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Penrose Lecture

  1. Aug 19, 2007 #1
    Greetings,

    In one of these threads, somebody commenting on the original thread provided a link to a Penrose lecture.

    I now cannot find which thread it was in.

    I'm also not sure - it was either in Genral Relativity or Cosmology threads.

    Does this ring a bell with anybody.

    What I'm looking for is the web address that has this lecture.

    I recall clicking on the web link and it started with what looked like a dry erase board full of notes.


    Thanks
    -Sparky_
     
  2. jcsd
  3. Aug 19, 2007 #2

    turbo

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  4. Aug 19, 2007 #3
    Turbo - 1
    Wow -

    no that's not the link but that is really good stuff.

    But that will keep me busy.

    I can keep surfing this site, I want to say the link was embedded in a discussion on the speed of light. I have gone through many of those threads in both the Cosmology section and General relativity section with no luck.

    I am hoping as members read this it will reasonate with somebody.

    -Sparky_
     
  5. Aug 19, 2007 #4

    George Jones

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    Are you thinking about Penrose's crunchless, cyclic universe?

    If so, three links were given in posts #32 and #33 here.
     
  6. Aug 30, 2007 #5
    warping of space

    hello!
    if u can plz help me, see if u can provide some information on warping of space, travelling through time in context of gravity
     
  7. Sep 4, 2007 #6

    cristo

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    I think you may have to be more specific. What level are you currently studying at. What have you heard that has made you interested to learn more? The question you ask is very, very broad, so it will help if you can narrow it somewhat.
     
  8. May 21, 2008 #7
    Can the Cyclic Universe mean Eternal Return?
     
  9. May 22, 2008 #8
    Are you refering to "The Nature of Space and Time" by Stephen Hawking and Roger Penrose which was quoted recently?
     
  10. May 22, 2008 #9

    marcus

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    I've never taken it to mean that, gorgos. But other people may. In the Penrose lecture that I suspect Sparky was asking about

    http://www.newton.cam.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/


    I didn't see any suggestion of Eternal Return. just the universe making a fresh randomized start over and over again. Maybe you could argue that if the possibilities are finite and it reboots infinitely often that eventually it would approximately (or even exactly?) repeat. But that seems so improbable one's tempted to ignore it.

    AFAICS people are a long way from proving empirically that the universe is cyclic in any sense. But they ARE making progress towards deciding if there was a bounce at the big bang.

    If there was a bounce, it could be a one-time event (infinite contraction followed by bounce followed by infinite expansion)------which would not be considered cyclic in the usual sense.
    So if it turns out the big bang was a bounce event, that still would not settle the issue about cyclic
     
  11. May 22, 2008 #10
    Just in case: Eternal return mean immortality.If Cyclic Universe can get scientific confirmation, it bring to cease-fire and armistice between Science and Religion.
     
  12. May 22, 2008 #11
    Nothing but proof in the exact word of the bible will create an armistice between science and religion. And as that does not seam very likely.

    If you disagree please explain but I can see no way that a cyclic universe will do anything to rectify the science and religion debate. I could never picture a good ole' southern baptist burning the Harry Potter books and then turning around and accepting a cyclic model of the universe.
     
  13. May 22, 2008 #12

    marcus

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    Hahahahah! That is a very funny thought. But GORGOS it would be constructive of you to create a link in philosophy forum or some more appropriate place and just post a simple link to it here.

    Actually sociology would be a good forum too, I think. Because an armistice between science folks and spokespersons for religion would actually be a SOCIOLOGICAL event. Whatever the logical merits, there is a human behavior issue of whether those people stop quarreling or continue.

    The point is that at PF we have an organization into forums that is fairly strict, for everybody's convenience. You can't continue, in a science thread like here, to discuss questions involving religion. Even if we LIKE the questions you raise, the mentors cannot allow it. So they have to move the thread, or lock it, or split it off and take part to the appropriate forum or something.

    What you need to do is put LINKS here to direct people to where you want to have a discussion about restoring harmony between science and religion. or whatever
     
  14. May 22, 2008 #13

    marcus

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    Just to keep on track, which is hard to do sometimes, I think this thread is nominally about PENROSE new cosmology idea. He gave this talk several places
    http://www.newton.cam.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/

    I even heard him give it live here at Berkeley. Same slides and everything. He gave it at Perimeter Institute too, as somebody pointed out.

    To the best of my understanding it is NOT CYCLIC in any familiar sense.

    I don't like Penrose idea but I have to report it as best I understand. I think it says that when our universe reaches its end of life, blank emptiness uniformly filled with cooling radiation, then it can spontaneously re-define itself as being in big bang mode. The trick is that once all matter has decayed into featureless radiation, you lose any ability to calibrate temperature. Uniform coldness is no different from uniform hotness.

    Penrose says in the title of his talk that the idea is outrageous and I do find that I am outraged if I think about it more than about two minutes. It doesnt work for me.

    I think George Jones has looked at this more carefully and may be able to correct me if I am wrong about this. But the way I see it, there is not even the faintest hint of an Eternal Repeat Performance in Penrose picture. The information in the mother is wiped out before the daughter can arise.

    The talk is a good one. It is not all about his outrageous idea. It has an excellent graphic treatment of entropy and the second law of thermodynamics, and a discussion of some other people's cosmology ideas. It is highly thoughtful and inciteful. It just drives headlong to strange conclusions. One can learn from it without accepting the conclusions.
     
    Last edited: May 22, 2008
  15. May 22, 2008 #14
    The information in the mother is wiped out before the daughter can arise.

    Who is the mother of the mother?
     
  16. May 22, 2008 #15

    marcus

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    heh heh, it sounds like the sort of question asked by people who are interested in religious or philos. issues rather than cosmology

    remember I am not telling you what I think. I am trying to discuss PENROSE idea as best I can (not being especially up on it)

    in the Penrose setup spacetime domains arise from prior domains that have reached a kind of uniform quiescent state

    so it is "mothers all the way back", or better said, there is no beginning, just an infinite sequence of mothers going back indefinitely

    ======================
    in scientific cosmology one does not ask what caused the whole thing to exist
    one takes existence for granted and tries to understand how it works
    and what possible alternatives are allowed by the system
    ======================

    Personally I am most interested in a different model. Not Penrose. It is a model that they are working on at Penn State in Ashtekar's group. In the version I'm thinking of, only one bounce occurs.
    There is a contracting phase which goes back indefinitely into the past. When the contraction reaches a critical density (where due to quantum effects gravity repels) contraction changes to expansion. The expanding phase is what we are in now, and it is due to continue indefinitely.

    The model is not intended to appeal to the imagination or to one's philosophical preconceptions. It is intended to be simple and to generate predictions of stuff we can observe at present-----about the microwave background etc.

    If it can be tested I shall be delighted either way it comes out-----whether it passes the test or is shot down. either way we win, because we learn something.

    There is tons more research about Ashtekar's model than there is about Penrose. So maybe other people feel the same way I do.

    If you want to see some of the research just do a keyword search at SLAC-Stanford with the keyword="quantum cosmology". Like this
    http://www.slac.stanford.edu/spires...+date+>+2005&FORMAT=WWW&SEQUENCE=citecount(d)

    this restricts to publication date > 2005
    and ranks the hits by how much the paper is cited in the literature (its rough importance)
    if you look at the top 20 hits, I guess none will be about Penrose model
    and the vast majority will be about Ashtekar's and the other Penn State group

    in cosmology where you have gotten rid of the singularity, theirs is the leading approach
    at least for the present :biggrin:

    if you want to engage with modern nonsingular cosmology (the approach getting the most attention these days) the quickest way is to get familiar with what that link pulls up and direct questions at it

    notwithstanding that Penrose is a brilliant creative mathematician
     
    Last edited: May 22, 2008
  17. May 22, 2008 #16

    Kea

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    Says who, Marcus?
     
  18. May 22, 2008 #17
    About Turok and Steinhardt Model.It seems to me it is not your favourite model.

    This my correspondence with Dr.Stennhardt:

    "Dear Dr. Steinhardt

    How about duration one cycle of Universe?

    We do not know for sure, but a reasonable guess is roughly 1 trillion
    years.

    Best regards,

    Paul Steinhardt"
     
  19. May 22, 2008 #18

    marcus

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    Bravo! great that you wrote Steinhardt, and also that he answered.

    About leading I should qualify that----what I mean is leading by the OBJECTIVE measures which everybody knows are imperfect but we can't do without.

    At the main international General Relativity and Cosmology conferences, whom do they invite to give the plenary talk about Quantum Cosmology?

    what group is publishing the most in peer-review journals and getting the most citations?

    We all know that these crude numerical measures don't tell the whole story. I tend to trust my own subjective judgement more than these objective measures. But rather than persuade you of my subjective opinion, what I meant in that case was the approach that is leading objectively, by the numbers.
     
  20. May 22, 2008 #19

    marcus

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    Maybe it wasn't clear that I based that on the Spires citation ranking. Let me repeat. From context it should be clear that I'm saying they're currently the leading approach for the objective reason that among other things they dominate the 20 topcited recent papers in Quantum Cosmology-----which for all practical purposes means nonsingular cosmology (models where you get rid of the singularity are mostly quantum these days.) It should be clear because I just gave the link right before:
    http://www.slac.stanford.edu/spires...+date+>+2005&FORMAT=WWW&SEQUENCE=citecount(d)

    If you want to get what I am saying before you respond, I urge go to that link and look for yourself what the topcited 20 papers are. Or the ten or however many you want to sample.

    Loop Quantum Cosmology will be the overwhelming majority of the recent topcite 20, or ten. whatever.
    (I checked, it is 9 out of the top 10, and 15 out of the top 20)


    It also has moved into lead position in the international conference scene.

    Notice however that any kind of measure like that could CHANGE :biggrin:.
    It is not an absolute or permanent thing. Next year it could be some other approach. Maybe one year is too soon but say 3 or 4 years from now.
    I'm indifferent to whether it is seen as good news or bad news. I'll report it as objectively as I can either way it goes.

    BTW Neil Turok was appointed Perimeter Institute Executive Director. I wonder if this will raise the stock of the Steinhardt-Turok ekpyrotic clashing brane cosmology model. That had been kind of fading from sight---will it make a comeback?

    If you go back to an earlier time period and do the same experiment, like in the late 1990s, you find stringy papers in the majority, as might be expected.
     
    Last edited: May 23, 2008
  21. May 23, 2008 #20
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