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Plack lenght and its inverse!

  1. Jul 15, 2014 #1
    Hello all,

    If Planck lenght (1.61619926 × 10(-35 )meters) places a theoretical limit on minimum possible distance does it also imply that we have a maximum theoretical limit on measurable length as inverse of planck length (1/Planck Length)..

    does there any such limit on the maximum theoretical distance exist for our Universe?

    Thank you for your inputs ..
  2. jcsd
  3. Jul 15, 2014 #2
    well, 1/planck length has units (1/meter) not (meter), so it isn't a distance at all.
  4. Jul 15, 2014 #3


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    Not true. To quote Wikipedia
  5. Jul 15, 2014 #4
    √ Agreed!

    @ Phinds - Wikipedia also states 'Profound significance of Planck length in theoretical physics' :)

    thank you both for responding ..
  6. Jul 15, 2014 #5


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    Yes but "profound significance" does NOT imply that it represents the smallest unit of length.
  7. Jul 15, 2014 #6


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    An inverse planck length would be several orders of magnitude larger than the observable universe. It is unclear [for obvious reasons] if it has any fundamental significance. The planck length is a somewhat contrived unit, as already noted. It is unclear if it has any fundamental significance. It is probably more of a convenient bookkeeping tool.
  8. Jul 15, 2014 #7


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    Re-read post #2. This is utterly incorrect.
  9. Jul 15, 2014 #8


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    Curvature is often measured as inverse area.

    A smallest length (if nature had one) would suggest a minimal detectable area, and thus an upper bound on curvature. Nature would have a "greatest possible curvature" in some sense.

    These smallest and largest things are not supposed to be exact, I think, but to have meaning as order of magnitude quantities, as *scales*.

    The central coefficient in the Einstein GR equation is a FORCE. Usually you see it as an inverse force constant on the lefthand side:

    It relates curvature (on the left) to energy density and the like (on the right). IOW it relates geometry to matter.

    I don't imagine folks understand this down to the level of precise detail, but in a general order of magnitude sense nature seems to have intrinsic scales of force, area, pressure, energy density, curvature, built into its very texture. At this point maybe it is just an intuitive feeling some people have. I believe we'll learn more.

    To me, the minimal area (and the related maximal curvature, maximal energy concentration) seem more meaningful than the minimal length. The length is just a conceptual way of approach, leading to the other quantities. Have to go, no time to try to make this more coherent.
    Last edited: Jul 15, 2014
  10. Jul 15, 2014 #9
    No, and the reason for this can be understood through this experiment:
    Suppose the earth receives a photon with a wavelength ##\gamma_1##. Since spacetime is expanding, we know that this photon had an original wavelength ##\gamma_2##, such that ##\gamma_2\lt\gamma_1##. This phenomenon is known as redshift. Nothing special.

    Now, here's the thing, if the earth receives from far away a photon whose wavelength is equal to Planck's length ##\ell_p##. This means that the photon before -- who traveled all this distance -- had a wavelength smaller than ##\ell_p##. And this contradicts the "Planck length [...] minimum possible distance".

    This problem is known as the Trans-Planckian problem, it's still unsolved and that's mainly because we have no theory of quantum gravity that can describe what happens at such scales.
  11. Jul 16, 2014 #10
    Thank You all again for your responses,

    To summarize what i learned here,

    -Theoretically and debatably distances lesser than Planck lenght can not exist within normally observed properties and framework of space time
    -Inverse of Planck length should just mean the number of Planck Length Units that can be included in a meter wide distance
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