Why Does E=mc2_ - Brian Cox [25]
The moon moves 4 centimeters farther away from the earth every year. Why? Picture the moon in your mind’s eye as being stationary above the surface of the spinning earth. The water in the oceans directly beneath the moon will bulge out a tiny bit toward the moon because the moon’s gravity is pulling it, and the earth will rotate once a day beneath this bulge. This is the cause of the ocean tides. There is friction between the water and the surface of the earth, and this friction causes the earth’s rate of spin to slow down. The effect is tiny but measurable; the earth’s day is gradually lengthening by approximately two-thousandths of a second per century. Physicists measure the rate of spin using angular momentum, so we can say that the angular momentum of the earth is reducing over time. Noether tells us that because the world looks the same in every direction (to be more precise, the laws of nature are invariant under rotations), then angular momentum is conserved, which means that the total amount of spin must not change. So what happens to the angular momentum the earth loses by tidal friction? The answer is that it is transferred to the moon, which speeds up in its orbit around the earth to compensate for the slowing down of the earth’s rotation. This causes it to drift slightly farther away from the earth. In other words, to ensure that the total angular momentum of the earth and moon system is conserved, the moon must drift into a wider orbit around the earth to compensate for the fact that the earth’s rate of spin is slowing down. This is a very real and quite fantastic effect. The moon is big, and it is drifting farther away from the earth as every year goes by to conserve angular momentum. Italian novelist Italo Calvino found it so wonderful that he wrote a short story called “The Distance of the Moon,” in which he imagined a time in the distant past when our ancestors could sail each night across the ocean in boats to meet the setting moon and clamber onto its surface using ladders. As the moon drifted farther away over the years, there came a night when the moon lovers had to make a choice between becoming trapped on the moon forever or returning to Earth. This surprising and, in the hands of Calvino, strangely romantic phenomenon has its explanation in the abstract concept of invariance and the deep connection between invariance and the conservation of physical quantities.
It is difficult to overstate the importance of the idea of invariance in modern science. At the heart of physics is the desire to produce an intellectual framework that is universal and in which the laws are never a matter of opinion. As physicists, we aim to uncover the invariant properties of the universe because, as Noether well knew, these lead us to real and tangible insights. Identifying the invariant properties is far from easy, however, because nature’s underlying simplicity and beauty are often hidden.
Nowhere in science is this truer than in modern particle physics. Particle physics is the study of the subatomic world; the quest for the fundamental building blocks of the universe and the forces of nature that stick them together. We have already met one of the fundamental forces, electromagnetism. Understanding it led us to an explanation for the nature of light that has launched us on the road to relativity. In the subatomic world there are two other forces of nature that hold sway. The strong nuclear force sticks the atomic nucleus together at the heart of the atom, and the weak nuclear force allows stars to shine and is responsible for certain types of radioactive decay; the use of radiocarbon dating to measure the age of things, for example, relies on the weak nuclear force. The fourth force is gravity, the most familiar perhaps, but by far the weakest. Our best theory of gravity today is still Einstein’s general theory of relativity and, as we shall see in the final chapter, it is a theory of space and time. These four forces act between just twelve fundamental