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  • 1.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    A no-go theorem for Poincare-invariant networks2015In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 32, no 20, article id 207001Article in journal (Refereed)
    Abstract [en]

    I explain why there are no Poincare-invariant networks with a locally finite distribution of nodes and links in Minkowski-spacetime of any dimension.

  • 2.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    A possibility to solve the problems with quantizing gravity2013In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 725, no 4-5, p. 473-476Article in journal (Refereed)
    Abstract [en]

    It is generally believed that quantum gravity is necessary to resolve the known tensions between general relativity and the quantum field theories of the standard model. Since perturbatively quantized gravity is non-renormalizable, the problem how to unify all interactions in a common framework has been open since the 1930s. Here, I propose a possibility to circumvent the known problems with quantizing gravity, as well as the known problems with leaving it unquantized: By changing the prescription for second quantization, a perturbative quantization of gravity is sufficient as an effective theory because matter becomes classical before the perturbative expansion breaks down. This is achieved by considering the vanishing commutator between a field and its conjugated momentum as a symmetry that is broken at low temperatures, and by this generates the quantum phase that we currently live in, while at high temperatures Planck's constant goes to zero.

  • 3.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    A relativistic acoustic metric for planar black holes2016In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 752, p. 13-17Article in journal (Refereed)
    Abstract [en]

    We demonstrate here that the metric of a planar black hole in asymptotic anti-de Sitter space can, on a slice of dimension 3 + 1, be reproduced as a relativistic acoustic metric. This completes an earlier calculation in which the non-relativistic limit was used, and also serves to obtain a concrete form of the Lagrangian.

  • 4.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Analog systems for gravity duals2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, no 12Article in journal (Refereed)
    Abstract [en]

    We show that analog gravity systems exist for charged, planar black holes in asymptotic anti-de Sitter space. These black holes have been employed to describe, via the gauge-gravity duality, strongly coupled condensed matter systems on the boundary of anti-de Sitter (AdS) space. The analog gravity system is a different condensed matter system that, in a suitable limit, describes the same bulk physics as the theory on the AdS boundary. This combination of the gauge-gravity duality and analog gravity therefore suggests a duality between different condensed matter systems.

  • 5.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Can we unify quantum mechanics and gravity?2013In: Physics world, ISSN 0953-8585, Vol. 26, no 10, p. 42-43Article in journal (Refereed)
    Abstract [en]

    The incompatibility of general relativity and quantum mechanics is perhaps the most important open problem in theoretical physics. Sabine Hossenfelder describes how physicists are working to unite these two perspectives in a theory of quantum gravity.

  • 6.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Disentangling the black hole vacuum2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, no 4, article id 044015Article in journal (Refereed)
    Abstract [en]

    We study the question of whether disentanglement of Hawking radiation can be achieved with any local operation. We assume that the operation we look for is unitary, from which follow restrictions on its action on basis vectors. This allows us to formulate requirements on the operation of disentanglement. We then show that these requirements can be fulfilled by a timelike boundary condition in the near-horizon area and that the local observer does not notice the presence of the boundary and does not encounter a firewall.

  • 7.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Gravity Can Be Neither Classical Nor Quantized2015In: Questioning the Foundations of Physics: Which of Our Fundamental Assumptions Are Wrong? / [ed] Aguirre, A.; Foster, B.; Merali, Z., New York: Springer, 2015, p. 219-224Chapter in book (Refereed)
    Abstract [en]

    I argue that it is possible for a theory to be neither quantized nor classical. We should therefore give up the assumption that the fundamental theory which describes gravity at shortest distances must either be quantized, or quantization must emerge from a fundamentally classical theory. To illustrate my point I will discuss an example for a theory that is neither classical nor quantized, and argue that it has the potential to resolve the tensions between the quantum field theories of the standard model and general relativity.

  • 8.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    HEAD TRIP2015In: Scientific American, ISSN 0036-8733, E-ISSN 1946-7087, Vol. 313, no 3, p. 47-49Article in journal (Refereed)
  • 9.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    How to Save the World2016In: How Should Humanity Steer the Future? / [ed] Anthony Aguirre, Brendan Foster, Zeeya Merali, Springer, 2016, p. 5-14Chapter in book (Refereed)
    Abstract [en]

    If you knew how humanity should steer the future, what difference would it make? The major challenge that humanity faces today is not that we lack ideas for what to do, as I am sure this essay contest will document. No, the major challenge, the mother of all problems, is to convert these ideas into courses of action. We fail to act in the face of global problems because we do not have an intuitive grasp on the consequences of collective human behavior, are prone to cognitive biases, and easily overwhelmed by data. We are also lazy and if intuition fails us, inertia takes over. Howmany people will read these brilliant essays? For the individual, evaluating possible courses of action to address interrelated problems in highly connected social, economic and ecological networks is presently too costly. The necessary information may exist, even be accessible, but it is too expensive in terms of time and energy. To steer the future, information about our dynamical and multi-layered networks has to become cheap and almost effortless to use. Only then, when we can make informed decisions by feeling rather than thinking, will we be able to act and respond to the challenges we face.

  • 10.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Minimal length scale scenarios for quantum gravity2013In: Living Reviews in Relativity, ISSN 1433-8351, E-ISSN 1433-8351, Vol. 16, no 1, article id 2Article in journal (Refereed)
    Abstract [en]

    We review the question of whether the fundamental laws of nature limit our ability to probe arbitrarily short distances. First, we examine what insights can be gained from thought experiments for probes of shortest distances, and summarize what can be learned from different approaches to a theory of quantum gravity. Then we discuss some models that have been developed to implement a minimal length scale in quantum mechanics and quantum field theory. These models have entered the literature as the generalized uncertainty principle or the modified dispersion relation, and have allowed the study of the effects of a minimal length scale in quantum mechanics, quantum electrodynamics, thermodynamics, black-hole physics and cosmology. Finally, we touch upon the question of ways to circumvent the manifestation of a minimal length scale in short-distance physics.

  • 11.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Quantum Superpositions of the Speed of Light2012In: Foundations of physics, ISSN 0015-9018, E-ISSN 1572-9516, Vol. 42, no 11, p. 1452-1468Article in journal (Refereed)
    Abstract [en]

    While it has often been proposed that, fundamentally, Lorentz-invariance is not respected in a quantum theory of gravity, it has been difficult to reconcile deviations from Lorentz-invariance with quantum field theory. The most commonly used mechanisms either break Lorentz-invariance explicitly or deform it at high energies. However, the former option is very tightly constrained by experiment already, the latter generically leads to problems with locality. We show here that there exists a third way to integrate deviations from Lorentz-invariance into quantum field theory that circumvents the problems of the other approaches. The way this is achieved is an extension of the standard model in which photons can have different speeds without singling out a preferred restframe, but only as long as they are in a quantum superposition. Once a measurement has been made, observables are subject to the laws of special relativity, and the process of measurement introduces a preferred frame. The speed of light can take on different values, both superluminal and subluminal (with respect to the usual value of the speed of light), without the need for Lorentz-invariance violating operators and without tachyons. We briefly discuss the relation to deformations of special relativity and phenomenological consequences.

  • 12.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Testing superdeterministic conspiracy2014In: EmQM13: emergent quantum mechanics 2013, 2014, p. 012018-Conference paper (Refereed)
    Abstract [en]

    Tests of Bell's theorem rule out local hidden variables theories. But any theorem is only as good as the assumptions that go into it, and one of these assumptions is that the experimenter can freely choose the detector settings. Without this assumption, one enters the realm of superdeterministic hidden variables theories and can no longer use Bell's theorem as a criterion. One can like or not like such superdeterministic hidden variables theories and their inevitable nonlocality, the real question is how one can test them. Here, we propose a possible experiment that could reveal superdeterminism.

  • 13.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    The Remote Maxwell Demon as Energy Down-Converter2016In: Foundations of physics, ISSN 0015-9018, E-ISSN 1572-9516, Vol. 46, no 4, p. 505-516Article in journal (Refereed)
    Abstract [en]

    It is demonstrated that Maxwell's demon can be used to allow a machine to extract energy from a heat bath by use of information that is processed by the demon at a remote location. The model proposed here effectively replaces transmission of energy by transmission of information. For that we use a feedback protocol that enables a net gain by stimulating emission in selected fluctuations around thermal equilibrium. We estimate the down conversion rate and the efficiency of energy extraction from the heat bath.

  • 14.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    The Soccer-Ball Problem2014In: SIGMA. Symmetry, Integrability and Geometry, ISSN 1815-0659, E-ISSN 1815-0659, Vol. 10, p. 074-Article in journal (Refereed)
    Abstract [en]

    The idea that Lorentz-symmetry in momentum space could be modified but still remain observer-independent has received quite some attention in the recent years. Motivated by Loop Quantum Gravity, this modified Lorentz-symmetry is being used as a phenomenological model to test possibly observable effects of quantum gravity. The most pressing problem in these models is the treatment of multi-particle states, known as the 'soccer-ball problem'.This article briefly reviews the problem and the status of existing solution attempts.

  • 15.
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Theory and Phenomenology of Space-Time Defects2014In: Advances in High Energy Physics, ISSN 1687-7357, E-ISSN 1687-7365, p. 950672-Article, review/survey (Refereed)
    Abstract [en]

    Whether or not space-time is fundamentally discrete is of central importance for the development of the theory of quantum gravity. If the fundamental description of spacetime is discrete, typically represented in terms of a graph or network, then the apparent smoothness of geometry on large scales should be imperfect-it should have defects. Here, we review a model for space-time defects and summarize the constraints on the prevalence of these defects that can be derived from observation.

  • 16.
    Hossenfelder, Sabine
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Frankfurt Institute for Advanced Studies, Germany.
    Zingg, Tobias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Helsinki, Finland.
    Analog models for holographic transport2019In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 100, no 5, article id 056015Article in journal (Refereed)
    Abstract [en]

    The gauge-gravity duality and analog gravity both relate a condensed matter system to a gravitational theory. This makes it possible to use gravity as an intermediary to establish a relation between two different condensed matter systems: the strongly coupled system from the gauge-gravity duality and the weakly coupled gravitational analog. We here offer some examples for relations between observables in the two different condensed matter systems. In particular, we show how the equations characterizing Green functions and fast order transport coefficients in holographic models can be mapped to those describing phenomena in an analog gravitational system, which allows, in principle, to obtain the former by measuring the latter.

  • 17.
    Hossenfelder, Sabine
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Frankfurt Institute for Advanced Studies, Germany.
    Zingg, Tobias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Analogue gravity models from conformal rescaling2017In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 34, no 16, article id 165004Article in journal (Refereed)
    Abstract [en]

    Analogue gravity is based on a mathematical identity between quantum field theory in curved space-time and the propagation of perturbations in certain condensed matter systems. But not every curved space-time can be simulated in such a way. For analogue gravity to work, one needs not only a condensed matter system that generates the desired metric tensor, but this system then also has to obey its own equations of motion. However, the relation to the metric tensor usually overdetermines the equations of the underlying condensed matter system, such that they in general cannot be fulfilled. In this case the desired metric does not have an analogue. Here, we show that the class of metrics that have an analogue is larger than previously thought. The reason is that the analogue metric is only defined up to a choice of parametrization of the perturbation in the underlying condensed matter system. In this way, the class of analogue gravity models can be vastly expanded.

  • 18.
    Hossenfelder, Sabineh
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Phenomenology of space-time imperfection. I. Nonlocal defects2013In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 88, no 12, p. 124030-Article in journal (Refereed)
    Abstract [en]

    If space-time is emergent from a fundamentally nongeometric theory it will generically be left with defects. Such defects need not respect the locality that emerges with the background. Here, we develop a phenomenological model that parametrizes the effects of nonlocal defects on the propagation of particles. In this model, Lorentz invariance is preserved on the average. We derive constraints on the density of defects from various experiments.

  • 19.
    Hossenfelder, Sabineh
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Phenomenology of space-time imperfection. II. Local defects2013In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 88, no 12, p. 124031-Article in journal (Refereed)
    Abstract [en]

    We propose a phenomenological model for the scattering of particles on space-time defects in a treatment that maintains Lorentz invariance on the average. The local defects considered here cause a stochastic violation of momentum conservation. The scattering probability is parametrized in the density of defects and the distribution of the momentum that a particle can obtain when scattering on the defect. We identify the most promising observable consequences and derive constraints from existing data.

  • 20. Kaltenbaek, Rainer
    et al.
    Aspelmeyer, Markus
    Barker, Peter F.
    Bassi, Angelo
    Bateman, James
    Bongs, Kai
    Bose, Sougato
    Braxmaier, Claus
    Brukner, Časlav
    Christophe, Bruno
    Chwalla, Michael
    Cohadon, Pierre-François
    Cruise, Adrian Michael
    Curceanu, Catalina
    Dholakia, Kishan
    Diósi, Lajos
    Döringshoff, Klaus
    Ertmer, Wolfgang
    Gieseler, Jan
    Gürlebeck, Norman
    Hechenblaikner, Gerald
    Heidmann, Antoine
    Herrmann, Sven
    Hossenfelder, Sabine
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Johann, Ulrich
    Kiesel, Nikolai
    Kim, Myungshik
    Lämmerzahl, Claus
    Lambrecht, Astrid
    Mazilu, Michael
    Milburn, Gerard J.
    Müller, Holger
    Novotny, Lukas
    Paternostro, Mauro
    Peters, Achim
    Pikovski, Igor
    Zanoni, André Pilan
    Rasel, Ernst M.
    Reynaud, Serge
    Riedel, Charles Jess
    Rodrigues, Manuel
    Rondin, Loïc
    Roura, Albert
    Schleich, Wolfgang P.
    Schmiedmayer, Jörg
    Schuldt, Thilo
    Schwab, Keith C.
    Tajmar, Martin
    Tino, Guglielmo M.
    Ulbricht, Hendrik
    Ursin, Rupert
    Vedral, Vlatko
    Macroscopic Quantum Resonators (MAQRO): 2015 update2016In: EPJ quantum technology, ISSN 2196-0763, Vol. 3, article id 5Article, review/survey (Refereed)
    Abstract [en]

    Do the laws of quantum physics still hold for macroscopic objects - this is at the heart of Schrodinger's cat paradox - or do gravitation or yet unknown effects set a limit for massive particles? What is the fundamental relation between quantum physics and gravity? Ground-based experiments addressing these questions may soon face limitations due to limited free-fall times and the quality of vacuum and microgravity. The proposed mission Macroscopic Quantum Resonators (MAQRO) may overcome these limitations and allow addressing such fundamental questions. MAQRO harnesses recent developments in quantum optomechanics, high-mass matter-wave interferometry as well as state-of-the-art space technology to push macroscopic quantum experiments towards their ultimate performance limits and to open new horizons for applying quantum technology in space. The main scientific goal is to probe the vastly unexplored 'quantum-classical' transition for increasingly massive objects, testing the predictions of quantum theory for objects in a size and mass regime unachievable in ground-based experiments. The hardware will largely be based on available space technology. Here, we present the MAQRO proposal submitted in response to the 4th Cosmic Vision call for a medium-sized mission (M4) in 2014 of the European Space Agency (ESA) with a possible launch in 2025, and we review the progress with respect to the original MAQRO proposal for the 3rd Cosmic Vision call for a medium-sized mission (M3) in 2010. In particular, the updated proposal overcomes several critical issues of the original proposal by relying on established experimental techniques from high-mass matter-wave interferometry and by introducing novel ideas for particle loading and manipulation. Moreover, the mission design was improved to better fulfill the stringent environmental requirements for macroscopic quantum experiments.

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