Unification of Forces

 

One of the landmark achievements in the history of physics was the 19th century theory of electromagnetism, based on the theory of Maxwell and experiments by Faraday and Oersted showing that magnetic effects could produce electric fields and electrical effects could produce magnetic fields. 

Fusion of ⚡ and 🧲 forces

The previously separate sciences of electricity and magnetism became linked under the common designation of electromagnetism. 

Fusion of electromagnetic and weak forces

In the 20th century, the attempt was made to carry this linking further to include other forces. First it was shown that electromagnetism and the weak force can be understood as two different aspects of the same force, called the electroweak force. If we study particle interactions at a high enough energy, these two forces behave similarly. It is convenient for us to regard them as separate forces for many of the effects we shall discuss, just as we often find it convenient to speak separately of electric and magnetic forces when we discuss electromagnetic phenomena. 

The theory of the electroweak force, which was proposed independently in 1967 by Stephen Weinberg (and for which he, along with Sheldon Glasgow, another originator of the theory, received the 1979 Nobel Prize in physics), suggested that, just as the photon is the carrier of the electromagnetic force, there should be heavy particles that carry the weak force, and these new particles should, on an energy scale of 100 GeV (about 100 times the rest energy of the proton), behave similarly to a high-energy photon. 

Discovery of W+, W- and Z Particles

In 1983, a research team at the European Center for Nuclear Physics (CERN), led by Carlo Rubbia and using experimental techniques developed by Simon van der Meer, discovered the predicted particles, now known as W⁺, W^- and Z", for which Rubbia and van der Meer were awarded the 1984 Nobel Prize in physics. The discovery of these particles provided the evidence for the unification of the electromagnetic and weak interactions into the electroweak interaction. 

Next the attempt was made to combine the strong and electroweak forces at a new higher level of unification. Theories that do so are called grand unified theories (GUTs), and at the present time there are many candidates for GUTs but none has as yet emerged as the correct one. Because the energy at which the forces merge is immense, perhaps 10¹⁵ GeV (10¹¹ times the energy of the largest particle accelerator yet built or even contemplated), we cannot do experiments to test the GUTs directly. We must therefore rely on tests at obtainable energies, where the effects are exceedingly small. One prediction of these theories is that the proton should not be a stable particle but should decay on a time scale greater than 10³¹ years. (Compare this number with the age of the universe, about 10¹⁰ years.) Searches for proton decay have been made by looking for a characteristic light signal that would accompany the decay of one of the protons in a large volume of water. So far such experiments have not observed proton decay, but they have placed a lower limit of 10³³ years on the decay lifetime. These results exclude certain of the GUTs, and experiments continue to try to verify the theories.

The final step in the unification would be to include gravity in the scheme to create a theory of everything (TOE). There is not yet a quantum theory of gravity, so it is difficult to anticipate the form that these theories might take, but they nevertheless provide challenges for theoretical speculation.