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Non-Boltzmann Statistics as an Alternative to Holography

2006, International Journal of Modern Physics A

An intriguing question related to black hole thermodynamics is that the entropy of a region shall scale as the area rather than the volume. In this essay we propose that the microscopical degrees of freedom contained in a given region of space, are statistically related in such a way that obey a nonstandard statistics, in which case an holographic hypothesis would not be needed. This could provide us with some insight about the nature of degrees of freedom of the geometry and/or the way in which gravitation plays a role in the statistic correlation between the degrees of freedom of a system.

Non-Boltzmann Statistics as an Alternative to Holography arXiv:gr-qc/0504004v2 21 Oct 2005 Marcelo Botta Cantcheff†,‡1 and Jose A.C. Nogales‡,⋆,2 † Instituto de Fı́sica Teórica (IFT/UNESP) Rua Pamplona 145 - Bela Vista 01405-900- Sao Paulo - SP - Brazil. ‡ Centro Brasileiro de Pesquisas Fı́sicas (CBPF), Departamento de Teoria de Campos e Partı́culas (DCP), Rua Dr. Xavier Sigaud 150, 22290-180, Rio de Janeiro, RJ, Brasil. ⋆ Instituto de Fı́sica, Facultad de Ciencias Puras y Naturales, Calle 27 Campus Universitario, Universidad Mayor de San Andrés, Casilla 8635, La Paz, Bolivia Abstract An intriguing question related to black hole thermodynamics is that the entropy of a region shall scale as the area rather than the volume. In this essay we propose that the microscopical degrees of freedom contained in a given region of space, are statistically related in such a way that obey a non-standard statistics, in which case an holographic hypothesis would be not needed. This could provide us with some insight about the nature of degrees of freedom of the geometry and/or the way in which gravitation plays a role in the statistic correlation between the degrees of freedom of a system. 1 2 [email protected] [email protected], [email protected] 1 We start this note by making the following crucial observation: when gravitational interaction of a given system is considered, it is impossible to isolate two subsystems (unless they are infinitely separated) 3 , so, to define statistical independence of two subsystems is actually problematic in this context. On the other hand, it is known that for systems whose microscopical components cannot be considered statistically independent (and in particular, those being highly correlated), it shall be considered a non-standard statistics (non-additive or non-extensive)[1]. Consequently, we can expect a deviation of the standard statistics when gravity is considered an important part of a system. In the last two decades, generalizations of the Boltzmann-Gibbs thermodynamics have been extensively explored [2]. In an ideal system in thermodynamic equilibrium, if the intensive variables are kept fixed and the side of the system is doubled, the extensive variables should then also double. This is not true for black holes [3, 4, 5, 6]. From the thermodynamic point of view, we mean macroscopic [7], the entropy of black holes is proportional to the area of the event horizon S ∝ A. We are going to analyze the statistical behavior of the degrees of freedom that compose this type of systems, in order to have consistency with the relation above. Let us explain more carefully the difference between additivity and extensivity [8]. Consider a physical quantity W (i) related with the subsystem i. For N of such subsystems, W is additive if we have: W (N) = N X W (i) , (1) i=1   where W (N) ≡ W {i}N 1 . Supposing that all subsystems are equal, W (N) = NW (1) . (2) In its turn, extensivity is defined by: |W (N)| <∞. N →∞ N lim 3 In contrast with ordinary thermodynamic systems. 2 (3) The physical quantities that do not fulfill this property are called non-extensive. We note that all quantities which are additive are also extensive, in fact |W (N)| = |W̄ | , N →∞ N lim (4) where the mean value, |W̄ |, is finite since W (i) < ∞ ∀i. In particular if the subsystems are equal, we get limN →∞ |WN(N )| = W (1). 1 Blak Hole thermodynamics and the holographic interpretation Roughly speaking, the argument which leads to the holographic principle is that if one assumes an uniform distribution of the gravitational degrees of freedom in a spatial region of volume V , or in other words, if, as suggested by quantum gravity, the spacetime is thought as filled of quantum geometric atoms like wormholes or more complex structures of Planck size ℓp more or less uniformly distributed, then the number of microstates is ∝ M V , where M is the number of states reachable by each microscopical component. Then, following the Boltzmann (extensive) statistics, one obtains that entropy S must scale as the volume V which would lead to a contradiction with the Bekenstein-Hawking law for the black hole entropy, S = A/4 in Plancks units [3, 5]. So, the holographic principle suggested by tHooft [9] essentially claims that all these degrees of freedom may be mapped to the boundary of that region, and consequently the corresponding entropy would scale as the area. As we can notice, an important part of this argument is the extensive BoltzmannGibbs statistics, and our proposal is precisely based on alternatives to this, in order to match consistency with the area law and to render the holographic viewpoint not necessary 4 . 2 Non-standard statistics and consistency with the area law. Now, we keep the assumption of uniformity in the distribution of the physical degrees of freedom in a spatial region of volume V , and drop out the holographic point of view 4 In this approach, we are not worried with the aspects concerning the information paradox which, in principle, could be avoided by the holographic principle. 3 about the possibility of mapping them to the boundary. According to this, if we suppose N identical degrees of freedom uniformly distributed in V , we have N ∝ V . Following the area law, we must have S(N) ∝ A ∝ N 2/3 . Thus, our main result is: S(N) ∼ N 2/3 S(1) , (5) which differs from (2). Substituting this into (3) we get lim N →∞ |S(N)| =0, N (6) which shows the extensivity of the entropy. Our conclusion is that the statistics that must be considered in a system where the gravitation plays an important role (as a Black Hole) is non-standard, in particular the entropy is a non-additive but extensive quantity. The law which rules the non additivity is given by (5). 3 Correlated Systems and Tsallis Statistics. Finally, we are going to discuss here a possible realization of the framework described above. As discussed in [1], for certain systems where the hypothesis of probabilistic independence is not applicable, the number of states is Γ ∼ N γ (γ > 0 , N ≫ 1). (7) On the other hand, the corresponding expression for the Tsallis entropy is S=k Γ1−q − 1 , 1−q (8) where q < 1 is a dimensionless parameter. If the degrees of freedom, geometric or not, are correlated by gravitational interaction such that (7) holds, we obtain that S ∼ N (1−q)γ (N ≫ 1) . (9) Thus, by requiring consistency with (5), we obtain: (1 − q)γ = 2/3. 4 (10) As a particular example, if we have a sort of Boolean system where each (distinguishable) degree of freedom may assume the states si = 0, 1 , i = 1, ..., N [9], and the constraint N X si = 1 (11) i=1 is also assumed, then, the possible states of the system are given by si = δik , for some k (1 ≤ k ≤ N). Then, we have Γ = N. Substituting this into (10), as a result one has: γ ∼ 1 ⇒ q = 1/3. On the other hand, notice that the q-statistics may be continuously extended to q > 1 values, and expression (8) becomes S=k Γq−1 − 1 . q−1 (12) In this case, one can verify a remarkable coincidence with a conjecture due to Tsallis et al [10, 8], about statistics of systems governed by a potential decreasing with the inverse square of the distance (as is the case of gravity). By using arguments quite different from here, they argued that q should be 5/3. But, on the other hand, we obtain precisely this value (q ∼ 5/3) for the simplified model we are considering above (with γ ∼ 1). We conclude this work by stressing that also the problems associated with the black hole thermodynamical instability could be by-passed by considering certain nonstandard statistics [11]. We would like to thank to CNPq for the invaluable financial help. Special thanks are due to A.L.M.A. Nogueira and J. A. Helayel by reading the manuscript. 5 References [1] C. Tsallis, ”What should a statistical mechanics satisfy to reflect nature”, [arXiv:cond-mat/0403012] [2] C. Tsallis, J. Stat. Phys. 52, 479-487(1988), ”Possible Generalization of Boltzmann-Gibbs Statistics”. See related references in http://www.cbpf.br/GrupPesq/StatisticalPhys/TEMUCO.pdf [3] J.D. Bekenstein, ”Black Holes and the Second Law”, Lett. Nuovo Cimento 4 , 737 (1972). [4] J.M. Bardeen, B. Carter, S.W. Hawking, ”4 Laws of Black Hole Mechanics”, Communications in Mathematical Physics 31,161-170 (1973). [5] S.W. Hawking, ”Particle Creation by Black Holes”, Communications in Mathematical Physics 43 , 199-220 (1975). [6] T. Jacobson, ”Thermodynamics of Spacetime- The Einstein Equation of State” Phys. Rev. Lett. 75,1260-1263 (1995). [7] M. Botta Cantcheff, On the Microscopical Structure of the Classical Spacetime., hep-th/0408151. T. Padmanabhan, Gravity As Elasticity Of Spacetime: A Paradigm To Understand Horizon Thermodynamics And Cosmological Constant, gr-qc/0408051. [8] C. Tsallis ”Is the entropy Sq extensive or nonextensive?”, proceedings of the 31st Workshop of the International School of Solid State Physics “Complexity, Metastability and Nonextensivity”, held at the Ettore Majorana Foundation and Centre for Scientific Culture, Erice (Sicily) in 20-26 July 2004, eds. C. Beck, A. Rapisarda and C. Tsallis (World Scientific, Singapore, 2005) arXiv/cond-mat/0409631 [9] G. ’T Hooft, ”Dimentional Reduction in Quantum Gravity”, [arXiv:grqc/9310026]. [10] ”Entropic Non-Extensivity: a Possible Measure of Complexity” C. Tsallis , Chaos, Solitons and Fractals , 13 (2002) 371 arXiv/cond-mat/0010150. [11] M. Botta Cantcheff and J. Nogales, work in progress. 6