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Is gravity an emergent phenomenon? 
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#37
Mar1711, 12:10 PM

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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/feat...iew_of_Gravity 


#38
Mar1711, 10:27 PM

PF Gold
P: 2,215




#39
Mar1811, 03:12 AM

P: 216

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/grqc/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 (DaviesUnruh 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 nonlocal 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/ 


#40
Mar3011, 12:44 AM

PF Gold
P: 2,215

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? 


#41
Mar3011, 02:48 AM

P: 216

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. 


#42
Mar3011, 10:31 PM

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#43
Mar3111, 02:29 AM

P: 216

In 2008, Fermilab particle astrophysicist Craig Hogan made waves with a mindboggling 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/brea...phicuniverse/ 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) 


#45
Apr111, 12:59 AM

Sci Advisor
P: 8,796

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. 


#46
Apr111, 09:21 AM

P: 216

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 nonlocal and we can't calculate how it curves the spacetime. 


#47
Apr111, 10:26 PM

PF Gold
P: 2,215

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


#48
Apr211, 04:12 AM

P: 216

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 


#49
Apr411, 06:15 AM

P: 371




#50
Apr411, 02:56 PM

P: 343

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 nonperturbative and nonstandard 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. 


#51
Apr411, 03:39 PM

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http://arxiv.org/abs/0911.0437 Fractal SpaceTime from SpinFoams 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 BarrettCrane (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 proofthe 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: "npoint 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. 


#52
Apr511, 06:33 AM

P: 343

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. 


#53
Apr611, 02:05 AM

P: 216

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 ? 


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