Carnegie Mellon theorist creates test for string theory

String theory has long been a subject of heated debate in particle physics because of its mostly unconfirmable predictions, which lead many experts to question its legitimacy. Now, in a controversial paper jointly published by researchers throughout the United States, scientists claim to have developed a test that may falsify the postulates of string theory and thereby change our perceptions of the universe.

The paper, titled “Falsifying Models of New Physics via WW Scattering” and published in Physical Review Letters on January 22, was co-authored by Jacques Distler from the University of Texas at Austin, Benjamin Grinstein from the University of California, San Diego, and Rafael A. Porto and Ira Rothstein, a graduate student and physics professor at Carnegie Mellon, respectively.

“Every theory like string theory is based on certain mathematical assumptions, and then what you can do is see what the consequences are of these assumptions,” Rothstein said.

The greatest quandary in physics today is how different the laws of physics are between massive and subatomic scales. General relativity, which describes the interactions of large masses and gravity, has long been at odds with quantum mechanics, which describes physical interactions at the atomic and subatomic level. The theories differ wildly in their interpretation of reality, yet each of them is the most accurate representation of the world at their respective scales.

Gravitational laws, for instance, dictate that an orbiting mass must continually lose energy and eventually spiral into the center of orbit. Electrons orbiting around the nucleus, however, have a quantified minimum orbital that appears to violate all laws of energy conservation.

String theory is the most successful of various quantum field theories that have attempted to reconcile these two interpretations of physical interaction. String theory postulates that the fundamental form of matter is not particle-shaped, but rather, string-shaped.

The vibration of these strings in multiple dimensions determines their excitation mode, which is the type of elementary particle they represent, such as bosons, pions, and fermions. Supersymmetry in string theory then relates the particles that transmit forces to the particles that make up matter.

Distler said that String theory is an is one possible ultraviolet completion of an existing Effective Field Theory that describes physics in terms of degrees of freedom on a particular energy scale. The experiment outlined in the paper attempts to determine the nature of the ultraviolet completion of string theory, which is how string theory should behave in high-energy situations.

“If the values lie outside some range, it tells you something very profound about physics at the high-energy scales, namely that what we’re seeing is not of the expected sort, like string theory,” Distler said.

The paper outlines a method of using the Large Hadron Collider (LHC) to measure the scattering of W-bosons. W-bosons, first detected at the European Organization for Nuclear Research (CERN) in 1983, are elementary particles that control the weak nuclear force.

Porto stated that the method in the paper relies on the absence of a light Higgs. Higgs bosons are theoretical particles that govern the difference between massive particles, such as bosons and protons, and massless particles such as photons.

He stated, "If a light Higgs is found we would have to include that into our calculations and pursue a different approach. Since this particle hasnt been discovered yet it isnt clear how massive it is, or if it is even there. New physics could be just around the corner, and our bounds would test it."

The postulate of the existence of a heavy Higgs boson simplifies calculations. Without this postulate, the calculation is complicated but still possible.

The LHC, which will be the largest particle accelerator in the world when it begins operation in Switzerland later this year, is hoped by many to be the first particle accelerator capable of detecting and analyzing Higgs bosons.

This paper is the first attempt to develop an experiment that may disprove string theory. Researchers are mostly expecting to see the bounds set by string theory satisfied by this experiment.

Distler said that a successful outcome in the experiment, however, should not be seen as anything more than a weak support for string theory. The results of the experiment can effectively rule out certain quantum field theories but not serve as verification.

Rothstein said, “I highly doubt that the bounds would be violated.... It seems highly unlikely.” At the same time, Rothstein thinks that it is important to test string theory. “Everyone always comes down on string theory because we never know if it is right,” he said, “but one thing that you might know is whether it is wrong.”

In the case that the results do falsify string theory, there is no shortage of viable alternatives. Loop quantum gravity, for example, is an alternate quantum field theory that breaks Lorentz invariance, one of the core assumptions of string theory. Other more esoteric theories may come to the forefront as well.

According to Distler, however, we should not expect to see results from the experiment for a few more years. However, researchers are very excited about the prospects of this experiment, regardless of the outcome. “If we don’t find the Higgs, a violation of our bounds will turn out to have a strong impact on our thinking, not just in high energy physics, and that would be just great,” Porto stated.

“I don’t expect to see the bounds violated either, but if they are, well, that’s how physics progresses,” Distler said.