Biological Homochirality and the Search for Extraterrestrial Biosignatures.

Most amino acids and sugar molecules occur in mirror, or chiral, images of each other, knowns as enantiomers. However, life on Earth is mostly homochiral: proteins contain almost exclusively L-amino acids, while only D-sugars appear in RNA and DNA. The mechanism behind this fundamental asymmetry of life remains unknown, despite much progress in the theoretical and experimental understanding of homochirality in the past decades. We review three potential mechanisms for the emergence of biological homochirality on primal Earth and explore their implications for astrobiology: the first, that biological homochirality is a stochastic process driven by local environmental fluctuations; the second, that it is driven by circularly-polarized ultraviolet radiation in star-forming regions; and the third, that it is driven by parity violation at the elementary particle level. We argue that each of these mechanisms leads to different observational consequences for the existence of enantiomeric excesses in our solar system and in exoplanets, pointing to the possibility that the search for life elsewhere will help elucidate the origins of homochirality on Earth.

The Consensus Problem in Polities of Agents with Dissimilar Cognitive Architectures.

Agents interacting with their environments, machine or otherwise, arrive at decisions based on their incomplete access to data and their particular cognitive architecture, including data sampling frequency and memory storage limitations. In particular, the same data streams, sampled and stored differently, may cause agents to arrive at different conclusions and to take different actions. This phenomenon has a drastic impact on polities-populations of agents predicated on the sharing of information. We show that, even under ideal conditions, polities consisting of epistemic agents with heterogeneous cognitive architectures might not achieve consensus concerning what conclusions to draw from datastreams. Transfer entropy applied to a toy model of a polity is analyzed to showcase this effect when the dynamics of the environment is known. As an illustration where the dynamics is not known, we examine empirical data streams relevant to climate and show the consensus problem manifest.

Science and awe.

Every scientific endeavor begins with mystery. Scientists engage in their research for a variety of reasons-as diverse as their research interests are. But at the core, we find the same sense of awe and wonder that inspires spiritual ways to look at the world.

The enigma of life: confronting marvels at the edges of science.

Einstein famously claimed that "the most incomprehensible thing about the universe is that it is comprehensible." This statement suggests that no amount of scientific explanation will suffice to make sense of the bizarre situation of the human mind within the universe. So what are the actual roles of awe and wonder within the framework of contemporary science? How, for instance, do awe and wonder inform scientists' understanding of the phenomena they are researching? What aspects of contemporary science are more likely to elicit wonder, and why? Is science rechanneling our innate thirst for knowledge and understanding toward more concrete and palpable realities, or is it aggravating the tension between truth and meaning by revealing the scope of our ignorance when it comes to probing the ultimate nature of reality? Physicist Marcelo Gleiser, experimental psychologist Tania Lombrozo, and physician Gavin Francis analyze the impact of awe and wonder on their own work and on the mindsets of their colleagues carrying out leading-edge scientific research.

The one and the many: the search for unity in nature.

The essence of physical reality-what the world consists of-has been a heated focus of contention for millennia. First with philosophers and then with physicists, the debate has been polarized since the beginning: while those loosely known as Platonists search for an underlying unity in nature, others caution that such unity is unachievable in practice and in principle. In this essay, we review both positions, arguing strongly for the latter in anticipation of experimental results from the Large Hadron Collider, the particle accelerator from the European Center for Nuclear Research. We further argue that, for the first time in history, the material essence of reality could be determined from an empirical standpoint as opposed to a purely dialectic one, settling the age-old debate.

The unification of physics: the quest for a theory of everything.

The holy grail of physics has been to merge each of its fundamental branches into a unified "theory of everything" that would explain the functioning and existence of the universe. The last step toward this goal is to reconcile general relativity with the principles of quantum mechanics, a quest that has thus far eluded physicists. Will physics ever be able to develop an all-encompassing theory, or should we simply acknowledge that science will always have inherent limitations as to what can be known? Should new theories be validated solely on the basis of calculations that can never be empirically tested? Can we ever truly grasp the implications of modern physics when the basic laws of nature do not always operate according to our standard paradigms? These and other questions are discussed in this paper.

Chiral polymerization in open systems from chiral-selective reaction rates.

We investigate the possibility that prebiotic homochirality can be achieved exclusively through chiral-selective reaction rate parameters without any other explicit mechanism for chiral bias. Specifically, we examine an open network of polymerization reactions, where the reaction rates can have chiral-selective values. The reactions are neither autocatalytic nor do they contain explicit enantiomeric cross-inhibition terms. We are thus investigating how rare a set of chiral-selective reaction rates needs to be in order to generate a reasonable amount of chiral bias. We quantify our results adopting a statistical approach: varying both the mean value and the rms dispersion of the relevant reaction rates, we show that moderate to high levels of chiral excess can be achieved with fairly small chiral bias, below 10%. Considering the various unknowns related to prebiotic chemical networks in early Earth and the dependence of reaction rates to environmental properties such as temperature and pressure variations, we argue that homochirality could have been achieved from moderate amounts of chiral selectivity in the reaction rates.

Toward homochiral protocells in noncatalytic peptide systems.

The activation-polymerization-epimerization-depolymerization (APED) model of Plasson et al. has recently been proposed as a mechanism for the evolution of homochirality on prebiotic Earth. The dynamics of the APED model in two-dimensional spatially-extended systems is investigated for various realistic reaction parameters. It is found that the APED system allows for the formation of isolated homochiral proto-domains surrounded by a racemate. A diffusive slowdown of the APED network induced, for example, through tidal motion or evaporating pools and lagoons leads to the stabilization of homochiral bounded structures as expected in the first self-assembled protocells.

Punctuated chirality.

Most biomolecules occur in mirror, or chiral, images of each other. However, life is homochiral: proteins contain almost exclusively L-amino acids, while only D-sugars appear in RNA and DNA. The mechanism behind this fundamental asymmetry of life remains an open problem. Coupling the spatiotemporal evolution of a general autocatalytic polymerization reaction network to external environmental effects, we show through a detailed statistical analysis that high intensity and long duration events may drive achiral initial conditions towards chirality. We argue that life's homochirality resulted from sequential chiral symmetry breaking triggered by environmental events, thus extending the theory of punctuated equilibrium to the prebiotic realm. Applying our arguments to other potentially life-bearing planetary platforms, we predict that a statistically representative sampling will be racemic on average.

Analytical characterization of oscillon energy and lifetime.

We develop an analytical procedure to compute all relevant physical properties of scalar field oscillons in models with quartic polynomial potentials: energy, radius, frequency, core amplitude, and lifetime. We compare our predictions to numerical simulations of models with symmetric and asymmetric double-well potentials in three spatial dimensions, obtaining excellent agreement. We also explain why oscillons have not been seen to decay in two spatial dimensions.

An extended model for the evolution of prebiotic homochirality: a bottom-up approach to the origin of life.

A generalized autocatalytic model for chiral polymerization is investigated in detail. Apart from enantiomeric cross-inhibition, the model allows for the autogenic (non-catalytic) formation of left and right-handed monomers from a substrate with reaction rates epsilon L and epsilon R, respectively. The spatiotemporal evolution of the net chiral asymmetry is studied for models with several values of the maximum polymer length, N. For N = 2, we study the validity of the adiabatic approximation often cited in the literature. We show that the approximation obtains the correct equilibrium values of the net chirality, but fails to reproduce the short time behavior. We show also that the autogenic term in the full N = 2 model behaves as a control parameter in a chiral symmetry-breaking phase transition leading to full homochirality from racemic initial conditions. We study the dynamics of the N--> infinity model with symmetric (epsilon L = epsilon R) autogenic formation, showing that it only achieves homochirality for epsilon > epsilon c, where epsilon c is an N-dependent critical value. For epsilon

Asymmetric spatiotemporal evolution of prebiotic homochirality.

The role of asymmetry on the evolution of prebiotic homochirality is investigated in the context of autocatalytic polymerization reaction networks. A model featuring enantiometric cross-inhibition and chiral bias is used to study the diffusion equations controlling the spatiotemporal development of left and right-handed domains. Bounds on the chiral bias are obtained based on present-day constraints on the emergence of life on early Earth. The viability of biasing mechanisms such as weak neutral currents and circularly polarized UV light is discussed. The results can be applied to any hypothetical planetary platform.

Prebiotic homochirality as a critical phenomenon.

The development of prebiotic homochirality on early-Earth or another planetary platform may be viewed as a critical phenomenon. It is shown, in the context of spatio-temporal polymerization reaction networks, that environmental effects--be they temperature surges or other external disruptions--may destroy any net chirality previously produced. In order to understand the emergence of prebiotic homochirality it is important to model the coupling of polymerization reaction networks to different planetary environments.

Resonant nucleation.

We investigate the role played by fast quenching on the decay of metastable (or false vacuum) states. Instead of the exponentially slow decay rate per unit volume, Gamma(HN) approximately exp([-E(b)/k(B)T] (E(b) is the free energy of the critical bubble), predicted by homogeneous nucleation theory, we show that under fast enough quenching the decay rate is a power law Gamma(RN) approximately [E(b)/k(B)T](-B), where B is weakly sensitive to the temperature. For a range of parameters, large-amplitude oscillations about the metastable state trigger the resonant emergence of coherent subcritical configurations. Decay mechanisms for different E(b) are proposed and illustrated in a (2+1)-dimensional scalar field model.

Resonant emergence of global and local spatiotemporal order in a nonlinear field model.

We investigate the nonequilibrium evolution of a scalar field in (2+1) dimensions. The field is set in a double-well potential in contact (open) or not (closed) with a heat bath. For closed systems, we observe the synchronized emergence of coherent spatiotemporal configurations, identified with oscillons. This initial global ordering degenerates into localized order until all oscillons disappear. We show that the synchronization is driven by resonant parametric oscillations of the field's zero mode and that local ordering is only possible outside equipartition. None of these orderings occur for open systems.

Nonequilibrium precursor model for the onset of percolation in a two-phase system.

Using a Boltzmann-like equation, we investigate the nonequilibrium dynamics of nonperturbative fluctuations within the context of Ginzburg-Landau models. As an illustration, we examine how a two-phase system initially prepared in a homogeneous, low-temperature phase becomes populated by precursors of the opposite phase as the temperature is increased. We compute the critical value of the order parameter for the onset of percolation, which signals the breakdown of the conventional dilute gas approximation.