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What is Loop Quantum Gravity?

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Question: What is Loop Quantum Gravity?
Answer: Loop quantum gravity (sometimes abbreviated LQG) is a theory which seeks to express the modern theory of gravity (i.e. Einstein's theory of general relativity) in a quantized format. The approach involves viewing spacetime as broken into discrete chunks. It is viewed by many as the most well-developed alternative to quantum gravity outside of string theory.

History of Loop Quantum Gravity

The beginning of loop quantum gravity is usually tracked back to 1986, when Abhay Ashtekar developed a quantum formulation of Einstein's general relativity field equations. In 1988, physicists Lee Smolin and Carlo Rovelli extended this line of work, and in 1990 showed that gravity is quantized under this approach, which can be viewed using Roger Penrose's spin networks.

In short, the spin network approach from loop quantum gravity shows spacetime as a series of pieces that are connected to each other. This can be visualized by dots (or nodes) representing the pieces of spacetime connected by lines - in other words, spacetime can be viewed as a network of quantum nodes. The smooth spacetime framework explicit in general relativity is what you see when you "zoom out" from the quantum scale up to the larger scale.

Implications of Loop Quantum Gravity

As with all theoretical physics exploring this realm, the physics and mathematics involved at this level is extremely complex. There is much debate over the merits of loop quantum gravity, especially as compared to other approaches such as string theory.

Primarily, loop quantum gravity enthusiasts lift up three successes of this approach:

  1. It quantizes the 3-dimensional spatial geometry of general relativity.
  2. It allows for a calculation of black hole entropy.
  3. Instead of an infinite singularity at the moment of the big bang, the theory predicts a big bounce.
These are successes in the sense of being rigorous proofs within mathematical physics, because have yet been experimentally verified (and, in the case of the big bounce prediction, it likely never will be).

The black hole entropy prediction was (and is) viewed by many as the greatest success of the theory, because loop quantum gravity was believed to provide an explicit way to describe the quantum states of a black hole, and yielded results that matched with the predictions about black hole entropy made by Stephen Hawking and others in the 1970s.

Some theoretical physicists, such as Brian Greene in Fabric of the Cosmos and Lee Smolin in Three Roads to Quantum Gravity, have expressed the viewpoint that loop quantum gravity and string theory will ultimately show to be different expressions of the same underlying physical structure. The hope is that research in these two areas will ultimately give insights into each other, allowing for a more comprehensive fundamental theory that describes the basic quantum theory that would allow for a successful unified field theory that will completely reconcile general relativity with the standard model of particle physics.

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