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It is postulated in Einstein’s relativity that the speed of light in vacuum
is a constant for all observers. However, the effect of quantum gravity could
bring an energy dependence of light speed, and a series of previous researches
on high-energy photon events from gamma-ray bursts (GRBs) and active galactic
nuclei (AGNs) suggest a light speed variation $v(E)=c\left(1-E / E_{\mathrm{LV}}\right)$ with $E_{\mathrm{LV}}=3.6 \times 10^{17} ~\mathrm{GeV}$. From the newly detected gamma-ray burst GRB 221009A, we find
that a $99.3~$GeV photon detected by Fermi-LAT is coincident with the sharp
spike in the light curves detected by Fermi-GBM and HEBS under the above
scenario of light speed variation, suggesting that this high energy photon was
emitted at the same time with a sharp spike of low energy photon emission at
the GRB source. Thus this highest energy photon event detected by Fermi-LAT
during the prompt emission of gamma ray bursts supports the linear form
modification of light speed in cosmological space.

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