# HUP; once more time

member 11137

## Main Question or Discussion Point

Sorry for this first year level question but something is really not clear in my head concerning the Heisenberg's uncertainty principle. First remark: my question here is neither a critic of this principle nor a tentative to collapse it. No; I consider the propagation of the light in the air or in vacuum. I stay at the origin of an inertial frame. I know via experiments two things: 1) the speed of light with a hight precision (c); 2) the trajectory of the light: it is automatically going straightforward as long as the beam does not encounter a mirror, a prism, a big concentration of matter, ... So: I can predict the position and the speed simultaneously ! What is wrong in this manner to present the reality ? Thanks for the help because I get some panic with this. Must I think that the trajectory only is an average one? I know: it must be unpleasant to always repeat the same things ...

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Do you know diffraction? If yes how can you predict with certainty the position of a photon within a beam of light?

Seratend.

jtbell
Mentor
The uncertainty principle deals with momentum, not velocity. The momentum of a photon is related to its wavelength by $p = h / \lambda$. So, even though the speed of a light beam is exactly known (in vacuum), its momentum can still be uncertain. This corresponds to an uncertainty in the wavelength. No light source is completely monochromatic. Even a laser has a small but finite spread in wavelengths.

member 11137
jtbell said:
The uncertainty principle deals with momentum, not velocity.
Of course !!! Oh I am so sorry: I think I need holydays... Thanks for the rapid answer; to seratend too.

DrChinese
The uncertainty principle deals with momentum, not velocity. The momentum of a photon is related to its wavelength by $p = h / \lambda$. So, even though the speed of a light beam is exactly known (in vacuum), its momentum can still be uncertain. This corresponds to an uncertainty in the wavelength. No light source is completely monochromatic. Even a laser has a small but finite spread in wavelengths.