Publications Topics:

Books

ASP Conference Series

Monograph Publications

IAU Publications

Books of Note

Purchase through the AstroShop

Journals

Publications of the ASP (PASP)

Magazines

Mercury Magazine

Archive

Guidelines for Authors

Order Mercury Issues

Mercury Advertising Rates

Newletters

The Universe in the Classroom

ASP E-mail Newsletters

Special Features

Astronomy Beat

Contact Us

Mercury, May/June 2000 Table of Contents

Fueled by scientific, technological, and even millennial fervor, some claim we are on the verge of figuring "it" all out. Stars, galaxies, large-scale structure, even the nature of the Universe itself. Might we really be so close to all the answers?

Chris Impey

What if...

What if the end of astronomy was in sight? A few more years pass, and the job is finished. We name every star and count every galaxy. We poke and prod stars so that all their life secrets are revealed. We scour nearby regions of the Milky Way for planets and life. We make successful models of all the exotica of the Universe. We understand galaxy formation after throwing the most powerful supercomputers at the problem. We finally agree on the parameters of the Big Bang model.

At a series of worldwide research conferences, consensus is reached on all the important issues. Each major research journal closes its doors with a final set of review articles. The great observatories shift their operations to public outreach, satellite tracking, and searching for Earth-crossing asteroids. At universities everywhere, faculty are retrained to work on environmental research and biomedicine. Graduate programs are phased out. Astronomers go home and take a well-deserved, and permanent, rest.

Perhaps this is idle fantasy, fueled by a millennial sense of closure. Yet there is no mistaking the confidence of astronomers as they work to understand the Universe. Astronomy is a vibrant science working with a powerful set of research tools. Some of the hardest problems are beginning to crack after a sustained onslaught. Astronomy is also the oldest science. Yet there is a strong feeling that finally, after 2500 years, we are approaching the end game. Could astronomy actually become a victim of its own success?

The debate over the end of astronomy is part of a larger argument about the fate of science. In the last couple of years, several authors have written pessimistic books about the future of the scientific enterprise (or, more generally, consider such works as Fukuyama' s The End of History and the Last Man, or Postman' s The End of Education) There are several arguments that the glory days of science are over. Each of them suffers from flaws and is subject to counter-arguments - the rebuttals do not get equal attention, however, since doom and gloom sell books.

One argument says that science cannot sustain its current rate of progress, that knowledge cannot grow without end. At some point, we will "hit a wall." Some scientists believe they can discern the overall outline of the jigsaw puzzle that represents our knowledge of the natural world. Perhaps only a few large pieces are missing. This is a weak argument since it assumes that we can infer the totality of knowledge from our present position of incomplete understanding. Early in the era of quantum physics, Heisenberg heard a radio interview where Pauli claimed that all the important problems had been solved. He sent Pauli a blank sheet of paper with the name "Titian" signed at the bottom and a note saying, "This is to show that I can paint like [Italian painter] Titian. Only the details are missing."

A variant of this argument holds that science has entered a period of diminishing returns. There are more scientists working today than ever before, and more money being spent on science than ever before. Indeed, with so many scientists, people ask, where are the Einsteins and the Darwins, missing the point that these singular figures are rare in any age. Current progress in most fields appears to be breathtaking. Perhaps the agenda of fundamental physics has entered a slower phase. But the claim of a general slowdown ignores the growing work across disciplinary boundaries, and it demeans the application of science to real-world problems.

It is harder to rebut the claim that "revolutions" in science are nearing their end. Here is the argument. Major advances in science - the layout of the periodic table, the discovery of the structure of DNA, the special and general theories of relativity - are special and become less likely as science progresses. Once an insight has come to Mendeleyev or Dalton or Rutherford, it cannot come to anyone else. Pivotal discoveries can only be made once. This sounds good, until we recall that no revolution in science was ever anticipated. At the turn of the 20th century, Michelson voiced the opinion that classical physics was complete and researchers had only the prospect of defining existing laws with increasing precision.

The Solar System provides an illustrative example. When unexplained perturbations were found in the orbit of Uranus, it was hypothesized that a more distant planet was tugging on Uranus. The discovery of Neptune was a dramatic confirmation of the power of Newton' s gravity law. When much smaller, unexplained perturbations were discovered in the orbit of Mercury, the result was a new theory of gravity. In hindsight, the difference between these episodes seems obvious. History is often illuminating but rarely predictive.

Another concern relates to the issues that face researchers now. Scientists appear to have made relatively slow progress on fundamental issues like consciousness and quantum gravity. The scientific method may have limited applicability to disciplines like psychology and economics. What if the problems that face scientists today are too difficult? This pessimism seems unfounded. Scientists have been continually resourceful in devising the conceptual tools to help them make progress. Several new branches of mathematics have been created to solve seemingly intractable scientific problems. There is no sign of an end to this ingenuity.

At heart, many scientists are reductionists or determinists, and this fuels the sense that all remaining problems are soluble. Physicists are simply awaiting the form of the final unification of gravity and the microscopic forces. The naturally occurring elements can combine in a vast number of ways, but the number of useful new materials is finite. Biology rests on the digital information coded in a few billion base pairs, and the industrial-scale project to unravel the human genome will soon be complete. Even the brain is under assault - understanding an electrical network with 1012 connections is a difficult but tractable problem.

The reductionist tendency of scientists leads them to think "inside the box." In this view, consciousness reduces to the working of a trillion neurons, human biology reduces to the infogmation content of a few billion base pairs, material science and chemistry reduce to the possible combinations of 92 elements, and physics reduces to the operation fo four forces. Illustration courtesy of the author.

Science does face fundamental limits. Some things are essentially impossible to compute, such as the quantum state of any large object, described atom by atom. The third law of thermodynamics will stand in the way of perfect efficiency in any physical process. Humans face limits to their view of the Universe imposed by the finite speed of light. There are some parts of the Universe that we have never seen, and we can never see distant objects as they are right now. Perhaps our brain function will prevent us from understanding the nature of consciousness. Few scientists are overly concerned with these limitations.

The Story So Far

A condensed history of astronomy conveys a sense of progress and unstoppable momentum. Astronomy is as old as human culture, for the night sky has always been used for navigation, for timekeeping, and as a repository of myths and legends. The science of astronomy began 2500 years ago when Greek philosophers pondered the mechanism of the heavens. After a slow start - the field was in the doldrums for more than a millennium due to the Dark Ages and the dazzling but misleading legacy of Aristotle - astronomy came of age during a 100-year span near the beginning of the Renaissance.

Copernicus used arguments of elegance and economy to displace Earth from the center of the Solar System (the evidence at the time was less than compelling). Dutch opticians built the first modest telescope, and Galileo had the inspiration to use it as a sense-extender for doing science. Newton created the theory of gravity that is still used for 99% of modern astronomy, a theory that is reliably used to send spacecraft throughout the Solar System.

In the last century, advances came in a crescendo. Hubble showed the vastness of the Universe by measuring distances to external galaxies. Physicists delved into the atom and discovered the reason that stars shine. Hertzsprung and Russell started to piece together the diverse life stories of stars. The Big Bang model emerged from calculations by Gamow and others, and was cemented by the serendipitous discovery of the microwave background radiation. Planetary science matured as NASA sent satellites to inspect many parts of the Solar System. Computers emerged as scientific tools to rival the brain and the telescope. On the cusp of the millennium, we witnessed the discovery of extrasolar planets and the refinement of the cosmological model.

Some will argue that the job is nearly done. Our theories of stellar structure and stellar evolution are indisputably mature. We can model everything from the "ringing" of a star like the Sun to the slow cooling of a white dwarf and the quaking crust of a neutron star. Perhaps stellar astronomers will be the first to head for the golf course. As the extrasolar planets pile up and the thrill of discovery fades, we will be reduced to cataloging their properties. The census of galaxies is now complete - fifty billion give or take a few billion - and modern telescopes are so powerful that the entire observable volume of the Universe has been surveyed. Cosmology has entered a phase of consolidation as we work to increase the precision of the existing cosmological parameters.

And yet, astronomy is an empirical science. All discussion of closure is surely misplaced when so few major discoveries were predicted. Black holes and neutron stars were anticipated theoretically, but the existence of billion-solar-mass back holes in the centers of galaxies was a total surprise. Who could have predicted gamma-ray events that outshine the entire Universe for a few brief moments? How are we to understand the acceleration mechanism of cosmic rays whose gyration radius is larger than a galaxy? And what about the stunning fact that exotic particles, undetected in any physics lab, greatly outweigh the normal matter content of the Universe?

Windows on the Universe

Astronomers have witnessed two transformations in their view of the Universe. Through the history of the human species, the naked eye has given us a view of roughly 6000 stars (somewhat more in the long span of time before our industry soiled the air and brightened the sky). The telescope allowed the collection of more photons and a consequently deeper view of the sky. A second part of this revolution came in the middle of the 19th century, when photography provided a way to make a permanent record of an image.

Transformations in our view of the Universe. The first was the invention of the telescope as a sense-extender for the eye. The second was the advance in electronics and detectors that allowed the electromagnetic spectrum to be viewed, often from space. The next is likely to be the routine detection of gravity waves. illustration courtesy of the author.

When we compare the naked-eye view of the sky with images of the Hubble Deep Fields (North and South) from the Hubble Space Telescope, the increase in depth is almost 25 magnitudes, or a factor of ten billion. This improvement comes from three factors: the gain in aperture of HST over the eye, the increase in efficiency of a CCD compared with the chemical detection of light, and the long time that HST can stare versus the eye' s rapid readout which is required to maintain the illusion of continuous motion.

These factors are, in turn, limited by economics, physics, and sociology. We are now witnessing an unprecedented burst in the construction of large telescopes. After fifty years when a single 5-meter and a single 6-meter telescope were the largest available, a dozen 8- to 12-meter telescopes have been built or will come online in a ten-year period (see "Telescopes of the 21st Century," Sep/Oct 1998, p. 10). As telescopes get larger, sharp images require fast electronics and a large number of actuators. Plans exist for telescopes up to fifty meters in diameter, and, in principle, there is no physical limit, but cost is a practical limitation. Going into space gives a darker (and, in the infrared, 1000 times quieter) background. However, collecting area in space costs ten to fifty times more than collecting area on the ground.

The Southern Hubble Deep Field, as seen by the Wide Field and Planetary Camera 2 of the Hubble Space Telescope. This picture, along with its twin in the northern sky, is the deepest image of the sky ever made. Almost every object is a distant galaxy, and none of them can be seen with the naked eye. Image courtesy of AURA/STScI and NASA.

The spur to build larger and larger telescopes comes because optical detectors are nearly perfect. Quantum efficiency is close to one hundred percent, and CCD read-noise is dominated by sky noise in most applications. When every photon is being recorded, the only options are to build larger telescopes or to tile larger areas of the focal plane. This second option leads to a new kind of diminishing return. The figure of merit of optical and near-infrared detectors - a product of solid angle and sensitivity - has been improving very rapidly. In fact, it is now barely worth starting projects that take longer than two to three years because the survey can be completed with new detector arrays faster than it could have been completed using old ones. Occasionally science progresses by procrastination.

Sociology comes in because it is difficult to use scarce resources on a multi-purpose observatory for a single application. The Hubble Deep Field did not arise out of the competitive time allocation process; it was a far-sighted decision by former Space Telescope Science Institute director, Robert Williams. Nevertheless, astronomers have bowed to the empirical nature of their subject. Surveys have always featured high on the research agenda.

Optical observations do face fundamental limits. Pencil-beam surveys are already sensitive to galaxies as bright as the Milky Way across the entire distance that light has traveled in the age of the Universe. Going deeper will only fill in the frame with closer, dimmer galaxies. (The search for the dimmest stars and brown dwarfs is, by contrast, a needle-in-the-haystack problem, best solved by wide-angle but shallower surveys.) Galaxies are extended, so the confusion limit will be reached even with diffraction-limited images from space. Current ultra-deep surveys are only a factor of ten away from the level of the extragalactic background light.

The second major transformation in astronomy has been the prying open of the electromagnetic spectrum. After centuries of viewing nature through an octave-wide slit, the birth of radio and electronic technology offered a new set of perspectives. Current facilities allow the detection of everything from meter-length radio waves to TeV gamma rays - a factor of a trillion in frequency or wavelength. Each new wavelength regime has revealed new classes of astronomical objects and unanticipated astrophysics. The only practical bounds on the electromagnetic spectrum are increasing cosmic noise at low energies and the pathlength to electron-positron pair production at high energies.

Astronomy's grasp has increased by a factor of 1010 in the dimension of sensitivity and 1012 in the dimension of wavelength. Is there anything left? Neutrino astronomy is in its infancy, and with roughly 100 neutrinos in every cubic centimeter of space, there is plenty to work with. We have not yet detected the fundamental dark-matter particle. But our understanding of the universe can only improve when we start working with the stuff that 95% of the Universe is made of.

Perhaps the most neglected dimension is time. Human lifetimes are such a small fraction of the timescale of cosmic evolution that astronomers often feel limited to a snapshot view of the Universe. Yet, the Universe is a hive of activity. Every year a new quasar is born, every day a new supernova goes off. A rich brew of astrophysics can be found over times ranging from fractions of a millisecond (fast pulsars) to seconds (gamma-ray busters) to hours (micro-lensed planets) to days (the event horizon of a massive black hole) to years (changes in energy flow in active galactic nuclei). We have barely scratched the surface of the phenomena that span this factor of 1014 in time.

The first solar "neutrinograph." Using data from the first 500 days of the Japan-based SuperKamiokande neutrino experiment, researchers at the Louisiana State University produced this image of the Sun. The resolution of this neutrino image is very low, with the size of the Sun's photosphere about the size of the central pixel. Image courtesy of R. Svoboda (Louisiana State University).

The upcoming experiments to detect gravitational waves remind us that there are unexplored ways to view the Universe. The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a pair of four-kilometer-long instruments designed to detect ripples in space-time. Gravity waves have been inferred to exist from the orbital decay of binary pulsars, but LIGO will offer us our first chance to "see" the Universe in terms of non-equilibrium gravity. This is a massive paradigm shift from the well-studied ways in which electromagnetic waves interact with matter. We expect to see signals from merging neutron stars and black holes, and perhaps from gravity waves in the early Universe. But the experimenters feel their greatest sense of anticipation from the things than cannot be predicted.

The Unfinished Revolution

All the Cassandras who predict the end of science reach for their metaphors. Science will hit a wall. There are only a few pieces of the jigsaw left to be found. We are peeling back the last few layers of the onion as we reach for the truth. Astronomy is the subject that most effectively mocks these pretensions. But there are signs of unfinished revolutions all across science.

In biology, the sequencing of the human genome is well on the way to completion. However, having the coded information in front of us does not mean we know what it means. The causes and mechanisms of disease are still dimly understood. It will take years to unravel the evolutionary strategies that life took four billion years to develop. Life on Earth operates with a high degree of specificity, using only twenty out of 10,000 possible amino acids and one of many potential replicating molecules. When we understand how the shape of complex molecules affects their interactions, we may be able to engineer their function and form, watching the emergent properties of auto-catalytic chemical networks. Designer biology awaits us.

Physics has an agenda that is unabashed reductionism. The field has been largely successful in explaining the rich diversity of the natural world in terms of four forces operating in an elegant mathematical framework. In fact, physicists have almost succeeded in explaining the mass of everyday matter as a secondary property that emerges from the pure energy of moving quarks and gluons. This agenda will take a big step forward if, as anticipated, the next generation of particle accelerators finds evidence of the Higgs field, which is presumed to generate the masses of both bosons and fermions.

However, the standard model of particle physics still contains around fifty parameters, many of which have to be fed in by hand (see "Elements of Elementary Particle Physics," Jul/Aug 1998, p. 9). A "theory of everything" must operate successfully at the level of the Planck mass, mpl=(hc/G)1/2=2.18 ´ 10-5 gram. The form of this theory is not yet clear, but it will probably replace particles and fields with a new type of entity that has its own rich phenomenology. Physicists have also been leading players in the new fields of chaos theory and non-linear dynamics. Cooperative phenomena lead to emergent properties on scales from the microscopic (superconductivity and Cooper pairs) to the human (decision and game theory) to the cosmic (the ecology of star and galaxy formation).

Modern science is powered by computation. Scientists of all types have been riding the wave of Moore' s law - a doubling in the speed of digital circuitry every eighteen months for the past thirty years. Continued gains using field-effect transistors will soon be limited by power dissipation in wires that connect them and by statistical effects as individual transistors are switched by only a handful of electrons. We are many orders of magnitude away from the information theory limit of about 1020 bit operations per second at a cost of 1 Watt of power dissipated. The next frontier is quantum computing, where information is coded at very high density using individual atoms. Logical operations and parallelism could be achieved using a superposition of quantum states.

All this refers only to hardware. The enormous promise of artificial intelligence has largely been unfulfilled. Computer science uses architectures and algorithms that have been in place for decades. Who knows what the limits will be when computers can program themselves and adapt their hardware to solve specific problems? Mathematics itself provides the most fertile ground for future advances. The debate over whether mathematics is invented or discovered is impossible to resolve. Either way, advances in pure math continue to drive scientific advances and their applications in the real world.

The Laser Interferometer Gravitational-Wave Observatory (LIGO), currently being commissioned at two, widely separated sites in the United States. The twin, four-kilometer-long arms of the Livingston, Louisiana, facility are shown here, and at the apex of the L-shpaed structure is its control center. This facility, when coupled with the one located in Hanford, Wasehington, will allow us to detect gravity waqves with high precision, giving a view of the Universe in terms of non-equilibrium gravity. Photo courtesy of Caltech/LIGO.

To return to astronomy, current confidence and concerns are exemplified by the state of cosmology. The Big Bang model is beyond reproach, resting as it does on three pillars of evidence: the Hubble relation between distance and redshift, the microwave background radiation, and nucleosynthesis of light elements in the Universe' s first minutes of existence. After decades of grappling with the cosmological parameters, astronomers feel they are homing in on the blueprint of the Creator (see "The Unspeakable Act of Creation," Mar/Apr 1998, p. 9). Upcoming surveys and microwave satellites are supposed to usher in an age of precision cosmology.

Yet the current state of affairs raises some deep questions. Most measurements indicate that the Universe is expanding at a rate given by a Hubble constant of H0 = 60 km/sec/Mpc (for every additional megaparsec of distance from the Milky Way, objects recede 60 km/sec faster due to the expansion of space). Recent research also indicates that space is flat, with a contribution of thirty percent to the closure density from matter (WM = 0.3) and seventy percent from some form of vacuum energy (Wl= 0.7). About ten percent of the Universe' s matter density is baryonic, while the majority is dark matter. Why do normal matter and dark matter have similar cosmic densities within an order of magnitude, even though the fundamental physics of the two types of particle is very different? Why is the entropy of the Universe so high, about 109 photons for every baryon? Why does the vacuum energy density of the Universe appear to be 120 orders of magnitude higher than quantum theory predicts? Why is the matter density so similar to the energy density, which must be a timing coincidence, given that the former declines as the Universe expands while the latter is constant?

Maybe the answers to these questions are just around the corner. Perhaps they involve years of struggle. It is likely that we will not fully understand our huge, old, and cold Universe until we understand its state in the first tiny fractions of a second after the Big Bang. Finally, a warning: consensus is not always correctness. We might be wrong in our thinking or we might be ignoring vital data that subvert the prevailing theory. Isn' t it just a little egotistical for the current crop of cosmologists to believe that they are the ones to finally crack all the difficult problems?

The End of the Beginning

Astronomy has seen spectacular advances in our perception of the Universe. For thousands of years, humans grew comfortable with the idea of a small universe wrapped around Earth. The Copernican revolution dislodged humans from the center of creation. We then learned that the stars were furnaces like the Sun, spread over thousands of lightyears. Subsequently we discovered vast stellar systems like our own, spread over millions of lightyears of empty space. Later we learned that we are not even made out of the kind of stuff that most of the Universe is made of.

Unfinished rebolutions. After discovering that we are located on just an average planet orbiting an average star in an average galaxy among fifty billion, we face the prospect of finding out whether or not biology is common in the Universe. In an empirical science like astronomy, revolutions cannot always be anticipated. Illustration courtesy of the author.

The humbling progression that began with the Copernican revolution is not yet over. Soon we will use new telescopes to inspect the atmospheres of extrasolar planets and see if they contain chemical traces of metabolisms at work. Is life a rare fluke or is it liberally sprinkled through the cosmos? We are just beginning to discover whether or not we live in a biological universe. Is our Universe unique? The Big Bang model predicts that the observable universe is much smaller than the physical Universe, and chaotic inflation postulates many universes emerging from quantum space-time foam, but we do not yet know whether or not science can address this question. Don't expect astronomers to retire in droves any time soon. Rather than the beginning of the end, it is the end of the beginning.

CHRIS IMPEY is a Professor of Astronomy and Deputy Department Head at the Steward Observatory of the University of Arizona in Tucson. He was born more than four decades ago in Edinburgh, Scotland, on Robert Burns' s Day. Since then he has authored numerous research papers on ultrafaint galaxies, quasars, and gravitational lenses, as well as writing with William Hartmann the introductory astronomy text, The Cosmic Journey (Wadsworth), and The Universe Revealed (Brooks/Cole). Away from astronomy, he is a reader of fiction and history, and a keen runner. He is also a traveler, preferring to roam off the beaten track in places such as Nepal, Patagonia, Russia, and Sumatra. He can be reached via email at cimpey@as.arizona.edu.