Researchers from the Institute of High Energy Physics of the Chinese Academy
of Sciences examined the validity of the theory of relativity with the
highest accuracy in a study entitled "Exploring Lorentz Invariance Violation
from Ultrahigh-Energy γRays Observed by LHAASO," which was published in the
latest issue of Physical Review Letters.
According to Einstein's theory of relativity, the fastest speed of matter in
the Universe is the speed of light. Whether that limit is breachable can be
tested by examining Lorentz symmetry breaking or Lorentz invariance
violation.
"Using the world's highest energy gamma rays observed by the Large High
Altitude Air-shower Observatory (LHAASO), a large-scale cosmic ray
experiment in Daocheng, Sichuan province, China, we tested Lorentz symmetry.
The result improves the breaking energy scale of Lorentz symmetry by dozens
of times compared with the previous best result. This is the most rigorous
test of a Lorentz symmetry breaking form, confirming once again the validity
of Einstein's relativistic space-time symmetry," said Prof. Bi Xiaojun, one
of the paper's corresponding authors. Prof. BI is a scientist at the
Institute of High Energy Physics and a member of the LHAASO collaboration.
What is the relationship between Lorentz symmetry and the theory of relativity?
Einstein's theory of relativity, the cornerstone of modern physics, requires
that physical laws have Lorentz symmetry. In the more than 100 years since
Einstein proposed his theory of relativity, the validity of Lorentz symmetry
has undergone numerous experimental tests.
However, there is an irreconcilable contradiction between general
relativity, which describes gravity, and quantum mechanics, which describes
the laws of the microscopic world. In order to unify general relativity and
quantum mechanics, theoretical physicists have made unremitting efforts and
have developed theories such as string theory and loop quantum gravity
theory. These theories predict that Lorentz symmetry is likely to be broken
at very high energies, which means relativity may need to be modified at
high energies.
Therefore, it is crucial to test the theory of relativity and develop more
fundamental laws of physics by looking for signals of Lorentz symmetry
breaking. However, according to these theories, the effect of Lorentz
symmetry breaking is only significant at the so-called Planck energy scale,
which is up to 1019 GeV (1 GeV = 1 billion electron volts).
Since artificial accelerators can only reach about 104 GeV, the effects of
Lorentz symmetry breaking are too weak to be tested in laboratories. But
there are very violent astrophysical processes in the universe where
particles can be accelerated to energies much higher than what man-made
accelerators can reach. Therefore, astrophysical observations are a natural
laboratory for looking for the effects of Lorentz symmetry breaking.
LHAASO is a large-scale cosmic ray experiment in China. During the process
of construction in 2021, the world's highest energy gamma ray event was
recorded by LHAASO, with its energy up to 1.4 PeV (1 PeV = 1015 electron
volts). At the same time as setting a world record, it also provided a
valuable opportunity for exploring the basic laws of physics, such as
Lorentz symmetry.
Lorentz symmetry breaking may cause high-energy photons to become unstable,
rapidly decaying into an electron-positron pair or into three photons. "In
other words, the high-energy photons automatically disappear on their
journey to Earth if Lorentz symmetry is broken, which implies the energy
spectrum we measured should be truncated at a particular energy," said Prof.
Bi.
The data from LHAASO show that the current gamma ray spectrum continues to
high energies above PeV, and no "mysterious" disappearance of any
high-energy gamma ray events has been found. This result shows that Lorentz
symmetry is still maintained when approaching the Planck energy scale.
Reference:
Zhen Cao et al, Exploring Lorentz Invariance Violation from Ultrahigh-Energy
γ Rays Observed by LHAASO, Physical Review Letters (2022).
DOI: 10.1103/PhysRevLett.128.051102
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Physics