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

Question about light

  1. Jun 19, 2005 #1
    so i just graduated high school taking only one physics class in 4 years. it was an academic level class and i only took it to fulfill a requirment, but im glad i took it because it got me thinking. i dont plan to do anything with phusics in the fuutre, but i do have some questions that will probably seem elementary to most of you. here's the question: i know light travels 299,792,458 meters per second, but what if it could somehow travel slower to say 299,742,450 m/s? what would happen? would it still be light? has this ever been done? physics is hard.
  2. jcsd
  3. Jun 19, 2005 #2
    Light travels slower that 3E8 m/s in all mediums except vaccuum.
  4. Jun 19, 2005 #3
    ok, i got that number off some website, but does that answer my question then?
  5. Jun 19, 2005 #4

    James R

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    In water, for example, light travels at about 225,500,000 metres per second.

    Can you still see light in water?
  6. Jun 19, 2005 #5
    Photons still travel at c, it's just that they interact with electrons in material which averages out so that it seems as if light travels slower.

    Forgive my ignorance, but how old does one have to be to "just graduate from High School?"
  7. Jun 19, 2005 #6
    i see, so is that slowest speed light travels?
  8. Jun 19, 2005 #7


    User Avatar
    Gold Member

    People claim to have stopped light.

    The misconception is that people say "Light travels at 300,000 km/second" and leave off "in a vacuum"
  9. Jun 19, 2005 #8
    disregard that last post, i didnt see masudr's post. I'm 18.
  10. Jun 19, 2005 #9
    how the f do you stop light, and how is it still light at that point?
  11. Jun 20, 2005 #10
    To answer your question, no it wouldn't be light. Light can only travel at one speed, so if it travels at another speed it's not light. If light did, somehow, slow down (I think -- and if I'm wrong someone will correct me) then it wouldn't be capable of existing.

    I'm going to take a stab at working through some equations to try to prove this, because if I make a mistake, maybe I can learn something from this too. :redface: But I haven't had any formal training, so don't take this on blind faith.

    The equation for the energy of a relativistic particle is [tex]E^2=m^2c^4+p^2c^2[/tex]. A photon doesn't have a rest mass (m = 0), so this reduces to [tex]E^2=p^2c^2[/tex] (Eq. 1).

    The equation for relativistic momentum (p) is [tex]p=\frac{mv}{\sqrt{1-\frac{v^2}{c^2}}}[/tex]. Since m is 0, the numerator (mv) will be 0, which at first may make it look like p will be 0, but if v = c, then the denominator will also be 0, and 0/0 is indeterminate, so as long as v=c, momentum doesn't have to be 0. If, however, v < c, then the denominator will become greater than 0, thus the momentum of a particle with zero rest mass moving at a speed less than the speed of light is zero. If a particle has both zero rest mass and zero momentum, then based on Eq. 1 it also has no energy. Something with no energy, no mass, and no momentum doesn't really seem to exist to me...
    Last edited: Jun 20, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook