Abstracts for July-August 2016

  • Local dark matter searches with LISA
    Massimo Cerdonio, Roberto De Pietri, Philippe Jetzer and Mauro Sereno
    The drag-free satellites of LISA will maintain the test masses in geodesic motion over many years with residual accelerations at unprecedented small levels and time delay interferometry (TDI) will keep track of their differential positions at a level of picometers. This may allow investigations of fine details of the gravitational field in the solar system previously inaccessible. In this spirit, we present the concept of a method for measuring directly the gravitational effect of the density of diffuse local dark matter (LDM) with a constellation of a few drag-free satellites, by exploiting how peculiarly it would affect their relative motion. Using as a test-bed an idealized LISA with rigid arms, we find that the separation in time between the test masses is uniquely perturbed by the LDM, so that they acquire a differential breathing mode. Such an LDM signal is related to the LDM density within the orbits and has characteristic spectral components, with amplitudes increasing in time, at various frequencies of the dynamics of the constellation. This is the relevant result in that the LDM signal is brought to non-zero frequencies.
  • We review some recent developments in the statistical mechanics of isolated quantum systems. We provide a brief introduction to quantum thermalization, paying particular attention to the eigenstate thermalization hypothesis (ETH) and the resulting single-eigenstate statistical mechanics. We then focus on a class of systems that fail to quantum thermalize and whose eigenstates violate the ETH: These are the many-body Anderson-localized systems; their long-time properties are not captured by the conventional ensembles of quantum statistical mechanics. These systems can forever locally remember information about their local initial conditions and are thus of interest for possibilities of storing quantum information. We discuss key features of many-body localization (MBL) and review a phenomenology of the MBL phase. Single-eigenstate statistical mechanics within the MBL phase reveal dynamically stable ordered phases, and phase transitions among them, that are invisible to equilibrium statistical mechanics and can occur at high energy and low spatial dimensionality, where equilibrium ordering is forbidden.
    (arXiv:1404.0686) This is the most readable introduction to many-body localization I’ve found. Gratifyingly, it describes the universal connection to thermalization and the ETH, rather than casting localization as just some peculiar wave phenomena.
  • What is called “orthodox” quantum mechanics, as presented in standard foundational discussions, relies on two substantive assumptions — the projection postulate and the eigenvalue-eigenvector link — that do not in fact play any part in practical applications of quantum mechanics. I argue for this conclusion on a number of grounds, but primarily on the grounds that the projection postulate fails correctly to account for repeated, continuous and unsharp measurements (all of which are standard in contemporary physics) and that the eigenvalue-eigenvector link implies that virtually all interesting properties are maximally indefinite pretty much always. I present an alternative way of conceptualising quantum mechanics that does a better job of representing quantum mechanics as it is actually used, and in particular that eliminates use of either the projection postulate or the eigenvalue-eigenvector link, and I reformulate the measurement problem within this new presentation of orthodoxy

    David Wallace investigates the weird way that quantum mechanics is actually put into practice as compared with how people attempt to formalize it: the collapse postulate (leading to an updated wavefunction) is almost never used in practice, even if the Born rule is. Some striking observations that motivate this:

    Firstly, collapse is conspicuously absent from second courses in QM, and in particular in courses on relativistic QM. This ought to strike a student as peculiar… the point is not that collapse is unsatisfactory in the relativistic regime. Of course it is;…But relativistic QM textbooks contain, not an unsatisfactory collapse rule, but no collapse rule at all. One concludes that the theory must be applicable without any mention of collapse….Secondly, the theoretical physics community has been worrying for forty years about the so-called “black hole information loss paradox”…At its heart, the paradox is simply that black hole decay is non-unitary and as
    such can’t be described within the Schrodinger-equation framework. But state vector collapse is also non-unitary!… One has the clear impression that (at least this part of) the theoretical physics community does not in fact think that dynamics is non-unitary in any other contexts in physics, rendering black hole decay uniquely problematic. Tempting though it might be for this advocate of the Everett interpretation to claim that the community has adopted the many-worlds theory en masse, a more mundane account is simply that (what they regard as) orthodox QM does not include the collapse postulate…Thirdly, modern quantum field theory largely abandons Hamiltonian methods in favour of the path-integral approach. But in that approach it is not even clear how collapse is to be defined (and, again, textbook presentations never seem to mention the issue), and yet the theory still seems to produce empirically successful predictions.

    Wallace goes on to discuss continuous measurement, which is often treated in a very ad hoc manner in textbooks (witness the opaque “Fermi’s Golden Rule”). He concludes with a claim that I have been pushing: the problem with orthodox quantum mechanics isn’t that it’s unintuitive, or that it violates causality, or that it’s indeterministic; the problem is that, as practiced, it’s vague:

    These examples probably strike the reader as uncomfortably opportunistic, even ad hoc. Indeed, they should so strike the reader. The ad hoc, opportunistic approach that physics takes to the interpretation of the quantum state, and the lack, in physical practice, of a clear and unequivocal understanding of the state — this is the quantum measurement problem…

  • Entanglement
    Robert Spekkens
    Abstract: (None)
    The rest of the these “abstracts” are actually just good introductory lectures from PI’s “It From Qubit” conference. In this one, Rob gives a good introduction to entanglement, starting with its first appearance in the context of EPR pairs and Bell inequalities, but then moving to the modern and more illuminating construction involving LOCC equivalence classes and the majorization partial order. I wish this talk existed when I was in grad school, because back then it took me weeks to piece together all these insights.
  • QI Basics
    Patrick Hayden
    Abstract: (None)
    This is an even more basic introduction to quantum information in general, with good discussion of the Fidelity and Trace distance from 12:00 to 25:45.

  • Abstract: If we imagine that the universe is truly eternal, special challenges arise for attempts to solve cosmological fine-tuning problems, especially the low entropy of the early universe. If the space of states is finite, the universe should spend most of its time near equilibrium. If the space of states is infinite, it becomes difficult to understand why our universe was in a particular low-entropy state. I will discuss approaches to addressing this problem in a model-independent fashion.

    I especially like Sean’s framing of the problem in the introduction. Also, this: “In that universe, the overwhelming majority of apple pies are Boltzmann apple pies, that fluctuate into existence, and you do not need an apple orchard to make an apple pie.”

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