The Grand Design

by Stephen Hawking

After uniting the weak and electromagnetic forces, physicists in the 1970s looked for a way to bring the strong force into that theory. There are a number of so-called grand unified theories or GUTs that unify the strong forces with the weak force and electromagnetism, but they mostly predict that protons, the stuff that we are made of, should decay, on average, after about 1032 years. That is a very long lifetime, given that the universe is only about 1010 years old. But in quantum physics, when we say the average lifetime of a particle is 1032 years, we don’t mean that most particles live approximately 1032 years, some a bit more and some a bit less. Instead, what we mean is that, each year, the particle has a 1 in 1032 chance of decaying. As a result, if you watch a tank containing 1032 protons for just a few years, you ought to see some of the protons decay. It is not too hard to build such a tank, since 1032 protons are contained in just a thousand tons of water. Scientists have performed such experiments. It turns out that detecting the decays and differentiating them from other events caused by the cosmic rays that continually shower us from space is no easy matter. To minimize the noise, the experiments are carried out deep inside places such as the Kamioka Mining and Smelting Company’s mine 3,281 feet under a mountain in Japan, which is somewhat shielded from cosmic rays. As a result of observations in 2009, researchers have concluded that if protons decay at all,...