To get string theory, you only need four physical assumptions

With just a few assumptions, string theory stands on its own.

Based on the idea that all subatomic particles are made of vibrating strings of energy, string theory is a candidate for a “theory of everything” that could merge disparate branches of physics under a single unifying framework. It has long been a controversial idea, with some scientists espousing it while others say there is a lack of evidence to support it.

A team of physicists reports a new detail that doesn’t settle the debate, but suggests that string theory is unique. Given only four fundamental physical assumptions, string theory is the only possible option for a theory of everything, theoretical physicist Clifford Cheung and his colleagues report in a paper accepted by Physical Examination Letters.

String theory has aroused skepticismfocusing in particular on the fact that his predictions so far have not been testable – and may never be. Without experimental evidence, physicists will never be able to say that string theory actually describes the real world. But “you can actually ask a question that’s the best thing to do,” says Caltech’s Cheung. “Given what we consider to be conservative or well-established principles, how unique or inevitable is string theory?

The work is part of a “recent wave of interest in understanding the particularities of string theory,” says Andrea Guerrieri, a theoretical physicist at City St George’s, University of London, who was not involved in the research. To explore this question further, researchers employ a strategy known as bootstrapping. Rather than starting with a proposed theory, physicists start with basic assumptions and follow them through to determine the characteristics of possible theories, figuratively pulling themselves up by their bootstraps.

Cheung and his colleagues focused on a quantity that a theory of everything, such as string theory, could predict, called scattering amplitudes. These are mathematical expressions that can be used to determine the probability that two particles will interact in a particular way.

In the new study, the researchers explored what types of scattering amplitudes are possible. Without any assumptions, a wide range of scattering amplitudes could be correct. By adding different hypotheses, the researchers attempted to work their way toward the scattering amplitudes predicted by string theory, without relying on the principle that the universe is made of strings.

Two of the hypotheses made by Cheung and his colleagues are generally well accepted by physicists. One, known as unity, comes from quantum mechanics and essentially means that the sum of the probabilities of all possible options in a given situation must add up to 100%. Another is Lorentz invariance, a principle of Einstein’s theory of special relativityimplies that the laws of nature are the same no matter where you are in the universe or how fast you travel.

Another, less established condition is that physics behaves well at extremely high energies, an area where some theories stop working, namely: general relativitythe theory that describes gravity. Finally, an engineering hypothesis, called “minimum zeros”, selects the simplest possible version of the scattering amplitude, bypassing more complex versions.

Based on their hypotheses, the researchers landed on two possible scattering amplitudes, called the Veneziano and Virasoro-Shapiro amplitudes. And there you have it: these are the amplitudes predicted by string theory.

“It’s very surprising that they were able to derive this in such a clear way,” says theoretical physicist Yu-tin Huang of National Taiwan University in Taipei. Even if some physicists may have had the intuition that string theory would be special from this point of view, “believing that it is true versus [being] able to show mathematically that it’s true, it’s different.

The result does not mean that string theory is correct. For example, one of the hypotheses could be false. But playing with such hypotheses is one way to explore potential theories, Guerrieri says.

This method of investigation flips the script of theoretical physics, Cheung says. “Usually we write a nice theory; we are thinking about a profound idea. » And then, in the end, the physicists calculate something that could be measurable in an experiment. “It’s the exact opposite of that.”