account for calculus and Mozart.
The Edge of Forever
From Carl Sagan’s “Cosmos, The Book”, Chapter 10, (Ballantyne Books paperback edition Nov.1985, pages 200-201 and 212-213).
“….”Some foolish men declare that a Creator made the world. The doctrine that the world was created is ill-advised and should be rejected.
If God created the world, where was he before creation?.....
How could God have made the world without any raw material? If you say that he made this first, and then the world, you are faced with an endless regression…….
Know that the world is uncreated, as time itself is, without beginning and end………”
{The Mahapurana (The Great Legend), Jinasena (India, ninth century)}
Ten or twenty billion years ago, something happened – the Big Bang, the event that began our universe. Why it happened is the greatest mystery we know. That it happened is reasonably clear. All the matter and energy now in the universe was concentrated at extremely high density – a kind of cosmic egg, reminiscent of the creation myths of many cultures – perhaps into a mathematical point with no dimensions at all. It was not that all the matter and energy were squeezed into a minor corner of the present universe; rather, the entire universe, matter and energy and the space they fill, occupied a very small volume. There was not much space for events to happen in.
In that titanic cosmic explosion, the universe began an expansion which has never ceased……..”
“….If the general picture of an expanding universe and a Big Bang is correct, we must then confront still more difficult questions. What were conditions like at the time of the Big Bang? What happened before that? Was there a tiny universe devoid of all matter, and then the matter suddenly created from nothing? How does that happen? In many cultures it is customary to answer that God created the universe out of nothing. But this is mere temporizing. If we wish courageously to pursue the question, we must of course ask next where God comes from. And if we decide this to be unanswerable, why not save a step and decide that the origin of the universe is an unanswerable question. Or, if we say that God has always existed, why not save a step and conclude that the universe has always existed?
Every culture has a myth of a world before creation, and of the creation of the world, often by the mating of gods or by the hatching of a cosmic egg. Commonly, the universe is naively imagined to follow human or animal precedent. Here, for example, are five small extracts from such myths, at different levels of sophistication, from the Pacific Basin:
In the very beginning everything was resting in perpetual darkness; night oppressed everything like an impenetrable thicket.
(The Great Father myth of the Aranda people of Central Australia)
All was in suspense, all calm, all in silence, all motionless and still; and the expanse of the sky was empty.
(The Popol Vuh of the Quiché Maya)
Na Arean sat alone in space as a cloud that floats in nothingness. He slept not, for there was no sleep, he hungered not, for as yet there was no hunger. So he remained for a great while, until a thought came to his mind. He said to himself, “I will make a thing”.
(A myth from Maiana, Gilbert Islands)
First there was the great cosmic egg. Inside the egg was chaos, and floating in chaos was P’an Ku, the Undeveloped, the divine Embryo. And P’an Ku burst out of the egg, four times larger than any man today, with a hammer and chisel in his hand with which he fashioned the world.
(The P’an Ku myths, China, around third century)
Before heaven and earth had taken form all was vague and amorphous….
That which was clear and light drifted up to become heaven, while that which was heavy and turbid solidified to become earth. It was very easy for the pure, fine material to come together, but extremely difficult for the heavy, turbid material to solidify. Therefore heaven was completed first and earth assumed shape after. When heaven and earth were joined in emptiness and all was unwrought simplicity, then without having been created things came into being. This was the great Oneness but all became different……….
(Huai-nan Tau, China, around first century, B.C.)
Theses myths are tributes to human audacity. The chief difference between them and our modern scientific myth of the Big Bang is that science is self-questioning, and that we can perform experiments and observations to test our ideas. But those other creation stories are worthy of our deep respect……….”
“….”Some foolish men declare that a Creator made the world. The doctrine that the world was created is ill-advised and should be rejected.
If God created the world, where was he before creation?.....
How could God have made the world without any raw material? If you say that he made this first, and then the world, you are faced with an endless regression…….
Know that the world is uncreated, as time itself is, without beginning and end………”
{The Mahapurana (The Great Legend), Jinasena (India, ninth century)}
Ten or twenty billion years ago, something happened – the Big Bang, the event that began our universe. Why it happened is the greatest mystery we know. That it happened is reasonably clear. All the matter and energy now in the universe was concentrated at extremely high density – a kind of cosmic egg, reminiscent of the creation myths of many cultures – perhaps into a mathematical point with no dimensions at all. It was not that all the matter and energy were squeezed into a minor corner of the present universe; rather, the entire universe, matter and energy and the space they fill, occupied a very small volume. There was not much space for events to happen in.
In that titanic cosmic explosion, the universe began an expansion which has never ceased……..”
“….If the general picture of an expanding universe and a Big Bang is correct, we must then confront still more difficult questions. What were conditions like at the time of the Big Bang? What happened before that? Was there a tiny universe devoid of all matter, and then the matter suddenly created from nothing? How does that happen? In many cultures it is customary to answer that God created the universe out of nothing. But this is mere temporizing. If we wish courageously to pursue the question, we must of course ask next where God comes from. And if we decide this to be unanswerable, why not save a step and decide that the origin of the universe is an unanswerable question. Or, if we say that God has always existed, why not save a step and conclude that the universe has always existed?
Every culture has a myth of a world before creation, and of the creation of the world, often by the mating of gods or by the hatching of a cosmic egg. Commonly, the universe is naively imagined to follow human or animal precedent. Here, for example, are five small extracts from such myths, at different levels of sophistication, from the Pacific Basin:
In the very beginning everything was resting in perpetual darkness; night oppressed everything like an impenetrable thicket.
(The Great Father myth of the Aranda people of Central Australia)
All was in suspense, all calm, all in silence, all motionless and still; and the expanse of the sky was empty.
(The Popol Vuh of the Quiché Maya)
Na Arean sat alone in space as a cloud that floats in nothingness. He slept not, for there was no sleep, he hungered not, for as yet there was no hunger. So he remained for a great while, until a thought came to his mind. He said to himself, “I will make a thing”.
(A myth from Maiana, Gilbert Islands)
First there was the great cosmic egg. Inside the egg was chaos, and floating in chaos was P’an Ku, the Undeveloped, the divine Embryo. And P’an Ku burst out of the egg, four times larger than any man today, with a hammer and chisel in his hand with which he fashioned the world.
(The P’an Ku myths, China, around third century)
Before heaven and earth had taken form all was vague and amorphous….
That which was clear and light drifted up to become heaven, while that which was heavy and turbid solidified to become earth. It was very easy for the pure, fine material to come together, but extremely difficult for the heavy, turbid material to solidify. Therefore heaven was completed first and earth assumed shape after. When heaven and earth were joined in emptiness and all was unwrought simplicity, then without having been created things came into being. This was the great Oneness but all became different……….
(Huai-nan Tau, China, around first century, B.C.)
Theses myths are tributes to human audacity. The chief difference between them and our modern scientific myth of the Big Bang is that science is self-questioning, and that we can perform experiments and observations to test our ideas. But those other creation stories are worthy of our deep respect……….”
Tuesday, January 6, 2009
Centuries of Search
“….A time will come when diligent research over long periods will bring to light things which now lie hidden. A single lifetime, even though entirely devoted to the sky, would not be enough for the investigation of so vast a subject….And so this knowledge will be unfolded only through long successive ages. There will come a time when our descendants will be amazed that we did not know things that are so plain to them……Many discoveries are reserved for ages still to come, when memory of us will have been effaced. Our universe is a sorry little affair unless it has in it something for every age to investigate……Nature does not reveal her mysteries once and for all.”
Seneca, Natural Questions, Book 7,(first century AD)
“……Some foolish men declare that a Creator made the world. The doctrine that the world was created is ill-advised and should be rejected.
If God created the world, where was he before creation?........How could God have made the world without any raw material? If you say he made this first, and then the world, you are faced with an endless regression……
Know that the world was uncreated, as time itself is, without beginning and end.”
The Mahapurana, The Great Legend, Jinsena, India, Ninth century
“…….we ought not to ask why the human mind troubles to fathom the secrets of the heavens……The diversity
of the phenomena of Nature is so great, and the treasures hidden in the heavens so rich, precisely in order that the human mind shall never be lacking for fresh nourishment.”
Johannes Kepler, Mysterium Cosmographicum
“……If a faithful account was rendered of Man’s ideas upon Divinity, he would be obliged to acknowledge, that for the most part the word “gods” has been used to express the concealed, remote, unknown causes of the effects he witnessed; that he applies this term when the spring of the natural, the source of known causes, ceases to be visible: as soon as he loses the thread of these causes, or as soon as his mind can no longer follow the chain, he solves the difficulty, terminates his research, by ascribing to his gods……When, therefore, he ascribes to his gods the production of some phenomenon…..does he, in fact, do any thing more than substitute for the darkness of his own mind, a sound to which he has been accustomed to listen with reverential awe?”
Paul Heinrich Dietrich, Baron von Holbach, Système de la Nature, London, 1770
“…….Probably all the organic beings which have ever lived on this earth have descended from one primordial form, into which life was first breathed…… There is grandeur in this view of life…..that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
Charles Darwin, The Origin of Species, 1859
“…….We inhabit a universe where atoms are made in the centers of stars; where each second a thousand suns are born; where life is sparked by sunlight and lightning in the airs and waters of youthful planets; where the raw material for biological evolution is sometimes made by the explosion of a star halfway across the Milky Way; where a thing as beautiful as a galaxy is formed a hundred billion times – a Cosmos of quasars and quarks, snowflakes and fireflies, where there may be black holes and other universes……How pallid by comparison are the pretensions of superstition and pseudoscience; how important it is for us to pursue and understand science, that characteristically human endeavor.”
Carl Sagan, Cosmos - The Book, (p.275), New York, 1985
Seneca, Natural Questions, Book 7,(first century AD)
“……Some foolish men declare that a Creator made the world. The doctrine that the world was created is ill-advised and should be rejected.
If God created the world, where was he before creation?........How could God have made the world without any raw material? If you say he made this first, and then the world, you are faced with an endless regression……
Know that the world was uncreated, as time itself is, without beginning and end.”
The Mahapurana, The Great Legend, Jinsena, India, Ninth century
“…….we ought not to ask why the human mind troubles to fathom the secrets of the heavens……The diversity
of the phenomena of Nature is so great, and the treasures hidden in the heavens so rich, precisely in order that the human mind shall never be lacking for fresh nourishment.”
Johannes Kepler, Mysterium Cosmographicum
“……If a faithful account was rendered of Man’s ideas upon Divinity, he would be obliged to acknowledge, that for the most part the word “gods” has been used to express the concealed, remote, unknown causes of the effects he witnessed; that he applies this term when the spring of the natural, the source of known causes, ceases to be visible: as soon as he loses the thread of these causes, or as soon as his mind can no longer follow the chain, he solves the difficulty, terminates his research, by ascribing to his gods……When, therefore, he ascribes to his gods the production of some phenomenon…..does he, in fact, do any thing more than substitute for the darkness of his own mind, a sound to which he has been accustomed to listen with reverential awe?”
Paul Heinrich Dietrich, Baron von Holbach, Système de la Nature, London, 1770
“…….Probably all the organic beings which have ever lived on this earth have descended from one primordial form, into which life was first breathed…… There is grandeur in this view of life…..that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
Charles Darwin, The Origin of Species, 1859
“…….We inhabit a universe where atoms are made in the centers of stars; where each second a thousand suns are born; where life is sparked by sunlight and lightning in the airs and waters of youthful planets; where the raw material for biological evolution is sometimes made by the explosion of a star halfway across the Milky Way; where a thing as beautiful as a galaxy is formed a hundred billion times – a Cosmos of quasars and quarks, snowflakes and fireflies, where there may be black holes and other universes……How pallid by comparison are the pretensions of superstition and pseudoscience; how important it is for us to pursue and understand science, that characteristically human endeavor.”
Carl Sagan, Cosmos - The Book, (p.275), New York, 1985
Tuesday, December 2, 2008
A New Account Of The Early Earth
By KENNETH CHANG
Published: December 1, 2008
The first 700 million years of Earth’s 4.5-billion-year existence are known as the Hadean period, after Hades, or, to shed the ancient Greek name, Hell.
That name seemed to fit with the common perception that the young Earth was a hot, dry, desolate landscape interspersed with seas of magma and inhospitable for life. Even if some organism had somehow popped into existence, the old story went, surely it would soon have been extinguished in the firestorm of one of the giant meteorites that slammed into the Earth when the young solar system was still crowded with debris.
Scars on the surface of the Moon record a hail of impacts during what is called the Late Heavy Bombardment. The Earth would have received an even more intense bombardment, and the common thinking until recently was that life could not have emerged on Earth until the bombardment eased about 3.85 billion years ago.
Norman H. Sleep, a professor of geophysics at Stanford, recalled that in 1986 he submitted a paper that calculated the probability of life surviving one of the giant, early impacts. It was summarily rejected because a reviewer said that obviously nothing could have lived then.
That is no longer thought to be true.
“We thought we knew something we didn’t,” said T. Mark Harrison, a professor of geochemistry at the University of California, Los Angeles. In hindsight the evidence was just not there. And new evidence has suggested a new view of the early Earth.
Over the last decade, the mineralogical analysis of small hardy crystals known as zircons embedded in old Australian rocks has painted a picture of the Hadean period “completely inconsistent with this myth we made up,” Dr. Harrison said.
Geologists now almost universally agree that by 4.2 billion years ago, the Earth was a pretty placid place, with both land and oceans. Instead of hellishly hot, it may have frozen over. Because the young Sun put out 30 percent less energy than it does today, temperatures on Earth might have been cold enough for parts of the surface to have been covered by expanses of ice.
In a new analysis, published in the current issue of the journal Nature, the zircons, the only bits of earth older than 4 billion years definitively known to have survived, provide another tantalizing hint about the Hadean period. Dr. Harrison and two U.C.L.A. colleagues, Michelle Hopkins, a graduate student, and Craig Manning, a professor of geology and geochemistry, report that minerals trapped inside zircons offer evidence that the processes of plate tectonics — the forces that push around the planet’s outer crust, forming and shaping the continents and oceans — had already begun.
“The picture that’s emerging is a watery world with normal rock recycling processes,” said Stephen J. Mojzsis, a professor of geology at the University of Colorado who was not involved with the U.C.L.A. research. “And that’s a comforting thought for the origin of life.”
With the old views of the Hadean period, the origin of life on Earth posed a huge problem. The earliest, and still debated, evidence for life lies within rocks in Greenland dated at 3.83 billion years. The rocks show a shift in the relative amounts of carbon-12, the usual form of carbon, and carbon-13, a less common but stable form of carbon. That shift was attributed to the presence of microorganisms, which would tend to concentrate the lighter carbon.
What was surprising, perhaps unbelievable, in the old views was that life started immediately at the end of the Late Heavy Bombardment, seemingly showing up the instant that it was possible.
In the new view of the early Earth, life could have emerged hundreds of millions of years earlier. “This means the door is open for a long, slow chemical evolution,” Dr. Mojzsis said. “The stage was set for life probably 4.4 billion years ago, but I don’t know if the actors were present.”
The revolution in early Earth studies comes largely from rocks in western Australia. The rocks are three billion years old, but they contain zircons that are older. Zircons, made primarily of the elements zirconium, oxygen and silicon, are extremely hard and durable and can survive conditions that erode, melt or otherwise transform the rock around them.
The zircons also contain enough uranium that they can be precisely dated by the decay of that uranium. In 2001, two groups, one led by Dr. Harrison and the other by John W. Valley of the University of Wisconsin, reported that the Australian zircons formed during the Hadean period as long ago as 4.4 billion years and were later embedded in the younger, 3-billion-year-old rocks.
The relative amounts of oxygen isotopes in the zircons points to the presence of water. Minerals like clays and carbonates that form in water prefer to incorporate oxygen-18 into their crystal structure, and the zircons contain relatively high levels of oxygen-18 compared to the more common oxygen-16.
In the U.C.L.A. study, the researchers studied tiny mineral grains trapped inside the zircons between 4 billion and 4.2 billion years ago as they were being formed. From the mix of elements they identified in the minerals, the scientists could calculate the depth and temperature at which the zircons crystallized — 1,300 degrees Fahrenheit at a depth of 15 miles — and the calculations showed a flow of heat coming out of that part of the Earth of 75 milliwatts per square meter.
That is too cool. The Earth during the Hadean period may not have been hellish, but it was hotter than today, and the heat flow should have been about three times the amount that was calculated.
That meant the zircons formed in a cool part of the crust. On Earth today, one such place is a subduction zone, where an ocean plate slides under a continental plate and is pushed into the mantle. The waterlogged ocean plate then melts at relatively low temperatures. The U.C.L.A. scientists believe that the high water content and the low temperatures inferred from the zircons thus point to the existence of such a subduction zone. And a subduction zone could not have existed unless some type of plate tectonics was already at work.
“It’s not a smoking gun,” Dr. Harrison said. “But we’re left without any other plausible explanation.”
Many geologists believe that the crust was too thin or the interior too hot for plate tectonics to occur back then. Neither Venus nor Mars shows obvious signs of plate tectonics, past or present, suggesting that only a limited range of planetary temperature and structure give rise to the phenomenon.
Dr. Sleep of Stanford said of the U.C.L.A. findings: “It may well be a subduction zone. It looks like a subduction zone.”
Dr. Valley has also concluded the Earth became cool and watery early in its history, but remains skeptical about the inferences about plate tectonics.
“To me, it’s not ruled out by anything,” he said, “but it’s far from proven with the certainty that Mark states it.” Dr. Valley said it was possible that some of the elements measured by the U.C.L.A. researchers might have infiltrated the zircons through tiny cracks.
If plate tectonics were overturning the Earth’s crust during the Hadean period, it would have shaped not just the land forms, but also the air and the climate.
In the 1980s, a climate model proposed a thick atmosphere of heat-trapping carbon dioxide, raising the average surface temperature to 185 degrees Fahrenheit, not quite boiling.
But if plate tectonics had already begun, much of the carbon dioxide would be trapped in carbonate rocks and then pushed into Earth’s interior. In 2001, a climate model by Dr. Sleep and Kevin Zahnle of the NASA Ames Research Center found that the late Hadean Earth then would have been somewhat chilly.
Neither near-boiling temperatures nor the chilly conditions make life impossible, but these factors could change ideas about how and when life started.
Earth, like the other planets, coalesced more than 4.5 billion years ago. It is commonly hypothesized that almost immediately, a Mars-size object about 4,000 miles wide hit it — a true cataclysm that vaporized much of the object and Earth. Some of the debris ejected into orbit became the Moon. The molten Earth cooled quickly, probably within a few million years, and nothing that large ever struck again.
Dr. Sleep said his calculations suggested that during the 700 million years of the Hadean period about 15 objects 100 miles wide or wider hit the Earth. About four of the objects were wider than 200 miles, and those collisions would have been violent enough to boil off most of the oceans. (By contrast, the more recent object that hit the Earth 65 million years ago and helped kill off the dinosaurs was about 6 miles wide.)
But in numerical simulations that will be presented this month at a meeting of the American Geophysical Union in San Francisco, Dr. Mojzsis and Oleg Abramov, a postdoctoral researcher at the University of Colorado, show that the Late Heavy Bombardment impacts were not quite as lethal as had been thought.
“Things are hurt really bad,” Dr. Mojzsis said. But the computer calculations indicated that even rocks up to 300 miles wide would not kill everything, that pockets would exist where organisms that thrive in high-temperature environments like hydrothermal vents could survive.
Genetic studies of current life support that notion, pointing to an organism that lived in a high-temperature environment as the last common ancestor. That does not mean that life started there, but that is almost certainly where survivors of the giant impacts would have huddled.
For the question of whether life existed during the Hadean period, researchers would like to find carbon and then perform an isotope analysis similar to what was done with the Greenland rocks. Despite analyzing 160,000 grain-size zircons, the U.C.L.A. researchers have not found carbon. (Another group has reported the presence of small diamonds, but that has not been confirmed.)
The search for more substantial amounts of Hadean rock also continues. Three months ago, researchers reported that a swath of bedrock in northern Quebec might be 4.28 billion years old, which would provide a mother lode of material to study. That bedrock includes intriguing structures known as banded iron formations, which are believed to occur only with the help of living organisms. But other scientists have questioned the age of the rocks, suggesting that they may really be 3.8 billion years old.
Dr. Mojzsis said “Hadean” might not be a misleading name for the earliest eon of Earth’s history, after all. The ancient Greek concept of hell was not one of fire and brimstone. “In Greek mythology, Hades was a dark, cold, mysterious place,” he said. “It seems to me the Hadean is living up to that moniker.”
Published: December 1, 2008
The first 700 million years of Earth’s 4.5-billion-year existence are known as the Hadean period, after Hades, or, to shed the ancient Greek name, Hell.
That name seemed to fit with the common perception that the young Earth was a hot, dry, desolate landscape interspersed with seas of magma and inhospitable for life. Even if some organism had somehow popped into existence, the old story went, surely it would soon have been extinguished in the firestorm of one of the giant meteorites that slammed into the Earth when the young solar system was still crowded with debris.
Scars on the surface of the Moon record a hail of impacts during what is called the Late Heavy Bombardment. The Earth would have received an even more intense bombardment, and the common thinking until recently was that life could not have emerged on Earth until the bombardment eased about 3.85 billion years ago.
Norman H. Sleep, a professor of geophysics at Stanford, recalled that in 1986 he submitted a paper that calculated the probability of life surviving one of the giant, early impacts. It was summarily rejected because a reviewer said that obviously nothing could have lived then.
That is no longer thought to be true.
“We thought we knew something we didn’t,” said T. Mark Harrison, a professor of geochemistry at the University of California, Los Angeles. In hindsight the evidence was just not there. And new evidence has suggested a new view of the early Earth.
Over the last decade, the mineralogical analysis of small hardy crystals known as zircons embedded in old Australian rocks has painted a picture of the Hadean period “completely inconsistent with this myth we made up,” Dr. Harrison said.
Geologists now almost universally agree that by 4.2 billion years ago, the Earth was a pretty placid place, with both land and oceans. Instead of hellishly hot, it may have frozen over. Because the young Sun put out 30 percent less energy than it does today, temperatures on Earth might have been cold enough for parts of the surface to have been covered by expanses of ice.
In a new analysis, published in the current issue of the journal Nature, the zircons, the only bits of earth older than 4 billion years definitively known to have survived, provide another tantalizing hint about the Hadean period. Dr. Harrison and two U.C.L.A. colleagues, Michelle Hopkins, a graduate student, and Craig Manning, a professor of geology and geochemistry, report that minerals trapped inside zircons offer evidence that the processes of plate tectonics — the forces that push around the planet’s outer crust, forming and shaping the continents and oceans — had already begun.
“The picture that’s emerging is a watery world with normal rock recycling processes,” said Stephen J. Mojzsis, a professor of geology at the University of Colorado who was not involved with the U.C.L.A. research. “And that’s a comforting thought for the origin of life.”
With the old views of the Hadean period, the origin of life on Earth posed a huge problem. The earliest, and still debated, evidence for life lies within rocks in Greenland dated at 3.83 billion years. The rocks show a shift in the relative amounts of carbon-12, the usual form of carbon, and carbon-13, a less common but stable form of carbon. That shift was attributed to the presence of microorganisms, which would tend to concentrate the lighter carbon.
What was surprising, perhaps unbelievable, in the old views was that life started immediately at the end of the Late Heavy Bombardment, seemingly showing up the instant that it was possible.
In the new view of the early Earth, life could have emerged hundreds of millions of years earlier. “This means the door is open for a long, slow chemical evolution,” Dr. Mojzsis said. “The stage was set for life probably 4.4 billion years ago, but I don’t know if the actors were present.”
The revolution in early Earth studies comes largely from rocks in western Australia. The rocks are three billion years old, but they contain zircons that are older. Zircons, made primarily of the elements zirconium, oxygen and silicon, are extremely hard and durable and can survive conditions that erode, melt or otherwise transform the rock around them.
The zircons also contain enough uranium that they can be precisely dated by the decay of that uranium. In 2001, two groups, one led by Dr. Harrison and the other by John W. Valley of the University of Wisconsin, reported that the Australian zircons formed during the Hadean period as long ago as 4.4 billion years and were later embedded in the younger, 3-billion-year-old rocks.
The relative amounts of oxygen isotopes in the zircons points to the presence of water. Minerals like clays and carbonates that form in water prefer to incorporate oxygen-18 into their crystal structure, and the zircons contain relatively high levels of oxygen-18 compared to the more common oxygen-16.
In the U.C.L.A. study, the researchers studied tiny mineral grains trapped inside the zircons between 4 billion and 4.2 billion years ago as they were being formed. From the mix of elements they identified in the minerals, the scientists could calculate the depth and temperature at which the zircons crystallized — 1,300 degrees Fahrenheit at a depth of 15 miles — and the calculations showed a flow of heat coming out of that part of the Earth of 75 milliwatts per square meter.
That is too cool. The Earth during the Hadean period may not have been hellish, but it was hotter than today, and the heat flow should have been about three times the amount that was calculated.
That meant the zircons formed in a cool part of the crust. On Earth today, one such place is a subduction zone, where an ocean plate slides under a continental plate and is pushed into the mantle. The waterlogged ocean plate then melts at relatively low temperatures. The U.C.L.A. scientists believe that the high water content and the low temperatures inferred from the zircons thus point to the existence of such a subduction zone. And a subduction zone could not have existed unless some type of plate tectonics was already at work.
“It’s not a smoking gun,” Dr. Harrison said. “But we’re left without any other plausible explanation.”
Many geologists believe that the crust was too thin or the interior too hot for plate tectonics to occur back then. Neither Venus nor Mars shows obvious signs of plate tectonics, past or present, suggesting that only a limited range of planetary temperature and structure give rise to the phenomenon.
Dr. Sleep of Stanford said of the U.C.L.A. findings: “It may well be a subduction zone. It looks like a subduction zone.”
Dr. Valley has also concluded the Earth became cool and watery early in its history, but remains skeptical about the inferences about plate tectonics.
“To me, it’s not ruled out by anything,” he said, “but it’s far from proven with the certainty that Mark states it.” Dr. Valley said it was possible that some of the elements measured by the U.C.L.A. researchers might have infiltrated the zircons through tiny cracks.
If plate tectonics were overturning the Earth’s crust during the Hadean period, it would have shaped not just the land forms, but also the air and the climate.
In the 1980s, a climate model proposed a thick atmosphere of heat-trapping carbon dioxide, raising the average surface temperature to 185 degrees Fahrenheit, not quite boiling.
But if plate tectonics had already begun, much of the carbon dioxide would be trapped in carbonate rocks and then pushed into Earth’s interior. In 2001, a climate model by Dr. Sleep and Kevin Zahnle of the NASA Ames Research Center found that the late Hadean Earth then would have been somewhat chilly.
Neither near-boiling temperatures nor the chilly conditions make life impossible, but these factors could change ideas about how and when life started.
Earth, like the other planets, coalesced more than 4.5 billion years ago. It is commonly hypothesized that almost immediately, a Mars-size object about 4,000 miles wide hit it — a true cataclysm that vaporized much of the object and Earth. Some of the debris ejected into orbit became the Moon. The molten Earth cooled quickly, probably within a few million years, and nothing that large ever struck again.
Dr. Sleep said his calculations suggested that during the 700 million years of the Hadean period about 15 objects 100 miles wide or wider hit the Earth. About four of the objects were wider than 200 miles, and those collisions would have been violent enough to boil off most of the oceans. (By contrast, the more recent object that hit the Earth 65 million years ago and helped kill off the dinosaurs was about 6 miles wide.)
But in numerical simulations that will be presented this month at a meeting of the American Geophysical Union in San Francisco, Dr. Mojzsis and Oleg Abramov, a postdoctoral researcher at the University of Colorado, show that the Late Heavy Bombardment impacts were not quite as lethal as had been thought.
“Things are hurt really bad,” Dr. Mojzsis said. But the computer calculations indicated that even rocks up to 300 miles wide would not kill everything, that pockets would exist where organisms that thrive in high-temperature environments like hydrothermal vents could survive.
Genetic studies of current life support that notion, pointing to an organism that lived in a high-temperature environment as the last common ancestor. That does not mean that life started there, but that is almost certainly where survivors of the giant impacts would have huddled.
For the question of whether life existed during the Hadean period, researchers would like to find carbon and then perform an isotope analysis similar to what was done with the Greenland rocks. Despite analyzing 160,000 grain-size zircons, the U.C.L.A. researchers have not found carbon. (Another group has reported the presence of small diamonds, but that has not been confirmed.)
The search for more substantial amounts of Hadean rock also continues. Three months ago, researchers reported that a swath of bedrock in northern Quebec might be 4.28 billion years old, which would provide a mother lode of material to study. That bedrock includes intriguing structures known as banded iron formations, which are believed to occur only with the help of living organisms. But other scientists have questioned the age of the rocks, suggesting that they may really be 3.8 billion years old.
Dr. Mojzsis said “Hadean” might not be a misleading name for the earliest eon of Earth’s history, after all. The ancient Greek concept of hell was not one of fire and brimstone. “In Greek mythology, Hades was a dark, cold, mysterious place,” he said. “It seems to me the Hadean is living up to that moniker.”
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