Why Does E=mc2_ - Brian Cox [64]
two protons (no electrons are around this time, so we have no chance to make them stick together in a hydrogen molecule). Left alone, they would fly apart in opposite directions because they both carry positive electric charge. So it seems pretty pointless to try to push them closer together. Even so, let us imagine pushing the protons closer together and investigate what happens. One way to do this would be to hurl them at each other with increasing speed. The force of repulsion between the protons gets larger and larger as the protons get closer and closer together. In fact, it doubles in strength for every halving of the distance. It therefore seems that our protons are always destined to be flung apart. If the electrical repulsion were the only force in nature, this would certainly be the case. There are, however, the strong and weak nuclear forces to contend with. When the protons get so close together that they are almost touching each other (protons are not solid balls, so we can even think of them as overlapping) something very remarkable happens. Not always, but some of the time, when we bring two protons together like this, one of the protons will spontaneously turn itself into a neutron and the excess positive electric charge (the neutron being electrically neutral, hence its name) is shed as a particle called a positron. Positrons are identical to the electron except that they carry positive charge. Also emitted is a particle called a neutrino. Compared to the proton and neutron, which have very similar masses, the electron and neutrino are very light and they whiz off into the sunset, leaving the proton and neutron behind. The details of this transmutation process are very well understood using the theory of weak interactions developed by particle physicists in the second half of the twentieth century. We will show how it works in the next chapter. All we need to know here is that the process can and does occur. Free from the electric repulsion, the proton and neutron can snuggle together under the influence of the strong nuclear force. A proton and neutron bound up like that is called a deuteron, and the process of a proton turning into a neutron with the emission of a positron (or vice versa, with the emission of an electron, which can also happen) is called radioactive beta decay.
How does all of that fit with our understanding of energy? Well, the two original protons each have a mass of 938.3 MeV/c2. 1 MeV is equal to 1 million eV (the “M” stands for “mega” or “million”). The conversion between MeV/c2 and kilograms is easy enough: 938.3 MeV/c2 corresponds to a mass of 1.673 x 10-27 kilograms.10 The two original protons have a total mass of 1876.6 MeV/c2. The deuteron has a mass of 1875.6 MeV/c2, and the energy associated with the 1 MeV remainder is carried away by the positron and neutrino, of which approximately half is used up to manufacture the positron since it has a mass of around ½ MeV/c2 (neutrinos have almost no mass at all). So when two protons convert into a deuteron, a relatively tiny fraction (around 1/40 of 1 percent) of the total mass is destroyed and converted into the kinetic energy of the positron and the neutrino.
Squeezing two protons together to make a deuteron is one way to liberate the energy bound up in the strong force, and it is an example of nuclear fusion. The term “fusion” is used to describe any process that releases energy as a result of fusing together two or more nuclei. In contrast to the energy released in a chemical reaction, which is a result of the electromagnetic force, the strong nuclear force generates a huge binding energy. For example, compare the ½ MeV released when a deuteron is formed to the 6 eV released in our hydrogen-oxygen explosion. This is in keeping: The energy released in a nuclear reaction is typically a million times the energy released in a chemical reaction. The reason that fusion doesn’t happen all the time in our everyday experience here on Earth is that, because the strong force operates only over short distances, it only kicks in when the