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Are we just a simulation? (Woit's message to the overlords: Dont turn us off!)

  1. Aug 19, 2007 #1


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    Maybe it's well-known that a group in Holland (at the Utrecht Institute for Theoretical Physics) have been running simplicial-model Monte Carlo simulations of the universe. So far they have generated literally millions of different universes----part of the idea requires large numbers because since they do quantum cosmology they need to be able to study random "average" properties in their universes.

    Still very primitive---getting the universe to assemble itself out of little 4D spacetime lego-blocks, and evolve. In the end, because the computer is finite, it runs out of spacetime lego blocks and must die.

    Ambjorn, Loll, and Jurkiewicz are the main authors.

    The Universe from Scratch
    Authors: R. Loll, J. Ambjorn, J. Jurkiewicz
    31 pages, 5 figures; review paper commissioned by Contemporary Physics and aimed at a wider physics audience
    (Submitted on 1 Sep 2005 (v1), last revised 14 Oct 2006 (this version, v3))

    Quantum Gravity, or The Art of Building Spacetime
    Authors: J. Ambjorn, J. Jurkiewicz, R. Loll
    22 pages, 6 figures. Contribution to the book "Approaches to Quantum Gravity", ed. D. Oriti, Cambridge University Press
    (Submitted on 28 Apr 2006)

    Reconstructing the Universe
    J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
    Physical Review D 72 (2005) 064014
    52 pages, 20 figures
    (Submitted on 17 May 2005 (v1), last revised 6 Jun 2005 (this version, v2))

    "We provide detailed evidence for the claim that nonperturbative quantum gravity, defined through state sums of causal triangulated geometries, possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale, and at the same time provides a nontrivial consistency check of the method of causal dynamical triangulations. A closer look at the quantum geometry reveals a number of highly nonclassical aspects, including a dynamical reduction of spacetime to two dimensions on short scales and a fractal structure of slices of constant time."

    Spectral Dimension of the Universe
    Authors: J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
    Phys.Rev.Lett. 95 (2005) 171301, 10 pages, 1 figure
    (Submitted on 12 May 2005 (v1), last revised 6 Jun 2005 (this version, v2))

    "We measure the spectral dimension of universes emerging from nonperturbative quantum gravity, defined through state sums of causal triangulated geometries. While four-dimensional on large scales, the quantum universe appears two-dimensional at short distances. We conclude that quantum gravity may be 'self-renormalizing' at the Planck scale, by virtue of a mechanism of dynamical dimensional reduction."

    Semiclassical Universe from First Principles
    Authors: J. Ambjorn, J. Jurkiewicz, R. Loll
    Phys.Lett. B607 (2005) 205-213, 15 pages, 4 figures
    (Submitted on 16 Nov 2004)

    "Causal Dynamical Triangulations in four dimensions provide a background-independent definition of the sum over space-time geometries in nonperturbative quantum gravity. We show that the macroscopic four-dimensional world which emerges in the Euclidean sector of this theory is a bounce which satisfies a semiclassical equation. After integrating out all degrees of freedom except for a global scale factor, we obtain the ground state wave function of the universe as a function of this scale factor."

    Emergence of a 4D World from Causal Quantum Gravity
    J. Ambjorn (1 and 3), J. Jurkiewicz (2), R. Loll (3) ((1) Niels Bohr Institute, Copenhagen, (2) Jagellonian University, Krakow, (3) Spinoza Institute, Utrecht)
    Phys.Rev.Lett. 93 (2004) 131301, 11 pages, 3 figures
    (Submitted on 21 Apr 2004 (v1), last revised 16 Sep 2004 (this version, v4))

    "Causal Dynamical Triangulations in four dimensions provide a background-independent definition of the sum over geometries in nonperturbative quantum gravity, with a positive cosmological constant. We present evidence that a macroscopic four-dimensional world emerges from this theory dynamically."

    Jan Ambjorn gave a recent talk describing the latest results in June 2007 at the Loops 07 conference.
    4d quantum gravity as a sum over histories
    The audio is available online.


    Anyway the idea of simulating the universe is in some sense a practical hands-on idea---it is just that so far we are doing it at a very primitive rudimentary level.

    It seems borderline lunacy to extrapolate this, at present, to a speculation about simulations so complex that subassemblages in them can develop powers of reasoning. Rarely does much good to speculate way far ahead.
    But, for better worse, some people at an English outfit called "Future of Humanity Institute" have been doing this and it has gotten into the New York Times Science section---with some controversy ensuing.

    Peter Woit, of the blog Not Even Wrong, has sent a message to the Overlords asking them not to turn off the simulation in which we exist. :biggrin:
    Last edited: Aug 19, 2007
  2. jcsd
  3. Aug 21, 2007 #2
    they can turn it off- it is much cheaper and simpler to run a simulation of all possible universes and all possible histories than a single state of a single history- so if the computer is turned off we still keep going because all the possible futures are encoded in the physical laws of the simulation- and simulation time is invariant of the computer's steps

    The rate of time flow perceived by an observer in the simulated universe is completely independent of the rate at which a computer runs the simulation ... Moreover, as emphasized by Einstein, it is arguably more natural to view our universe not from the frog perspective as a 3-dimensional space where things happen, but from the bird perspective as a 4-dimensional spacetime that merely is. There should therefore be no need for the computer to compute anything at all — it could simply store all the 4-dimensional data, i.e., encode all properties of the mathematical structure that is our universe. Individual time slices could then be read out sequentially if desired, and the “simulated” world should still feel as real to its inhabitants as in the case where only 3-dimensional data is stored and evolved

    ~Max Tegmark
    Last edited: Aug 21, 2007
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