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Speed of light question

  1. Nov 5, 2012 #1
    Just a random question regarding the speed of light. I am not a physics major or a college student, just curious.

    I was watching a program on the science channel (I think) and it was talking about how the speed of light is absolute. The example it gave was traveling in a car at a high rate of speed then turning on a flash light facing in the same direction as the car. So if the car was going 100 mph it wouldn't add on to the speed of "C", since it is absolute.

    So, my question is since the earth is moving in our solar system and our sun is moving in our galaxy and our galaxy is moving in space, how can we know the true value of "C" if we don't know what stationary is? Does this make sense? I might be looking at this the wrong way but I figure someone here might know.

  2. jcsd
  3. Nov 6, 2012 #2
    I'm not 100% sure on this but imagine the whole universe as a giant car. Everything in it seems stationary to someone inside the car. Similar to that, the only way to answer your question is to view our universe from another universe.

    IDK, just a guess, I might be wrong. xD
  4. Nov 6, 2012 #3


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    Statonary is whatever reference frame from which you measure the speed of light. The speed of light is invariant irrespective of the reference frame.
  5. Nov 6, 2012 #4


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    Terms like stationary can only be applied to objects in reference to other objects. For example the chair I am sitting on is stationary relative to me but not to a car driving by my house.
  6. Nov 6, 2012 #5
    Hi, welcome to physicsforums. :smile:

    It's really a "relativity" question. According to that theory the "locally" measured speed of light is always c, if it is measured with a standard reference system. Indirectly, your question was recently discussed here:

    Does that help?
  7. Nov 21, 2012 #6
    You don't need to know what stationary is. Imagine you're standing on the earth and you measure the speed of light. Shoot a light beam one way, the opposite way, every which way, and you will measure the same speed of light. THEN, accelerate in some direction and while moving, do the same experiment. Shoot the light forward, back, up, down...you'll get the same result. The speed of light is the same.

    That's enough to tell you that no matter what you do, the speed of light is the same.
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