4 ms·

A priori, the Planck length is a a length like any other. It's perfectly fine to talk about that scale in classical physics, without involving QM. > Pure class

A priori, the Planck length is a a length like any other. It's perfectly fine to talk about that scale in classical physics, without involving QM. > Pure classical physics doesn't have a smallest length scale. Quantum mechanics doesn't have a smallest length scale, either, as far as we know.

amai 3y ago
No, Planck length is not a length like any other. Any other length gets Lorentz contracted, but the Planck length does not. It is a constant in any reference frame. And if you assume hbar->0 as for classical mechanics the Planck length will be zero. So classical mechanics doesn't have a finite Planck length. Quantum mechanics doesn't have a smallest length scale, but Quantum Electrodynamics has, namely the Compton wavelength. And this is what goes into the derivation of the Planck length. So with only classical theory you simply can't derive the Planck length. Classical theory doesn't have a smallest length scale.
- codethief 3y ago
  > Any other length gets Lorentz contracted, but the Planck length does not. Are you claiming that the value of the constant l_P is the same in every reference frame or that objects of length equal to l_P will have the same length (= l_P) in every reference frame? The former is trivially true, the latter is definitely false unless you assume new physics. See also the discussion in https://physics.stackexchange.com/questions/4094/is-the-planck-length-lorentz-invariant https://physics.stackexchange.com/questions/4094/is-the-plan... > but Quantum Electrodynamics has, namely the Compton wavelength Citation needed. (Besides, the Compton wavelength of what particle exactly?) > So with only classical theory you simply can't derive the Planck length. So classical mechanics doesn't have a finite Planck length. I don't need to derive anything. In classical mechanics I am free to define a constant with value equal to 1.616255(18)×10−35 m and call it the Planck length. Whether or not it has any physical significance is irrelevant. (I mean, questions of physical significance so far have never stopped proponents of the Planck length in QM, either.)

amai 3y ago

No, Planck length is not a length like any other. Any other length gets Lorentz contracted, but the Planck length does not. It is a constant in any reference frame. And if you assume hbar->0 as for classical mechanics the Planck length will be zero. So classical mechanics doesn't have a finite Planck length. Quantum mechanics doesn't have a smallest length scale, but Quantum Electrodynamics has, namely the Compton wavelength. And this is what goes into the derivation of the Planck length. So with only classical theory you simply can't derive the Planck length. Classical theory doesn't have a smallest length scale.

codethief 3y ago
> Any other length gets Lorentz contracted, but the Planck length does not. Are you claiming that the value of the constant l_P is the same in every reference frame or that objects of length equal to l_P will have the same length (= l_P) in every reference frame? The former is trivially true, the latter is definitely false unless you assume new physics. See also the discussion in https://physics.stackexchange.com/questions/4094/is-the-planck-length-lorentz-invariant https://physics.stackexchange.com/questions/4094/is-the-plan... > but Quantum Electrodynamics has, namely the Compton wavelength Citation needed. (Besides, the Compton wavelength of what particle exactly?) > So with only classical theory you simply can't derive the Planck length. So classical mechanics doesn't have a finite Planck length. I don't need to derive anything. In classical mechanics I am free to define a constant with value equal to 1.616255(18)×10−35 m and call it the Planck length. Whether or not it has any physical significance is irrelevant. (I mean, questions of physical significance so far have never stopped proponents of the Planck length in QM, either.)