Unity and symmetry

Apart from mass, the electromagnetic photon and the weak Z boson are the same particle—in two manifestations.

In this series of Physics in a Nutshell articles, Don and I have talked about each of the four fundamental forces of nature: electromagnetism, the strong force, the weak force and gravity. However, these four forces are not truly distinct. Many physicists are motivated by the idea that they are really four manifestations of a single principle, yet to be discovered. It's already clear that two of them, electromagnetism and the weak force, are related by a unifying principle, known as electroweak symmetry.
Perhaps the deepest idea in physics is Noether's Theorem, which states that symmetries in the laws of physics imply the existence of conserved quantities. For instance, the fact that an isolated experiment performed today would yield the same result as the same experiment performed tomorrow—time translation symmetry—is ultimately responsible for the conservation of energy. Since mass is a form of energy, this fact about the nature of time implies that matter is persistent and may be thought of as a substance.
Similarly, the conservation of electric charge is due to a symmetry. It is called gauge symmetry, and, if you're familiar with electronics, this is the reason that all voltages are measured relative to an arbitrary ground level. A circuit operating between 0 volts and 5 volts is the same as the circuit between 100 volts and 105 volts. When magnetism is added to the mix, gauge symmetry becomes more complicated, since you can trade electric voltages for magnetic potentials.
In the 1970s and '80s, physicists discovered that the weak force can also be added to the mix. The gauge symmetry of the weak force is much more complex than that of electromagnetism, but the two are actually factors of a single equation. An experimental consequence of this is that the photons of electromagnetism and the Z bosons of the weak force are really the same particle (or halfandhalf mixtures of two fundamental particles, depending on how you want to look at it). A photon can behave like a Z boson and viceversa, under the right circumstances.
However, the photon is massless and the Z boson is so massive that only a handful of highenergy accelerators have ever created them. If the symmetry were exact, they would both be massless. Since electric and weak charges do exist, it is widely believed that the laws of physics have electroweak symmetry, but something in the environment "breaks" the symmetry.
The Higgs mechanism is one possible explanation. This idea is that we are immersed in a field that interacts with Z bosons, giving them an effective mass. It is thus impossible to do a truly isolated experiment. Since the particle discovered last year seems to be the longsought Higgs boson, we may finally be able to test this theory.
Perhaps electroweak unification is only the first step. It would be intellectually satisfying if all forces derive from a single principle, but more importantly, that principle would reveal the conservation laws that give rise to matter itself.
—Jim Pivarski
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