Gravitational Properties of Light and Ultra-Relativistic Matter
Gravitational Properties of Light and Ultra-Relativistic Matter
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Einstein’s equations tell us that light must gravitate [1] and that the momentum of ultra-relativistic matter contributes much more to its gravitational field than its rest mass. Gravitational fields due to such relativistic sources differ notably from gravitational fields sourced by slow-moving matter.
For example in [2,3], Martin Wilkens, Ralf Menzel and I have investigated the gravitational field of laser pulses and found that all physical effects are confined to spherical shells expanding with the speed of light and these shells are imprints of the spacetime events representing emission and absorption of the pulse. Furthermore in [4], we have shown that polarization entanglement of light sources can affect the light-light scattering cross section in Perturbative Quantum Gravity.
In follow-up articles with my collaborators from Tübingen, Daniel Braun and Fabienne Schneiter, we have shown that circular polarization of a focused light beam leads to frame dragging and gravitational spin-spin coupling of light [5,6].
These effects are of fundamental interest for two reasons: firstly, the properties of light are inherent in modern physics (as the basis for General Relativity) and one may gain some deeper understanding of gravity by taking also the gravitational properties of light into account, and secondly, the great control over quantum properties of light available in modern labs suggests that quantum properties of the gravitational field may be explored by experimentally investigating the gravitational properties of light when such experiments may become possible in the future.
Presently, a measurement is more realistic for the gravitational field of relativistic particle beams, like those in the beam of the Large Hadron Collider (LHC) at CERN, as we have recently been able to show as part of my collaboration with Daniel Braun and Felix Spengler [7].
[1] Tolman R.C., Ehrenfest P., Podolsky B. "On the gravitational field produced by light." Physical Review 37.5 (1931): 602.
[2] Rätzel D., Wilkens M., Menzel R. "Gravitational properties of light - The gravitational field of a laser pulse" New J. Phys. 18 2 023009 (2016), doi.org/10.1088/1367-2630/18/2/023009; Preprint arxiv.org/abs/1511.01023
[3] Rätzel D., Wilkens M., Menzel R. "Gravitational properties of light: The emission of counter-propagating laser pulses from an atom" Phys. Rev. D 95 (2017), 8 084008, doi.org/10.1103/PhysRevD.95.084008; Preprint arxiv.org/abs/1607.01310
[4] Rätzel D., Wilkens M., Menzel R. "The effect of entanglement in gravitational photon-photon scattering" EPL 115 (2016) 51002, doi.org/10.1209/0295-5075/115/51002; Preprint arxiv.org/abs/1511.01237
[5] Schneiter F., Rätzel D., Braun D. "The gravitational field of a laser beam beyond the short wavelength approximation" Class. Quantum Grav. 35 195007 (2018), doi.org/10.1088/1361-6382/aadc81; Preprint arxiv.org/abs/1804.08706
[6] Schneiter F., Rätzel D., Braun D. "Rotation of polarization in the gravitational field of a laser beam—Faraday effect and optical activity" Classical and Quantum Gravity 36 (20), 205007 (2019), doi.org/10.1088/1361-6382/ab3523; Preprint arxiv.org/abs/1812.04505
[7] Spengler F., Rätzel D., Braun D. “Perspectives of measuring gravitational effects of laser light and particle beams” New Journal of Physics 24.5 (2022): 053021, https://doi.org/10.1088/1367-2630/ac5372; Preprint: arxiv.org/abs/2104.09209
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