Seminars
Winter term 2017/18
Is Standard Semiclassical Einstein Gravity Viable?
Abstract: It will be examined whether standard semiclassical Einstein gravity (SSEG), represented by the standard semiclassical Einstein equation, constitutes a selfconsistent and empirically adequate theory of mattergravity coupling. This will be done from two points of view: SSEG as a fundamental proposal, and SSEG as a meanfield approximation to a (putative) quantum theory of gravity. It will be shown that, from either point of view, there are compelling reasons to question the selfconsistency and empirical adequacy of SSEG. Finally, the implications will be discussed for one of the most prominent applications of SSEG  Hawking's 1975 argument (and refinements thereof) that black holes evaporate due to semiclassical gravitational backreaction of emitted Hawking radiation.
The chronon in Mtheory
Abstract: In this seminar we discuss some aspects of chronon physics in an Mtheory model. We start considering the possibility that quantum fluctuations are stretched on very large distances allowing a quantum mechanical treatment of physics on certain macroscopic scales. A crucial element of our analysis is the relativity of time. Indeed, the presence of a 5D black hole (with its gravitational field) leads us to a scenario where small quantum fluctuations produced near the black hole become very large for an asymptotic observer in harmony with the relativity of time. In the deep IR region, gravity shows new phenomena (related to an orbifold of time) which cannot be described through a field theory on the brane and, in this sense, these phenomena resemble the actionatadistance of Newtonian gravity when interpreted from the standpoint of Sbrane physics. If an observer on the brane tries to analyze quantum gravity in his ground state, the gauge fixing procedure shows that all the matter fields and all the interactions of the Standard Model become redundant: the only physical degree of freedom for quantum gravity in this case is the time (namely the chronon) with its two dimensions (parametrized by the dilaton and the radion). Remarkably, the dynamics of the chronon is governed by a modified Schroedinger equation and, in this sense, the Schroedinger equation is the most fundamental equation of physics. A dilatonic signal traveling in the bulk can bring information from the future (of a different dilatonic time dimension) to our present dilatonic time. The selection of the dilatonic time dimension is related to the value of the chameleonic radion which is stabilized by some UV dynamics. Therefore, a modification of the environment produces a small shift in the radionic coordinate and this selects a totally different dilatonic time dimension (this mechanism is reminiscent of a butterfly effect). A number of consequences of this approach will be discussed.
Is quantum theory exact, or approximate?
Abstract: Why does the wavefunction of a quantum system collapse during a measurement? It maybe possible to answer this question, as in the manyworlds interpretation, without modifying the theory. In this case, quantum theory is exact. On the other hand, it maybe that the theory has to be modified, as in the phenomenological model of Spontaneous Collapse. This model proposes that every quantum particle in nature undergoes spontaneous collapse, and that collapse is a fundamental property of nature, along with SchrÃ¶dinger evolution. The predictions of this model differ from those of quantum theory, and we review the current experimental bounds on these models. We also review work which attempts to provide a theoretical underpinning for this phenomenological model, including the possible role of gravity, and a possible interconnection between the measurement problem and the problem of time in quantum theory.
Quantum dynamics of the outermost dust shell in the LTB model
Abstract: An action for the outermost shell in the Lema\^{i}treTolmanBondi model for spherically symmetric, inhomogeneous dust collapse is derived starting from the EinsteinHilbert action. The resulting Hamiltonian is then quantized and shown to admit exact solutions. Because the coordinates are fixed (to the ones naturally provided by the dust) one can construct an Hilbert space, and wavefunctions have the usual probability interpretation. Restricting to positive ADM energies, all solutions contained in this Hilbert space avoid the classical singularity: The shell never collapses to a point. Furthermore, a wave packet centered initially around the classical trajectory is shown to bounce, meaning after collapse to some nonzero minimal radius (which can lie below the apparent horizon), it expands again in a timereversal symmetric fashion. Hence it can be said, in a slight misuse of terminology, that this model features a quantum transition from black hole to white hole. Finally some implications of this bouncing behavior are discussed: the nature of the horizon, and the lifetime of the temporary 'grey' hole from the perspective of a comoving and exterior observer, respectively.
Date  Time  Speaker  Topic  Room 

April 10  12:00 
Maaneli Derakhshani (Utrecht)

Is Standard Semiclassical Einstein Gravity Viable?  Konferenzraum 1 (Neubau) 
April 24  12:00 
Andrea Zanzi (Ferrara)

The chronon in Mtheory  Konferenzraum 1 (Neubau) 
April 27  16:30 
T. P. Singh (Tata Institute, Mumbai)

Is quantum theory exact, or approximate?  Room 0.03 new building 
May 15  11:30 
Anirudh Gundhi

Master Colloquium  Konferenzraum 1 (Neubau) 
May 15  12:00 
Yurii Dumin (Moscow)

Is the cosmological Lambdaterm a new fundamental constant?  Konferenzraum 1 (Neubau) 
May 29  12:00 
Tim Schmitz

Quantum dynamics of the outermost dust shell in the LTB model  Konferenzraum 1 (Neubau) 
Past seminars
Winter term 2017/18
Summer term 2017
Winter term 2016/17
Summer term 2016
Winter term 2015/16
Summer term 2015
Winter term 2014/15
Summer term 2014
Winter term 2013/14
Summer term 2013
Winter term 2012/13
Summer term 2012
Winter term 2011/12
Summer term 2011
Winter term 2010/11
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Summer term 2009
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Summer term 2007
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Winter term 2003/04
Summer term 2003