Cosmology and the Allegory of the Cave

Chapter 5F

Cosmology and the Allegory of the Cave

As I mentioned in the previous posting, the more details we know about the universe the more hints we benefit from in trying to devise new theories to explain its origin and its evolution. And of course we have seen already that space physics and space astronomy are essential if we are to attain a worthwhile understanding of the nature of the universe (cosmology), for much of the critical information is transmitted in wavelengths that our atmosphere keeps from Earth-bound instruments. And to those two space sciences we should add astro-chemistry, for this other science is necessary if we are to understand the composition of the galactic medium, for example.

Furthermore, it is not plausible to say that we have a complete idea of the nature of the universe unless we have a far better understanding of phenomena such as quasars and black holes which obviously may have great consequences for our theories of gravitation, for what decent idea of the universe would ignore the gravity that holds it together? Nor is it plausible to hold onto certain ideas of the origins of the universe without the guidance of reliable theories of gravitation. As we will see below, space science is essential not only for the study of violent events like quasars and black holes but for the more general task of transforming our theories of gravitation. Thus, since unification schemes must deal with questions of the origin and evolution of the universe, space astronomy is clearly fundamental science by even the narrowest and strictest of criteria. Much rides on the telescopes that our rockets may take into the heavens.

To summarize this point, since cosmology is essential to the completion of the program of unification in physics, it is also essential to the pursuit of fundamental physics, even by notions of pre-Dark-Matter-Dark-Energy days. Thus even if we accept the most extreme view, that particle physics is the most fundamental science, it seems that space science is unavoidable, and therefore just as fundamental. Our investigation of the microcosm eventually takes us to the stars.

Of course, there is the possibility that all present unification schemes are entirely misguided. But if that is so, space astronomy will be particularly helpful in pointing to areas of physics where new directions would be fruitful. Without that help our models of the evolution of the universe will remain far too speculative to determine the strengths and weaknesses of any potential unification of the basic forces of physics. This point supports the claim made in Chapter 3 to the effect that space science helps transform theoretical science. Science needs to rub against the world, for such friction polishes and sharpens the rough guesses that humans make about their universe.[1] And as we can see now, at the edge of the universe we find the end of a journey through space science that begins here and brings us back.

But what if we give up on the unification of the basic forces of physics? Even so space science would be as fundamental as Earth-bound physical science. One reason is the opportunity to develop several aspects of the two main physical theories of this century: quantum physics and general relativity. I will leave general relativity for a section of its own later in the chapter. As for quantum physics, it explains micro-phenomena and is thus bound to come to terms with the first few moments of the origin of the universe, when the universe was so small that quantum events would have profound effects upon its subsequent evolution. Even without the goal of unification, different ideas on the nature of fundamental particles, their creation and their ways of interaction, could be refined so as to make distinct predictions about how the universe should have turned out. In general, to the extent that microphysics decides what processes underlie the macro-phenomena of the large universe, then the study of those phenomena ought to serve as an independent testing ground for our theories of the small.

Another reason is that the possibility of studying black holes and other strange objects presents extraordinary opportunities to challenge all of physics. It has been said that in black holes all of physics comes to an end. The reason is that in a runaway gravitational collapse, which is presumably what exists in a black hole, matter and energy disappear into a single geometrical point at the center of the black hole. This point, called a "singularity," obviously contains no space. And it contain no time either since, as we will see below, time slows down in the presence of a strong gravitational field. Where the field is practically infinite, time simply does not "happen." But the laws of physics make no sense outside of time and space. Thus we seem to have a situation in which matter-energy is no longer subject to the laws of physics as we can presently conceive of them. These are strange possibilities indeed, and there is nothing like the serious consideration of strange possibilities to loosen the grip of entrenched ideas.

Moreover, another serious complication arises. As the matter and energy in the black hole collapse towards the singularity, a moment comes when they are compressed into a volume so small that quantum effects begin to dominate. This would mean, for example, that the account of the previous paragraph could not be right, since the Heisenberg uncertainty relation between position and momentum would rule out any deterministic prediction about the behavior of matter in such a small volume. Indeed, it could be thought that a similar problem may show up at the beginning of the universe. The result is that we seem to have a conflict between the two main theories of physics: general relativity and quantum physics. A future compromise, quantum gravity, has been the goal of many theoretical physicists, particularly string theories, but without any success so far.

To pass up the opportunity to enrich our cosmology so immensely would be far more than folly for the scientist who wishes to understand the universe. In the Republic, Plato describes a group of men who are chained facing the back wall of a cave. By the entrance to the cave there is a road, and beyond the road a fire that projects on the wall the shadows of the objects that pass in front of the cave. The men spend their time trying to determine what those objects are from the shapes they see before them. One day a man is set free and turned around. In reaching the outside world he is at first taken with fright, but soon he adapts to the sunlight and marvels not only at the objects whose shadows he had seen before, but at the many that had not even crossed his imagination, let alone his line of sight. And the question is, would this man go back voluntarily to his chains in the cave? Would he be satisfied with the guessing games based on what he now knows are mere shadows?

In a certain sense the atmosphere and the gravity well of our planet have been our cave and our chains. One of the great space pioneers, the Russian rocket theorist Tsiolkovsky spoke of the chains of gravity and spent his life trying to break them. He and the other space pioneers have made it possible for us to come out and see the universe as if for the first time. Perhaps even then the universe will remain a complete mystery to us. But can we as cosmologists afford to reject the chance they offer to us and take back our place in front of the cave wall?

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[1] This is a highly objectionable aspect of String Theory: that it makes no contact with the universe we actually observe. Perhaps it will eventually, though.

COSMOLOGY AND THE PROGRESS OF PHYSICS

Space 5E

COSMOLOGY AND THE PROGRESS OF PHYSICS

As we may recall, one objection against my thesis was that we apply truly fundamental physics to settle issues in space astronomy and physics, not the other way around. This objection has two corollaries. First, our truly important scientific views are not likely to be affected in significant ways by what we do in space. Second, as a consequence of space science, some of our views of the universe may well change, but they are of such remote or esoteric phenomena that the opportunity for practical consequences, even in the long run is slim at best.

Let us take up first the matter of significance. There is a pecking order in the natural sciences, with physics clearly at the top. This is so because of three reasons. One is historical: physics led the way in the scientific revolution and presumably set the standards for subsequent science. A second reason is that physics deals with processes that are fundamental to the natural world; it deals with what all objects have in common. It is not surprising, then, that important changes in physics tend to be felt in many scientific places. The third reason is that the mathematical and experimental rigor of physics, coupled with extraordinary feats of the imagination, maintains the prestige of physics very high.

Even if such high prestige is well deserved, there is an unfortunate tendency to think that other sciences have much to learn from physics but little to teach it. And so we not only give priority to physics but even within physics we may downgrade what the received wisdom does not consider fundamental. For several decades, the most pressing problems were thought to reside in the very small, for the simple reason that the smallest components of matter are presumed to be the building blocks of the universe. These problems are the province of particle physics, and for some years the leading view to explain the nature of particles has been the Standard Model. This model contains 12 basic particles, fermions, which come in two classes: quarks and leptons. The theory of quarks goes by the strange name of “chromodynamics.” The name is strange because it appears to refer to the dynamics of color, but the “color” in question is a property of quarks that cannot be seen (charm is another whimsical property of quarks, and the name "quark" itself is a nonsense word from a poem by James Joyce). The word “lepton” means light, as the electron, the most famous lepton, is supposed to be. But now there are “heavy” leptons. To these whimsically named particles, proponents of the Standard Model have added the hypothetical Higgs particle, presumed to give mass to the other particles. Quarks are the building blocks of hadrons, e.g. protons and neutrons, which together with electrons form atoms. The model also accounts for three of the basic forces: electromagnetic, and the weak and strong nuclear forces. To do this it posits the existence of four force-carrying particles named bosons, which include the photon and the gluon, which “glues” quarks to other quarks.

The so-called fundamental physics contained in the Standard Model, however, does not account for gravity (more on this soon). A rather popular way to bring gravity and quantum physics together has been string theory, which supposes that particles are really little loops or strings. It requires the existence of many dimensions in its attempt to give a consistent account of quantum gravity. Unfortunately that account does not yet have any empirical evidence to support it, nor does it propose many experimental tests that would allow us to acquire that evidence. Even more fanciful views would have many universes existing side by side, so to speak, although such views boast of no more evidence to their credit than plain string theory.

The importance of quarks, and later of strings, was that they presumably explained the variety and properties of sub-atomic particles. More specifically they pointed the way toward a unified account of the basic forces that act between particles, which physicists often call the "basic" forces of nature, except, once again, for gravity. Fundamental research is then research about those forces, and that is the research done at gigantic and very expensive particle accelerators. Since space science is also expensive, there has been an uneasy feeling that space science has taken money away from truly fundamental research. Not that particle physics is the only kind of research a respectable physicist can undertake. But other physics shines by reflected glory, so to speak, and thus the more removed from the center of the discipline the less important it is thought to be.[1]

This uneasy feeling about space science has been losing ground dramatically, and for very good reasons. Let us leave aside, for the moment, the discovery of Dark Matter and Dark Energy, which force us to do fundamental physics in a way that requires the active participation of space science. Apart from those considerations, then, we should take into account the apparent goals of fundamental physics before the astonishing finds of the space-based telescopes. The basic forces of nature are, once more, the electromagnetic, the weak, the strong, and the gravitational. Current physical theory has brought a more or less unified account of the first two (the electro-weak), and there is hope that the strong nuclear force can also be similarly unified with those two. But gravity is not so tractable. The best hope for a unified account of all the forces requires a state of the universe in which all the forces are of comparable strength. But today gravity is far weaker.[2] Many proponents of the standard model trust that eventually in collisions at very high energies in new particle accelerators they will eventually find the so-called Higgs particle. To some extent, though, these ideas have been influenced by scientific beliefs about the beginning of the universe, for then we find a clear case of the required strong gravity, "supergravity," in interaction with the other forces.

Now, to study the beginning of the universe we first make hypotheses about it on the basis of which we predict how the universe should have evolved (e.g., how "lumpy" it should be, what kinds of galaxies should be born and how they should be clustered, what ratio of matter to antimatter should exist, and what other objects, say, quasars and black holes, we should find). We then must observe the universe in order to determine which of the competing hypotheses explains it better (again, this was in the “happy days” before the present obsession with Dark Matter and Dark Energy). It is obvious, though, that the sharper a description we have of the universe, the more sophisticated our testing of those hypotheses about its origin. It should be clear also that the more details we know about the universe the more hints we benefit from in trying to devise new theories to explain its origin and its evolution.

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[1] For an account of the contemporary low status of the planetary sciences (until rather recently), see Stephen G. Brush, “Planetary Science: From Underground to Underdog,” Scientia, 1978, vol. 113, p.771.

[2] Taking the value of the strong force to be 1, the other values are as follows: electromagnetic, 10^-2; weak, 10^-6; gravity, 10^-40.

A NEW VISION OF THE UNIVERSE

Chapter 5D

A NEW VISION OF THE UNIVERSE

When Galileo turned his telescope to the heavens a new sense was born. Galileo's telescope gave us the moons of Jupiter, the phases of Venus, and thousands of new stars. But this new sense was no mere addition, for it helped usher in a view that contradicted direct sensory experience. As Galileo himself said, "…there is no limit to my astonishment, when I reflect that Aristarchus and Copernicus were able to make reason so to conquer sense that, in defiance of the latter, the former became mistress of their belief."[1] But in the telescope he found a "superior and better sense than natural and common sense".[2] This new sense, furthermore, was not just a refinement of sight, but rather an alternative that agreed with the Copernican view, unlike plain sight. Indeed, whereas to plain sight the magnitude of Mars changed little--a troublesome fact for the Copernican view according to which the distance between Mars and the Earth varied considerably--in looking through the telescope the magnitude behaved as if God had been a Copernican. Likewise, the phases of Venus explained why the magnitude of Venus remained constant: when Venus is closest we only see a small portion of its disk lighted, but as it moves away from us we see more and more of its disk lighted, until when it is furthest from Earth we see it fully lighted.

In this way a new technology came to the rescue of an idea that was to transform our view of nature most profoundly. Not that the telescope was free from reasonable question, for on the contrary, given what was known about optics and perception at the time, its reliability was a lucky assumption made plausible more by Galileo's enthusiasm than by his argument. Among other things, we should keep in mind that the brain does not merely experience the pattern of light that strikes the retina: it interprets that pattern on the basis of past experience, expectation, and environmental clues. It also imposes constancies of shape, color, and size upon the visual image. When peering into the skies, however, many of those clues are absent and thus the untrained eye can experience many illusions. And indeed many people looking through Galileo's telescope did not see what he saw.[3] Moreover, it was generally thought that perception cannot be trusted when the natural medium through which the information travels has been tampered with (e.g., a haze or a drunken brain). Galileo's telescope was theoretically suspect, then, because it clearly altered the natural medium through which the light from distant objects was transmitted to the eye.

But the telescope did open up many avenues of observation and investigation that would not have been there otherwise. It was a promise from the heavens better kept in the course of the new science than perhaps Galileo had a right to imagine. For him it was too striking a coincidence that the telescope would so match the new astronomy of Copernicus. For others, whose fundamental views were at stake, it was a case of a distorting instrument of observation presuming to support a refuted and obnoxious view of the cosmos.

To most of us now it seems fortunate that, as Galileo put it, Copernicus "with reason [theory] as his guide...resolutely continued to affirm what sensible experience seemed to contradict."[4] Many of those who were in a position to choose decided in favor of the most exciting of the alternatives (in accordance with a principle that NASA scientist Brian Toon fondly calls "Sagan's Razor"). Whether they had other, more compelling reasons I shall not discuss here. Suffice it to say that the invention by Newton of the reflecting telescope and the refinement of both kinds of telescopes permitted not only the discovery of many new objects in the universe, but also a shift in perspective about its nature. And having embarked on a different approach to nature, the new scientists also had the motivation and opportunity to develop the auxiliary sciences (e.g., optics, electromagnetism) that eventually established the reliability of the telescope as well.

Such reliability is rather limited, as we have learned. Apart from obvious problems like background lights and bad weather, optical telescopes until recently had to contend with the effects of columns of air within them and of gravitational distortions of their very large mirrors. Worse still, even though a large telescope such as the 5-meter telescope on Mount Palomar could in principle separate the images of objects in the sky as close to each other as 0.02 arc second (this is called angular resolution), the motion of air molecules blur the path of light through the atmosphere to the point that the best angular resolution we can achieve is about 50 times worse than that. Such was the situation at the point when space technology could begin to place optical telescopes in orbit. Today those space telescopes work in concert with new generations of Earth-bound telescopes in which computers compensate for some of the distortions of the atmosphere and in which combinations with other such telescopes (in interferometers) allows the collection of much greater amounts of light.

Moreover we know now things that Galileo could not have the tools to imagine. Light is a form of electromagnetic radiation and the Earth's atmosphere blocks many other forms of that radiation: More specifically, it absorbs the X-ray and gamma frequencies, as well as most of the ultraviolet and several bands of the infrared. The envelope of gases that made life possible in the beginning, and has protected it ever since, has given us only a small window into the universe at large. Through that window we have seen and we have dreamt, and through that window we have also adjusted our views of nature. Galileo's new sense became better and better, and when it seemed that it was reaching its limitations, in the middle of the 20th century, we discovered radio astronomy, and then infrared astronomy.

By then we had found that the sun was not the center of the universe either, that it was one among billions of stars in the outskirts of a rather common galaxy, and that the galaxies were receding from each other, that is, that the universe was expanding. This last discovery prompted Einstein to recognize what he called his greatest mistake. The idea that the universe might change in size had seemed so preposterous at the time he proposed his general theory of relativity that he introduced a constant into the theory to ensure that the universe would appear static in it.

So much resulted from the evolution of Galileo's instrument. But then, with the discovery of radio and infrared astronomy, new kinds of objects and new kinds of activity in objects already known gave us a glimpse of what a look at the universe in the full electromagnetic spectrum may do. Quasars and pulsars, and the radiation left over from the big bang began to tell us about a universe far stranger than we had imagined. With the advent of space exploration we can go above the atmosphere to examine the universe in the full electromagnetic spectrum. We can also do far better astronomy in the more traditional wavelengths (radio, optical, and infrared).

For example, in the visible range, NASA's Hubble Space Telescope, unencumbered by most of the handicaps of its terrestrial counterparts, swept a volume 350 times larger and produced images ten times finer than most traditional telescopes could at that time. Even more sophisticated future space telescopes may allow us to detect planets the size of Earth around stars within 10 light years from us. Radio astronomy, which allows us to study regions obscured by dust, as well as some very cold or very hot objects, is not hampered much by the atmosphere, but it is limited by the size of the Earth. Even though the angular resolution of the largest radio telescopes is very poor (about 10 arc seconds), they can be placed together in large arrays (with the telescopes at large distances from each other) that are equivalent to gigantic telescopes. The largest such array (an interferometer called the Very Long Baseline Array) has a baseline almost as long as the diameter of the Earth and attains a resolution of 0.001 arc seconds. To attain a finer resolution we need a larger baseline, and the only way to secure that is to place some radio telescopes in space (with the first such extensions the resolution of the Very Long Baseline Array will improve by a factor of 100).

Infrared astronomy permits us to study the incredible energy released in the center of remote galaxies and other events taking place at the edge of the universe. For the objects in question are receding so fast from us that their light has been shifted into the infrared spectrum. But infrared astronomy is also of importance closer to home, as we will see in the last section of this chapter.

Let us see now some of the ways in which this developments help us understand how space physics and space astronomy are fundamental sciences.

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[1] Galileo Galilei. Dialogue Concerning the Two Chief World Systems. University of California Press, Berkeley. 1953 edition. P. 328.

[2] Ibid., p. 335.

[3] Feyerabend

[4] Galileo, op. cit., p. 335.

Planetary Science and Fundamental Physics

Chapter 5C

Planetary Science and Fundamental Physics

In the case of planetary science, as Stephen Brush reminds us, the attempt to satisfy intellectual curiosity brought about many of the most significant advances of the past few centuries. In 1746, for example, d'Alambert won a prize for making one of the first consistent uses of partial differential equations. His was an essay on winds. The work of Laplace at the end of the 18th century resulted from the calculations in celestial mechanics of Clairut, Euler, and Lagrange. Legendre and Laplace originated the use of spherical harmonics in potential theory while trying to calculate the gravitational attraction of the Earth. This use was later of great value in electricity and in quantum mechanics. And many of Poincare's major works on mathematical analysis were inspired by problems in planetary mechanics.[1]

Similar remarks may be made about Gauss, one of the world's greatest mathematicians, who did much work in geodetic surveys and terrestrial magnetism. As Brush points out, "even an advocate of pure science might concede that Gauss' geophysical work provided the stimulus for some of his contributions to geometry and potential theory, just as his early work on the computation of orbits led to a major contribution to probability theory, the `Gaussian distribution', and could thereby be justified."[2] James Clerk Maxwell, clearly a giant of physics, won the Adams Prize with an essay on the stability of the rings of Saturn. This work was the basis for his kinetic theory of gas viscosity and eventually of his theory of transport processes. He later returned to the problem of the rings of Saturn and applied to it the methods he developed in his kinetic theory of gases.

A case of particular interest to Brush in establishing the connection between pure science and planetary science is the 19th century problem of the dependence of thermal radiation on the temperature of the source. This problem was of great significance to planetary science, of course, because the sun is the greatest source of radiation in the solar system. The outcome of the attempt to determine the surface temperature of the sun "was Stefan's suggestion (1879) that the data could best be represented by assuming that the rate of emission of energy is proportional to the 4th power of the absolute temperature."[3] Further experimental investigation, and Boltzman's theoretical derivation of such a formula, led to the subsequent work on the frequency distribution of black-body radiation. The search for the law that would govern such distribution can thus be seen as the genesis of Planck's quantum theory.

These remarks on the history of physics vindicate the claim that the study of the heavens at all levels has been a driving force in the development of fundamental science. This may come as a surprise to some. But it should not be a surprise if we consider the variety of interactions between cosmology and other areas of science, and between the different levels of cosmological research themselves. The study of the cosmos leads to the discovery of fundamental principles of physics, further development of physics in turn leads to new investigative tools of the cosmos, and so on. We may speak here of a dialectical relationship between areas and levels of science which results in the dynamic growth of science. In the case of planetary science we find no exception: fundamental research on physics and cosmology leads to changes in our ideas about the solar system and its planets. On the other hand, those ideas in turn give many hints as to useful lines of "pure" inquiry. This result is not merely part of the historical record: if anything it should receive greater prominence as space exploration multiplies our means of investigating the cosmos.

Indeed we will see presently that space science provides an excellent opportunity to enhance the relationship between astrophysics and the rest of fundamental physics.

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[1] S.G. Brush. “Planetary Science: From Underground to Underdog.” Scientia, 113: 771-787 (1979).

[2] Brush, ibid.

[3] Brush, ibid.

Dark Problems

Chapter 5B

Dark Problems

AN EXAMPLE: THE DARK SIDE

It had been suspected for some time that galaxies had far more matter than we could determine from what was visible. Once we were able to look at galaxies in the full spectrum we realized that perhaps as much as 90% of some galaxies’ mass was unaccounted for (one main reason for this realization is that the outer stars in a galaxy are going so fast that without that much extra mass to keep them in, they would be flung into intergalactic space). But apparently most of that missing mass (now called “dark matter”) cannot be seen, for it does not interact with ordinary matter, except through gravity. In other words, most of the matter in the universe is completely different from anything we have known until now (it is not made up of protons, neutrons, electrons and the like). To study dark matter it is necessary to observe the universe with space telescopes, as will be explained below.

To make matters worse, it was discovered that the expansion of the universe is accelerating, in violation of the sensible belief that gravitational attraction should slow down and perhaps even reverse the expansion initiated in the most famous of all explosions, the Big Bang. A new form of energy, Dark Energy, which we understand even less than we understand Dark Matter, presumably accounts for that perplexing expansion. Since dark energy and dark matter take up most of the universe, our precious Standard Model tries to explain the entire universe on the basis of the less than ten percent of matter that it is acquainted with. Imagine that you come to a new place and get to know ten percent of it. All you know about the rest is that it is completely different from the small portion you know. How confident would you feel about explaining the whole on the basis of the one part you can handle? And then add the complication that Dark Energy make up two thirds of the universe!

Again, to study dark energy we need to go into space, sooner or later. Some of the work of surveying the universe can be done with terrestrial telescopes, but the findings of such surveys will have to be corroborated and supplemented by telescopes in orbit, as we will see below. This means that to have much of a chance to come up with a fundamental explanation of the universe we need to do space science. Some physicists still hope that in the new particle accelerators, which will produce very high energies, violent collisions will yield some dark matter particles. And, who knows, their hopes may perchance be realized, but since we do not know what dark matter is, those physicists sound a bit too optimistic. And even then we would still have the even bigger puzzle of dark energy.

To do away with the problem of Dark Energy, some physicists have proposed to replace the Theory of General Relativity with another theory in which gravity is not a constant. This hypothesis, which is rejected by most physicists, would of course represent a radical transformation of fundamental physics brought about by space astronomy and physics. Either way, space science should receive significant credit for the serendipity that will result from the soon-to-be new physics.

SPACE SCIENCE AND THE TRADITION OF FUNDAMENTAL PHYSICS

Let us begin our discussion of Claim (1) by remembering that the connection with astrophysics has been a trademark of modern physical science from its inception and throughout most of its history.

Although it is well known that Copernicus proposed that the sun and not the Earth was at the center of the universe, his motivation is not so well understood. It was not that his system could account for the position of the planets clearly better than the Ptolemaic system, for even Copernicus acknowledged that the matter was not settled. Nor was it obvious either, in spite of the claims by Pierre Duhem and others, that his system was vastly simpler. It is true that the Ptolemaic system employed a variety of mechanisms--epicycles, eccentrics, deferents, equants--to account for the paths of the planets, but with the exception of the equant so did the Copernican system. (See figures).[1] The difference was that the Ptolemaic system often had alternative combinations of such mechanisms for different aspects of the behavior of the same planet. This would seem outrageous to someone weaned on the notion that only one such mechanism could be correct. But the mere talk of correctness assumes that we can inquire about the real nature of the heavens.

We feel entitled to make that assumption rather freely today. But that was not the case in Copernicus' day. From the time of Ptolemy (second century AD.), the inquiry about the reality of the heavens had been looked upon with suspicion. The reason was that whereas the progress of mathematical astronomy made it possible to calculate with increasing precision the positions of the planets, the accounts of why the planets moved as they did had broken down not long after Aristotle had proposed the interaction of concentric spheres made of his quintessence (about 350 BC.).

According to Ptolemy himself, mathematics can apply only to "changes in form: i.e. in trajectory, shape, quantity, size, position, time, and the rest."[2] As to the actual nature of things there is little that science can do because they either take "place far above us, among the highest things in the universe, far away from the objects we directly observe with our senses," or else, as the objects of (terrestrial) physics, those "material things...are so unstable and difficult to fathom that one can never hope to get philosophers to agree about them."[3] Questions about the nature of the heavenly objects must lead one back to the ultimate source of all change, and thus they can only be answered by theology. Therefore science gains little to profit from asking them. And they are also distinct in kind from the sorts of questions that physics tries to answer, whose underlying principles, if any, did not seem amenable to mathematical treatment.

In the long run the Copernican revolution accomplished several important changes in points of view. For one thing it insisted in investigating the nature of the behavior of the heavenly objects. And it did so by looking for mutual underlying mathematical principles for both the heavens and the Earth. The success of this gross violation of Ptolemy's methodological rules turned on Copernicus' belief that astrophysics was possible. Eventually Newton succeeded where Aristotle had failed, and astrophysics became the shining example that new branches of physics had to follow.

Confusions about Copernicus' motivation were created mainly by Osiander's preface to Copernicus' masterpiece, On the Revolutions. Fearing a confrontation with theological dogma, Osiander urged the readers to "permit these new hypotheses to become known together with the ancient hypotheses," and to do so because Copernicus' hypotheses are "admirable and also simple, and bring with them a huge treasure of skillful observations." But the Copernicus' reader, Osiander wrote, should not accept as the truth "ideas conceived for another purpose."[4] All these admonitions by Osiander contradict Copernicus' own words and belie his attempt to discover the truth about the heavens by rational means instead of revelation.

In the centuries following Copernicus, astrophysics continued as a driving force of fundamental theory. Newton is, of course, the most prominent example. His laws of dynamics applied equally to terrestrial and heavenly objects, and his law of gravitation was a striking statement of the discovery that the force that kept us glued to the surface of the Earth was the same that made the stars and planets keep their appointed rounds.

We begin to see why this view of science is distorted when we realize that fundamental questions often cannot be asked without the appropriate technology and will not be asked without the right kind of inspiration and motivation. But this realization leads to another: that a whole host of activities are potentially as crucial to scientific progress as work that aims to solve problems within the most prestigious field of the time. Researchers who create new technology or new opportunities may contribute just as much to keep intact the dynamic character of science. And inspiration has often come as much from the planets as from the stars.

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[1] T.S. Kuhn. (1957) The Copernican Revolution. Harvard University Press.

[2] S. Toulmin and J. Goodfield (196 ) The Fabric of the Heavens.

[3] Ibid.

[4] Ibid.

Space Physics and Astronomy

Chapter 5A

Space Physics and Astronomy

According to the journal Science, Rashid Sunyaev, a famous Russian astrophysicist, “once heard the chair of his department say that ‘astronomy was an absolutely useless science.’”[1]

After the spectacular successes of the space telescopes and the new generation of Earth-bound telescopes, the public may be surprised to learn that not long ago many scientists regarded space astronomy and space physics with some suspicion. Quite a few physicists, for example, felt that all those billions of dollars for space astronomy should have supported the construction of a new generation of particle accelerators instead – particle accelerators dealt with truly basic science. I presume that a good many of those physicists may now agree that the money spent in space telescopes has been money well spent. But it is important to see why they are right in having changed their minds.

The reason is that, in the pursuit of cosmological knowledge, physics and astronomy done in space affect the transformation of our views in a very important respect: They provide a framework within which to challenge our most fundamental terrestrial sciences. For to understand the formation and evolution of the universe we need to see how the basic laws of nature are expressed in it. At the same time, to have a good grasp of the basic laws of nature we need to see how well we can describe the universe by using them. In a second respect they affect that transformation in an even more radical way: astronomy and physics done in space allow us to discover phenomena that we could not have discovered otherwise and that will force us to develop a new physics.

THE OBJECTION TO SPACE SCIENCE

As we recall from Chapter 3, a scientific critic might argue that, since these sciences examine far-away objects, the ensuing transformation of our views is unlikely to pay off for the inhabitants of the Earth. For example, space astronomy and physics constitute a prime example of attempting to satisfy our intellectual curiosity -- they aim to describe features of the universe that many people find interesting, sometimes fascinating. The problem for my thesis is precisely that these space sciences fit my points about curiosity so well while apparently failing to satisfy my expectation about practical results in the long run. Surely, the objection continues, it is by no means obvious that the transformation of our theories about black holes, quasars, and intergalactic gas will be of much application on the Earth.

There is, according to the objection, a great difference between Earth-bound physics and these space sciences. Consider anew the example of how Einstein’s revolution in physics led to lasers and their application in medicine: That revolution transformed our understanding of the basic principles of matter, of principles that apply down here. It is not surprising, then, that our panorama of problems and opportunities was bound to change as a result of the transformation of our thought. The principles of fundamental science (e.g. particle physics) apply down here because they apply everywhere. By contrast, space astronomy and space physics merely apply to stars, galaxies, and quasars the fundamental principles of matter discovered by Earth-bound physics -- thus they are derivative sciences. My thesis about serendipity would then apply only to fundamental science. Therefore, space astronomy and space physics cannot be justified by my general philosophical argument.

For almost a century now the most fundamental and empirically successful description of matter has been given by the so-called Standard Model, which explains the universe in terms of its building blocks (particles) and the fields (strong, weak, electromagnetic and gravitational) that allow those blocks to interact. The main experimental tools of the Standard Model have been giant particle accelerators that smash those particles at speeds close to that of light and then theorize from the resulting debris. When the choice came between spending billions to build even more powerful accelerators or spending billions to put up telescopes in orbit, the feeling among many physicists was that, interesting though astronomy may be, taking money away from the terrestrial tools that would allow us to advance the Standard Model further was tantamount to blunting fundamental science’s cutting edge.

My response will be as follows: (1) space physics and astronomy have distinct scientific advantages over terrestrially bound sciences; (2) these scientific advantages show that space physics and astronomy are fundamental science in the same sense that terrestrially bound physics is because (a) you cannot do terrestrial physics properly without doing space science, and (b) the theoretical and experimental articulation of physics needs challenge, a challenge that space science has provided and will continue to provide. Space science has made the Standard Model due for a change.

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[1] “News Focus: In the Afterglow of the Big Bang.” Science, vol 327, January 1, 2010: 27.

The solar system as a laboratory

Chapter 4O

The solar system as a laboratory

We often extrapolate with profit the bits and pieces we learn about the Earth to our understanding of other planets. Ideas about aspects of the Earth do serve as the basis for hypotheses about what we may find elsewhere. But this ability only strengthens our dependence on planetary science. For since those ideas about the Earth are also ideas about how a particular planet behaves, their worth is often appreciated only by seeing how they apply to other planets. Thus the solar system provides us both with an appropriate context in which to interpret our observations of the Earth and with valuable opportunities to test our ideas about the Earth.

Furthermore, in order to understand the evolution of the Earth, we need to know what factors originate and shape planetary environments. But it is difficult to determine the range of those factors when we only see them in operation on the Earth. Although, as we saw earlier, we can carry out some experiments on the Earth's weather (and perhaps on its geophysics) the range of global controlled experiments is likely to be very restricted for a long time to come. That is a problem, for in science when we have an idea we want to see how it works, we want to look at it from different angles. Fortunately for us, the solar system can take the place of the laboratory. If we want to know how the mass of a planet influences its tectonics, we look at several planets with a variety of masses and examine their tectonics. We cannot vary the global conditions of our planet at will. But we can look at other worlds in which those variations occur naturally and see how other factors are correlated with them.

On Earth, the oceans provide a buffer for heat and supply most of the water vapor in the atmosphere. What other influences are there on planetary weather? The study of planets without oceans begins to answer that question. How much of a factor is the planet's rotation? Jupiter, with its gigantic, three-layered atmosphere, rotates every ten hours. In Venus, by contrast, the day is the equivalent of 243 Earth days (the atmosphere, however, rotates 60 times faster than the planet). Computer models of weather systems that control for this and other factors have been tested by the actual performance of the atmospheres of the other planets. This gives us to some degree the analog of manipulating our own atmosphere to test our ideas about the Earth's weather.

Atmospheric density is another important influence on a planet's weather. On Venus, which has a very dense atmosphere (about ninety times that of Earth), the temperature variations are minimal, 10 to 20 degrees Celsius, between poles and equator. This characteristic surely contributes to the absence of some of the weather patterns familiar on Earth. On the other hand on Mars the atmosphere is very thin, less than one-hundredth that of Earth. Apart from making practically impossible the presence of liquid water on the surface, the thin atmosphere does not provide a barrier to break the atmospheric waves. Whereas on the Earth atmospheric waves encounter resistance and form eddies, on Mars they can run their course, so to speak. The result is that on Mars the weather patterns are far more regular, repeatable, and periodic than on the Earth. Since the structures that organize the Earth's weather are not periodic, our weather is very difficult to predict. Nevertheless we can hope that the comparative study of planetary atmospheres will continue to identify the factors that contribute to the behavior of our own weather.

The point is not that comparative planetology guarantees better weather forecasting, though it might, but that in giving us a much wider experience with weather systems it may transform our views about weather. It is the transformation of those views that may provide a great impetus, some day, to much improved weather forecasting. And similar remarks may apply in the case of improvements in our knowledge of tectonics with respect to earthquake prediction. It may of course turn out that all the knowledge in the universe would not suffice to improve the prediction of weather and earthquakes past a certain level of precision. But the improvements may be great before we reach that level. A strategy that aims to give us the deepest understanding of these phenomena and the limits of their predictability is surely the best one to follow. That is what comparative planetology offers.

The transformation and fine-tuning of our views about the Earth is by no means the only benefit likely to accrue from the exploration of the solar system. We will also increase the precision with which we can observe and predict a variety of environmental states. This increase will come, partly because we will be better able to specify relevant factors and parameters of the global environment, as a result of our comparative study of the gravitation, magnetism, atmospheres, morphology, topography, geology and chemistry of the Earth and other planets; and partly because of the advances in the technology necessary to carry out that comparative study. At the present time, for example, lasers that bounce off satellites can be used to measure the movement of the tectonic plates and the vibrations of the ground around volcanoes. In this and many other new ways, the scientific exploration of the solar system will make it easier for us to ask more fruitful questions and secure more precise answers in our quest to understand and monitor the world to which we were born.[1]

Moreover, we have also seen that when trying to understand our own planet we often are unaware of many of the crucial factors that affect it. That is so because those factors often are the result of trends and forces that have developed sometimes for billions of years, or because they are hidden from our view or caused by interactions with the rest of the solar system. So we make up theories to guide our thinking. But those theories have little contact with the results that might shape them in fruitful directions if we limit ourselves to direct observations of the Earth. A generation ago we had observed up close only one planet: the Earth. Today we have examined over forty planetary bodies in the solar system. Those other bodies give us the opportunity to test the mettle of our ideas about our own world. The inevitable transformation of those ideas is therefore a transformation of our understanding of the Earth.

It is perhaps sensible to adopt a posture of skepticism toward the sensational predictions of catastrophe. The world has never suffered a scarcity of doomsayers. But it may well be that humans do have an increasingly greater impact upon the environment. And it also seems that the present rate of change is higher than what would have been brought about by other natural processes. The evidence is inconclusive but enough to make it at least prudent to look into these matters. The Earth may have seen greater changes in its past than we are liable to inflict upon it; but we should not rest assured that we will endure whatever we carelessly bring about.

We are rather in the position of a blind man in a china shop. If he moves he loses; if he does not move he loses too. Neither recklessness nor paralysis is to be recommended. To avoid them both he needs to know what the shop is like and how he can move about in it. To give him sight would be the greatest gift. The Earth is our china shop, and the satisfaction of our curiosity through space science can help us see where we are going. Wisdom requires that we accept that gift.

[1] One interesting illustration of this point can be found, once again, in the environmental problem of CO₂. Most of the dire scenarios include the melting of the polar caps. But determining how the caps would melt needs at least two kinds of investigations: (1) a way of measuring changes in the ice cap that can be correlated with increases in global temperature, and (2) a general theory about the formation and evolution of ice caps that will allow us to infer trends from such measurements. Fortunately space exploration has given us the means to take accurate measurements that would be practically impossible otherwise: polar satellites that make the precise comparisons needed for a fine determination of changes in the ice caps.

Leslie's Probability

Leslie’s intuitions about probability seem to be of a kind with the case of the double lottery winner. Suppose that you buy lottery tickets for Tuesday and Friday. There are on both occasions ten million possible numbers, so your chances of winning on Tuesday or Friday are each one in ten million. And now suppose that you win on Tuesday. What are your chances that you will win on Friday also? Most people will feel that the chances of winning on Friday after having won on Tuesday must be far less than your chances when you won on Tuesday: one in ten million times ten million (1/10¹⁴). But this is a mistake. It is one thing to ask, what are your chances that you will win in both Tuesday and Friday? The answer is: 1/10¹⁴. It is another thing to ask, what are the chances that you will win on Friday after winning on Tuesday? Well, you have one ticket and there are 10 million possible numbers. So your chances should be one in ten million (10⁷). But people like Leslie find this hard to accept. They think that there is something valid about the intuition that tells us that after winning the lottery once, the chances of winning it again have to be far smaller than they were the first time. The difficulty comes from thinking, in this case, that if you won on Tuesday, to win again on Friday really means that you will have won on both Tuesday and Friday and that should have an extremely small probability (even compared to the small probability of winning one lottery to begin with).

To dispel this intuition, let us look at one example in which the closer we get to an unlikely combined result, the greater its probability. Suppose you flip a fair coin a hundred times. What are the chances that you will get a hundred heads? 1/2¹⁰⁰. That is an extremely small number. Suppose now that on the first flip of the coin you get heads. What are the chances now that you will get 100 heads? There are 99 flips left. That means that you will have to get heads on all 99. Your chances then will be 1/2⁹⁹. That is a very small number, too, but it is twice as large as the number you had before (because the denominator, 2^99, is half of 2¹⁰⁰: 2x2⁹⁹ is 2¹⁰⁰). After getting 2 heads, your chances of getting 100 heads will be 1/2⁹⁸, which is a very small number, but twice as large as that of your second flip and 4 times as large as your first’s. The more heads you get, the closer you get to getting 100 heads, the more your chances improve of getting 100 heads. Suppose you have flipped 98 heads, so you have only two flips left. Your chances will be 1/2²=1/4. You have gotten 99 heads? Your chances of getting 100 heads depend on your getting heads on your last try. That is 1/2.

Of course, the chances of getting 99 heads and one tail will at that point also be 1/2. At the start, there were 2¹⁰⁰ possible combinations; getting 100 heads was just one of them (1/2¹⁰⁰). But once you got heads on your first try, you eliminated half the possible combinations, that is 2⁹⁹ hitherto possible combinations. Check it: 2⁹⁹ + 2⁹⁹ = 2¹⁰⁰. For an easier example: 2² + 2² = 2³ (4 + 4 = 8) and 2³ + 2³ = 2⁴ (8 + 8 = 16). If your base were 3, then you would have to add three numbers: 3^{2 +} 3² + 3² = 3³ (9 + 9 + 9 = 27). If your base were 4 you would have to add four numbers, and so on. Now, when you get your second head in a row, you eliminate half of the existing possible combinations left up to that point, that is, you eliminate 2⁹⁸ additional possible combinations. And so on: every time you get heads, you eliminate half of the possible combinations still in existence up to that flipping of the coin. Until, finally, when you only have one flip to go, there are only two possible combinations left. The first will give you 99 heads and then tails. The second will give you100 heads as the result of getting heads on your last flip. Both combinations are equally probable. Therefore, your chances of getting 100 heads when you have already gotten 99 are far greater than when you first got a head, while the chances of getting a head on your last flip are exactly the same as the chances of getting a head on your first flip: 1/2.

Let us remember this reasoning when we begin to think about the probability that life could evolve from inorganic materials.

Venus and the End of the World

Chapter 4M

Venus and the End of the World

Before concluding the discussion on these aspects of comparative planetology on too happy a note, I should mention that the hypothesis that Venus once had oceans, as appealing and reasonable as it may be, is by no means universally accepted. For it depends crucially on the plausible reading of the ratio of deuterium to common hydrogen as evidence that Venus has lost a substantial amount of water. But this is not the only possible reading.

Some scientists have argued that the same ratio of deuterium to hydrogen could be caused by a continuous re-supply of water to the atmosphere of Venus. They find this hypothesis more plausible because, at the present rate of escape, water would disappear from Venus altogether in a few hundred million years. This would mean that right now we are witnessing the very end of a long process. Most of us would not be bothered by this prospect, but some people feel that we should treat with suspicion all lucky coincidences in science. Imagine, Grinspoon tells us, that

You show up at a house where no one seems to have been home for a long time, but you hear water running. You go upstairs and find a vigorously draining bathtub which has only an inch of water in it. Now, it is possible that you showed up just as the last bit of water was running out -- but isn’t it more likely that the tap has been left running?[1]

The intuition behind this reasoning is a peculiar view of probability that sounds quite reasonable at first. Suppose that an urn contains two white balls and 98 black balls, while another contains two white balls and 9998 black balls. Without knowing which urn is which, you reach into one of them and come up with a white ball. You should conclude that in all probability you have reached into the smaller urn, for the chances of getting a white ball are two in a hundred, which are much higher than the two in ten thousand that you would find in the larger urn. It is not reasonable to believe that you have reached into the larger urn, since getting a white ball out of it is so much more unlikely.

These intuitions about probability come with their own problems, however. The first problem is that it turns scientific reasoning on its head. In the case of Venus, it leads us to expect that the amount of water we find is normal (that it has been like that for a long time). It leads us to assume that things are pretty much as we find them (a steady-state) because otherwise the situation would be very unusual and thus unlikely.

Notice how differently we reason in science. When we first observe any phenomenon, we take pains to determine whether our sample is representative. The history of science is littered with ideas that seemed promising but went nowhere because they were based on the assumption that we were looking at the normal state of things (this is the fallacy of induction). That is, normally we have to demonstrate that our sample of observations indeed represents the usual state of affairs. We are not allowed to take that for granted. Otherwise we may conclude, say, that we have discovered a new branch of the homo family based on one fossil with a peculiar skeleton, only to find out much later that it was the skeleton of an individual with a bone disease (a true case). We worry about whether the Viking and Venera landers ended up in representative locations in Mars and Venus, whether the Galileo probe went into a section of Jupiter that is like the rest of the atmosphere (it didn’t). Nature is rich and what we come to observe may actually be as unusual as flowers, birds, and bees are in the solar system: they exist in only one world – Earth – out of the many that orbit the sun.

The kind of reasoning that leads to a steady-state view of water in Venus also leads to some very strange views when applied elsewhere. John Leslie, for example, has concluded in The End of the World that the human species is likely to become extinct very soon.[2] He reasons that if the human species were to live for a long time, let’s say millions of years, then the amount of people alive today would be an insignificant percentage of the total amount of human beings that will ever be alive. Thus, he thinks, belonging to such an unusual group of humans (those of today) would be extremely unlikely. On the other hand, if the world were to end within fifty years or so, we would be part of the largest group of humans who will ever be alive (the six billion or so alive today are far more than all the rest of the humans who have ever lived put together). And it is more likely, then, that if you were to pick a human at random he would belong to the overwhelming majority than to a very small minority. Therefore the human species is far more likely than not to become extinct very soon.

Many sensible people would consider Leslie’s argument a reductio ad absurdum of the kind of probabilistic reasoning under examination here. Nevertheless, he, like others, sticks to his probabilistic intuitions, despite counterarguments like the following. Suppose the devil places ten people in a room and tells them that he will kill them all if he gets double sixes in a roll of the dice. If they survive, he will then place 100 people in the room, and then 1,000, and so on, always multiplying by ten. And every time the devil will roll the dice in hopes of getting a double six. Now, most of us will think that the chances of any one group getting killed will be 1 in 36, but according to Leslie, the ill-fated chances of the group of ten thousand have to be far greater than those of the group of ten. Leslie concludes not that there is something very wrong with his reasoning, but that he has encountered a paradox of probability. And he remains worried about the end of humanity, just in case the paradox should resolve itself in the direction of his reasoning.

There is no paradox, however. The probability estimate that takes into account a causal mechanism has priority (in this case, that the devil will kill all the people in the room if he rolls double sixes), as do all probability estimates that clearly have more relevance. For example, suppose that an ordinary man goes to the hospital to have an appendectomy. He is informed by his doctors that the chances that the operation will go well are 98%, for their survey shows that that 98 out of every 100 patients who undergo the operation do well. But suppose now that the patient is 89 years old and suffering from cancer and its debilitating effects. Surely the previous estimate would not apply to him. Let’s say, for the sake of argument, that statistical records have been kept for people in his condition and that people in his condition survive less than 5% of the time. This latter estimate is the one that should guide his decision to undergo the operation, for it is based on the factors most relevant to him. It would be preposterous of him to say, “But my chances might still be 98%.”

It may turn out that the amount of water of Venus in a steady state, but to have confidence in that idea we need independent scientific reasons in its favor, not peculiar intuitions about probability. A candidate to keep the Venusian atmosphere re-supplied with water is volcanism. A good possible outside source of water is the combination of comets and their fragments, for they are basically a mixture of ice water and other compounds. As Grinspoon points out, however, comets collide with planets often but not continuously and, thus, we would not know whether that hypothesis is consistent with our present readings of Venusian water (comets would supply water in spurts). Furthermore, this approach needs to assume that the exceptionally large ratio of deuterium to hydrogen would exist in Venus for any given short period of time. Otherwise we would have to conclude again that Venus has been losing water for a long time. Now, some scientists say that in first coming up with the “oceans in Venus” hypothesis we assume the original Venusian deuterium-to-hydrogen ratio should be close to Earth’s, but that this assumption may be incorrect: after all, they are different planets. This point is fine as far as it goes. But the ratio is 120 times greater on Venus than on Earth! It is difficult to imagine what could account for such a phenomenal difference in natural ratios between the two planets. The original ocean hypothesis must continue to be considered most reasonable until further notice.

This proliferation of ideas is by no means restricted to planetary geologies. But perhaps we can see this point better by discussing several illustrations in connection with another point, namely that the variety we find in the solar system permits us to test our ideas of the Earth.

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[1]. D.H. Grinspoon, op. cit., p. 109.

[2]. J. Leslie, The End of the World, Routledge, 1996. Leslie believes that this statistical reasoning should make us more fearful of possible cosmic cataclysms, such as giant asteroid impacts and space-time-gobbling new universes growing inside our universe, as well as man-made catastrophes such as nuclear war.

Martian Systems

Chapter 4L

Martian Systems

The global understanding we may thus gain is fine-tuned by our exploration of still more planets. Let us consider Mars now. Since Mars was smaller and less dense than the Earth, it did not have available as much internal energy as the Earth. And since Mars is further from the sun, it receives less sunlight. To compensate, if Mars were to have as comfortable a climate for life, it would have to have a much greater greenhouse effect than the Earth. This would require truly large amounts of CO₂ in the atmosphere. At the present time the CO₂ in Mars is 50 times per unit volume that of the Earth's atmosphere. That sounds like much, but it produces only a puny greenhouse effect. The reason is that since the atmosphere is already so cold and thin, the Martian water is frozen at the poles or spread as permafrost under large areas of the surface. This means that the initial boost that CO₂ gives to the greenhouse effect is not multiplied by the action of water vapor in trapping even larger amounts of infrared radiation.

It seems, however, that earlier in its history, when Mars' internal energy was much higher, Martian volcanoes might have filled the atmosphere with as much as 100 times as much CO₂ as today.[1] This factor would have raised the density and temperature enough to permit liquid water and large amounts of water vapor, and hence much more of a greenhouse effect. As we look at Mars now, that appears to have been the case. For years, spacecraft photographs showed what seemed to be riverbeds and suggested other indications of significant amounts of liquid water in the past, perhaps even an ocean. After the recent exploration of the Martian surface by robots, the case for water in Mars is very strong.

This view of Mars is strengthened by the recent discovery that Mars at one time did have plate tectonics as well. It seems, then, that life could have existed on Mars. If so, why did not Martian life control the climate the way Earth's presumably did? Mars apparently did not have enough energy to run the cycles that have made a sustainable biosphere on Earth possible, just as it did not have enough heat to support the motion of tectonic plates for billion of years. Life, if it ever existed on Mars, was thus powerless to stop the ultimate collapse of its global environment.

The history of Mars should prove most instructive. If Martian life did exist, its perils might tell a tale as dramatic as it would be fascinating. For in the old layers of Martian rock, human geologists may one day find the mark of life just as they do in very old rocks in our home planet. And those rocks would provide a record of a series of interactions between life and the environment in which the mechanisms of the "thermostat" finally collapsed. Just as we learn much about the human brain by studying those brains that break down through injury or disease, we can learn about a terrestrial planet's global environment by studying terrestrial planets in which the global environment broke down.

As we will see in Chapter 6, the consequences of Martian life for our understanding of Earth’s biology would be truly extraordinary, even if we can find only fossils. The geological exploration of Mars is made even more tantalizing by the suggestion that at least one Martian meteorite, ALH84001, contains evidence of past life on our sister planet.

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[1]. Some researchers have suggested that the Earth's early atmosphere, following the initial heavy bombardment by asteroids, also had a very high percentage of CO₂. The ensuing greenhouse would then compensate for the dimmer sun. Life eventually removed much of the CO₂, thus preventing a runaway temperature when the sun's luminosity increased. This hypothesis runs contrary to other ideas on the composition of the early atmosphere, according to which a primitive atmosphere would exhibit either a highly reducing mixture of methane, ammonia, water and molecular hydrogen (similar to that of Jupiter, Saturn and the other planetary gas giants) or else a mildly reducing mixture of carbon monoxide, carbon dioxide, nitrogen, and water, with not much molecular hydrogen. (In this context a mixture is reducing to the extent that it contains hydrogen). To decide between these and perhaps other alternatives it will be helpful to study not only the histories of Mars and Venus, but the largely methane atmospheres of Titan and Triton, the large moons of Saturn and Neptune respectively. The reason these matters are so worth looking into is that knowing more about the composition of the primitive atmosphere can tell us much about the origin and evolution of the global environment of a planet; in this case, of our planet.

Consider also one of the most interesting aspects of Jupiter's atmosphere: the famous Red Spot. In a dense atmosphere with winds of hundreds of miles per hour, how could a storm, which is what the Red Spot is, remain stable for centuries, perhaps for many thousands of years? The answer seems to be that the fast spin of Jupiter (once every ten hours) produces very strong Coriolis forces, which in turn produce the turbulent winds that drive the gigantic eddy of gas otherwise known as the Red Spot. As the planet spins, many smaller eddies develop, but these eddies eventually feed the Red Spot. In the midst of turbulence the Red Spot has achieved stability within the Jovian atmosphere. Thus stability arises from chaos. But interesting as this may be, what significance does it have for people on Earth? The significance is that space scientists see many parallels between the dynamics of the Red Spot and some weather patterns in the atmosphere of the Earth. In particular, these scientists see parallels to systems of high pressure that sit still for weeks or even months. Understanding this phenomenon, known as "blocking", would be a great help in forecasting the weather here on Earth. Of course, it may still turn out that what we learn about the stability of the Red Spot does not apply to stationary high-pressure systems on the Earth.