Wed Aug 23, 2017

Physics Colloquium: Prof. Neil Johnson, University of Miami

4:00 PM - 5:00 PM
118 Nieuwland Science Hall

New Terrorism Reveals New Physics
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In this talk, I will try to convince you that buried in arguably the most urgent challenge facing society, i.e. global terrorism, lies a wealth of interesting physics related to many-body out-of-equilibrium systems, complex dynamical networks, critical phenomena, kinetic theory, and even Green's Functions and Feynman diagrams [1-10]. Moreover there now exist detailed spatiotemporal datasets on global violent events which back up these claims. During the talk, I will try to emphasize the point that this is not just about providing something new for Physics, but rather that Physics is uniquely positioned to offer key insights into this important ‘many-person problem’ in a way that the traditional disciplines associated with analyzing terrorism can never do.

Posted In: Colloquia

Mon Aug 28, 2017

Nuclear Physics Seminar: Dr. Juergen Stein, Target Systemeletronik GmbH, Germany

4:00 PM - 5:00 PM
124 Nieuwland Science Hall

Statistical Signal Processing of Scintillation Detectors in Gamma Ray Spectrometry

Dr. Juergen Stein
Target Systemeletronik GmbH, Germany

Hosted by Prof. Wiescher

Posted In: Nuclear Seminar

Tue Aug 29, 2017

Astrophysics Seminar: Dr. Maxim Barkov, Purdue University

12:30 PM - 1:30 PM
184 Nieuwland Science Hall

Pulsar/Stellar wind collision in 3D and The origin of the X-ray-emitting object moving away from PSR B1259-63

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For the first time, we simulate in 3 dimensions the interaction of isotropic stellar and relativistic pulsar winds along one full orbit, on scales well beyond the binary size. We used the code PLUTO to carry out relativistic hydrodynamical simulations in 2 and 3 dimensions of the interaction between a slow dense wind and a mildly relativistic wind with Lorentz factor 2, along one full orbit in a region up to ~100 times the binary size. The simulations in 3 dimensions confirm previous results in 2 dimensions, showing: a strong shock induced by Coriolis forces that terminates the pulsar wind also in the opposite direction to the star; strong bending of the shocked-wind structure against the pulsar motion; and the generation of turbulence. The shocked flows are also subject to a faster development of instabilities in 3 dimensions, which enhances shocks, two-wind mixing, and large-scale disruption of the shocked structure. In addition to the Kelvin-Helmholtz instability, discussed in the past, we find that the Richtmyer-Meshkov and the Rayleigh-Taylor instabilities are very likely acting together in the shocked flow evolution.

     A mysterious X-ray-emitting object has been detected moving away from the high-mass gamma-ray binary PSR B1259-63, which contains a non-accreting pulsar and a Be star whose winds collide forming a complex interaction structure. Given the strong eccentricity of this binary, the interaction structure should be strongly anisotropic, which together with the complex evolution of the shocked winds, could explain the origin of the observed moving X-ray feature. We propose here that a fast outflow made of a pulsar-stellar wind mixture is always present moving away from the binary in the apastron direction, with the injection of stellar wind occurring at orbital phases close to periastron passage. This outflow periodically loaded with stellar wind would move with a high speed, and likely host non-thermal activity due to shocks, on scales similar to those of the observed moving X-ray object. Such an outflow is thus a very good candidate to explain this X-ray feature. This, if confirmed, would imply pulsar-to-stellar wind thrust ratios of  0.1, and the presence of a jet-like structure on the larger scales, up to its termination in the interstellar medium.

Posted In: Astrophysics Seminar

Wed Aug 30, 2017

Physics Colloquium: Prof. Tom McLeish, Durham University

4:00 PM - 5:00 PM
118 Nieuwland Science Hall

Are there Ergodic limits to Evolution?

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There is a fruitful and developing programme exploring analogies between evolution and the statistical mechanics of complex matter [1,2].  Both treat exponentially large spaces of configurations (possible genomes/possible microstates) which is explored by stochastic dynamics (random mutation/thermal motion).  Both admit maximisation/minimisation criteria (fitness/free-energy).  There are even evolutionary analogies of temperature (inverse population size) [3]  and free energy (“free fitness”) [4]. 

     Beneath the notion of thermodynamic equilibrium is the assumption of ‘ergodicity’ -  the exploration of configuration space in a representative way, with an induced characteristic timescale to attain equilibrium.  ‘Ergodic times’ are typically much less than the exponentially long times required for a complete search.  For example, for a polymer of N subunits, the timescale for equilibration increases as N2 rather than the zN of a complete search.  The strong dependence of these timescales on system size means that, even if a large system is not in equilibrium, some of its subsystems may be.  We consider an illustrative example of the insect compound eye, which also carries the advantage that its optimum (in acuteness of vision) relies on physics and can be calculated [5].

     The ergodic exploration of systems and subsytems within evolution generates a discussion of timescales for representative exploration of genotypic spaces.  This in turn suggests a strong connection to the phenomenon of convergent evolution, and the conjecture that this might be expected in subsystems whose fitness is optimised by well-defined phenotypical constraints, and whose ergodic time is less than the relevant evolutionary time. 

 

[1] R.A. Fisher, Trans. Roy. Soc. Edinb., 52, 399-433.

[2] N.H. Barton and J.B. Coe, J.Theor. Biol. 259, 317–324 (2009).

[3] G. Sella and Hirsh AE, Proc Natl Acad Sci USA, 102, 9541–9546. (2005).

[4]  B.S. Khatri, T.C.B. McLeish and R.P. Sear, PNAS, 106, 9564-9569 (2009).

[5] T.C.B. McLeish, J. Roy. Soc. Interface Focus, 20150041 (2015).

Posted In: Colloquia

Tue Sep 5, 2017

Particle Physics Seminar: Dr. Kevin Pedro, Fermilab

4:00 PM - 5:00 PM
415 Nieuwland Science Hall

Search for supersymmetry in multijet events with missing transverse momentum in proton-proton collisions at 13TeV

Dr. Kevin Pedro
Fermilab…

Posted In: Particle Physics Seminar

Wed Sep 6, 2017

Physics Colloquium: Dr. Christopher Wesselborg, Managing Editor, Physical Review C

4:00 PM - 5:00 PM
118 Nieuwland Science Hall

Title TBA

Dr. Christopher Wesselborg
Managing Editor of Physical Review C…

Posted In: Colloquia

Mon Sep 11, 2017

Nuclear Physics Seminar: Dr. Aaron Gallant, Lawrence Livermore National Laboratory

4:00 PM - 5:00 PM
124 Nieuwland Science Hall

Title TBA

Dr. Aaron Gallant
Lawrence Livermore National Laboratory

Hosted by Prof. Brodeur

Posted In: Nuclear Seminar

Tue Sep 12, 2017

Astrophysics Seminar: Cameron Liang

12:30 PM - 1:30 PM
184 Nieuwland Science Hall

Title TBA

Mr. Cameron Liang
Graduate Student
University of Chicago

Hosted by Dr. Lehner

Posted In: Astrophysics Seminar

Wed Sep 13, 2017

Colloquium: Graduate Physics Conference Talks

4:00 PM - 5:00 PM
118 Nieuwland Science Hall

Annual Graduate Physics Students Research Conference

Details forthcoming

Posted In: Colloquia and Special Lectures and Events

Tue Sep 19, 2017

Astrophysics Seminar: Multiple Presenters

12:30 PM - 1:30 PM
184 Nieuwland Science Hall

One-Minute Talks

Multiple Presenters

Hosted by Dr. Placco

Posted In: Astrophysics Seminar

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