Fri 14 Feb 13:00: Third law of black hole mechanics for supersymmetric black holes
The third law of black hole mechanics asserts that it is impossible for a non-extremal black hole to become extremal in finite time (in classical General Relativity). A proof of this law was claimed in the 1980s. However, counterexamples to this law were found recently: gravitational collapse of a massless charged scalar field can produce an exactly extremal Reissner-Nordstrom black hole in finite time, passing through an intermediate phase in which the solution is exactly Schwarzschild at the horizon. These examples involve matter with a large charge to mass ratio. What about theories, such as supersymmetric theories, with an upper bound on the charge to mass ratio of matter? In this case I have proved that one cannot form a supersymmetric black hole (such as extremal Reissner-Nordstrom) in finite time. Thus a third law holds for supersymmetric black holes. The proof involves ideas related to quasi-local energy. In this talk I shall review all of these developments.
- Speaker: Harvey Reall, DAMTP, Cambridge
- Friday 14 February 2025, 13:00-14:00
- Venue: Potter room/Zoom: https://cam-ac-uk.zoom.us/j/86544434784?pwd=dE3XiqwmaWGPz9w5pXNkKk93XsmtJv.1.
- Series: DAMTP Friday GR Seminar; organiser: Xi Tong.
Wed 12 Feb 13:30: A model-theoretic approach to Roth's theorem
The ultraproduct construction is a useful tool in model theory to study the asymptotic behavior of a class of structures. In the particular case of a class of finite groups, the ultralimit of the normalized counting measure yields a translation-invariant Keisler measure on internal sets, which has played a crucial role in the recent years in several applications of model-theoretic techniques to additive combinatorics.
In this talk, we present a model-theoretic result that resonates with Croot-Sisask’s almost periodicity technique for a general group equipped with a Keisler measure under some mild assumptions. We then show how to use this result to obtain, via an ultrafilter construction, a non-quantitative proof of Roth’s theorem on arithmetic progressions of length three. The core idea of our model-theoretic version of almost periodicity is the stability-like behaviour of a convolution of sets. We will not assume prior knowledge of model theory for this talk.
In the first part of the talk, aimed at a general (non-logic) audience, we will recall the ultraproduct construction of finite groups, as well as Łoś’s theorem, dense internal subsets and the main features of stable relations, in order to briefly outline how to prove a non-quantitative version of Roth’s theorem.
The second part of the talk will focus on a more detailed explanation of some aspects of the proofs, in particular the notions of dense and random elements and their features. If time permits, we will explain how some of these techniques can be adapted to study the collection of starting points of arithmetic progressions in the primes and in the square-free integers.
- Speaker: Amador Martin-Pizarro (Albert-Ludwigs-Universität Freiburg)
- Wednesday 12 February 2025, 13:30-15:00
- Venue: MR4, CMS.
- Series: Discrete Analysis Seminar; organiser: Julia Wolf.
Thu 22 May 14:00: Walter Kohn: the theoretical physicist who created DFT and won the Nobel Prize for Chemistry
Abstract not available
- Speaker: Prof. Sir David Clary, FRS (University of Oxford)
- Thursday 22 May 2025, 14:00-15:30
- Venue: TCM Seminar Room.
- Series: Theory of Condensed Matter; organiser: Bo Peng.
Thu 13 Feb 14:00: Quantum phases of matter under non-unitary dynamics
Recent breakthroughs in the development of digital quantum devices promise to grant computational capacities far beyond the reach of classical architectures, and open unprecedented possibilities to study quantum many-body systems. This swift progress is fueling intense interest in the complex interplay of unitary quantum dynamics and non-unitary processes arising naturally in experiments, such as dissipation stemming from coupling to the environment or projective measurements performed on the system. This talk illustrates the rich dynamical phase diagrams that can emerge in these non-unitary settings. In the first part, we address the challenges of protecting quantum coherence against environmental noise, and explore the dynamical phase diagram of dissipative quantum many-body systems. In contrast to the general expectation that in an open system coherent information is quickly lost to the dissipative environment, we construct a regime of open quantum dynamics, functioning as a quantum error-correcting code which is dynamically protected against generic boundary noise. We comment on the implications of these results for designing robust quantum devices. We then turn to the effects of local measurements performed on the system. Specifically, we demonstrate that appropriately chosen projective measurements can imprint highly non-trivial order on quantum many-body systems, realizing the out-of-equilibrium counterpart of spontaneous symmetry breaking and symmetry protected topological order.
- Speaker: Izabella Lovas, ETH Zurich
- Thursday 13 February 2025, 14:00-15:00
- Venue: TCM Seminar Room.
- Series: Theory of Condensed Matter; organiser: Gaurav.
Fri 28 Feb 14:00: Lewis Lectures 2025 - Lecture II - "Tales of the Unexpected: New Perspectives on Electrochemistry at Carbon Electrodes and Membranes"
A wide variety of carbon materials are used in electrochemistry, with diverse applications that include (bio)electroanalysis and sensors, batteries and fuel cells, and membranes. The family of carbon materials is broad, spanning sp2 and sp3 materials, and includes 1D carbon nanotubes, 2D graphene (and non-carbon analogues) and 3D graphite and conducting diamond, along with amorphous carbon and various composites. The electronic properties of each of these materials are further influenced by local structure and defects, method of preparation, and (for 1-D and 2-D materials) the conducting support, the number of layers, and their arrangement. Ultimately, all of these factors can influence interfacial charge transfer and electrochemistry. In this lecture, I shall discuss our work in this area, which has established a new paradigm for structure-activity across a wide range of carbon materials and electrochemical processes. We combine high resolution electrochemical imaging data with information from other microscopy and spectroscopy techniques applied to the same area of an electrode surface, in a correlative-electrochemical microscopy approach, to produce highly resolved and unambiguous pictures of electrode activity at the nanoscale. The new models of electrochemistry offer surprises, overturn longstanding dogma, unify observations across length scales, and provide a foundation for future rational applications of carbon electrodes.
- Speaker: Prof. Patrick Unwin, University of Warwick
- Friday 28 February 2025, 14:00-15:00
- Venue: Dept of Chemistry, Wolfson Lecture Theatre .
- Series: Materials Chemistry Research Interest Group; organiser: Sharon Connor.
Thu 27 Feb 14:00: Lewis Lectures 2025 - Lecture I - New Ways of Seeing Electrochemistry
Electrochemistry is an important, complex and beautiful subject, and a fascinating scientific area to explore! Electrochemical processes and electrified interfaces are at the heart of living systems, batteries, fuel cells and electrolyzers. Electrochemical devices are found in diagnostic and sensor platforms, from measuring glucose in blood, to trace gases in air, and for new generation DNA /RNA sequencing. And there is still much more to learn about electrochemistry. From the earliest days, electrochemists sought to visualise processes at electrochemical interfaces, and this remains true today; there is an increasing variety of microscopy techniques that have been developed to investigate electrodes and electrified interfaces in-situ and operando. In this lecture, I will concentrate on the main scanned electrochemical probe microscopes that find increasing use in labs around the world. I shall provide a personal perspective on the development of these techniques, their capabilities and highlight key applications, from materials chemistry to biophysical processes at living cells. A major focus of the lecture will be scanning electrochemical cell microscopy (SECCM) and its role at the centre of a multimicroscopy strategy that can be used to dissect structure-activity at the nanoscale in unprecedented detail. Key discoveries from SECCM in fundamental electrochemistry, (electro)catalysis, corrosion, and charge storage will be highlighted, and I shall outline future directions for this technique and its role in a new era of high throughput nanoscale electrochemistry.
- Speaker: Prof. Patrick Unwin, University of Warwick
- Thursday 27 February 2025, 14:00-15:00
- Venue: Dept of Chemistry, Wolfson Lecture Theatre .
- Series: Materials Chemistry Research Interest Group; organiser: Sharon Connor.
Tue 11 Feb 14:30: 17T7 as a Galois group over Q through Hilbert modular forms
The inverse Galois problem asks whether every finite group can be realised as the Galois group of a finite Galois extension of Q. For a long time, the so-called group 17T7, acting transitively on a set of 17 elements, was the smallest group in the transitive group ordering for which no such extension of Q was known. In this talk, I will describe joint work with Edgar Costa, Noam Elkies, Timo Keller, Sam Schiavone, and John Voight, in which we use certain Hilbert modular forms to find such an extension.
- Speaker: Raymond van Bommel (Bristol)
- Tuesday 11 February 2025, 14:30-15:30
- Venue: MR13.
- Series: Number Theory Seminar; organiser: Rong Zhou.
Fri 14 Feb 14:00: Gaussian and non-Gaussian universality, with applications to data augmentation Note unusual location
The term Gaussian universality refers to a class of results that are, loosely speaking, generalized central limit theorems (where, somewhat confusingly, the limit law is not necessarily Gaussian). They provide useful tools to study certain problems in machine learning. I will give a short overview of this idea and then present two types of results: One are upper and lower bounds that map out where Gaussian universality is applicable and what rates of convergence one can expect. The other is the use of these techniques to obtain quantitative results on the effects of data augmentation in machine learning problems.
Note unusual location
- Speaker: Peter Orbanz (UCL)
- Friday 14 February 2025, 14:00-15:00
- Venue: Centre for Mathematical Sciences MR15, CMS.
- Series: Statistics; organiser: Qingyuan Zhao.
Wed 19 Mar 14:15: The Schubert variety of a hyperplane arrangement
I’ll tell you about some of my favorite algebraic varieties, which are beautiful in their own right, and also have some dramatic applications to algebraic combinatorics. These include the top-heavy conjecture (one of the results for which June Huh was awarded the Fields Medal), as well as non-negativity of Kazhdan—Lusztig polynomials of matroids.
- Speaker: Nicholas Proudfoot, University of Oregon
- Wednesday 19 March 2025, 14:15-15:15
- Venue: CMS MR13.
- Series: Algebraic Geometry Seminar; organiser: Dhruv Ranganathan.
Thu 03 Apr 16:00: 2024 Novo Nordisk Prize Lectures
Shankar and David will deliver the 2024 Novo Nordisk Lectures. Shankar’s will talk on Decoding DNA and David the applications of physical sciences to biomedicine, next generation DNA sequencing and beyond. This will be followed by a drinks reception hosted by the Novo Nordisk foundation.
- Speaker: Professors Sir Shankar Balasubramanian and Sir David Klenerman
- Thursday 03 April 2025, 16:00-17:30
- Venue: Dept of Chemistry, (BMS) Bristol Myers Squibb Lecture Theatre.
- Series: Chemistry Departmental-wide lectures; organiser: Balasubramanian-Admin.
Fri 04 Apr 10:30: Frontiers in Biophysics and Chemical Biology symposium
Funded by the Novo Nordisk Foundation and chaired by the 2024 Novo Nordisk Prize winners, Professors Sir Shankar Balasubramanian and Sir David Klenerman, this one day symposium brings together world leading scientists in the field of biophysics and chemical biology at the Yusuf Hamied Department of Chemistry.
The speakers are Professor Ed Boyden, MIT ; Professor Jason Chin, University of Cambridge; Professor Thomas Carrell from LMU , Munich; Professor Chuan He, University of Chicago; and Professor Xiaowei Zhang, Harvard University.
To register: https://www.eventbrite.co.uk/e/frontiers-in-biophysics-and-chemical-biology-tickets-1224872329109?aff=oddtdtcreator
- Speaker: Professor Ed Boyden, MIT; Professor Jason Chin, University of Cambridge; Professor Thomas Carrell from LMU, Munich; Professor Chuan He, University of Chicago; and Professor Xiaowei Zhang, Harvard University.
- Friday 04 April 2025, 10:30-17:00
- Venue: Dept of Chemistry, (BMS) Bristol Myers Squibb Lecture Theatre.
- Series: Chemistry Departmental-wide lectures; organiser: Balasubramanian-Admin.
Wed 12 Feb 14:00: Short-term, high-resolution sea ice forecasting with diffusion model ensembles
Sea ice plays a key role in Earth’s climate system and exhibits significant seasonal variability as it advances and retreats across the Arctic and Antarctic every year. The production of sea ice forecasts provides great scientific and practical value to stakeholders across the polar regions, informing shipping, conservation, logistics, and the daily lives of inhabitants of local communities. Machine learning offers a promising means by which to develop such forecasts, capturing the nonlinear dynamics and subtle spatiotemporal patterns at play as effectively—if not more effectively—than conventional physics-based models. In particular, the ability of deep generative models to produce probabilistic forecasts which acknowledge the inherent stochasticity of sea ice processes and represent uncertainty by design make them a sensible choice for the task of sea ice forecasting. Diffusion models, a class of deep generative models, present a strong option given their state-of-the-art performance on computer vision tasks and their strong track record when adapted to spatiotemporal modelling tasks in weather and climate domains. In this talk, I will present preliminary results from a IceNet-like [1] diffusion model trained to autoregressively forecast daily, 6.25 km resolution sea ice concentration in the Bellingshausen Sea along the Antarctic Peninsula. I will also touch on the downstream applications for these forecasts, from conservation to marine route planning, which are under development at the British Antarctic Survey (BAS). I welcome ideas and suggestions for improvement and look forward to discussing opportunities for collaboration within and beyond BAS .
[1] Andersson, Tom R., et al. “Seasonal Arctic sea ice forecasting with probabilistic deep learning.” Nature communications 12.1 (2021): 5124. https://www.nature.com/articles/s41467-021-25257-4
- Speaker: Andrew McDonald, University of Cambridge and British Antarctic Survey
- Wednesday 12 February 2025, 14:00-15:00
- Venue: BAS Seminar Room 2; https://ukri.zoom.us/j/96472472041.
- Series: British Antarctic Survey - Polar Oceans seminar series; organiser: Dr Birgit Rogalla.
Wed 26 Feb 16:15: Porous nitride semiconductors for novel light sources
Porous semiconducting nitrides are effectively a new class of semiconducting material, with properties distinct from the monolithic nitride layers from which devices from light emitting diodes (LEDs) to high electron mobility transistors are increasingly made. The introduction of porosity provides new opportunities to engineer a range of properties including refractive index, thermal and electrical conductivity, stiffness and piezoelectricity. Quantum structures may be created within porous architectures and novel composites may be created via the infiltration of other materials into porous nitride frameworks. A key example of the application of porous nitrides in photonics is the fabrication of high reflectivity distributed Bragg reflectors (DBRs) from alternating layers of porous and non-porous GaN. These reflectors are fabricated from epitaxial structures consisting of alternating doped and undoped layers, in which only the conductive, doped layers are electrochemically etched. Conventionally, trenches are formed using a dry-etching process, penetrating through the multilayer, and the electrochemical etch then proceeds laterally from the trench sidewalls. The need for these trenches then limits the device designs and manufacturing processes within which the resulting reflectors can be used. We have developed a novel alternative etching process, which removes the requirement for the dry-etched trenches, with etching proceeding vertically from the top surface through channels formed at naturally-occurring defects in the crystal structure of GaN (see Figure). This etch process leaves an undoped top surface layer almost unaltered and suitable for further epitaxy. This new defect-based etching process provides great flexibility for the creation of a variety of sub-surface porous architectures on top of which a range of devices may be grown. Whilst DBR structures enable improved light extraction from LEDs and the formation of resonant cavities for lasers and single photon sources, recent development also suggests that thick, sub-surface porous layers may enable strain relaxation to help improve the efficiency of red microLEDs for augmented reality displays. Meanwhile, the option of filling pores in nitride layers with other materials provides new opportunities for the integration of nitrides with emerging photonic materials, such as the hybrid-perovskite semiconductors, with perovskites encapsulated in porous nitride layers demonstrating greatly improved robustness against environmental degradation.
- Speaker: Rachel Oliver (DMSM)
- Wednesday 26 February 2025, 16:15-17:15
- Venue: Small Lecture Theatre, Cavendish Laboratory, J.J. Thomson Avenue.
- Series: Cavendish Quantum Colloquium; organiser: Lucas Sá.
Thu 03 Apr 16:00: 2024 Novo Nordisk Prize Lectures
Abstract not available
- Speaker: Professors Sir Shankar Balasubramanian and Sir David Klenerman
- Thursday 03 April 2025, 16:00-17:30
- Venue: Dept of Chemistry, (BMS) Bristol Myers Squibb Lecture Theatre.
- Series: Chemistry Departmental-wide lectures; organiser: Balasubramanian-Admin.
Tue 11 Feb 14:00: Title to be confirmed
Abstract not available
- Speaker: Wendelin Werner (Cambridge)
- Tuesday 11 February 2025, 14:00-15:00
- Venue: MR12.
- Series: Probability; organiser: ww295.
Wed 30 Apr 14:00: Title to be confirmed
Abstract not available
- Speaker: Dr Bjarne Bergh, DAMTP, University of Cambridge
- Wednesday 30 April 2025, 14:00-15:00
- Venue: MR5, CMS Pavilion A.
- Series: Information Theory Seminar; organiser: Prof. Ramji Venkataramanan.
Wed 28 May 11:15: Title to be confirmed
Abstract not available
- Speaker: Paul Goddard - University of Warwick
- Wednesday 28 May 2025, 11:15-12:00
- Venue: Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.
- Series: Quantum Matter Seminar; organiser: Mads Fonager Hansen.
Thu 06 Feb 17:00: Formalisation of Combinatorial Optimisation in Isabelle/HOL: Network Flows
Combinatorial optimisation (CO) is a sub-area of discrete mathematics. Basic examples for CO problems are finding a shortest path or a minimum spanning tree in a graph. So-called network flows or variations of matching would be more advanced problems. There are also abstract concepts like matroids that offer an algebraic point of view and a uniform foundation for some of the more concrete problems.
Since the considered structures are finite, it is a natural aim to compute a solution efficiently. That implies an overlap with the theory of algorithms, especially running time analysis.
This talk is mainly about the Isabelle/HOL formalisation of a specific CO problem, namely, minimum cost flows, which are a subtype of network flows. Among others, this includes Orlin’s Algorithm, which is a most efficient method to compute a minimum cost flow in general networks. Also, the running time argument for this advanced algorithm and some reductions among flow problems were formalised.
- The Isabelle proof scripts can be found in this GitHub repo: https://github.com/mabdula/Isabelle-Graph-Library
- The formalisation is described in this paper: A Formal Analysis of Capacity Scaling Algorithms for Minimum Cost Flows by Mohammad Abdulaziz and Thomas Ammer, ITP 2024
=== Hybrid talk ===
Join Zoom Meeting https://cam-ac-uk.zoom.us/j/87143365195?pwd=SELTNkOcfVrIE1IppYCsbooOVqenzI.1
Meeting ID: 871 4336 5195
Passcode: 541180
- Speaker: Thomas Ammer (King's College London)
- Thursday 06 February 2025, 17:00-18:00
- Venue: MR14 Centre for Mathematical Sciences.
- Series: Formalisation of mathematics with interactive theorem provers ; organiser: Jonas Bayer.
Fri 14 Mar 10:30: TBC (Mini-course talk 2) Please note the unusual time and place.
Abstract not available
Please note the unusual time and place.
- Speaker: Jonny Evans (Lancaster)
- Friday 14 March 2025, 10:30-11:30
- Venue: CMS, MR15.
- Series: Differential Geometry and Topology Seminar; organiser: Ailsa Keating.
Thu 13 Mar 11:15: TBC (Mini-course talk 1) Please note the unusual time and place.
Abstract not available
Please note the unusual time and place.
- Speaker: Jonny Evans (Lancaster)
- Thursday 13 March 2025, 11:15-12:15
- Venue: CMS, MR14.
- Series: Differential Geometry and Topology Seminar; organiser: Ailsa Keating.