Quantum Gravity

After implementing an effective minimal length, we will present a new class of spacetimes, describing both neutral and charged black holes. As a result, we will improve the conventional Schwarzschild and Reisner-Nordstroem spacetimes, smearing out their singularities at the origin. On the thermodynamic side, we will show how the new black holes admit a maximum temperature, followed by the ``SCRAM phase'', a thermodynamic stable shut down, characterized by a positive black hole heat capacity. As a consequence, also for the neutral solution, in place of the runaway behavior of the temperature, one finds that the evaporation ends up with a zero temperature extremal black hole, i.e. a final configuration entirely governed by the minimal length. For the charged case, both the Hawking and Schwinger pair creation will be discussed in this new scenario. We will also analyze the above solutions in the presence of extra dimensions and the connections with the production of mini black holes, which is foreseen in the extreme energy hadron collisions at the LHC in the next few months. Finally we will discuss further developments and possible connections with other approaches in this field.

The relation between loop quantum gravity (LQG) and ordinary quantum field theory (QFT) on a fixed background spacetime still bears many obstacles. When looking at LQG and ordinary QFT from a mathematical perspective it turns out that the two frameworks are rather different: Although LQG is a true continuum theory its Hilbert space is defined in terms of certain embedded graphs which are labeled by irreducible representations of SU(2). The natural arena for ordinary QFT, on the other hand, is a Fock space which strongly uses the metric properties of the underlying continuum spacetime. In this talk I will review this issue and show how one can use Born--Oppenheimer methods to further progress towards an understanding of (matter) quantum field theories from first principles.

How sure are you that spacetime is continuous? One approach to quantum gravity, causal set theory, models spacetime as a discrete structure: a causal set. This talk begins with a brief introduction to causal sets, then describes a new approach to modelling a quantum scalar field on a causal set. We obtain the Feynman propagator for the field by a novel procedure starting with the Pauli-Jordan commutation function. The candidate Feynman propagator is shown to agree with the continuum result. This model opens the door to physical predictions for scalar matter on a causal set.

We review some recent results on tachyon nonperturbative solutions of the nonlocal, lowest-level, effective action of string field theory. It is shown how nonlocality is encoded in a spacetime diffusion equation and how the latter emerges from the symmteries of the full, background-independent theory.

In 1981 Bill Unruh showed that the equation of motion for sound waves in a convergent fluid flows is given by a wave equation in an acoustic metric geometry. More importantly it is possible to set up sonic horizons in transsonic flows, and thus in principle to mimic experimentally the black-hole evaporation process. Almost 30 years later we have set up an experiment at the University of British Columbia to find out if indeed it is possible to detect (traces of) black hole radiation. We will discuss the necessary theoretical milestones, such as the effective field theory description for water to explore the robustness of Hawking radiation, and the avoidance of shock waves using surface waves rather then sound waves. In theory we should be able to detect the classical component of the thermal emission of black holes in our system and answer the ultimate question: How much do we trust our effective quantum field theory in curved spacetimes?