Physics has a problem: gravity. Unlike the other three fundamental forces (weak nuclear interaction, strong nuclear interaction and electromagnetism), gravity plays by its own rules, warping spacetime and compelling Einstein to devise unique ways of handling object interactions at scale. Even worse? Gravity does all this with nary a particle in sight, frustrating physicists’ efforts for a unified understanding of our universe.
String theory offers a potential tie-breaker in the four-force fight by replacing three-dimensional point-like particles with one-dimensional constructs. But is our universe really knotty by nature? Here’s what you need to know about the science and sensation of strings.
One of Everything, Please
Finding a “theory of everything” remains a long-standing goal for physicists; such a theory could connect seemingly disparate phenomena under a single ruleset, providing insight about how the universe began, how it works right now and where it’s headed. Unfortunately, unification has proved uniquely frustrating — Einstein was unable to find a unified theory before his death and other efforts, such as quantum chromodynamics (QCD) focused on specific force mechanics over complete modeling.
String theory offers an alternative explanation that supersedes the Standard Model by forcing gravity to play by the rules. String-based science got its start with Gabriele Veneziano in 1968, who discovered a new way to approach strong nuclear force interactions: By reimagining point-like particle masses as one-dimensional strings. Removed from their third-dimension shackles, these open-ended or closed-loop strings were free to match mathematical predictions and explain strong force experimental results.
Researchers John Schwarz and Michael Green realized these theoretical strings could also be applied to gravity and published a paper in 1984 that jump-started the field of string theory. In 1985, Edward Witten co-authored another paper with more in-depth analysis and in the decade that followed, strings became immensely popular, although several notable critics — including Richard Feynman and Stephen Hawking — lamented the lack of experimental data.
Despite its role in governing universal conditions, gravity refuses to play nice. But string theory offers a way to describe “gravitons,” which are fundamental particles that carry the force of gravity. In string theory, the vibration of tiny strings — on the order of millions to billions times smaller than current particles — account for the nature of all matter. Strike the right tone and you get a boson. Strike another and an electron emerges. Hit exactly the right note and you get a graviton.
Gravitons permit a quantum theory of gravity, one that aligns with the behavior of the other three fundamental forces. Observationally, this makes sense; as noted by astrophysicist Ethan Siegel, the way two massive bodies gravitate is eerily similar to the action of electrically charged particles that attract or repel, while gravitational waves closely match the behavior of waves of light and water. Instead of requiring a special theory for gravity that forces mathematical and mental gymnastics, string theory creates a unified force model that tidies up gravity’s spacetime mess.
Like all theories, however, strings aren’t neatly tied up. As Physics.org points out, for string theory to work it not only presupposes the existence of one-dimensional strings but also requires an additional seven dimensions that are so tightly folded up into infinitesimally small spaces they’re virtually undetectable. Work on string models eventually produced five competing theories; Edward Witten suggested in 1995 that these five theories were in fact variations on a theme — a fundamental, 11-dimensonial model he called “M-theory” that included general equations to explain all behavior in the universe. So far, however, it’s proved elusive.
Unraveling the Evidence
Lacking the ability to experimentally verify extra dimensions and even the existence of one-dimensional strings, how does the theory stack up?
While its equations are often described as “elegant,” there’s been little progress made after the initial burst of excitement during the late ’80s and early ’90s. In addition, current string models suggest a universe that isn’t expanding and postulate the existence of 10500 universes, all of which bear only passing similarity to our own. Still, it’s not all bad — that many theoretical doppelgangers vastly increases the potential for trips across inter-universal lines.
But string theory also has its share of scientific success stories. Along with a viable path to quantum gravity, Live Science points to the use of strings in describing early-universe processes like the Big Bang. String-based models were also the first to describe black hole entropy in a way that wasn’t just a rough estimate but provided a “spot-on counting,” according to Space. The theory could also help further our understanding of quantum teleportation. Recent experiments have proven that it works, but strings may provide more data about underlying universal mechanics.
Is the universe knotty by nature? Or will further work fray the edges of string theory and its model of quantum gravity? No matter the eventual outcome, strings play a critical part in untangling universal mysteries.