Reading Comprehension Practice Test #1

As any good contractor will tell you, a sound structure requires stable materials. But atoms, the building blocks of everything we know and love—bunnies, brownies, and best friends—don’t appear to be models of stability. Why are some atoms, like sodium, so hyperactive while others, like helium, are so aloof ? Why do the electrons that inhabit atoms jump around so strangely, from one bizarrely shaped orbital to another? And why do protons, the bits that give atoms their heft and personality, stick together at all? We are told that every atom has a tiny nucleus containing positively charged protons and uncharged neutrons, swarmed by a cloud of speedy electrons. We are also told that like charges, such as protons, repel each other with a force that shoots up to infinity as they get closer. Even worse, you can’t get much closer than two protons in the nucleus of an atom. So what’s keeping atomic nuclei from flying apart? Obviously, some other force must be at work inside the atom, something that we can’t detect at our human scale. Physicists call this the “strong nuclear force.” But where does it come from? In order for this force to account for the binding of protons in the nucleus, it must have certain interesting features. First, it can’t have any sizeable effect beyond the radius of the atom itself, or it would play havoc with the nuclei of adjacent atoms, destroying matter as we know it. Second, it must perfectly balance the repulsive force of electricity at an “equilibrium point” of about 0.7 × 10⁻¹⁵ meters, the average distance between bound protons, in order to create a stable nucleus. Third, it must repel at even shorter distances, or else neutrons (which don’t have any electrostatic repulsion to balance the strong nuclear force) would collapse into each other. The graph shows the behavior of such a force relative to the repulsive electrostatic force. In 1935, Japanese physicist Hideki Yukawa proposed that the nuclear force was conveyed by a then-undiscovered heavy subatomic particle he called the pi meson (or “pion”), which (unlike the photon, which conveys the electrostatic force) decays very quickly and therefore conveys a powerful force only over a very short distance. Professor Yukawa’s theory, however, was dealt a mortal blow by a series of experiments conducted at Los Alamos National Laboratory in the early 1990s that demonstrated that pions carry force only over distances greater than the distance between bound protons. The pion was a plumber’s wrench trying to do a tweezer’s job. Current atomic theory suggests that the strong nuclear force is most likely conveyed by massless particles called “gluons” according to the theory of quantum chromodynamics, or QCD for short. According to QCD, protons and neutrons are composed of smaller particles called quarks, which are held together by the aptly named gluons. This quark-binding force has a “residue” that extends beyond the protons and neutrons themselves to provide just enough force to bind the protons and neutrons together. If you’re hoping that QCD ties up atomic behavior with a tidy little bow, you may be just a bit disappointed. As a quantum theory, it conceives of space and time as tiny chunks that occasionally misbehave, rather than smooth predictable quantities, and its mathematical formulas are perhaps as hard to penetrate as the nucleus itself. In line 7, “ aloof ” most nearly means