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

Bojowald's letter in Nature Physics (July 2007)

  1. Jul 2, 2007 #1

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    This thread is distinct from discussion of the mainstream media coverage of the letter that just appeared in Nature Physics.

    I just want to get more precise about the actual issues.

    So far, not having a subscription, I don't have access to the advance online publication in Nature Physics (NP), but there is a preprint of a longer article at arxiv which cites the letter and mentions some relevant conclusions.

    http://arxiv.org/abs/0706.1057
    Effective equations for isotropic quantum cosmology including matter
    Martin Bojowald, Hector Hernandez, Aureliano Skirzewski
    42 pages

    The NP letter is reference [29] in this paper. See for example pages 34 and 37
    ==quote==
    ...In such a situation, it is important to know the precise state,
    i.e. all parameters c1, c2, α1, α2, δ1 and δ2, in order to determine the quantum nature of
    the bounce. However, the relevant squeezing parameters cannot be fully determined from
    using the state at only one side of the bounce [29] due to exponential suppression factors
    of some of the integration constants in (29)–(32) if one restricts φ to a fixed sign. Thus,
    the precise quantum nature of the bounce may always depend on what assumptions one
    makes for a state...

    ...

    At first perturbative order, there is no strong back-reaction for a Wheeler-DeWitt quantization,
    or for most of a single collapsing or expanding branch in a loop quantization.
    But quantum back-reaction effects are noticeable for squeezed states around
    the bounce of loop quantum cosmology due to the influence of the growing branch on the
    collapsing one. Since squeezing parameters of a bouncing state are not determined by
    properties in one of the two large volume regime only [29], no general statement about
    the presence of a semiclassical bounce is available. There are quantum parameters whose
    influence is negligible at large volume but which will become important near a bounce and
    determine its fate.
    ==endquote==
     
    Last edited: Jul 2, 2007
  2. jcsd
  3. Jul 2, 2007 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    the letter is listed in the table of contents for the July issue
    http://www.nature.com/nphys/index.html
    but access is subscribers-only

    a science information service called Eureka Alert has this
    Penn State press release with a couple of interesting quotes
    http://www.eurekalert.org/pub_releases/2007-07/ps-whb062907.php

    I would attempt to filter out anything that is merely the journalist's interpretation, but the quotes from Bojowald himself could give some clue

    ==exerpt from PSU press release==
    The model's equations require parameters that describe the state of our current universe accurately so that scientists then can use the model to travel backward in time, mathematically "un-evolving" the universe to reveal its state at earlier times. The model's equations also contain some "free" parameters that are not yet known precisely but are nevertheless necessary to describe certain properties. Bojowald discovered that two of these free parameters are complementary: one is relevant almost exclusively after the Big Bounce and the other is relevant almost exclusively before the Big Bounce. Because one of these free parameters has essentially no influence on calculations of our current universe, Bojowald colludes that it cannot be used as a tool for back-calculating its value in the earlier universe before the Big Bounce.

    The two free parameters, which Bojowald found were complementary, represent the quantum uncertainty in the total volume of the universe before and after the Big Bang. "These uncertainties are additional parameters that apply when you put a system into a quantum context such as a theory of quantum gravity," Bojowald said. "It is similar to the uncertainty relations in quantum physics, where there is complementarity between the position of an object and its velocity -- if you measure one you cannot simultaneously measure the other." Similarly, Bojowald's study indicates that there is complementarity between the uncertainty factors for the volume of the universe before the Big Bounce and the universe after the Big Bounce. "For all practical purposes, the precise uncertainty factor for the volume of the previous universe never will be determined by a procedure of calculating backwards from conditions in our present universe, even with most accurate measurements we ever will be able to make," Bojowald explained. This discovery implies further limitations for discovering whether the matter in the universe before the Big Bang was dominated more strongly by quantum or classical properties.

    "A problem with the earlier numerical model is you don't see so clearly what the free parameters really are and what their influence is," Bojowald said. "This mathematical model gives you an improved expression that contains all the free parameters and you can immediately see the influence of each one," he explained. "After the equations were solved, it was rather immediate to reach conclusions from the results."
    ...
    ==endquote==
     
    Last edited: Jul 2, 2007
  4. Jul 2, 2007 #3

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    This article definitely looks interesting. I thought I was in luck earlier and would be able to log in "via athens" and read it-- sadly that didn't work! I'll be going into university tomorrow, though, and so should be able to read it there.

    That said, I know nothing about quantum cosmology! Hopefully, though, the fact that this is in nature should make it a little more comprehendable. I'll let you know how I get on!!
     
  5. Jul 2, 2007 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    another angle on Bojowald's recent results
    July 2007 Scientific American news item

    http://www.sciam.com/article.cfm?articleID=84DB833A-E7F2-99DF-3E5612DACCB0F990&chanID=sa007

    I think the uncertainty about some aspect of a prior contracting phase is overblown. Cup half full----in the model it seems that something pre-bounce is knowable, if not all. We have been coping with quantum uncertainty tradeoffs since Heisenberg was a postdoc---however it sounds to journalists it is not a curtain of mystery.

    but filtering out the SciAm dramatics, maybe there are a few more bits of the story here
     
  6. Jul 2, 2007 #5
    so no closer to a first cause then ?

    How about rewinding back to the big bang and starting over again ? Would identical initial conditions dictate the same universe we see now ?
     
  7. Jul 2, 2007 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    to give an operational meaning to "initial conditions" you have to be talking about measurements that someone could at least theoretically make

    he says he has uncovered a kind of "complementarity" between certain pre- and post- bounce quantities----like the complementarity between position and momentum described by Heis.uncert.princ.

    the more you try to pin down on one side, the more spread you get at the other side

    this doesn't apply to every kind of measurment, he just says he found one kind of measurment which you cant simultaneously be certain about, because of this complementarity tradeoff.

    I can't reply unequivocally to your question but I would say that from what I have heard the suggestions are that NO
    there is even in principle (even philosophically) no complete determinism IN THIS MODEL

    that is, there is no operational meaning to saying that a full set of prior-to-bounce conditions would 100% determine the post-bounce
    (the uncertainty devil somehow intervenes)
    ===============

    I also sense a lot of problems with the FIRST CAUSE idea, that you mentioned.
    Numerous people have disposed of it IIRC as a contradiction in terms.

    If there is a creation event then probably, as Newton suspected, it has to be OUTSIDE OF THE TIMELINE.

    in any case we don't have to worry about "first cause" ideas because they are not relevant to THIS MODEL which belongs to the philosophical sector in which time goes back forever---to "minus infinity"---without beginning (just as Newton's time did and the way it usually has in all of the European scientific tradition)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Bojowald's letter in Nature Physics (July 2007)
  1. New Bojowald LQC paper (Replies: 0)

  2. Icfi 2007 (Replies: 0)

Loading...