Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question for Daniele

  1. Nov 17, 2004 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    what do you think of the new AJL paper
    the computer runs seem to give rise to Hawking's type of quantum gravity
    does spinfoam, as you see it, tie into this?
  2. jcsd
  3. Nov 28, 2004 #2
    Are you talking about Daniele Oriti? There's a Daniele now online. Is him?
  4. Nov 28, 2004 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    that's a mystery isnt it?
    probably it isnt either Daniele -----there are two Loop Daniels:

    Daniele Oriti, that we may know from spin foams and his thesis (with its great quotes from Groucho Marx and Homer Simpson)

    but also there is
    Daniele Collosi who just co-wrote a paper with Carlo Rovelli
    I used that article to start the "Ontology" thread.
    It is about the contingent existence of particles.

    But quite likely our D is just some other D. who knows?
  5. Dec 2, 2004 #4
  6. Dec 2, 2004 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I regret to say, meteor, that I think we must assume that
    our D is neither Daniele Oriti nor Daniele Colosi, but is one of
    thousands of other Daniels in the world who might also be
    interested in Quantum Gravity, but of whom we know nothing.
  7. Dec 8, 2004 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Daniele Oriti has answered that question

    I just saw that the genuine (not substitute) Oriti has answered the question
    asked here about the relation of his "causal" spinfoam to the
    Simplicial Gravity: Dynamical Triangulation approach of Ambjorn Jurkiewicz Loll.

    today oriti posted this
    The Feynman propagator for quantum gravity: spinfoams, proper time, orientation, causality and timeless-ordering

    and look on page 8:

    ---quote oriti---
    Notice that, if such a relationship is found, not only we would have strenghtened the argument for the validity of our constuction and paved the way for further developments, but we would have also obtained, as a side result (!), the first definition to be found in the literature, to the best of our knowledge, of a proper time expression for the action of simplicial gravity, that reduces to the Regge action when the additional variable is integrated out.

    Another application of the proper time formalism could be the issue of Wick rotation in spin foam models, and more gnerally in quantum gravity, since it seems to be the right parameter in which to analytically continue the amplitudes to define an “euclideanized”model. Also, the generalised (proper time dependent) formulation of spin foam models could represent a new starting point for bridging the gap between spin foam models and other approaches to quantum gravity in which proper time plays a significant role, as for example the Lorentzian dynamical triangulations, that have achieved recently important results concerning the issue of continuum limit.

    The causal models seem indeed to be the at the point of convergence of simplicial quantum gravity, dynamical triangulations and causal sets, in addition to canonical loop quantum gravity, and therefore represent the easiest context in which to analyse the relationships between all these approaches.

    Finally, if the orientation-independent models can be understood as defining the matrix elements of the projector operator onto physical quantum gravity states, then an intriguing possibility is to interpret the new models as defining the matrix elements of an “evolution operator”, whose property could be studied to understand for example whether a notion of unitary evolution is feasible in Quantum Gravity and the “scattering”between quantum gravity perturbations, in absence of an external time coordinate, but with a clearly identified notion of causality.

    ACKNOWLEDGMENTS I would like to thank gratefully Etera Livine and Florian Girelli for discussions and encouragement, and the organizers and staff of the DICE 2004 Workshop for a stimulating and truly enjoyable meeting.


    I also want to quote from page 7

    ---quote oriti---

    Let us summarise what we have presented. We have discussed in which sense causality may be thought of preexisting (!) time at the most fundamental level, and what notion of causality, interpreted as a ’seed’ from which the usual continuum notion of causality in terms of lightcone structure will emerge in a semiclassical approximation, can instead replace it in deep quantum gravity regime; we have argued that such a fundamental notion of causality can be implemented most naturally in a sum over-histories context; with the aim of implementing this idea in the spin foam context, we have linked the notion of causality with the orientation of the 2-complex on which the spin foam is based....

    this has some strong basic ideas
    that causality comes before time
    that causality is linked to a geometric idea, namely the ORIENTATION of a simplicial complex.

    have to go, back later

    thanks to that Daniele, if he ever shows up here. really interesting paper
    Last edited: Dec 8, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook