Toward the Unification of Forces

Overview

Scientists develop theories as attempts to understand the physical world. The more observed phenomena a theory can account for, the more promising it is. One of the main goals of modern physics is to construct a single theory that would explain the four known forces of nature: electromagnetism, gravity, and the strong and weak nuclear forces. The philosophical implications of such a fundamental understanding of the workings of the universe have led many physicists to speak metaphorically of "reading the mind of God." But the ultimate "theory of everything" has yet to be found.

Background

The main objective of science is to find the underlying explanations for the phenomena we observe in nature. The wonderful variety of these phenomena might lead one to conclude that the underlying causes are just as numerous. Who would imagine, for example, that the long neck of the giraffe has anything to do with the sweet smell of flowers? Yet the theory of evolution explains them both quite nicely.

In the physical sciences, the behavior of objects is understood in terms of forces. A force is a vector quantity, meaning it has both magnitude (amount) and direction. A force is an impetus that tends to cause an object to move. The study of astronomy proceeded for thousands of years without anyone realizing that the motions of the planets are governed by the same gravitational force that keeps our feet on the ground. Finally, Isaac Newton (1642-1727) made this great leap in a realization symbolized by the famous legend of the falling apple.

Similarly, until the nineteenth century, electricity, magnetism, and optics were studied as three completely separate disciplines. There was no obvious reason for early experimenters to suppose that electrical charge, magnetized iron, and lenses were related in any way. However, James Clerk Maxwell's (1831-1879) theory of electromagnetic waves traveling at the speed of light swallowed up the entire field of optics and explained electricity and magnetism as different manifestations of the same force.

Thus the two known forces of nature at the end of the nineteenth century were the electromagnetic force and the gravitational force. Both of these forces presented the same philosophical problem: that of action at a distance. If you use a hammer to exert a force directly upon a nail, you would not be surprised to find the nail moving into the wood. On the other hand, the hammer couldn't cause the nail to move just by being somewhere in its vicinity. Yet the gravitational force holds the Earth fast in its orbit around the Sun, despite the 93 million miles (150 million km) of space in between. Likewise, there will be an attraction between a positive electrical charge and a negative one, whether they are in a metal wire or in empty space.

To get around this difficulty, nineteenth-century physicists began to use a new terminology to describe forces. Instead of regarding them solely in terms of the interaction between two objects, such as the Sun and the Earth, they began thinking in terms of force fields. Any object with mass has a gravitational field, whether or not another object is present. The force field interacts with other objects; for example, the gravitational field of the Sun keeps the Earth in its thrall. A charged particle such as an electron has an electrical field around it that acts on other charges.

Impact

The early twentieth century saw a revolution in physics. Albert Einstein (1879-1955), in his general theory of relativity, described gravitational fields in terms of the curvature of space and time. Not satisfied with merely superseding Newtonian mechanics, Einstein strove to develop a unified field theory that would include electromagnetism as well. He spent the last 25 years of his life working on this project, and he worried in his final years that he had failed either to construct such a theory or prove that one could not exist.

Einstein's blind spot was quantum mechanics. By the 1930s two more forces of nature had been discovered, both operating at the scale of the atomic nucleus. The strong force holds the nuclear particles together. The weak force participates in their decay. Quantum mechanics describes the behavior of atomic particles in terms of probabilities and maintains that it is impossible to determine both position and velocity precisely. Such a formulation was profoundly offensive to Einstein's idea of an ordered universe. His famous response was that "God does not play dice with the universe." By refusing to consider quantum mechanics, the great man condemned his unified field theory efforts to failure.

Physicists had better luck applying Einstein's special theory of relativity to electromagnetism. The mathematics of special relativity becomes important for objects moving very quickly, at a velocity comparable to the speed of light, which is 186,000 miles per second, or c. While nothing can go faster than c, massless particles can achieve this speed, and subatomic particles can come close. In quantum mechanics, particles' positions are expressed as probabilistic wave functions, leading to the idea of wave-particle duality. Light, or sub-atomic particles like electrons, become quantities that sometimes act like particles and sometimes act like waves.

In relativistic quantum field theories (RQFTs), every field has an associated particle, or quantum. The quantum of the electromagnetic field is the photon, a massless packet of energy. According to the theory of quantum electrodynamics (QED), the most successful relativistic quantum field theory, the photon is the carrier of the electromagnetic force.

Grand unified theories (GUTs) attempt to reconcile three of the four known forces: electromagnetism, the strong force, and the weak force. One important step in this direction has been the standard model of twentieth-century particle physics. While it continues to be extremely useful, it does have its deficiencies, including a number of parameters that must be juggled to keep the model in line with experimental values. However, during the last decades of the twentieth century, there were a number of important experimental confirmations of the standard model's predictions, and no experiments that contradicted them.

In the standard model, electromagnetism and the weak force are unified and called the electroweak force. Sheldon Glashow (1932- ) first proposed this unification in 1961. The quanta of the electroweak field, and thus the carriers of the electromagnetic and weak forces, are called intermediate vector bosons (IVBs). A problem with Glashow's theory was that the weak force required massive carriers to account for its short range, while the photon, carrier of the long-range electromagnetic force, is massless.

This conundrum was resolved independently by Steven Weinberg (1933- ) and Abdus Salam (1926- ). At low energies, these forces appear separate because of a field called the Higgs field. The Higgs field requires carriers with mass for the weak force and massless photons to carry the electromagnetic force. However, above a high threshold energy called the electroweak scale, about 100 billion electron volts, the Higgs field vanishes and all the IVBs are massless. Glashow, Weinberg, and Salaam shared the 1979 Nobel Prize in physics for this work.

The standard model also concerns itself with the strong force. The strong force holds together the hadrons (protons and neutrons) in the atomic nucleus. Each hadron is made of three fundamental particles called quarks. Quarks come in three categories called "colors," and so the theory of the strong force is called quantum chromodynamics (QCD). The carrier of the strong force between quarks is called the gluon.

GUTs generally hold that all three forces will be unified at some grand unification scale, perhaps at energies on the order of a quadrillion electron volts. This picture, in which increasing energies reveal first one force and then the next to be part of a unified whole, motivate particle physicists to propose ever more powerful accelerators. Unfortunately it is often difficult to induce non-physicists to share their enthusiasm, at least on a consistent basis. The much-anticipated Superconducting Super Collider was cancelled in 1993 after 14 miles of tunnels had already been dug in Texas, and $2 billion spent on the project. On a more theoretical level, particle physicists and cosmologists have found common interests in discussing the forces that existed in the extremely high energies of the early universe.

Supersymmetry theories unify matter particles, called fermions, with the force carrying bosons. It offers a framework for including not only the strong, weak, and electromagnetic forces of the grand unified theories, but also gravity, with its hypothetical force carrier, the graviton. Thus, supersymmetry may be a part of any four-force "theory of everything." Unfortunately, the current supersymmetry theories, constructed based on relativistic quantum field theories, have yielded mathematical infinities in inconvenient places. This may be a fault of the supersymmetry theories themselves, but is just as likely to be a problem with conventional RQFTs.

In string theory, first discussed in the late 1960s, the hadrons were understood as different oscillation modes of a "string" 10²⁰ times smaller than the diameter of the atomic nucleus. The strings hold the quarks together but are too small to be observed, and their effects become apparent only at extremely high energies. At lower energies, string theory reduces to the standard model. However, troublesome features of early string theories included as many as 26 spatial dimensions, and faster-than-light particles called tachyons.

In the 1980s string theory was combined with supersymmetry to yield superstring theory. The problematic tachyons disappeared, and the inclusion of supersymmetry pointed the way to bringing in gravity as well. Superstring theory doesn't solve all the problems, though; it introduces particles and interactions that have not been observed and does not seem to result in a solution as unique as one might expect a "theory of everything" to be.

The conception of gravity as a property of the very fabric of space-time continues to cause problems in developing a universal theory. Some physicists have suggested that gravity is not a separate force at all, but rather a byproduct of the other three. It is also possible that Einstein's formulation may break down at the Planck scale, corresponding to energies of approximately 10¹⁹ GeV.

As physicists consider the basic questions of the universe, many have the sense of treading on hallowed ground. The past few decades have seen an unprecedented number of books on physics invoking the name of God in their titles. Perhaps it is not surprising, then, that the universe does not give up its secrets easily. However, it is in the nature of humanity to keep looking for the ultimate answers. "I want to know how God created this world," wrote Einstein. "I want to know His thoughts. The rest are details."

SHERRI CHASIN CALVO