10 Novembre – Thesis defense - Paul Azar

14 h30 Amphi 3 - building A9 (Talence campus)

Study of the gravitational influence of a light pulse.

In 2016, the first direct detection of gravitational waves took place thanks to the combined efforts of the many scientists of LIGO. Since then, the giant interferometers of LIGO and VIRGO regularly detect these gravitational waves generated by cataclysmic astrophysical phenomena such as the merger of two black holes or neutron stars. In order to better understand the mechanism at the origin of these waves, i.e. general relativity, laboratory experiments have been imagined. These experiments aim to create enough mass acceleration to produce a measurable gravitational deformation. However, mass acceleration is not the only mechanism that can generate gravitational deformations in the laboratory, nor in space.
In 1962, Gertsenshtein shows that it is possible to have the generation of gravitational waves by interaction of an electromagnetic wave with a static magnetic field. By investigating this demonstration, we could observe that any electromagnetic wave produces a gravitational deformation. In this thesis we are interested in such a type of gravitational deformation. We address it by modeling a light pulse by a cylinder of homogeneous energy density, moving at the speed of light c.
We implement a method for solving the linearized Einstein equations based on the D'Alembertian Green's function, which leads us to an exact analytical determination of the gravitational deformation generated by a cylinder of light on its propagation axis. The solutions thus found are valid even in the unsteady regime, and thus allow us to know more about the setup of some gravitational potentials. We thus determine the mode of establishment of the gravitational potential of a static object of constant energy density which would suddenly appear in space, but also the shape and the amplitude of the gravitational deformations generated by a light pulse. These calculations are confirmed by comparison with the solution of the Schwarzschild metric which gives us the same results far from the studied objects. Breaking our study into that of a cylinder of constant energy density and that of a cylinder of oscillating energy density, we conduct a variational study on the different quantities characteristic of such a light emission. We are thus interested in the relationship between length and width of the light pulse, its propagation time, but also its wavelength, its intensity and its power. Through these considerations, we estimate and try to optimize the performance of a laboratory gravitational deformation generation experiment using a power laser.
As an opening to this thesis, we propose various calculations that could develop and support the results of our research. We conclude by showing the interest of the study of these deformations generated only by an electromagnetic part. Not just in the laboratory, where we detail the various paths followed to date to detect the very high frequency gravitational deformations we are studying, but also in astrophysics, where extremely intense electromagnetic phenomena can take place, such as gamma-ray bursts.

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