We study the dynamics of an incommensurate chain sliding on a periodic lattice, modeled by the Frenkel Kontorova hamiltonian with initial kinetic energy, without damping and driving terms. We show that the onset of friction is due to a novel kind of dissipative parametric resonances, involving several resonant phonons which are driven by the (dissipationless) coupling of the center of mass motion to the phonons with wavevector related to the modulating potential. We establish quantitative estimates for their existence in finite systems and point out the analogy with the induction phenomenon in Fermi-Ulam-Pasta lattices.PACS numbers: 45.05.+x, 46.55.+d, 46.40.Ff The possibility of measuring friction at the atomic level provided by the Lateral Force Microscopes [1] and Quartz Crystal Microbalance [2] has stimulated intense research on this topic [3]. Phonon excitations are the dominant cause of friction in many cases [4]. Most studies are carried out for one-dimensional non-linear lattices [5][6][7][8][9][10][11][12] and in particular for the Frenkel-Kontorova (FK) model [13], where the surface layer is modeled by a harmonic chain and the substrate is replaced by a rigid periodic modulation potential. The majority [6][7][8][9][10][11][12] examines the steady state of the dynamical FK model in presence of dissipation representing the coupling of phonons to other, undescribed degrees of freedom.We study the dynamics of an undriven incommensurate FK chain. Our aim is to ascertain whether the experimentally observed superlubricity [14] can be due to the blocking of the phonon channels caused by an incommensurate contact of the two sliding surfaces. Therefore we do not include any explicit damping of the phonon modes, since we wish to find out if they can be excited at all by the motion of the center of mass (CM). In an earlier study, Shinjo and Hirano [5] found a superlubric regime for this model, where the chain would slide indefinitely without dynamic friction but with a recurrent exchange of kinetic energy between CM and a single internal mode. We will show that their finding is oversimplified by either too short simulation times or too small system sizes. The inherent non-linear coupling of the CM to the phonons leads to an irreversible decay of the CM velocity, albeit with very long time scales in some windows. The dissipative mechanism is driven by the coupling of the CM to the modes with modulation wavevector q or its harmonics, ω nq , and consists in a novel kind of parametric resonances with much wider windows of instabilities than those deriving from the standard Mathieu equation [15]. The importance of harmonic resonances at ω nq has been pointed out before [6,8,10], with the suggestion [10] that they could be absent in finite systems due to the discreteness of the phonon spectrum. However, it has not been realized that they act as a driving term for the onset of dissipation via subsequent complex parametric excitations which we shall describe, establishing quantitative estimates for their existence in ...

The Schön misconduct case has been widely publicized in the media and has sparked intense discussions within and outside the scientific community about general issues of science ethics. This paper analyses the Report of the official Committee charged with the investigation in order to show that what at first seems to be a quite uncontroversial case, turns out to be an accumulation of many interesting and non-trivial questions (of both ethical and philosophical interest). In particular, the paper intends to show that daily scientific practices are structurally permeated by chronic problems; this has serious consequences for how practicing scientists assess their work in general, and scientific misconduct in particular. A philosophical approach is proposed that sees scientific method and scientific ethics as inextricably interwoven. Furthermore, the paper intends to show that the definition of co-authorship that the members of the Committee use, although perhaps clear in theory, proves highly problematic in practice and raises more questions that it answers. A final plea is made for a more self-reflecting attitude of scientists as far as the moral and methodological profile of science is concerned as a key element for improving not only their scientific achievements, but also their assessment of problematic cases.

We show that invariance properties of the Lagrangian of an incommensurate system, as described by the Frenkel-Kontorova model, imply the existence of a generalized angular momentum that is an integral of motion if the system remains floating. The behavior of this quantity can therefore monitor the character of the system as floating (when it is conserved) or locked (when it is not). We find that, during the dynamics, the nonlinear couplings of our model cause parametric phonon excitations that lead to the appearance of Umklapp terms and to a sudden deviation of the generalized momentum from a constant value, signaling a dynamical transition from a floating to a pinned state. We point out that this transition is related but does not coincide with the onset of sliding friction, which can take place when the system is still floating.

Evolution has met with considerable religious opposition for 150 years and is still controversial among various religious groups. This article tries to understand the evolution controversy by reframing it as a phenomenon of public understanding of science. Three paradigms were used as hypotheses for the rejection of evolution by Dutch Protestant Christians: knowledge deficit, attitude deficit and trust deficit. Ten Dutch Protestants rejecting evolution were interviewed about their views concerning evolution and science. It was found that the main reason for rejecting evolution was an a priori decision to trust the Bible more than science. Any views on science and evolution were based on this decision, so all three hypotheses, which suggest an a posteriori decision, were found to be not sufficient to explain the rejection of evolution, even though both a knowledge deficit and a trust deficit were found for some participants. However, all respondents felt that their a priori decision was supported by scientific facts. All respondents stated that evolution does not meet the criteria for good science and is therefore as unscientific as the belief in creation. Excluding evolution from science allows the respondents to retain their positive attitudes towards science.

We study the propagation of quantum fields on κ-Minkowsi spacetime. Starting from the noncommutative partition function for a free field written in momentum space we derive the Feynman propagator and analyze the non-trivial singularity structure determined by the group manifold geometry of momentum space. The additional contributions due to such singularity structure result in a deformed field propagation which can be alternatively described in terms of an ordinary field propagation determined by a source with a blurred spacetime profile. We show that the κ-deformed Feynman propagator can be written in terms of vacuum expectation values of a commutative nonlocal quantum field. For sub-Planckian modes the κ-deformed propagator corresponds to the vacuum expectation value of the time-ordered product of non-local field operators while for trans-Plankian modes this is replaced by the Hadamard two-point function, the vacuum expectation value of the anti-commutator of non-local field operators. * Electronic address: michele.arzano@roma1.infn.it † Electronic address: consoli.