Ian Shipsey give an update on the department and introduces the next three talk on 'AI in Physics'.

In this talk Subir Sarkar will explain how deflagration supernovae have been used to infer that the Hubble expansion rate is accelerating, and critically assess whether the acceleration is real and due to `dark energy’.

In this talk, Philipp Podsiadlowski will explain how this energy (sometimes) creates a visible fireball, before going on to explain the role of supernovae in the production of the heaviest elements in the periodic table.

In this talk, James Binney will outline the physics that leads to prodigeous release of energy in core-collapse and deflagration supernovae.

To study the Higgs boson at the LHC we also need to understand how highly energetic quarks and gluons interact, among themselves and with the Higgs. These interactions are described by quantum field theory, a beautiful mathematical framework that combines quantum mechanics with Einstein’s theory of special relativity. In recent years, our understanding of quantum field theory has progressed significantly, allowing us to develop a new generation of accurate theoretical predictions for key LHC...

We learn about the Higgs Boson and its interactions at the LHC by examining the debris produced by colliding protons head-on at unprecedented high energies. However, we know from our theory of strong interactions - quantum chromodynamics (QCD) - that protons themselves are highly complex bound states of more fundamental 'quarks', held together by the force carriers of QCD, the 'gluons'. The question is then: how do we go from the collision of these complicated protons to a theoretical...

Over the past two years, CERN’s Large Hadron Collider (LHC) has started to directly probe a qualitatively new class of interactions, associated with the Higgs boson. These interactions, called Yukawa interactions, are unlike any other interaction that we have probed at the quantum level before. In particular, unlike the electromagnetic, weak and strong forces, they have an interaction strength that does not come in multiples of some underlying unit charge. Yukawa interactions are believed...

The coding theorem from algorithmic information theory (AIT) - which should be much more widely taught in Physics! - suggests that many processes in nature may be highly biased towards simple outputs. Here simple means highly compressible, or more formally, outputs with relatively lower Kolmogorov complexity. I will explore applications to biological evolution, where the coding theorem implies an exponential bias towards outcomes with higher symmetry, and to deep learning neural networks,...

Active systems, from cells and bacteria to flocks of birds, harvest chemical energy which they use to move and to control the complex processes needed for life. A goal of biophysicists is to construct new physical theories to understand these living systems, which operate far from equilibrium. Topological defects are key to the behaviour of certain dense active systems and, surprisingly, there is increasing evidence that they may play a role in the biological functioning of bacterial and...

Ian Shipsey delivers the welcome speech for the Saturday Mornings of Theoretical Physics.

Siddharth Parameswaran, Associate Professor, Physics Department. The usual picture of entropy in statistical mechanics is that it quantifies our degree of ignorance about a system. Recent advances in cooling and trapping atoms allow the preparation of quantum systems with many interacting particles isolated from any external environment. Textbook discussions of entropy — that invoke the presence of a “large” environment that brings the system to thermal equilibrium at a fixed temperature ---...

John Chalker, Head of Theoretical Physics, gives a talk on entropy. Thermodynamics and statistical mechanics give us two alternative ways of thinking about entropy: in terms of heat flow, or in terms of the number of micro-states available to a system. John Chalker will describe a physical setting to illustrate each of these. By applying thermodynamics in a realm far beyond its origins, we can use the notion of an ideal heat engine to find the temperature of a black hole. And by applying...

Alexander Schekochihin, Professor of Theoretical Physics, gives a talk on entropy. When dealing with physical systems that contain many degrees of freedom, a researcher's most consequential realisation is of the enormous amount of detailed information about them that she does not have, and has no hope of obtaining. It turns out that this vast ignorance is not a curse but a blessing: by admitting ignorance and constructing a systematic way of making fair predictions about the system that rely...

This talk reviews the developments in quantum information processing.

This talk reviews testing and developing ideas in quantum computing using laser-manipulated trapped ions.

This talk explains how qubits are used to represent numbers in a way that permits 'quantum-mechanical parallel' computing. We show how this can used to achieve fast factorisation of large numbers, and hence the breaking of current codes. We end by explaining how entangled pairs of particles can be used to provide an alternative and entirely secure cryptographic system.

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...

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...

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...

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...

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...

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...

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