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Bogdan Bancia. Quantum gravity. Ad Astra, 2019.

Abstract: The current understanding of gravity is based on Albert Einstein's general theory of relativity, which is formulated within the framework of classical physics. On the other hand, the other three fundamental forces of physics are described within the framework of quantum mechanics and quantum field theory, radically different formalisms for describing physical phenomena. 

While a quantum theory of gravity may be needed to reconcile general relativity with the principles of quantum mechanics, difficulties arise when applying the usual prescriptions of quantum field theory to the force of gravity via graviton bosons. The problem is that the theory one gets in this way is not renormalizable (it predicts infinite values for some observable properties such as the mass of particles) and therefore cannot be used to make meaningful physical predictions. As a result, theorists have taken up more radical approaches to the problem of quantum gravity, the most popular approaches being string theory and loop quantum gravity.  Although some quantum gravity theories, such as string theory, try to unify gravity with the other fundamental forces, others, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces.

Strictly speaking, the aim of quantum gravity is only to describe the quantum behavior of the gravitational field and should not be confused with the objective of unifying all fundamental interactions into a single mathematical framework. A quantum field theory of gravity that is unified with a grand unified theory is sometimes referred to as a theory of everything (TOE). While any substantial improvement into the present understanding of gravity would aid further work towards unification, the study of quantum gravity is a field in its own right with various branches having different approaches to unification.

One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10 to -35 meter, a scale far smaller, and equivalently far larger in energy, than those currently accessible by high energy particle accelerators.

Therefore physicists lack experimental data which could distinguish between the competing theories which have been proposed and thus thought experiment approaches are suggested as a testing tool for these theories.


O ipoteza care mi s-a parut interesanta: Suprafluidul cuantic si gravitatia cuantica

Am gasit pe internet o ipoteza ce incearca sa explice gravitatia facand o analogie intre materia intunecata (dark matter) si heliul superfluid (fenomen cuantic macroscopic)

Cu totii stim ca heliul lichid la temperatura de 2.17 K devine un suprafluid avand vascozitate nula si poate curge singur din recipientul in care se afla, invingand gravitatia. Curge singur din recipient deoarece tensiunea superficiala a suprafluidului este nula iar forta de atractie cu peretii vasului este mult mai mare decat forta gravitationala (efect cuantic al suprafluidului)

Suprafluiditatea este explicata din punct de vedere cuantic (teoria cuantica a suprafluiditatii), asa incat ne-am putea gandi prin analogie la ipoteza unui suprafluid cuantic ce umple spatiul inter-planetar, inter-stelar si inter-galactic, in incercarea de a explica gravitatia cuantica

Toata materia din Univers se afla imersata in acest suprafluid cuantic, iar legile cuantice care il descriu dicteaza distributia materiei precum si deplasarea acesteia in Univers

O alta ipoteza plauzibila pentru a explica gravitatia la nivel cuantic este ipoteza spatiului-timp cuantic. In cadrul acestei ipoteze spatiul-timp este presupus a fi un efect cuantic.

Dupa cum stim vidul cuantic (spatiul-timp) este plin de fluctuatii cuantice, - generare de particule pereche "materie-antimaterie" - care se anihileaza rezultand fotoni, fenomen masurat in laborator (efectul Casimir, forta Casimir). Deci pana acum putem deduce ca spatiul-timp are o structura cuantica iar daca ne mai gandim la radiatia Hawking, putem spune ca intre spatiu-timp cuantic si materie exista un schimb de particule (radiatia Hawking si evaporarea gaurilor negre)

Ipoteza presupune ca pe parcursul evolutiei spatiu-timpului, de la Big Bang si pana in prezent, structura lui granulara (fluctuatiile cuantice) a inceput sa capete o forma organizata, particulele concentrandu-se in anumite zone ale spatiu-timpului rezultand energie, apoi materie (Universul nostru). Atata timp cat putem vorbi despre evolutia Universului in termeni cuantici, putem vorbi despre o teorie a gravitatiei cuantice

Keywords: quantum gravity

Posted by Bogdan Bancia


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