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Spatial Expansion Interpretation with Quantum Graphity Model

  1. Jun 19, 2015 #1
    The way the Quantum Graphity model has been explained to me is that I should picture "space" as being represented by the quantum graph, with points in the graph being nodes of communication/interaction between various fields. So, as space expands, do the points become farther apart, separated by "nothing", or do more points "fill in". It seems that there couldn't be "nothing" because it would create vacuum energy and virtual particle production.

    How should I visualize this?
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  3. Jun 19, 2015 #2


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    There isn't a known description here. The problem is that we don't have a good handle on quantum gravity, and without understanding quantum gravity, we really can't say whether or not space-time is discrete, and if it is discrete what that looks like.

    Certainly it is the case in General Relativity that a cubic meter of space when our universe was five billion years old is the same as a cubic meter of space today (aside from the matter/radiation contained within). So one could argue that the same must be true of a quantum theory of gravity, but I don't think anybody knows for sure.
  4. Jun 19, 2015 #3
    I gather that Einstein dedicated his later years to that, and made progress but reached no conclusions.
  5. Jun 19, 2015 #4
    If we broaden the discussion to any of the the quantum gravity models, including basic loop gravity, my confusion remains more or less the same. If "space" has a discrete structure, how does the expansion of space occur. Do the "quanta" of space expand correspondingly, or do the quantity of spacial quanta increase to create expanding space.
  6. Jun 19, 2015 #5


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    Hard to see how your question would apply to LQG. Rovelli is a central figure in contemporary covariant LQG (aka spin foam QG) and he has explained the Loop does not say what geometry is "made of" but rather describes how it responds to measurement.
    You do not imagine that space or spacetime "is" little grains, or little loops, or some kind of ball and stick contraption, or little triangles and pyramids stacked together, or a "simplicial complex" or a "spin network" of nodes and links.
    You are trying to do quantum geometry which means how nature responds to measurement of lengths, angles, areas, volumes. geometric observables.

    Bohr said quantum theory is not concerned with what Nature "is" but with what we can say about it what we can measure, what we can predict, how our interactions and measurements affect other measurements. So Rovelli was just extending Bohr's teaching to geometry, not just matter particles and radiation.

    Your question assumes that space is "made" of some kind of hardware that needs to expand if space expands.
    but actually when geometry expands, distances expand. And there is nothing that says they should not. GR is all about dynamic geometry.
    We have no right to expect lengths, angles, areas etc to remain always the same or always in the same relation to each other.

    When distances expand we say "space expands" but space is not a THING. Nothing material has expanded. There is no mechanical underlay that we need to describe, like "creation of new points".

    That's LQG. I don't know if some other QG has space actually made of little toy objects or grains. So then your question would apply. Do they expand or are more created? But that would be some other kind of QG, I'm not sure which.
    If I think of active areas of QG research, non-string, I think most would be like LQG in that. IMHO Asymptotic Safety QG, anyway, maybe Causal Dynamical Triangulations too.
  7. Jun 20, 2015 #6


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    Don't know about Q. Graphity. Haven't seen or heard much about it for 5 or 6 years. Don't know what its quantum operators, its observables look like.
    Maybe it is more like what you have in mind. Space "made of" something. But I didn't respond (until you said "broaden the discussion" because I'm not all that familiar with it.)

    Maybe there will be a talk about it at the upcoming Loops 2015 conference. Here
    Loops conference organizers often invite a few talks from other lines of QG research
    (eg AsymSafeQG, NonCommutative Geometry, Causal Sets, CDT, Effective field theory, these are ordinarily not large enough QG efforts to have their own conference so they may give talks at Loops conference. It is good for everybody to share ideas and results across lines. Something with Q. Graphity might come up next month at Loops but I didn;t see any announcement)
    Last edited: Jun 20, 2015
  8. Jun 20, 2015 #7
    Thanks Marcus. I'll check out what the conference has to offer on the subject.
  9. Jun 23, 2015 #8
    I realize that I shouldn't pursue this line of thought, but I'd like to try anyway (without sliding down the slippery slope into philosophy).

    I believe you've hit the core essence of my question here, but I'm still stuck in a perpetual fog of confusion about this subject. So, let me reframe my question somewhat.

    You've stated that I shouldn't think of space as being a "thing", but I can't quite shake that concept entirely. When you (professional physicists/cosmologists/academics) consider "space", are you not ultimately thinking about it as a distribution of some type of field? It seems that I've seen several sources stipulate that "empty space" is a meaningless reference... that it doesn't (and can't) exist because there is always the "quantum foam" (at least) to give it structure. In that sense anyway, isn't space a "thing" of sorts? So, if we simply consider the quantum foam as a structural component that "expands" as space expands, how does that occur? Whether we think of space as unitary or discrete in nature, it must always contain the quantum foam of energy and virtual particles, if I understand that concept correctly. So, does the distribution of quantum foam "disperse" as space expands, with the virtual particles popping into and out of existence "further apart" at the same frequency, or do virtual particles occur at greater frequency? Is the quantum foam something that I can think of as having a relative "density" of sorts? Is this really a question about the vacuum energy content of a given volume of space. If so, how does that distribution/value of energy change as space expands?

    I recognize that there's a lot of questions there, but I'd appreciate any input that might help me wrap my mind around this concept accurately.
  10. Jun 23, 2015 #9
    I suppose that I should clarify that I'm trying to utilize the virtual particle distribution and production frequency as a relative measuring device because they seem to me to be somewhat independent "scale-wise" from the expanding space... in the sense that they are dimensionless fundamental point particles.
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