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

Speed vs. light

  1. Jul 9, 2010 #1
    I have always been confused about this. On here people constantly talk about traveling in a space ship that goes .5c, or.9999c etc. But how can this be, no matter how fast one travels light will always travel at c compared to your reference frame. Does this not mean that no matter how fast you travel your speed will always equal 0 compared to light?
  2. jcsd
  3. Jul 9, 2010 #2


    User Avatar

    Staff: Mentor

    You are correct. So the speed of a spaceship (or anything else, for that matter) is never measured with respect to the speed of light, it is always measured relative to stationary objects.
  4. Jul 9, 2010 #3
    I don't think so. Your speed equals 300000 km/s compared to light.

    Edit: Sorry binbots, I didn't understood your question. Russ is right.
    Last edited: Jul 9, 2010
  5. Jul 10, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    When people talk about a space ship travelling at 0.5c or 0.9999c, they mean with respect to some other frame (say the earth). In that frame you are traveling at 0.5c or 0.9999c and light, of course, is traveling at c.

    In your frame you are at rest and light travels at speed c with respect to you.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook