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What is the gravitational field ?

  1. Jun 14, 2010 #1
    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) ?
  2. jcsd
  3. Jun 14, 2010 #2
    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:

    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,)

    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 relevent, but it sure is conceptually understandable....
    Last edited by a moderator: Apr 25, 2017
  4. Jun 14, 2010 #3
    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...
  5. Jun 15, 2010 #4
    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.
  6. Jun 18, 2010 #5
    I think we have a language barrier here.

    Quantum information and the holographic principle are not directly related to particle/antiparticles....although quantum thoery underlies them ....I understand none of the rest of the last post....
  7. Jun 18, 2010 #6
    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.
  8. Jun 27, 2010 #7
    Sorry I have no idea what most of your questions mean....maybe a language barrier?
  9. Jun 27, 2010 #8


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    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.
    Last edited: Jun 27, 2010
  10. Jun 27, 2010 #9

    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.

    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.
  11. Jun 27, 2010 #10


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    Nicely put, vacuumcell! I'll go with what you said, without the slightest quibble.
  12. Jun 28, 2010 #11
    A fine welcome to Physics Forums. Thank you very much marcus!
  13. Jun 30, 2010 #12
    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.
  14. Jun 30, 2010 #13
    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 ?
  15. Jun 30, 2010 #14


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    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.
  16. Jul 1, 2010 #15
    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 ?
  17. Jul 1, 2010 #16
    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 [Broken]
    Professor Sachs is 90 years old but he has a very original mind.
    Last edited by a moderator: May 4, 2017
  18. Jul 1, 2010 #17


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    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 possess 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.
  19. Jul 1, 2010 #18
    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.
    Last edited by a moderator: May 4, 2017
  20. Jul 1, 2010 #19
    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:
    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.
    Last edited by a moderator: Apr 25, 2017
  21. Jul 1, 2010 #20
    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 [Broken] for more details.
    Last edited by a moderator: May 4, 2017
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