Acknowledgments................................................................ix Preface........................................................................xiii Prologue From Myth to Reality..................................................1 One Einstein's Toolkit, and How to Use It......................................27 Two The Realm of the Nebulae...................................................52 Three Let's Do Cosmology!......................................................89 Four Discovering the Big Bang..................................................102 Five The Origin of Structure in the Universe...................................130 Six Dark Matter—or Fritz Zwicky's Greatest Invention.....................174 Seven Dark Energy—or Einstein's Greatest Blunder.........................202 Eight The Modern Paradigm and the Limits of Our Knowledge......................229 Nine The Frontier: Major Mysteries That Remain.................................253 Appendixes.....................................................................263 Glossary.......................................................................281 Bibliography...................................................................291 Index..........................................................................295
Overconfidence among the Cognoscenti at the Dawn of the Twentieth Century
As the nineteenth century drew to a close and the new century dawned, an intellectual ferment spread across the disciplines comprising western culture. Art, music, literature, and science were radically transformed in the Modernist period to a degree comparable to the changes that occurred in the Renaissance and the Enlightenment. The revolutionary expansion of our cosmic consciousness, which we will detail in the next chapter, was paralleled by the revolution in our scientific tools—the laws of physics. But at first no one saw the changes that were coming in both physics and astronomy. The complacent, turn-of-the-century belief that the laws of physics were essentially nailed down, with only refinements to follow, was blown apart by Albert Einstein, who single-handedly initiated the revolutions of quantum mechanics, special relativity, and general relativity. In the next twenty years, he and his colleagues established the new physics that provided the foundations for our modern cosmology.
Let's begin our exploration of the physics revolution in 1894. Professors have gathered for a lecture in a new, neo-gothic building, a curious amalgam of the ancient and the modern at the just established University of Chicago. In his ceremonial, opening address, Albert Michelson, the director of the laboratory, said:
The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote. Future discoveries must be looked for in the sixth place of decimals.
How wrong he was. Michelson himself went on to become the most accomplished optical experimentalist of his age and to perform the Michelson-Morley experiment that helped to confirm Einstein's revolutionary new physics. Simon Newcomb, head of the United States Nautical Almanac Office, had dismissed the possibility of future astronomical discoveries somewhat earlier, in 1888, but then later gave us the best measurement of the speed of light.
Even today, books proclaiming the end of scientific discovery are published. As recently as 1996 one of us was asked on a popular television program to comment on a new book, The End of Science, which maintained that we had come to the end of discovery and in fact were at the end of the investigation of the big questions. It was fairly easy to make the case that we still did not know the answers to the really big questions that any child might ask: How did the world begin? How will it end? What is the world made of? Are we alone in the universe? Clearly, big questions remain to be answered.
But in the late nineteenth century, because Newton's laws of motion and gravitation seemed to have successfully explained dynamical phenomena in the heavens and on the Earth, it really did appear to many physicists and to astronomers that the operation of physical laws was clear and that our knowledge of the universe was essentially complete. The subject of astronomy was primarily understood to be measuring the positions of the planets on the sky, computing their expected positions, and comparing the two approaches with honesty and care. True, the calculations could be tediously complex; but everything could be accounted for in principle, even if the applied mathematicians still had to work out the details to six or more decimal places.
Everything also appeared to be satisfactory with the laws of electricity and magnetism. A bushy-bearded Scotsman, James Clerk Maxwell, had brilliantly unified electricity and magnetism in 1861 with the introduction of Maxwell's equations, thereby providing a new synthesis of existing physical laws. The science of thermodynamics, so useful in the industrial revolution for understanding heat, temperature, and steam engines, could likewise be considered mature, with little potential for further development. It seemed to some that physics was being killed off by its own success; instead of pursuing natural philosophy, the physicists would have to occupy themselves doing applied science.
Scientists had no way of knowing that their physics and astronomy was, in fact, hugely incomplete and that great breakthroughs awaited them. To the extent that scientists at the time were aware of not having the full picture, the knowledge gaps seemed slight. For example, in astronomy there was a slight worry over the motion of the planet Mercury, the elliptical orbit of which rotated with respect to the Sun at a slightly faster rate than predicted by Newton's laws. But this "advance of the perihelion of Mercury" appeared to many as a small detail that the mathematicians would surely resolve, given more time. Looking further out, many felt that the mysterious spiral nebulae—milky patches in the heavens that piqued the interest of natural philosophers and astronomers from Immanuel Kant to William Herschel—were planetary systems in the making. The idea proposed by Kant and others that they were "island universes" comparable to our own Milky Way galaxy had been dropped, as had William Herschel's claim that nebulae were star clusters. In 1885, the astronomer and writer Agnes Mary Clerke had this to say on the structure of nebulae:
The question whether the nebulae are external galaxies hardly any longer needs discussion. It has been answered by the progress of discovery. No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way. A practical certainty has been attained that the entire contents, stellar and nebular, of the sphere belong to one mighty aggregation.
As the nineteenth century passed into history, the conviction was widespread that the entire universe consisted of just one island, the Milky Way, and that we, on planet Earth revolving around the Sun, were centrally located in that galactic system. The facts of astronomy and the laws of physics were completely determined.
* Revolution in Physics: The Inception of Quantum Theory and Relativity
The startling new discoveries that transformed the dominant, but incomplete world-view of the new century did not emerge from a laboratory or even a telescope; rather, they were the result of pure thought. Einstein detected two glaring inconsistencies of logic within the scientific canon. First, he realized that two sets of experiments analyzing the paths taken by ordinary light were apparently inconsistent with one another. And second, he realized that Maxwell's equations, which explained all experimental data in the fields of electricity and magnetism, were actually inconsistent with Newton's laws.
In the first breakthrough, he realized that some accepted experiments were inconsistent with the wave nature of light that was fundamental to Maxwell's equations—and this led him to postulate a particle aspect to light rays, a development that immediately pushed physics toward the quantum interpretation of microscopic phenomena and which ultimately upended all of classical physics.
This all happened in 1905, Einstein's annus mirabilis, during a four-month frenzy of research activity while he was still employed as a clerk in the Swiss patent office in Bern. Here, in translation, is his summary of the breakthroughs, extracted from a letter he wrote in May 19o5 to his friend Conrad Habicht:
Why have you still not sent me your dissertation ... you wretched man? I promise you four papers in return. The first deals with radiation and the energy properties of light and is very revolutionary ... The second paper is a determination of the true sizes of atoms ... The third proves that bodies on the order of magnitude 1/1000 mm, suspended in liquid, must already perform a random motion that is produced by thermal motion. The fourth paper is only a draft at this point, and is on electrodynamics of moving bodies that employs a modification of the theory of space and time.
The first paper, which is, according to some scholars, the most revolutionary in the history of physics, contains Einstein's suggestion that light comes not just in waves but in tiny packets of energy, quanta of light, later called photons. At this moment in history, Max Planck had already hinted that energy is "composed of a very definite number of equal finite packages" as he stated in December 19oo at the Berlin Physical Society. But well-established diffraction experiments—in which a beam of light, passing through two slits in an opaque wall, can be seen to interfere with itself—had demonstrated the wave nature of light.
At the heart of Einstein's first 1905 paper was the central question bedeviling physics: is the universe made of particles, or is it the unbroken continuum of electromagnetic and gravitational fields described by classical physics? Einstein effectively argued that light could be both. That is, the particulate nature of light is an intrinsic property of light itself rather than a description of how light interacts with matter. Henceforth physicists would adopt this duality: light could behave as either a continuous wave motion (classical physics) or as a stream of quanta (quantum physics). Einstein's work made necessary the developments, which are still not complete, that could unify the classical and quantum modes of describing nature. Other physicists, including Paul Dirac, did succeed in the next decades, uniting quantum mechanics with electricity and magnetism. However, a quantum theory of gravity continues to elude our grasp.
Einstein's second and third papers provided evidence, from already accepted experiments, for the reality of atoms and molecules. The theoretical physicist Max Born recalled in 1949, "At the time atoms and molecules were still far from being regarded as real." The three papers thoroughly demonstrated the exceptional creativity of young Einstein; but, important as these works of 1905 were, our interest lies primarily in Einstein's fourth paper on special relativity, which startled the world by proposing new properties for space and time, thereby introducing revolutionary new ideas about the nature of the universe to the world and forever altering our concepts of physical reality. The invention of special relativity successfully effected the merger of Newton's dynamics with electricity and magnetism.
But none of this touched on gravity, a central component of Newton's laws and at the heart of all cosmic investigations. Einstein realized the defect and remedied it a decade later with general relativity, providing, at last, the update for Newton that was consistent with all known macroscopic experiments and also consistent in its formulation with Maxwell's laws. However, as we noted, the final union of a gravity and quantum mechanics has not yet been achieved, even to the present day, and it remains the holy grail of string theory and its rivals.
It is worth spending a little more time in these early years, since the stage was set then, philosophically, for all the subsequent reformulations of our concepts of space, time, and causality.
* Special Relativity
Two aspects of classical physics deeply impressed Einstein. First, there was Newton's comprehensible clockwork universe in which causes produced effects, forces acted on objects, and in theory everything could be predicted from initial conditions by the mathematical rules governing gravity, mass, force, and motion. Einstein liked this strict causality: "the profoundest characteristic of Newton's teaching," as he put it.
The second great pillar of classical physics to impress young Einstein was electromagnetism, the field in which Michael Faraday and lames Clerk Maxwell blazed the trail. In the mid-nineteenth century, Newton's mechanics was joined by Faraday's work on electricity and magnetism (the latter pioneered by America's first great scientist, Benjamin Franklin, in the mid-eighteenth century). Faraday showed that an electric current produces magnetism, and a changing magnetic field can induce a current in a conducting wire. Maxwell's equations united electrical and magnetic phenomena by showing that the coupling between the two is achieved by an electromagnetic wave.
At an experimental level the equations of Newton's mechanics and Maxwell's electromagnetism were known to be correct to a high degree of precision. But Einstein realized that, at a deeper level, they were actually in conflict. They were logically inconsistent with one another. Over time, Einstein came to a profound conclusion—that either Newton or Maxwell (or both of them for that matter) must be wrong somewhere; and eventually he resolved the conflict by thought experiments.
In the hope of finding what was wrong, Einstein considered how the experiments of two observers at motion with respect to one another must differ. First, he supposed that in one frame of reference an investigator finds that both Maxwell's and Newton's theories are exactly correct to an infinite degree of precision. He was then able to show that for the second, equally qualified observer, moving at some constant velocity with respect to the first observer, both theories would not be true. The experiments of the second observer would show that both theories could not be true: one or both were false. This followed from the intrinsic mathematical framework of the two theories and led to an impossible situation: the two theories might be correct for one observer, but then one or the other would be incorrect for another who is moving at a constant velocity with respect to the first one.
Which is the "right" observer? Einstein realized Newton's and Maxwell's theories were logically inconsistent, and he suspected that it was Newton's theory that was false. No physical experiments were performed and none were needed. In fact, the predicted failures would have been so small as to be undetectable given the experimental techniques of the time. This logical inconsistency between the two sets of accepted laws led him to invent special relativity as a first attempt to fix the Newtonian paradigm.
Unlike the majority of theoretical physicists, Einstein's working methods did not involve covering dozens of pages a day with trial-and-error equations. Rebellious of authority and skeptical of anything that could not be observed, he said of himself that he preferred "to think in pictures." Further thought experiments convinced him that the conflict between Newton's laws of mechanics and Maxwell's equations concerned the speed of light. Why, he wondered, did Maxwell's field equations include the velocity of light when Newton's did not?
He began with two postulates, the first of which is the principle of relativity: the fundamental laws of physics must be the same for all observers moving at a constant velocity with respect to each other. This is based on the intuition of Galileo and is called "Galilean Relativity." There is no experiment do that will tell you your absolute velocity, although observations can measure velocity with respect to some other observer: an experimentalist below decks in a smoothly traveling ship could never determine the ship's speed. Einstein's second postulate was that the speed of light through empty space is independent of the state of motion of the emitting body. At first Einstein had great difficulty in reconciling these two postulates with his thought experiments, which involved moving trains (a tradition that lives on in today's textbooks). His frequent visits to the huge Zurich train station with its synchronized clocks provided the setting. The solution really did come in a flash, and we'll use Einstein's thought experiment to explain it.
An observer is standing on the embankment of a railway track exactly halfway between two distant towns, A and B. This stationary observer sees two bolts of lightning simultaneously strike church steeples in towns A and B. At the same moment, there is a train on the tracks midway between A and B, and an observer inside the train is at the midway point. The train is traveling from A to B. The train observer is therefore moving toward B and away from A. While the light from B is rushing toward the observer on the train, the motion of the train will take the observer closer to the onrushing light signal from B and farther away from the light signal from A. The train observer therefore sees the flash from B before the flash from A.
This leads to a conclusion that would become immensely important in observational cosmology. Events that are simultaneous at one point of reference are not simultaneous for a moving observer at exactly the same spatial location. This may seem trivial, but it is not. It means that there is no absolute time. Time does not go tick-tock all over the universe independent of the observer's frame of reference. This shattered a premise that Newton had made in Principia more than two centuries earlier.
The equations of motion now became a little more complicated than the Newtonian versions. In order to write equations that would work perfectly in frames of reference that are moving at constant velocity with respect to each other, Einstein had to attach a fourth dimension, time, to the three spatial dimensions. That deft move altered the representation of geometry in the universe from three-dimensional graph paper to a four-dimensional space-time continuum. The future implications for cosmology were immense because, as we shall see, modern cosmology involves the interpretation of observations from parts of the universe that are both remote in time and are moving at enormous velocities relative to our vantage point in the Milky Way.
There is a fifth paper from the 1905 annus mirabilis. September finds Einstein penning another letter to Habicht in which he casually remarks that mass is a direct measure of the energy contained in a body, and he states that light carries mass with it. His paper, published in the journal Annalen der Physik, fills just three pages, but contains the most famous equation in physics, E = mc2, imprinted on thousands of T-shirts to this day. This formula means that the energy, E, equivalent to a body of mass, m, can be found by multiplying it by c2, the square of the speed of light. Because the velocity of light is so great, very small changes in the mass of an object can release or absorb enormous amounts of energy. The truth of this equation would be realized in atomic weapons, nuclear power, and as we will see, the shining of the Sun and most stars. It provided the essential clue to those scientists, half a century later, who tried and succeeded in solving the ancient problem of how the Sun continues to shine, of how it has sufficient fuel to continue, when all conventional sources of energy would have been exhausted long ago.
Excerpted from Heart of Darkness by Jeremiah P. Ostriker Simon Mitton Copyright © 2013 by Jeremiah P. Ostriker and Simon Mitton. Excerpted by permission of PRINCETON UNIVERSITY PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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