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

Quantum Gravity

  1. Nov 28, 2008 #1
    I have just read the book Three Roads to Quantum Grabity... but my mind has gone blank, the three methods are string theory (M-Theory), loop quantum gravity and one other. I cant remember the last :(:(

    Any help would be great.,..
  2. jcsd
  3. Nov 28, 2008 #2
    the third road is not simply defined...but is based on those who take an independent raod...via fundamental questions such as "what is time" 'What is space" ..such as Roger Penrose and his "spin networks" which I think can now be viewed also as evolving from some loop approaches...
  4. Nov 28, 2008 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The difficulty that caused the blankout was you said method instead of road.

    He does not present 3 already invented theories of QG. So you are not looking for a 3rd theory (to go with ST and with LQG).

    He presents 3 directions from which one might come to find an (as yet unknown) theory.
    And he gives examples of people approaching from each of the three directions.
    But in only two cases does the example involve a formal QG theory-under-construction.

    The third direction is exemplified by Bekenstein and Unruh, who thought deeply about horizons and temperature and entropy----they came to unexpected conclusions about space and time and information. Conclusions which are fundamental and do not depend on any particular theory setup or method or scheme.

    A. One can come at QG from General Relativity
    The example he gives is how he and his friends invented LQG, by taking seriously the ideas essential to General Relativity. (ideas like there is no fixed preordained geometry, geometry is the gravitational field itself, and that individual points are nothing but nametags given to particular relationships among events or among the field lines. Apart from relationships there is no space.)

    B. One can come at QG from conventional Quantum Mechanics
    The example he gives is the development of String theoretics. I should say that in both A and B the examples are just that, examples. He might have chosen some other QG method to illustrate either road. The aim is to describe how your thinking is influences by the road you come on, and the direction you come from.

    C. One can come at QG by asking original questions that transcend any particular methodology.
    The examples he gives are how Bekenstein and Unruh approached it. By unusual thought experiments they were able to conclude things about the temperature and entropy of a black hole and the temperature felt by an observer simply because he is accelerating.
    Another example is so-to-speak a holographic idea, that the information about what is inside a volume can somehow be represented on the surface surrounding the volume. As I recall he discusses that too.

    He is showing you three roads or three ways to approach this still unknown thing.
    He is not presenting three already formed methods for you to "buy". At least that is how I recall it. It is a fine book. It would be worth reading over again!
    Last edited: Nov 28, 2008
  5. Nov 29, 2008 #4
    thanks alot for the help, I get it now!! :)
  6. Nov 29, 2008 #5


    User Avatar

    I agree with Marcus last words on that book. More than anything I think it was an inspiring book, that sets the readers own imagination in spin. If I am not mistaken, this is one of Smolins objectives - to encourage and stimulate the reader to ask questions and think about these problems rather than to sell the reader one specific idea. Great book.

    I think the fact that he reasons generally about the problems, from different angles, gives a more honest illustration of the fact the book is about open problems. Perhaps this gives the reader the courage to at least _look for_ their own road?

  7. Nov 29, 2008 #6
    I thought this was a decent overview of Quantum Gravity which includes various approaches to QG by the Perimeter Institute of Theoretical Physics-

    http://www.perimeterinstitute.ca/en/Outreach/What_We_Research/Quantum_Gravity/ [Broken]
    Last edited by a moderator: May 3, 2017
  8. Nov 30, 2008 #7


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I think you are right about it being a decent overview. Thanks for the link!
    Last edited by a moderator: May 3, 2017
  9. Dec 1, 2008 #8
    so are you lot physics students? Or just have great interest in it...
  10. Dec 1, 2008 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    There is a wide range, ampatel. Some indicate they are young (under 20) studying physics for the first time. Some say they are are old (over 65) retired from various professions and simply interested observers of the research scene. And then amongst the rest there is a huge variety. We do have a few self-announced professional physicists---grad students, post docs, even some faculty who occasionally post here. On the other hand, many others do not say who they are or what their real-world connection to physics is, if any. One has to form an impression of the level of knowledge from reading the individual post.

    To me this great variety, with a challenging level of the unknown, is part of what makes the PF (PhysicsForums) community so interesting.

    I have no idea who you are, nor do you me, but I think you already contributed something interesting to us by bringing up the subject of that Smolin book: Three Roads. I hope you will bring up other interesing questions and topics in future.
  11. Dec 1, 2008 #10
    yea thanks for all the help... i'm going to study physics at degree next year!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook