GRAVITY HAS ALWAYS appeared the odd-force-out in physics. It is strong enough over large distances to organize the world of planets, stars and galaxies. Yet it is phenomenally weaker than other forces. In the microcosm of atoms and subatomic particles, gravity plays no perceptible role.
Today there is a revolution brewing in physical theory which is radically changing our understanding of gravity. We have known for centuries that gravity is what keeps the moon in its orbit. What we are beginning to learn now is how, mechanistically, the earth achieves that feat. The new theory also fits gravity into the other, mysterious forces of nature. This "unified-field theory" is the "holy grail" of physics. Einstein died still seeking it. Some of the fundamental forces have already been unified in the last decade, and it appears that the last force to fall into place will be gravity.
Theoretical physicists are hard at work on a theory of "quantum gravity," called the "superforce" by physicist Paul Davies. In his book with that title, he writes, "for the first time in history we have within our grasp a complete scientific theory of the whole universe in which no physical object or system lies outside a small set of scientific principles." All the mysterious subatomic particles will live in its framework and interact with each other according to well defined rules.
The "superforce" is not an extra-strong force. It is a theory that would relate gravity to the rest of nature in a comprehensive, mathematically stated law, as elegantly as EeMC2 expresses the relationship between energy and mass. The four fundamental forces become aspects of one force -- and this unified force we call the "superforce."
In superforce theories, the very fabric of space and time acquires a complex structure far beyond our normal apprehension of space as uniform emptiness.
Physicists working on the superforce speak seriously of the universe having 11 dimensions -- the three space dimensions we're aware of, time, and then seven more space dimensions. They say we are normally unaware of the other dimensions because they manifest themselves only to a small extent, and then only in the world of subatomic particles.
The superforce also connects with new thinking about the origin of the universe that describes the first moments of time and space 10-20 billion years ago at the start of the "Big Bang" which created space and time.
Summarizing the profound new direction of physics, Davies states, "The world, it seems, can be built more or less of structured nothingness. Force and matter are manifestations of space and time. If true, it is a connection of deepest significance."
Gravity was the first force of nature to come under scientific scrutiny back in the 17th century. Our entire scientific and technological age was ignited by the study of the motion of bodies under its influence. Ascribing mathematically precise laws to the inner workings of nature was a revolutionary concept essential to technological development. But the road to success was not easy. Galileo needed the courage to challenge the Aristotelian view that heavier bodies fall faster than lighter ones. In establishing Aristotle's error -- proving that all bodies accelerate toward the ground identically -- Galileo started a path that Newton and then Einstein pursued with a vengeance.
Isaac Newton, a sickly ninth child born in the year Galileo died -- 1642 -- was a 23- year-old college student escaping to the countryside to avoid the bubonic plague. In that interlude, Newton formulated his law of universal gravitation: All objects exert gravitational attraction on each other. The pull of gravity between two objects increases the closer they are. And if one object comes twice as close to another, the attraction is not doubled, but squared.
It was astounding to be able to describe the motion of heavenly bodies with rules that would also govern small objects. This law sufficed for centuries to describe accurately the course of planets and their moons, the tides and the fall of apples to the ground.
In 1916, Einstein, in his theory of general relativity, further revealed that this mysterious force of gravity even acted on light: In a famous 1919 eclipse observation, it was confirmed that a star's light would curve a predicted amount as it passed near our sun. Einstein explained gravity as the curvature of four-dimensional space-time caused by matter.
But the question with which science is still wrestling is: How does this happen? We can describe in great detail the electrical forces that make a radio work. We still only have the vaguest idea, however, by exactly what mechanism the sun holds the earth in its orbit. The latest thinking is that all matter emits particles of force called gravitons and gravitinos that go out in the universe and create the effects that we understand as gravity.
It is the answer to this question -- how, mechanistically, does gravity work? -- that may finally unlock the riddles of all the forces in the universe, and how space and time were created in the Big Bang.
The modern history of making the forces that control nature fit together starts with the 19th-century notion that electromagnetic waves, such as sunlight, traveled through an invisible medium in space called "the ether" much the way sound travels through the air. The trouble with that theory turned out to be that unlike sound, the speed of light is unnervingly constant for all observers, no matter how fast they were traveling toward the source of that light.
Einstein resolved the paradox by dispensing with the ether and formulating an intimate relation between space and time -- his theory of special relativity. Common sense notions of time became extinct. "Now" for us here on Earth is not the same as "now" in a speeding spaceship. The flow of time was inextricably connected with space. Events occurred in four-dimensional "space-time" rather than in space alone.
At about the same time, the notion of the atom being a hard billiard ball had been dealt blows by accumulating contradictory evidence. It began to show structure -- an electrically charged nucleus surrounded by electrons. A new theory -- quantum mechanics -- had to be developed to explain atomic phenomena.
In quantum mechanics, light, for example, could have dual attributes of particle and wave. It also does not behave according to cause and effect. Rather, its behavior is described by the laws of statistics. Subatomic particles, which are also covered by quantum mechanics, make transitions in their states of motion, position, spin, etc. that are impossible to predict. They behave unlike anything in the "real" world. The probability of occurrence of a behavior can be computed with great precision, but the exact time, location, form, etc. of the behavior cannot be predicted.
Quantum mechanics, in essence, tells us that at the subatomic level, "things just happen. Don't ask why." In quantum mechanics, scientists are confronted with physical behavior that they have to admit exists, but that, to the average person, just seems to make no sense. Events occur that have no observable cause. Particles appear to act on each other instantaneously over vast distances. Some of these particles, by the definitions in the theories scientists have been forced to create, will never be detectable no matter what our technology, because they exist in a zone of uncertainty that any conceivable detection experiment will never be able to get into.
This capriciousness continues to shake our world view. Einstein, though a major contributor to the development of quantum mechanics, refused to believe that "God plays dice with the universe." Today we know that God does.
As a theory, relativity is simple and elegant. Quantum mechanics, in contrast, is so bizarre that one physicist claimed, "Anybody who says he understands quantum mechanics, doesn't." Yet quantum mechanics is essential to modern technology from semiconductors to biochemistry. And it now appears that it is central to our understanding of gravity.
A pioneering theory of quantum mechanics has been developed to explain why one negatively charged electron will repel another negatively charged electron, much as the south pole of a magnet will repel the south pole of another magnet. The theory is that electrons give off "exchange particles." And it is these "exchange particles" that actually reach out and repel the other electrons at a distance.
In the case of electrons, these "exchange particles" that do the repelling are called "photons." And these photons, in turn, are merely visible light.
In this example, there are two kinds of particles -- the electron, and the photon that it throws off. The electron is the kind of particle that theorists think of as being associated with matter. The photon is the kind of particle that theorists think of as being mainly a force carrier. All kinds of particles that belong to the "mass-y" general class are lumped together as "fermions." All kinds of particles that are mainly force carriers are lumped together as "bosons."
Now, as it turns out, electromagnetism is not the only force that can be explained as the action of particles on each other. Two more of the "big four" forces in the universe -- the strong nuclear force, responsible for holding the atomic nucleus together, and the weak nuclear force, responsible for the decay of some particles -- can also be explained in this particle mode. They, too, are manifestations of "mass-y" fermion particles emitting mainly force-carrying "boson" particles, which actually produce the results that we understand as basic forces.
So exchange particles appear to explain electromagnetic force and the two nuclear forces, but until recently there was no corresponding exchange particle for the last of the major forces in the universe -- gravity.
Here is where quantum mechanics begins to fit into our understanding of the Big Bang that started the universe. Up until now, "unifying" these forces has meant that we understand them in basically the same way. As a result of particles that were discovered at the powerful European CERN nuclear accelerator in 1983, researchers now begin to think that at the earliest instant of the Big Bang, these forces were literally unified. The electromagnetic and the weak force really were the same force. Only one kind of force existed between particles at that time. This was all before the first trillionth of a second of time, when temperatures were 1,000 trillion degrees above absolute zero.
As the universe rapidly expanded, it cooled. Apparently, at these lower energy levels, these forces came to acquire their specialization in order to perform the functions they now do.
Now there has also been a conjectured unification of the strong nuclear force with the electro-weak force in so-called "Grand Unification Theories" or GUTs. If these theories are valid, they produce a prediction that all protons in the universe will ultimately decay. Thus, to check the theory, scientists have been hard at work to detect the extremely rare decay of a proton. Some believe that evidence of decay has been found.
If this force unification were ever literally true -- if all four forces, including gravity, were once really one -- it would have been even earlier in the Big Bang. It would have been an almost-inconceivable one ten-million-trillionth-of-a-trillionth of a second of time.
So now scientists think they have three of the four major forces brought into the fold -- both conceptually and possibly literally. Only gravity is still the odd- force out. If someone could make gravity fit, he or she would explain the "superforce."
Theorists since the early 1970s have been trying to find a way to express the mechanism of gravity in quantum mechanical terms. In short, the question is, are there particles of gravity? And if so, how exactly do they work on each other? The brilliant British physicist, Stephen Hawking, is one who believes that a complete superforce theory will soon be at hand that will be able to explain all of nature.
Yet the pendulum of physics is swinging again, bringing in new geometries of higher dimensional space to explain the particle interactions of the superforce. In particular, a five dimensional space-time geometry, developed by Kaluza and Klein in the 1920s to tie gravity and electromagnetism together, is now being dusted off to understand the superforce. By adding seven space dimensions to four-dimensional space-time, physicists have been able to describe their menagerie of particles as the curling of space-time at incredibly small scales. We are unaware of these added dimensions because their manifestations are far too small to observe.
Down at dimensions a billion trillion times smaller than that of an atomic nucleus, space has a foam-like structure, according to this theory. Gravitational theorist Bryce DeWitt, writes that, " . . . An observer who attempts to penetrate the fourth spatial dimension is almost instantly back where he started: Indeed, it is meaningless to speak of such an attempt, because the very atoms of which the observer is composed are larger than the cylindrical circumference (of the fourth dimension). The fourth dimension is simply unobservable as such."
Our notions of space and time are sorely tested by quantum mechanics' attempt to explain the superforce. Tiny wormholes, tears and lumps may appear in a statistically fluctuating fabric of space-time at the minute level. Bryce DeWitt writes, "In a universe governed by quantum gravity, the curvature of space-time and even its very structure would be subject to fluctuations. Indeed, it is possible that the sequence of events in the world and the meaning of past and future would be susceptible to change."
Paul Davies writes that, "The orderly arrangement of points, the smooth continuity of the space of classical geometry, disappears in the froth of space-time. Instead we have a melee of half-existing ghost spaces all jumbled together. In this chaotic shifting sea, the common sense notion of 'place' fades completely away."
If the superforce seems remarkable to you, it has also put many physicists in awe. The final, numbing suggestion it offers involves mass-related "fermion" particles giving off the force-related "boson" particles. Nothing, the physicists thought, could be more fundamentally different in their behavior. Yet now it is suspected that if we gain an understanding of the superforce, these two radically different classes of particle might be tied together by an underlying symmetry rule!
The superforce may not be the last important development in gravity research, though it might well be the final theory. Might we get control over gravity someday, much as we have mastered electromagnetism? Physicist Robert Forward has speculated that it might be possible to communicate via ripples in space-time -- gravity waves.
Now the prospect seems remote, because high densities of matter moving at near light velocity would have to be obtained to make gravity-wave transmitters. But gravity waves could penetrate the solid body of a planet with ease. Forward has suggested that tiny black holes left over from the origin of the universe might hover in the cores of planets and asteroids. If we could mine asteroids for them, electrically charge them, and move them around, we might have a source of high-density mass for future gravitational engineering projects.
What about antigravity? According to Robert Forward, there are at least three ways of getting antigravity through known physical laws, and other potential ways through speculative extensions of physics. Unfortunately, antigravity would be a costly proposition, requiring exceedingly high-density materials (the density of some dwarf stars). If we could fashion a very dense flat slab of material and support it on super-strong pillars (made of diamond?), the region beneath the slab would have a near zero gravity field due to the cancellation of Earth's gravity by the upward pull of the slab.
This Newtonian antigravity machine might not be very practical (except for amusement parks), but Newton himself could have conceived it. Forward has also described dense, twisting doughnuts of mass and other configurations that would give rise to antigravity fields allowed by special and general relativity. Whether humanity could ever harness these stupendous machines for space flight or other purposes is an open question.
Gravity was once the dusty curiosity of high-school physics labs with their inclined planes, spring scales and stop watches. It now appears that science has come full swing to view gravity as the central creative force in the universe. We knew that gravity stoked the fusion fires of stars, that it made planets and moons round and that it plotted the course of galaxies. We did not suspect that an enlarged view of gravity would be the key to all physics and the origin of all things.