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

Special relativity

  1. Jun 21, 2009 #1
    if a flashlight is moving at half the speed of light and sending out light at a relative speed of c and then the flashlight runs into an asteroid and wham, it stops moving relative to the asteroid, does the light that has already been sent out put on its retro rockets so that is can resume its speed at c relative to the smashed flashlight?
  2. jcsd
  3. Jun 21, 2009 #2
    That is the theory of relativity, time changes not velocity.
  4. Jun 21, 2009 #3
    Light leaves the flashlight traveling at c, and it travels at c with respect to both the flashlight and asteroid. The main difference is measuring while traveling with the flashlight you would get one frequency and measuring with the asteroid you would get another higher frequency. This is called the relativistic Doppler shift.
  5. Jun 21, 2009 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The main premise of relativity is that all obsevers measure light to be traveling at c no matter what their speed is relative to another observer. This doesn't change if an observer's speed changes. Phrases like the ones above will cause you confusion.
  6. Jun 21, 2009 #5
    You have not grasped the idea that the speed of light is universal. This is VERY important to understanding relativity.

    Simply put, light moves at c from all reference points. It does not matter if the flashlight was moving at .5c or .999999c or .0000000001c, light moves at c at all costs.

    If you were racing with a beam of light, and you moved at .9999c, you would say that light is moving .0001 c faster than you right? Wrong. It is moving at c relative to you. You would wonder how is it possible that its moving at c relative to you even tho u are moving at .9999c...well that is because space and time change in order for light to remain at c from ANY reference point.
  7. Jun 21, 2009 #6
    i always wondered about the possibility of intervening events as far as conflicting observations.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook