Scientists have been searching for a ‘new space-time structure’ deep beneath the South Pole. This is what they found
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Scientists have peered into the fabric of spacetime to search for new physics that may be written in the signatures of elusive “ghost particles,” with the help of a giant observatory that stretches nearly a mile below the South Pole, a new study reports.
Although this multi-year experiment did not find any new physics imprinted in these spectral particles, known as neutrinos, it still provides unprecedented insight into shadowy regions of the cosmos that have remained out of sight until now. The new research sheds particular light on the quest to describe gravity using quantum mechanics, as this so-called “quantum gravity” is a major key to unlocking some of the universe’s greatest mysteries.
The IceCube Neutrino Observatory, the largest neutrino telescope in the world, has been operating for a decade at the South Pole. The detector is made up of thousands of sensors that reach about 2,500 meters below the Antarctic ice – roughly the length of 28 football fields – where they capture energetic neutrinos produced in explosive events at the edge of time and space.
Now, the IceCube Collaboration, a team that includes more than 400 scientists, has announced the results of a “search for a new space-time structure” that explored regions of space that were previously “inaccessible to human technologies, according to study published on Monday in Nature Physics.
“IceCube is really special, because it can see neutrinos coming from a long distance and with really high energy,” Teppei Katori, an IceCube team member and experimental particle physicist at King’s College London and co-author of the study’s paper, told Motherboard .
“We use these two properties; that neutrinos can travel the longest distance in space and with the highest energy,” he continued. “It’s a big guess, but it’s believed that these particles are very sensitive to everything within space-time.”
Neutrinos are so light that their masses are almost imperceptible, earning them the nickname “ghost particles.” For this reason, they are able to effortlessly pass through planets, stars and other forms of matter without slowing down or changing direction. This makes neutrinos very difficult to detect with conventional instruments, even though there are so many of them in the universe that about 100 trillion pass through your body every second.
Most of the neutrinos around Earth are fired by the Sun, but there is another class of high-energy “astrophysical neutrinos” that come from pyrotechnic facilities called “cosmic accelerators” located many billions of light-years from Earth. These accelerators may be objects like blazars, which are galactic centers that eject jets of light and energy, although the exact sources of astrophysical neutrinos are still unknown.
Neutrinos come in three different “flavors” that are associated with fundamental particles in the universe called electrons, muons and taus. Scientists have long suspected that changes in the flavor of astrophysical neutrinos could open a window into regions of spacetime that might defy what’s known as Lorentz symmetry, an important foundation of Albert Einstein’s special theory of relativity.
Lorentz symmetry essentially means that the cosmos should look the same to two observers traveling at a constant speed relative to each other. In other words, the universe on a large scale is basically isotropic and homogeneous, although it appears more diverse on smaller scales, including the planetary perspective we experience as humans on Earth. Researchers are obsessed with discovering the breaking of this symmetry because it could unravel the long-sought missing link between gravity and the standard model of particle physics that governs quantum mechanics.
“For the last 100 years, people have been trying to find evidence that Lorentz symmetry is not true, and no one can find it,” Katori explained. “This is one of the most traditional studies of modern physics – people challenging the theory of spacetime.”
“If something is wrong in Lorentz symmetry or something is out of Lorentz symmetry, you might have some connection, for the first time, with gravity in the Standard Model,” he added. “Quantum gravity is something that many people hope is really the next generation, or the open door to the next stage.”
Astrophysical neutrinos offer a promising test of Einstein’s theories because they could encounter unexplored regions of spacetime affected by quantum gravity. Neutrinos passing through such regions could potentially switch flavors in surprising ways that would leave a record of space-time anomalies in their signatures that could be read by scientists capturing them on Earth.
“Neutrinos change flavors even without this space-time effect,” Katori noted. “We’re looking for anomalous changes, or unforeseen ways of changing. That is the focus of this research.”
IceCube’s search found no anomalies in the neutrino flavor conversion, leaving the notion of Lorentz symmetry intact for now. While Katori said these results were somewhat “disappointing” – who wouldn’t want to find new physics? – it’s still an important finding. IceCube was able to “unequivocally reach the parameter space of physics motivated by quantum gravity,” the study said. In other words, the results have opened a new trail in the theoretical domain of quantum gravity that will have all kinds of applications for scientists in different fields.
“We believe these are great results,” said Katori. “We have the highest sensitivity and we’re also the first experiment to get to a region — or ‘phase space,’ the technical word — that really looks for it,” referring to Lorentz symmetry violations.
“I’m so relieved it’s finally out,” he continued. “From taking data and other issues, it’s just such a long effort.”
Even as this initial experiment comes to a bittersweet end, a new beginning emerges from beneath the Antarctic ice, as well as from other instruments around the world. The IceCube collaboration plans to re-search their dataset using new machine learning techniques that could pinpoint anomalies missed in this study. The team also hopes to dramatically expand the size of IceCube to produce an even larger data set that could eventually reveal traces of space-time anomalies that point to quantum gravity.
“In my opinion, there is still a chance,” Katori said. “This analysis is the first iteration of its type. We built this as an analysis framework and developed the code, but in a sense we haven’t done the best because things are still evolving.”
“I believe there’s a chance it can improve,” he noted, “but I can’t guarantee how much.
Meanwhile, the new results show that it is possible to probe spacetime itself using slippery particles from deep space, providing a way to explore a host of other potential models and experiments.
“Although the motivation of this analysis is to look for evidence of quantum gravity, the formalism we used is model-independent, and our results may place constraints on a variety of new physical models, including the new long-range force, neutrino-dark energy coupling, neutrino scattering and dark matter, violation of the equivalence principle and so on,” concluded the IceCube collaboration study.
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