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 self-consistent and empirically adequate theory of matter-gravity coupling. This will be done from two points of view: SSEG as a fundamental proposal, and SSEG as a mean-field 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 self-consistency 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 back-reaction of emitted Hawking radiation.

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The chronon in M-theory

Abstract: In this seminar we discuss some aspects of chronon physics in an M-theory 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 action-at-a-distance of Newtonian gravity when interpreted from the standpoint of S-brane 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.

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Is quantum theory exact, or approximate?

Abstract: Why does the wave-function of a quantum system collapse during a measurement? It maybe possible to answer this question, as in the many-worlds 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 Schrödinger 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 inter-connection between the measurement problem and the problem of time in quantum theory.

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TBA

Abstract: TBA

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Is the cosmological Lambda-term a new fundamental constant?

Abstract: TBA

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Quantum dynamics of the outermost dust shell in the LTB model

Abstract: An action for the outermost shell in the Lema\^{i}tre-Tolman-Bondi model for spherically symmetric, inhomogeneous dust collapse is derived starting from the Einstein-Hilbert 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 non-zero minimal radius (which can lie below the apparent horizon), it expands again in a time-reversal 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 co-moving and exterior observer, respectively.

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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 M-theory 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 Lambda-term 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
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