Higgs and the Mass of the Universe

In all the recent hoopla about the long-sought Higgs boson, you often hear it said that it is responsible for the mass of the universe. This is not true. Assuming it exists, the Higgs boson is actually responsible for only a small fraction of the total mass of the universe.

This is not to say that the Higgs boson is not important. The main role of the Higgs in the standard model of elementary particles is to provide for the symmetry breaking of the unified electroweak force by giving mass to the weak bosons and splitting the electromagnetic and weak nuclear forces. It also gives mass to the other elementary particles. If elementary particles did not have mass, they would all be moving at the speed of light and never stick together to form stuff like stars, cats, and you and me.

The mass of the universe, however, is not simply the sum of the masses of the elementary particles that constitute matter. Einstein showed that the mass of a body is equal to its rest energy. If that body is not elementary but composed of parts, then its rest energy as a whole will be the sum of all the energies of its parts. This sum will include the kinetic and potential energies of the parts in addition to their individual rest energies.

Now, for the bodies of normal experience, such as your neighbor's cat, the kinetic and potential energies of their parts are small compared to their rest energies. So, so for all practical purposes, the total mass of a cat is equal to the sum of the masses of its parts.

This is even true at the microscopic scale. The masses of chemical elements are, typically, thousands of MeV (million electron volts, in energy units), while the kinetic and binding energies are a few tens of electron volts. Only when you get down to the nuclei of the chemical elements do you get kinetic and potential energies that are measurable fractions of their rest energies.

Inside nuclei, we have nucleons--protons and neutrons--that are themselves composed of quarks. Since quarks do not appear as free particles outside nucleons, their masses must be estimated from studying the effects of their mutual interactions on the masses and other properties of nucleons and other particles that are composed of quarks. Fortunately, there are only six quarks but hundreds of particles made from these quarks to provide data to pin down quark properties. By using supercomputers, physicists have obtained reliable estimates of quark masses. The result: the masses of the quarks inside a proton or neutron constitute only 1 percent of its mass.

The objects familiar to most humans, including most scientists, have masses that are essentially given by the number of protons and neutrons they contain. So, we can say that only 1 percent of that mass arises from the masses of quarks. Furthermore, this normal stuff is itself only 5 percent of the total mass of the universe.

Now, where does that leave Mister Higgs? While the Higgs mechanism gives masses to elementary particles, other processes may contribute to the masses of quarks. I need not get into these. Even if all the mass of a quark comes from the Higgs mechanism, the Higgs contribution to the mass of the universe is less than one part in 2,000.