THE ELECTROWEAK THEORY

Another major advance in our understanding of the nature of the basic interactions came in 1964 when Steven Weinberg, Abdus Salam and Sheldon Glashow discovered a basic connection between the electric and weak interactions. Einstein had spent the latter part of his life trying without success to unify, find a common basis for, the electric and the gravitational force. It came somewhat as a surprise that the electric and weak interactions, which appear so different, had common origins. Their theory of the two forces is known as the electroweak theory.
In the electroweak theory, if we heat matter to a temperature higher than 1000 billion degrees, we will find that the electric and weak interaction are a single force. If we then let the matter cool, this single electroweak force splits into the two separate forces, the electric interaction and the weak interaction. This splitting of the forces is viewed as a so called phase transition, a transition in the state of matter like the one we see when water turns to ice at a temperature of 0°C.
The temperature of the phase transition for the electroweak force sounds impossibly hot, but it is attainable if we build a big enough accelerator. The

Figure 1: Paths for the large particle accelerators at CERN. The Geneva airport is in the foreground.

cancelled superconducting supercollider was supposed to allow us to study the behavior of matter at these temperatures.
One of the major predictions of the electroweak theory was that after the electric and weak interactions had separated, electric forces should be caused by zero rest mass photons and the weak interaction should be caused by three rather massive particles given the names W ⁺ , W ^– and Z ⁰ mesons. These mesons were found, at their predicted mass, in a series of experiments performed at CERN in the late 1970s.
We have discussed Yukawa’s meson theory of forces, a theory in which the range of the force is related to the rest mass of the particle responsible for the force. As it turns out, Yukawa’s theory does not work for nuclear forces for which it was designed. The gluons have zero rest mass but because of their interaction, gives rise to a force unlike any other. What Yukawa’s theory does describe fairly well is the weak interaction. The very short range of the weak interaction is a consequence of the large masses of the weak interaction mesons W ⁺ , W ^– and Z⁰ . (The W mesons are 10 times as massive as a proton, the Z⁰ is 11 times as massive.)