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A quick question about Loop Quantum Gravity

  1. Oct 4, 2008 #1
    There's an article about Loop Quantum Gravity in this month's Scientific American, and I have a quick question. They assume space itself is made out of "atoms" with definite size (Planck's length if I'm not mistaken). So how exactly does it work with the expansion of the Universe? Do these "space-atoms" move away from each other? Then what would be the stuff between them (non-space)? Or are there more "space-atoms" being created? Or do the "space-atoms" themselves get bigger? In that case, it's unclear to me against what their increasing size can be judged since they themselves are space. Thanks.
  2. jcsd
  3. Oct 4, 2008 #2


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    In actual LQG they do not assume that space is made out of atoms. The Scientific American is a wonderful magazine but it gives popularized accounts which are verbal and to a large extent metaphorical, sometimes making them to some extent misleading.

    I think the moral is probably don't ever take a verbal metaphor for the real thing. It is never a substitute for the mathematical model.

    The mathematics of LQC (loop quantum cosmology) is not all that difficult. You might want to have a look. It may be that the article you are talking about is not about LQG but about LQC, somewhat different.

    Here is a keyword search for all the quantum cosmology research papers published since 2005 ordered by citation count so you get the most important ones first (SLAC Stanford data base).
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    Talking about plancksize atoms of space is a great way to give people INTUITION about how the mathematics of LQC works. The math is not based on assuming atoms but it works out somewhat as if---it gets rid of the bang singularity and matches the standard cosmology model as soon as a few momnts have passed. It might be right or it might be wrong, don't know, interest and research in it has been on the rise in the past 2 or 3 years.
    Last edited by a moderator: May 3, 2017
  4. Oct 4, 2008 #3
    Ya I sort of have the same intuitive question as Phosopher does about expansion but not from quantum gravity fueling. Just from regular common sense expansion experience. I have a big intuitive notch missing when trying to gulp ideas like "Space itself being created" or "there was no space created yet". BIG problems. Like time, if there was "NONE" what would there be? And when there is "SOME", we still can sense it...how? We cant feel it, we can't see it, smell it, or taste it, yet we exist in it! So we know its there by some deeper sense than mathematical theory. And just that "Deep inner sense" is all we ever have to ascertain all phenomena. And based on that alone there must be some way to compare space's expansion in measure. If space is expanding, its everywhere no? Right in front of us as we corkscrew through it. So in a vacuum as galaxies move apart why can't the vacuum medium remain as is? I mean what can you possibly compare galaxy motion alone to, versus the entire space motion?
    (Especially at this distance!) But let me remind you "That distance" is always right here around us in front of us at + - 20miles per sec! So are there any measurable expanses of "space itself" real in science at this time?
  5. Oct 4, 2008 #4


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    As you say, you are asking about ordinary classic expansion that you hear about in standard cosmology. There is a thread about that in the Cosmology forum called "the physical meaning of expansion in cosmology".

    I will get the link for you.

    Your question is not about quantum gravity, as you point out, so it is off topic here. But would not be off topic in that other thread.
    Last edited: Oct 4, 2008
  6. Oct 6, 2008 #5
    Phsopher and others are absolutely correct. The atoms of space - elements of the "spin network" - that loop quantum gravity assumes to be the elementary building blocks of space don't look like ordinary atoms we know, but they share their basic properties such as having a characteristic size and distance from each other (assumed to be the Planck length). That's the typical length of the edges of the "spin network", for example.

    This simple property that the spin network shares with the "ordinary atoms" is enough to see that conventional cosmology is impossible in the framework. The spin network is a material, analogous to solids, and it would have to create new atoms (or links of the "spin network") for the Universe to expand.

    There are other ways to see that all models assuming that the empty space is made out of anything similar to "atoms" contradict basic, well-established, and somewhat "obvious" features of physics. The most obvious one is that the atoms pick a preferred reference frame, much like the luminiferous aether did in the 19th century. That's excluded by special relativity and all evidence supporting special relativity. It's enough for the atoms to be able to carry a nonzero entropy in empty space and relativity is doomed because the entropy density is the time-component of a 4-vector. If it is nonzero, Lorentz invariance must be (heavily) broken, by an amount proportional to the (gigantic) Planckian entropy density. The "material" of the spin network would also slow down moving objects hugely: anything made out of anything resembling normal atoms is closer to a crystal than a vacuum.

    Every hint that theories with structure filling empty space able to carry nonzero entropy (information about the detailed arrangement of the atoms) can be viable 103 years after special relativity are balderdash. LQG and dozens of other naive models of physics have been dead for more than a century but some very slow people haven't yet noticed.
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