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Big Bounce

  1. Sep 18, 2008 #1
    Re the article by Bojowald in the October issue of Scientific American about Loop Quantum Gravity. He states that one of the consequences of the theory is that the speed of light is a function of wavelength. Any experimental evidence, or even hint of evidence, that anyone knows of?
  2. jcsd
  3. Sep 18, 2008 #2


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    Do you have a link for the article?

    This slight dependence has been discussed for several years and is estimated only to show up (if at all) with very high energy gammaray, like GeV or better TeV, and only after the photons have been traveling for on the order of a billion years. this gives the slightly faster ones a chance to get out ahead and arrive a little bit sooner, in theory.

    It is, according to what I have read, a possible consequence of SOME but not all QG models.
    (and not only Loop QG models, but some others). Maybe Bojowald has something clear and definite to say (I have not read the article) but to the best of my knowledge the situation is not clear.

    You ask about HINTS of evidence. Well certainly, there was the August 2007 publication of a paper by some 100 people where they claimed to have seen just the kind of delay that is predicted. They were the collaboration that runs a Gammaray telescope called "magic" located in the Canary Islands. German Spanish Italian astrophysicists mostly.

    they were observing a flare of TeV gammaray from an active galactic nucleus Makarian 501 (if I remember right) and the more energetic photons (they thought) came in a few minutes later than the others (they thought).

    Another gammaray telescope has tried to find some similar event but has not!

    Also I believe people are looking for this kind of thing in the gammaray burst data from the GLAST satellite----now renamed the Fermi Mission. It went into orbit earlier in 2008.
  4. Sep 18, 2008 #3


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    I got a link that worked to get the entire article, with illustrations, and FREE!

    there are 5 or 6 pages plus three pages of illustration.

    Better get it now if you are interested. It might start being pay-per-view in a few days.

    the title of the article by Bojowald is Big Bang or Big Bounce?

    It is the cover story for the October 2008 SciAm issue

    The headline on the cover is more catchy, it says "Forget the Big Bang!"

    Here's a quote from the lead of the article:
    Big Bang or Big Bounce?: New Theory on the Universe's Birth
    Our universe may have started not with a big bang but with a big bounce—an implosion that triggered an explosion, all driven by exotic quantum-gravitational effects

    By Martin Bojowald

    Key Concepts

    * Einstein’s general theory of relativity says that the universe began with the big bang singularity, a moment when all the matter we see was concentrated at a single point of infinite density. But the theory does not capture the fine, quantum structure of spacetime, which limits how tightly matter can be concentrated and how strong gravity can become. To figure out what really happened, physicists need a quantum theory of gravity.
    * According to one candidate for such a theory, loop quantum gravity, space is subdivided into “atoms” of volume and has a finite capacity to store matter and energy, thereby preventing true singularities from existing.
    * If so, time may have extended before the bang. The prebang universe may have undergone a catastrophic implosion that reached a point of maximum density and then reversed. In short, a big crunch may have led to a big bounce and then to the big bang.

    [then the main part of the article starts]

    Last edited: Sep 18, 2008
  5. Sep 18, 2008 #4
    Let me quote from the article on page 51 :"According to loop gravity, a light wave cannot be continuous; it must fit on the lattice of space. The smaller the wavelength, the more the lattice distorts it. In a sense, the spacetime atoms buffet the wave. As a consequence, light of different wavelenghts travels at different speeds."
  6. Sep 18, 2008 #5


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    The article is available here: http://www.sciamdigital.com/index.c...LEID_CHAR=43E0E374-3048-8A5E-10B6B56CC6F83B0F (though the full article doesn't appear to be freely available). [*]
    Bojowald only mentions this somewhat in passing at the end of the article. I'll paraphrase, for those of you without access: he basically says that since the space consists of a lattice in LQG, a light wave cannot be continuous. The speed of light is thus changed, with that of shorter wavelength being affected (by this restriction to the discrete lattice) most. This ties in with what marcus mentions above, regarding only the high energy light being affected by an amount large enough to be measure in future. He then points to a past sciam paper by William B. Atwood, Peter F. Michelson and Steven Ritz, which discusses using gamma ray bursts to try and see this effect.

    [*] Edit: Looks like you found a free version, and faster than I could reply!
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