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Plank length / space expansion question

  1. Nov 5, 2014 #1

    Grinkle

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    Do models of the expansion of space-time manifest that expansion by an increase in Plank-length, or by additional Plank-lengths appearing in between existing particles?

    I think it must be the latter because the Plank-length is not an empirically measured value and the constants it derives from do not change. IIRC anyway.

    Same question for inflation, if the answer is any different.

    If I think of a black hole and the whole Hawking entropy / hologram thing, as the black hole grows, it adds Plank areas to its horizon, so my intuition tells me that its sensible to think of expansion and inflation both as processes that add Plank lengths (additional Plank lengths each of the same size I mean) to all dimensions.

    Actually, for all I can imagine, the value of an individual Plank length may be observed by some entity outside our space-time to fluctuate wildly and I might have no way to notice. Maybe its even a silly question I'm asking.
     
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  3. Nov 5, 2014 #2

    phinds

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    You seem to be hung up on the Plank length for some reason. It is just an arbitrary, man-made unit of measure just like the foot and the meter. Reality does not care how we label it, it just does what it does. What we try to do is come up with units that have various utility for us. Out units of measure do not change just because it happens that things on a cosmological scale are getting farther apart.

    The space between particles, or planets or solar systems, or even galaxies within a local cluster, does not change with the expansion of space, which only affects objects which are not gravitationally (or otherwise) bound.
     
  4. Nov 5, 2014 #3

    Grinkle

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    phinds -

    My perspective -

    I think of the Plank length as the smallest distance that is physically meaningful. From Wikipedia -

    "the Planck length is, in principle, within a factor of order unity, the shortest measurable length – and no theoretically known improvement in measurement instruments could change that."

    Smaller distances are not physically realizable. No particle can be measured to be at a location to greater precision than the Plank distance. If I'm wrong about that, then its just me being hung up, as you say.
     
  5. Nov 5, 2014 #4

    Grinkle

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    And you answered my question. Because we do not observe objects expanding if there is any significant force to bind them together, it must be that extra distance or extra space or extra Plank lengths or whatever appear and particles that are attracted to each other "snap" across this new space to retain their previous spacing. If the Plank length itself were changing, unless my understanding of quantum physics is totally off (which it may be) then no snapping as I describe it would be possible.

    I had forgotten the observation you noted - appreciate the reminder.
     
  6. Nov 5, 2014 #5

    phinds

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    Yes, but I don't see what and of that has to do with whether or not space can expand by a smaller amount. The whole issue of whether or not space is a "thing" that can bend / expand / etc is a contentious issue and not pinned down, nor is the issue of whether or not there is any granularity to the expansion, but if there IS granularity, there is absolutely no reason be believe that it is one Plank length or has anything to do with any man-made arbitrary unit of measurement.

    It is possible that you are getting sidetracked by the fact that the Plank length is defined in terms of the speed of light and other physical constants. So what? So is the meter.
     
  7. Nov 6, 2014 #6

    Grinkle

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    I am being sidetracked by my perception that the Plank length is postulated to be the size of one grain of space, and the Plank time is postulated to be the size of one grain of time.

    After doing some looking for specific reference to that postulation, I can't find one - the only special thing about the Plank values are they result from scaling other constants to a value of "1" - which may just be nothing more than interesting numerology, as you are flagging.

    As an aside, I note that I find the notion of granular space-time very compelling. It strikes me as more likely than continuity at scales arbitrarily small, but that is of course a subjective aesthetic comment.
     
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