Is gravity an emergent phenomenon?

[marcus]

or do they...?

Nearly all theories of quantum gravity seem to imply that spacetime emerges from an effectively two dimension theory either by starting from two dimensional degrees of freedom, in string theory or LQG, or by predicting that spacetime is two dimensional on small scales e.g. CDT, asymptotic safety or Horava gravity. So two dimensions seems to be an input or an output in all the top theories of quantum gravity. The reason for this is very simple; two dimensions is the dimension in which Newton's constant is dimensionless. The problem for strings and LQG is to get from the two dimensional degrees of freedom to the standard model. In strings one has to compactify the extra dimensions in a clever way whereas LQG faces the problem of recovering classical GR from its highly non-perturbative and non-standard stating point.

CDT and AS are much more conservative and have both already shown that they have classical spacetime as an appropriate limit. The challenge for these theories is to understand the underlying microscopic degrees of freedom that they seem to be uncovering.

This is intriguing and even inspiring---it may contain an important insight. However it seems to me that LQG could be more in the situation of CDT and AS. That is, according to Modesto's work, 2D emerges at small scales. Steve Carlip's review of QG spontaneous dimensional reduction echoed and cited Modesto on this.
http://arxiv.org/abs/0911.0437
Fractal Space-Time from Spin-Foams
Elena Magliaro, Claudio Perini, Leonardo Modesto
(Submitted on 2 Nov 2009)
"In this paper we perform the calculation of the spectral dimension of spacetime in 4d quantum gravity using the Barrett-Crane (BC) spinfoam model. We realize this considering a very simple decomposition of the 4d spacetime already used in the graviton propagator calculation and we introduce a boundary state which selects a classical geometry on the boundary. We obtain that the spectral dimension of the spacetime runs from approximately 2 to 4..."

So they found the dimensionality (measured by a diffusion process) to be around 2 at very small scale and 4 at macro scale. This is similar to what Loll et al found for CDT, using the same measure of dimensionality.

Also quite a lot of evidence has accumulated that Loop gets ordinary gravity at large scale, but no rigorous proof---the eyes and tees still need to be dotted and crossed on that. Rovelli's February review discusses the current situation:
http://arxiv.org/abs/1102.3660
E.g. starting on page 18 with section D3: "Large distance expansion", and continuing into sections E1 and E2: "n-point functions" and "cosmology".Opinions can of course differ but offhand I would say the situation with Loop is more comparable with CDT on that score (than you suggest) and less comparable with String.

 

[Finbar]

Opinions can of course differ but offhand I would say the situation with Loop is more comparable with CDT on that score (than you suggest) and less comparable with String.

I think the difference between CDT and LQG is that LQG starts at the level of the microscopic description in terms of spin foams which don't yet describe classical general relativity. One needs to some how coarse grain the spin foam in such a way that the classical metric of say de sitter space comes out. CDT starts at the level of a regularised spacetime and then aims to show that there is a good continuum limit.

I'm certainly not saying string and loop are similar. Only that they are both somehow putting in the two dimensionality of spacetime by hand when thy choose the microscopic degrees of freedom which they would like to quantize. CDT and AS only choose the coarse grained degrees of freedom and aim to uncover the microscopic ones.

 

[czes]

Lee Smolin:
"We apply a recent argument of Verlinde to loop quantum gravity, to conclude that Newton's law of gravity emerges in an appropriate limit and setting. This is possible because the relationship between area and entropy is realized in loop quantum gravity when boundaries are imposed on a quantum spacetime. "
http://arxiv.org/abs/1001.3668

I have not read such article of another LQG authors. Instead of the Holographic Principle there is a Big Bounce of Bojowald and an infinite dense singularity.
Do Rovelli and Ashtekar reject the Holographic Principle in LQG ?

 

[baywax]

Is space-time (re: distance) an emergent property?

 

[atyy]

Is space-time (re: distance) an emergent property?

In general relativity, gravity=spacetime, so that is the same question as the original.

When the gravity=spacetime identification is made, it is assumed that there is matter, or at least test particles, clocks and rulers.

 

[baywax]

In general relativity, gravity=spacetime, so that is the same question as the original.

When the gravity=spacetime identification is made, it is assumed that there is matter, or at least test particles, clocks and rulers.

Ha! Very good, thank you. Is beer an emergent phenomenon?! (or part of the foam?!)

 

[atyy]

Ha! Very good, thank you. Is beer an emergent phenomenon?! (or part of the foam?!)

Beer is fundamental. Foam is emergent.

 

[baywax]

Beer is fundamental. Foam is emergent.

If all we can observe is emergent, what are we missing? Surely mathematical formulas do not thoroughly represent or depict the fundamental source of all that is emergent. Is it like throwing flour into the wind and saying we know what the wind looks like?

 

[alt]

Originally Posted by czes

The emergent gravity by R.T.Cahill, (2011)
The dynamical theory for this 3-space involves G, which determines the dissipation rate of space by matter, and alpha, which experiments and observation reveal to be the fine structure constant. For the 1st time we have a comprehensive account of space and matter and their interaction - gravity.
http://arxiv.org/abs/1102.3222

Pretty interesting. Is anyone subscribed to this site?

Cahill is an interesting Aussie scientist. If you click onto his name at the top of the page of the above link, you get to all his papers, including;

A Quantum Cosmology: No Dark Matter, Dark Energy nor Accelerating Universe

Why, I recall a couple of years ago, emblazoned over the front page of our newspapers, he had defeated the speed of light .. or something .. (most likely in one of his other papers).

Anyway, I clicked onto this thread in the hope of understanding gravity more - but now I understand in less.

 

[zhermes]

Cahill is an interesting Aussie scientist.

Cahill is not a scientist.

 

[alt]

Cahill is not a scientist.

Lol .. Is that it ? Who said elsewhere of another contributor ..

"an incredibly empty statement lacking important, key elements" ?

You could have as easily inferred I used the term in a broad sense, befitting the following fine definitions;

thefreedictionary.com
a person who studies or practises any of the sciences or who uses scientific methods

Wordweb;
A person with advanced knowledge of one or more sciences

World English Dictionary
a person who studies or practises any of the sciences or who uses scientific methods

Dictionary.com
an expert in science, especially one of the physical or natural sciences.

Wikipedia
A scientist in a broad sense is one engaging in a systematic activity to acquire knowledge. In a more restricted sense, a scientist is an individual who uses the scientific method.[1] The person may be an expert in one or more areas of science.

 

[zhermes]

Who said elsewhere of another contributor ..

I certainly appreciate the detective work; I was trying to imply an obvious finality to the statement. I still can't say I understand why it even needs to be backed.

[/U]a person who studies or practises any of the sciences or who uses scientific methods

All of your definitions here are just tautologies, effectively: 'a scientist is an 'ist' of science'.
The key is that what Cahill does, is far from science. Ignoring all evidence in desperate reaches for overly forced explanations is a far cry from any type of scientific method. Every one of his ideas is systematically built on conjecture and---I'd dare say---a personal desire for a particular solution, instead of physical motivation.

But, as you already pointed out, this is flimsy and subjective. More objective is that most of his work isn't rigorously (if at all) peer-reviewed. In just about all of his 'articles' his only citations are to his other articles. He never has coauthors who behave any differently. He never has a single shred of empirical evidence to motivate his standpoint as apposed to a simpler, more accepted explanation. (While 'accepted' doesn't mean correct, Occam's razor suggests that he needs to work a little harder when he's fighting against 'accepted'--he doesn't).

A scientist in a broad sense is one engaging in a systematic activity to acquire knowledge. In a more restricted sense, a scientist is an individual who uses the scientific method.[1] The person may be an expert in one or more areas of science.
[/I]

Cahill, in my opinion, is in no way searching for knowledge. Often, on physicsforums (e.g.), people post wild ideas to explain already solved problems. They're not looking for 'knowledge', they're looking for gratification. Cahill does the same, on a larger scale. Instead of learning the 'knowledge' of science, he attempts to fabricate it. What Reginald T. Cahill does is an insult to ever true scientist, or person who authentically strives to be a scientist, out there.

 

[alt]

I certainly appreciate the detective work; I was trying to imply an obvious finality to the statement. I still can't say I understand why it even needs to be backed.


All of your definitions here are just tautologies, effectively: 'a scientist is an 'ist' of science'.
The key is that what Cahill does, is far from science. Ignoring all evidence in desperate reaches for overly forced explanations is a far cry from any type of scientific method. Every one of his ideas is systematically built on conjecture and---I'd dare say---a personal desire for a particular solution, instead of physical motivation.

But, as you already pointed out, this is flimsy and subjective. More objective is that most of his work isn't rigorously (if at all) peer-reviewed. In just about all of his 'articles' his only citations are to his other articles. He never has coauthors who behave any differently. He never has a single shred of empirical evidence to motivate his standpoint as apposed to a simpler, more accepted explanation. (While 'accepted' doesn't mean correct, Occam's razor suggests that he needs to work a little harder when he's fighting against 'accepted'--he doesn't).


Cahill, in my opinion, is in no way searching for knowledge. Often, on physicsforums (e.g.), people post wild ideas to explain already solved problems. They're not looking for 'knowledge', they're looking for gratification. Cahill does the same, on a larger scale. Instead of learning the 'knowledge' of science, he attempts to fabricate it. What Reginald T. Cahill does is an insult to ever true scientist, or person who authentically strives to be a scientist, out there.

Detective work ? Hardly. I frequent that forum more than any other.

I disagree a great deal with your last paragraph, but am hardly in a position to try to defend RTC. As you say, much is subjective.

 

[baywax]

Is all that we can observe/calculate and imagine emergent?

 

[Feullieton]

Is all that we can observe/calculate and imagine emergent?

I guess the question here is about the correct way of talking about physical properties.

In the age of alchemy they didn't refer to formulas of chemical reacions, instead is was all about the reaction and how it appears to us. When we use formulas, we say that what appears as chemical reaction is actually emerging from more fundamental interactions of molecules. We write these interactions as chemical formulas. Still, the first thing we know is the apearance - that what we observe.
In general science goes like this: after we have made observation, we propose our hypothesis. Then using our hypothesis we calculate to predict further observations and observe again to test that. If all goes as planed and predictions succeed, we have developed a theory, which states law. And now we say: our observable phenomena is emerging from this law, which we call law of nature.

So referring to our general thread question, gravity is emergent as far as we are working with consistent theory which derrives gravity from more fundamental phenomena.

My personal point of view is that gravity as emergent phenomena is a road worth pursuing. We have spent too much time stuck with two inconsistent theories of nature, without any good success making sense out of it. So unless they find graviton particle, it would be favourable to see consistent and rigorous research in gravity emerging from universe entropy or quantum statistics.

 

[baywax]

I guess the question here is about the correct way of talking about physical properties.

In the age of alchemy they didn't refer to formulas of chemical reacions, instead is was all about the reaction and how it appears to us. When we use formulas, we say that what appears as chemical reaction is actually emerging from more fundamental interactions of molecules. We write these interactions as chemical formulas. Still, the first thing we know is the apearance - that what we observe.
In general science goes like this: after we have made observation, we propose our hypothesis. Then using our hypothesis we calculate to predict further observations and observe again to test that. If all goes as planed and predictions succeed, we have developed a theory, which states law. And now we say: our observable phenomena is emerging from this law, which we call law of nature.

So referring to our general thread question, gravity is emergent as far as we are working with consistent theory which derrives gravity from more fundamental phenomena.

My personal point of view is that gravity as emergent phenomena is a road worth pursuing. We have spent too much time stuck with two inconsistent theories of nature, without any good success making sense out of it. So unless they find graviton particle, it would be favourable to see consistent and rigorous research in gravity emerging from universe entropy or quantum statistics.

Very interesting! My understanding of the unit "graviton" is that it is just that, a unit of measurement designed to make the study of the phenomenon gravity easier. This is much the same as the unit "photon".

 

[Feullieton]

Very interesting! My understanding of the unit "graviton" is that it is just that, a unit of measurement designed to make the study of the phenomenon gravity easier. This is much the same as the unit "photon".

I just might want to note that your statement of graviton being unit of measurement is not quite correct if regarded as statement about nature. What you might have wanted to say or state is statement about our method - that notion of graviton is a means to measure certain physical property. Technically it is not quite correct either - for if we stay in the realm of those theories (that state the existence of graviton) we have to recognize how this notion is used in particular theory - and it is not used as measurement unit - the measurement unit stays unit of mass - kilogram or whatever fancy mass unit they use (electronvolt/c^2).
Graviton and photon is that which is measured and therefore observed. They are the physical property itself.

Correct understanding of what we know and are able to know is very important!