Quantum Loop Gravity?

[l0rca]

What is this theory? What sort of predictions does it make? I've heard of it before, but I could never find a good explanation. I'm hardly sure what the 'loop' means. What sort of assumptions does this theory make? I get the impression that it's somehow superior to "String Theory" and other sorts of things.

 

[Cuddles]

The Wiki article is quite good. It is one of a set of theories that try to combine quantum physics with general relativity, and so come up with a theory for quantum gravity.

Unfortunately, like most thing involving the word "quantum", if you can't understand the maths behind it then there is really no sensible way of describing it. The term "loop" comes not from the idea that everything is made up of tiny loops, although this is a common misconception, it is simply the name of a certain type of mathematical quantisation. Depending on who you ask, QLG may not even try to be a ToE like string theory, according to many people it is simply a theory of gravity and does not describe matter at all.

Basically, as Wiki says :

In a nutshell, loop quantization is the result of applying C*-algebraic quantization to a non-canonical algebra of gauge-invariant classical observables.

If you can't understand that then you can't understand quantum loop gravity.

 

[l0rca]

(I'm bringing this thread back from the dead, kind of, mostly because the reply makes sense in the context of my kicking it, and aside from the principle of bumping threads, there's no practical reason I can think not to.)

I've been lightly reading about Quantum Physics recently, and I'm not sure if a certain understand I've been coming around to is actually correct, so I wanted to ask: in Quantum Loop, does spacetime emerge from the wavefunction, or does the wavefunction emerge from spacetime? I think that I'm clear that in the Standard Model, the Higgs-Boson particle is largely responsible for the emergence of spacetime, but I'm still not positive.

 

[Cuddles]

I've been lightly reading about Quantum Physics recently, and I'm not sure if a certain understand I've been coming around to is actually correct, so I wanted to ask: in Quantum Loop, does spacetime emerge from the wavefunction, or does the wavefunction emerge from spacetime?

Loop quantum gravity is the result of quantising a 4-dimensional space-time in a certain way. You need the spacetime before anything is available to be quantised.

I think that I'm clear that in the Standard Model, the Higgs-Boson particle is largely responsible for the emergence of spacetime, but I'm still not positive.

No. The Higgs particle (assuming it actually exists) is responsible for mass, in the same way that photons are responsible for electromagnetism and gluons are responsible for the strong force. Spacetime exists regardless of the particles and forces that exist within it.

 

[69dodge]

Loop quantum gravity is the result of quantising a 4-dimensional space-time in a certain way. You need the spacetime before anything is available to be quantised.

Is this a description of the historical process by which people developed the theory of loop quantum gravity, or is it a description of what that theory says about how nature works?

It sounds like the former, but I suspect l0rca was asking about the latter.

 

[l0rca]

No. The Higgs particle (assuming it actually exists) is responsible for mass, in the same way that photons are responsible for electromagnetism and gluons are responsible for the strong force. Spacetime exists regardless of the particles and forces that exist within it.

I ask because I read this on Wikipedia:

In some theories of particle physics, even such basic structures as mass, space, and time are viewed as emergent phenomena, arising from more fundamental concepts such as the Higgs boson or strings.

 

[fuelair]

http://relativity.livingreviews.org/Articles/lrr-1998-1/

http://www.newscientist.com/channel/fundamentals/mg19125645.800-you-are-made-of-spacetime.html

(1997 and 2006 summaries one by person in field other by Science sources.)

 

[Cuddles]

Is this a description of the historical process by which people developed the theory of loop quantum gravity, or is it a description of what that theory says about how nature works?

It sounds like the former, but I suspect l0rca was asking about the latter.

It is both. However, it's really more of a philosophical question than a scientific one. Does spacetime existing allow it to have properties, or do the properties cause spacetime to exist? If you ever manage to find a sensible answer to this, I have another about a chickens and eggs.

I ask because I read this on Wikipedia:

That's a bit misleading really. Mass is not a basic structure in the way space and time are. Mass is not really any different from something like electric charge, it is simply a property of things which exist within spacetime. While spacetime can arise from certain fundamental assumptions in some theories, mass just isn't in the same class. It's sort of like saying that asphalt gives rise to the fundamental structure of roads, therefore cars give rise to velocity.

 

[l0rca]

It is both. However, it's really more of a philosophical question than a scientific one. Does spacetime existing allow it to have properties, or do the properties cause spacetime to exist? If you ever manage to find a sensible answer to this, I have another about a chickens and eggs.

I agree it's a philosophical question. I would say that certain very basic forms of phenomena (which could loosely be described as matter) give rise to spacetime, or manifolds in general. For instance, in Quantum Loop, we sometimes talk of different inter-connected webs of spacetime, which through different energy-emergences, we get matter. I'm actually sort of writing a paper about this, from a materialist perspective, where random, generally quantum-sized phenomena could give rise to manifolds through a sense of "metaphysical selection," where chaotic emergence with energy patterns with the most viable form of replication and other complimentary properties would be more capable of dominating virtual space, or at least eventually giving rise to higher forms of matter.

That was probably really redundant, but I'm being rushed at the moment....

 

[INRM]

What's a Higgs Boson?

 

[arthwollipot]

What's a Higgs Boson?

The last particle of the Standard Model yet to be isolated. There are hopes that the new Large Hadron Collider might do so when it comes on line (sometime this year, I had heard).

The Higgs boson is the particle which is responsible for giving matter mass. Apparently it is supposed to impart inertia to a particle by dragging on surrounding spacetime or something.

I don't follow the full story since I'm not a high-energy physicist.

 

[Cuddles]

What's a Higgs Boson?

Basically what Arthwolipot said. It's a particle predicted to exist that is responsible for causing mass, similar to how gluons are responsible for the strong force. However, it's not really a standard model particle, since the Higgs boson or its equivalents are predicted by almost every seriously considered theory.