Warped Passages - Lisa Randall [38]
Before physicists could look inside matter and deduce its composition, they needed technological advances to create sensitive measuring instruments. But every time they developed more accurate technological tools, structure—more elementary constituents—emerged. And every time physicists had access to tools that could probe still smaller sizes, they discovered yet more fundamental ingredients: substructure, constituents of the previously known structural elements.
The goal of particle physics is to discover matter’s most basic constituents and the most fundamental physical laws obeyed by those constituents. We study small distance scales because elementary particles interact at these scales, and it’s easier to disentangle fundamental forces. At large scales, the basic ingredients are bound into composite objects, which makes fundamental physical laws difficult to disentangle and therefore more obscure. Small distance scales are interesting because new principles and connections apply there.
Matter is not simply a Russian doll with smaller copies of similar entities inside. Smaller distances reveal truly novel phenomena. Even the workings of the human body—the heart and the circulation of the blood, for example—were badly misconstrued until scientists such as William Harvey cut people open in the 1600s and looked inside. Recent experiments have done the same thing with matter, exploring smaller distances where new worlds operate via more fundamental physical laws. And just as the blood’s circulation has important consequences for all human activity, the fundamental physical laws have important consequences for us on larger scales.
We now know that all matter is made up of atoms, which combine through chemical processes into molecules. Atoms are very small, about an angstrom, or one-hundredth of a millionth of a centimeter in size. But atoms are not fundamental: they consist of a central, positively charged nucleus which is surrounded by negatively charged electrons (see Figure 30). The nucleus is far smaller than the atom, occupying only about one hundred thousandth of the atom’s size. And the positively charged nucleus is itself composite: it is made from positively charged protons and neutral (uncharged) neutrons, collectively known as nucleons, which are not very much smaller than the nucleus in size. This was the picture of matter that scientists held before the 1960s, and is very likely the blueprint you learned about in school.
This template for the atom is correct, although, as we will see later, quantum mechanics gives a more interesting picture of an electron’s orbits than any picture you can draw. But we now know that even the proton and neutron are not fundamental. Contrary to Gamow’s quote in the introduction, the proton and neutron contain substructure, more fundamental ingredients known as quarks. The proton contains two up quarks and one down quark, while the neutron contains two down quarks and one up quark (see Figure 31). These quarks are bound together through a nuclear force known as the strong force. The electron, the other component of the atom, is different. So far as we can tell, it is fundamental: the electron cannot be divided into smaller particles and contains no substructure within.
Figure 30. The atom consists of electrons circulating around a tiny nucleus. The nucleus is composed of positively charged protons and charge-neutral neutrons.
Figure 31. The proton and neutron are composed of more elementary quarks bound together through the strong force.
The Nobel Prize-winning physicist Stephen Weinberg coined the term “Standard Model” to label the well-established particle physics theory that describes the interactions of these fundamental building blocks of matter—the electron, the up quark, and the down quark—as well as other fundamental particles that we will get to momentarily. The Standard Model also describes three of the four forces through which the elementary particles interact: electromagnetism, the