Loop Quantum Gravity

[Tim Tyler]

Michael Price directed my attention to Loop Quantum Gravity (LQG).

Apparently it's a discrete theory - or is about as close to being one
as conventional mainstream physics has got for quite a while, anyway.

In particular it divides the continuum of space up as follows:

``In LQG, the fabric of spacetime is a foamy network of interacting loops
mathematically described by spin networks. These loops are about 10^-35
meters in size, called the Planck scale. The loops knot together forming
edges, surfaces, and vertices, much as do soap bubbles joined together.
In other words, spacetime itself is quantized.''

- http://en.wikipedia.org/wiki/Loop_quantum_gravity

I also liked this bit about the Bekenstein bound:

``LQG's interpretation of black hole entropy is that the spacetime fabric
that make up the black hole horizon is quantized per Planck area, and
the Bekenstein-Hawking entropy represents the degrees of freedom
present in each Planck quanta. LQG does not offer an explanation why
the interior of the black hole carries no volume-extensive entropy.
Instead, it assumes that the interior does not contribute. The
spacetime is truncated at the event horizon, and consistency requires
to add Chern-Simons theory at the event horizon. A calculation within
Chern-Simons theory leads to the desired result for the entropy,
proportional to the horizon area.''

"The interior does not contribute" - quite right ;-)
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[Lubos Motl]

On Mon, 18 Oct 2004, Tim Tyler wrote:

> I also liked this bit about the Bekenstein bound:
>
> ``LQG's interpretation of black hole entropy is that the spacetime fabric

> that make up the black hole horizon is quantized per Planck area, and ...

Thanks, it was my text.

> "The interior does not contribute" - quite right ;-)

An important point of that text was to say that there is no justification
for this assumption in loop quantum gravity. The entropy of black holes
scale with their "surface", and most of us believe that in some sense it
means that the interior does not contribute to the entropy. But such a
belief is very different from a derivation, and loop quantum gravity must
rely upon the belief. Its derivation has as much output as much input we
inserted.

Loop quantum gravity in general should belong to this newsgroup - it is
also an example of a biased approach that "everything must be discrete in
the most naive way - made of 'atoms'".

See
http://en.wikipedia.org/wiki/Objections_to_the_theory_of_loop_quantum_gravity

for critical comments about loop quantum gravity.
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