What is the gravitational field ?

[czes]

I have read the papers of Verlinde, Jacobson, Beckenstein, Smoot and many other. They use the Unruh vacuum. It seems that Unruh vacuum is nothing but a thermal vacuum in the sense of thermodynamics. It contains vacuum fluctuations creating the virtual particle-antiparticle pairs. Verlinde wrote that gravity is an entropic force.
Does it mean the gravitational field is an equivalent to the entropy of the vacuum with its virtual particles-antiparticles ?
If the gravity is relatively measured what about the density of the vacuum (virtual particle-antiparticle pairs) ?

 

[Naty1]

Does it mean the gravitational field is an equivalent to the entropy of the vacuum with its virtual particles-antiparticles ?

I think in general "yes" but am unsure why you mention particle antiparticle pairs. I don't know if you are implying they are especially relevant.

Have you seen this thread:
https://www.physicsforums.com/showthread.php?t=403081&highlight=verlinde


and the reference cited in post #5:

"It From Bit - Entropic Gravity For Pedestrians"
-- http://www.scientificblogging.com/ha...ty_pedestrians



which says this about a simple model tetrahedron universe:

(but which Marcus may not buy, I'm not sure,)

So, the two particles being together at the same vertex creates a smaller entropy reduction compared to the case of the two particles being separate. In other words, two particles together at one vertex corresponds to significantly more states than two particles at separate vertices. This is all that is needed for a tendency for both particles to stick together.

which makes sense if you buy that entropy (states) tends to increase over time...

I don't know if the example is accurate, and the illustration relevant, but it sure is conceptually understandable...

 

[Naty1]

Verlinde says that gravity [and mass and position, and other constituents of our universe, say forces for example] are emergent macroscopic manifestations of a vastly larger microscopic degrees of freedom..."it from bit" is how Wheeler summarized it. As entropy increases, things appear ..are manifested...on the macroscopic scale...

In the little "it from bit" tetrahedon example the blogger assumes the particles already exist...I don't know the implications of the different assumptions...:


Verlinde makes certain assumptions, like space is emergent, time exists...around minute 13) I have no idea what happens to his ideas if he reverses those assumptions, for example...and as noted in the referenced thread above, others see omissions and maybe flaws in his arguments...

I sure don't fully understand all that is being implied, but ADS/CFT correspondence has always suggested to me a deep (as yet not understood) underlying connection between dimensional space and gravity...and it's connection with the holographic principle and Beckensteins work seems to tie in bits as well...

 

[czes]

Thank you Naty1 for the links.There are so many ideas now.
My question about virtual particle-antiparticle pairs in the vacuum is because of the quantum information amount. We calculate the amount of the information on the surface of the event horizon (Surface/4 Planck length squared). How to calculate this number in our 3D space ?
I think the Compton wave length is here important and may be we may calculate the discretness of the space this way.
If the vacuum is a medium for the light so we have to calculate the approximate, relative density of the vacuum build of virtual appearing and disappearing virtual pairs.

 

[Naty1]

My question about virtual particle-antiparticle pairs in the vacuum is because of the quantum information amount.

I think we have a language barrier here.

Quantum information and the holographic principle are not directly related to particle/antiparticles...although quantum theory underlies them ...I understand none of the rest of the last post...

 

[czes]

Quantum information and the holographic principle are not directly related to particle/antiparticles...although quantum theory underlies them

It is just my question. Is it a possibility to refer the quantum information with the virtual particle and antiparticle ?
May be it is strange but if we can calculate the amount of the information in bits on the surface of the event horizon so we have to calculate it in 3D space too.
Virtual particles are not defined but what is their origin ?
I assume the Compton wave length of the particle is non-local and it has to manifest its existence anyway. May be this manifestation are the virtual particles and antiparticles.
Is it any eqivalence there ?
We can calculate amount of the non-local Compton waves if we know the length of the wave and the distance between particle and observer.
Compton wave length is in all quantum calculations.

 

[Naty1]

Sorry I have no idea what most of your questions mean...maybe a language barrier?

 

[marcus]

I have read the papers of Verlinde, Jacobson, Beckenstein, Smoot and many other. They use the Unruh vacuum. It seems that Unruh vacuum is nothing but a thermal vacuum in the sense of thermodynamics. It contains vacuum fluctuations creating the virtual particle-antiparticle pairs. Verlinde wrote that gravity is an entropic force.
Does it mean the gravitational field is an equivalent to the entropy of the vacuum with its virtual particles-antiparticles ?
If the gravity is relatively measured what about the density of the vacuum (virtual particle-antiparticle pairs) ?

This is your first post heading this thread. It is interesting that you did not mention geometry.

One way to think of the gravitational field (probably the standard way that experts have looked at it since 1915 GR appeared) is that the gravitational field is the spacetime geometry.

There is no other: Euclidean space geometry or Lorentzian 4D geometry are simply the zero gravitational field. They are the "flat" which (according to the 1915 GR theory) is what occurs when there is either only negligible amounts of matter around, or no matter at all. But we have no right to expect that geometry will always be flat, or that it will not expand or contract.

So there has to be some theory that tells us what geometry to expect--what makes geometry the way it is. So far GR is the best theory to do this.

That is one answer to your question: What is the gravi field? It is the dynamically changing geometry of space. The gravitational field is not IN space, it IS space. (Or more exactly spacetime.)

The other fields are defined on the gravi field. They live in it. There is no space(time) as a separate entity.

Verlinde does not contradict this--he just focuses on a simplified problem of the Newton approximation to gravity. He cites Jacobson, which is the fundamental work to which all the entropic force papers hark back. Jacobson derived classical General Rel, which is what I've been talking about, from thermodynamics. So far the "entropic force" gambit has not gotten away from the classic 1915 GR viewpoint, adopted here, but simply found a subterranean connection to thermodynamics.

 

[vacuumcell]

(probably the standard way that experts have looked at it since 1915 GR appeared) is that the gravitational field is the spacetime geometry.

I think it's the other way round: Spacetime geometry is a structural property of the gravitational field, but only when the latter is not too strong. Thus we can have gravity without any kind of well defined spacetime. Put another way, spacetime is a property of the gravitational field which emerges at sufficiently low energies.

Lorentzian 4D geometry are simply the zero gravitational field.

My understanding is that by a "Lorentzian" geometry we usually mean a geometry described by a metric with Lorentizian signature and not that the spacetime is globally flat.

 

[marcus]

Nicely put, vacuumcell! I'll go with what you said, without the slightest quibble.

 

[vacuumcell]

A fine welcome to Physics Forums. Thank you very much marcus!

 

[fundamentally]

There exist a nice article of Dennis Sciama on inertia that at least explains the motor behind gravity. Gravity is about the mutual influence of items in universe. That influence appears in the action S that manipulates the items.

 

[czes]

That is one answer to your question: What is the gravi field? It is the dynamically changing geometry of space. The gravitational field is not IN space, it IS space. (Or more exactly spacetime.)

The other fields are defined on the gravi field. They live in it. There is no space(time) as a separate entity.

Verlinde does not contradict this--he just focuses on a simplified problem of the Newton approximation to gravity. He cites Jacobson, which is the fundamental work to which all the entropic force papers hark back. Jacobson derived classical General Rel, which is what I've been talking about, from thermodynamics. So far the "entropic force" gambit has not gotten away from the classic 1915 GR viewpoint, adopted here, but simply found a subterranean connection to thermodynamics.

The gravitational field is not IN space, it IS space. This is what I thought. I agree.
If then, the field contains vacuum energy. Is it possible that there is a correlation (correspondence) between gavitational field and vacuum energy ?
I mean the virtual particle-antiparticle pairs appear due to that gravitational field (spacetime) and we observe the curvature of the space time.
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.

Gravitational field = spacetime = vacuum = thermodynamics of the virtual pairs ?

 

[bapowell]

The energy of the vacuum is a purely quantum mechanical phenomenon. The classical gravity described by Einstein's equations need have nothing to do with vacuum energy. That being said, we believe that both quantum mechanics and GR are relevant to the universe, however, as you probably know, the two theories are not, at present, reconciled. Instead, physicists take a 'semi-classical' approach -- gravity takes the form of a classical field (spacetime geometry) and then quantum matter fields are actors on the classical spacetime stage. This is not a trivial task -- the subject of quantum field theory in curved backgrounds is fraught with technicalities and nuance. In certain spacetimes, in particular those with horizons (black holes, de Sitter space, accelerated observers), virtual particles from the vacuum can become 'real', and constitute thermal radiation. However, other spacetimes exist in which the quantum vacuum does nothing remarkable at all, for example, good old fashioned decelerating expansion. So, it's not correct to suggest that spacetime geometry, as an entity, has a temperature. Instead, quantum fields (specifically the quantum vacuum) when existing in spacetimes with nontrivial features (specifically horizons), can lead to the thermal emission of particles.

 

[czes]

I assume the virtual particle-antiparticle pairs in the vacuum are real things though we do not observe them directly.
I assume they are created because of the interaction (relation) between the non-local information of the Compton waves of the particles. Therefore we perceive the gravitational field as the background spacetime as Marcus wrote.
The space in our Universe is just a Vacuum with its virtual particle-antiparticle pairs and contains separated real particles . There isn't other space but vacuum.
What other spactime could exist ?
I don't think it is possible in our Universe.
May be it is not sure the Vacuum / Gravitational field correspondence and we have to discuss it.
Any way Vacuum is everywhere, isn't it ?

 

[fundamentally]

May be the books of Mendel Sachs may bring new insight. He claims that he can unify gravity and the other fields by taking the metric as represented by a four-vector with quaternionic coefficients. He then uses all 16 coefficients of the metric instead of the restriction that Einstein applied. See http://www.compukol.com/mendel/publications/publications.html
Professor Sachs is 90 years old but he has a very original mind.

 

[bapowell]

In a standard Friedmann-Robertson-Walker universe we do not detect thermal radiation arising from the vacuum. Of course the fluctuations are there, but they do not manifest themselves thermally. When you have a spacetime with horizons, ie a black hole, then you can indeed observe them directly -- this is Hawking radiation.

I don't understand why you simultaneously refer to them as "virtual particle-antiparticle pairs and contain separated real particles". I don't know what that could even mean.

In my previous post I was simply trying to logically separate the notions of spacetime and vacuum. As I mentioned, in real life spacetime is indeed filled with quantum vacuum fluctuations. But spacetime as an entity, as it arises from GR, does not logically possesses or necessitate vacuum energy. Furthermore, quantum field theory works just fine in special relativity, and here Minkowski space is not dynamic like the spacetimes of GR, i.e. there's no gravity in SR, but we have a quantum vacuum. The two concepts of a dynamical spacetime and quantum vacuum energy are not logically or physically linked.

 

[czes]

May be the books of Mendel Sachs may bring new insight. He claims that he can unify gravity and the other fields by taking the metric as represented by a four-vector with quaternionic coefficients. He then uses all 16 coefficients of the metric instead of the restriction that Einstein applied. See http://www.compukol.com/mendel/publications/publications.html
Professor Sachs is 90 years old but he has a very original mind.

Thank you for this link. Professor Sachs wrote: " Dark matter is explained in terms of a sea of particle-antiparticle pairs, each in a particular (derived) ground state."
It is just how I understand the spacetime and vacuum:
Dark Energy is our spacetime (Vacuum)and the difference in the distribution of the virtual pairs creates Dark Matter effect. We observe it by gravitational effect.

 

[czes]

I don't understand why you simultaneously refer to them as "virtual particle-antiparticle pairs and contain separated real particles". I don't know what that could even mean.

In my previous post I was simply trying to logically separate the notions of spacetime and vacuum. As I mentioned, in real life spacetime is indeed filled with quantum vacuum fluctuations. But spacetime as an entity, as it arises from GR, does not logically possesses or necessitate vacuum energy. Furthermore, quantum field theory works just fine in special relativity, and here Minkowski space is not dynamic like the spacetimes of GR, i.e. there's no gravity in SR, but we have a quantum vacuum. The two concepts of a dynamical spacetime and quantum vacuum energy are not logically or physically linked.

General Relativity alone is not compatibile to Quantum Mechanics. Therefore I would like to use the Quantum Gravity in physics.
In my idea there isn't distance nor time. There is the relation between the quantum information only. The quantum fluctuations manifest as virtual particle-antiparticle pairs and it is our spacetime. In that spacetime may move real particles.
The real particles (matter) are not fundamental. They are taken out of the vacuum when we concentrate more virtual pairs (information) together that the virtual particles and antiparticles become separated.
According to Wheeler and Beckenstein everything is made of information which we observe as space, time, matter and energy.
If the Gravitational field is different close to Earth and far away it has to indicate something. If there is space as a vacuum and there are fluctuations in that vacuum I assume the gravitational field is our imagination of the fluctuations distribution.

We know that quantum information is non-local. It means it may be everywhere but with a different probability. The succesivity of the information creates its probability of the distribution and we perceive it as a distance. The less probable information we perceive as a more distant.

We start to search the refractive index of the vacuum:
http://arxiv.org/abs/gr-qc/0604009
http://www.iscap.columbia.edu/pages_html/Seminars2007-2008Content/columbia0408.pdf
The paper is from 2006. I proposed in 2007 on an another Forum the "space as a virtual plasma". Verlinde in 2010 shows that gravity may be an enropic force as in a thermodynamic.
I assume the quantum gravity will succesfully use the vacuum and thermodynamics of the virtual pairs.

 

[fundamentally]

I think that space and time belong to different things. Space comes from position observations. Time relates to the thing that pushes states around in Hilbert space. Spacetime is an artificial construct. That is why we are confronted with Minkowski and Lorentzian metrics. See http://www.scitech.nl/English/Science/Exampleproposition.pdf for more details.

 

[czes]

I think that space and time belong to different things. Space comes from position observations. Time relates to the thing that pushes states around in Hilbert space. Spacetime is an artificial construct. That is why we are confronted with Minkowski and Lorentzian metrics. See http://www.scitech.nl/English/Science/Exampleproposition.pdf for more details.

Yes, it is in the Hilbert space.
But where the Hilbert space come from ?
I am an adherent of the Hologram Space where the Hilbert space is created by the relation of the information. The space may recess there:
If there is more information between the objects the distsnce grows. A particle move in the space because it absorbs and emits the virtual pairs from the vacuum. If it absorbs more than emits it is accelerating and the observer does see Unruh effect. The deceleration causes an opposite effect and an object emits more than absorbs.
An extremal example is Compton scattering.

General Relativity is a useful approximation but it fails in Quantum Mechanics where the information doesn't move it just appears and disappears.
I agree with Marcus when he wrote that spacetime is a gravitational field.
My question goes farther - is there a possibility to unify that spacetime with a Vacuum and its thermodynamics of the virtual pairs ?
I think , we find it.

 

[bapowell]

My question goes farther - is there a possibility to unify that spacetime with a Vacuum and its thermodynamics of the virtual pairs ?
I think , we find it.

I've answered this question above. You should consider reading other people's posts when engaging in a thread on the forums.

 

[czes]

In my previous post I was simply trying to logically separate the notions of spacetime and vacuum. As I mentioned, in real life spacetime is indeed filled with quantum vacuum fluctuations. But spacetime as an entity, as it arises from GR, does not logically possesses or necessitate vacuum energy. Furthermore, quantum field theory works just fine in special relativity, and here Minkowski space is not dynamic like the spacetimes of GR, i.e. there's no gravity in SR, but we have a quantum vacuum. The two concepts of a dynamical spacetime and quantum vacuum energy are not logically or physically linked.

I agree that spacetime in GR is smooth and empty. But it is a useful approximation and I do not write about it. I write about a spacetime in Quantum Gravity where the vacuum energy is important.
When we start from a proper Quantum Gravity we come to the General Relativity.
When we start from General Relativity we can't come to Quantum Gravity.
It is the basic difference.
Therefore I want to find a proper Quantum Gravity which agrees with GR. The Vacuum with the thermodynamics of the virtual pairs which create the refractive index may do it, I assume.
There are many ideas how the space may behave. The vacuum with its virtual particles-antiparticles is an experimentally confirmed phenomenon and each theory has to contain it. The problem is how to calculate the virtual pairs in the vacuum that way that it show a gravitational field effect.
Therefore my question about the structure of the vacuum.

 

[fundamentally]

Just a remark: Wave mechanics is quite similar to Fourier optics, but Fourier optics taken one dimension higher. In that sense the camera obscura resembles a black hole. The opening in the wall is a holographic screen.

 

[fundamentally]

Hilbert spaces enter QM because physical items obey quantum logic. The set of quantum logical propositions is lattice isomorph with the set of closed subspaces in Hilbert space. In this way all items in universe get a representation in Hilbert space. What occurs in universe can be represented in Hilbert space!

 

[bapowell]

Therefore I want to find a proper Quantum Gravity which agrees with GR.
The Vacuum with the thermodynamics of the virtual pairs which create the refractive index may do it, I assume.

Virtual particles are not in general thermal. I've given you an example of a spacetime in which they are not. Therefore, in what limit does your quantum gravity theory reduce to GR?

There are many ideas how the space may behave. The vacuum with its virtual particles-antiparticles is an experimentally confirmed phenomenon and each theory has to contain it. The problem is how to calculate the virtual pairs in the vacuum that way that it show a gravitational field effect.

This has been done in so-called semi-classical treatments in which spacetime is classical and the matter fields are treated quantum mechanically. Are you suggesting something different than the work of Hawking, Unruh, Davies, etc?

Therefore my question about the structure of the vacuum.

The structure of the vacuum or the gravitational properties of the vacuum? In order to do this, you need a theory of gravity. What will you use in place of GR? After all, you say that your starting place will not be GR, since GR must emerge from your quantum gravity. There is not currently a theory of quantum gravity. You keep touting the importance of spacetime -- what is the nature/origin of spacetime in your quantum gravity theory? How is it separate from the quantum fields that exist in it?

 

[czes]

Virtual particles are not in general thermal. I've given you an example of a spacetime in which they are not. Therefore, in what limit does your quantum gravity theory reduce to GR?

This has been done in so-called semi-classical treatments in which spacetime is classical and the matter fields are treated quantum mechanically. Are you suggesting something different than the work of Hawking, Unruh, Davies, etc?

The structure of the vacuum or the gravitational properties of the vacuum? In order to do this, you need a theory of gravity. What will you use in place of GR? After all, you say that your starting place will not be GR, since GR must emerge from your quantum gravity. There is not currently a theory of quantum gravity. You keep touting the importance of spacetime -- what is the nature/origin of spacetime in your quantum gravity theory? How is it separate from the quantum fields that exist in it?

I agree the virtual pairs alone are not thermal. The difference in the absorption is thermal. As I wrote the accelerating object when it absorbs more than emits perceives a themperature. It isn't in an equilibrium when absorption=emission.
Why an object accelerates in the gravitational field ?
It accelerates because the distribution of the virtual pairs is denser close a massive object because a probability of the interaction between information of the Compton wave. Therefore an object absorbs more virtual pairs towards the massive object. It is what Verlinde wrote. Gravity is an entropic force.

I do not want to change GR. It is a good theory for macro world but in micro world we need Quantum Gravity. In my idea the spacetime is created of the vacuum. Vacuum is the physical spacetime of the mathematic.
You may calculate the motion of an object in the spacetime and it is correct. If you want know the origin of the spacetime you have to search the vacuum. There isn't another spacetime but a vacuum in our real Universe.
general Relativity is a good theory but it doesn't show an origin of the spacetime.

Unruh-Davies effect, Compton scattering indicate that Vacuum is not an imaginary empty spacetime. There are fluctuations (vacuum energy) which interact with a moving object.
I identify this fluctuation with an interaction of the non-local information of the Compton wave length (time).
For example:
(lp / l x ) * (lp / l y ) = -a Fg / Fe
where: lp=Planck length, lx,ly=Compton wave length of the particle x and y
a=alfa=fine structure constant
Fg=gravitational interaction, Fe=electromagnetic interaction.
http://www.cramerti.home.pl/

Due this above equation each oscillation of the particle creates a non-local information about this oscillation everywhere but its distribution is due to probability inversely proportional to the squared distance between the objects.
The distance is not fundamenta,l it is created by a number of the non-local interactions of the quantum information between the objects.
I assume the interaction between the non-local quantum information creates the virtual particle-antiparticle pair. We observe the virtual pairs, not the virtual particles. We do not know what is the virtual particle and what is the fundamental information because we can't observe them. Therefore i write virtual pairs, not virtual particles which may be everywhere and anything (non-locality).

My idea is the same as the holographic principle of Gerard 't Hooft, Beckenstein, Verlinde, Smoot. All of them write about vacuum with the virtual pairs. Here is a good article about inertia:
http://www.calphysics.org/articles/chown2007.html

 

[czes]

Just a remark: Wave mechanics is quite similar to Fourier optics, but Fourier optics taken one dimension higher. In that sense the camera obscura resembles a black hole. The opening in the wall is a holographic screen.

It is very interesting example. I have to study it closer.
Also the Hilbert space is a very good mathematical tool for physics in our Universe we do not need something else.

 

[ameliaswank]

this questions triggers in mind from quite a long time... and now i have got some very useful information regarding that... specially the part of information

"Unruh-Davies effect, Compton scattering indicate that Vacuum is not an imaginary empty spacetime. There are fluctuations (vacuum energy) which interact with a moving object.
I identify this fluctuation with an interaction of the non-local information of the Compton wave length (time).
For example:
(lp / l x ) * (lp / l y ) = -a Fg / Fe
where: lp=Planck length, lx,ly=Compton wave length of the particle x and y
a=alfa=fine structure constant
Fg=gravitational interaction, Fe=electromagnetic interaction"

is far more informative this example is really helpful in making things clear... Great work guyss

 

[czes]

this questions triggers in mind from quite a long time... and now i have got some very useful information regarding that... specially the part of information

"Unruh-Davies effect, Compton scattering indicate that Vacuum is not an imaginary empty spacetime. There are fluctuations (vacuum energy) which interact with a moving object.
I identify this fluctuation with an interaction of the non-local information of the Compton wave length (time).
For example:
(lp / l x ) * (lp / l y ) = -a Fg / Fe
where: lp=Planck length, lx,ly=Compton wave length of the particle x and y
a=alfa=fine structure constant
Fg=gravitational interaction, Fe=electromagnetic interaction"

is far more informative this example is really helpful in making things clear... Great work guyss

The idea of the "Virtual Plasma" seems crasy but it explains all mysteries like Dark Energy, Dark Matter, Space Recession, Space Inflation...
We have consider it seriously since Nobel prize laureates 't Hooft, Smoot and other physicists like Beckenstein, Verlinde are engaged in the holographic principle.

If a virual pair of the virtual plasma is just an interaction between non-local information about Compton wave length and due to above equation causes a length contraction of the Planck length and time dilation of Planck time, we can calculate retationship for a static Black Hole from thermodynamics of the Vacuum:

Lets mass of an object (M) / average mass of the particle (m) is a number of particles in an object (n=M/m).
Due to limitation on measurement the uncertainty in position must be greater than half of the reduced Compton wavelength (h/2mc).
One particle causes a relative dilation lp / (h/2mc) interfering with another particle (non-local quantum information).
How many particles n=M/m emits non-local information and stops the time or covers the radius (R/lp) of the object ?
(M/m) [(lp / (h/2mc)]= (R/lp)
From this simple calculation we receive known solution for static Schwarzschild Black Hole:
2MG / c^2 = R
http://www.cramerti.home.pl/

It suggests that the most basic effect causing the curvature of the space is just a refractive index of the vacuum (virtual plasma). Therefore it has its own relativistic mass (Dark Matter) and we observe gravitational field effect.

The gravitational field is here an observed effect of the vacuum (virtual plasma).
Is it written properly ?