Physics: Fundamental Forces and the Synthesis of Theory

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Physics: Fundamental Forces and the Synthesis of Theory

Introduction

Since the beginnings of modern science in the 1500s and 1600s, physicists have tried to unify their theories, that is, to explain the known properties of energy and matter with fewer theories. For example, in the seventeenth century English physicist and mathematician Isaac Newton (1642–1727) unified Italian astronomer and physicist Galileo Galilei's (1564–1642) equations for the movements of accelerating everyday objects with German astronomer and mathematician Johannes Ke-pler's (1571–1630) equations describing the motions of the planets. Newton showed that his own three laws of motion, plus his law of universal gravitation, could explain everything that the other two theories had explained and more.

In the nineteenth century, Scottish physicist James Clerk Maxwell (1831–1879) showed that electric and magnetic forces could be described by a single, unified, mathematically precise theory. In the twentieth century, still more ambitious attempts at unification began as scientists sought to reconcile the theory of relativity with quantum physics and to explain the four fundamental forces (gravity, electromagnetic, weak, and strong) with a single, unified theory. The ultimate goal, which may still be decades away or permanently unattainable, is a Theory of Everything—or a finished description of the fundamental nature of Nature.

Historical Background and Scientific Foundations

The drive for unified physical theory has been at the center of Western science since the Middle Ages. Historians of science speculate that the nature of Christian religious doctrine, which framed European philosophical and scientific thought for over a thousand years, encouraged the assumption—by no means universal in human cultures—that nature is ruled by laws and that these laws can be comprehended by human beings.

One of the basic concerns of Western physics since the time of the ancient Greeks has been force. The existence of forces—pushes and pulls—has always been intuitively obvious, but it was not until Newton that the concept received a strict, mathematical definition. Newton defined the total force acting on an object as the time rate of change of that object's momentum (the product of its mass and velocity). In other words, a force is something that can make an object speed up, slow down, or change direction.

Newton used his definition of force, along with his law of universal gravitation, to show that the same gravitational force that governs falling apples and other objects here on Earth also guides the planets around the sun. In doing so he achieved the first great unification in modern physics, the synthesis or bringing-together of planetary mechanics with everyday mechanics.

Scientists in Newton's day already knew of the attractive and repulsive forces of electrical charges and magnets, but had no useful theory about them; magnetic and electric forces seemed to be two quite different things. In the early nineteenth century, English scientist Michael Faraday (1791–1867) and French scientist André-Marie Ampère (1775–1836) showed that moving electric charges produce magnetic fields. This proved that the two kinds of force, electrical and magnetic, were closely related. Faraday attempted to show that electromagnetism and gravity are also related, but failed.

Building on the discoveries of Faraday and others, Maxwell, in a series of papers published in the 1850s and 1860s, showed that a single group of equations, known today as Maxwell's equations, could describe all known features of the electrical and magnetic forces. He had achieved the first true unification of two distinct forces.

Next, German—American physicist Albert Einstein (1879–1955) showed that space and time, previously thought to be absolutely different things, were closely related. His theory unifying space and time, Special Relativity, appeared in 1905; soon afterward he showed that the force of gravity could be understood in terms of the curvature of space and time by objects. This theory, General Relativity, he perfected in 1915. Einstein shared Faraday's dream of unifying the gravitational force with electromagnetics; he spent the last thirty years of his life trying to work out a theory that would do so. His efforts were premature, however, and did not succeed.

Even as Einstein went to work on a unified field theory, which he was never to find, physicists already knew of two other apparently fundamental forces. The first was the strong or nuclear force. The need for such a force became clear with the discovery of the neutron 1932 by English physicist James Chadwick (1891–1974). Physicists then knew that the nucleus of the atom must be made up of protons, which are positively charged, and neutrons, which have no electrical charge. Because like charges repel, the protons in the nucleus would fly apart, pushed by repulsive forces of between 50 and 100 pounds acting on each particle, if they were not held together by some even stronger force—the strong force. The strong force is so called because at short distances it is stronger than any of the other forces; it drops off quickly with distance, however, so that outside the atomic nucleus the behavior of matter is dominated by electromagnetic and gravitational forces.

The existence of a fourth force, called the weak force, was proposed in 1934 by Italian physicist Enrico Fermi (1901–1954) to explain beta decay, a type of radioactive decay in which an atomic nucleus breaks up into smaller pieces, giving off a high-speed electron (beta particle) in the process.

Thus, by the 1930s four fundamental or basic forces were known. For about 40 years, these remained separate in physical theory. Finally, in the 1970s, Pakistani physicist Abdus Salam (1926–1996), American physicist Sheldon Glashow (1932–), and American physicist Steven Weinberg (1933–) succeeded in describing the electromagnetic force and the weak force as manifestations of a single, underlying force, the electroweak force. They were jointly awarded a Nobel Prize in physics in 1979 for their work.

In 1978, physicists coined the term Grand Unified Theories (GUTs) to describe physical theories seeking to unify the electroweak force with the strong force. Several such theories have been devised in the decades since, but as of 2007 none had been experimentally proven to be correct. Also since the 1970s, physicists have been trying to devise what are sometimes called Theories of Everything (TOEs), theories that would unify the strong and electroweak force with the weakest of all the forces, gravitation. As of 2007, these efforts had not yet succeeded.

The Fundamental Forces and the Standard Model

The four fundamental forces, as currently described, are as follows:

Strong force. The strong force acts only on the particles called quarks, gluons, and hadrons. Its range is very short (about 10–15 meter). It glues together atomic nuclei despite the mutual repulsion of their protons.
Weak force. The weak force acts on quarks and leptons. (Electrons are a type of lepton.) It is one ten-trillionth as strong as the strong force, hence its name, and acts at an even shorter range (about 10–18 meter). It is involved in some radioactive processes.
Electromagnetic force. The electromagnetic force acts on all electrically charged particles, including electrons and protons. It is the force that attaches atoms to each other in molecules and so makes chemistry and biology possible. It is relatively strong—one percent as strong as the strong force—and can act over distances far greater than the width of an atom's nucleus.
Gravitation. Gravitation is by far the weakest of all the fundamental forces; even the “weak” force is about 10 26 times stronger than gravity. However, gravity always acts positively, it acts at long range, and it acts on all particles, so its effects are important. Gravity shapes planets and stars into globes and ignites nuclear reactions in the hearts of stars through pressure-generated heat.

As of the early 2000s, all definite knowledge about the fundamental forces and particles had been encapsulated in a scheme known as the Standard Model, a sort of parts list for the universe. The Standard Model was developed in the 1970s and has been adjusted repeatedly since. In modern physics, all forces other than gravity are described as exchanges of particles, so the Standard Model is a list both of the particles that make up matter and of the particles that mediate the exchange of forces. To see how forces might actually consist of particle exchanges, imagine two people standing on wheeled stools throwing a heavy ball to each other: the people would be gradually pushed apart.

The Standard Model has been extended to account for the unification of the electromagnetic and weak forces as the electroweak force, but it does not account for gravity. Several Theories of Everything that would explain all fundamental particles and interactions have been proposed, but testing them experimentally has proved difficult. Gravity remains the greatest challenge. It is elegantly described by the theory of general relativity, but reconciling relativity with quantum mechanics—the physics of the very small, of which the Standard Model is one product—has not yet been possible.

Modern Cultural Connections

The drive toward unification in physics has produced a series of revolutions in our understanding of the physical world. These revolutions are not of merely theoretical interest: Newton's unification allows us to build and control satellites or space probes; Maxwell's unification of the electric and magnetic fields is fundamental to radio and modern electronics; and Einstein's unification of matter and energy provided the initial insights into the capacity of nuclear weapons and nuclear power.

For over half a century, the push for final unification in physics has motivated large bodies of fresh theoretical and experimental work. The primary candidate for total unification today is M-theory, which posits 11 dimensions (10 of space, one of time), and its associated string theories, which explain matter in terms of tiny, vibrating stringlike entities. M-theory has not yet been tested experimentally, and it is not clear whether it even can be.

Although earlier unifications in physics have affected society by making possible new technologies, it is harder to pinpoint the social consequences of recent attempts toward unification of the fundamental forces. These have not yet given rise to new technologies and may never do so. The social effects of the effort towardthe synthesis of theory in physics are therefore not tangible or technological, at least so far. Success would tend to bear out Western civilization's ancient faith in the rationality of the universe and of the capacity of the human mind to understand that rationality. How it would ultimately impact art, philosophy, politics, or other non-technical realms is speculation.

Primary Source Connection

The Superconducting Super Collider (SSC) was a particle accelerator that was partially constructed near Waxa-hachie, Texas. Plans for the SSC called for an underground, ring-shaped tunnel complex with a 54 mile (87 km) circumference and an energy of 20 TeV per beam. Researchers hoped the SSC would help particles physicists observe elementary particles, like the Higgs boson, which had never been observed but was predicted by the Standard Model.

The project quickly outgrew its initial budget estimates and became mired in political controversy over its ultimate utility and cost. When Congress killed the SCC project in 1993, many researchers mourned the lost opportunity to study fundamental questions about the universe and sharply criticized politicians and the media for not understanding the scientific importance of the SSC. The SCC was never completed, leaving over 14 miles of tunnels and 17 large shafts in the ground at the time the project was abandoned.

SSC, R.I.P.

The good news for Superconducting Super Collider fans in 1993 was that Congress voted to protect the $640 million that had been earmarked for the SSC in this fiscal year. The bad news was that the money is to be spent ripping the project apart. “The SSC as we know it is dead,” said Louisiana senator J. Bennett Johnston, a longtime SSC supporter, after the fatal vote in the House of Representatives. “It cannot be revived.”

The SSC was arguably the most ambitious and certainly one of the most costly pure-science projects ever undertaken. The 54-mile underground tunnel circling Waxa-hachie, Texas—now one-quarter complete—was to hold 10,000 superconducting magnets capable of accelerating two small clouds of protons toward each other at nearly the speed of light. The energy of the resulting collision would have rivaled the energy of the universe immediately after the big bang and created a shower of exotic elementary particles. Physicists expected that one of those particles would be the Higgs boson—which, if it exists, would be the key to understanding the origin of mass. No existing accelerator packs enough of a punch to make a Higgs, and most physicists had pinned their hopes on the SSC.

IN CONTEXT: DARK MATTER, DARK ENERGY, AND NEW PHYSICS

In 1933, Swiss astronomer Fred Zwicky (1898–1974) noticed that the spiral shapes of the galaxies could be explained by the gravitation of their visible stars and gas; there must be some form of invisible or “dark” matter clumped around the galaxies that exerts gravitational force. In the 1980s and 1990s, observations from space satellites confirmed Zwicky's idea. Not only is there dark matter out there, the new data showed, but it outweighs all the ordinary, visible matter in the universe by a factor of at least 5. Our own galaxy, the Milky Way, has about 10 times as much dark matter as visible matter (stars, gas clouds, planets).

Clearly, dark matter exists and is very important. Yet nobody knows what it is. As of 2007, the most popular theory was that it is a form of non-baryonic cold matter. Baryons are protons and neutrons, which form the nuclei of ordinary atoms; in physics, “cold” simply means moving at speeds not close to the speed of light. On this theory, dark matter consists of large numbers of some unknown type of particle drifting in space.

Adding one mystery to another, in 1998 observations by scientists at Lawrence Berkeley National Laboratory in California and others showed for the first time that the expansion of the universe is accelerating—getting a little faster all the time. The only possible explanation is an invisible energy field pervading the cosmos, nicknamed dark energy.

Neither dark matter nor dark energy can be accounted for by existing physics. Until the mysteries of gravitation, dark matter, and dark energy are solved, it is highly unlikely that any unifying physical theory will explain the three still-fundamental forces—the strong force, electroweak force, and gravity—as aspects of a single, underlying force.

News of the SSC's death left much of the physics community stunned and disheartened—and not just the 150 physicists who were directly employed by the project but the thousands who work on particle physics at universities. Many of these physicists believe that only the SSC would have been able to provide the experimental data they needed to move forward. “It's tragic,” says Lisa Randall, a particle physics theorist at MIT. “We were just at the point at which we were hoping to answer some of the fundamental questions that have been on our agenda for years, and now it's possible we'll never have those answers.”

Europe's Large Hadron Collider, currently on the drawing board, will produce about a third of the energy of the SSC; some put its chances of finding the Higgs at one in three. Thus unless the European project gets lucky, physicists might have to give up—for the time

IN CONTEXT: THE DOUBTERS

Some scientists have taken a dim view of the quest for total unification in physics, arguing that it has sucked resources from other work and may not even be attainable. In the early 1990s, Howard Georgi (1947–), one of the first physicists to propose a Grand Unified Theory uniting the electroweak force with the strong force and co-proposer of the Super-symmetric Standard Model in 1981, wrote: “The legacy of grand unification, which in my view is very bad for the field of particle physics, is that it is considered reasonable—and even fashionable—for someone who calls him or herself a particle theorist to spend full time speculating about the world at distances much smaller than anything that we will ever be able to study in the laboratory.”

While most scientists continue to support the quest for a theory that would unify all forces and account for all particles, there remain dissidents, including Georgi, who argue that a Theory of Everything may never be found.

being, at least—on unraveling this outstanding mystery: Why does matter have mass?

Some say most of the collateral damage from the SSC's demise will be felt by the upcoming generation of would-be physicists. “A lot of bright people are drawn into studying physics by the dream of discovering the fundamental laws of nature, even if most of them ultimately end up working on something else,” says Steven Giddings, a theorist who currently works on black holes at the University of California at Santa Barbara. “If when I was in college I had gotten the message that our society lacks the will to pursue these fundamental questions anymore, I might well have gone to law school.”

Still, not all physicists and certainly not all scientists mourn the SSC. Some had come to resent the mammoth project's drain on overall science funds, and some even questioned its chances of success. Some wondered whether the results SSC was after were really that much more important than research in solid-state physics, nuclear physics, astrophysics, or geophysics—projects with much lower price tags. The $640 million that will be spent on the SSC in this fiscal year, for instance, is more than the National Science Foundation expects to spend this year on all the Earth sciences and astronomy combined.

Critics of the project even included some particle physicists. “There is a group of people who felt the SSC was too big and too dependent on brute force,” says Richard Blankenbecler, head of the theory group at the Stanford Linear Accelerator Center. “But at the time it was designed, there weren't any competing ideas, and people were just too impatient to get at the data.” Blankenbe-cler adds that a new linear collider under development at SLAC “may well end up a smaller, cheaper way of answering some of the same questions. People in this field are clever. They will come up with new ideas and bounce back.”

But to its supporters, the death of the SSC was a shattering event. Some of them saw the project's demise as a gloomy portent for the future of science in general, even though funding for basic research in this country is at an all-time high. Giddings points out that the cost of the SSC would have been about $4 for every person in the United States over each of the next eight years. “That's less than the cost of a movie, and a lot less than the cost of a subscription to DISCOVER,” he says. “How much would you pay to know what the world is made of?”

David H. Freedman

freedman, david h. “ssc, r.i.p.” discover 15, no. 1 (january 1994): 101.