Fermilab Today Friday, Jan. 10, 2014
Physics in a Nutshell


This graphic of strings and the Superman emblem was chosen to introduce today's topic: superstrings. The kitten is there just to get your attention.

One of the oldest scientific questions in history is "What are the ultimate building blocks of the universe?" Today's article talks about a cool idea called "superstrings," tiny subatomic strings that play a cosmic and subatomic symphony. Superstrings are a possible answer to the ancient question surrounding the identity of the universe's smallest components. Understanding this answer requires some historical context.

The first recorded debate on the subject was written down about 2,500 years ago in Greece, with a philosopher named Democritus making the most accurate guess when he postulated discrete units he called atomos. The question was left to the realm of the philosophers for millennia until the 1700s, when modern chemistry began to shed light on the topic using empirical techniques. With the identification of the atoms of the chemical elements, central features of Democritus' model were validated.

Over the next two-plus centuries, scientists proposed and discovered increasingly smaller components that make up our universe. They discovered the familiar proton, neutron and electron that make up atoms. Then they learned that the proton and neutron contained even smaller components called quarks. They also discovered that the two types of quarks that make up protons and neutrons, called the up and down quarks, weren't the only quarks out there.

Today we know the building blocks of matter can be classified into two types called quarks and leptons, each of which consists of six examples. The six quarks are called up, down, charm, strange, top and bottom. The six leptons are called electron, electron neutrino, muon, muon neutrino, tau and tau neutrino. With so many fundamental particles, a new organizing principle is needed to make sense of what was understood to be the universe's building blocks.

So what the heck is the story with these particles of the Standard Model, the modern-day atomos of Democritus? What role do they play in our understanding of the subatomic world?

Well, we don't know the answer to that in detail. We do know of patterns. Up, charm and top quarks all have the same electrical charge but rather different masses, with up being the lightest and top the heaviest. Down, strange and bottom also have identical electric charge and increasing mass. The electron, muon and tau lepton exhibit the same behavior. Naturally, when we see recurring patterns like this, we go looking for an explanation. One such explanation is superstrings.

Unlike what we've seen before, with each particle being composed of an even smaller one, superstrings break the pattern. Superstring theory postulates that the ultimate building block of matter consists of tiny, tiny "strings" that vibrate. Strings that vibrate the least are the quarks and leptons with the smallest mass, while the heavier particles have more energetic vibrations. Employing a musical metaphor, it's as if the electron might be a B-flat, while the bottom quark might be an F-sharp above that.

There are some bizarre consequences to this idea. Early calculations using superstring theory yielded nonsense. For example, when adding up all the things that could happen when two strings interacted, physicists found that there was more than a 100 percent chance something would happen. Since 100 percent is the maximum, that was a very bad outcome. These calculations were done in ordinary three dimensional space.

Then a curious soul played around with the same calculation, but using more than three spatial dimensions. Scientists found that when four dimensions were invoked, the answer was more sensible, but still above 100 percent. By adding more and more dimensions, the answer became more and more reasonable. When 10 dimensions were invoked, the result gave the expected 100 percent. This is the reason people talk about 10 dimensions of space and time.

Physicists put forward five different versions of superstring theory, but it turned out that by invoking one additional dimension, all the five versions turned out to be the same. This is the reason you might have heard that there are 11 dimensions of space and time.

It is important to note that the idea of superstrings has not been proven empirically. It could well be wrong. However, the idea that the universe could have a single building block (the string) and that all the observed particles of nature are just different vibrations is a very attractive idea. Another benefit of superstring theory is that it incorporates gravity very easily. Other ideas unifying the various forces have a tough time with gravity. This isn't a reason to believe in superstrings, but it is a reason to find the theory to be attractive.

The story of superstrings is much bigger than I can describe here. I've only given the quickest possible description. If the idea of the entire universe being a cosmic melody played by tiny strings interests you, you might be interested in reading Brian Greene's book The Elegant Universe.

—Don Lincoln

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