Past Quantum Field Theory Seminar

27 February 2018
Cecile Huneau

In this talk, I will present the construction of a family of solutions to
vacuum Einstein equations which consist of an arbitrary number of high
frequency waves travelling in different directions. In the high frequency
limit, our family of solutions converges to a solution of Einstein equations
coupled to null dusts. This construction is an illustration of the so called
backreaction, studied by physicists (Isaacson, Burnet, Green, Wald...) : the
small scale inhomogeneities have an effect on the large scale dynamics in
the form of an energy impulsion tensor in the right-hand side of Einstein
equations. This is a joint work with Jonathan Luk (Stanford).

  • Quantum Field Theory Seminar
20 February 2018
Simon Wood

Some of the most studied examples of conformal field theories
the Wess-Zumino-Witten models. These are conformal field theories exhibiting
affine Lie algebra symmetry at non-negative integers levels. In this talk I
discuss conformal field theories exhibiting affine Lie algebra symmetry at
certain rational (hence fractional) levels whose structure is arguably even
more intricate than the structure of the non-negative integer levels,
one is prepared to look beyond highest weight modules.

  • Quantum Field Theory Seminar
13 February 2018
Tim Palmer

Hardy's axiomatic approach to quantum theory revealed that just one axiom
distinguishes quantum theory from classical probability theory: there should
be continuous reversible transformations between any pair of pure states. It
is the single word `continuous' that gives rise to quantum theory. This
raises the question: Does there exist a finite theory of quantum physics
(FTQP) which can replicate the tested predictions of quantum theory to
experimental accuracy? Here we show that an FTQP based on complex Hilbert
vectors with rational squared amplitudes and rational phase angles is
possible providing the metric of state space is based on p-adic rather than
Euclidean distance. A key number-theoretic result that accounts for the
Uncertainty Principle in this FTQP is the general incommensurateness between
rational $\phi$ and rational $\cos \phi$. As such, what is often referred to
as quantum `weirdness' is simply a manifestation of such number-theoretic
incommensurateness. By contrast, we mostly perceive the world as classical
because such incommensurateness plays no role in day-to-day physics, and
hence we can treat $\phi$ (and hence $\cos \phi$) as if it were a continuum
variable. As such, in this FTQP there are two incommensurate Schr\"{o}dinger
equations based on the rational differential calculus: one for rational
$\phi$ and one for rational $\cos \phi$. Each of these individually has a

simple probabilistic interpretation - it is their merger into one equation
on the complex continuum that has led to such problems over the years. Based
on this splitting of the Schr\"{o}dinger equation, the measurement problem
is trivially solved in terms of a nonlinear clustering of states on $I_U$.
Overall these results suggest we should consider the universe as a causal
deterministic system evolving on a finite fractal-like invariant set $I_U$
in state space, and that the laws of physics in space-time derive from the
geometry of $I_U$. It is claimed that such a  deterministic causal FTQP will
be much easier to synthesise with general relativity theory than is quantum

  • Quantum Field Theory Seminar
21 November 2017
Alexander Strohmaier

I will review some classical results on geometric scattering
theory for linear hyperbolic evolution equations
on globally hyperbolic spacetimes and its relation to particle and charge
creation in QFT. I will then show that some index formulae for the
scattering matrix can be interpreted as a special case of the  Lorentzian
analog of the Atyiah-Patodi-Singer index theorem. I will also discuss a
local version of this theorem and its relation to anomalies in QFT.
(Joint work with C. Baer)

  • Quantum Field Theory Seminar
7 November 2017
Gabriele Veneziano

I will start with a quick reminder of what we have learned so far about
transplanckian-energy collisions of particles, strings and branes.
I will then address the (so-far unsolved) problem of gravitational
bremsstrahlung from massless particle collisions at leading order in the
gravitational deflection angle.
Two completely different calculations, one classical and one quantum, lead
to the same final, though somewhat puzzling, result.


  • Quantum Field Theory Seminar
24 October 2017
Kasia Rejzner

In this talk I will present recent results obtained within the
framework of perturbative algebraic quantum field theory. This novel
approach to mathematical foundations of quantum field theory allows to
combine the axiomatic framework of algebraic QFT by Haag and Kastler with
perturbative methods. Recently also non-perturbative results have been
obtained within this approach. I will report on these results and present
new perspectives that they open for better understanding of foundations of

  • Quantum Field Theory Seminar
17 October 2017
Jorma Louko

How long does a uniformly accelerated observer need to interact with a
quantum field in order to record thermality in the Unruh temperature?
In the limit of large excitation energy, the answer turns out to be
sensitive to whether (i) the switch-on and switch-off periods are
stretched proportionally to the total interaction time T, or whether
(ii) T grows by stretching a plateau in which the interaction remains
at constant strength but keeping the switch-on and switch-off
intervals of fixed duration. For a pointlike Unruh-DeWitt detector,
coupled linearly to a massless scalar field in four spacetime
dimensions and treated within first order perturbation theory, we show
that letting T grow polynomially in the detector's energy gap E
suffices in case (i) but not in case (ii), under mild technical
conditions. These results limit the utility of the large E regime as a

probe of thermality in time-dependent versions of the Hawking and
Unruh effects, such as an observer falling into a radiating black
hole. They may also have implications on the design of prospective
experimental tests of the Unruh effect.

Based on arXiv:1605.01316 (published in CQG) with Christopher J
Fewster and Benito A Juarez-Aubry.

  • Quantum Field Theory Seminar
13 June 2017
Roger Penrose

In the cosmological scheme of conformal cyclic cosmology (CCC), the equations governing the crossover form each aeon to the next demand the creation of a dominant new scalar material that is postulated to be dark matter. In order that this material does not build up from aeon to aeon, it is taken to decay away completely over the history of the aeon. The dark matter particles (erebons) would be expected to behave as essentially classical particles of around a Planck mass, interacting only gravitationally, and their decay would be mainly responsible for the (~scale invariant)

temperature fluctuations in the CMB of the succeeding aeon. In our own aeon, erebon decay ought to be detectable as impulsive events observable by gravitational wave detectors.

  • Quantum Field Theory Seminar