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String theory attempts to unify general relativity and quantum theory.Popular in the 1990s, string theory fell out of favor as it failed to provide testable predictions and required ten dimensions for the math to work out.A new study uses a “bootstrap” approach and finds that using only four physical assumptions about the universe produces scattering amplitudes previously predicted by string theory.
The known universe—or at least our understanding of it—is governed by two overarching theories. The first, courtesy of Albert Einstein, is the theory of general relativity, which accurately describes the physics of space-time at massive scales. The second is quantum field theory, which concerns itself with the realm of the subatomic. Both theories are remarkably accurate in their own scientific spheres of influence, but when physicists try to reconcile them with each other, they run into more than a few problems.
The main sticking point is gravity. While the other fundamental forces of the universe feature “force carriers,” such as photons, gluons, or w and z bosons, gravity has no such particle—at least, not that we know of. Simply put, we have no quantum theory of gravity, and without it, the ever-elusive “theory of everything” will remain out of reach. However, in the past half-century, that hasn’t stopped physicists from proposing methods of uniting the quantum and general relativity realms.
String theory proposes that, at scales a billion billion times smaller than a proton, the universe is composed of tiny, vibrating strings that give rise to different particles—gravity included. First proposed in the late 1960s, the idea reached its zenith in the 90s. But string theory never produced testable predictions and required ten dimensions to really work, which isn’t exactly an easy thing to test empirically.
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Although the fever around string theory cooled in the subsequent decades, the idea hasn’t faded entirely, and now a new study published in the journal Physical Review Letters theoretically demonstrates that string theory actually emerges from “almost nothing.” In other words, with only a few physical assumptions, string theory becomes an attractive candidate for uniting quantum mechanics and general relativity.
“We didn’t start with any assumptions about strings at all, but then the solution contained the cornerstone signatures of strings,” California Institute of Technology’s (Caltech) Clifford Cheung, the lead author of the study, said in a press statement. “The strings just fell out.”
This isn’t the much-needed experimental evidence required for string theory to graduate into a more serious hypothesis. Instead, the new research relies on what’s known as a “bootstrap” approach, where physicists start from basic assumptions believed to be true and follow them through to see what theories emerge. According to Science News, the researchers explored what types of scattering amplitudes—mathematical objects used to determine the probability that particles will interact in a particular way—are possible given four such assumptions.
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The first two are well accepted: unitarity (from quantum mechanics, meaning all probabilities must add up to 100 percent) and Lorentz invariance (from Einstein’s special relativity, meaning the laws of physics apply universally regardless of location or speed). The second two are slightly bigger leaps. One is that physics remains well-behaved at extremely high energies—a realm where some theories, such as general relativity, break down. The final assumption, called “minimal zeroes,” selects the simplest possible scattering amplitudes rather than more complex ones. With just these four assumptions, the authors derived two specific scattering amplitudes—the Veneziano and Virasoro-Shapiro amplitudes—that were predicted by string theorists decades earlier, including the infinite tower of particles first described by Italian theoretical physicist Gabriele Veneziano in the late 1960s.
“The precise details of string theory emerged automatically, including the infinite tower of massive spinning particles that form the ‘harmonics’ of the string that the theory is famous for,” New York University’s Grant Remmen, a co-author of the study, said in a press statement.
Of course, this doesn’t mean we’ve found the theory of everything. But when the answers we’re looking for play out at scales far too small to observe, it helps to know that a small set of reasonable assumptions about the universe leads, almost inevitably, to strings.
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Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.