Theoretical Physics - From Outer Space to Plasma

This talk reviews how to deal with quantum systems that are coupled to the outside world, as in reality all systems are. We first introduce density operators and explain how quantum states give rise to them. We then turn to measures of entanglement that can be computed from a density operator, and show that entanglement grows with time. Finally, we show how the interaction with the environment gives rise to the phenomenon of decoherence.

This talk reviews the modern formulation of the basic ideas of quantum mechanics. We start by explaining what quantum amplitudes are, how they lead to the idea of a quantum state and how these states evolve in time. We then discuss what happens when a measurement is made before describing correlated ('entangled') systems. Applying these ideas to two-state systems ('qubits') we point out that the complexity of computing the evolution of an N qubit system grows like exp(N)

Dr Ralph Schoenrich will talk about the chemical evolution side Spiral density waves patterns re-distribute stars throughout the entire system, making it impossible to know a star's origin from just its kinematics. However, stars are more than just points in phase space: every star is labelled with the elemental abundances of the gas cloud from which it was formed. Over the last few years a number of observational campaigns have started to measure these labels for millions of stars in our own Galaxy's disc. Ralph Schoenrich will describe how chemodynamical models are being used to piece together the evolution of our Galactic environment from presolar times to the present.

Dr John Magorrian will talk about the dynamics of galaxy discs In galaxy discs it is energetically favourable for angular momentum to move outwards and mass to move inwards. This transportation is effected by spiral arms, but what causes them? Simple linear response calculations demonstrate that even the smallest perturbation is amplified manyfold, while the differential rotation of the disc means that the response is stretched out into a spiral-like pattern. John Magorrian will introduce the notion of the disc as a resonant cavity, within which spiral density perturbations rattle back and forth.

Prof James Binney FRS will talk about stellar systems: a new state of matter The long range of gravity means that many concepts from undergraduate statistical mechanics do not apply: energy is not extensive; there is no microcanonical or canonical ensemble. Stars and dark matter particles have long mean free paths, which means that to a very good approximation their motion is determined by the mean-field gravitational potential. James Binney will identify a hierarchy of timescales, explaining how the Boltzmann equation for the full 6N-dimensional many-particle phase space distribution function can be reduced to an evolutionary equation of a function of a mere 3 variables that is governed by the resonances among the particles' orbital frequencies.

In this talk, Siddharth Parameswaran discusses how a topological approach to 2D systems reveal that they can indeed become superfluid, and lead to surprising and beautiful universal results whose implications continue to resonate today. Superfluids spontaneously break a continuous symmetry linked to the conservation of particle number in a many-body system. Standard lore holds that such symmetries must remain unbroken at any temperature above absolute zero in a two-dimensional material, such as a thin sheet or film, apparently precluding superfluidity in such systems.

Stephen Blundell reviews a theory of superconductivity that was developed in Oxford in the 1930’s by Fritz London. The idea is that under certain conditions quantum coherent effects can become manifest on a large scale. In an effect such as superconductivity, this idea can be put to use in such applications as magnetic resonance imaging, in which a living human patient is inserted inside a quantum coherent wave function. He will explain how coherent effects can be measured in real superconductors.

John Chalker discusses how the laws of quantum mechanics lead us from the microscopic world to macroscopic phenomena. The notion that atoms of a given isotope are indistinguishable has profound consequences in the quantum world. For liquids made of identical bosons, indistinguishability forces the particles into a quantum condensate at low temperature, where they all dance in perfect synchrony. Treated gently, such a condensate has no viscosity: once it is set in motion --say around a circular pipe -- flow will persist indefinitely (so long as the fluid is kept sufficiently cold!).

John March-Russell gives a talk about gravitational wave signals of stringy physics, a ‘soundscape’ connected to the landscape of string vacua.

Subir Sarkar reviews the detection of the ‘chirrup’ signal from a pair of merging massive black holes by the Laser Interferometer Gravitational-Wave Observatory, as well as subsequent experimental developments.