Modern Cosmology: John and Mary Gribbin -- Four Books -- Review

Galaxies:  A Very Short Introduction

By John Gribbin.  Oxford, New York:  Oxford University Press.  2008.  121 pp.

Stardust:  Supernovae and Life -- the Cosmic Connection

By John and Mary Gribbin.  New Haven and London:  Yale University Press.  2000.  238 pp. 

The Birth of Time:  How Astronomers Measured the Age of the Universe

By John Gribbin.  New Haven and London:  Yale University Press.  1999.  237 pp.

Fire on Earth:  Doomsday, Dinosaurs, and Humankind.  How Asteroid and Comet Collisions Have Shaped Human History -- And What Dangers Lie Ahead.  

By John and Mary Gribbin.  New York:  St. Martin's Press.  1996.  264 pp.

I have been interested in the origin of the universe since I was about twelve years old, when I read Martin Gardner's little book Relativity for the Million.  It presented some thought experiments to introduce the ideas of Einstein's Special Theory of Relativity and its relation to cosmology.  I began a discussion of these ideas with the pastor of my local church, whom I looked up to, admired, and liked.  It was the beginning of the end of my religious faith.  I learned the difference between the scientific mindset, which values questions, curiosity, imagination, observation, and experimentation, versus the religious mentality, which values tradition and simplicity, frowns on curiosity, questions, and innovation, and further attempts to crush them through the invocation of authority and belligerence.  I saw early on with my own eyes that science is unconditionally superior.  While science has many limitations and cannot solve every human and social problem -- in fact, many problems it cannot even address -- religion solves nothing, and actually impedes solutions and perpetuates misunderstandings and falsehoods that prevent people from gaining insight and taking the steps necessary to solve their problems.  The field of cosmology is the clearest illustration of this, but it was not possible to see this with such transparency before the middle of the twentieth century.  John Gribbin documents the development of the modern understanding of the universe through experiments, advances in theory, technological innovations, and the personal struggles of individual scientists.  It is a fascinating human story of progress and advance, illustrating through many examples how science is a very human enterprise that moves forward by increments and at times by surprising leaps, rife with uncertainties, ambiguities and disagreements at every stage.  It is totally different from the religious approach that starts from divine revelation through individual prophets, who are taken ever after as authoritative and unquestionable. 

We are fortunate to be the first people in history to know in the broad outline where it all came from, how it all got here, and where it is all going.  No other people that have ever lived have had as clear and complete a picture of those basic questions as we do.  Our answers are not complete and much is still very controversial and ambiguous, but over the last one hundred or so years, the broad outline has become sharper and the big picture is becoming increasingly well defined and detailed. 

John Gribbin with his wife, Mary, have written many books on scientific topics for general readers.  These are four of their better ones that together tell the story of modern cosmology and its relationship to life on earth.  These four very readable, interesting texts give the broad overview of the current scientific picture of the origin of the world in nontechnical, well-written prose. 

Galaxies is a concise summary of the modern understanding of the cosmos.  It is told from a historical perspective that describes the development of the ideas, the problems confronting science, the conflicting views and interpretations of data, and how advances in technology and crucial experimental observations have gradually resolved many of these issues and established the framework of our current understanding of the universe.  Scientific theories are not just fantasies, imaginative fairy tales.  They start as real dilemmas which are then gradually surmounted by thought, observation, experimentation, and logical inference.  Gribbin is very good at presenting the scientific dilemmas and then describing the observational data from experiments, observations from telescopes, and mathematical calculations that yield decisive solutions to these problems, which in turn build a platform from which further refinements can be made.  Looking at it historically enables one to see how our understanding of the development of the universe is clearly progressive.  For example, we know now that our Milky Way galaxy is not the limit of the universe.  There are billions of other galaxies outside the Milky Way extending to unimaginable distances.  Methods have been developed that allow us to approximate the vast distances across the universe, which are described in fascinating historical detail in The Birth of Time, but here, too, in Galaxies, the development of some of those methods is described.  We know that the Universe is approximately 14 billion years old.  It is not eternal.  There was a beginning and the universe has grown and evolved over the span of its life.  Edwin Hubble's observations at the Mount Wilson Observatory in the 1920s led to the stunning conclusion that the universe is expanding.  This does not mean that things like galaxies are simply moving farther and farther apart, but that space itself is expanding like a piece of elastic, powered by dark energy.  Gribbin provides an excellent explanation of what physicists call "dark matter," how its existence was inferred from the study of galaxies, and the importance it has in our understanding of the dynamics of the universe.  We now know that the universe has a shape:  that it is flat, and that the galaxies in the universe are distributed in discernible patterns.  These patterns are related to the distribution of "dark matter" -- something no one has seen and no one has described, but whose existence can be inferred from observations of what can be seen and measured.  Dark matter is one of the continuing mysteries of the scientific understanding of the universe.  The fate of the galaxies and of the universe itself is one of the unresolved problems confronting science today.  But we know there are several possibilities and those possibilities depend on the cosmological constant, which is a number used to balance the equations in Einstein's theory of General Relativity, according to whether the Universe is static, expanding, shrinking, slowing down, or speeding up.  Currently the cosmological constant is known to imply that the universe is expanding, and not only is it expanding, but it is speeding up in its expansion.   It is not known if this will continue indefinitely, that is, whether the cosmological constant is really constant, or if it is subject to change.  If the cosmological "constant" can change over time, then the universe's expansion may at some time long in the future, begin to slow down, or even collapse on itself.  The expansion of the Universe creates "dark energy," which counters the force of gravity that tends to pull the universe together.  Dark energy is what is making the Universe expand.  If enough dark energy is being created by the expansion, then the Universe will go on expanding and may even accelerate.  But if the expansion is slow enough and the amount of dark energy is below a certain threshold, then the force of gravity within the Universe will one day slow it down and it will begin to collapse.  This is one of the major unknowns in the cosmological story of today.  John Gribbin's little book is a very concise and clear summary of these issues.  It is a credit to science and its approach to understanding the physical world in terms of impersonal laws that can be discerned and described in ever expanding generality that has given us a deeper, more accurate understanding of the Universe and our place in it than any people at any time have ever had. 

Stardust is the story of how the elements that make up our bodies and everything else that exists on earth were created in stars.  We are literally stardust.  All of the chemical elements that make up our bodies and everything around us in the earth, with the exception of hydrogen and some helium, were created in the hot interiors of stars.  Stardust tells the story of how the elements are created in stars and dispersed into space by supernova explosions.  The clouds of gas left in the wake of a supernova eventually begin to coagulate under gravitation and form new stars and planets.  Our sun and solar system are offspring of such an event. 

There are reasons why rare earths such as gold and platinum are rare and why the abundance of the various elements is distributed in the way that it is.  It has to do with the workings of the nuclear furnaces within stars which create the lighter elements such as helium, carbon, oxygen, and nitrogen, in greater abundance.  These also happen to be the elements that form the molecules of living organisms.  Space is full of organic molecules such as ammonia, formaldehyde, ethyl alcohol, formic acid, amino acids, and even purines and pyrimidines, which are the chemical building blocks of DNA.  This knowledge did not begin to accumulate until the 1960s and these latter discoveries did not come until the 1990s.  So the understanding that space, and in particular stars, is the place of origin for the chemical ingredients of life is a very recent development. 

These discoveries came about mainly through spectroscopy, which is the analysis of light emitted by a star (or anything) in order to determine its chemical composition.  Gribbin gives an excellent explanation of spectroscopy.  He relates the patterns of spectral lines to the quantum theory of the atom in very clear, simple, straightforward language.  In addition to spectroscopy, the discovery of radioactivity played an important role in dating, especially rocks and things that have lived on earth in the past.  Gribbin explains all of these different measuring sticks and the context of their historical development and how they were used to measure the distances to stars, and how the gradual accumulation of these measurements and improvements in observational technique have led only over the last hundred or so years to the realization of how vast the universe actually is.  It has taken a very long time to develop the ideas, technology, observations, and leaps of imagination that are the ingredients of the modern conception of the universe.  Seen historically, as Gribbin presents it, one grasps the logic and the plausibility of the differing viewpoints based on the observations and understandings of the time.  The importance of the personalities of scientists engaged in the research also becomes increasingly clear.  Science becomes a very human enterprise in Gribbin's hands that is inherently intriguing.  It is not necessary to jazz it up or make it sexy.  The problems and questions that are the quests of scientists are inherently captivating. 

The so-called Big Bang theory of the origin of the universe was not consolidated and accepted by a broad spectrum of scientists until the 1960s with the discovery of the "background radiation," left over from the Big Bang.  It had been predicted by George Gamov in the 1940s, and its discovery was a major validation of the Big Bang conception of the beginning of the universe.  But it wasn't until calculations were carried out that described how the fundamental elements that make up the universe, hydrogen and helium, were synthesized in the Big Bang, that the theory became firmly established as the dominant model of the creation of the universe.  The calculations were carried out, interestingly, by a team that included the astronomer Fred Hoyle, who had been a prominent adversary of the Big Bang theory.  These calculations of the synthesis of hydrogen and helium in the Big Bang correspond to observations that measure the proportion of hydrogen and helium in the oldest stars giving further substantiation to the theory.  It took a long time for astronomers to realize that the stars are made mostly of hydrogen and helium.  Gribbin points out how stubborn the conviction was that the stars contained a large portion of heavy elements much like the earth.  As early as the mid 1920s it had been shown by Cecelia Payne and William McCrea that stars must be made up largely of hydrogen.  However, this discovery was resisted and did not become fully accepted until the 1950s, simply because many people could not imagine how it could possibly be true.  It is one of many cases where prejudice influences the acceptance or rejection of a scientific theory in the face of calculations and observations that tend to support it.  Scientific ideas start out as guesses, leaps of imagination that attempt to make connections between things that seem inexplicable and things which are understood.  Observations and measurements help to decide between competing speculations.  Much of scientific progress depends on the development of instruments and technologies that allow one to make more precise measurements and observations.  But the human element also plays a decisive role in shaping science and in enabling and impeding its progress.  Gribbin's style of presenting the technological and theoretical context of a pending scientific problem gives his books an extremely readable quality that draws the reader into an unraveling mystery.  Much effort  and expenditure is being spent these days trying to interest American young people in science and technology.  All manner of techniques are being applied to make science entertaining or "fun."  It is unnecessary and misguided.  Scientific questions are inherently interesting if presented simply and straightforwardly as intriguing problems.  Why does the sun shine?  Why does it keep shining and not burn out?  What is it made out of?  How far away is it?  How far away are the stars?  These questions could not be answered until well into the twentieth century.  The answers depend on theoretical developments in science as well as developments in technology that enable the relevant observations, as well as unexpected accidents, and the personalities of individual scientists.  If you teach science this way it becomes interesting by default.  Children's curiosity will naturally gravitate toward it. 

The discovery of radiation in the 1890s lent to physics a new source of energy, which heretofore had been unknown.  It was important for two reasons: on the one hand it was a new energy source that helped explain how the sun could keep shining as long as it does, and because of the nature of radioactive elements to decay in "half-lives," for the first time it became possible to get accurate dates for the ages of rocks.  This was necessary to explain a growing realization that the sun and the earth must be much older than previously thought, and if so, how is it that the sun continues to shine for such a long time?  Einstein's famous equation from Special Relativity that E=mc2 provided the key to understanding the enormous amounts of energy that are stored up in matter and the potential for their being released in nuclear reactions.  Gribbin masterfully exposits these developments and explains their interrelation.  One realizes from his account what an exciting time the turn of the twentieth century must have been to be a scientist. A revolution was taking place in the way people conceived of time and of the place of the earth in the universe.  Exciting discoveries and theoretical innovations were occurring in rapid succession.  It was the epicenter of an earthquake in the way people understood the world in its very foundations.  The shock waves from this revolution continue to resound through modern intellectual life.  These developments in scientific ideas in the last years of the nineteenth century and the early years of the twentieth century heralded a transformation in the way human beings viewed the world and their place in it that superceded any innovations in human thought  that have occurred in human history prior.  What has happened in science over the last one hundred and twenty or so years have set people living today apart from all of humanity that has lived before us, in that we are able to see our place in the scheme of things with greater clarity and greater depth than any people who have come before us.  We know how we got here and we know the destiny of the earth, the sun, and the universe with much greater accuracy and completeness than has ever been imagined by anyone who lived before us.  It is a towering achievement of modern science and John Gribbin gives a lucid and inspiring account of its construction. 

The heart of Stardust is the long chapter on supernovae.  This chapter is detailed and challenging.  There are two main types of supernovae and the processes by which they transform and disintegrate are complicated.  But it is through the process of catabolism of supernovae that the heavy elements on the periodic chart are synthesized.  It is only in supernovae that these heavier elements (heavier than Iron 56) are created because smaller stars do not have enough mass to attain the temperatures and the complicated nuclear processes necessary to fuse these heavy elements and send them sprawling into space.  Stars are nuclear furnaces that transform elemental hydrogen into helium and heavier elements.  A star the size of the sun, for example, will create lighter elements like carbon, oxygen, and some nitrogen in abundance, but nothing heavier.  Only stars that are at least four times as massive as the sun can create heavier elements (up to iron 56) and for the heaviest elements a star has to start out with at least eight to ten times the mass of our sun.  These are the stars that become supernovae, and their lifespan is relatively short compared to the sun.  Hydrogen and helium make up 99 percent of the atomic mass in the universe.  The elements from lithium to iron together make up less than one percent of the atomic mass of the universe, and the heaviest elements (everything heavier than iron) make up one thousandth of one percent of the atomic mass of the universe.  If nickel 28 is excluded from this heaviest group, they would only contribute one ten thousandth of one percent of the total atomic mass of the universe.  The heavy elements truly are rare earths.  This distribution of the abundance of the various elements of matter is predicted by theory and confirmed by observation.  It would not be possible to understand any of this without relativity theory and the quantum theory of the atom, which were developed in the first three decades of the twentieth century.  However the synthesis of the elements in stars was not clarified until late in the twentieth century.  In 1987 a supernova was observed in the Large Magellanic Cloud, which is a companion galaxy to our Milky Way.  The observations and measurements made from this supernova event yielded dramatic confirmation of theoretical predictions made about the synthesis of heavy elements in supernovae and left no doubt that the elements are manufactured in stars and spread through the galaxy through supernova explosions.

The elements that are created in the nuclear furnaces of supernovas are then blasted into space in the death agonies of these huge stars and quickly begin combining to form larger, more complex compounds, including organic molecules like ammonia, formaldehyde, methanimine, and glycine, an amino acid that is a building block of proteins.  Organic molecules of increasing complexity continue to be found in interstellar space.1  Gribbin regards it as "one of the most profound discoveries ever made" that the Milky Way Galaxy is "packed with the raw materials for life, and these raw materials are the inevitable product of the process of star birth and star death." (p. 214)  But these raw materials do not inevitably lead to complex life all around the universe like mushrooms springing up in a field.  The conditions under which complex life can evolve are rather rare. 

 His recounting of the process of the formation of our sun, the earth, and the planets of our solar system from a vast cloud of galactic gas and dust, the remnants of primordial supernova explosions is a very readable, logical presentation.  He explains how comets rained down upon the earth for the first 500-600 million years of its existence and brought the water that created the oceans as well as the organic molecules that eventually became life.  A chance event was crucial for preparing the earth to be the womb of life roughly ten million years into its life.  A large object somewhat bigger than the planet Mars struck the earth at a glancing blow.   The impact remelted the surface of the earth and blasted enormous amounts of silicate material into space.  It allowed the iron in the earth's crust to sink to the center of the earth.  The light silicate matter coalesced to form the moon and the earth's relatively thin crust, which is prone to cracking, with the result that the surface of the earth is governed by plate tectonics.  This also gave the earth the tilt of its axis, which gives us our seasons and it spun the earth into its 24 hour cycle that gives us our day.  All of these developments were crucial for the development of life, but none of them were in any way necessary.  We are indeed a lucky accident, and probably a rare one in the vast reaches of the universe. 

The final appendix in this book you can skip.  This recounts some rather fantastical speculations based on some consequences of the equations of relativity theory in black holes.  I regard this discussion and these wild fantasies of "nesting universes" connected by wormholes through black holes as reflecting the limitations of relativity theory and the domain of current science, rather than a description of anything that might be "real."  The heart of this book is the description of how the elements are created in stars and the relationship between this process of nuclear creation and the synthesis of organic molecules in interstellar space that are the germs of life. 

The Birth of Time is the story of how the age of the universe was calculated. This was a research project that Gribbin himself participated in, and which was very much in contention until the end of the 20th century.  But it is also the story of how our modern conception of time was built on the scientific developments in theory and observation that occurred in the 19th and 20th centuries.  He begins with theological attempts to date the earth from biblical genealogies in the seventeenth century, and how observations of fossils in rocks on the tops of mountains led to questions being raised against these results.  A rift began to develop between the scientific account of the origin of the earth and the religious one that sharpened and deepened in the coming centuries. This was the one played out between me and my pastor when I was barely a teenager in the 1960s and underlines how conservative and backward looking religious thinking is.  In the nineteenth century scientists began to realize that the earth had to be much older than they had previously imagined, but they had no clue how much older.  Gribbin describes the slow process of the human imagination's awakening to the vastness of the universe and the unimaginable extent of time.  It has taken human beings a long time to grasp how big the universe is and how long it has existed prior to our appearance. 

As late as the 1910s, only a century ago, astronomers still believed that the Milky Way encompassed the entire universe. Gribbin explains in very clear language the historical development of techniques that enabled people to measure the distances to astronomical objects such as stars, nebulae, and galaxies.   It was not a smooth process.  Mistakes were often made in interpreting observations that led to errors in calculating the distances to objects.  The errors were usually overcome through technological developments that allowed for more accurate observation.  It is an uneven process and it illustrates how science progresses in fits and starts against the psychological resistances that often impede that progress. 

He describes one case in considerable detail that captures the challenges scientific problems present on many levels.  In August of 1885 a bright star appeared in the Andromeda nebula.  It was observed and photographed by numerous astronomers around the world.  It faded gradually over the next several months disappearing from sight in February of 1886.3  We now know that this star was a supernova, a huge exploding star of unfathomable brightness.  But at the time it was debated whether "nebulae" such as Andromeda were really part of the Milky Way galaxy or objects outside of it.  If this nebula were outside of the Milky Way galaxy, as far away as some astronomers were suggesting, then such a star would have a brightness beyond the comprehension and experience of any astronomical observers to that time.  It would be as bright as a billion stars like the sun put together.  We now know that supernovae can burn as brightly as one hundred billion suns put together, but such a possibility was beyond credulity in the late nineteenth century.  Inexperience, limited observation, poverty of imagination, and wishful thinking are some of the human barriers that science must overcome.  But overcome them it does as Gribbin illustrates in case after case and this is how human understanding of the world and our place in it is pushed forward. 

The very success that science has made over the last couple of centuries may be reaching a barrier point due to the sheer size and expense of the equipment now needed to push back the limits of observation.   Steven Weinberg, in an excellent article in the New York Review of Books, May 12, 2012, discusses what he calls a "crisis" in basic research due to cuts in funding for large particle accelerators such as the Superconducting Super Collider and telescopes such as the James Webb Space Telescope.  He included a picture of Ernest Rutherford holding the apparatus he used to disintegrate a nitrogen nucleus in 1917.  Many astronomers built their own telescopes up until about the end of the nineteenth century.  The equipment needed today to make the observations needed to further scientific progress costs many billions of dollars and is beyond the resources of even wealthy individuals.  Scientific progress no longer depends simply on the curiosity and dedication of individual scientists, but rather on public policies and political will to commit substantial public resources to the effort.  In the United States, especially, such willingness is not easily forthcoming.2  

The final third of the book details the struggle to nail down the calculation of the age of the Universe.  There was a contradiction between the age of the universe calculated by Edwin Hubble in 1929, from his measurements of the red-shifts in the light from distant galaxies and the age of the earth calculated by geologists from measurements of radioactive decay in rocks.  Hubble pegged the Universe at about 2 billion years old, but geologists knew that the earth itself had to be approximately 4 billion years old.  This anomaly simmered until 1952 and Gribbin tells the story of how it was unraveled.  It was a rather complex chain or reasoning and research.  It depended upon the construction of the 200 inch reflecting telescope at Mount Palomar and the acute observational skill of German born astronomer, Walter Baader, who came to realize that Hubble's calculations were based on a mistake that was due to the limitations of the telescopes of his time.  Baader, using the 200 inch reflector at Mount Palomar was able to discern two types of Cepheid variable stars, one type much older than the other.  Cepheids are stars which have masses between 5 and 20 times that of our sun.  The stars are unstable because they have used up most of their hydrogen fuel, so the stars expand and contract in a regular cycle, which makes the star appear brighter and dimmer viewed from earth.  The larger the star, the longer the period of its cycle.  The apparent brightness of a star seen from earth also depends on its distance from the earth.  If one has two Cepheids that have the same period but are at different distances from the earth, their relative brightness to an observer on earth will vary in a way that can be mathematically calculated.  In other words, if you have two Cepheids with the same period, and you know the distance to one of them, you can calculate the distance to the other one.  This provided one of the first measuring sticks for distances to remote objects in the universe, and it was developed in the early decades of the twentieth century.  There were a number of problems with this method of calculating cosmic distances that gave misleading results.  Over time they were resolved and Gribbin provides a detailed discussion of Cepheids and their use as a measuring stick for the cosmos.  The periods of variability in the light from these stars were instrumental in Hubble's construction of the cosmic yardstick that he used to measure distances and time across the Universe.  But in failing to make this distinction between the two types of variable stars, simply due to limitations in his equipment, Hubble ended up with a much younger estimate for the age for the Universe than is accurate.  Baader was able to refine Hubble's estimate with the 200 inch reflector at Mount Palomar by discerning different classes of Cepheid variable stars.  The refinement process for this age estimate continued through the twentieth century and Gribbin details the steps of this process. 

The last two chapters of the book are rather challenging.  They may daunt the casual reader, but the serious reader will be delighted with the detail and intensity of the discussion.  Gribbin himself was involved in the research that finally converged on an estimate of about thirteen and a half billion years (plus or minus a billion or so) for the age of the Universe.  It depended on refinements to several different measuring techniques.  The central quest to determine the Hubble Constant, which defines the rate at which space, and thus the Universe, is expanding, was difficult and contentious.  Different methods and different researchers were arriving at considerably different values for this number.  If the number is too high, the Universe ends up being younger than some of its oldest stars -- which are dated by another method. 

One other important discovery that should be mentioned is the cosmic background radiation referred to earlier.  This is radiation left over from the original Big Bang with which the Universe began.  The existence of this radiation was predicted by George Gamov in the 1940s and its discovery in the 1960s was the decisive factor that persuaded many scientists that there really was a Big Bang and that the Universe really had a beginning.  It illustrates how scientific theories progress by making predictions that can then be verified or refuted by observation.  Sometimes the observations are years or decades in coming, sometimes they require the development of sophisticated measuring instruments, but in principle this is the way science moves forward.  It is a highly successful approach to understanding the world. 

Fire on Earth brings us much closer to home, but continues to affirm that the Earth is very much an astronomical body and the fate of the earth and the life forms upon it very much depend on astronomical events. It is a very readable, fascinating story beginning with the Chicxulub comet impact of 65 million years ago and how it brought about the extinction of the dinosaurs which opened the way for mammals -- and us -- to become the predominant life form on earth.  But the Chicxulub impact is only the starting point, the paradigmatic case, one might say, for a much broader discussion of the role comets have played in the development of life and its upheavals in the course of the life of the earth.  The Tunguska event (1908), which is much less known, is a much more immediate representative of the effect comets can have on our lives and the fate of life on our planet.  On the morning of June 30, 1908, a comet less than the diameter of a football field entered the earth's atmosphere over Siberia north of Lake Baikal.  It exploded about three and a half miles above the earth in a blast that is estimated at least 1000 times as powerful as the Hiroshima bomb.  It devastated virtually uninhabited forest land over 1200 square miles and could have easily obliterated a large city.  Another impact in the same remote region of Siberia occurred in 1947.  It is known as the Sikhote-Alin meteorite.  It was considerably smaller than the Tunguska meteor but it still created an explosion greater than that of Hiroshima.  In April of 1972 the earth had a near miss with an object much bigger than the Tunguska meteor.  It streaked through the earth's atmosphere over the Rocky Mountains creating a spectacular fireball and leaving a rain of small meteors in its wake, but it went on through the earth's atmosphere and sped away into space without striking the earth.  Three significant encounters with cosmic objects within a century Gribbin sees as cause for concern and it gives credence to biblical stories of Sodom and Gomorrah being destroyed by fire raining down from heaven.  Such a possibility is very real and such ancient stories of destruction by fire from heaven and the ancient view of comets' appearances as harbingers of doom and destruction are credible. 

He goes on through succeeding chapters to explain the history of comet impacts on the life of the earth, starting with the moon.  Through telescopes it can be seen that the moon is scarred by thousands of impact craters.  The moon itself is thought to have been formed in the aftermath of a cataclysmic collision between the earth and a very large object, perhaps as large as the planet Mars, which gave the earth the tilt of its axis, which gives us our seasons, and set the earth spinning on its axis, which results in our 24 hour day.  The water that comprises our oceans is thought to have been brought to earth by way of comets impacting upon the earth.  Comets are balls of rock and ice.  They differ from asteroids in that asteroids are merely rocky.  Most, if not all, of the water has evaporated from an asteroid.  Gribbin describes the growth in scientific knowledge of asteroids and comets, including the work of Edmund Halley, for whom the famous comet is named, the Kuiper Belt, which is a region beyond the orbit of Neptune containing billions of comets, and the discovery of the Opik-Oort Cloud, which is a region far beyond our solar system containing trillions of comets, very few of which ever enter our neighborhood of the solar system.  However, some among them, which Gribbin calls "supercomets," are exceptionally large -- as much as 200 miles across.  Were one of these to fall into the inner solar system, the havoc it could wreak would be beyond imagining.  The meteor showers which are visible from earth at certain times of the year are actually debris from comets in orbits around the sun which in turn cross the earth's orbit and result in our meteor showers.  This discovery was made in the nineteenth century and developed further in the twentieth. 

One of the recurring themes throughout the book is the possibility of disaster to the earth and its lifeforms due to the impacts of comets and their effects on climate, the atmosphere, sea levels, and the temperature of the planet.  There have been five massive extinctions of life on Earth since the first fish evolved.  They occurred at 438 million years ago, 360 million years ago, 250 million years ago, 215 million years ago, and 65 million years ago -- this most recent one being the end of the era of the dinosaurs.  There have also been many less massive, but still significant waves of extinctions in the intervening years.  Gribbin discusses the possibility that these extinctions could have been the result of comet impacts upon the earth and even the possibility that such impacts occur in cycles due to the orbits of cosmic objects and their relation to the sun. 

The last chapter is probably the weakest and deals with what we might do to ward off the possibility of being extinguished by a comet.  The research that is being done to detect comets and asteroids that might impact the earth is interesting, and the realization of the possible effects such impacts might have is sobering, but the cockamamie schemes to defend the earth against such impacts strike me as little more than special effects fantasies that are better suited to movies.  However, this book is a captivating, thought provoking read that will give you a new perspective on the fate of life on the earth. 

These four books together provide a broad, accessible overview of the development of cosmological thinking over the last hundred and fifty years.  They are well-written, detailed, historical, and offer a considerable depth without being overly technical.  I gave all four of them to my nephew, who is thirteen. 

 1.  Matson, John (2009)  Complex organic molecules detected in interstellar space.  Scientific American.  April 22, 2009

2.  Weinberg, Steven (2012)  The Crisis in Big Science.  The New York Review of Books, Vol. 59, No. 8.  May 10, 2012.  pp. 59-62.

3.   Beesley, D. E. (1985)  Isaac Ward and S. Andromedae.  Irish Astronomical Journal, Vol. 17, No. 2,  pp. 98-102.