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De Sitter Relativity: a New Road to Quantum Gravity-Aldrovandi and Pereira take risk

  1. Nov 17, 2007 #1


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    de Sitter Relativity: a New Road to Quantum Gravity--Aldrovandi and Pereira take risk

    Aldrovandi and Pereira are at the ITP at Sao Paolo State University.

    They have taken a risky step and proposed to modify General Relativity so that it should be locally de Sitter instead of locally Lorentzian (or Poincaresque). What they get are two results that look too good to be true (and I am sure they know this, they are not novices.)

    They get a reasonable value of the cosmo constant. And what is even more risky: they retrodict the high energy GAMMARAY DELAY INITIALLY FOUND BY the new Canary Island Cherenkov telescope called "MAGIC".

    MAGIC is a good consortium of excellent German Italian Spanish etc astrophysicists and they have an excellent instrument, currently the largest optical dish in the world. they are 130 people and they have published and delivered many respectable papers. But in August this year, they claimed to have observed a 4 minute delay in some 2 TeV photons coming from Makarian 501. This could very easily be a WRONG OBSERVATION and their announcement was strongly condemned by vocal physics bloggers. The commentators disapproved of MAGIC for publishing on the basis of just one observed flare event and also for taking Ellis and Mavromatos on board, who mentioned there might be some QUANTUM GRAVITY EXPLANATION. The commentators opined that Ellis and Mavromatos were premature and that the consortium should have waited and observed more flares. Above all they should not have mentioned a possible QG implication.*

    Now Aldrovandi and Pereira have gone and calculated that there SHOULD HAVE BEEN THIS DELAY because of the deSitterness of spacetime. This could bring disapproval down on their heads and even ridicule. What if MAGIC repeats the observation with another flare, and doesn't find the delay the next time???!!!

    then A & P will look foolish. And they surely know this because they are IMHO the two top theoretical physics/cosmology people in Brazil. I have been watching their papers and I think they are tops and they are obviously seasoned mature research people, not newcomers.

    So this was a deliberate calculated step to take. Evidently they really believe that you should modify Gen Rel to be deSitter and that this will give you numbers for the cosmological constant (which nobody else can explain) and for the MAGIC 4 minute delay (which almost nobody else seems to believe.) I have to respect this. All we can do is watch and see what happens to this idea.

    de Sitter Relativity: a New Road to Quantum Gravity
    R. Aldrovandi, J. G. Pereira
    17 pages
    (Submitted on 14 Nov 2007)

    "The Poincare' group generalizes the Galilei group for high-velocity kinematics. The de Sitter group is here assumed to go one step further, generalizing Poincare' as the group governing high-energy kinematics. Algebraically, this is done by supplementing spacetime translations with proper conformal transformations. This change in special relativity implies concomitant changes in general relativity -- yielding a de Sitter general relativity. The source current turns out to include now, in addition to energy-momentum, the proper conformal current, which appears as the origin of the cosmological constant. In consequence, it is no longer a free parameter, and can be determined in terms of other quantities. When applied to the propagation of ultra-high energy photons, de Sitter general relativity gives a good estimate of the time delay observed in extragalactic gamma-ray flares. It can, for this reason, be considered a new approach to quantum gravity."

    *I didn't agree with all the scolding that went on in the blogs. I was glad MAGIC published the paper, which contained interesting information.
    Last edited: Nov 17, 2007
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  3. Nov 17, 2007 #2


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    I believe this paper deserves our focused attention.
    And so request posters NOT to introduce links to other papers that you think might be somehow related to this one, or otherwise clutter this thread with links and comment not focused on this paper.

    I'm more than usually concerned about thread getting distracted here, so if you have some other ideas and papers to mention, please start another thread about them rather than bringing them up here.

    Aldrovandi and Pereira do a couple of remarkable things in this paper that I want to point out right away.

    THEY GIVE US A NEW EQUIVALENCE PRINCIPLE. (last paragraph on page 3)

    THEY GIVE US A NEW EINSTEIN EQUATION. (eqn #27 on page 6)
    General Relativity always impresses one as a real theory, because it is based on definite principles and has a main equation. There are frameworks and philosophies nowadays that don't have a comparable degree of definition---aren't derived from a small number of principles and don't have equations (like newtonian mechanics has, for instance). So this is a valuable aspect of GR. What they are telling us is that their new GR is ALSO like that.

    The paper is in a sense very easy to read, at least substantial parts of it. It has a straightforwardness and simplicity that i like very much.

    The NEW EQUIVALENCE PRINCIPLE is simply the old equivalence principle with the word deSitter replacing Poincare.

    In the presence of a gravitational field, it is always possible to
    find a local coordinate system in which the laws of physics reduce to those of a de Sitter
    special relativity.

    Their NEW EINSTEIN EQUATION is also very simple. It is just the usual one including a Lambda term on the left as you have to with nonzero cosmo these days, PLUS on the right there is a new matter term (the conformal current K) which is added in with the usual energy momentum T term. I don't want to have to do the LaTex, it is just equation (27).

    They have a nice thing in equation (32) which is ANALOGOUS TO THE FINE STRUCTURE CONSTANT alpha.
    They call it alphag------the gravitational alpha.
    Last edited: Nov 17, 2007
  4. Nov 18, 2007 #3


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    I'm worried about their derivation of the value of the cosmological constant.
    At best it only seems to get within an order of magnitude, unless you make some unexplained choices of estimates which I do not as yet understand.

    This is on page 9.

    Then on page 10 they begin exploring why there could have been that delay in gammaray arrival from Makarian 501 (if it was not just a fluke.)

    On page 10 they start a new section Photon kinematics in deSitter relativity

    got to keep remembering that it is deSitter GENERAL relativity they are talking about---not just DSR (deformed special relativity) that we are used to hearing about.

    there is a new element in the picture called the conformal current which appears to have the physical units of force (energy per unit length) and be related to the expansion of space.

    It would be nice to have someone else's perspective on this paper.

    Unless someone who's faster at LaTex than I is willing to, I will transcribe the new Einstein Equation in LaTex, since it might be useful to have in this thread. This is their eqn (27) on page 6

    [tex]R_{(T)\mu\nu} - \frac{1}{2}g_{\mu\nu}R_{(T)} - \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}(T_{\mu\nu} - \frac{1}{l^2}K_{(\mu\nu)})[/tex]
    Last edited: Nov 18, 2007
  5. Nov 18, 2007 #4


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    If MAGIC repeats, the most important observation will be the correlation of the length of the delay to the redshift of the emitting body. If the delay is proportional to the redshift of the source, then Fritz Zwickey's "tired light" idea needs a new review.
  6. Nov 18, 2007 #5
    Not a great fan. If there is a fixed length scale, shouldn't it be plancks length, and not some cosmological constant derived length. As for the observation, yeah, physicists often embrasses themselves building theories on uncomfired observations, rembemer PVLAS. But its the early worm who'll get the Nobel prize, so you can hardly blaim them.
  7. Nov 18, 2007 #6


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    Fraid we have to have the lengthscale l (not Planck) because it is built into deSitter space.
    In practice it is a large length on the order of 10 billion LY, as I recall. It's called various things like the radius of deSitter space.

    it is closely related to the cosmological constant Lambda, which is the reciprocal of an area. If you look at the usual Einstein equation, in which Lambda was first defined, you see by its units that Lambda has to be one over a length squared.

    And in the case of the simplest solution with positive Lambda---case where there is no matter at all---that length has to be the deSitter radius (what we are writing l).

    I wish lowercase l did not look so much like numeral one 1. Maybe we need a different font, or italics, just for that one symbol.
  8. Nov 18, 2007 #7
    Aldrovandi and Pereira have cojones, that's for sure. I don't want to jump the gun and say they're right or wrong; although I am a proponent of Lorentzian GR and I don't very much like the idea of the DeSitter universe, they do have a very compelling argument.

    There are some shaky spots to this paper, first off the way they found their cosmological term puzzles me a bit. Plus, their cosmological term has some implications that I don't believe were addressed. I don't have a couple hours to read this paper word for word (when I do, I'll post here again), so I apologize if I missed anything.

    As a little disclaimer, this may be completely wrong because I'm bad at physics:

    1. Dark energy density = 2 x energy density, this doesn't seem right to me, I can't recall the specific instances, but are there papers which say otherwise? Even though they do say it's just for dust, this is just a tough thing for me to grasp.

    2. Supersymmetry and some theories in that field require a cosmological constant of exactly 0. How does this paper address this?

    3. Since it's positive, that means there's a finite amount of entropy the universe can undergo. (Because of the holographic principle)

    4. "in spite of the rough assumptions made — is quite close to the observed value." Whenever I see words like "rough assumptions" or "rough estimate" I always look for some other sources of error. Their order of magnitude may be right, since the constant is very small, but there just isn't enough testing to convince me that it's right or wrong. After further observations from MAGIC and discussion about this paper we will find out.

    More on this later as I read deeper into this paper.

    As a side note:

    In recent news of people "jumping the gun", there's already work being done with E8 theory. Posting more about this for conversation on the thread we have for it so I don't derail this one.
  9. Nov 19, 2007 #8
    Does anyone know when MAGIC will be repeating their measurements?
  10. Nov 19, 2007 #9


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    I don't. they may have already collected more data and be keeping quiet about it.
    they could also just be taking their time analyzing it. I totally don't know and feel some suspense and frustration.

    I wonder if Aldrovandi or Pereira has heard anything.

    What you said about it took balls to post a paper like this...I'd definitely agree without question.

    I wonder if it would be appropriate to write the contact person of the MAGIC group and ask.
    It would say on the August 2007 paper who the contact person is for questions regarding that paper.

    I guess we may as well know more detail about the authors
    Jose Geraldo Pereira and Ruben Aldrovandi are at the Institute of Theoretical Physics at Sao Paolo
    The generic acronym for that is "ITP" but in Portuguese it is IFT Instituto Física Teórica
    Here are Pereira's 79 papers from the SPIRES database
    I see he has one with 100+ cites and many with 25+ cites.
    Aldrovandi was Pereira's PhD thesis advisor. He is the senior of the two.
    Here are Aldrovandi's 76 papers from the same database

    I admit these are kind of mundane details but it helps to get to know the people.
    As I recall Pereira was the organizer and welcome speaker at some particle physics conference that Christine told us about where there were several international speakers including Lee Smolin. He has some stature in the Brazil theory community.

    He has published a book co-authored with Aldrovandi, as well as those many articles.

    I see Physical Review Letters, Physical Review D, GRG, CQG, among the journals he has published in.

    Here's a picture!

    Born 1953.
    Postdoc at Center for Relativity University of Texas Austen---very good place.

    E-mail: jpereira*ift.unesp.br.
    Institute webpage: www.ift.unesp.br/users/jpereira/

    I sense the guy's weight and it makes me more interested in this paper than before---but I was already pretty interested.
    Last edited: Nov 19, 2007
  11. Nov 20, 2007 #10


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    Disappointing news. A friend of mine communicated with someone who ought to know (not anyone in the MAGIC collaboration but who would be in contact) and the word is that there is no new data as yet. So one should not be expecting a new paper any time soon that would help resolve the issue (one way or the other.)

    I don't know about you but I find this frustrating and discouraging---I'd like to see the question settled one way or the other.

    It must be tough on the two authors A & P waiting it out.
    Last edited: Nov 20, 2007
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