Loop Quantum Gravity: Understanding How Gravity Occurs

In summary, Tyger posted a thread proposing a simplified system of dimensional units that incorporates quantum mechanical phase, length, time, and plane angle. Another user responded by saying that all of these units may not be necessary. In a separate thread, Tyger asked why gravity occurs in the world. Another user responded by discussing a theory that suggests gravity is a result of maintaining a constant Volumetric Rate of Expansion, with the dimensions l3/mt. This theory proposes that gravity couples to a more basic quantity, quantum mechanical frequency, and that the rate of expansion is l3/Ω, where Omega is quantum mechanical phase. The conversation then turns to the topic of loop quantum gravity and its potential as a background independent model of quantum geometry.
  • #1
Tyger
398
0
I've posted a thread related to a simplified system of dimensional units using quantum mechanical phase, a length, possibly a time and plane angle;

You don't need all that junk.

and another about why gravity may occur in the world:

Why does Gravity occur?

Let's see what happens when we "marry" the two sets of ideas.

The second post says that Gravity happens because nature tries to maintain a quantity called the Volumetric Rate of Expansion, constant. In ordinary units it has the dimensions l3/mt but if we suppose that gravity couples to a more basic quantity than inertia, namely quantum mechanical frequency, then in the new units the rate of expansion is just l3/Ω, where Omega is quantum mechanical phase.

This just means that whenever quantum mechanical phase increments by an amount, say 2[pi]radians the volume of the Universe increases by approx. the cube of one classical electron radius.

What do you loop quantum gravity people think about this?
 
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  • #2
Originally posted by Tyger
I've posted a thread related to a simplified system of dimensional units using quantum mechanical phase, a length, possibly a time and plane angle;

You don't need all that junk.

and another about why gravity may occur in the world:

Why does Gravity occur?

Let's see what happens when we "marry" the two sets of ideas.

The second post says that Gravity happens because nature trys to maintain a quantity called the Volumetric Rate of Expansion constant. In ordinary units it has the dimensions l3/mt but if we suppose that gravity couples to a more basic quantity than inertia, namely quantum mechanical frequency, then in the new units the rate of expansion is just l3/Ω, where Omega is quantum mechanical phase.

This just means that whenever quantum mechanical phase increments by an amount, say 2πradians the volume of the Univers increases by approx. the cube of one classical electron radius.

What do you loop quantum gravity people think about this?

Hello Tyger, you clearly have a theory here about GRAVITY as distinct from geometry. Because my interest is so much towards quantum geometry I cannot find much to say.

I am most interested in theories that suggest something about how the curvature of space evolves---how geometry itself arises.
these are the "background independent" models of geometry that don't begin by assuming a static spatial framework for things to happen in.

String theory has never seemed very interesting or promising to me for this very reason. It begins by a committment to a static, un-dynamic geometry: standard Minkowski space!. It can't seem to get started without it. So I don't think it has much chance of being valid. Nature is not flat unexpanding space of special relativity in which light always goes in straight lines. Naive to expect that kind of approach to work.

I think it is pretty clear that "gravity is geometry" and that a quantum theory of gravity must be a quantum geometry and must start with no prejudiced preconceived notion of the shape of the universe. Particularly true for *quantum cosmology*!

Ashtekar calls "Loop Quantum Gravity" by the more accurate name of Quantum Geometry---it seems more descriptive to me.

Right now the field is hot and has been having a run of surprising results. Quantizing area/volume. Predicting BH entropy and/or vibration frequency. Eliminating matter field divergences----getting rid of the need for QFT renormalizations. And most recently overcoming the Big Bang singularity----loop quantum cosmology goes all the way back to time zero without blowing up and has spacetime dissolve into something else for negative time.

Loop quantum geometry is not merely offering an explanation of the "force" of gravity or some schematic way of relating the "force" of gravity to some other forces---in the fixed arena of a static space. It gives equations for the evolution of curved and expanding space and it predicts interesting numbers----and it is on a roll right now.

So while its predictions may be tested experimentally and eventually proven incorrect and some refinement or alternative to it (like spin foams in some as yet unformulated version) prove superior, it is currently the most interesting background independet model of quantum geometry in my view. By far.
Cant think of any alternative. No real competition as far as I know.

I take a skeptical view of the truth of theories that are largely untested and still under development. so I am not a "believer" of any description. So I can't speak for the "loop" people whose opinion you asked.

But I can react as someone who expects a background-free quantum geometry of SOME type to be the next major development in physics.

Such a thing must be developed, I believe, before there can be any valid successor to the "Standard Model" because today's particle fields are only defined in a flat preconceived space. So we have no acceptable theories of particles or forces----only something very ad hoc.

So imagining a valid "TOE" defined in unvarying flat space is premature and naive.

Your theory has some appeal though----at least it works without postulating invisible extra dimensions conveniently rolled up so one doesn't trip on them. You could learn something by playing aorund with it.

I should tell you that the Planck unit of energy density (same as pressure, same as momentum flux) is guess what

c7/G2hbar

seems to be energy density (and its look-alike pressure) that
determines curvature. looking at the dimensions may suggest how this works to someone, if not to me. You have been doing a lot of dimensional reasoning I think----where your current model of gravity came from.
 
  • #3
The said faction of space that supports the unit, will either be in expansion or contraction, the Varying Volume/Length evolves as a two-way traffic system.

Alpha moves to Omega as Omega moves to Alpha? :wink:
 
  • #4
The Big Bang scenario

says that matter existed before space, as we know it, appeared in the Universe. If that is valid then the correct description should start with matter and with the passage of time have empty space (the ubiquitious vacuum) appear at a later time, expanding all the while toward its current state.

Of course it's possible that some parts of the scenario are wrong. We don't really know how matter arrived in the world, or whether it is being made right now by some process.

But that's one of the big problems with Cosmology, devising all the possible scenarios and then finding some kind of experimental evidence to select among them.
 
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  • #5


Originally posted by Tyger
says that matter existed before space, as we know it, appeared in the Universe. If that is valid then the correct description should start with matter and with the passage of time have empty space (the ubiquitious vacuum) appear at a later time, expanding all the while toward its current state.

Of course it's possible that some parts of the scenario are wrong. We don't really know how matter arrived in the world, or whether it is being made right now by some process.

But that's one of the big problems with Cosmology, devising all the possbible scenarios and then finding some kind of experimental evidence to select among them.

You might find this interesting. There has been a successful quantizing of the big bang which gets thru it without the model blowing up
so there is evolution of the universe before time zero

see last paragraph on page 25 of
arxiv/gr-qc/0202077
"in which the universe bounces off in order to enter an expanding branch. ...whether it expands forever or recollapses in order to start a new such process depends..."

and pictures of a wave function passing the time-zero point (where the classical singularity was) on page 16 of
arxiv/gr-qc/0303073

these are recent papers (this year and last year) by Martin
Bojowald who is a postdoc of Astekar and getting results very fast at present. he sort of leads the pack in loop quantum cosmology.

i find myself slowly stopping to believe in the singularity at time-zero that we have always been told about (because classical GR predicts it)

Bojowald's models include matter----matter and geometry are coupled throughout, so one does not preceed the other.

in another paper he has found a quantum geometrical mechanism for inflation---accelerated exapansion early on. he is going thru a very productive phase apparently with guidance from Ashtekar---on the order of about ten innovative papers last year
 

1. What is loop quantum gravity?

Loop quantum gravity is a theory that attempts to reconcile Einstein's theory of general relativity with quantum mechanics. It proposes that space and time are made up of discrete, indivisible units, or "loops", rather than being continuous.

2. How does loop quantum gravity explain gravity?

According to loop quantum gravity, gravity is not a force between objects, but rather a curvature of space-time caused by the presence of matter and energy. The theory suggests that this curvature is made up of tiny loops that interact with each other, creating what we perceive as gravity.

3. How is loop quantum gravity different from string theory?

Loop quantum gravity and string theory are both attempts to create a theory of quantum gravity, but they take different approaches. While string theory proposes that the fundamental building blocks of the universe are strings, loop quantum gravity suggests that they are loops. Additionally, loop quantum gravity does not require extra dimensions, unlike string theory.

4. What are some potential implications of loop quantum gravity?

If loop quantum gravity is proven to be a valid theory, it could lead to a deeper understanding of the fundamental nature of the universe. It may also help to solve some of the current problems in theoretical physics, such as the incompatibility of general relativity and quantum mechanics.

5. Has loop quantum gravity been proven?

No, loop quantum gravity is still a theoretical framework and has not yet been proven. However, there is ongoing research and experimentation to test its predictions and determine its validity. So far, the results have been promising, but more evidence is needed before it can be accepted as a complete theory of quantum gravity.

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