The universe is mostly vacuum. In the remote regions between galaxies, you would be lucky to find a single atom in a space the size of the Louisiana Superdome. The atoms are packed more densely in our own world of solids and liquids and gases, but even here it is not as crowded as you might think. A close-up of an atom would reveal that the nucleus, which carries 99.9 percent of the weight, hovers in the center of the atom like a BB suspended in the Superdome; except for a few electrons that waft about the stadium like ghostly gnats, the rest is empty space.
So since we and our world are made of such insubstantial stuff, it is surprising how much thought and energy (not to mention money) scientists spend trying to unravel the riddle of matter. Shouldn’t they be worrying instead about the nature of the vacuum, which is by a wide margin the major constituent of the universe?
In fact, many have been doing just that, and what they find is startling. The vacuum is a far busier place than it would appear to be. That is not news, although it is a twentieth-century discovery, a consequence of quantum mechanics. What is new is the realization, just within the past few years, that this dynamic vacuum can be shaped and manipulated. Soon it may become a working component of novel electronic devices. Modern physics has transformed the vacuum from a passive stage for the affairs of matter into an active entity. The vacuum is; but it also does.
That is a fitting resolution of an ancient duality, of a battle that has surged back and forth across the heart of physics and philosophy for nearly 2,500 years. Unlike the existence of matter, which has not often been called into question, the existence of the vacuum has been the subject of controversy since science began.
The vacuum was invented as part of the atomic hypothesis by the Greek philosopher Leucippus and his student Democritus in the fifth century b.c. Not much of their work has survived, but one fragment by De-mocritus retains its freshness and power: By convention there is sweet, by convention there is bitter, by convention hot and cold, by convention color; but in reality there are only atoms and the void. All these centuries later, this insight remains a succinct summary of the modern conception of matter.
Democritus needed the void to make sense of the world. If matter were really as unbroken and continuous as it seems, where would a knife find room to begin the process of slicing into a piece of wood, for example? How would milk find room to dissolve in water? Such puzzles are solved convincingly if there is a vacuum between atoms--to accommodate the edge of the knife, or to allow milk atoms to intermingle with water atoms. Nevertheless, the ideas of Democritus soon ran into trouble. The trouble was, you couldn’t see atoms. As a result atoms and the vacuum that separates them were rejected by philosophers who had less intuitive insight into the architecture of the world.
The most powerful opponent of the atomic theory was Aristotle, who found a total void philosophically unacceptable and who, unfortunately, was destined to be regarded as the supreme authority on scientific matters for more than 1,000 years. Aristotle filled the vacuum with the ether. The ether was the stuff of the stars and the heavens, but it also permeated the four elements--earth, fire, air, and water--of our lowly world.
As an idea it had remarkable staying power. Even when the four elements turned out not to be elemental, the ether survived, just as it survived the demise of Aristotle’s original reason for invoking it. By the seventeenth century his philosophical objections to the vacuum had been deflated; scientists had actually made a vacuum, or something close to one, with the help of the newly invented vacuum pump, a device that sucks air out of a sealed container. Although these experimental vacuums were not perfect (we still can’t make a perfect vacuum), it had become at least possible to imagine a space that was totally empty. But the ether lived on.Indeed, it rose again to become an essential component of nineteenth- century physics.
The instrument of its revival was the wave theory of light. When light was found to consist of waves, the question naturally arose: Waves of what? We are used to sound waves in air, water waves in the ocean, and even amber waves of grain, but we cannot imagine waves of void. Yet light, unlike sound, can travel through apparently empty space--it reaches us from the sun, for instance. Space, physicists reasoned, could not really be empty. It must be filled with an ether.
This ether, though, had more definite physical properties than Aristotle’s. It was known that sound waves move faster in a denser medium, such as water, than in a thinner one like air. Since the speed of light is so tremendously high--186,000 miles per second--the ether had to be exceedingly firm, even solid. And yet planets move through it without encountering any detectable resistance. It was strange stuff indeed, this ether, at the same time denser than steel and more tenuous than air, but the physicists of a century ago could see no way to do without it.
From 1887 on, however, they began to have serious doubts about its reality. In that year the American physicists Albert Michelson and Edward Morley conducted an ingenious experiment designed to prove the existence of the ether. If Earth was moving through a stationary ether, they figured, it should be feeling an ether wind. And when light was bucking this head wind, it should move more slowly than when it was cutting across the wind. Michelson and Morley built a device called an interferometer that could measure this tiny effect. They found nothing; the speed of light was constant. So the ether’s foundations were shaken.
In 1905 Albert Einstein brought the whole 2,400-year-old house down. In the introduction to his first paper on the special theory of relativity, which ushered in a revolution in physics, he declared simply that the ether hypothesis was superfluous. To critics who might object that waves need a medium to carry them, he replied, in effect: That may be true for some waves, but for light it just doesn’t happen to be so. Exit the ether.
The vacuum thus cleansed remained empty for a quarter of a century, but then it began to fill up again--this time with the conceptual fruits of quantum theory. Quantum mechanics was introduced in 1925 as a replacement for Newton’s mechanics in the description of atomic phenomena such as the emission and absorption of light. It turned out to have implications for the vacuum as well. The key concept here is the famous uncertainty principle, which forbids the position and speed of a particle from being determined with certainty at the same time. As Tom Stoppard put it in his play Hapgood: When you know what it’s doing, you can’t be certain where it is, and when you know where it is, you can’t be certain what it’s doing.
The uncertainty principle is not just a description of our mental limitations, though; the position of a particle really is inherently uncertain. One consequence is that particles and other systems in motion have what is called a zero point energy. The internal vibrations of a molecule, for example, can never be eliminated altogether. There always remains a last, irreducible quivering, like that of an aspen leaf in the wind. The position of the molecule remains uncertain even when it is practically at rest. This zero point motion has actually been measured in the laboratory, where it manifests itself as a blurring of the light given off by molecules.
Zero point energy affects the vacuum as well. Light, according to the theory James Clerk Maxwell put forth in the nineteenth century, consists of electric and magnetic fields that oscillate and feed on each other as they travel along. According to quantum theory the oscillating fields are afflicted with zero point energy. So they never quite vanish. A vacuum that is utterly dark is nonetheless suffused with an electromagnetic field that fluctuates in gentle random waves of all wavelengths, each wavelength with its own zero point energy. What’s more, since there is an infinity of these vacuum fluctuations, the sum total of all zero point energies, even in a compact volume of, say, a cubic inch, must be infinite. The impossibly dense ether has been replaced by an infinite energy density pervading the entire universe.
Not surprisingly, free-spirited inventors have proposed all kinds of ideas for harvesting energy from the vacuum. But most physicists dismiss such schemes as fanciful. The vacuum energy seems to be like the gold in Fort Knox--it is there in abundance, but you can’t get at it without breaking a few laws, in this case the laws of physics. So physicists feel comfortable ignoring it.
But quantum mechanics predicts a phenomenon even more exotic than electromagnetic vacuum fluctuations. Occasionally a fluctuation carries enough energy to materialize into a pair of new particles. All of a sudden, for a brief moment, a negative electron and its antimatter twin, a positron, pop up out of nowhere. Together they preserve the electrical neutrality of the vacuum, and in an instant they annihilate each other and vanish without a trace. If there happens to be a strong positive electric charge nearby, however, the electron will be attracted to it, and the positron repelled, so during its brief lifetime the pair may line up like a compass needle. Thus the vacuum becomes momentarily polarized.
The dynamic vacuum is like a quiet lake on a summer night, its surface rippled in gentle fluctuations, while all around electron-positron pairs twinkle on and off like fireflies. It is a busier and friendlier place than the forbidding emptiness of Democritus or the glacial ether of Aristotle. At the same time it represents a synthesis of their antithetical concepts and a treaty in their long-running war: Democritus was right to insist that the world consists of atoms and the void, and Aristotle was right to say there is no such thing as absolute emptiness. As a theoretical conception the dynamic vacuum has always held great appeal. But its existence had to be proved in the laboratory.
The experimental discovery of the modern vacuum has unfolded as a drama in three acts, the first of which took place right after World War II, at Columbia University. In 1947 Willis Lamb and his assistant Robert Retherford applied the radar technology developed during the war to the study of hydrogen. Hydrogen has only a single electron, and by 1947 there was a well-developed theory to explain the quantized energy jumps the electron makes as it absorbs or emits radiation. By shining microwaves onto hydrogen and observing which wavelengths were absorbed and which were not, Lamb and Retherford measured the energy levels precisely. They found a minute discrepancy between theory and experiment, amounting to one part in a million--the so-called Lamb shift.
The discrepancy was immediately attributed to the effect of the vacuum. As the electron circles the hydrogen nucleus (a single proton), it is exposed to the fluctuations of the vacuum, which cause it to jiggle back and forth ever so slightly. This effect blurs the electron’s path, changing its energy by a minute amount. To make matters more complex, the polarization of the vacuum also contributes to the Lamb shift, but because it results from only those exceptional fluctuations that happen to carry high energy, the effect is much smaller. The precise agreement between the experimental value of the Lamb shift and its theoretical explanation in terms of vacuum fluctuations and polarization is clear evidence for the influence of the void on atoms.
The second act of the drama took place in 1948 in an industrial laboratory. Hendrik Casimir, working at the Philips Research Laboratories in the Netherlands, was trying to determine if the vacuum might be responsible for a detectable force between two neutral atoms. Because atoms are complex, three-dimensional things, Casimir first tried to simplify the problem by imagining instead two idealized, two-dimensional objects: two large, parallel metal plates in a vacuum. He assumed that no gases were present to push on the plates, that they were cool enough for thermal radiation to be negligible, that they were both electrically neutral, and that their mutual gravitational attraction was too small to matter--in short, that there was nothing in classical physics to cause a force between the plates.
When Casimir considered the quantum-mechanical vacuum, however, he realized that a range of electromagnetic fluctuations that exist outside the plates would be excluded from the gap between them. And this restriction would have a measurable effect.
The Casimir effect, as it is now called, is not at all obvious, and it is hard to find an everyday analogy for it, but maybe a somewhat artificial one will help. Imagine an enormous water-filled tank in which you can somehow generate waves of every wavelength (the distance from one crest to the next) and send them off in all directions. In the center of the tank there is a flat, vertical wall. Waves of all kinds, from tiny ripples to long rollers, push the wall from the left. But similar waves push from the right, so the forces cancel.
Now consider another flat wall, parallel to the first and ten feet away from it. In the space between the walls the water ripples just as merrily as on the outside (again, don’t worry about how the ripples are generated). But there are no long rollers. The reason is simple: To conserve the total amount of water, waves must have both crests and troughs. So only waves whose entire wavelength can fit in the ten-foot gap will grow there. The net result is that there are long waves striking the walls from the outside which are not counterbalanced on the inside. Those waves push the two parallel walls together.
Metal plates exposed to vacuum fluctuations behave very similarly. A simple calculation led Casimir to the surprising prediction that the exclusion of long-wavelength fluctuations from the gap between two neutral plates causes the plates to attract each other. His claim was soon put to the test, and the force, though exceedingly feeble, was found to be real. In spite of its puniness, the Casimir effect is so surprising that more than 40 years later it continues to exert a powerful fascination on physicists. In 1987 Casimir proudly cited 483 references to his discovery.
Meanwhile the curtain has risen on the third act in the drama of the dynamic vacuum. The latest developments concern the effect of a vacuum on the absorption and emission of radiation by individual atoms. When an atom is excited, having absorbed energy by exposure to heat or light or an electric spark, it soon decays back to its original state. In the process it spontaneously emits light or radio waves into the surrounding vacuum. (This is what goes on in a light bulb.) Spontaneous emission used to be thought of, common-sensically, as an intrinsic property of the atom, as inevitable as a bomb’s explosion once the fuse is lit.
But what happens if the vacuum isn’t prepared to receive the radiation? The Casimir effect suggests how that situation might arise. If the vacuum happens to be enclosed in a metallic cage so narrow that not even one wavelength of the radiation can fit in it, the vacuum will not be receptive. The spontaneous decay of the atom will be prevented.
This is the crux of the modern exploitation of the dynamic vacuum. It rests on the realization that spontaneous emission is not something an atom does all alone but is instead the result of an interaction between the atom and the vacuum. Spontaneous emission can be artificially inhibited by adjusting the geometry of the vacuum (for example, by varying the gap between two plates). It can also be enhanced, surprisingly enough, by putting excited atoms in a container that resonates to their specific wavelength, the way an organ pipe resonates to a certain wavelength of sound. In such a container an excited atom will emit radiation and return to its ground state more quickly than normal. It’s as if the vacuum were pulling radiation out of the atom.
In the MIT laboratory of Daniel Kleppner, one of the leaders in this research, the focus is on the first process, emission inhibition. Kleppner’s device consists of two parallel copper plates, approximately six inches long and a sixteenth of an inch apart. A beam of cesium atoms is excited to a state of high energy and then directed into the gap between the plates. Just before the atoms enter the channel, they are oriented in such a way that their spontaneous radiation, if they emit any, will be directed perpendicular to the plates. At the far end of the plates a monitor counts how many cesium atoms have radiated and how many have made it through the gap without radiating.
When the gap is relatively wide, most of the atoms radiate and decay back to their ground states during the six-inch trip. The monitor shows a steady, weak signal revealing that few of the cesium atoms remain excited. But when the gap is narrowed until it is only half a wavelength wide--that is, until it can accommodate no more than a single crest or trough--things suddenly change. The monitor signal shows that many more atoms remain excited--apparently because spontaneous emission has been inhibited.
Nabil Lawandy and Jordi Martorell at Brown University have achieved similar results with a different experimental design--a lattice of polystyrene balls, each about a millionth of an inch in diameter, stacked like cannonballs in a bath of water. The balls were so small they made the water look like milk. Lawandy and Martorell introduced molecules of dye into the tiny spaces between the balls and then excited the molecules with a burst of laser light. Normally the molecules would have quickly returned to their ground state by emitting red light. But in the tight spaces between the balls there is not much room for red light. The result, the researchers reported in October 1990, is that the dye molecules remain in their excited state almost twice as long as they normally would. Since the normal lifetime is only a few billionths of a second, the effect is not spectacular. But it is a first step toward the complete suppression of spontaneous emission.
The payoff, if that goal can be achieved, would be improved lasers. The last three letters in the word laser stand for stimulated emission of radiation and refer to the principle of the device’s operation. Stimulated emission is an artificially induced type of radiation, and it is spoiled by the random, undisciplined competing process of spontaneous emission. If spontaneous emission could be eliminated, lasers could attain unprecedented levels of efficiency, purity, and power.
Better lasers are just one of several technologies that may emerge from our newly discovered power to modify the vacuum. From a more fundamental point of view, though, the manipulation of the vacuum illustrates one of the principal lessons of modern physics. Contrary to its fragmented appearance, the world is, at its most basic level, a connected unit. The walls of a container, the vacuum it encloses, and the atom in its center can no longer be imagined as separate entities. Like other great philosophical dividing lines--between subject and object, mind and body-- the line between matter and vacuum has become blurry.
By intuition and reason Democritus penetrated to the very heart of nature when he saw that reality is to be found in atoms and the void; by intuition and reason Aristotle realized that no space could be absolutely empty. But neither man could have foreseen what we are learning today: that stars and atoms and the vacuum are all part of a single, seamless whole.
