In the framework of Einstein's theory of relativity, light propagates in the
same way in all directions, whatever the photon energy is. This invariance is
called "Lorentz invariance" as it has been described mathematically by the
1902 Nobel physics prize laureate H.A. Lorentz. However, according to some
theories trying to match the two pillars of modern physics, general relativity
and quantum mechanics, this fundamental law could be violated. Investigating a
possible violation of this law is an active research area in physics since
decades, in particular in the particle physics domain. More recently,
astrophysical tests were proposed, in particular by analyzing the properties
of the light emitted by distant sources, such as gamma-ray bursts
(GRB).
INTEGRAL observed the gamma-ray burst GRB041219A burst which is located at
more than 300 million light years from the Earth, and has put a very strong
constraint on the possibility to obtain such a violation of the Lorentz
invariance. Indeed, the IBIS telescope onboard INTEGRAL, thanks to its Compton
imaging mode, has measured the polarization of the GRB gamma-ray emission in
two adjacent energy bands and showed, that there is statistically no
difference between these two measurements (see Figure). On the other hand,
according to some quantum gravity theories, Lorentz invariance violation
should induce an energy dependent rotation of the linear polarization during
its long travel in vacuum from the GRB to us. INTEGRAL observations have then
enabled us to put a limit on this phenomenon - the "vacuum birefringence
effect" - to ξ < 1.1x10-14, that is 105 times more
constraining than what was previously determined, reinforcing one of the
pillars of Einstein's general relativity. The picture shows a contour plot of
the composed error on polarisation angle and polarisation fraction for two
energy bands, as observed by INTEGRAL/IBIS during a 10 second interval of the
GRB041219A prompt emission. The X shows the best fit position for the energy
band (200-250) keV. Contour levels are at the 67, 90, and 95 % level (from
white towards black filled areas). The + and dotted, dashed, and dash-dotted
lines show the best fit parameters obtained for the second energy band
(250-325) keV, consistent at 95% with the first energy band best
fit.
