Introduction to What Is the Fundamental Nature of Space?
What does the space between this book and your eyes consist of? Although it looks like nothing at all, just empty space, we know that there are air molecules floating around. However, what if we were to get rid of all the air molecules and create a complete vacuum between the book and your eyes? Then surely that volume of space would be truly nothing—an empty void with no properties of its own. Right?
It may surprise you to learn that this “common sense” view of space is, in fact, wrong. Modern physics has shown that space—even an absolute vacuum—has a number of surprising properties. In fact, a vacuum is full of activity, with various kinds of particles winking in and out of existence and tiny bundles of energy interacting with each other. In the late 1990's, researchers were trying to gain a more complete understanding of the bizarre conditions that characterize the fundamental nature of space, one of the great remaining mysteries of science.
Our usual notion of space goes back to Aristotle and the other ancient Greek philosophers, who believed that space is a uniform void with three dimensions—length, width, and height. The long scientific process that ultimately changed this view began in the 1600's, with the work of the Italian physicist and astronomer Galileo Galilei and the English mathematician and scientist Sir Isaac Newton. Galileo described the motion of free-falling objects, and Newton expanded Galileo's findings into a universal theory of gravitation.
Newton discovered that the force of gravity between two bodies, such as the sun and the Earth, depends on the mass of the bodies and the distance between them. But he did not know how gravity works, beyond being some mysterious force that acts between objects. One reason why Newton was unable to discover how gravity works was that he accepted the classical notion of space as a uniform void that cannot change. It was not until the early 1900's that scientists abandoned this concept of space.
Einstein's View of Space
In 1915, the German-born scientist Albert Einstein developed the general theory of relativity, which explained gravity in terms of curved spacetime. Spacetime was a concept derived from Einstein's 1905 special theory of relativity, which demonstrated that time is a relative concept. For example, the theory implied that time would appear to move slower for a spaceship full of people traveling near the speed of light (299,792 kilometers [186,282 miles] per second) than for a stationary observer that the spaceship passes. The theory also implied a four-dimensional universe, in which the three dimensions of space are unified with the dimension of time.
Einstein's general theory stated that a massive object, such as the sun, curves spacetime around itself, much as a bowling ball would form a depression in a rubber sheet. Smaller objects, such as Earth and the other planets, move around the large object along paths in the curved spacetime, just as marbles would roll around the depression in the rubber sheet.
Therefore, gravity is not a mysterious force, as Newton thought it was. Rather, general relativity shows that the gravitational attraction between objects is actually a geometric effect. These objects fall toward one another along the curves that their masses produce in spacetime.
Quantum Mechanics
Although general relativity explains the effects of gravity around planets, stars, galaxies, and other large masses, it cannot account for phenomena that occur at very small distances, in the realm of atoms and subatomic particles. These effects are described by quantum mechanics, a branch of physics developed in the early 1900's by a number of physicists, including Niels Bohr of Denmark, Erwin Schrodinger of Austria, and Werner Heisenberg of Germany. Quantum mechanics explains how atoms absorb and give off units of energy and momentum called quanta. Quanta act as both particles and waves. Quantum mechanics also explains the behavior of the various particles that make up atoms.
Physicists have developed a quantum theory, known as the Standard Model of Particle Physics, to describe all of the fundamental particles that make up atoms. According to this theory, protons and neutrons, which make up an atom's nucleus (core), are themselves made up of tiny particles called quarks. There are six known kinds of quarks, varying in mass and electric charge. In addition, atoms contain negatively charged particles, called electrons, that orbit the nucleus.
The Standard Model also describes the forces at work in the subatomic realm, including the electromagnetic force (which holds atoms and molecules together), the strong force (which holds protons and neutrons together), and the weak force (which causes certain forms of radioactive decay). The forces are transmitted by particles called bosons. For example, bosons known as photons carry the electromagnetic force. The Standard Model does not include a description of gravity, which in the subatomic realm is much weaker than the other forces.
Another particle in the Standard Model is the Higgs boson, which had not yet been observed as of 1999. Particle physicists believe these bosons are generated by a field—the Higgs field—that permeates the vacuum of space, giving mass to all particles of matter.
Scientists thus have one theory (general relativity) to describe the behavior of large masses such as planets and another theory (quantum mechanics) to explain the behavior of atoms and subatomic particles. What is needed, physicists say, is a single theory that would somehow combine these two separate concepts. This “unified theory” would describe the fundamental principles that physicists believe underlie the workings of both general relativity and quantum mechanics.
Superstrings
Physicists began searching for such a theory soon after the development of quantum mechanics. However, progress was limited until the early 1980's, when John Schwarz of the California Institute of Technology in Pasadena and Michael Green of Queen Mary College in England began to develop an idea called string, or superstring, theory. This theory not only combined general relativity and quantum mechanics, it also sought to describe exotic entities that make up all the particles in the universe.
String theory states that quarks, electrons, and other particles of matter, rather than being pointlike, are actually tiny linelike objects called strings. These strings are incredibly small. Each is approximately the size of the Planck length, the smallest possible distance in spacetime—less than one billionth of one billionth the radius of an electron. Just as the strings of a violin can vibrate at different frequencies and produce varying musical notes, the tiny strings in spacetime can vibrate in many ways to create different types of elementary particles.
Besides making up all particles of matter in spacetime, according to the theory, strings also make up all the force-carrying particles that act on matter. Furthermore, the theory proposes that strings move within a curved spacetime. Therefore, string theory naturally incorporates the explanation of gravity provided by general relativity.
Physicists theorize that strings exist in two basic forms—open and closed. An open string has two free ends, while a closed string forms a loop with no free ends. Strings interact by splitting and joining, so that one string can break into two, and two strings can combine to form one. In addition, open strings can close up, and closed strings can break open. These interactions and motions help determine the kinds of particles and forces the strings give rise to.
One prediction that string theory makes is that spacetime has more than four dimensions. In fact, the mathematical description of string theory doesn't work unless physicists assume that strings vibrate in 10 dimensions. But where are these other dimensions, and why can't we see them? Physicists working on string theory propose that we cannot see the extra dimensions because they are wrapped up in tiny balls. Each ball may be the size of the Planck length, and there is a ball at each point in the three-dimensional space that we observe. Physicists believe that the properties of these rolled-up dimensions determine some of the properties of the elementary particles that occur in nature.
Quantum Geometry
Another approach to combining quantum mechanics with general relativity is known as quantum geometry, a theory begun by physicist Abhay Ashtekar at Syracuse University in Syracuse, New York, in the mid-1980's. Unlike string theory, which attempts to describe the underlying quantum nature of all the particles and forces that exist in spacetime, quantum geometry describes the fabric of spacetime itself as having quantum characteristics. According to this theory, space at its smallest distances resembles a weave of cloth with interlocking threads. The interconnections and interactions of the threads of the weave give rise to the spacetime continuum (continuous whole) at the heart of general relativity. Physicists are not sure how the behavior of these energetic threads of spacetime might relate to particles of matter or force-carrying particles.
Because the threads themselves make up spacetime, the tiny areas between the threads are regions where space and time do not exist at all. This is a concept that boggles the minds of even physicists!
Quantum geometry does not require the existence of extra dimensions to describe spacetime, as string theory does. Instead, it sees spacetime as a four-dimensional background on which elementary particles exist. However, the theory does not explain what makes up the particles, as string theory does. Some physicists speculate that the ideas of string theory and quantum geometry might eventually be incorporated into a single theory, though, as of 1999, physicists did not know how this might be accomplished.
As the year 2000 approached, physicists presented the world with a view of space almost unimaginably different from what most people take for granted. Theorists expected this conception to evolve further, leading to an ever greater understanding of the fundamental nature of space. It will be a challenge to the physicists of the new millennium to arrive at such an understanding, which could help us to better comprehend the universe as a whole.