Of course, the fact that some forms of radiation can cause sufficient damage to kill the living cells they impinge can be a good thing. The cells in a piece of meat from the butcher are nonliving, but the bacteria within the meat that can cause salmonella or other diseases are very much alive. Exposure to radiation does no significant damage to the cells in the food that are already dead but can penetrate and kill the bacteria living in the food, thus making the food much safer to eat. The exposure to radiation does not convert the stable nuclei in the food’s atoms into unstable nuclei, and they will not in turn emit their own radioactivity. A material that has been irradiated does not (with very few exceptions) itself become radioactive.
A lot of the harm of nuclear radiation is caused when either negatively charged beta rays or positively charged alphas or gamma-ray photons collide with atoms and cause them to lose their electrons. This process is called “ionization,” and when an atom loses some or all of its electrons, its chemical properties can be radically changed. Sometimes these changes are beneficial. The Earth is constantly being bombarded with cosmic rays, which are primarily (though not exclusively) high-energy protons that come from sources as close as our sun and as distant as other galaxies. When these protons (a few of which have energies of a million trillion electron Volts) strike the atmosphere, they can generate a slew of other elementary particles moving near the speed of light. When some of these particles strike the DNA in our cells, they can cause ionizing damage and alter the chemical properties of our genetic code. If the affected DNA is in a sperm or egg cell, these changes may be passed along to offspring. In this way exposure to cosmic rays is a natural source of genetic mutation, leading to biological modifications that can be harmful but occasionally improve an organism’s fitness to its environment.
But beneficial mutations that do not harm the original organism and lead to genetic alterations that improve the offspring’s reproductive success are extremely rare. More commonly, chemical modifications induced by ionizing radiation can destroy cells themselves or induce deleterious alterations in chromosomes or DNA. This damage often leads to the formation of malignant cancerous tumors, quite different from the runaway cell growth presented in science fiction movies such as The Amazing Colossal Man or The Attack of the Giant Leeches.
CHAPTER ELEVEN
Man of the Atom
Before there was physicist Jon Osterman, there was physicist Philip Solar. In the 1986 DC comic book Watchmen, Osterman was disintegrated by the accidental removal of his intrinsic field at Gila Flats and reconstructed himself as the superpowered Dr. Manhattan. In the 1962 Gold Key comic book series Solar—Man of the Atom, Phillip Solar was exposed to a lethal dose of radiation in a sabotaged nuclear research experiment at Atom City, yet survived, though he acquired “quantum powers.” In issue # 2 he is vaporized by an atomic bomb blast but manages through sheer force of will to reconstitute himself as, well, Dr. Solar, which was his name after all. As a survivor of graduate school myself, I can empathize with Osterman and Solar’s inclination to retain the title associated with their Ph.D.’s, in the lab or as a superpowered hero. Once you’ve passed through the crucible of a graduate school candidacy exam, having to reassemble yourself up from the subatomic level is not as challenging as you might think.
In writer Alan Moore’s initial outline of the DC comic book miniseries Watchmen, he intended to use comic book characters created by Charlton, another comic book publisher. Charlton had declared bankruptcy, and the company had been acquired by DC Comics, home of Superman and Batman. Moore’s initial outline for Watchmen made direct use of the Charlton characters, but the editors at DC Comics, seeing that some of these characters would not make it out of the miniseries unscathed, instructed Moore to 8 instead employ alternate versions of the Charlton heroes. Dr. Manhattan is the analog of Captain Atom, an air force captain, who was disintegrated and (I’m sure you can see this coming at this stage) through force of will was able reassemble himself into a quantum-powered superbeing.
Captain Atom’s powers were quantum based only in that he was able to manipulate energy, which he employed primarily for flight, superstrength, and energy blasts. Dr. Solar, though not a Charlton character, seems to be a closer antecedent for Dr. Manhattan, as Solar was also able to change size (Dr. Solar # 10 and # 11), split himself into multiple copies of himself (Dr. Solar # 12), and manipulate matter and energy, though unlike the blue Dr. Manhattan, Dr. Solar’s skin turned green when he used his powers. There are just enough differences among Captain Atom, Dr. Solar, and Dr. Manhattan that it is unlikely that they are all the same person, on three different versions of Hugh Everett’s many worlds, though further study appears warranted.
One of the more accurate manifestations of quantum mechanical powers was presented in “Solar’s Midas Touch,” in 1965’s Dr. Solar, Man of the Atom # 14. In this tale an underwater nuclear reactor pile went critical when one of the control rods (whose role is to absorb neutrons, decreasing the rate of uranium fission, as described in Chapter 9) broke. Dr. Solar, whose powers are normally energized by exposure to radiation, went underwater to fix the reactor but found himself weakened by the reactor’s radioactivity (through a process not clearly explained in the comic). Eventually he was rescued by a worker wearing a lead-lined safety suit, who would have done the job in the first place if Dr. Solar hadn’t attempted to “save the day.” The additional radiation he absorbed from the reactor temporarily endowed Solar with a new superpower. As illustrated in Figure 27, whenever Dr. Solar comes into physical contact with an object, he transmutes it into the next element up the periodic table. In Figure 27, he transforms gold, with seventy-nine protons, into mercury, with eighty protons; earlier he grasps a copper rod (twenty-nine protons) and converts it into zinc (atomic number 30); and even when flying he begins to choke when the oxygen (atomic number 8) turns into fluorine gas (with nine protons). This newfound power of Dr. Solar’s appears to be the abil-ity to initiate beta decay of the neutrons in any object he touches, inducing elemental transmutation via the weak nuclear force, an aspect of Watchmen’s “intrinsic field” that we have not discussed much yet.
Figure 27: In Dr. Solar, Man of the Atom # 14, an additional nuclear accident endows Dr. Philip Solar (wearing the scuba suit and visor) with the temporary ability to induce beta decay via the weak nuclear force in any object he comes into direct contact with, thus transmuting gold into mercury.
We saw in Chapter 9 that neutrons, through the strong force, hold the nucleus together by binding with protons and other neutrons and overwhelming the electrostatic repulsion that would, in their absence, cause the protons to fly out of the nucleus. Protons also exhibit the strong force, but without neutrons there is not sufficient binding energy to hold together a nucleus consisting only of protons. Neutrons themselves are not stable outside of a nucleus. A neutron sitting alone in the lab will decay into a proton and an electron with a half-life of about ten and one quarter minutes. The electron will be moving very near the speed of light, and when this process occurs within a nucleus, it is the source of the beta rays emitted from unstable isotopes.
As the total mass and energy of an isolated system must remain unchanged in any process, a “stationary” neutron43 can decay only to fundamental particles with less mass than the neutron’s. A neutron will thus decay into a proton, which has a slightly smaller mass, while a “stationary” proton could not decay into a heavier neutron. However, as the neutron is electrically neutral, and the proton is positively charged, the decay must also generate a negatively charged electron, in order for the total electrical charge to remain unchanged before and after the decay (we have not needed to invoke this principle before now, but another conservation principle in physics, comparable to conservation of energy or conservation of angular momentum, is conservation of charge, in that the net electrical charge can be neither created nor destroyed in any process). An electron is nearly two thousand times lighter than a proton, less than the mass difference between neutro
ns and protons, so adding an electron to the decay is still consistent with mass conservation. While a neutron decaying into a proton and an electron means that mass and electrical charge are balanced during the decay, examination of the kinetic energy of the proton and the high-speed electron (that is, the beta ray) and comparison to the rest-mass energy of the neutron indicates that some energy went missing in the process—not a lot, but enough to notice, and enough to cause trouble.
When physicists in the late 1920s discovered this phenomenon and realized that it appeared to violate the principle of conservation of energy, they were faced with two choices: (1) either abandon conservation of energy, at least for neutron decay processes, or (2) invent a miracle particle that was undetectable by instrumentation of the time but that carried off the missing energy. In 1930, Wolfgang Pauli (whose exclusion principle I address in the next section) suggested going with option 2. Knowing that this ghost particle had to be electrically neutral and had to have very little or no mass, Enrico Fermi called it the “little neutral one” in Italian, or “neutrino.”44
Detectors were eventually constructed to observe these particles, and their existence was confirmed in 1956. These particles not only really exist, aside from photons they are the most common particle in the universe. Their interactions with matter are governed by the weak nuclear force, which is one hundred billion times weaker than electromagnetism (the force by which electrons interact with matter). Neutrinos consequently barely notice normal matter (it takes more than two light years of lead—that is, a length of more than ten trillion miles—to stop one). If you hold out your thumb and blink, during that time period more than a billion neutrinos will pass through your thumbnail.
Dr. Solar, after his radiation overdose, must have gained an uncontrolled ability to induce beta decay in any object with which he came into contact. If a gold atom, with seventy-nine protons, seventy-nine electrons, and 118 neutrons, has one of its neutrons spontaneously decay into a proton and an electron, then it will have eighty protons, eighty electrons, and 117 neutrons. The lightest, stable configuration of mercury has eighty protons, eighty electrons, and 118 neutrons, so Dr. Solar will have created an unstable isotope of mercury in Figure 27. The half-life of this isotope of mercury with 117 neutrons is roughly two and a half days, so there will be time for Solar to finish his adventure and try to restore the transmuted mercury back to its original golden state. While transforming one element into its periodic-table neighbor via neutron beta decay is not quite the alchemist’s dream of transmuting lead into gold (normal beta decay would convert platinum, with seventy-eight protons, into gold, with seventy-nine protons, so, depending on world exchange prices, you may wind up losing money on the deal), a process known as “reverse beta decay” would turn mercury into gold. While we cannot initiate such a conversion on Earth at will, fortunately this inverse process occurs constantly in the center of the sun, keeping the sun shining and providing the basis of all life.
The light from the sun—which is transformed by photosynthesis into chemical energy stored within plants, which in turn provides us with the energy we need to maintain our metabolisms—originates from nuclear transformations in the star’s core. Four protons, that is, hydrogen nuclei, subjected to the extreme pressures and temperatures at the center of the sun, are fused together to form helium nuclei. But a helium nucleus consists of two protons and two neutrons, not four protons. Recall that neutrons are necessary as mediators of the strong nuclear force that holds the nucleus together. Thus, to make helium out of hydrogen, you first have to combine two protons and then through reverse beta decay convert one of the protons into a neutron.
I argued above that a single proton cannot convert into a neutron, as the mass of the proton is less than that of the neutron, and lighter objects cannot decay into heavier products. If two protons collide, the weak force operates on the protons, turning one into a neutron through reverse beta decay, as illustrated in Figure 28. The proton and neutron, subject to the strong force, become bound (now a deuterium nucleus—an isotope of hydrogen) and lower their en-ergy compared to an isolated proton and neutron. This lower energy is reflected in a smaller mass for the deuterium nucleus, relative to a free proton and neutron. While the mass difference is very small, through E = mc2 the energy difference of the bound deuterium is significant, and it emits a 2.225-million-electron-Volt gamma-ray photon during formation. In addition to the neutron generated by the weak force, the reaction creating a deuterium nucleus yields an antimatter electron (which has a positive electrical charge like a proton, but the mass of an electron) and a neutrino.
Figure 28: Sketch of the nuclear reactions in the center of the sun by which protons (hydrogen nuclei) combine to form alpha particles (helium nuclei). In step (a), two protons (represented by open circles) tunnel together, where the weak force converts one proton (open circle) into a neutron (dark circle). The proton and neutron then form a bound deuterium nucleus, with the release of a gamma ray photon (the positron and neutrino released are not shown for simplicity). The deuterium can then collide with another proton in step (b) and form a bound proton-proton-neutron nucleus, termed helium-3. In step (c) we indicate a possible reaction where two helium-3 nuclei collide and form a stable helium-4 nucleus (two protons and two neutrons), with the release of two protons and another gamma ray. Similar mechanisms result in the fusion of helium nuclei to synthesize heavier elements, such as carbon and oxygen, and up.
The weak force extends over a length scale roughly one thousand times smaller than that of the strong force, which itself acts only over distances less than the diameter of a nucleus. Two protons, both being positively charged, repel each other, and the closer they are, the greater the repulsive force. So one must force the two protons very close together, overcoming their electrical repulsion, in order for there to be an opportunity for the weak force to transform, through reverse beta decay, one of the protons into a neutron. The temperatures and pressures in the center of the sun are enormous, so that there are many opportunities for high-velocity collisions between two protons. However, even at the center of the sun the proton speeds are not sufficient to overcome the electrical repulsion when they draw too close. How do they manage to get past this electrical barrier? Through quantum mechanical tunneling!45 Just as the alpha particles in radioactive decay use tunneling to escape the strong-force barrier around the nucleus that keeps the protons and neutrons together, the two protons that join together, forming the simplest isotope of hydrogen, must tunnel to overcome the barrier of their mutual repulsion.
The deuterium nucleus created in the center of the sun is stable and continues to collide with other protons. Combining this deuterium with another proton forms a nucleus with two protons (that is, helium) but only one neutron (making it helium 3, a lighter isotope of helium). Here again quantum-mechanical tunneling is required to get the second proton close enough to the deuterium nucleus, overcoming the electrical proton-proton repulsion, for the strong force to hold the second proton in the now larger nucleus. The lower energy of this bound state results in the release of another gamma-ray photon. This reaction is much more likely than for two deuterium nuclei to combine to form normal helium (two protons and two neutrons).
There are then many different ways that the helium 3 or deuterium nuclei can interact to form a stable helium nucleus, all of which involve quantum mechanical tunneling to get the positively charged nuclei close enough for the strong force to operate, resulting in the release of a great deal of energy in the form of kinetic energy of the nuclei, gamma rays, and neutrinos. The neutrinos pass right through the sun and head off in all directions, while the gammas heat up the nuclei and electrons in the center, accelerating them and causing them to emit electromagnetic radiation at all wavelengths. The light created in the center of the sun is scattered many, many times before reaching the surface, where it then takes the brief, eight-and-a-half-minute journey to Earth. Before reaching the surface, the average photon spends forty thousand year
s colliding with the dense nuclear matter in the sun’s interior. The outward energy pressure counteracts the inward gravitational pull and keeps the diameter of the sun fairly stable.
In addition to providing us with energy, this fusion process is the mechanism by which elements heavier than helium are synthesized. Our sun is actually a second-generation star that formed after a much larger star passed through its life cycle and “went supernova.” Our sun converts a great deal of hydrogen as it generates energy—approximately six hundred million tons per second. But eventually stars exhaust their supply of hydrogen, and the star collapses until the temperature and pressure rise to the point where helium nuclei begin to fuse, forming carbon. The process continues, generating nitrogen, oxygen, silicon, and other heavy elements up the periodic table to iron and nickel. However, the larger the nucleus created, the less energy is released per reactant, and at the iron/nickel point, the outward flow of energy is insufficient to counteract the inward gravitational pull. At this stage the star collapses onto itself; in the process, all elements heavier than iron are created, and there is an explosive outpouring of energy as the star becomes a supernova, releasing as much energy in a period of several weeks as our sun does over its entire lifetime. It is from the elements synthesized in a much larger star that lived and underwent a violent demise that the planets and sun of our solar system formed.
The Amazing Story of Quantum Mechanics Page 14
