With just a handful of assumptions, string theory stands alone.

Based on the idea that all subatomic particles are made up of vibrating strings of energy, string theory is a candidate for a “theory of everything” that could merge disparate branches of physics under one unifying framework. It’s long been a controversial idea, with some scientists embracing it and others arguing that evidence for it is lacking.

A team of physicists reports a new detail that doesn’t settle the debate, but does suggest that string theory is unique. Given just four basic physics assumptions, string theory is the only possible option for a theory of everything, theoretical physicist Clifford Cheung and colleagues report in a paper accepted to Physical Review Letters .

String theory has garnered skepticism , most famously centering on the issue that its predictions thus far have not been testable — and may never be.…