Illustration of Loop Quantum Gravity

Dr Benjamin Bahr

Dr Benjamin Bahr

Senior Research Associate in Theoretical Physics

Telephone: 01223 338200


Benjamin A Bahr was born in Eckernförde, Germany in 1980, where he also grew up. In 1999 he started studying Physics and Computer Science at the University of Kaiserslautern until the Pre-Diploma ("Vordiplom") in 2001, whereafter he changed to the University of Göttingen in order to specialise in Mathematical Physics.

After studying in Göttingen for one year, he moved to Cambridge in 2002 in order to obtain the CASM, participating in Part III of the Mathematical Tripos as a student of Darwin College.  Courses he attended were picked from the DAMTP, such as Quantum Field Theory and General Relativity, but also from DPMMS, such as Lie Algebras, Differential Geometry and C*-Algebras.

After the year in Cambridge he returned to Göttingen and obtained his German Diploma with Professor Buchholz in the field of Algebraic Quantum Field Theory in 2004.

He was accepted in the IMPRS of the Max-Planck-Institute for Gravitational Physics in Potsdam, where he wrote his PhD from 2005 until 2008 under the supervision of Professor Thomas Thiemann in the field of Loop Quantum Gravity.

Since October 2008 on, Benjamin Bahr has been working in the Department of Applied and Theoretical Physics, in the General Relativity group run by Gary Gibbons.

Dr Bahr's research interests lie in the field of Quantum Gravity, i.e. the attempt to fuse Quantum Theory and General Relativity into a unified framework. These two theories are the two pillars of modern theoretical physics, and the source of our understanding of physical phenomena, from the behaviour of the smallest elementary particles to the movement of galaxies and the evolution of the cosmos itself. However, both theories are by themselves incomplete and describe only part of the physical world. In particular, both lead to contradictions and inconsistencies if used to describe situations in which both quantum effects and relativistic effects should play a role, such as the interior of black holes or the Big Bang, the proposed beginning of space and time. Physicists have very little understanding about the exact nature of the Big Bang, and hope that a Quantum Gravity theory will help to unravel e.g. the question of how and why the Big Bang took place, and what - if at all - existed before.

Although there is no complete Quantum Gravity Theory at the moment, there are several approaches, the most prominent ones being String Theory, Loop Quantum Gravity and the Spin Foam state sums. Most of them seem to indicate that on microscopic scales one has to abandon the notion of a continuous space and time, and maybe even the distinction between space-time and matter itself.

Currently, Benjamin Bahr is working on the question of how to construct a canonical formulation of Regge Calculus, a discretization of General Relativity. This is closely connected to Spin Foams on the one side, since those theories start with constrained BF theory which is discretized in exactly the same way. It is connected with Loop Quantum Gravity on the other side, since that approach starts from a canonical formulation of General Relativity. A canonical formulation of Regge Calculus would lead to a Quantum Theory quite similar to Loop Quantum Gravity, although the diffeomorphism- and Hamilton constraints would be discretized right from the start, which would help to overcome the difficulties one has with the continuous versions of these constraints in the LQG approach. Also, one could understand the connection between the LQG approach and the Spin Foam approaches much better.

The methods one has to employ as a mathematical physicist working in this direction are manifold - they range from functional analysis and function spaces, over differential geometry and algebraic topology up to category theory.

Publications list

  1. Bahr, B.: The Hot Bang state of massless fermions 2006 Lett.Math.Phys. 78
    39-54 [arXiv:math-ph/0504074]
  2. Bahr, B., Thiemann, T.: Approximating the physical inner product of Loop Quan-
    tum Cosmology
    2007 Class. Quant. Grav. 24 2109-2138 [arXiv:gr-qc/0607075]
  3. Bahr, B., Korsch, H.-J.: Quantum mechanics on a circle: Husimi phase space
    distributions and semiclassical coherent state propagators
    2007 J. Phys. A: Math.
    Theor. 40 3959-3985 [arXiv:quant-ph/0611116]
  4. Bahr, B., Thiemann, T.: Gauge-invariant coherent states for Loop Quantum
    Gravity I: Abelian gauge groups
    [arXiv:gr-qc/0709.4619], to be published in Class.
    Quant. Grav.
  5. Bahr, B., Thiemann, T.: Gauge-invariant coherent states for Loop Quantum
    Gravity II: Non-abelian gauge groups
    [arXiv:gr-qc/0709.4636], to be published in
    Class. Quant. Grav.
  6. Bahr, B., Thiemann, T.: Automorphisms in Loop Quantum Gravity [arXiv:gr-