Good elementary description of LQG

[mathman]

January 2004 issue of Scientific American contains a good elementary description of LQG. One point he makes in favor of LQG in contrast to string theory is that LQG does not require a background of contiuous space-time, while string theory does. Comments?

 

[marcus]

Originally posted by mathman
January 2004 issue of Scientific American contains a good elementary description of LQG. One point he makes in favor of LQG in contrast to string theory is that LQG does not require a background of contiuous space-time, while string theory does. Comments?

Correct, Loop gravity doesn't need any predetermined geometry. It is frequently constructed on a manifold but that manifold (continuum) doesn't need to have a pre-established metric or any specific shape for the construction to get started. So in that sense (actually a pretty important feature) the theory is independent of fixed metric background geometry. The geometry of space is left free to evolve dynamically.

this reflects the situation in the original 1915 theory of General Relativity, which is background independent and invariant under smooth mappings ("diffeomorphisms")

LQG is an attempt to preserve the essential features of GR while quantizing it----really just a quantized version of GR as close as possible to the original

Hope some others will reply. thanks for starting a thread on the SciAm article! I am just going out and cannot comment myself but I did read and thought it was pretty good. Wd be glad to see other's comments.

 

[alexsok]

Originally posted by marcus
Correct, Loop gravity doesn't need any predetermined geometry. It is frequently constructed on a manifold but that manifold (continuum) doesn't need to have a pre-established metric or any specific shape for the construction to get started. So in that sense (actually a pretty important feature) the theory is independent of fixed metric background geometry. The geometry of space is left free to evolve dynamically.

this reflects the situation in the original 1915 theory of General Relativity, which is background independent and invariant under smooth mappings ("diffeomorphisms")

LQG is an attempt to preserve the essential features of GR while quantizing it----really just a quantized version of GR as close as possible to the original

Hope some others will reply. thanks for starting a thread on the SciAm article! I am just going out and cannot comment myself but I did read and thought it was pretty good. Wd be glad to see other's comments.

Could you scan it perhaps m8? :)

 

[marcus]

Originally posted by alexsok
Could you scan it perhaps m8? :)

Actually I don't own a copy! I read it down at
the public library.

Does anybody else know if the article is online
somewhere? Or will it be online eventually at SciAm?

[edit, I checked again at sciam.com and all I could find online
is the first half-dozen sentences or so, as a teaser:]

-------quote from sciam.com-------
Atoms of Space and Time
By Lee Smolin

We perceive space and time to be continuous, but if the amazing theory of loop quantum gravity is correct, they actually come in discrete pieces

Little more than 100 years ago most people--and most scientists--thought of matter as continuous. Although since ancient times some philosophers and scientists had speculated that if matter were broken up into small enough bits, it might turn out to be made up of very tiny atoms, few thought the existence of atoms could ever be proved. Today we have imaged individual atoms and have studied the particles that compose them. The granularity of matter is old news.

In recent decades, physicists and mathematicians have asked if space is also made of discrete pieces. Is it continuous, as we learn in school, or is it more like a piece of cloth, woven out of individual fibers? If we could probe to size scales that were small enough, would we see "atoms" of space, irreducible pieces of volume that cannot be broken into anything smaller? And what about time: Does nature change continuously, or does the world evolve in series of very tiny steps, acting more like a digital computer?...continued at Scientific American Digital

---------end of quote--------

 

[alexsok]

Originally posted by marcus
Actually I don't own a copy! I read it down at
the public library.

Does anybody else know if the article is online
somewhere? Or will it be online eventually at SciAm?

Let me put it this way: are there some novel details down there, unseen previously anywhere else?

 

[marcus]

Originally posted by alexsok
Let me put it this way: are there some novel details down there, unseen previously anywhere else?

It is a non-technical overview for
wide audience and the value of that kind of article is
usually in
the examples and mental images chosen to make things seem
understandable and familiar.

For me the explanations were in many cases novel! The basic
ideas of loop gravity were presented in what (for me) was
a fresh and visually intuitive way.

But technically speaking I don't think there was anything new,
just a different way of describing known things for newcomers to the subject.

Maybe someone else will respond, or even type in some exerpts. I will have another look and maybe find some new detail to mention. I don't have any good way to scan the article but if you give us time I am sure more about it will become available----and maybe some bits and pieces. Sorry I don't have anything that immediately fills the bill.

 

[alexsok]

Originally posted by marcus
It is a non-technical overview for
wide audience and the value of that kind of article is
usually in
the examples and mental images chosen to make things seem
understandable and familiar.

For me the explanations were in many cases novel! The basic
ideas of loop gravity were presented in what (for me) was
a fresh and visually intuitive way.

But technically speaking I don't think there was anything new,
just a different way of describing known things for newcomers to the subject.

Maybe someone else will respond, or even type in some exerpts. I will have another look and maybe find some new detail to mention. I don't have any good way to scan the article but if you give us time I am sure more about it will become available----and maybe some bits and pieces. Sorry I don't have anything that immediately fills the bill.

That's just fine m8, ain't pushing no one :)
I think Rovelli's preliminary "Quantum Gravity" book pretty much covers the whole technical aspect of the theory :)

But nevertheless, it would be nice if someone could post some bits & pieces as you say, or excerpts from that article.

 

[MathematicalPhysicist]

i purchased this issue and read it, for my account it's short and explain about the main propeties of a discontious spacetime (ie area,volume) and it talks about spin networks.
the author of this article is lee smolin.