The Cosmic Landscape (6 page)

If you actually try this experiment at home, you may find that it’s not so easy as I made it sound. Two things make it difficult. The interference pattern can be seen only if the slits are very narrow and very close together. Don’t expect to succeed by cutting holes with a can opener. Secondly, the source has to be very small. The old, low-tech way of making a small source is to pass the light through a very small pinhole before allowing it to fall on the sheet with the slits. A much better way is to use a high-tech laser. A laser pointer is ideal. Laser light passing through very carefully made slits produces excellent zebralike interference patterns. The main problem in doing the experiment will be holding everything steady.

Now we are going do the entire optical exercise over again, but this time we will turn down the intensity of the source to such a low level that individual photons come through, one at a time. If we expose the film for a short time, a few darkened dots appear where individual photons landed on the film. Expose it again, the same way, and the dots will get denser. Eventually we will see the patterns of the first experiment reproduced. Among other things the experiment confirms Einstein’s idea that light is composed of discrete photons. Moreover, the particles arrive randomly, and only when enough have accumulated do we see that a pattern is reproduced.

But these particle-like photons behave in a most unexpected way. When both slits are opened, not a single particle arrives at the locations where the destructive interference took place. This is despite the fact that photons do arrive at these places when only one slit is open. It seems that opening the left slit prevents the photons from going through the right slit and vice versa.

To express it differently, suppose the point X is a point on the film where destructive interference takes place. The photon can get to X if the left slit is open. It can get to X if the right slit is open. A sensible person would expect that if both were opened, a photon would be even more likely to get to X. But no—not a single photon appears at X no matter how long we wait. How does the photon, about to go through the left slit, know that the right slit is open? Physicists sometimes describe this peculiar effect by saying that the photon does not go through either of the slits, but instead “feels out” both paths, and at certain points, the contribution of the two paths cancel. Whether or not this helps your understanding, interference is a very weird phenomenon. You get used to the weirdness of quantum mechanics if you work with it for forty or more years. But stopping to reflect on it, it’s weird!

Nature seems to be organized in a hierarchical way: big things made out of smaller things made out of yet smaller things until we reach the smallest things that we are able to uncover. The ordinary world is full of such hierarchies. An automobile is nothing but its parts: wheels, engine, carburetor, and so on. The carburetor, in turn, is built from smaller parts such as idle screws, choke levers, jets, and springs. As far as one can tell, the properties of the smaller things determine the behavior of the larger. This view, that the whole is the sum of its parts and that nature can be understood by reducing it to the simplest and smallest components, is called
reductionism.

Reductionism has the status of a dirty word in many academic quarters. It stirs up passions almost as powerful as those that evolution excites in certain religious circles. The notion that all of existence is no more than inanimate particles touches the same insecurities as the similar idea that we humans are mere vehicles for our selfish genes. But, like it or not, reductionism does work. Every auto mechanic is a reductionist, at least during working hours. In science the power of reductionism is phenomenal.
¹⁰
The basic laws of biology are determined by the chemistry of organic molecules like DNA, RNA, and proteins. Chemists reduce the complex properties of molecules to those of atoms, and then physicists take over. Atoms are nothing but collections of electrons orbiting atomic nuclei. As we learn in elementary science courses, nuclei are composites of protons and neutrons. They, in turn, are made of quarks. How far does this “Russian nesting doll” view of nature go? Who knows? But twentieth-century physics has succeeded in pushing reductionism to the level of the so-called elementary particles. By the Laws of Physics, I mean the laws of these so-far smallest building blocks. It will be important to have a clear idea of whatthese laws are before we can begin to question why they are the laws.

The language of theoretical physics is mathematical equations. It’s hard for physicists to conceive of any form for a theory other than an equation or a small set of equations. Newton’s equations, Maxwell’s equations, Einstein’s equations, Erwin Schrödinger’s equation—these are some of the most important examples. The mathematical framework for elementary-particle physics is called
quantum field theory.
It is a difficult mathematical subject loaded with very abstract equations. In fact the equations of quantum field theory are so complicated that one may get the feeling that equations are not really the right way to express the theory. Luckily for us the great Richard Feynman had exactly that feeling. So he invented a pictorial way to visualize the equations. Feynman’s way of thinking is so intuitive that the main ideas can be summarized without a single equation.

Dick Feynman was a genius of visualization (he was also no slouch with equations): he made a mental picture of anything he was working on. While others were writing blackboard-filling formulas to express the laws of elementary particles, he would just draw a picture and figure out the answer. He was a magician, a showman, and a show-off, but his magic provided the simplest, most intuitive way to formulate the Laws of Physics.
Feynman diagrams
(see page 38) are literally pictures of the events that take place as elementary particles move through space, collide, and interact. A Feynman diagram can be nothing more than a few lines describing a couple of colliding electrons, or it can be a vast network of interconnected branching, looping trajectories describing all of the particles making up anything from a diamond crystal to a living being or an astronomical body. These diagrams can be reduced to a few basic elements that summarize everything that is known about elementary particles. Of course there is more than just the pictures—there are all the technical details of how they are used to make precise calculations, but that’s less important. For our purposes a picture is worth a thousand equations.

A quantum field theory begins with a cast of characters, namely, a list of elementary particles. Ideally the list would include all elementary particles, but that is not practical: we are fairly certain that we don’t even know the complete list. But not too much is lost by making a partial list. It is like a theater performance: in reality every story involves everyone on earth, past and present, but no sane author would try to write a play with several billion characters. For any particular story some characters are more important than others, and the same is true in elementary-particle physics.

The original story that Feynman set out to tell is called Quantum Electrodynamics, or QED for short, and it involves only two characters: the electron and the photon. Let me introduce them.

The Electron

In 1897 the British physicist J. J. Thomson made the first discovery of an elementary particle. Electricity was well known before that point, but Thomson’s experiments were the first to confirm that electric currents are reducible to the motion of individual charged particles. The moving particles that power toasters, lightbulbs, and computers are, of course, electrons.

For dramatic effects it’s hard to beat electrons. When a giant lightning bolt rips across the sky, electrons flow from one electrified cloud to another. The roar of thunder is due to a shock wave caused by the collision of rapidly accelerated electrons with air molecules blocking their path. The visible lightning bolt consists of electromagnetic radiation that was emitted by agitated electrons. The tiny sparks and crackling noises due to static electricity, on a very dry day, are manifestations of the same physics on a smaller scale. Even ordinary household electricity is the same flow of electrons, tamed by electrically conducting copper wires.

Every electron has exactly the same electric charge as every other electron. The charge of the electron is an incredibly small number. It takes an enormous number of electrons—about 10
¹⁹
per second—to create a common electric current of one amp. There is an oddity about the charge of the electron that has puzzled and troubled generations of undergraduates studying physics: the electron’s charge is
negative.
Why is that? Is there something intrinsically negative about the electron? In fact the negativity of the electron charge is not a property of the electron but rather a definition. The trouble dates back to Benjamin Franklin, who was the first physicist to realize that electricity was a flow of charge.
¹¹
Franklin, who knew nothing of electrons, had no way of knowing that what he called
positive current
was actually a flow of electrons in the opposite direction. For this reason we have inherited the confusing convention of a negative electron charge. As a consequence, we physics professors constantly have to remind students that when electric current flows to the left, electrons move to the right. If this boggles your mind, blame it on Ben Franklin and then ignore it.

If all electrons were suddenly to disappear, a great deal more than toasters, lightbulbs, and computers would fail. Electrons play another very profound role in nature. All ordinary matter is made of atoms, which in turn are made of electrons—each electron whirling around the atomic nucleus like a ball on a rope. Atomic electrons determine the chemical properties of all the elements listed in the periodic table. Quantum Electrodynamics is more than the theory of electrons: it is the basis for the theory of all matter.

The Photon

If the electron is the hero of QED, the photon is the sidekick that makes the hero’s deeds possible. The light emitted by a lightning bolt can be traced to microscopic events in which individual electrons shake off photons when they are accelerated. The entire plot of QED revolves around one fundamental process: the emission of a single photon by a single electron.

Photons also play an indispensable role in the atom. In a sense that will become clear, photons are the ropes that tether the electrons to the nucleus. If photons were to be suddenly eliminated from the list of elementary particles, every atom would instantly disintegrate.

One of the main goals of QED was to understand the detailed properties of simple atoms, especially hydrogen. Why hydrogen? Hydrogen, having only a single electron, is so simple that the equations of quantum mechanics can be solved. More complex atoms with many electrons, all exerting forces on one another, could be studied only with the aid of powerful computers, which didn’t exist when QED was being formulated. But to study any atom, one more ingredient must be added—the nucleus. Nuclei are made of positively charged protons and electrically neutral neutrons. These two particles are very similar to each other, apart from the fact that the neutron has no electric charge. Physicists group these two particles together and give them a common name: the
nucleon.
A nucleus is essentially a blob of sticky nucleons. The structure of any nucleus, even of hydrogen, is so complicated that physicists like Feynman decided to ignore it. They concentrated instead on the much simpler physics of the electron and photon. But they couldn’t do away with the nucleus altogether. So they introduced it not as an actor, but as a stage prop. Two things made this possible.

First, the nucleus is much heavier than an electron. It is so heavy that it is almost immobile. No big mistake is made if the nucleus is replaced by an immovable point of positive electric charge.

Second, nuclei are very small by comparison with atoms. The electron orbits the nucleus at about 100,000 nuclear diameters and never gets close enough to be affected by the complicated internal nuclear structure.

According to the reductionist view of particle physics, all the phenomena of nature—solids, liquids, gases, living as well as inanimate matter—are reduced to the constant interaction and collision of electrons, photons, and nuclei. That’s the action and the whole plot—actors crashing into one another, bouncing off one another, and here and there, giving birth to new actors out of the collision. It is this banging away of particles by other particles that Feynman diagrams depict.

“If you come to a fork in the road, take it.”

— YOGI BERRA

We have the actors, we have the script, and now we need a stage. Shakespeare said, “All the world’s a stage,” and as usual, the Bard got it right. The set for our farce is the whole world: for a physicist that means all of ordinary three-dimensional space. Up-down, east-west, and north-south are the three directions near the surface of the earth. But a stage direction involves not only
where
an action takes place, but also
when
it takes place. Thus, there is a fourth direction to
space-time:
past-future. Ever since Einstein’s discovery of the Special Theory of Relativity, physicists have been in the habit of picturing the world as a four-dimensional space-time that encompasses not only
the now,
but also all of the future and the past. A point in space-time—a where and a when—is called an
event.