Why Does E=mc2_ - Brian Cox [70]
Perhaps the whole of physics as we know it represents only the tip of the iceberg, or maybe we really are closing in on a “theory of everything.” Whichever is the case, one thing is certain: We currently have a theory that is demonstrably proven, after a vast and painstaking effort by thousands of scientists around the world, to work across a very broad range of phenomena. It is an astonishing theory, for it unifies so much, yet its central equation can be written on the back of an envelope.
FORMULA 7.1
We’ll call this central equation the master equation, and it lies at the heart of what is now known as the Standard Model of Particle Physics. Although it is unlikely to mean much to most readers at first sight, we can’t resist showing it above.
Of course, only professional physicists are going to know what’s going on in detail in the equation, but we did not show it for them. First, we wanted to show one of the most wonderful equations in physics—in a moment we will spend quite some time explaining why it is so wonderful. But also it really is possible to get a flavor of what is going on just by talking about the symbols without knowing any mathematics at all. Let us warm up by first describing the scope of the master equation: What is its job? What does it do? Its job is to specify the rules according to which every particle in the entire universe interacts with every other particle. The sole exception is that it does not account for gravity, and that is much to everyone’s chagrin. Gravity notwithstanding, its scope is still admirably ambitious. Figuring out the master equation is without doubt one of the great achievements in the history of physics.
Let’s be clear what we mean when two particles interact. We mean that something happens to the motion of the particles as a result of their interaction with each other. For example, two particles could scatter off each other, changing direction as they do so. Or perhaps they might spin into orbit around each other, each trapping the other into what physicists call a “bound state.” An atom is an example of such a thing, and in the case of hydrogen, a single electron and a single proton are bound together according to the rules laid down in the master equation. We heard a lot about binding energy earlier in the previous chapter, and the rules for how to calculate the binding energy of an atom, molecule, or atomic nucleus are contained in the master equation. In a sense, knowing the rules of the game means we are describing the way the universe operates at a very fundamental level. So what are the particles out of which everything is made, and just how do they interact with each other?
The Standard Model takes as its starting point the existence of matter. More precisely, it assumes the existence of six types of “quark,” three types of “charged lepton,” of which the electron is one, and three types of “neutrino.” You can see the matter particles as they appear in the master equation: They are denoted by the symbol ψ (pronounced “psi”). For every particle there should also exist a corresponding antiparticle. Antimatter is not the stuff of science fiction; it is a necessary ingredient of the universe. It was British theoretical physicist Paul Dirac who first realized the need for antimatter in the late 1920s when he predicted the existence of a partner to the electron called the positron, which should have exactly the same mass but opposite electrical charge. We have met positrons before as the byproducts of the process whereby two protons fuse to make the