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!

Speed of sound in a vacuum

  1. Feb 21, 2012 #1
    Does anyone have an explanation for why kinetic energy carried by electromagnetic radiation travells at the speed of light?

    My understanding of the speed of sound is that the denser the medium, the faster the wave velocity. Since this is just kinetic energy propagating through a medium, how is it different when applied to electromagnetic radiation? If it is no different, does that mean that a vacuum has infinite density?
  2. jcsd
  3. Feb 21, 2012 #2
    you seem to have confusion between sound waves and light waves. when you say electromagnetic radiation, you are talking about light wave. light can travel in vacuum. sound cannot travel in vacuum, it needs medium to propagate.
  4. Feb 21, 2012 #3
    True, sound waves (kinetic energy dispersing through a medium) requires something to travel through. So how can photons transfer kinetic energy (and at such velocity) without a medium. Correct me if I am wrong (very possible) but photons carry kinetic energy as shown by the photo electric effect and the fact that they have mass when not in a rest state. Also, if a laser is directerd onto a suspended metal surface the surface will rotate?
  5. Feb 21, 2012 #4


    User Avatar
    Gold Member

    That's just one of the wild things about light, it requires no medium to travel through.

    Photons do not have mass, but they do have an energy and momentum (they simply are not given by [itex]KE = {{1}\over{2}}mv^2[/itex] and [itex]p = mv[/itex]).
  6. May 8, 2012 #5
    You might have been confused. Sound propagate in vacuum but light can(300,000m/s)
  7. May 8, 2012 #6
    Yes, it will. There's something called radiation pressure.
    This pressure of light will be doubled for all angles of incidence if the light is both absorbed and reflected.
    However, I think the formula of solar radiation pressure on that page is wrong.It should be F= -p(c+1)Ar
    where p is the force per unit area,
    c is the coefficient of reflectivity
    Last edited: May 8, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook