John Baez: LQG Talk Online: Quantum Geometry, Spin Foams & Black Holes

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In summary, John Baez gave a talk on loop quantum gravity, quantum geometry, and spin foams. The talk is available online (PS format) and is a tutorial with lots of pictures and good explanatory language. It is one of the best introductions to LQG and allied QG approaches that I've seen and the math is there but in its most basic and intuitive form. The slides are not too abbreviated to read discursively and the summary given here says part of it but does not indicate how conceptual and pictorial the talk actually is. Check it out!
  • #1
marcus
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A week ago in Dublin John Baez gave one of the plenary talks

the title was:
Loop Quantum Gravity, Quantum Geometry and Spin Foams

this talk is now available online (PS format) from Baez website

http://math.ucr.edu/home/baez/lectures.html#lqg

click on where it says:
"You can see the transparencies of this talk:... - Postscript version."

the talk is a tutorial with lots of pictures and good explanatory language

It is one of the best introductions to LQG and allied QG approaches that I've seen---the math is there but in its most basic and intuitive form.

The slides are not too abbreviated to read discursively---some care was taken. The summary given here says part of it but does not indicate how conceptual and pictorial the talk actually is. Check it out!

"The last few years have seen the techniques of loop quantum gravity applied to a growing number of problems. They have been used to compute the entropy of various sorts of black holes, including those with non-minimally coupled matter, for which the entropy is not proportional to the area. Perhaps more importantly, loop quantum gravity has given us a clear picture of the quantum geometry of the horizon, which accounts for the microscopic degrees of freedom responsible for black hole entropy. There are tantalizing connections to Hod's work on quasinormal modes, but these remain mysterious. Combined with traditional ideas on quantum cosmology, loop quantum gravity has led to new insights on how quantization can eliminate the singular behavior of geometry at the big bang - perhaps with testable consequences. Predictions of Lorentz symmetry violation may also be experimentally testable, but these remain controversial. Finally, the dynamics of the theory is being studied with the help of spin foam models. My talk will survey all this work with a bare minimum of technical details."
 
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The smallest allowed area is

[tex]8\pi\gamma\sqrt{\frac{1}{2}(\frac{1}{2} +1)} = 5.17004[/tex]

In what units? :wink:
Planck of course

so the Planck area isn't the smallest measurable area, 5.17 is, instead

this is with the latest Immirzi---credited to Domagala, Lewandowski, and Meissner. A nice thing about Baez shortsweet tutorial on LQG is that it is up to date. Field is moving so fast that tutorials get old after a year or so.

Surveys latest LQG calculations of Black Hole entropy---in cases where it is NOT just A/4, but they calculated it anyway. And reproduce the semiclassical results in all cases with the same Immirzi.

Also fairly up to date report of Bojowald and others LQ Cosmology results.
 
  • #3
marcus said:
A week ago in Dublin John Baez gave one of the plenary talks
baez is super cool!
i don't understand any of this stuff (yet), but his site is really worth hanging out in even if only for the ambience!

his crackpot page is hilarious and for someone like me who is trying to get back into physics after a long, long absence all his advice on books, ideas, physics, math etc etc etc is just extremely inspirational (as is pf)!

this is really a great place:
http://math.ucr.edu/home/baez/

in friendship,
prad
 
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  • #4
POST MORE BAEZ LINKS

right on, phirst!
 

1. What is LQG and how does it relate to quantum geometry?

LQG, or loop quantum gravity, is a proposed theory that attempts to reconcile general relativity and quantum mechanics by treating spacetime as a collection of discrete loops. It is closely related to quantum geometry, which studies the quantum properties of geometric structures such as space and time.

2. What are spin foams and how do they fit into LQG?

Spin foams are a mathematical tool used in LQG to describe the quantum states of spacetime. They are 2-dimensional structures that represent the interactions between the discrete loops that make up spacetime. They are an essential part of the LQG framework and are used to make predictions about the behavior of spacetime at the quantum level.

3. How does LQG address the issue of black holes?

LQG provides a unique perspective on black holes by treating them as objects with discrete quantum properties, rather than singularities with infinite density. It also suggests that black holes may have a discrete surface area, and that the information that falls into a black hole may be encoded on this surface. LQG offers a potential solution to the black hole information paradox, which has been a longstanding problem in physics.

4. What are the main challenges facing LQG as a theory?

One of the main challenges facing LQG is its lack of experimental evidence. As a theory that attempts to unify general relativity and quantum mechanics, it is difficult to test in current experimental settings. Additionally, there are still many mathematical and conceptual issues that need to be resolved before LQG can be considered a complete theory.

5. How does the concept of quantum geometry impact our understanding of the universe?

The concept of quantum geometry challenges our traditional understanding of space and time as continuous, smooth entities. It suggests that at a fundamental level, the fabric of the universe is made up of discrete, quantized units. This has implications for our understanding of gravity, the behavior of matter at the quantum level, and the structure of the universe as a whole.

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