I posted a few comments on Lubos post here:
https://www.physicsforums.com/showthread.php?p=272083#post272083
right now it just looks like Lubos post, "Re:Penrose critique of string theory" is simple animosity.
But we can just wait and see, maybe Lubos is in fact connecting with something real and Penrose
has made some serious criticisms.
(I just couldn't find any evidence of that, no quotes or page references)
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In the meantime, until we learn more about that, let's look at the
issue of Penrose thought as it relates to Loop Gravity!
Way back when they were just getting started Rovelli and Smolin
found they could use the "spin networks" idea of Penrose. I've forgotten when that was, around 1990 was it?
But that's ancient history and Penrose has made a whole zoo of creative mathematics which Hawking has used to do his thing, and other people have used to do their things. If Penrose is a "grandfather" of LQG well that is just because he is a general purpose grandfather of much late-20th century development.
I don't see him as partisan to LQG or biased in its favor particularly or even having had a big influence on Smolin----I guess he is stimulating company and the Loop people have benefited, along with others, with lots of conversations. But I wouldn't make a big point of the influence.
What I see now as a major impact of Penrose on LQG is something nobody has mentioned----and i could be wrong of course, very likely am wrong---which is his invoking the
second law of thermodynamics to challenge Bojowald's LQC story of a bounce.
If there was a contracting phase that would have had a lot of entropy and then there was this
cross-over from contraction to expansion, which takes the place of the old classical big bang singularity, and expansion begins with the gravitational field and matter density spread out rather evenly and everything very dense and hot-----and right after crossover the universe is supposed to have very
low entropy.
Now entropy is supposed to increase. so at the classical bigbang ex-singularity the entropy is supposed to be the lowest it has ever been. And entropy has been increasing for 13.7 billion years till now.
In his 2nd Princeton talk, titled "Faith", Penrose challenges the bounce idea on this basis. How could you have a collapsing U with high entropy and then suddenly, after a brief quantum muddle, come out with an expanding U with very low entropy.
http://www.princeton.edu/WebMedia/lectures/
(scroll down to October 2003)
Dont take my word on this! Listen to the 2nd Princeton lecture! Or probably you can read the same argument in the book, which I haven't seen yet.
My attitude is challenges to strong people (like Bojowald) are good and I think this is a serious challenge. Please correct me if you know better.
I think something interesting may come out of it.
So this, curiously enough, is how I see Penrose impact on Loop right now.
the 2nd Law is in a class of Eternal Physics, for me. And the LQC bounce---getting rid of the BB singularity---is one of the most important features of the Loop scene. And Penrose is someone who has thrown down a challenge
to this bounce, based on Eternal Physics. This is very much to my taste as a watcher. How much better can it get?