The God Particle: If the Universe Is the Answer, What Is the Question?

by Leon Lederman

  In the same way, take the neutron, which has a "half-life" of 10.3 minutes, meaning that if you start with 1,000 neutrons, half have disintegrated in 10.3 minutes. But a given neutron? It can decay in 3 seconds or 29 minutes. Its exact time of decay is unpredictable. Einstein hated this idea. "God does not play dice with the universe," he said. Other critics said, suppose there is, in each neutron or each electron, some mechanism, some spring, some "hidden variable" that makes each neutron different, like human beings, who also have an average lifetime. In the case of humans, there are plenty of not-so-hidden things—genes, clogged arteries, and so on—which in principle can be used to predict an individual's day of demise, barring falling elevators, disastrous love affairs, or an out-of-control Mercedes.

  The hidden-variable hypothesis has been essentially disproven for two reasons: no such variables have shown up in all the billions of experiments done on electrons and new, improved theories related to quantum-mechanics experiments have ruled them out.

  THREE THINGS TO REMEMBER ABOUT QUANTUM MECHANICS

  Quantum mechanics can be said to have three remarkable qualities: (1) it is counterintuitive; (2) it works; and (3) it has aspects that made it unacceptable to the likes of Einstein and Schrödinger and that have made it a source of continuing study in the 1990s. Let's touch on each of these.

  1. It is counterintuitive. Quantum mechanics replaces continuity with discreteness. Metaphorically, instead of a liquid being poured into the glass, it is very fine sand. The smooth hum you hear is the beating of huge numbers of atoms on your eardrums. Then there is the spookiness of the double-slit experiment, already discussed.

  Another counterintuitive phenomena is "tunneling." We talked about sending electrons toward an energy barrier. The classical analogue is rolling a ball up a hill. If you give the ball enough initial push (energy), it will go over the top. If the initial energy is too low, the ball will come back down. Or picture a roller coaster with the car stuck in a trough between two terrifying rises. Suppose the car rolls halfway up one rise and loses power. It will slide back down, then almost halfway up the other side, then oscillate back and forth, trapped in the trough. If we could remove friction, the car would oscillate forever, imprisoned between the two insurmountable rises. In quantum atomic theory such a system is known as a bound state. However, our description of what happens to electrons aimed at an energy barrier or an electron trapped between two barriers must include probabilistic waves. It turns out that some of the wave can "leak" through the barrier (in atomic or nuclear systems the barrier is either an electrical or a strong force), and therefore there is a finite probability that the trapped particle will appear outside the trap. This was not only counterintuitive, it was considered a major paradox, since the electron on its way through the barrier would have negative kinetic energy—a classical absurdity. But with evolving quantum intuition one responds that the condition of the electron "in the tunnel" is not observable and therefore not a question for physics. What one does observe is that it does get out. This phenomenon, called tunneling, was used to explain alpha-radioactivity. It is the basis of an important solid state electronic device known as a tunnel diode. Spooky as it is, this tunnel effect is essential to modern computers and other electronic devices.

  Point particles, tunneling, radioactivity, double slit anguish—all of these contributed to the new intuitions that quantum physicists needed as they fanned out in the late 1920s and '30s with their new intellectual armaments to seek unexplained phenomena.

  2. It works. As a result of the events of 1923–1927, the atom was understood. Even so, in those pre-computer days, only simple atoms—hydrogen, helium, lithium, and atoms in which some electrons are removed (ionized)—could be properly analyzed. A breakthrough was made by Wolfgang Pauli, one of the wunderkinder, who understood the theory of relativity at the age of nineteen and became the "enfant terrible" of physics as an elder statesman.

  A digression on Pauli is unavoidable at this point. Noted for his high standards and irascibility, Pauli was the conscience of physics in his time. Or was he just candid? Abraham Pais reports that Pauli once complained to him that he had trouble finding a challenging problem to work on: "Perhaps it's because I know too much." Not a brag, just a statement of fact. You can imagine that he was tough on assistants. When one new young assistant, Victor Weisskopf, a future leading theorist, reported to him at Zurich, Pauli looked Weisskopf over, shook his head, and muttered, "Ach, so young and already you are unknown." After some months, Weisskopf presented Pauli with a theoretical effort. Pauli took one glance and said, "Ach, that isn't even wrong!" To one postdoc he said, "I don't mind your thinking slowly. I mind your publishing faster than you think." No one was safe from Pauli. In recommending a fellow to be assistant to Einstein, who was, in his later years, deep into the mathematical exotica of his fruitless quest for a unified field theory, Pauli wrote: "Dear Einstein, This student is good, but he does not clearly grasp the difference between mathematics and physics. On the other hand, you, dear Master; have long lost this distinction." That's our boy Wolfgang.

  In 1924 Pauli proposed a fundamental principle that explained the Mendeleev periodic table of the elements. The problem: we build up the atoms of the heavier chemical elements by adding positive charge to the nucleus and electrons to the various allowed energy states of the atom (orbits, in the old quantum theory). Where do the electrons go? Pauli announced what has become known as the Pauli exclusion principle: no two electrons can occupy the same quantum state. Originally an inspired guess, the principle turned out to be a consequence of a deep and lovely symmetry.

  Let's see how Santa, in his workshop, makes the chemical elements. He has to do this right because he works for Her, and She is tough. Hydrogen is easy. He takes one proton—the nucleus. He adds an electron, which occupies the lowest possible energy state—in the old Bohr theory (which is still useful pictorially) the orbit with the smallest allowed radius. Santa doesn't have to be careful; he just drops the electron anywhere near the proton and it "jumps" eventually to this lowest "ground" state, emitting photons on the way. Now helium. He assembles the helium nucleus, which has two plus charges. So he needs to drop in two electrons. And with lithium it takes three electrons to form the electrically neutral atom. The issue is, where do these electrons go? In the quantum world, only certain states are allowed. Do they all crowd into the ground state, three, four, five ... electrons? This is where the Pauli principle comes in. No, says Pauli, no two electrons can be in the same quantum state. In helium, the second electron is allowed to join the first electron in the lowest energy state only if it spins in the opposite sense to its partner. When we add the third electron, for the lithium atom, it is excluded from the lowest energy level and must go into the next lowest level. This turns out to have a much larger radius (again a la Bohr theory), thus accounting for lithium's chemical activity—namely, the ease with which it can use this lone electron to combine with other atoms. After lithium we have the four-electron atom, beryllium, in which the fourth electron joins the third in its "shell," as the energy levels are called.

  As we proceed merrily along—beryllium, boron, carbon, nitrogen, oxygen, neon—we add electrons until each shell is filled. No more in that shell, says Pauli. Start a new one. Briefly, the regularity of chemical properties and behaviors all comes out of this quantum buildup via the Pauli principle. Decades earlier, scientists had derided Mendeleev's insistence on lining the elements up in rows and columns according to their characteristics. Pauli showed that this periodicity was precisely tied to the various shells and quantum states of electrons: two can be accommodated in the first shell, eight in the second, eight in the third, and so on. The periodic table did indeed contain a deeper meaning.

  Let's summarize this important idea. Pauli invented a rule for how the chemical elements change their electronic structure. This rule accounts for the chemical properties (inert gas, active metal, and so on), tying them to the numbers and states of the electrons, especi
ally those in the outermost shells, where they are most readily in contact with other atoms. The dramatic implication of the Pauli principle is that if a shell is filled, it is impossible to add an additional electron to that shell. The resistive force is huge. This is the real reason for the impenetrability of matter. Although atoms are way more than 99.99 percent empty space, I have a real problem in walking through a wall. Probably you share this frustration. Why? In solids, where atoms are locked together via complicated electrical attractions, the imposition of your body's electrons on the system of "wall" atoms meets Pauli's prohibition on having electrons too close together. A bullet is able to penetrate a wall because it ruptures the atom-atom bonds and, like a football blocker, makes room for its own electrons. Pauli's principle also plays a crucial role in such bizarre and romantic systems as neutron stars and black holes. But I digress.

  Once we understand atoms, we solve the problem of how they combine to make molecules, for example, H20 or NaCl. Molecules are formed via the complex of forces among electrons and nuclei in the combining atoms. The arrangement of the electrons in their shells provides the key to creating a stable molecule. Quantum theory gave chemistry a firm scientific base. Quantum chemistry today is a thriving field, out of which has come new disciplines like molecular biology, genetic engineering, and molecular medicine. In materials science, quantum theory helps us explain and control the properties of metals, insulators, superconductors, and semiconductors. Semiconductors led to the discovery of the transistor, whose inventors fully credit the quantum theory of metals as their inspiration. And out of that discovery came computers and microelectronics and the revolution in communications and information. And then there are masers and lasers, which are complete quantum systems.

  When our measurements reached into the atomic nucleus—a scale 100,000 times smaller than the atom—the quantum theory was an essential tool in that new regime. In astrophysics, stellar processes produce such exotic objects as suns, red giants, white dwarfs, neutron stars, and black holes. The life story of these objects is based on quantum theory. From the point of view of social utility, as we have estimated, quantum theory accounts for over 25 percent of the GNP of all the industrial powers. Just think, here are these European physicists obsessed with how the atom works, and out of their efforts come trillions of dollars of economic activity. If only wise and prescient governments had thought to put a 0.1 percent tax on quantum technological products, set aside for research and education ... Anyway, it does indeed work.

  3. It has problems. This issue has to do with the wave function (psi, or ψ and what it means. In spite of the great practical and intellectual success of quantum theory, we cannot be sure we know what the theory means. Our uneasiness may be intrinsic to the mind of man, or it may be that some genius will eventually come up with a conceptual scheme that makes everyone happy. If it makes you queasy, don't worry. You're in good company. Quantum theory has made many physicists unhappy, including Planck, Einstein, de Broglie, and Schrödinger.

  There is a rich literature on the objections to the probabilistic nature of quantum theory. Einstein led the battle, and in a long series of efforts (not easy to follow) to undermine the uncertainty relations, he was continually thwarted by Bohr, who had established what is now called the "Copenhagen interpretation" of the wave function. Bohr and Einstein really went at it. Einstein would invent a thought experiment that was an arrow to the heart of the new quantum theory, and Bohr; usually after a long weekend of hard work, would find the flaw in Einstein's logic. Einstein was the bad boy, the needier in these debates. Like a troublemaking kid in catechism class ("If God is all-powerful, can She build a rock so heavy that not even She can lift it?"), Einstein kept coming up with paradoxes in the quantum theory. Bohr was the priest who kept countering Einstein's objections.

  The story is told that many of their discussions took place during walks in the forest. I can see what happened when they encountered a huge bear. Bohr immediately drew a pair of $300 Reebok Pump running shoes out of his backpack and began lacing them up. "What are you doing, Niels? You know you can't outrun a bear," Einstein logically pointed out. "Ah, I don't have to outrun the bear, dear Albert," responded Bohr. "I only have to outrun you."

  By 1936 Einstein had reluctantly agreed that quantum theory correctly describes all possible experiments, at least those that can be imagined. He then switched gears and decided that quantum mechanics cannot be a complete description of the world, even though it does correctly give the probability for various measurement outcomes. Bohr's defense was that the incompleteness that worried Einstein was not a fault of the theory but a quality of the world in which we live. These two debated quantum mechanics into the grave, and I'm quite sure they are still at it unless the "Old One," as Einstein called God, out of misplaced concern settled the problem for them.

  Einstein and Bohr's debate requires books to tell, but I will try to illustrate the problem with one example. A reminder about Heisenberg's fundamental tenet: no attempt to make a simultaneous measurement of where a particle is and where it is going can ever be entirely successful. Design a measurement to locate the atom, and there it is, as precise as you like. Design a measurement to see how fast it is going—presto, we get its speed. But we can't have both. The reality that these measurements reveal depends on the strategy that the experimenter adopts. This subjectivity challenges our cherished beliefs in cause and effect. If an electron starts at point A and is seen to arrive at point B, it seems "natural" to assume it took a particular path from A to B. Quantum theory denies this, saying the path is unknowable. All paths are possible, and each has its probability.

  To expose the incompleteness of this ghostly-trajectory idea, Einstein proposed a crucial experiment. I cannot do justice to his concept, but I'll try to get across the basic idea. It's called the EPR thought experiment, for Einstein, Podolsky, and Rosen, the three inventors. They proposed a two-particle experiment, in which one particle's fate is tied to the other's. There are ways of creating a pair of particles flying apart from each other so that if one spins up the other must spin down, or if one spins right the other must go left. We send one particle speeding off to Bangkok, the other to Chicago. Einstein said, okay, let's accept the idea that we can't know anything about a particle until we measure it. So we measure particle A, in Chicago, and discover that it spins right. Ergo, we now know about particle B, in Bangkok, whose spin is about to be measured. Before the Chicago measurement, the probability of spin left versus spin right was 50 percent. Now, after Chicago, we know that particle B spins left. But how does particle B know the result of the Chicago experiment? Even if it carries a little radio, radio waves travel at the speed of light, and it would take some time for the message to arrive. What is this communicating mechanism, that doesn't even have the courtesy to travel at the velocity of light? Einstein called this "spooky action at a distance." The EPR conclusion is that the only way to understand the connection of A happenings (the decision to measure at A) with the outcome of B is to provide more details, which quantum theory cannot do. Aha! cried Albert, quantum mechanics isn't complete.

  When Einstein hit him with EPR, even traffic in Copenhagen stopped while Bohr pondered this problem. Einstein was trying to finesse the Heisenberg uncertainty relation by measuring an accomplice particle. Bohr's eventual rejoinder was that one cannot separate the A and B events, that the system must include A, B, and the observer who decides when to make the measurements. This holistic response was thought to have some ingredients of Eastern religious mysticism, and (too many) books have been written about these connections. The issue is whether the A particle and the A observer, or detector, have a real Einsteinian existence or are irrelevant intermediate ghosts before measurement. This particular issue was resolved by a theoretical breakthrough and (aha!) a brilliant experiment.

  Thanks to a theorem developed in 1964 by a particle theorist named John Bell, it became clear that a modified form of the EPR thought experiment could actually be done in
the lab. Bell devised an experiment that would predict different amounts of long-distance correlation between A and B particles depending on whether Einstein's or Bohr's point of view was right. Bell's theorem has almost a cult following today, partly because it fits on a T-shirt. For example, there's at least one women's club, probably in Springfield, that meets every Thursday afternoon to discuss Bell's theorem. Much to Bell's chagrin, his theorem was heralded by some as "proof" of paranormal and psychic phenomena.

  Bell's idea resulted in a series of experiments, the most successful of which was carried out by Alan Aspect and colleagues in 1982 in Paris. The experiment in effect measured the number of times detector A results correlated with detector B results, that is, left spin and left spin or right spin and right spin. Bell's analysis enabled one to predict this correlation using the Bohr interpretation of a "complete-as-can-be" quantum theory as opposed to the Einstein notion that there must be hidden variables that determine the correlation. The experiment clearly showed that Bohr's analysis was correct and Einstein's wrong. Apparently these long-distance correlations between particles are the way nature works.

  Did this end the debate? No way. It rages today. One of the more intriguing places where quantum spookiness has arisen is in the very creation of the universe. In the earliest phase of creation, the universe was of subatomic dimensions, and quantum physics applied to the entire universe. I may be speaking for the masses of physicists in saying that I'll stick to my accelerator research, but I'm mighty glad someone is still worrying about the conceptual foundations of quantum theory.

  For the rest of us, we are heavily armed with Schrödinger, Dirac, and the newer quantum field theory equations. The road to the God Particle—or at least its beginning—is now very clear.