En Route to a Grand Unified Theory: The Unification of Electromagnetism and the Weak Nuclear Force at the Turn of the 1970s.

Overview

Sheldon L. Glashow (1932- ), Steven Weinberg (1933- ), and Abdus Salam (1926-1996) jointly received the 1979 Nobel Prize in physics "for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including inter alia the prediction of the weak neutral current." The Glashow-Weinberg-Salam theory of electroweak interactions is, in fact, the first experimentally proven scheme at unifying in a single set of fundamental laws two of nature's four basic forces. This major step in theoretical physics greatly enhanced our comprehension of the universe and lies at the foundation of subsequent attempts at unifying all natural forces.

Background

Aristotle thought that all matter in the universe was made of four basic elements—earth, air, fire, and water—upon which only two forces acted: gravity—the tendency for earth and water to sink—and levity—the tendency for air and fire to rise. While levity lost its "scientific" credentials over the centuries, gravity became recognized as one of nature's universal forces. In Philosophiae Naturalis Principia Mathematica (1687), Isaac Newton (1642-1727) postulated a law based on the principle of action at a distance in order to explain how (though not why) every particle of matter in the universe is attracted toward one another. The more massive and closer to each other two material bodies are, the more they will feel the strength of that ubiquitous force. Today gravity is best understood by Albert Einstein's (1879-1955) general theory of relativity.

During the nineteenth century electricity and magnetism were synthesized into a complete and unique set of equations that now stand for electromagnetism, the second of the natural forces. This achievement was described in a Treatise on Electricity and Magnetism (1873), written by one of the most influential physicists of the last century, James Clerk Maxwell (1831-1879). Electromagnetism is the force that binds, for instance, negatively charged electrons to a positively charged nucleus to create what is called an atom. As with gravity, the distance over which the electromagnetic force acts is infinite though, contrary to the former, it can be either attractive or repulsive. Nowadays the theory that explains the nature of this force is called quantum electrodynamics (QED), one of the most successful theoretical concepts ever designed by physicists.

The weak nuclear force, third on our list of basic forces, is responsible for radioactivity, which is the property of unstable atoms to release energetic particles after the spontaneous disintegration of their nuclei. An example of the weak nuclear force is beta decay, which briefly converts a neutron into a proton by emitting an electron and an antineutrino. In 1933 the Italian physicist Enrico Fermi (1901-1954) developed a theory of beta decay that tried to explain what was at the time a not-so-well understood phenomenon. Only in the late 1960s, however, did a field theory proposed by Glashow, Weinberg, and Salam come to be viewed as the solution to the problem. (We will discuss this first field theory, also known as the electroweak force, later on.)

The fourth and final natural force is the strong nuclear force, which holds quarks together in the proton and neutron and glues the latter two into atomic nuclei. It is also responsible for nuclear fusion (which powers the Sun) and fission (used in atomic bombs). The strong nuclear force was the last of the basic forces to receive some kind of formal theoretical description in what is now called quantum chromodynamics (QCD), which was put forward during the mid-1970s. (The "chromo" prefix is explained by the fact that quarks composing atomic nuclei come in three different virtual "colors": red, green, and blue by analogy with the three primary colors of light.)

These four natural forces constitute our physical world; lying at the foundation of matter and energy and therefore of life itself. Their unification could lead to nothing less than the complete understanding of the origin of the universe. This is one of the reasons why the Glashow-Weinberg-Salam theory is so fundamental to our continual quest for knowledge.

Impact

In the early 1970s the Glashow-Weinberg-Salam theory of electroweak interactions was brought to the forefront of the physics community by experiments that were conducted at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, and at the European Organization for Nuclear Research (CERN) near Geneva, Switzerland. These experiments had one essential aim—finding whether weak neutral currents existed or not. The answer was given in 1973; it was, for some at least, an astonishing yes.

The neutral current is predicted by the Glashow-Weinberg-Salam theory, which was advanced independently by the three authors late in the 1960s. Their theory was first designed to give a complete account of the weak nuclear force in a conceptual framework akin to QED. In accordance with the latter theory, the attractive or repulsive force experienced between two charged particles is transmitted by a so-called force particle, or vector boson—a photon in the particular instance of QED. Hence, when an electron feels the repulsion of another electron, the phenomenon manifests itself through the exchange of a photon. Similarly, to make use of the aforementioned beta decay, the archetype of weak interactions, the Glashow-Weinberg-Salam theory assumes that the neutron radiates a force particle called W-, causing the former particle to turn into a proton; that W- particle then rapidly decays into an electron and an antineutrino.

Glashow, Weinberg, and Salam did more than explain all known weak nuclear interactions by suggesting the existence of two force particles, W- and W+. Their theory predicted yet another vector boson, Z₀, which describes, for instance, how a neutrino (a neutral particle) scatters an electron by emitting such a Z₀. The acknowledgement of this neutral current in 1973 launched a new era of modern physics. According to this original gauge theory, the electroweak force, which unifies electromagnetism and the weak nuclear force—since then recognized as two different aspects of the same interaction—can today be described as being mediated by a set of four force particles: the photon, W-, W+, and Z₀. All three of the new vector bosons were discovered experimentally at CERN in 1983.

Following the explanatory success of the theory of electroweak interaction, quantum chromodynamics (QCD) was developed during the mid-1970s in order to find a concise conceptual scheme for the strong nuclear force. (The strong nuclear force is mediated by force particles called gluons.) These two theories nowadays form what is labeled the "Standard Model." As it stands, the Standard Model is completely successful in its description of subatomic interactions—so successful, in fact, that experimental evidence that would contradict it has yet to be detected. But can this Standard Model actually be the final physical reality scientists are able to propose? Since the Standard Model is, in fact, composed of two distinct theories, why not try, some have asked, to unify them both into a unique theory?

Many attempts at a Grand Unified Theory (GUT) that would unite the electroweak force to the strong nuclear force have failed. In fact, a number of GUTs do exist but they are, to say the least, highly speculative. But what about gravity? Since the beginning of our discussion, gravity has been completely left out of the theoretical picture. The main reason for this situation is that no one, not even Einstein himself, was ever able to combine gravity (the theory of the very big) and quantum mechanics (the theory of the very small) into a single theory in which the principle of action at a distance would ultimately be replaced by a force particle called the graviton. Nevertheless, mathematical physicists continue to hope to find not only a GUT but a TOE a "Theory of Everything" as it is sometimes comically referred to, that would encompass the four natural forces. Many have put their faith of a TOE in superstring theories but, unfortunately for now, to no avail.

How could a Grand Unified Theory be corroborated experimentally? The energy required to verify any of the GUTs would call for a particle accelerator as big as the solar system! These machines have been at the forefront of particle physics since the 1930s. The latest prototype to be proposed would have been a gigantic 53-mile- (85 km) diameter machine built in Texas at a cost of at least five billion dollars. Called the Superconducting Supercollider (SSC), it would have been used to probe the structure of matter in regions a hundred thousand times smaller than the diameter of a proton. The U.S. government canceled the project a few years ago.

The ultimate theoretical unification of all natural forces will face in the coming years extraordinary conceptual and technical challenges but also—and perhaps essentially—tremendous pressure from society. How can we agree, people could argue, to build a machine costing billions of dollars when there is so much poverty and social injustice in the world? This question will have to be answered someday. One thing, however, is certain; science has always relied on observation and measurement to further its understanding of the macrocosm and the microcosm. The dialogue between theory and experiment is as vital as the two sides of a coin; one does not go without the other. During the twentieth century, the theories and the apparatus for practicing science have grown ever more complex and the fact is that if we do not build those gargantuan machines needed to probe nature, nothing excitingly new will come out from science. Weinberg said to a journalist a few years ago that today is a "terrible time for particle physics." Does this mean, therefore, that rather than reaching the edges of science during the next millennium, we will be facing the end of science instead? The Glashow-Weinberg-Salam theory of electroweak interaction opened new vistas in theoretical and experimental physics in the late 1960s and early 1970s. In the coming decades, however, if humankind wants to take the next step in its current journey to the last frontiers of science, it will have to answer ethical, scientific, and economic questions of the utmost importance.

JEAN-FRANÇOIS GAUVIN