Ulvi Yurtsever

Earth before life

BackgroundA recent study argued, based on data on functional genome size of major phyla, that there is evidence life may have originated significantly prior to the formation of the Earth.ResultsHere a more refined regression analysis is performed in which 1) measurement error is systematically taken into account, and 2) interval estimates (e.g., confidence or prediction intervals) are produced. It is shown that such models for which the interval estimate for the time origin of the genome includes the age of the Earth are consistent with observed data.ConclusionsThe appearance of life after the formation of the Earth is consistent with the data set under examination.ReviewersThis article was reviewed by Yuri Wolf, Peter Gogarten, and Christoph Adami.

Holographic entropy bound and local quantum field theory.

I show how the holographic entropy bound can be derived from elementary flat-spacetime quantum field theory when the total energy of Fock states is constrained gravitationally. This energy constraint makes the Fock space dimension (whose logarithm is the maximum entropy) finite for both bosons and fermions. Despite the elementary nature of my analysis, it results in an upper limit on entropy in remarkable agreement with the holographic bound, and also provides a microscopic deviation of a more general entropy bound recently introduced by Gour.

Test fields on compact space‐times

In this paper, some basic aspects of (Lorentzian) field theory on compact Lorentz manifolds are studied. All compact space‐times are acausal, i.e., possess closed timelike curves; this makes them a useful testbed in analyzing some new notions of causality that will be introduced for more general acausal space‐times. In addition, studying compact space‐times in their own right raises a wide range of fascinating mathematical problems some of which will be explored. It will be shown that it is reasonable to expect Lorentzian field theory on a compact space‐time to provide information on the topology of the underlying manifold; if this is true, then this information is likely to be ‘‘orthogonal’’ (or complementary) to the information obtained through the study of Euclidean field theory.

Geometry of chaos in the two-center problem in general relativity.

The now-famous Majumdar-Papapetrou exact solution of the Einstein-Maxwell equations describes, in general, N static, maximally charged black holes balanced under mutual gravitational and electrostatic interaction. When N=2, this solution defines the two-black-hole spacetime, and the relativistic two-center problem is the problem of geodesic motion on this static background. Contopoulos and a number of other workers have recently discovered through numerial experiments that, in contrast with the Newtonian two-center problem, where the dynamics is completely integrable, relativistic null-geodesic motion on the two-black-hole spacetime exhibits chaotic behavior Here I identify the geometric sources of this chaotic dynamics by first reducing the problem to that of geodesic motion on a negatively curved (Riemannian) surface.

Signalling, entanglement and quantum evolution beyond Cauchy horizons

Consider a bipartite entangled system, half of which falls through the event horizon of an evaporating black hole, while the other half remains coherently accessible to experiments in the exterior region. Beyond complete evaporation, the evolution of the quantum state past the Cauchy horizon cannot remain unitary, raising the questions: how can this evolution be described as a quantum map, and how is causality preserved? What are the possible effects of such non-standard quantum evolution maps on the behaviour of the entangled laboratory partner? More generally, the laws of quantum evolution under extreme conditions in remote regions (not just in evaporating black-hole interiors, but possibly near other naked singularities and regions of extreme spacetime structure) remain untested by observation, and might conceivably be non-unitary or even nonlinear, raising the same questions about the evolution of entangled states. The answers to these questions are subtle, and are linked in unexpected ways to the fundamental laws of quantum mechanics. We show that terrestrial experiments can be designed to probe and constrain exactly how the laws of quantum evolution might be altered, either by black-hole evaporation, or by other extreme processes in remote regions possibly governed by unknown physics.

Remarks on the averaged null energy condition in quantum field theory.

The averaged null energy condition has been recently shown to hold for linear quantum fields in a large class of spacetimes. Nevertheless, it is easy to show by using a simple scaling argument that ANEC as stated cannot hold generically in curved four-dimensional spacetime, and this scaling argument has been widely interpreted as a death-blow for averaged energy conditions in quantum field theory. In this note I propose a simple generalization of ANEC, in which the right-hand-side of the ANEC inequality is replaced by a finite (but in general negative) state-independent lower bound. As long as attention is focused on asymptotically well-behaved spacetimes, this generalized version of ANEC is safe from the threat of the scaling argument, and thus stands a chance of being generally valid in four-dimensional curved spacetime. I argue that when generalized ANEC holds, it has implications for the non-negativity of total energy and for singularity theorems similar to the implications of ANEC. In particular, I show that if generalized ANEC is satisfied in static traversable wormhole spacetimes (which is likely but remains to be shown), then macroscopic wormholes (but not necessarily microscopic, Planck-size wormholes) are ruled out by quantum field theory.

A simple proof of geodesical completeness for compact space‐times of zero curvature

Any Riemannian metric on a compact manifold is geodesically complete. By contrast, it is widely known that incomplete compact Lorentzian manifolds exist. In this article it will be proven that a compact manifold with a smooth Lorentz metric must be complete if the metric is flat everywhere. This question of completeness; which has significant implications for the classification of (compact) Lorentzian space forms, has remained unresolved until quite recently, when it was finally established by Carriere as a special case of an even more general result. Here, a simple, alternative proof of this special case of Carriere’s theorem is given; the proof requires minimal mathematical machinery but still involves the use of some intriguing connections between the topology and global geometry of a compact flat space‐time.

Exploiting the quantum Zeno effect to beat photon loss in linear optical quantum information processors

We devise a new technique to enhance transmission of quantum information through linear optical quantum information processors. The idea is based on applying the Quantum Zeno effect to the process of photon absorption. By frequently monitoring the presence of the photon through a QND (quantum non-demolition) measurement the absorption is suppressed. Quantum information is encoded in the polarization degrees of freedom and is therefore not affected by the measurement. Some implementations of the QND measurement are proposed.

A remark on kinks and time machines

We describe an elementary proof that a manifold with the topology of the Politzer time machine does not admit a nonsingular, asymptotically flat Lorentz metric.

Thwarting the photon number splitting attack with entanglement enhanced BB84 quantum key distribution

We develop an improvement to the weak laser pulse BB84 scheme for quantum key distribution, which utilizes entanglement to increase the security of the scheme and enhance its resilience to the photon-number-splitting attack. This protocol relies on the non-commutation of phase and number to detect an eavesdropper performing quantum nondemolition measurement on photon number. The potential advantages and disadvantages of this scheme are compared to the coherent decoy state protocol.