Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What happens when you point a laser-pointer into the sky?

  1. Feb 6, 2007 #1
    Suppose it's a very clear night. You take a laser-pointer, and briefly allow it to "shine" into the sky.

    Will the photons emitted from your laser actually make it millions of miles into space? Or will they somehow "dissipate" and not make it that far?

    It would be very cool to think that just you or I could actually cause something to travel off into space and even long after we're gone, that that beam of light will still be traveling off into the stars.
  2. jcsd
  3. Feb 6, 2007 #2
    Theoretically I think it's possible for the photons to travel through space forever, but in reality they're probably going to give up all their energy in an interaction with matter after a finite travel time.
    Last edited: Feb 6, 2007
  4. Feb 6, 2007 #3


    User Avatar
    Gold Member

    If they make it out of the atmo, they're virtually home free. I think it would take lightyears of space to encounter that much matter again.
  5. Feb 7, 2007 #4
    I would agree with the others and say that once out of the atmosphere, your photons will keep going until intercepted by matter. However, the atmosphere is pretty thick, so there would be quite a bit of scattering and whatnot for a little laser pointer.
  6. Feb 7, 2007 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Why use a laser pointer? Astronauts have been observing street lights from our cities since we started lofting them above the atmosphere...

    By the way, your laser pointer has already attracted extraterrestrials. They are hovering in the skies over Mexico City even now...
  7. Feb 7, 2007 #6
    I'm fairly certain that light from a laser pointer can make it out of the atmosphere in a virtually coherent form. However, even "empty" space contains some gas. So eventually all of the light would interact with matter.
  8. Feb 7, 2007 #7


    User Avatar
    Gold Member

    Yup, this has happened already. I remember reading or someone telling that a laser on earth was focused on the moon. I believe this is how they determined the distance from earth to moon and also concluded that the moon is moving away from earth at approx. 3.8cm(?) per year.

    EDIT: the experiment is called Lunar Laser Ranging Experiment
  9. Feb 7, 2007 #8
    Yes, this is true. As I recall, mirrors were placed on the moon by NASA astronauts who landed there in the early 70's, so as to facilitate this.

    Obviously, the gas density in space is small enough that a laser beam would be able to travel quite a distance before scattering very much.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook