# What is the basic idea of loop quantum gravity? [duplicate]

I have researched a little on loop quantum gravity (LQG), but all I got were very ambiguous explanations about loops and nodes that all end with mathematical equations.

If someone could give me a non-mathematical, clear idea of LQG, I would really appreciate it.

• You can explain it to your granny, according to this duplicate: physics.stackexchange.com/q/2407
– user108787
Commented Jul 14, 2016 at 20:53

Loop quantum gravity (LQG) is an attempt to unify general relativity and quantum mechanics. LQG attempts to treat gravity as a feature of spacetime as it is in general relativity, rather than as a force. The output of the theory is a picture of spacetime where space is granular (as a result of the quantization). Just as light can only be emitted in packets (quanta) space can only be traveled through by a minimum distance (in simpler terms, there is a minimum distance one can travel; you cannot travel an infinitely small distance). This minimum distance is the Planck length ($10^{-35}$ meters).

Space is considered a very fine fabric, "woven" of tiny, finite loops that are only visible at the Planck length. The networks of loops that make up space are called spin networks, and the evolution of these networks over time is called spin foam. Spin networks represent the quantum state of the gravitational field. An image one can have is that every "node" in the spin network (a node is equivalent to a loop) is an elementary quantum of volume and every link between the nodes is a quantum area surrounding this volume. Spin networks quantitize quantum geometry, and spin foam does the same thing for spacetime. (There is more information about spin foam here and more information about spin networks here.)

Therefore, LQG predicts that space itself has an atomic structure. LQG is developing in two main directions: canonical LQG and covariant LQG, also known as spin foam theory. There are about 30 research groups worldwide working on both types of LQG. LQG has been applied to cosmology in what is known as loop quantum cosmology. This has applied LQG ideas to the early universe and the Big Bang, its most significant implication being that the evolution of the universe can be continued beyond the Big Bang in what is called a Big Bounce (you can learn more about loop quantum cosmology here).

Let me know if there's anything specific you wanted to know about that I haven't addressed, though I think I've given a decent overview here. This website is very helpful and goes more in-depth into LQG.

Hope this helps!

• Its a very nice summary, and it passes the granny test :). You might do a quick edit and change all the LGQ to LQG. I read Smolins book on LQG, ( 3 Roads to QG) and to me at least, he does not explain how long the minimum distances take to travel. i.e. do you cross at normal light speed. I assume you do, but I still am not sure if that's part of the theory, (at the Planck scale anythings possible) but I will read up myself from the links in your post. Thanks.
– user108787
Commented Jul 15, 2016 at 0:52
• @count_to_10, thanks for catching that mistake; I've changed all the LGQs to LQGs. I can look into your other question. Commented Jul 15, 2016 at 12:43
• Thanks heather, no need to take up your time. I self study, so on the way to finding something out, I usually discover a 100 other (basic) things I should know. Regards
– user108787
Commented Jul 15, 2016 at 13:20