> 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

> 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.)