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Nickyv2423

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Can it be either?

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- Thread starter Nickyv2423
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In summary, loop quantum gravity is a theory that attempts to reconcile the principles of quantum mechanics and general relativity. It suggests that space and time are made up of discrete, quantized units, rather than being continuous as predicted by general relativity. Some argue that this theory is non-local, meaning that it allows for instantaneous actions at a distance, while others argue that it is a local quantum field theory, meaning that interactions are limited by the speed of light. The debate continues, with further research and experiments needed to determine the true nature of loop quantum gravity.

- #1

Nickyv2423

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Can it be either?

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- #2

julian

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In GR the coordinate time (as well as the spatial coordinates) can in principle be discarded from the formulation of the theory without loss of physical content.

In GR Dirac observables are smeared over all of spacetime, therefore Dirac observables are not local with respect to the unphysical coordinates ##t,x^a##.

You are able to specify a phyical meaning of a local region where a measrement takes place however not specified in terms of some coordinates rather in terms of other measurements, usually in terms of matter degrees of freedom representing material reference systems.

In quantum gravity physical locality can be accommodated while coordinate locality is completely lost.

Something often overlooked is while mathematically in LQG they seem to be able to quantise geometry without matter, when it comes to physical observables matter seems indispensable.

In GR Dirac observables are smeared over all of spacetime, therefore Dirac observables are not local with respect to the unphysical coordinates ##t,x^a##.

You are able to specify a phyical meaning of a local region where a measrement takes place however not specified in terms of some coordinates rather in terms of other measurements, usually in terms of matter degrees of freedom representing material reference systems.

In quantum gravity physical locality can be accommodated while coordinate locality is completely lost.

Something often overlooked is while mathematically in LQG they seem to be able to quantise geometry without matter, when it comes to physical observables matter seems indispensable.

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- #3

Nickyv2423

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Are you saying it's a local or non local quantum field theory? Because if it's a local quantum field theory of quantum gravity, it has to be wrong.julian said:

In GR Dirac observables are smeared over all of spacetime, therefore Dirac observables are not local with respect to the unphysical coordinates ##t,x^a##.

You are able to specify a phyical meaning of a local region where a measrement takes place however not specified in terms of some coordinates rather in terms of other measurements, usually in terms of matter degrees of freedom representing material reference systems.

In quantum gravity physical locality can be accommodated while coordinate locality is completely lost.

Something often overlooked is while mathematically in LQG they seem to be able to quantise geometry without matter, when it comes to physical observables matter seems indispensable.

- #4

atyy

Science Advisor

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Nickyv2423 said:Are you saying it's a local or non local quantum field theory? Because if it's a local quantum field theory of quantum gravity, it has to be wrong.

At present, there is no theorem forbidding gravity from being a local QFT.

There is an offshoot of LQG called group field theory, with more recent developments called tensor field theory. Mathematically they are related to QFT, but in physical interpretation, they are likely to be nonlocal.

https://arxiv.org/abs/1603.07278

Vincent Rivasseau

https://arxiv.org/abs/1109.4812

Razvan Gurau, James P. Ryan

https://arxiv.org/abs/1610.09758

Edward Witten

[PLAIN]https://arxiv.org/abs/1611.04032[/PLAIN]

https://arxiv.org/abs/1611.04032

Razvan Gurau

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- #5

Nickyv2423

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According to Lubos Motl, there is.atyy said:At present, there is no theorem forbidding gravity from being a local QFT.

Here is a quote from his website concerning asymptotic safety in quantum gravity. He goes into why it can't work because it is a local QFT of quantum gravity...

"There has been a lot of other progress in the recent decade or so. I think that we have realized not only string theory works but even if you assumed that the right theory of quantum gravity is something else, many of the qualitative properties of string theory are necessary for any consistent theory of quantum gravity. We have a much better understanding for the detailed reasons why string theory is the only possible consistent theory of quantum gravity.

Quantum gravity cannot be described as a local field theory in the bulk because of many reasons, including

- the infinitely many types of terms that could be added; Weinberg discussed some partial successes but I don't think that there exists any known sensible UV fixed point for gravity; after all, its non-existence was the reason why so many people began to look at Hořava's non-relativistic extension of it recently;
- the wrong scaling of the entropy: scale-invariant field theories always have a volume-extensive entropy density and it seems impossible to guarantee that the entropy bounds will be imposed, i.e. that the black hole with its area-extensive entropy remains the record-holder for the total entropy in a volume (and therefore the ultimate stage of a collapse);
- the information preservation during the Hawking radiation that implies that physics of quantum gravity must allow for some kind of nonlocal effects that are able to get the information out of the black hole; these effects are impossible if the causal structure dictated by a metric tensor (quantum field) strictly holds;
- wrong trans-Planckian, very high-energy scattering amplitudes; the probability to create two particles in such a collision should exponentially decrease, as seen from general black hole thermodynamics, but that won't happen in a local theory in the bulk that is scale-invariant in the UV; the latter would lead to power laws."

Loop quantum gravity is a theory that aims to unify the principles of general relativity and quantum mechanics. It proposes that space and time are made up of tiny, indivisible units called "loops" and that these loops interact to create the fabric of the universe.

This is a topic of ongoing debate and research in the scientific community. Some argue that loop quantum gravity is local, meaning that interactions between particles are limited to their immediate surroundings. Others argue that it is non-local, meaning that interactions can occur over long distances.

There is currently no definitive evidence that supports loop quantum gravity as a non-local theory. However, some scientists point to the theory's ability to resolve certain issues in quantum mechanics, such as the measurement problem, as an indication of its non-local nature.

Loop quantum gravity and string theory are both attempts to reconcile general relativity and quantum mechanics. However, they approach this goal in different ways. Loop quantum gravity focuses on the discrete structure of space and time, while string theory proposes that the fundamental building blocks of the universe are one-dimensional strings.

Currently, there is no way to directly test loop quantum gravity experimentally. However, scientists are working on developing mathematical models and simulations that could potentially provide evidence for or against the theory. Additionally, some aspects of loop quantum gravity, such as the effects of gravity on the quantum scale, could potentially be observed through experiments with advanced technology in the future.

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