Posted on Nov 15, 2011

Proof has beenfound for unifying quantum principle by John Cardy, who proposed a far-reaching principle to constrain all possible theories of quantum particles and fields. Cardy's "a-theorem" says that the number of ways in which quantum fields can be energetically excited is always greater at high energies than at low energies. Cardy's theorem has held up for almost 25 years, and may have been proved earlier this year. His theorem could help physicists long struggling to relate physics at the high-energy, short-distance scale of quarks to the physics at longer-distance, lower-energy scales, such as that of protons and neutrons.

If the solution holds, it will have implications for any previously unknown particles that may be discovered at the Large Hadron Collider (LHC) at CERN, Europe’s particle physics lab near Geneva, Switzerland.

“I’m pleased if the proof turns out to be correct," says Cardy, a theoretical physicist at the University of Oxford, UK. “I’m quite amazed the conjecture I made in 1988 stood up.”

The proposed proof, from theorists Zohar Komargodski and Adam Schwimmer of the Weizmann Institute of Science in Rehovot, Israel, was put forward in July 2011, and is now gradually gaining acceptance as other theoretical physicists have a chance to validate it. “I think it’s quite likely to be right,” said Nathan Seiberg, a theoretical physicist at the Institute of Advanced Study in Princeton, New Jersey.

A version of it had already been proven in two dimensions, but Cardy proposed that it might hold in four dimensions — such as the three dimensions of space and one of time that exist in the space in which we live. However, the theorem seemed to be have been killed off in 2008, when two physicists found a counter-example3: a quantum field theory that didn’t obey the rule.

Now that Cardy's conjecture has some firm backing, it is likely to be applied more widely than before. The most fruitful territory will be the many quantum field theories that have been proposed to unify physics beyond the standard model, including supersymmetry, in which known particles are paired with extra-heavy counterparts that have yet to be found.

The a-theorem would help because given predictions from a theory at low energy, it would constrain what the predictions at high energy should look like, and vice versa.

CERN physicists hope that the LHC will find evidence for supersymmetry or other particles beyond the standard model, and when that happens theorists will need all the help they can get to explain them.

The Daily Galaxy via nature.com, oxford university, scientificamerican.com and universetoday.com

Image credit: With thanks to PASIEKA/SPL