What is Planck length: Definition and 81 Discussions

In physics, the Planck length, denoted ℓP, is a unit of length in the system of Planck units that was originally proposed by physicist Max Planck, equal to 1.616255(18)×10−35 m. The Planck length can be defined from three fundamental physical constants: the speed of light, the Planck constant, and the gravitational constant. It is also the reduced Compton wavelength of a particle with Planck mass. Regardless of whether it represents some fundamental limit to the universe, it is a useful unit in theoretical physics.

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  1. R

    What Happens Below the Planck Length?

    This comes from Alex Vilenskin's Many Worlds in One: I didn't think there was such a distance below the Planck length.
  2. D

    Understanding Planck Length and Time in Relation to Spacetime and Acceleration

    Planck length and Planck time are constants, correct? If spacetime changes for an observer who accelerates to subluminal velocities, what happens to these units? If they're constant I'm guessing they can't be "stretched" or "squashed". So do they change in number for a given measurement (say...
  3. D

    Planck length - what is it really?

    Hello everyone. After reading most of the existing topics on the Planck length, I still haven't found what the Planck length really is. Some references say nothing can possibly be smaller than the Planck length, whereas others say it's just a threshold below which "special" things happen...
  4. T

    Does Planck length and irrational solutions mean time can't be reversed?

    If there is an irrational solution to an equation for where a particle should be, for example from an ODE, then what effect does Planck length have on that? Does the actual position of the particle get rounded to an a multiple of the Planck length? If it does, wouldn't that imply there is a loss...
  5. B

    From the Planck length to the Observable Universe

    Here is a study in distances based on powers of ten of the meter. It gives easy-to-see graphic examples of relative size, from the Plank length out to the Observable Universe. Imho, a great teaching aid. http://www.newgrounds.com/portal/view/589217
  6. B

    'Quantum grains' smaller than 'Planck length'

    How is it possible that 'quantum grains' are many orders of magnitude smaller than 'strings' or the 'Planck length'?
  7. C

    Hypothetical situation of planck length object and lightspeed

    Initially known theories: 1. General consensus: Planck length is smallest measurable unit of length 2. Objects approacing light speed contract in length Scenario: Some hypothetical mass measured to be a Planck length is accelerated infinitely close to light speed. Questions: Can a...
  8. T

    Singularities, Density, and the Planck Length

    It is generally accepted that a star of sufficient mass collapsing in on itself will form a black hole (singularity) where density is infinite. I see a few problems arising with this, and I would like to have them clarified. 1.) Density=mass/area If the mass of any star is finite, how can an...
  9. nomadreid

    Planck length - Greene versus classics

    Planck length -- Greene versus classics Brian Greene, in Chapter 10 of "The Elegant Universe", offers a sketch of string theory's version of why massive particles have a minimum size of the Planck length. I give my summary of the argument in the next paragraph, and the question in the following...
  10. S

    Is the Planck Length real? How do we know?

    I've heard of Stephen Hawking talking about the Planck Length and how nothing could exist at a level smaller than that length. I'd like to know how we know this to be certain. It seems that history has always repeated itself in that when humans see as far as we possibly see we simply just...
  11. T

    Can Extra Dimensions in String Theory Be Smaller Than a Planck Length?

    Can an extra dimension of empty space be smaller than a Planck length? Is that why string theory allows those extra dimensions to be smaller than a Planck length? Because they are empty? I remember reading posters on these forums say that space itself can expand faster than the speed...
  12. H

    Planck length and the speed of light

    How can the Planck length and the speed of light both be universal constants? Will the Planck length change depending on your frame of reference? :)
  13. F

    Exploring the Planck Length & Quantum Effects

    "The Planck length is the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate. This is the ‘quantum of length’, the smallest measurement of length with any meaning." What I am wondering about this description is the word "dominate"...
  14. LarryS

    Wavelength Less Than Planck Length?

    It seems like if you accelerated a massive particle to a high enough velocity (momentum), you could get it's wavelength to be less than the Planck Length. Does that make sense? As always, thanks in advance.
  15. N

    Cosmological Constant and Planck Length in relativity

    There seems to be a difference in the way relativity views Planck length as frame dependent and the cosmological constant, an energy density?, as invariant...Any insights appreciated! There is a well known contradiction between relativity and the Planck length: Wikipedia explains the...
  16. D

    Exploring the Planck Length-to-Mass Ratio in Quantum Physics

    does anyone know something about the ratio of Planck length to Planck mass : signification, use in quantum relationships...
  17. M

    Derive Planck Length in terms of c, G, and h_bar

    Homework Statement a. The first part of this problem was to derive the escape speed (Vesc) for a star of mass M and radius R. b. The second part is where I am having trouble. It says to equate the Vesc calculated above and derive a formula for Planck length in terms of c, G, and h_bar...
  18. R

    Does the Planck length grow with the scale factor?

    What happens to the Planck length due to the expansion? Does it grow with the scale factor?
  19. J

    How can the Planck length be claimed to be the smallest length?

    Many arxiv papers state that the Planck length is the smallest measureable length. On the other hand, the gravitational length L=2Gm/c^2 associated with every electron or proton is 19 or 22 orders of magnitude smaller than the Planck length. Nobody seems to doubt either of the two statements...
  20. D

    Is Space Discrete or Continuous in a Planck World?

    On a ruler, you could move from the 1cm marking on a ruler to the 2cm marking, or you could move from the 0.5cm marking to the 1.5cm marking - both are 1cm movements but you cannot go from 1.5cm to 1cm in a 1cm movement. If we had Planck co-ordinates, could you move from 1 => 2 and also 1.5 =>...
  21. S

    Deriving the Planck Length, Time, and Mass

    How are the Planck length, Planck time, and Planck mass "derived"? I know their values, but I don't understand where we get them from.
  22. S

    Susskind on 'variation in Planck length'?

    In "Black Holes, Information and the String Theory Revolution" by Leonard Susskind and James Lindesay, the authors give, (Ch. 15: Entropy of Strings and Black Holes, pg. 170): "The string and Planck length scales are related by g^2* (l_s)^D-2 = (l_p)^D-2 (15.0.23) " They then find...
  23. S

    Susskind on 'variation in Planck length'?

    In "Black Holes, Information and the String Theory Revolution" by Leonard Susskind and James Lindesay, the authors give, (Ch. 15: Entropy of Strings and Black Holes, pg. 170): "The string and Planck length scales are related by g^2* (l_s)^D-2 = (l_p)^D-2 (15.0.23) " They then find...
  24. K

    Newton's Third Law, Gravity, and the Planck Length

    If you have a object that is about 10^36 less massive than the Earth (about 10^-12 kg) and say it exerts a force on the Earth about 10^-11 Newtons, this would cause the Earth to accelerate about 10^-36 m/s^2. If this object was away from earth, and say if these objects were initial stationary...
  25. H

    Smallest Distance Possible: Planck Length

    Is it appropriate, if I defined Planck Length as the smallest distance possible for a matter to travel?
  26. F

    Why Half a Planck Length is Wrong

    I know the concept of half a Planck length is wrong. But could someone explain exactly why it is wrong. Also could someone tell me what we conclude about the photon that has moved at C in a Planck time (for example) Do we conclude that the photon was at A and is now at B after the passing of a...
  27. C

    What is Planck time, Planck length, Planck particle

    Can someone explain what are they: For egs. What is Planck time, PLanck length, Planck particle etc.
  28. E

    Deriving Planck Length: Smallest Possible Length?

    Ok so I understand how we derive Planck Length using the natural constants G,h, and c. However, how do we know this indeed represents the smallest possible length an object in the universe? Or does Planck length just represent the smallest possible length under which gravity is still influential?
  29. E

    Explaining Plank Length & Its Dependence on Constants

  30. M

    Exploring the Planck Length Theory Controversy

    Is the Planck length a commonly accepted theory or is it controversial in any way?
  31. C

    How was the Planck length (or time) calculated?

    Can anyone please shed some light on how the Planck length or Planck time were found? I understand why they have to exist, but what predicts their values? Is it even a prediction or can we actually calculate the Plank length with experiment? Thanks,
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