New research synthesizes different aspects of causality in quantum field theory

In current quantum field theory, causality is usually defined by the disappearance of field switches for spatial separations. Two researchers from the University of Massachusetts and the Federal Rural University of Rio de Janeiro recently conducted a study to analyze and synthesize some of the key aspects of causality in quantum field theory. Their article, published in Physical Review Letters, is the result of their study of a theory of quantum gravity commonly called "quadratic gravity".

"Like the ingredients of the standard model, quadratic gravitation is a renormalizable quantum field theory, but it has special properties," John Donoghue, one of the researchers who conducted the study, told Phys.org. "The small violation of causality is the most important of these and our goal was to better understand it, and in the process we realized that some of the ideas are of more general interest and we decided to write our understanding in the form of physical review letter, to share these ideas more widely. "

The paper by Donoghue and his colleague Gabriel Menezes synthesizes many aspects of causality that have been part of quantum field theory for several decades. The realization that there may be microscopic violations of causality in some theories goes back to the 1960s, particularly the work of physicists T. D. Lee and G. C. Mèche. In their study, Donoghue and Menezes also drew on a more recent study by Donal O'Connell, Benjamin Grinstein and Mark B. Wise.

So far, most theoretical discussions of causality, especially the "time arrow", have argued that the laws of physics have no preference for the flow of time. However, this particular hypothesis is not applicable to quantum physics, where a direction for causal effects is present.

"The various factors of i in the quantization procedures are related to the direction of the causal action, which leads to the" arrow of causality "in quantum physics," Donoghue explained. "This connection is not discussed very often."

Donoghue and Menezes were intrigued by the fact that the macroscopic feeling of causality, which also applies to classical physics, is due to the underlying structure of quantum theory. In their recent article, they examined this particular aspect of causality to better understand its meaning and implications.

"The idea that there may be dual causality arrows in the same theory is even more obscure," said Donoghue. "However, this happens in a very simple context where the Lagrangian of the theory has more derivative powers than usual.This is what happens in quadratic gravity, but it could also occur in other theories. . "

Even though the meaning of causal influence is a convention associated with the choice of a description of the measure of time, its existence is a necessary condition based on the laws of quantum physics. In this context, Donoghue and Menezes observed that the arrow of causality could potentially be violated by contradictory conventions.

"Perhaps the most important implication of our study is that we have collected evidence of causal uncertainty due to the space-time fluctuations that can occur in a quantum theory of gravity," said Menezes. "This would provide us with a deep intuitive understanding of the origins of causality."

About a decade ago, O'Connell, Grinstein, and Wise completed a study that was based in part on a series of lectures by Sidney Coleman. They specifically suggested that, in a wave packet description of a diffusion process with mixed causal arrows, it can be verified that the decay products can be detected earlier than expected from the moment of production and that the associated probability of detection decreases exponentially over time. In their study, Donoghue and Menezes have deepened this idea.

"One of the implications of our study is that, if the ideas put forward by O'Connell and his colleagues, as well as by other research teams, can in principle be observed, there is no conflict with the experiences in the case of gravity because the phenomenon occurs at order energies of the Planck scale, 15 orders of magnitude more than the energy range accessible to the LHC, "said Menezes.

The recent study by Donoghue and Menezes offers a general and useful discussion of causality and the causality arrow, focusing on how a given theory can have arrows forwards and backwards. This discussion addresses the subject of time reversal in field theory to inform various physics studies. This could also help to clarify the quantum theory of quadratic gravity, which still has many unanswered questions.

Overall, Donoghue and Menezes suggest that mixed conventions in individual physics theories might actually be possible and that future studies should deepen this topic. Researchers are currently working on a project to fully explore the phenomenon of causality due to the quantum fluctuations of the gravitational field.

"There are other technical considerations that we need to address regarding this description of quantum gravity as a renormalizable quantum field theory," Menezes said. "One of them concerns the stability of quadratic gravity in the curved bottoms, which has already been studied by other authors, and I hope it will be part of this future work as well. , the most intriguing investigation we hope to conduct will be the study of the effect of causal uncertainty in the primitive universe. "

News Source

https://phys.org/news/2019-11-aspects-causality-quantum-field-theory.html

Journal References

1. John F. Donoghue et al. Arrow of Causality and Quantum Gravity, Physical Review Letters (2019). DOI: 10.1103/PhysRevLett.123.171601
2. T.D. Lee et al. Negative metric and the unitarity of the S-matrix, Nuclear Physics B (2002). DOI: 10.1016/0550-3213(69)90098-4
3. Benjamín Grinstein et al. Causality as an emergent macroscopic phenomenon: The Lee-WickO(N)model, Physical Review D (2009). DOI: 10.1103/PhysRevD.79.105019