In the last 30 days

• Open/close Lectures on holographic methods for condensed matter physics

  Sean A Hartnoll 2009 Class. Quantum Grav. 26 224002 Tag this article

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  These notes are loosely based on lectures given at the CERN Winter School on Supergravity, Strings and Gauge theories, February 2009, and at the IPM String School in Tehran, April 2009. I have focused on a few concrete topics and also on addressing questions that have arisen repeatedly. Background condensed matter physics material is included as motivation and easy reference for the high energy physics community. The discussion of holographic techniques progresses from equilibrium, to transport and to superconductivity.

• Open/close String theory: a perspective over the last 25 years

  Sunil Mukhi 2011 Class. Quantum Grav. 28 153001 Tag this article

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  This review provides some historical background and then reviews developments in string theory over the last 25 years or so. Both perturbative and non-perturbative approaches to string theory are surveyed and their impact on how we view quantum gravity is analysed.

• Open/close Loop quantum gravity: the first 25 years

  Carlo Rovelli 2011 Class. Quantum Grav. 28 153002 Tag this article

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  I give a synthetic presentation of loop quantum gravity. I present the aims of the theory and compare the results obtained with the initial hopes that motivated the early interest in this research direction. I give my own perspective on the status of the program and attempt for a critical evaluation of its successes and limitations.

• Open/close Does an atom interferometer test the gravitational redshift at the Compton frequency?

  Peter Wolf et al 2011 Class. Quantum Grav. 28 145017 Tag this article

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  Atom interferometers allow the measurement of the acceleration of freely falling atoms with respect to an experimental platform at rest on Earth's surface. Such experiments have been used to test the universality of free fall by comparing the acceleration of the atoms to that of a classical freely falling object. In a recent paper, Müller et al (2010 Nature 463 926–9) argued that atom interferometers also provide a very accurate test of the gravitational redshift (or universality of clock rates). Considering the atom as a clock operating at the Compton frequency associated with the rest mass, they claimed that the interferometer measures the gravitational redshift between the atom-clocks in the two paths of the interferometer at different values of gravitational potentials. In this paper, we analyze this claim in the frame of general relativity and of different alternative theories. We show that the difference of 'Compton phases' between the two paths of the interferometer is actually zero in a large class of theories, including general relativity, all metric theories of gravity, most non-metric theories and most theoretical frameworks used to interpret the violations of the equivalence principle. Therefore, in most plausible theoretical frameworks, there is no redshift effect and atom interferometers only test the universality of free fall. We also show that frameworks in which atom interferometers would test the redshift pose serious problems, such as (i) violation of the Schiff conjecture, (ii) violation of the Feynman path integral formulation of quantum mechanics and of the principle of least action for matter waves, (iii) violation of energy conservation, and more generally (iv) violation of the particle-wave duality in quantum mechanics. Standard quantum mechanics is no longer valid in such frameworks, so that a consistent interpretation of the experiment would require an alternative formulation of quantum mechanics. As such an alternative has not been proposed to date, we conclude that the interpretation of atom interferometers as testing the gravitational redshift at the Compton frequency is unsound.

• Open/close Focus section on quantum gravity - 25 years of quantum gravity

  Joseph Samuel 2011 Class. Quantum Grav. 28 150301 Tag this article

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  The problem of quantum gravity has been with us for over 80 years. After quantum theory was established in the 1920s, it was successfully applied to the electromagnetic field. Over the years there have been many attempts to bring gravity into the fold. There has been work on the Hamiltonian formulation of general relativity, perturbative approaches to quantum gravity and more. Much intellectual effort went into understanding conceptual and technical problems stemming from the general covariance of the theory. However, in earlier decades, the subject of quantum gravity was relatively on the fringes of theoretical physics research, pursued by a small and diverse community of people.

  In the mid 1980s the situation changed dramatically. The subject of quantum gravity came to the forefront of fundamental physics research, no longer a backwater but the mainstream. Quantum gravity was widely acknowledged as the last frontier of fundamental physics and attracted the brightest young people. Unlike in previous decades, workers in this area were no longer isolated groups or individuals ploughing lonely furrows, but organised into coherent `programmes' for a concerted attack on the problem. The main programmes coincidentally were all formulated in the mid 1980s. The two `programmes' covered in this section are string theory and loop quantum gravity. String theory was born an offshoot of Hadronic models in particle physics and reflects the particle physicists view that gravity is just one more interaction to be encompassed by a unified theory. Loop quantum gravity reflects the general relativist's conviction that gravity is different and should not be treated as a perturbation about Minkowski spacetime. Each of these approaches has its proponents, adherents and critics. It is now about a quarter of a century since these programmes started. It is perhaps a good time to take stock and assess where we are now and where each of these programmes is headed.

  The idea in this focus section is to get a comparative perspective on these programmes, by asking our reviewers to critically evaluate progress in their programmes over the last 25 years (1986–2011). This section features invited review articles from physicists who have been associated with these programmes from their inception. They were invited to write a retrospective review: what were the initial hopes? To what extent have these hopes been realised? What were the major successes, surprises, and disappointments? The emphasis is on what has come out of the programme rather than technical developments internal to the programme. We hope that the reader, whatever his/her persuasion, will be able to form a panoramic view of quantum gravity research today within these two programmes. We hope to complement this view with a topical review of causal sets in the future.

• Open/close Atom interferometry and the gravitational redshift

  Supurna Sinha and Joseph Samuel 2011 Class. Quantum Grav. 28 145018 Tag this article

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  From the principle of equivalence, Einstein predicted that clocks slow down in a gravitational field. Since the general theory of relativity is based on the principle of equivalence, it is essential to test this prediction accurately. Müller, Peters and Chu claim that a reinterpretation of decade old experiments with atom interferometers leads to a sensitive test of this gravitational redshift effect at the Compton frequency. Wolf et al dispute this claim and adduce arguments against it. In this paper, we distill these arguments to a single fundamental objection: an atom is not a clock ticking at the Compton frequency. We conclude that atom interferometry experiments conducted to date do not yield such sensitive tests of the gravitational redshift. Finally, we suggest a new interferometric experiment to measure the gravitational redshift, which realizes a quantum version of the classical clock 'paradox'.

• Open/close Quantum gravitational decoherence of matter waves

  Charles H-T Wang et al 2006 Class. Quantum Grav. 23 L59 Tag this article

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  One of the biggest unsolved problems in physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance has made the issue extremely evasive, though various attempts have been made to relate the loss of matter wave coherence to quantum spacetime fluctuations. We present a new approach to the gravitational decoherence near the Planck scale, made possible by the recently discovered conformal structure of canonical gravity. This leads to a gravitational analogue of Brownian motion whose correlation length is given by the Planck length up to a scaling factor. With input from recent matter wave experiments, we show the minimum value of this factor to be well within the expected range for quantum gravity theories. This suggests that the sensitivities of advanced matter wave interferometers may be approaching the fundamental level due to quantum spacetime fluctuations, and that investigating Planck scale physics using matter wave interferometry may become a reality in the near future.

• Open/close Surprising connections between general relativity and condensed matter

  Gary T Horowitz 2011 Class. Quantum Grav. 28 114008 Tag this article

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  This article is intended to introduce gravitational physicists to recent developments in which general relativity is being used to describe certain aspects of condensed matter systems, e.g., superconductivity.

• Open/close Polymer and Fock representations for a scalar field

  Abhay Ashtekar et al 2003 Class. Quantum Grav. 20 L11 Tag this article

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  In loop quantum gravity, matter fields can have support only on the 'polymer-like' excitations of quantum geometry, and their algebras of observables and Hilbert spaces of states cannot refer to a classical, background geometry. Therefore, to adequately handle the matter sector, one has to address two issues already at the kinematic level. First, one has to construct the appropriate background-independent operator algebras and Hilbert spaces. Second, to make contact with low-energy physics, one has to relate this 'polymer description' of matter fields to the standard Fock description in Minkowski space. While this task has been completed for gauge fields, important gaps remained in the treatment of scalar fields. The purpose of this letter is to fill these gaps.

• Open/close A rigorous derivation of gravitational self-force

  S E Gralla and R M Wald 2011 Class. Quantum Grav. 28 159501 Tag this article

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  The full text of this corrigendum is provided in the pdf file.