Is gravity an emergent phenomenon?

[baywax]

If this has been covered or is so fundamental a question that its dumb... please excuse me...... otherwise, an answer is appreciated, thank you!

Actually I just realized its probably a property if it is emergent at all.

Another two properties I'd like to know about in terms of emergence are light and heat. Are these two emergent properties at all?

 

[zhermes]

If this has been covered or is so fundamental a question that its dumb... please excuse me...... otherwise, an answer is appreciated, thank you!

Actually I just realized its probably a property if it is emergent at all.

Another two properties I'd like to know about in terms of emergence are light and heat. Are these two emergent properties at all?

Out of those three, heat is the only 'emergent' property.

 

[lightarrow]

If this has been covered or is so fundamental a question that its dumb... please excuse me...... otherwise, an answer is appreciated, thank you!

Actually I just realized its probably a property if it is emergent at all.

I'd say gravity it's not an emergent phenomenon, because it's not generated by the complex interaction of simple systems. In GR gravity is just the space-time curvature produced by mass-energy-momentum.

 

[StevieTNZ]

How does gravity arise?

 

[Pengwuino]

While there has been research done that shows hints of the possibility that gravity can be understood as a thermodynamic phenomena, it's certainly not mainstream. Black holes have a temperature. This has lead to speculation about what this really means for gravity as a whole. Nothing has really come of it, however.

 

[rkirby7]

Yes, gravity is an emergent phenomena as are all observable phenomena. The language of emergency allows all of nature to be explained in terms of one observable emerging from an underlying organizational feature of constituent building units. These building blocks of matter have emergent arrangements in three dimensional space that improve the use of space relative to the disorganized units. The result is energy lowering for the system and stabilization of the organizational matrix.

 

[baywax]

While there has been research done that shows hints of the possibility that gravity can be understood as a thermodynamic phenomena, it's certainly not mainstream. Black holes have a temperature. This has lead to speculation about what this really means for gravity as a whole. Nothing has really come of it, however.

Wow, thanks everyone...

So, on the microscopic level of the quantum universe gravity continues to exist and therefore is not an emergent property... ie loop quantum gravity?

In what way is it that light escapes the label of "emergent property"?

 

[atyy]

Light is an emergent property. We know this because the quantum field theory (quantum electrodynamics) that describes light at low energies is mathematically inconsistent at high energies. So quantum electrodynamics is just an approximation to another theory, and "emerges" from that still unknown theory.

There are constructions in which QED emerges at low energy from something different. However those theories don't get other sorts of matter correct (yet?), so it is still unknown how QED in our universe emerges.

 

[baywax]

Light is an emergent property. We know this because the quantum field theory (quantum electrodynamics) that describes light at low energies is mathematically inconsistent at high energies. So quantum electrodynamics is just an approximation to another theory, and "emerges" from that still unknown theory.

There are constructions in which QED emerges at low energy from something different. However those theories don't get other sorts of matter correct (yet?), so it is still unknown how QED in our universe emerges.

By "unknown" theory, do you mean "unproven"?

 

[zhermes]

From a purely semantic point of view, I don't think the uses of 'emergent' as it has been applied to gravity (@rkirby) and light (@atyy) are correct:

See:
http://plato.stanford.edu/entries/properties-emergent/"
http://cscs.umich.edu/~crshalizi/notabene/emergent-properties.html"
Laughlin, Robert (2005), A Different Universe: Reinventing Physics from the Bottom Down, Basic Books

An emergent property is one which arises from a collection or system, each element of which does not exhibit said property, and is fundamentally at a higher level of abstraction.

Yes, gravity is an emergent phenomena as are all observable phenomena. The language of emergency allows all of nature to be explained in terms of one observable emerging from an underlying organizational feature of constituent building units...

And in this case, I don't see what 'underlying organizational feature' the poster is referring to... Perhaps the poster is suggesting that gravity emerges from the fundamental properties of space-time; if this is the case, I would reply that gravity is simply tautological with the nature of space-time, nor is space-time any time of conglomerate or assembly of smaller elements.

Light is an emergent property. We know this because the quantum field theory (quantum electrodynamics) that describes light at low energies is mathematically inconsistent at high energies. So quantum electrodynamics is just an approximation to another theory, and "emerges" from that still unknown theory...

This line of reasoning suggests that our understanding of light is simply incomplete, not that it 'emerges' per se from more fundamental principles. If one assumes that gravity is not emergent (it doesn't matter for this example if that is true or not), then it is no more or less emergent in an incomplete Newtonian understanding of gravity than in a full general relativistic framework.

Additionally, from a particle perspective, if the photon and graviton are viewed as fundamental particles---they clearly cannot be emergent.

Now, even from a non-semantic perspective, I don't see that its in any way informative to view light or gravity as an emergent property. We know of no underlying system which in a macroscopic sense gives rise to these phenomenon. If such systems exist, the case may need to be reconsidered, but at the moment the point would be purely speculative, unverifiable and irrefutable---and thus not science.

 

[baywax]

From a purely semantic point of view, I don't think the uses of 'emergent' as it has been applied to gravity (@rkirby) and light (@atyy) are correct:

See:
http://plato.stanford.edu/entries/properties-emergent/"
http://cscs.umich.edu/~crshalizi/notabene/emergent-properties.html"
Laughlin, Robert (2005), A Different Universe: Reinventing Physics from the Bottom Down, Basic Books

An emergent property is one which arises from a collection or system, each element of which does not exhibit said property, and is fundamentally at a higher level of abstraction.


And in this case, I don't see what 'underlying organizational feature' the poster is referring to... Perhaps the poster is suggesting that gravity emerges from the fundamental properties of space-time; if this is the case, I would reply that gravity is simply tautological with the nature of space-time, nor is space-time any time of conglomerate or assembly of smaller elements.


This line of reasoning suggests that our understanding of light is simply incomplete, not that it 'emerges' per se from more fundamental principles. If one assumes that gravity is not emergent (it doesn't matter for this example if that is true or not), then it is no more or less emergent in an incomplete Newtonian understanding of gravity than in a full general relativistic framework.

Additionally, from a particle perspective, if the photon and graviton are viewed as fundamental particles---they clearly cannot be emergent.

Now, even from a non-semantic perspective, I don't see that its in any way informative to view light or gravity as an emergent property. We know of no underlying system which in a macroscopic sense gives rise to these phenomenon. If such systems exist, the case may need to be reconsidered, but at the moment the point would be purely speculative, unverifiable and irrefutable---and thus not science.

As far as I know,

"Light is created when electrons absorb the energy of photons. The excess energy from absorbing this photon results in light because an electron can only absorb so much energy and then has to release the rest."

http://answers.askkids.com/How_Stuff_Works/how_is_light_created

So from this microscopic, quantum level of interaction between electrons and photons, light emerges. This sounds like the formula for an emergent property.

 

[zhermes]

"Light is created when electrons absorb the energy of photons. The excess energy from absorbing this photon results in light because an electron can only absorb so much energy and then has to release the rest."

While most of this statement is true, per se; it is in no way an explanation for how light is created. Also, its just a bad description of physical processes.

Let me paraphrase the above quotation: 'light is created when light hits an electron, because electrons can only have so much energy'.
1) It starts out with light, thus its not especially helpful in presenting a primary cause of light.
2) This is an accurate example of a single type of electron-photon scattering interaction, which isn't especially interesting or insightful about the physical processes.
3) An electron can hold an arbitrarily large amount of energy, but not necessarily stably---in a particular situation.

But anyway, that is all somewhat unrelated... It still suggests nothing about light emerging from more fundamental processes or systems.

So from this microscopic, quantum level of interaction between electrons and photons, light emerges. This sounds like the formula for an emergent property.

Here you are just stipulating that light "emerges" from something, and then using that as evidence that light "emerges" is general. That doesn't quite work.
Again, you're starting with photons (photons = light) and thus it doesn't make sense to say light emerges from light. Also, as I tried to illustrate with the references I cited, interactions don't warrant "emergent phenomena".

As a final note, askkids.com isn't a source which holds much water.

 

[baywax]

1) It starts out with light, thus its not especially helpful in presenting a primary cause of light.

Again, you're starting with photons (photons = light) and thus it doesn't make sense to say light emerges from light.

I was under the impression that:

"In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force."

http://en.wikipedia.org/wiki/Photon

and that the photon is not restricted to the description of a unit of light.

 

[zhermes]

"In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force."

All true.

and that the photon is not restricted to the description of a unit of light.

'Light' can mean the visible spectrum (although this would make the common phrase 'visible light'
redundant), the visible and near-visible (IR and UV) spectrum, or colloquially the entire electromagnetic wave spectrum. Photons apply to the entire spectrum, not just the visible---if that's what you are saying, then that is correct.

There is still nothing here to suggest that light 'emerges' from photons, or anything else for that matter.

 

[lightarrow]

I was under the impression that:

"In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force."

http://en.wikipedia.org/wiki/Photon

and that the photon is not restricted to the description of a unit of light.

The photon is the quantum of excitation of the electromagnetic field. First come the field, then the photon.

 

[zhermes]

First come the field, then the photon.

The two are inextricable. Neither came first, nor second.

 

[lightarrow]

The two are inextricable. Neither came first, nor second.

Yes, they are inextricable, but what I mean is that to talk of "a quantum of excitation of an em field", you first must have the concept of "the em field".

 

[zhermes]

Yes, they are inextricable, but what I mean is that to talk of "a quantum of excitation of an em field", you first must have the concept of "the em field".

I do appreciate what you're saying, but I think (as wittgenstein would point out) this is just a feature of our language and way of thinking about things, not of the underlying physics.

 

[baywax]

Yes, they are inextricable, but what I mean is that to talk of "a quantum of excitation of an em field", you first must have the concept of "the em field".

Does this article help?

Origin of Light
Xiao-Gang Wen¤ Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
(Dated: June 15, 2001)

The existence of light (a massless U(1) gauge boson) is one of unresolved mysteries in nature. In this paper, we would like to propose that light is originated from certain quantum orders in our vacuum. We will construct quantum spin models on lattice to demonstrate that some quantum orders can give rise to light without breaking any symmetries and without any fine tuning. Through our models, we show that the existence of light can simply be a phenomenon of quantum coherence in a system with many degrees of freedom. Massless gauge fluctuations appears commonly and naturally in strongly correlated quantum systems which originally contain no gauge fields.

http://dao.mit.edu/~wen/pub/light.pdf

 

[zhermes]

Does this article help?
http://dao.mit.edu/~wen/pub/light.pdf

No, it doesn't. A speculative, unrefereed, non-mainstream or established article from 10 years ago with numerous typos and grammatical errors in the abstract (alone), is not helpful.

 

[baywax]

No, it doesn't. A speculative, unrefereed, non-mainstream or established article from 10 years ago with numerous typos and grammatical errors in the abstract (alone), is not helpful.

Here's a little more about one of the authors...

http://en.wikipedia.org/wiki/Xiao-Gang_Wen

 

[zhermes]

</topic>

 

[fzero]

No, it doesn't. A speculative, unrefereed, non-mainstream or established article from 10 years ago with numerous typos and grammatical errors in the abstract (alone), is not helpful.

I'll agree that it's speculative, but the article was published in PRL: http://prl.aps.org/abstract/PRL/v88/i1/e011602

 

[baywax]

If mass is an emergent phenomenon/property, wouldn't that make gravity the same?

 

[baywax]

If mass is an emergent phenomenon/property, wouldn't that make gravity the same?

Here's what wiki says about mass being an emergent property:

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. In some interpretations of quantum mechanics, the perception of a deterministic reality, in which all objects have a definite position, momentum, and so forth, is actually an emergent phenomenon, with the true state of matter being described instead by a wavefunction which need not have a single position or momentum. Most of the laws of physics themselves as we experience them today appear to have emerged during the course of time making emergence the most fundamental principle in the universe and raising the question of what might be the most fundamental law of physics from which all others emerged. Chemistry can in turn be viewed as an emergent property of the laws of physics. Biology (including biological evolution) can be viewed as an emergent property of the laws of chemistry.

http://en.wikipedia.org/wiki/Emergence

 

[fzero]

If mass is an emergent phenomenon/property, wouldn't that make gravity the same?

Gravity couples to energy, not just mass. That's how massless particles like the photon interact with gravity. So emergent gravity does not directly follow from emergent mass.

 

[baywax]

gravity couples to energy, not just mass. That's how massless particles like the photon interact with gravity. So emergent gravity does not directly follow from emergent mass.

e=mc2?

 

[fzero]

e=mc2?

No, that's the expression from special relativity for the energy of a massive particle at rest. The central equation of general relativity is

R_{\mu\nu} - g_{\mu\nu} R = \frac{8\pi G}{c^4} T_{\mu\nu} .

The quantities on the LHS describe the curvature of spacetime, while T_{\mu\nu}, called the stress-energy tensor, describes the energy and pressure of the matter that acts as the source for gravity. You can read a bit more at http://en.wikipedia.org/wiki/Einstein_field_equations#Mathematical_form The point is that T_{\mu\nu} does not depend only on mass, but also involves electromagnetic energy, fluid pressure, and anything else that contributes to the energy of a collection of matter.

 

[baywax]

No, that's the expression from special relativity for the energy of a massive particle at rest. The central equation of general relativity is

R_{\mu\nu} - g_{\mu\nu} R = \frac{8\pi G}{c^4} T_{\mu\nu} .

The quantities on the LHS describe the curvature of spacetime, while T_{\mu\nu}, called the stress-energy tensor, describes the energy and pressure of the matter that acts as the source for gravity. You can read a bit more at http://en.wikipedia.org/wiki/Einstein_field_equations#Mathematical_form The point is that T_{\mu\nu} does not depend only on mass, but also involves electromagnetic energy, fluid pressure, and anything else that contributes to the energy of a collection of matter.

OK... this is really interesting. But, does this mean matter, an emergent phenomenon, gives rise to gravity as well as electromagnetic energy etc... and does that make gravity emergent as well?

Further to this... is energy an emergent property?

Thank you!

 

[atyy]

OK... this is really interesting. But, does this mean matter, an emergent phenomenon, gives rise to gravity as well as electromagnetic energy etc... and does that make gravity emergent as well?

No. The RHS of the Einstein equation is the energy of matter, which may not be emergent. The LHS is the gravitational field, which may also not be emergent. (Actually the gravitational field is in the RHS too, since the energy of matter cannot be defined without it. However this indicates the mutual interaction of matter and gravity, not the emergence of one from the other.)

However, there are research programmes in which gravity and matter both emerge from more fundamental degrees of freedom (ie. the Einstein equation emerges as a limit of other equations). String theory is an example of such a programme.

The contrasting programme in which gravity is not emergent is called Asymptotic Safety.

 

[czes]

The two are inextricable. Neither came first, nor second.

A photon moves in a Vacuum, which is not empty. There are fields of the virtual particles-antiparticles.
The fields of virtual pairs are more fundamental than a photon.
The question is:
What is a relation between a gravitational field and Vacuum (density of the virtual particles-antiparticles) ?

 

[_PJ_]

I think gravity, which really describes a mutual acceleration between energetic* objects, is not so much an emergent phenomena of spacetime or GR, but is something deeper that contributes to, or even forms a foundation for the structure of spacetime even at the quantum level.
Taking a cue from the WardConstruction, if the smallest spacetime curvature (essentially the effect of the instanciation of a single graviton) is evaluated and, by naturally, as gravity does, interacting with spacetime, it therefore causes decoherence in any entangled system, possibly offering insight into a solution to Quantum measurement problem as well as perhaps suggesting why Gravity has no 'charge', - is always attractive.

As to what Gravity "IS", I cannot hope to imagine. I don't think it's as easily explained as a "Force", such as the others. I think it's wholly more exotic.

*(since even bosons can be affected by gravity, distinguishing between mass and energy is impractical, so I take momentum in terms of energy and velocity - since no object is truly at rest this also prevents any issues with objects cited as 0 rest mass.)

 

[baywax]

No. The RHS of the Einstein equation is the energy of matter, which may not be emergent. The LHS is the gravitational field, which may also not be emergent. (Actually the gravitational field is in the RHS too, since the energy of matter cannot be defined without it. However this indicates the mutual interaction of matter and gravity, not the emergence of one from the other.)

However, there are research programmes in which gravity and matter both emerge from more fundamental degrees of freedom (ie. the Einstein equation emerges as a limit of other equations). String theory is an example of such a programme.

The contrasting programme in which gravity is not emergent is called Asymptotic Safety.

In layman's terms what you're saying is that you can't have one without the other when it comes to gravity, matter and energy. So there is no causal relationship here, where phenomena can arise from one microsystem, moreover, these seemingly separate properties are all one and the same since none can exist independently. Is that why it's hard to determine if they are emergent or not?

 

[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

 

[baywax]

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?

 

[baywax]

This looks like the full story, PDF

http://www.ptep-online.com/index_files/2011/PP-24-05.PDF

 

[czes]

So it’s not necessarily crazy, even if still very speculative, to suppose that thermodynamics and information will serve as the bridge for bringing gravity and quantum physics together. Einstein’s equations link energy to matter and matter to gravity, and the new work connects matter and energy to information and entropy. These links imply that Einstein’s equations are more about information than energy, the physicists write. “In other words, information might be a more profound physical entity than matter or field.”
http://www.sciencenews.org/view/feature/id/63190/title/A_New_View_of_Gravity

 

[baywax]

So it’s not necessarily crazy, even if still very speculative, to suppose that thermodynamics and information will serve as the bridge for bringing gravity and quantum physics together. Einstein’s equations link energy to matter and matter to gravity, and the new work connects matter and energy to information and entropy. These links imply that Einstein’s equations are more about information than energy, the physicists write. “In other words, information might be a more profound physical entity than matter or field.”
http://www.sciencenews.org/view/feature/id/63190/title/A_New_View_of_Gravity

Yes, however, "information" is an anthropomorphic word for "energy". Don't you agree? Its all in how or even "if" the energy is received, interpreted and/or perceived. And, what does this have to do with my original question about gravity being emergent?

 

[czes]

The gravity may be emergent from the vacuum energy disstribution.
The change of refractive index of the vacuum caused by the presence of matter has exactly the same effect on the path of light as the curvature of space in Einstein's General Relativity.
http://arxiv.org/abs/gr-qc/0604009

If a particle absorbs more information from the vacuum than emits, it is accelerating toward the source of the information (“denser vacuum”). The massive object creates the vacuum around due to the Compton wavelength of its rest mass particles. If the emission is faster than the absorption the particle decelerates then (Davies-Unruh effect). So the vacuum is transformed into a real, detectable thing.
http://www.calphysics.org/articles/chown2007.html

The vacuum as a volume is an illusion made of relation between the information on a flat screen.
Also the non-local process of producing a holographic reconstruction is explained purely in terms of interference and diffraction. Thus, someone looking into the hologram "sees" the objects even though they are no longer present. The hologram is not an image, but an encoding system which enables the scattered light field to be reconstructed. Images can then be formed from any point in the reconstructed beam either with a camera or by eye.
May be we are a part of an Holographic Universe ?
http://www.hologram.glt.pl/

 

[baywax]

The gravity may be emergent from the vacuum energy disstribution.
The change of refractive index of the vacuum caused by the presence of matter has exactly the same effect on the path of light as the curvature of space in Einstein's General Relativity.
http://arxiv.org/abs/gr-qc/0604009

If a particle absorbs more information from the vacuum than emits, it is accelerating toward the source of the information (“denser vacuum”). The massive object creates the vacuum around due to the Compton wavelength of its rest mass particles. If the emission is faster than the absorption the particle decelerates then (Davies-Unruh effect). So the vacuum is transformed into a real, detectable thing.
http://www.calphysics.org/articles/chown2007.html

The vacuum as a volume is an illusion made of relation between the information on a flat screen.
Also the non-local process of producing a holographic reconstruction is explained purely in terms of interference and diffraction. Thus, someone looking into the hologram "sees" the objects even though they are no longer present. The hologram is not an image, but an encoding system which enables the scattered light field to be reconstructed. Images can then be formed from any point in the reconstructed beam either with a camera or by eye.
May be we are a part of an Holographic Universe ?
http://www.hologram.glt.pl/

you might want to pursue this in the philosophy forum.

Now that I have asked about emergent phenomena and properties I wonder if there's an answer to this question that's nagging me...

What is the quantum state or "microcosmic world" a product of? Does it all emerge from energy?

 

[czes]

What is the quantum state or "microcosmic world" a product of? Does it all emerge from energy?

There are many evidences that quantum state and energy emerge from the quantum information relation.
Bekenstein's topical overview "A Tale of Two Entropies" describes potentially profound implications of Wheeler's trend in part by noting a previously unexpected connection between the world of information theory and classical physics. Bekenstein summarizes that "Thermodynamic entropy and Shannon entropy are conceptually equivalent: the number of arrangements that are counted by Boltzmann entropy reflects the amount of Shannon information one would need to implement any particular arrangement..." of matter and energy. The only salient difference between the thermodynamic entropy of physics and the Shannon's entropy of information is in the units of measure; the former is expressed in units of energy divided by temperature, the latter in essentially dimensionless "bits" of information, and so the difference is merely a matter of convention.

 

[baywax]

There are many evidences that quantum state and energy emerge from the quantum information relation.
Bekenstein's topical overview "A Tale of Two Entropies" describes potentially profound implications of Wheeler's trend in part by noting a previously unexpected connection between the world of information theory and classical physics. Bekenstein summarizes that "Thermodynamic entropy and Shannon entropy are conceptually equivalent: the number of arrangements that are counted by Boltzmann entropy reflects the amount of Shannon information one would need to implement any particular arrangement..." of matter and energy. The only salient difference between the thermodynamic entropy of physics and the Shannon's entropy of information is in the units of measure; the former is expressed in units of energy divided by temperature, the latter in essentially dimensionless "bits" of information, and so the difference is merely a matter of convention.

Same Bekenstein?

http://www.phys.huji.ac.il/~bekenste/

 

[czes]

In 2008, Fermilab particle astrophysicist Craig Hogan made waves with a mind-boggling proposition: The 3D universe in which we appear to live is no more than a hologram.
Now he is building the most precise clock of all time to directly measure whether our reality is an illusion.
http://www.symmetrymagazine.org/bre...s-to-test-hypothesis-of-holographic-universe/

The Compton wave seems to be a fundamental unit of which our Observable Universe is built. The proton's Compton wavelength is a basic lattice and other particles have to correspond with the proton's wavelength. It is a question now.
Hogan wants to measure the quantum states made of the Compton wavelength of the proton (10^-15 m)

 

[baywax]

In 2008, Fermilab particle astrophysicist Craig Hogan made waves with a mind-boggling proposition: The 3D universe in which we appear to live is no more than a hologram.
Now he is building the most precise clock of all time to directly measure whether our reality is an illusion.
http://www.symmetrymagazine.org/bre...s-to-test-hypothesis-of-holographic-universe/

The Compton wave seems to be a fundamental unit of which our Observable Universe is built. The proton's Compton wavelength is a basic lattice and other particles have to correspond with the proton's wavelength. It is a question now.
Hogan wants to measure the quantum states made of the Compton wavelength of the proton (10^-15 m)

OK, I'll bite.

Are illusions an emergent phenomenon?

 

[atyy]

In layman's terms what you're saying is that you can't have one without the other when it comes to gravity, matter and energy. So there is no causal relationship here, where phenomena can arise from one microsystem, moreover, these seemingly separate properties are all one and the same since none can exist independently. Is that why it's hard to determine if they are emergent or not?

No, I meant that in general relativity, gravity=spacetime is not emergent.

To me, an emergent property is one of which it makes sense to talk about sometimes, but not all the time. For example, when we study resistors and capacitors, it makes sense to talk about resistance, Ohm's law and all that. However, resistance is not a fundamental property. If we blow the resistor up into its individual atoms, then we have to use atomic physics or something other than Ohms law. So Ohmic resistance is emergent.

Within general relativity, it always makes sense to talk about gravity=spacetime, even in extreme circumstances like black holes (let's ignore the singularity for now). So in general relativity, gravity=spacetime is not emergent, although it is curved by matter.

Of course general relativity could be wrong, and perhaps it doesn't make sense to talk about gravity=spacetime under all conditions. The AdS/CFT conjecture is an example of a theory in which gravity is emergent.

 

[czes]

No, I meant that in general relativity, gravity=spacetime is not emergent.

Within general relativity, it always makes sense to talk about gravity=spacetime, even in extreme circumstances like black holes (let's ignore the singularity for now). So in general relativity, gravity=spacetime is not emergent, although it is curved by matter.

Of course general relativity could be wrong, and perhaps it doesn't make sense to talk about gravity=spacetime under all conditions. The AdS/CFT conjecture is an example of a theory in which gravity is emergent.

There is locality in General Relativity and the particle is defined in the spacetime=gravity.
Therefore there are some problems with singularity, dark matter, dark energy...
In quantum mechanics there isn't locality. The particle is not defined in the spacetime and the gravity is an emergent phenomenon. The particle is non-local and we can't calculate how it curves the spacetime.

 

[baywax]

No, I meant that in general relativity, gravity=spacetime is not emergent.

To me, an emergent property is one of which it makes sense to talk about sometimes, but not all the time. For example, when we study resistors and capacitors, it makes sense to talk about resistance, Ohm's law and all that. However, resistance is not a fundamental property. If we blow the resistor up into its individual atoms, then we have to use atomic physics or something other than Ohms law. So Ohmic resistance is emergent.

Within general relativity, it always makes sense to talk about gravity=spacetime, even in extreme circumstances like black holes (let's ignore the singularity for now). So in general relativity, gravity=spacetime is not emergent, although it is curved by matter.

Of course general relativity could be wrong, and perhaps it doesn't make sense to talk about gravity=spacetime under all conditions. The AdS/CFT conjecture is an example of a theory in which gravity is emergent.

Maldacena duality, this is deep physics. I thought about how gravity=spacetime is potential energy. I wondered if its like a rubber band that sits unused then someone with some energy stretches it and it's potential energy is actualized. Its not a product of energy, necessarily, but can be made to act like/with energy. If matter is emergent perhaps that doesn't mean gravity is automatically emergent as well. Would that be Asymptotic Safety? Gravity is microscopic in nature and therefore easily viewed as non-emergent.

Is there any theory that suggests all energy is a product of space=time?

 

[czes]

In General Relativity the particle is localized in the spacetime=gravity. We have the energy then.
On the fundamental quantum level there isn't a locality and therefore there isn't a distance and no spacetime=gravity. There is a quantum information and relation between them. The energy emerges when we compare the frequencies of the information.

It seems that most fundamental is the information (quantum state) which may move toward the increasing entropy which creates the gravity. It is discussed according to
article: Gravity is not entropic force:
http://arxiv.org/abs/1009.5414

 

[dimension10]

How does gravity arise?

Different theories have different answers.

 

[Finbar]

Different theories have different answers.

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 cleaver 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.