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Saturday, December 25, 2010

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?

[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.

Tuesday, December 14, 2010


Space 5E


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.

[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.

Monday, December 6, 2010


Chapter 5D


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.

[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.

Sunday, December 5, 2010

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.

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

[2] Brush, ibid.

[3] Brush, ibid.