Nelson, Philip C.
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Publication Spontaneous Expulsion of Giant Lipid Vesicles Induced by Laser Tweezers(1997) Moroz, J David; Nelson, Philip C; Bar-Ziv, Roy; Moses, ElishaIrradiation of a giant unilamellar lipid bilayer vesicle with a focused laser spot leads to a tense pressurized state which persists indefinitely after laser shutoff. If the vesicle contains another object it can then be gently and continuously expelled from the tense outer vesicle. Remarkably, the inner object can be almost as large as the parent vesicle; its volume is replaced during the exit process. We offer a qualitative theoretical model to explain these and related phenomena. The main hypothesis is that the laser trap pulls in lipid and ejects it in the form of submicron objects, whose osmotic activity then drives the expulsion.Publication Hidden Symmetry in Topological Gravity(1991) Distler, Jacques; Nelson, Philip CTalk presented by P.N. at “Mathematics and Physics of Strings,” Berkeley and “Topology and Geometry in Theoretical Physics,” Turku (Finland), 1991.Publication The Syncytial Drosophila Embryoas a Mechanically Excitable Medium(2013-10-01) Idema, Timon; Dubuis, Julien; Kang, Louis; Manning, M. Lisa; Nelson, Philip C; Lubensky, Tom; Liu, Andrea J.Mitosis in the early syncytial Drosophila embryo is highly correlated in space and time, as manifested in mitotic wavefronts that propagate across the embryo. In this paper we investigate the idea that the embryo can be considered a mechanically-excitable medium, and that mitotic wavefronts can be understood as nonlinear wavefronts that propagate through this medium. We study the wavefronts via both image analysis of confocal microscopy videos and theoretical models. We find that the mitotic waves travel across the embryo at a well-defined speed that decreases with replication cycle. We find two markers of the wavefront in each cycle, corresponding to the onsets of metaphase and anaphase. Each of these onsets is followed by displacements of the nuclei that obey the same wavefront pattern. To understand the mitotic wavefronts theoretically we analyze wavefront propagation in excitable media. We study two classes of models, one with biochemical signaling and one with mechanical signaling. We find that the dependence of wavefront speed on cycle number is most naturally explained by mechanical signaling, and that the entire process suggests a scenario in which biochemical and mechanical signaling are coupled.Publication Modular Forms and the Cosmological Constant(1986-10-01) Moore, G; Harris, J; Nelson, Philip C; Singer, IThe vacuum amplitude of the heterotic string in a flat background vanishes for the first twenty orders of string perturbation theory. The proof relies on the algebraic geometry of modular forms.Publication Time to Stop Telling Biophysics Students That Light Is Primarily a Wave(2018-01-01) Nelson, Philip CStandard pedagogy introduces optics as though it were a consequence of Maxwell’s equations, and only grudgingly admits, usually in a rushed aside, that light has a particulate character that can somehow be reconciled with the wave picture. Recent revolutionary advances in optical imaging, however, make this approach more and more unhelpful: How are we to describe two-photon imaging, FRET, localization microscopy, and a host of related techniques to students who think of light primarily as a wave? I was surprised to find that everything I wanted my biophysics students to know about light, including image formation, x-ray diffraction, and even Bessel beams, could be expressed as well (or better) from the quantum viewpoint pioneered by Richard Feynman. Even my undergraduate students grasp this viewpoint as well as (or better than) the traditional one, and by mid-semester they are already well positioned to integrate the latest advances into their understanding. Moreover, I have found that this approach clarifies my own understanding of new techniques.Publication Hard Spheres in Vesicles: Curvature-Induced Forces and Particle-Induced Curvature(1997-07-01) Dinsmore, A. D; Nelson, Philip C; Wong, D. T; Yodh, A. GPublication Biological Physics Student Edition 2020: Contents and Prefaces(2020-05-21) Nelson, Philip CBrief Contents; Detailed Contents; To the Student; To the InstructorPublication Rigid Chiral Membranes(1992-12-01) Nelson, Philip C; Powers, ThomasStatistical ensembles of flexible two-dimensional fluid membranes arise naturally in the description of many physical systems. Typically one encounters such systems in a regime of low tension but high stiffness against bending, which is just the opposite of the regime described by the Polyakov string. We study a class of couplings between membrane shape and in-plane order which break 3-space parity invariance. Remarkably there is only one such allowed coupling (up to boundary terms); this term will be present for any lipid bilayer composed of tilted chiral molecules. We calculate the renormalization-group behavior of this relevant coupling in a simplified model and show how thermal fluctuations effectivelyreduce it in the infrared.Publication How to Integrate the Fermionic String Measure(1989-03-01) La, HoSeong; Nelson, Philip CWe show how to treat boundary divergences in heterotic string theory covariantly and unambiguously. The method applies even to theories with nonvanishing tadpoles; in this case the Fischler-Susskind mechanismsuffices to ensure unambiguous amplitudes. No splitting or projection of supermoduli space is needed.Publication Direct Determination of DNA Twist-Stretch Coupling(1996-11-01) Kamien, Randall; Lubensky, Tom; Nelson, Philip C; O'Hern, Corey S.The symmetries of the DNA double helix require a new term in its linear response to stress: the coupling between twist and stretch. Recent experiments with torsionally constrained single molecules give the first direct measurement of this important material parameter. We extract its value from a recent experiment of Strick et al. [Science 271 (1996) 1835] and find rough agreement with an independent experimental estimate recently given by Marko. We also present a very simple microscopic theory predicting a value comparable to the one observed.