Exploring the possibility of probing fundamental space-time symmetries via gravitational wave memory

As predicted by general relativity, the passage of gravitational waves can leave a measurable change in the relative positions of objects. This physical phenomenon, known as gravitational wave memory, could potentially be exploited to study both gravitational waves and spacetime.

Researchers from the Gran Sasso Science Institute (GSSI) and the International School for Advanced Studies (SISSA) recently conducted a study in which they investigated the possibility of using gravitational wave memory to measure spacetime symmetries, fundamental properties of spacetime that remain the same after specific transformations. Their paper, published in Physical assessment letterssuggests that these symmetries can be probed through the observation of displacement and spin memory.

“I have long been curious about the phenomenon of gravitational wave memory and the connection of the associated low-energy physics to quantum mechanics,” Boris Goncharov, co-author of the paper, told Phys.org. “I first heard about Weinberg’s soft graviton theorem from Prof. Paul Lasky at Monash University in Australia, during my Ph.D., when I was discussing gravitational wave memory. Then I learned about the so-called “Infrared Triangle” that connects the soft theorem with gravitational wave memory and symmetries of spacetime at infinity of gravitational wave sources.”

Weinberg’s soft graviton theorem and the “infrared triangle” are mathematical formulations that describe the same physical phenomenon: gravitational wave memory. As part of their recent study, Goncharov and his colleagues set out to investigate the possibility of exploiting gravitational wave memory to probe spacetime symmetries.

“This phenomenon plays a role in an ongoing attempt to describe a hundred-year-old, unsinkable and yet incompatible with the microscopic world Einstein’s theory of gravity – general relativity – as a quantum field theory at the asymptotic edge of spacetime,” Goncharov said.

“This approach to a unification in physics seems to me substantial and promising; I find it very exciting. Our specific project arose while discussing new developments in this field with Prof. Laura Donnay, a co-author of the publication.”

Reviewing previous literature in this area, the researchers found that a growing number of distant spacetime symmetries were being discussed, but it was unclear which of these symmetries and their associated memory terms occur in nature. Although several physicists had explored the possibility of detecting gravitational wave memory, Goncharov and his colleagues were uncertain about what physics could be constrained using their measurements.

“The idea that we could test these spacetime symmetries was central to our study,” Goncharov explained. “Another aspect is that I and Prof. Jan Harms are members of the Einstein Telescope collaboration, for which it was important to investigate the observational prospects of gravitational wave memory. The Einstein Telescope is the next generation European ground-based gravitational wave detector planned for the 2030s.”

Until now, researchers had not introduced a conventional approach to measure spacetime symmetries via the observation of gravitational wave memory effects. The recent paper by Goncharov and his colleagues aimed to fill this apparent gap in the literature.

“There was much earlier important work focused on (a) predicting when and with what instruments we can detect various gravitational wave memory terms, (b) how gravitational wave memory effects can be calculated analytically or using numerical relativity, and (c) how different models of spacetime symmetries yield gravitational wave memory terms,” Goncharov said. “However, a discussion of spacetime symmetries based on the observed memory effects seemed to be a gap in the literature.”

The recent work of these researchers can be seen as a proof of principle. In their paper, they introduce new observational tests that can be used to investigate spacetime symmetries, while also outlining possible limitations of their proposed approach that can be addressed in the future.

Overall, their study suggests that the pool of tests of general relativity could be expanded, and it provides a number of useful calculations that could be performed using data collected by different gravitational wave detectors.

Goncharov and his colleagues hope that their paper will open further discussions about spacetime symmetries and gravitational wave memory and other topics within their research community. These discussions could potentially pave the way to the unification of different physics theories.

“I am currently starting a study on gravitational wave memory with Pulsar Timing Arrays (PTAs) together with Sharon Tomson (a new PhD student at my current institute, AEI in Hannover, Germany) and Dr. Rutger van Haasteren.”

PTAs are astronomical observation tools that collect very stable and regular signals from pulsars (i.e. rapidly spinning neutron stars) using radio telescopes on Earth. These neutron stars act as very precise clocks, as they are sensitive enough to pick up delays and advances of radio pulses caused by the propagation of gravitational waves through the Milky Way.

“PTAs are galactic-scale detectors, which currently appear to be gradually picking up a collective hum of slowly inspiring supermassive binary black holes in the nearby Universe. The signal yields slow variations in the arrival times of the pulses that are most prominent on timescales of several years to decades,” Goncharov added.

“A striking merger of supermassive binary black holes in a nearby galaxy could produce a gravitational wave burst with memory, which could be detected by PTAs. Although such bursts are very rare, we hope to extract some useful information from the data by placing constraints on their existence.”

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