Is it possible that the speed of light is faster in some places in the universe and slower in other places?

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The new measurements confirm that no matter where you are or how fast you are moving, the laws of physics apply at the highest energies ever explored. The record-breaking gamma-ray observations demonstrate the stability of Lorentz invariance, part of Einstein's theory of relativity, which predicts that the speed of light is constant everywhere in the universe. The High Altitude Cherenkov Observatory (HAWC) in Puebla, Mexico, detected gamma rays from a distant galactic source, and the study is published in the journal Physical Review Letters.

"How relativity behaves at very high energies has real consequences for the world around us," said Pat Harding, an astrophysicist in the Neutron Science and Technology Group at Los Alamos National Laboratory and a member of the Cherenkov Observatory Scientific Collaboration. "Most models of quantum gravity say that relativistic behavior will break down at very high energies. Our observations of photons at these high energies push the relativistic energy scale up by a factor of more than a hundred, and Lorentz invariance is an essential part of the Standard Model of physics."

However, some physical theories beyond the Standard Model suggest that Lorentz invariance may not hold at the highest energies. If Lorentz invariance is violated, many exotic phenomena become possible. For example, the propagation speed of gamma rays can be faster or slower than the conventional speed of light. If the speed is faster, these high-energy photons will decay into lower energy particles and therefore never reach the Earth. The Cherenkov Observatory Gamma-ray Observatory has recently detected some astrophysical sources that produce photons exceeding 100 TeV.

"That's a trillion times the energy of visible light and far higher than any accelerator on Earth can deliver. Because the Cherenkov Observatory can see these gamma rays, it extends the scale of Lorentz invariance by a factor of 100. Detecting higher-energy gamma rays at astronomical distances will allow for even more stringent checks of relativity. As the Cherenkov Observatory continues to acquire more data in the coming years and incorporates Los Alamos-led improvements in detector and analysis techniques at the highest energies, we will be able to study this physics further."

Due to the high energies and great distances from the sources, astrophysical observations offer a unique opportunity to examine possible signatures of Lorentz invariance violation (LIV). Superluminal LIV can cause photons to decay at high energies. The High Altitude Cherenkov Observatory (HAWC), one of the most sensitive gamma-ray instruments currently operating above 10 TeV, has found evidence for at least four astrophysical sources emitting 100 TeV photons. For the strongest limits set, these observations rule out an energy scale for superluminal LIV: 2.2×10^31 eV, which is more than 1800 times the Planck energy and an improvement of 1 to 2 orders of magnitude over previous limits.

Illustration: This composite image shows a sky view of ultra-high-energy gamma rays. Arrows point to four gamma-ray sources with energies exceeding 100 TeV, which come from within our own Milky Way galaxy (courtesy of the Cherenkov Observatory Collaboration). Located on a photo of 300 large water tanks at the Cherenkov Observatory, which contain sensitive light detectors that can measure the particle rain created by gamma rays hitting the atmosphere more than 10 miles overhead.

Boco Park | Research/From: Los Alamos National Laboratory

Reference journal: Physical Review Letters

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