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Scientists have peered into the structure of space-time to search for new physics that can be written into the fingerprints of elusive “ghost particles”, with the help of a giant observatory stretching nearly a mile under the South Pole, according to a new study.
Although this years-long experiment found no new physics imprinted in these spectral particles, known as neutrinos, it still represents an unprecedented glimpse into the mysterious worlds of the universe that have so far remained out of sight. In particular, the new research sheds light on the quest to describe gravity using quantum mechanics, because so-called “quantum gravity” is a key key to unlocking some of the universe’s biggest mysteries.
The IceCube Neutrino Observatory, the world’s largest neutrino telescope, has been operating for a decade at the South Pole. The detector consists of thousands of sensors that reach about 2,500 meters below the Antarctic ice – 28 football fields long – where they pick up energetic neutrinos arising in explosive events from the edge of space and time.
Now, the IceCube collaboration, a team of more than 400 scientists, has announced the results of a “search for a new structure of space and time” that searched regions of the universe previously “inaccessible to human technologies, according to the Study published on Monday in Nature Physics.
Teppei Katori, IceCube team member and experimental particle physicist at King’s College London, as well as a co-author of the study said on a call with Motherboard.
“We use these two properties; neutrinos can travel the longest distance in the universe and with the highest energy.” “It’s a big assumption, but it is believed that these particles are very sensitive to anything within spacetime.”
Neutrinos are so light in weight that their masses are almost imperceptible, which made them nicknamed “ghost particles”. For this reason, they can easily pass through planets, stars, and other forms of matter without slowing down or changing direction. This makes it very difficult to detect neutrinos using conventional instruments, despite their abundance in the universe that about 100 trillion of them pass through your body every second.
The Sun releases most neutrinos around the Earth, but there is another class of high-energy “astrophysical neutrinos” that come from fiery objects called “cosmic accelerators” that lie several billion light years from Earth. These accelerators could be objects like blazars, galactic centers that release jets of light and energy, although the exact sources of astrophysical neutrinos remain unknown.
Neutrinos come in three different “flavors” 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 challenge what’s known as Lorentz symmetry, an important cornerstone of Albert Einstein’s special theory of relativity.
Essentially, Lorentz symmetry means that the universe must appear symmetric to two observers traveling at a constant speed relative to each other. In other words, the universe at large scales is fundamentally isotropic and homogeneous, although on smaller scales it appears more diverse, including the planetary perspective we experience as humans on Earth. Researchers are so obsessed with discovering violations of this symmetry that they may uncover the long-awaited missing link between gravity and the Standard Model of particle physics that governs quantum mechanics.
Cattori explained, “For the past 100 years, people have tried to find evidence that the Lorentz symmetry is not true, and no one can find it.” “This is one of the most traditional studies of modern physics – people are challenging this space-time theory.”
“If there is something wrong with Lorentz symmetry or something is beyond Lorentz symmetry, you may have some connection, for the first time, with gravity in the Standard Model,” he added. “Quantum gravity is something that many people really hope will be the next generation, or an open door to the next.”
Astrophysical neutrinos offer a promising test of Einstein’s theories because they may encounter unexplored regions of spacetime affected by quantum gravity. Neutrinos passing through these regions can change flavors in surprising ways that will leave a record of space-time anomalies in their signatures that can be read by scientists picking them up on Earth.
“The neutrinos change flavors even without the influence of spacetime,” Katori noted. “We are looking for anomalous changes, or unexpected ways to change. That is the focus of this research.”
The IceCube research found no anomalies in neutrino flavor shifting, leaving the idea of Lorentz symmetry intact for the time being. While Cattori said these results were somewhat “disappointing” – who wouldn’t want to find new physics, after all? It’s still an important discovery. According to the study, IceCube was able to “unambiguously access the parameter space of quantum gravity physics”. In other words, the results have led to a new path in the theoretical field of quantum gravity that will have all kinds of applications for scientists across fields.
“We think these are great results,” Cattori said. “We have a high sensitivity and we are also the first experiment to get to a region – or ‘phase space,’ the technical word – to really look for it, referring to violations of Lorentz symmetry.
“I am very relieved to finally publish it,” he continued. “From data collection and other issues, it’s just a lengthy effort.”
Even as this initial experiment comes to a bittersweet end, a new beginning is emerging under the Antarctic ice, as well as from other instruments around the world. The IceCube collaboration plans to research their dataset again using new machine learning techniques that may be able to identify anomalies not overlooked in this study. The team also hopes to significantly expand the IceCube in order to obtain a larger data set that may finally reveal traces of space-time anomalies indicative of quantum gravity.
“In my opinion, there is still a chance,” Cattori said. “This analysis is the first iteration of this kind. We made this a framework for analysis and developed the code, but in a sense, we didn’t do our best because things were still developing.”
“I think there is an opportunity to improve it, but I can’t guarantee how much,” he noted.
Meanwhile, the new findings show that it is possible to examine spacetime itself using slippery particles from the distant universe, providing a way to explore a range of other potential models and experiments.
“Although the motivation of this analysis is to search for evidence of quantum gravity, the formalism we used is model independent, and our results could place limitations on various new physical models, including a new long-range force, dark neutrino energy coupling, scattering Neutrino dark matter, parabolic principle violation and so on,” the IceCube collaboration on the study concluded.
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