‘Impressive’: Tibetan Plateau telescope tests light’s speed – and Einstein’s theory of special relativity
- Large High Altitude Air Shower Observatory helps team test Einstein’s theory, finding no evidence that particles’ speed changes depending on energy
- Lhaaso, commissioned in 2021, operates as most sensitive ultra-high-energy gamma ray detector in the world
The speed of light appeared to remain constant during the most powerful cosmic explosion ever observed, based on the world’s most sensitive high-energy light detector, according to Chinese scientists who tested Einstein’s theory of relativity.
Their study, which was posted to the preprint platform arXiv this month but is not yet peer-reviewed, provides the latest evidence that the universe is a symmetrical place – also known as Lorentz symmetry, a pillar of Einstein’s theory of special relativity – leaving scientists empty-handed in their search for violations of this symmetry.
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Nicolas Yunes, a theoretical physicist at the University of Illinois Urbana-Champaign in the US, said Lorentz violation was a phenomenon predicted by researchers who sought to develop a quantum gravity theory to unify general relativity and quantum mechanics.
“If detected, such a violation would be a groundbreaking discovery, which would shake the very foundations of physics,” Yunes said. “To date, no such violation has been observed.”
The results by the Lhaaso team placed “the strongest constraints” on such violations, and implied that a linear-type Lorentz violation was “probably truly not present in nature”, Yunes said.
In Einstein’s relativity theory, Lorentz symmetry stipulates that observers see the same laws of physics from any given direction or frame of reference, as long as the observed object is moving at a constant speed.
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As a result of Lorentz symmetry, people always observe light to be travelling at 300 million metres per second, whether they are driving on a highway or flying through space in a spacecraft. The constant speed of light is a cornerstone of modern physics and forms the basis of our understanding of space and time.
Every experiment conducted to date has verified Lorentz symmetry. However, this rule might not hold in extreme scenarios, such as when tiny particles zip through the vast universe at incredibly high energies and yield to a mysterious, unknown force.
While such a force may be extremely weak, it could increase with the energy of the particles and accumulate to detectable levels over great travelling distances, the researchers wrote.
The Lhaaso collaboration, led by the Institute of High Energy Physics in Beijing and with 280 participants from around the world, then divided these photons into 10 groups according to their energy ranges to look for lags in their arrival time.
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The researchers’ analysis of the data, based on two different methods, showed that changes in the speed of light at high energies was very limited. The lower limits of energies at which Lorentz violation might occur would have been much higher than current predictions, they found.
In other words, based on their detections of the gamma-ray photons, the team did not find definitive evidence to support the theory of variable speed of light at the energies observed.
Yunes said it was “particularly impressive” that the new study could constrain the linear-type violations at a scale 10 times higher than the Planck scale, whose units set the universe’s minimum limit. Quadratic-type violations were also constrained “at a weaker, although not less impressive, scale”, he said.
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Lhaaso was built by Chinese scientists at an altitude of 4,410 metres (14,470 feet) on Haizi Mountain in Daocheng county, Sichuan province. It contains four types of detectors, including about 5,200 electromagnetic particle detectors, 1,200 muon detectors, 18 wide-field Cherenkov telescopes and a 78,000-square-metre (839,600 square foot) water Cherenkov detector array.
The facility was commissioned in 2021 and has been operating as the most sensitive ultra-high-energy gamma ray detector in the world.
