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In the fifth century B.C., the Greek philosopher Leucippus of Miletus proposed that everything was made of tiny, indivisible particles called atoms. Atoms were experimentally identified in the 19th and 20th centuries, but it turned out they were not the fundamental building blocks of the universe. They instead were divisible into protons, neutrons, and electrons, begging the question: What were those particles made of? Then came the discovery of quarks, the even smaller particles making up subatomic particles, in the 1960s.
But these discoveries still left many questions unanswered. While quantum physics (the study of very small systems) was able to describe most of the forces governing tiny particles like quarks, electrons, and neutrinos — the strong, weak, and electromagnetic forces — there was no such theory that also described the force that most noticeably affects us at the large scale: gravity.
String theory grew out of an attempt in the 1960s to describe the strong force, which holds quarks together to make up protons and neutrons. But physicists soon realized that this theory made more sense if the objects in it were spread out like strings rather than single-point particles, says Dean Rickles, professor of history and philosophy of science at the University of Sydney. Furthermore, when strings were incorporated into this theory, one of the ways the strings vibrated matched the behavior of the graviton — the hypothetical particle believed to carry the force of gravity. Inspired by this finding, scientists began developing string theory as a way to unify quantum physics and gravity.
Put simply, string theory says that everything is made of tiny, vibrating strings — and that the way they vibrate dictates what particles they make up, explains Geraint Lewis, professor of astrophysics at the University of Sydney. “The easiest way to think about it is that if we have a violin string, each harmonic corresponds to a type of particle.” These strings vibrate in “quite strange ways,” Lewis adds, because they exist in many dimensions. “We always have this idea of something simple like a guitar string. But when we have a string in 11 or 26 dimensions, they’re moving around back and forth, but their oscillations can be quite complicated.”
Furthermore, there are different kinds of strings: Some are closed to form loops and correspond to forces, while others are open and correspond to matter particles. You can think of strings like energy tubes that rotate and stretch, and the longer they are, the more energy is in them, says Rickles. Energy, in this case, is defined as “something that can do something, that can do work,” he explains. “You can make something move with that string.”
One of the strangest conclusions drawn from string theory thus far is that there are many more spatial dimensions than the three we typically notice. “If you want to write the mathematics to include all the different aspects of physics, you can’t get that with strings vibrating in one dimension or two dimensions or three dimensions,” says Lewis. Depending which version of string theory you’re going on, there are 11 or more spatial dimensions. We just don’t notice them because they’re curled up into tiny spaces.
Another confusing finding of string theory? That there are many possible ways the universe could operate, says Rickles. “They thought they had this unique theory of everything, then they realized there are billions and billions of possible ways the world could be,” he says. Physicists typically draw conclusions about which way string theory works based on our observations of the universe we’re in. For example, certain ways dimensions might curl up would not allow complex structures like stars and life to exist, so they rule it out. But this is controversial because it violates the Copernican principle, which says you shouldn’t treat humans as special.
On top of string theory allowing for all these different universes, there are five different string theories that seem to work out mathematically, begging the question of which one really describes our universe. An umbrella theory called M-theory has been proposed as a way to tie all these string theories together, says Rickles. “The idea is that each of those five consistent superstring theories is really just a different way of looking at one and the same underlying theory, much like the proverbial blind men and the elephant think they are feeling different objects, but it’s really just the same thing taken from different perspectives.”
This all comes with one major caveat: Nobody has actually observed these strings. “At the moment, we have no real proof that string theory is correct,” Lewis admits. “Some people would look at the mathematics and think it has a beauty to it — there’s something in the equations which they feel is pointed in the right direction for the description of all the physics in the universe — but the problem is, they haven’t got to a stage where you can make a prediction that would distinguish string theory from the other contenders.”
However, we may be able to get more solid answers one day in the not-so-distant future. One prediction of string theory that could potentially be tested at some point is supersymmetry — the idea that fermions (objects made of matter, like quarks) and bosons (forces, like photons, that fermions exchange) are made of the same things and hence would look the same at the small scale, says Rickles. But you’d need to be able to see the world at a very tiny scale that our technology does not yet allow for to directly verify string theory.
Nevertheless, if scientists can make progress in developing string theory, they may be able to answer age-old questions like, “What happened before the big bang?” In fact, some interpretations of string theory challenge the most commonly held views of the universe’s origins. “String cosmologies seem to show that when you go to what looks like zero radius, the point of the big bang, it sort of expands on the other side,” says Rickles. String theory is also being used in labs for purposes such as modeling quark-gluon plasmas, which are notoriously hard to deal with using standard quantum theory of point particles.
At this point in the game, all we can say for sure about string theory is that it has extensive and fascinating potential. “I don’t know if it’s the right answer,” says Lewis. “But if it is, it will change everything.”