Warped Passages (26 page)

Between the late 1920s and the 1940s, the English physicist Paul Dirac and the Americans Richard Feynman and Julian Schwinger—as well as Sin-Itiro Tomonaga working independently in post-war Japan—developed the quantum mechanical theory of the photon.
They named the branch of quantum theory that they developed
quantum electrodynamics
(QED). Quantum electrodynamics includes all the predictions of the classical electromagnetic theory as well as particle (quantum) contributions to physical processes—that is, interactions that are generated by exchanging or producing quantum particles.

QED predicts how photon exchange produces the electromagnetic force. For example, in the process illustrated in Figure 47, two electrons enter the interaction region, exchange a photon, and then emerge with their resultant paths (speed and direction of motion, for example) influenced by the electromagnetic force that was communicated. Field theory associates numbers with each part of the diagram so that we can use it to make quantitative predictions. This picture is an example of a
Feynman diagram
, named after Richard Feynman, and is a pictorial way of describing interactions in quantum field theory. (Feynman was so proud of his invention that he had some diagrams painted on his van.)

Figure 47.
The Feynman diagram, on the right, has several interpretations. One interpretation (reading bottom to top) is that two electrons enter an interaction region, exchange a photon, and two electrons leave, as illustrated schematically on the left. (This diagram can also be interpreted in terms of electron-positron annihilation.)

Not all QED processes involve a photon that is destroyed, however. In addition to the ephemeral
intermediate
or
internal particles
^*
—like the photons leading to electromagnetic interactions that are produced and almost immediately destroyed—there are also real,
external
photons, particles that enter or leave an interaction region.
Sometimes those particles are deflected and sometimes they turn into other particles. Either way, the particles that enter or leave are real physical particles.

Quantum Field Theory

Quantum field theory, the tool with which we study particles,
^*
is based on eternal, omnipresent objects that can create and destroy those particles. These objects are the “fields” of quantum field theory. Like the classical electromagnetic fields that inspired their name, quantum fields are objects that permeate spacetime. But quantum fields play a different role. They create or absorb elementary particles. According to quantum field theory, particles can be produced or destroyed anywhere and at any time.

For example, an electron or a photon can appear or disappear anywhere in space. Quantum processes allow the number of charged particles in the universe to change through particle creation or destruction. Each particle is created or destroyed by its own particular field. In quantum field theory, not only electromagnetism but all forces and interactions are described in terms of fields, which can create new particles or eliminate particles that were already present.

According to quantum field theory, you can think of particles as excitations of the quantum field. Whereas the
vacuum
, a state with no particles, contains only constant fields, states with particles present contain fields with bumps and wiggles corresponding to the particles. When the field acquires a bump, a particle is created, and when it absorbs this bump to become constant once again, the particle is destroyed.

The fields that create electrons and photons must exist everywhere to guarantee that all interactions can occur at any point in spacetime. This is essential because interactions are
local
, which is to say that only particles in the same place can participate. Action at a distance would be more like magic. Particles don’t have ESP—they have to be in contact to interact directly.

Electromagnetic interactions do occur between distant charges that are not in direct contact, but only through the auspices of the photon or some other particle that has direct contact with both of the interacting charged particles. In that case, charges appear to affect each other instantaneously, but only because the speed of light is so fast. Really, the interaction only occurred through local processes; the photon first coincided with one of the charged particles and then the other. The field therefore had to create and destroy the photon at the precise locations of the charged particles.

Antiparticles and the Positron

Quantum field theory also tells us that for each particle, a counterpart must exist, known as an antiparticle. Tom Stoppard talks about antiparticles in his play
Hapgood
: “When a particle meets an anti-particle, they annihilate each other, they turn into energy-bang, you understand.” Any science fiction fan knows about antiparticles—they are what you make guns from to destroy the universe and are also what powers
Star Trek
’s USS
Enterprise
.

Those last applications are fictitious, but antiparticles are not. Antiparticles are truly a part of the particle physics view of the world. In field theory and the Standard Model, they are as essential as particles. In fact, antiparticles are just like particles, except that all their charges are opposite.

Paul Dirac first encountered antiparticles when he developed the quantum field theory describing the electron. He found that a quantum field theory that is consistent with both quantum mechanics and special relativity necessarily includes antiparticles. He hadn’t deliberately added them. When he incorporated special relativity, the theory spit them out. Antiparticles are a necessary consequence of relativistic quantum field theory.

Here’s a rough argument for why antiparticles follow from special relativity. Charged particles can travel forwards and backwards in space. Naively, special relativity would therefore tell us that those particles should be able to travel forwards and backwards in time as well. But so far as we know, neither particles nor anything
else we are aware of can actually travel backwards in time. What happens instead is that oppositely charged antiparticles replace the reverse-time-traveling particles. Antiparticles reproduce the effects the reverse-time-traveling particles would have so that even without them, quantum field theory’s predictions are compatible with special relativity.

Imagine a movie of a current of negatively charged electrons traveling from one point to another. Now imagine running the movie in reverse. Negative charge would then travel backwards, or, equivalently (so far as the charge is concerned), positive charge would travel forwards. A current of
positrons
, the positively charged antiparticles of electrons, produces this positively charged forward-traveling current and therefore acts like a time-reversed electron current.

Quantum field theory tells us that if any type of charged particle exists, such as an electron, so must a corresponding antiparticle with opposite charge. For example, since an electron carries charge -1, the positron has a charge of +1. The antiparticle is like the electron in all repects aside from its charge. A proton also has a charge of +1, but it is 2,000 times heavier than an electron and therefore could not be its antiparticle.

As Stoppard said, antiparticles do indeed annihilate particles when the two come into contact. Because the charges of a particle and its antiparticle always add up to zero, when a particle meets an antiparticle, they can annihilate each other and be destroyed. The particle and antiparticle together carry no charge, so Einstein’s relation
E = mc2
tells us all the mass can convert into energy.

On the other hand, energy can convert into a particle-antiparticle pair when the energy is sufficient to produce them. Both particle annihilation and particle creation occur in high-energy particle accelerators, where physicists conduct the experiments that study heavy particles, particles too massive to be found in ordinary matter. In these colliders, a particle and an antiparticle meet and annihilate each other, thereby creating a burst of energy from which new particle-antiparticle pairs emerge.

Because matter—and atoms in particular—are composed of particles and not antiparticles, antiparticles such as positrons are generally not found in nature. But they can be produced temporarily at
particle colliders, in hot regions of the universe, and even in hospitals, where positron emission tomography (PET) is used to scan for signs of cancer.

Gerry Gabrielse, a colleague of mine in the Harvard physics department, makes antiparticles all the time in the basement of Jefferson Laboratories, where I work. Thanks to the work of Gerry and others, we know at a very high level of precision that antiparticles really are like their particle counterparts in mass and gravitational pull, despite their opposite charge. But there aren’t enough of them to do any harm. I can assure science fiction fans that these antiparticles do far less damage to the building than the perpetual construction of new labs and offices, which is always preceded by a large amount of visible and audible destruction.

Electrons, positrons, and photons are the simplest and most accessible particles. It is no coincidence that electric forces and electrons were the first Standard Model ingredients that physicists understood. The electron, positron, and the photon are not the only particles, however, and electromagnetism is not the only force.

I listed the known particles and nongravitational forces
^*
in Figures 32 and 33. I left gravity out of the picture because it is qualitatively different from the other forces and must be treated separately. Despite the prosaic names of two of the forces—the weak force and the strong force—they have many interesting properties. In the next two sections, we’ll see what they are.

The Weak Force and the Neutrino

Even though you don’t notice the weak force in your daily existence because it is indeed weak, it is essential to many nuclear processes. The weak force explains some forms of nuclear decay, such as that of potassium-40 (found here on Earth, with a decay that is sufficiently slow—about a billion years on average—to continue to heat the Earth’s core) and, indeed, of the neutron itself. Nuclear processes
change the structure of the nucleus, and through such processes the number of neutrons in a nucleus changes, releasing a large amount of energy. This energy can be harnessed for nuclear power or nuclear bombs, but has other purposes as well.

For example, the weak force plays a role in the creation of heavy elements, which are created during cataclysmic supernova explosions. The weak force is also essential for stars, including the Sun, to shine: it kicks off the chain of reactions that convert hydrogen to helium. The nuclear processes that are triggered by the weak force help make the composition of the universe continuously evolve. From our knowledge of nuclear physics, we can deduce that about 10% of the universe’s primordial hydrogen has been used as nuclear fuel in stars. (Happily, the 90% that remains guarantees that the universe won’t need to rely on foreign energy sources any time soon.)

Despite its importance, scientists identified the weak force only relatively recently. In 1862, William Thomson (later Lord Kelvin
^*
), one of the most respected physicists of his day, grossly underestimated the age of the Sun and the Earth because he didn’t know about nuclear processes originating from the weak force (which, in fairness to him, had not yet been discovered). J.J. Thomson based his estimate on the only known source of illumination, incandescence. He deduced that the energy that had been available could not have supported the Sun for more than about 30 million years.

Charles Darwin didn’t like this result. He had come up with a minimum age far closer to the correct one by estimating the number of years required for erosion to wash away the Weald, a valley in the south of England. Darwin’s estimate of 300 million years had the further appeal for him that it allowed enough time for natural selection to provide the large range of species found on Earth.

However, everyone—including Darwin himself—assumed that Thomson, the physicist of stellar reputation, was correct. Darwin was so persuaded by Thomson’s calculation and reputation that he removed his own time estimates from later editions of his book
The Origin of Species
. Only after Rutherford’s discovery of the significance
of radiation
^*
was Darwin’s idea for an older age vindicated and the age of the Earth and the Sun established as about 4.5 billion years—far larger than Thomson’s estimate, and Darwin’s.

In the 1960s, the American physicists Sheldon Glashow and Steven Weinberg, and the Pakistani physicist Abdus Salam, all working independently (and not necessarily harmoniously), developed the
electroweak theory
, a theory that explains the weak force and provided insight into the force of electromagnetism.
^†
According to the electroweak theory, the exchange of particles called
weak gauge bosons
produces the effects of the weak force, just as photon exchange communicates electromagnetism. There are three weak gauge bosons. Two are electrically charged, the W+ and W-(the W stands for weak force, and the + or - sign is the gauge boson’s charge). The other one is neutral and is called the Z (because of its zero charge).