Christian Corda

Christian Corda is an astrophysicist and theoretical Physicist. His current research interests concern mainly gravitational physics starting from Einstein’s general theory of relativity (GTR), including gravitational waves (GWs), extended theories of gravity (ETG) and quantum physics of black holes (BHs).

The GTR is considered by physicists, together with quantum theory, the best scientific theory of all and represents the best, current description of gravitation in modern physics. One of the most important goals of modern physics, if not the most important, is finding a final, unified theory that allows all that is usually thought of as elementary particles and fundamental forces to be written in terms of a single interaction. This was also the famous dream of Albert Einstein, Stephen Hawking and other famous scientists.

It is a common conviction that, in order to achieve such a prestigious goal, a fundamental step is reconciling quantum theory, which is the subset of physics which explains the physical behaviours at the molecular, atomic and sub-atomic levels, with the GTR. This is still an open question. The demand for consistency between quantum theory and the GTR indicates the need for a full quantum theory of gravity (QTG). At the present time, an absolute QTG which implies a total unification of various interactions has not been obtained. In addition, the GTR assumes a non-quantum description of the matter which is totally inappropriate at subatomic scales, which are the scales of BHs and of the relic Universe. Various unification approaches have been suggested, but without palpable observational evidences in a laboratory environment on Earth. Starting from these considerations, one defines the ETG some attempts to extend the GTR.

On one hand, the ETG are considered an intermediate step between the non-complete GTR and the final QTG. On the other hand, the ETG can, in principle, help to understand some unsolved astrophysical and cosmological problems like the famous dark matter and dark energy.  In this picture, even the nascent GW astronomy, which started with the famous, recent announcement by the LIGO  Collaboration of the first, historical GW detection, could, in principle, be important to confirm or rule out the physical consistency of the GTR or of any other theory of gravitation. This is because, in the context of the ETG, some differences between the GTR and the others theories can be pointed out by analysing the GWs. In his paper Int. Journ. Mod. Phys. D, 18, 14, 2275-2282 (2009) Christian has indeed shown that if advanced projects on the GW detection, like LIGO and others, will improve their sensitivity, allowing to really perform a detailed GW astronomy, then the received, detailed signals will be the definitive test for the GTR or, alternatively, a strong endorsement for the ETG.

Another important issue in the framework of a QTG is the BH quantum physics, which arises from the remarkable ideas of Jacob Bekenstein and Stephen Hawking. In fact, researchers in quantum gravity have the intuitive, common conviction that, in some respects, BHs are the fundamental bricks of quantum gravity in the same way that atoms are the fundamental bricks of quantum mechanics. This similarity suggests that the BH mass should have a discrete spectrum. On the other hand, the analogy generates an immediate and natural question: if the BH is the nucleus of the “gravitational atom” in quantum gravity, what are the “electrons”? In his research papers Class. Quantum Grav. 32, 195007 (2015), Adv. High En. Phys. 867601 (2015), Ann. Phys. 353, 71(2015) and others, Christian gives an intriguing answer to that question, showing that the oscillations of the horizon of the Schwarzschild BH  “triggered” by the emission of Hawking quanta and by the potential absorptions of neighbouring particles can be considered as the “electrons” of that “gravitational atom”. Thus, Christian shows that the intuitive picture is more than a picture as the oscillations of the horizon can be really interpreted in terms of BH quantum levels discussing a BH model somewhat similar to the semi-classical Bohr model of the structure of a hydrogen atom. This issue has important consequences on the famous BH information puzzle of Stephen Hawking, who, in 1976 claimed that physical information is completely loss in BH evaporation. Christian has indeed shown that BHs are well defined quantum mechanical systems, having an ordered, discrete quantum spectrum. In addition, the time evolution of the Bohr-like BH is governed by a time dependent Schrödinger equation which permits information to come out in BH evaporation. Also the famous entanglement problem connected with the BH information paradox is solved through this approach. Another fundamental feature of the Bohr-like BH is the discreteness of the BH horizon area as the function of the BH principal quantum number, which is consistent with various models of quantum gravity where the space-time is fundamentally discrete. Christian is now attempting to generalize the Bohr-like BH to other kinds of BHs. In fact, the analogy between the BH and the hydrogen atom found by Christian works only for the simplest BH, that is the Schwarzschild static BH. The extension of the analysis to charged and rotating BHs is the next step, while the dream is to quantize the system Hawking radiation – horizon’s oscillations without any semi-classical approximation, that is considering the full Einstein’s equations of the GTR. In fact, Bohr model is an approximated model of the hydrogen atom with respect to the valence shell atom model of full quantum mechanics. In the same way, the Bohr-like BH model should be an approximated model with respect to the definitive, but at the present time unknown, BH model arising from a full QTG.

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Contact Christian at
cordac [dot] galilei at gmail /dot/ com
christian {dot} corda at-sign ronininstitute [dot] org