arXiv subjects
Publications and source records attributed to Shahar Hod.
A fundamental bound on the relaxation time τof a perturbed thermodynamical system has recently been derived, τ\geq \hbar/πT, where $T$ is the system's temperature. We demonstrate analytically that black holes saturate this bound in the extremal limit and for large values of the azimuthal number m of the perturbation field.
From information theory and thermodynamic considerations a universal bound on the relaxation time $τ$ of a perturbed system is inferred, $τ\geq \hbar/πT$, where $T$ is the system's temperature. We prove that black holes comply with the bound; in fact they actually {\it saturate} it. Thus, when judged by their relaxation properties, black holes are the most extreme objects in nature, having the maximum relaxation rate which is allowed by quantum theory.
We study {\it analytically} the asymptotic quasinormal spectrum of fermionic fields in the Kerr spacetime. We find an analytic expression for these black-hole resonances in terms of the black-hole physical parameters: its Bekenstein-Hawking temperature $T_{BH}$, and its horizon's angular velocity $Ω$, which is valid in the asymptotic limit $1 \ll ω_I \ll ω_R$. It is shown that according to Bohr's correspondence principle, the emission of a Rarita-Schwinger quantum ($s=3/2$) corresponds to a fundamental black-hole area change $ΔA=4\hbar \ln 2$, while the emission of a Weyl neutrino field ($s=1/2$) corresponds to an adiabatic quantum transition with $ΔA=0$.
We advocate the idea that Unruh's quantum radiation, whose theoretical discovery was originally motivated by the physics of black holes, may have important implications on the structure and dynamics of elementary particles. To that end, we analyze the Unruh radiation effect experienced by an accelerated particle in atomic and nuclear systems. For atomic systems, the effect is negligible as compared to the characteristic energy of the system. On the other hand, it is found that a quark inside a nucleon may experience Unruh radiation whose energy is of the same order of magnitude as the quark's own mass.
Black-hole quasinormal modes have been the subject of much recent attention, with the hope that these oscillation frequencies may shed some light on the elusive theory of quantum gravity. We study {\it analytically} the asymptotic quasinormal spectrum of a {\it charged} scalar field in the (charged) Reissner-Nordström spacetime. We find an analytic expression for these black-hole resonances in terms of the black-hole physical parameters: its Bekenstein-Hawking temperature $T_{BH}$, and its electric potential $Φ$. We discuss the applicability of the results in the context of black-hole quantization. In particular, we show that according to Bohr's correspondence principle, the asymptotic resonance corresponds to a fundamental area unit $ΔA=4\hbar\ln2$.
We suggest that quantum transitions of black holes comply with selection rules, analogous to those of atomic spectroscopy. In order to identify such rules, we apply Bohr's correspondence principle to the quasinormal ringing frequencies of black holes. In this context, classical ringing frequencies with an asymptotically vanishing real part ω_R correspond to virtual quanta, and may thus be interpreted as forbidden quantum transitions. With this motivation, we calculate the quasinormal spectrum of neutrino fields in spherically symmetric black-hole spacetimes. It is shown that ω_R->0 for these resonances, suggesting that the corresponding fermionic transitions are quantum mechanically forbidden.
We analyze the dynamics of random walks with long-term memory (binary chains with long-range correlations) in the presence of an absorbing boundary. An analytically solvable model is presented, in which a dynamical phase-transition occurs when the correlation strength parameter μreaches a critical value μ_c. For strong positive correlations, μ> μ_c, the survival probability is asymptotically finite, whereas for μ< μ_c it decays as a power-law in time (chain length).
We study analytically the quasinormal mode spectrum of near-extremal (rotating) Kerr black holes. We find an analytic expression for these black-hole resonances in terms of the black-hole physical parameters: its Bekenstein-Hawking temperature T_{BH} and its horizon's angular velocity Ω, which is valid in the intermediate asymptotic regime 1<<ω<<1/T_{BH}.
The celebrated area-entropy formula for black holes has provided the most important clue in the search for the elusive theory of quantum gravity. We explore the possibility that the (linear) area-entropy relation acquires some smaller corrections. Using the Boltzmann-Einstein formula, we rule out the possibility for a power-law correction, and provide severe constraints on the coefficient of a possible log-area correction. We argue that a non-zero logarithmic correction to the area-entropy relation, would also imply a modification of the area-mass relation for quantum black holes.
We analyze the dynamics of random walks in which the jumping probabilities are periodic {\it time-dependent} functions. In particular, we determine the survival probability of biased walkers who are drifted towards an absorbing boundary. The typical life-time of the walkers is found to decrease with an increment of the oscillation amplitude of the jumping probabilities. We discuss the applicability of the results in the context of complex adaptive systems.
In the evolutionary minority game, agents are allowed to evolve their strategies (``mutate'') based on past experience. We explore the dependence of the system's global behavior on the response time and the mutation threshold of the agents. We find that the precise values of these parameters determine if the strategy distribution of the population has a U-shape, inverse-U-shape, or a W-shape. It is shown that in a free society (market), highly adaptive agents (with short response times) perform best. In addition, ``patient'' agents (with high mutation threshold) outperform ``nervous'' ones.
Motivated by novel results in the theory of correlated sequences, we analyze the dynamics of random walks with long-term memory (binary chains with long-range correlations). In our model, the probability for a unit bit in a binary string depends on the fraction of unities preceding it. We show that the system undergoes a dynamical phase-transition from normal diffusion, in which the variance D_L scales as the string's length L, into a super-diffusion phase (D_L ~ L^{1+|alpha|}), when the correlation strength exceeds a critical value. We demonstrate the generality of our results with respect to alternative models, and discuss their applicability to various data, such as coarse-grained DNA sequences, written texts, and financial data.
Motivated by novel results in the theory of black-hole quantization, we study {\it analytically} the quasinormal modes (QNM) of ({\it rotating}) Kerr black holes. The black-hole oscillation frequencies tend to the asymptotic value $ω_n=mΩ+i2πT_{BH}n$ in the $n \to \infty$ limit. This simple formula is in agreement with Bohr's correspondence principle. Possible implications of this result to the area spectrum of quantum black holes are discussed.
In the evolutionary version of the minority game, agents update their strategies (gene-value $p$) in order to improve their performance. Motivated by recent intriguing results obtained for prize-to-fine ratios which are smaller than unity, we explore the system's dynamics with a strategy updating rule of the form $p \to p \pm δp$ ($0 \leq p \leq 1$). We find that the strategy distribution depends strongly on the values of the prize-to-fine ratio $R$, the length scale $δp$, and the type of boundary condition used. We show that these parameters determine the amplitude and frequency of the the temporal oscillations observed in the gene space. These regular oscillations are shown to be the main factor which determines the strategy distribution of the population. In addition, we find that agents characterized by $p={1 \over 2}$ (a coin-tossing strategy) have the best chances of survival at asymptotically long times, regardless of the value of $δp$ and the boundary conditions used.
Black-hole quasinormal modes (QNM) have been the subject of much recent attention, with the hope that these oscillation frequencies may shed some light on the elusive theory of quantum gravity. We compare numerical results for the QNM spectrum of the (rotating) Kerr black hole with an {\it exact} formula Re$ω\to T_{BH}\ln 3+Ωm$, which is based on Bohr's correspondence principle. We find a close agreement between the two. Possible implications of this result to the area spectrum of quantum black holes are discussed.
The study of societies of adaptive agents seeking minority status is an active area of research. Recently, it has been demonstrated that such systems display an intriguing phase-transition: agents tend to {\it self-segregate} or to {\it cluster} according to the value of the prize-to-fine ratio, $R$. We show that such systems do {\it not} establish a true stationary distribution. The winning-probabilities of the agents display temporal oscillations. The amplitude and frequency of the oscillations depend on the value of $R$. The temporal oscillations which characterize the system explain the transition in the global behavior from self-segregation to clustering in the $R<1$ case.
Motivated by novel results in the theory of complex adaptive systems, we analyze the dynamics of random walks in which the jumping probabilities are {\it time-dependent}. We determine the survival probability in the presence of an absorbing boundary. For an unbiased walk the survival probability is maximized in the case of large temporal oscillations in the jumping probabilities. On the other hand, a random walker who is drifted towards the absorbing boundary performs best with a constant jumping probability. We use the results to reveal the underlying dynamics responsible for the phenomenon of self-segregation and clustering observed in the evolutionary minority game.
Hawking's black hole information puzzle highlights the incompatibility between our present understanding of gravity and quantum physics. However, Hawking's prediction of black-hole evaporation is at a semiclassical level. One therefore suspects some modifications of the character of the radiation when quantum properties of the {\it black hole itself} are properly taken into account. In fact, during the last three decades evidence has been mounting that, in a quantum theory of gravity black holes may have a discrete mass spectrum, with concomitant {\it discrete} line emission. A direct consequence of this intriguing prediction is that, compared with blackbody radiation, black-hole radiance is {\it less} entropic, and may therefore carry a significant amount of {\it information}. Using standard ideas from quantum information theory, we calculate the rate at which information can be recovered from the black-hole spectral lines. We conclude that the information that was suspected to be lost may gradually leak back, encoded into the black-hole spectral lines.