by Michio Kaku
Today, lasers can be found everywhere: grocery checkout counters, hospitals, computers, rock concerts, satellites in space, etc. Not only can vast amounts of information be carried on laser beams, you can also transmit colossal amounts of energy, sufficient to burn through most materials. (Apparently, the only limitation to the energy of a laser is the stability of the lasing material and the energy that drives the laser. So, with the appropriate lasing substance and power source, one could in principle create a laser beam similar to the ones seen in science fiction movies.)
What Is Life?
Erwin Schrödinger was a pivotal figure in formulating quantum mechanics. But Schrödinger was also interested in another scientific problem that had fascinated and dogged scientists for centuries: What is life? Could quantum mechanics answer this age-old mystery? He believed that one by-product of the quantum revolution would be the key to understanding the origin of life.
Throughout history, scientists and philosophers believed that there was some sort of life force that animated living things. When a mysterious soul entered a body, it suddenly became animate and acted human. Many believed in something called dualism, where the material body coexisted with a spiritual soul.
Schrödinger, however, believed that the code of life was hidden inside some master molecule that obeyed the laws of quantum mechanics. Einstein, for example, banished the ether from physics. Likewise, Schrödinger would try to banish the life force from biology. In 1944, he wrote a pioneering book, What Is Life?, that had a profound effect on a new generation of postwar scientists. Schrödinger proposed to use quantum mechanics to answer the most ancient of questions about life. In that book, he saw that a genetic code was somehow being transported from one generation of living organisms to the next. He believed that this code was stored not in a soul but in the arrangement of molecules in our cells. Using quantum mechanics, he theorized about what this mysterious master molecule could be. Unfortunately, not enough was known about molecular biology in the 1940s to answer this question.
But two scientists, James D. Watson and Francis Crick, read the book and were fascinated by the search for this master molecule. Watson and Crick realized that molecules were so small that it was impossible to see or manipulate one. This is because the wavelength of visible light is much larger than a molecule. But they had another quantum trick up their sleeve: X-ray crystallography. The wavelength of X-rays is comparable in size to molecules, so by shining X-rays at a crystal of organic materials, the X-rays would be scattered in many directions. But the pattern of the scatter contained information about the detailed atomic structure of the crystal. Different molecules produced different X-ray patterns. A skilled quantum physicist, by looking at photographs of the scatter, could then surmise what the structure of the original molecule was. So although you could not see the molecule itself, you could decipher its structure.
Quantum mechanics was so powerful that one could determine the angle at which different atoms bound together to create molecules. Like a child playing with Tinkertoys or Legos, one could then build up, atom for atom, chains of these atoms stuck together to reproduce the actual structure of a complex molecule. Watson and Crick realized that the DNA molecule was one of the main constituents of the nucleus of a cell, so that was a likely target. By analyzing the crucial X-ray photos taken by Rosalind Franklin, they were able to conclude that the structure of the DNA molecule was a double helix.
In one of the most important papers published in the twentieth century, Watson and Crick were able to use quantum mechanics to decode the entire structure of the DNA molecule. It was a masterpiece. They demonstrated conclusively that the fundamental process of living things—reproduction—could be duplicated at the molecular level. Life was encoded on the strands of DNA found in every cell.
That was the breakthrough that made it possible to achieve the holy grail of biology, the Human Genome Project, which has given us a complete atomic description of a person’s DNA.
As Charles Darwin had predicted in the previous century, it was now possible to construct the family tree of life on Earth, with every living thing or fossil a member of one branch of this tree. All of this was the product of quantum mechanics.
So the unification of the laws of quantum physics not only revealed the secrets of the universe, it also unified the tree of life.
The Nuclear Force
We recall that Einstein was unable to complete his unified field theory, in part because he was missing a huge piece of the puzzle, the nuclear force. Back in the 1920s and 1930s, almost nothing was known about it.
But in the postwar era, buoyed by the astounding success of QED, physicists turned their attention to the next burning problem—applying the quantum theory to the nuclear force. This would be a difficult and arduous task, since they were starting from scratch and needed entirely new powerful instruments to find their way in this unknown territory.
There are two kinds of nuclear forces, the strong and the weak. Since the proton has positive charge, and since positive charges repel each other, the nucleus of the atom might ordinarily fly apart. What holds the nucleus together, overcoming electrostatic repulsion, are the nuclear forces. Without them, our entire world would dissolve into a cloud of subatomic particles.
The strong nuclear force is sufficient to keep the nucleus of many chemical elements stable indefinitely. Many have been stable since the beginning of the universe itself, especially if the number of protons and neutrons are in balance. However, some nuclei are unstable for a number of reasons, especially if they have too many protons or neutrons. If they have too many protons, then the electric repulsion will cause the nucleus to fly apart. If the nucleus has too many neutrons, then their instability can cause it to decay. In particular, the weak nuclear force is not strong enough to hold the neutron together permanently, so eventually it falls apart. For example, half of any collection of free neutrons will decay in fourteen minutes. What is left are three particles: the proton; the electron; and another mysterious new particle, the anti-neutrino, which we will discuss later.
Studying the nuclear force is exceedingly difficult, since the nucleus is about one hundred thousand times smaller than an atom. To probe inside a proton, physicists needed a new tool, the particle accelerator. We saw how years before Ernest Rutherford used the rays emitted by radium encased in a lead box to discover the nucleus. To explore deeper inside the nucleus, physicists needed even more powerful sources of radiation.
In 1929, Ernest Lawrence invented the cyclotron, the forerunner of the giant particle accelerators of today. The basic principle behind the cyclotron is simple. A magnetic field forces protons to move in a circular path. At each cycle, the protons are given a small boost of energy by an electric field. Eventually, after many revolutions, the beam of protons can reach millions and even billions of electron volts. (The basic principles of a particle accelerator are so straightforward that I even built an electron particle accelerator, a betatron, when I was in high school.)
This beam, in turn, is eventually directed at a target, where it smashes into other protons. By sifting through the enormous debris from this collision, scientists were able to identify new, previously undiscovered particles. (This process of shooting beams of particles to smash protons apart is a clumsy, imprecise operation. It has been compared to throwing a piano out the window, and then trying to determine all the piano’s properties by analyzing the sound of the crash. As clumsy as this process is, it is one of the only ways we have to probe the interior of the proton.)
When physicists first smashed protons with a particle accelerator in the 1950s, they found, to their dismay, an entire zoo of unexpected particles.
It was an embarrassment of riches. Nature, it was believed, was supposed to become simpler the deeper you searched, not more complex. To the quantum physicist, it seemed that perhaps nature really was malicious after all.
> Frustrated by this flood of new particles, Robert Oppenheimer declared that the Nobel Prize in Physics should be given to the physicist who did not discover a new particle that year. Enrico Fermi declared that “if I had known there would be so many particles with Greek names, I would have become a botanist rather than a physicist.”
Researchers were drowning in subatomic particles. The mess prompted some physicists to claim that perhaps the human mind was not smart enough to understand the subatomic realm. After all, they argued, it is impossible to teach a dog calculus, so perhaps the human mind is not powerful enough to understand what’s happening in the nucleus of an atom.
Some of the confusion began to be clarified with the work of Murray Gell-Mann and his colleagues at the California Institute of Technology (Caltech), who claimed that, inside the proton and neutron, there were three even smaller particles called quarks.
It was a simple model, but it worked spectacularly well in arranging the particles into groups. Like Mendeleyev before him, Gell-Mann could predict the properties of new strongly interacting particles by looking at the gaps in his theory. In 1964, another particle predicted by the quark model, called the omega-minus, was actually found, verifying the basic correctness of this theory, for which Gell-Mann won the Nobel Prize.
The reason the quark model was able to unify so many particles is because it was based on a symmetry. Einstein, we recall, introduced a four-dimensional symmetry that turned space into time and vice versa. Gell-Mann introduced equations containing three quarks; when you interchanged them inside an equation, the equation remained the same. This new symmetry described the reshuffling of three quarks.
Polar Opposites II
The other great physicist at Caltech, Richard Feynman, who renormalized QED, and Murray Gell-Mann, who introduced the quark, were polar opposites in their personality and temperament.
In the popular media, physicists are universally portrayed either as mad scientists (like Doc Brown in Back to the Future) or hopelessly inept nerds, as in The Big Bang Theory. However, in actuality physicists come in all shapes and sizes and personality types.
Feynman was a colorful gadfly, ever the showman and the clown, full of ribald stories of his outrageous stunts, told in a rough working-class accent. (During World War II, he once cracked the safe containing the secrets of the atomic bomb at the Los Alamos National Laboratory. Inside the safe, he left a cryptic note. When officials found this note the next day, it set off a major alarm and panic at the nation’s top secret laboratory.) Nothing was too unorthodox or outrageous for Feynman; out of curiosity, he once even sealed himself in a hyperbaric chamber to see if he could have an out-of-body experience.
Gell-Mann, however, was the opposite, ever the gentleman, precise in his words and manners. Bird-watching, collecting antiques, linguistics, and archaeology were his favorite pastimes, not reciting hilarious stories. But as different as they were in character, they both had the same drive and determination, which helped them to penetrate the mysteries of the quantum theory.
Weak Force and Ghostlike Particles
Meanwhile, great strides were being made in understanding the weak nuclear force as well, which is about a million times weaker than the strong force.
The weak force, for example, is not powerful enough to hold the nuclei of many types of atoms together, so they fall apart and decay into smaller subatomic particles. Radioactive decay, as we have seen, is the reason the inside of the Earth is so hot. The fierce energy of thundering volcanoes and terrible earthquakes comes from the weak nuclear force. A new particle had to be introduced to explain the weak force. A neutron, for example, is unstable and eventually decays into a proton and an electron. This is called beta decay. But in order for the calculations to work out, physicists needed to introduce a third particle, a shadowy particle called the neutrino.
The neutrino is sometimes called the ghost particle, because it can penetrate entire planets and stars without being absorbed. At this very instant, your body is being radiated by a flood of neutrinos from deep space, some of which traveled through the entire planet Earth. In fact, some of these neutrinos could penetrate a block of solid lead that stretches from the Earth to the nearest star.
Pauli, who predicted the existence of the neutrino in 1930, once lamented, “I have committed the ultimate sin. I have introduced a particle that can never be observed.” As elusive as this particle is, it was finally discovered experimentally in 1956 by analyzing the intense radiation emitted from a nuclear power plant. (Although the neutrino hardly interacts with ordinary matter, physicists compensated for this by exploiting the vast number of neutrinos emitted in a nuclear reactor.)
To make sense of the weak nuclear force, physicists once again introduced a new symmetry. Since the electron and neutrino were a pair of weakly interacting particles, it was proposed that they could be paired, giving us a symmetry. Then this new symmetry, in turn, could be coupled to the older symmetry of Maxwell’s theory. The resulting theory was called the electroweak theory, which unified electromagnetism with the weak nuclear force.
This electroweak theory of Steven Weinberg, Sheldon Glashow, and Abdus Salam won them the Nobel Prize in 1979.
So light, instead of being united with gravity, as Einstein had hoped, actually preferred to be united with the weak nuclear force.
Thus, the strong force was based on Gell-Mann’s symmetry, which binds the three quarks together to make protons and neutrons, while the weak nuclear force was based on a smaller symmetry, the rearranging of the electron with the neutrino, which is then combined with electromagnetism.
But powerful as the quark model and the electroweak theory were in describing the zoo of subatomic particles, this still left a huge gap. The burning question was: What holds all these particles together?
Yang-Mills Theory
Because the Maxwell field had so much success in predicting the properties found in electromagnetism, physicists began to study a new, more powerful version of Maxwell’s equation. It was proposed by Chen Ning Yang and Robert L. Mills in 1954. Instead of just one field, written down by Maxwell in 1861, it introduced a family of these fields. The same symmetry that Gell-Mann used to rearrange the quarks in this theory was now used to rearrange this new collection of Yang-Mills fields into one another.
The idea was simple. What holds the atom together is the electric field, which is described by Maxwell’s equations. Then perhaps what holds the quarks together is a generalization of Maxwell’s equations—that is, the Yang-Mills fields. The same symmetry that describes the quarks is now applied to the Yang-Mills field.
However, for several decades, this simple idea languished because, when calculating the properties of the Yang-Mills particles, the result was again infinite, just like we saw in QED. Unfortunately, the bag of tricks introduced by Feynman was not enough to renormalize the Yang-Mills theory. For years, physicists despaired of finding a finite theory of the nuclear force.
Finally, an enterprising Dutch grad student, Gerard ’t Hooft, had the courage and raw stamina to plow through this thicket of infinite terms and, via brute force, renormalize the Yang-Mills field. By then, computers were advanced enough to analyze these infinities. When his computer program spit out a series of zeros representing these quantum corrections, he knew he must be right.
News of this breakthrough caught the immediate attention of physicists. Physicist Sheldon Glashow would exclaim, “Either this guy’s a total idiot, or he’s the biggest genius to hit physics in years!”
It was a tour de force that would win ’t Hooft and his adviser, Martinus Veltman, the Nobel Prize in 1999. Suddenly, there was a new field that could be used to bind together the known particles in the nuclear force and explain the weak force. When applied to quarks, the Yang-Mills field was called the gluon, because it acted like a glue to bind the quarks together. (Computer simulations show t
hat the Yang-Mills field condenses into a taffy-like substance, which then holds the quarks together, like glue.) To do this, one needed quarks coming in three types, or colors, obeying Gell-Mann’s three-quark symmetry. So a new theory of the strong force began to gain wide acceptance. This new theory was christened quantum chromodynamics (QCD), and today this represents the best-known representation of the strong nuclear force.
Higgs Boson—The God particle
So gradually, a new theory was emerging from all this chaos, called the Standard Model. The confusion surrounding the zoo of subatomic particles was lifting. The Yang-Mills field (called the gluon) held the quarks together in the neutron and proton, and another Yang-Mills field (called the W and Z particles) described the interaction between electrons and neutrinos.
But what prevented final acceptance of the Standard Model was the lack of the final piece of the jigsaw puzzle of particles, called the Higgs boson, or sometimes the God particle. Symmetry was not enough. We need a way to break that symmetry because the universe we see around us is not perfectly symmetrical.
When we look at the universe today, we see the four forces all working independently of one another. Gravity, light, and the nuclear forces, at first glance, seem to have nothing in common. But as you go back in time, these forces begin to converge, perhaps leaving only one force at the instant of creation.
