Science magazine reports:

For decades, string theory promised a "theory of everything" that described all particles and forces as tiny vibrating strings. Physicists hoped it could also solve one of the field's deepest problems: reconciling quantum mechanics with gravity. But as string theory grew increasingly elaborate — and experimentally unreachable — many physicists lost hope.

Now, some researchers are revisiting the theory from first principles. In a paper in press at Physical Review Letters, Clifford Cheung, a physicist at the California Institute of Technology, and colleagues lay out a small set of assumptions about the universe and show that they inevitably give rise to string theory.... Cheung's study, along with another one posted to arXiv in January, starts with two reasonably conservative assumptions: that the probabilities of all possible outcomes of an event add up to 100%, and that the laws of physics are consistent for observers moving at different speeds. Each group then posits additional assumptions that have not been borne out by observations. Cheung's analysis invokes "ultrasoftness," the idea that the probability of certain particle interactions drops off at a particular rate at high energies. The second study, led by University of Michigan physicist Henriette Elvang, instead assumes "supersymmetry," a maximal coupling between matter and forces. Both groups conclude the only theory that can satisfy their assumptions is one that looks like string theory...

Cheung and Elvang stress that their aim is not to prove the inevitability of string theory. "I don't have a dog in the fight; I just work here," Cheung says. Rather, the goal is to explore the space of possible theories under rigid constraints — regardless of whether they reflect reality... The one thing the researchers all agree on is that the field would benefit from more alternative models to string theory. Cheung sees the agnostic, bottom-up exploration as a step in that direction. "You can either give up on the problem because it's too culturally toxic, or you can ask: If you want to find an alternative, what do you need?" he says. "Now, we know exactly what to do."

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