GUTs and TOEs

GUTs and TOEs may sound like funny looking body parts, but for scientists, they are acronyms for Grand Unified Theories and Theories of Everything. These theories aspire to unify all of physics into a single idea.

Let's talk about GUTs. No, I'm not talking about that extra weight you put on when you visited your mom over the holidays. I'm talking about nothing less than the ultimate goal of all Fermilab physicists. I'm talking about a Grand Unified Theory.

There is a long history of unification in science. If you think about it, it's not at all obvious that the phenomenon that pulls you to the ground and gives you weight is related in any way to the march of the planets and occasional comet across the night sky. It took the genius of Isaac Newton in the late 1600s to say that these two seemingly different phenomena can both by described by his universal theory of gravity. Scientists say that Newton "unified" the phenomena of celestial motion and how a dropped apple behaves on Earth.

In the mid-1800s, a large number of scientists studied the phenomena of electricity and magnetism. Electricity is represented by a fleeting stroke of lightning, static cling and the behavior of a battery connected to wires, while magnetism is seen in the dogged determination of a compass to point northwards. After reviewing decades of work by other scientists, James Clerk Maxwell realized how everything was interconnected and wrote a set of equations that showed that electricity and magnetism were really one and the same thing. His equations unified the two phenomena, resulting in our now-familiar electromagnetism. The beauty of this unification was made even more evident when it was shown that electromagnetism also explained the behavior of light and all of chemistry.

The pattern repeated itself in the 1960s, when physicists were able to show that electromagnetism and the weak force were intimately related. Scientists now talk about the "electroweak" force. Again, two seemingly unrelated phenomena were unified. The recent observations of what might be the Higgs boson give us even more reason to believe that this unification is real.

Given the history, it is natural to ask, "Is it possible that the remaining three known forces—gravity, the strong nuclear force and the electroweak force—might be but a single force, made to appear different because of how we are looking at it?" If this idea is true, the resultant, single, overarching theory that explains it is called the Theory of Everything. A TOE is the famous theory that is so simple and so overarching, the equation of the universe could be written on a t-shirt.

However, prior to discovering a TOE, scientist will first need to merge just the strong nuclear force and the electroweak force. This unification is called a GUT or Grand Unified Theory, and it represents an ideal toward which particle physics aspires. There's no guarantee that the universe needs to have a single underlying principle, but the history leads us to think it is likely. If the GUT/TOE ideas are true, our current best guess is that we will be able to prove it only at very high energy. Different theories predict a different unification energy, but the more established ones predict a GUT energy of about 10¹⁶ GeV and a TOE energy of about 10¹⁹ GeV. In comparison, the LHC explores the relatively low energy of 10⁴ GeV. Thus, at a minimum, unification is expected to occur at energies a trillion times higher than current experiments can attain.

The prediction for the unification energy can be modified considerably if physicists discover additional dimensions of space and time. Under that interesting proposal, it might be possible that the unification energy could be not very much higher than what we can explore with the LHC.

You should not believe in grand unification or in a theory of everything. They're just ideas, indeed ideals, and they might be wrong. However, they remain the ultimate goal of all particle physicists, and noble goals they are.

—Don Lincoln

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