1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Photons and speed

  1. Feb 27, 2010 #1
    This is an interesting question that I can't seem to sort out. Since photons are massless, they travel at the speed of light(3*10^8 m/s). Why must photons travel at the highest possible speed (the speed of light)? Why is it when a photon is created, it doesn't start off at a speed zero and then once a force(whatever the magnitude) is acted upon it cause it to move at the speed of light? How could this happen?

    Thanks! :smile:
  2. jcsd
  3. Feb 27, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, if that bothers you (I don't see WHY it should bother you - you could just as well ask why massive objects DO accelerate, right ?), think of photons as having a very very tiny mass. They will then accelerate as you propose, but they will do so extremely swiftly, so that the time for them to "get up speed" is very small. Their speed will then by a tiny tiny fraction below light velocity, but that will not be noticeable. Now think of them to be "lighter and lighter". In the end, there is no observable phenomenon that will be noticeably different from the photons without any mass.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook