Warped Passages - Lisa Randall [80]
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).
As with photon exchange, weak gauge boson exchange produces forces that can be attractive or repulsive, depending on the particles’ weak charges. Weak charges are numbers that play the same role for the weak force that electric charge plays for the electromagnetic