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Shoot Light

  1. Dec 11, 2009 #1
    If there is a target at a distance d and I shoot light at it, does it remain at the same point in space as light proceeds toward it?

    For example, if the target in in line with the revolution of the earth around the sun, will the target move away at 18.55 miles per second as the light proceeds toward it?
  2. jcsd
  3. Dec 11, 2009 #2


    Staff: Mentor

    In general, no. The only case where it would remain at the same distance is if it were at rest in the frame in which the distance was measured.
  4. Dec 11, 2009 #3
    Naturally, I was assuming we were on the earth.

    So, light takes off at c and the earth moves also and so does the target.

    When light is shot at the target, and light proceeds in free space, will the target remain at rest even though the earth is in orbit around the sun? In other words, will the target move away with the motion of the earth as the light speeds toward it?
  5. Dec 11, 2009 #4
    Are you asking if the target will accelerate or not depending on whether or not we shoot a light at it? Before the light even reaches it?

    Can you rephrase? I'm sure that's not what you mean to ask.
  6. Dec 11, 2009 #5
    The target is on the earth.
  7. Dec 11, 2009 #6
    I still don't understand the question. If the target is stationary relative to earth's surface, then its motion is the same as earth's surface relative to the sun, ie both orbital and rotational motion. But that has nothing to do with shooting light at it.
  8. Dec 11, 2009 #7
    Light is shot at a target.

    I assume light shoots.

    Now, when light shoots at a target in the direction of the earth's orbit, will the target move?

    If you say no, then the earth is geocentric.
  9. Dec 11, 2009 #8
    Of course the target will continue its existing motion if no force acts on it, but that has nothing to do with light being shot at it.

    I still must not understand the question.
  10. Dec 11, 2009 #9


    Staff: Mentor

    The source will move with its own motion, and the target will move with its own motion. Neither are constrained to any specific value and neither influence the speed of the light pulse.
  11. Dec 11, 2009 #10


    Staff: Mentor

    Obviously the earth is geocentric. That's the definition of geocentric.
  12. Dec 11, 2009 #11
    You mean it doesn't depend on whether I say no or not? :eek:
  13. Dec 11, 2009 #12


    Staff: Mentor

    Correct. The answer to scenario has nothing to do with whether or not the earth is geocentric nor does what you say the answer to the scenario is.

    The earth is geocentric by definition. I would think that is blatantly obvious.
  14. Dec 12, 2009 #13
    LOL. As far as I can tell the answer to the scenario (what is the motion of the target?), has nothing to do with any light being shot at it as well.

    What is the point of the question? Is the target trying to dodge the light? :confused:
  15. Dec 12, 2009 #14


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    Staff: Mentor

    Perhaps the OP is backwardsly asking if the speed of light is dependent on the speed of the observer?

    In the scenario given, the target and shooter are stationary with respect to each other. All that other stuff about the earth's rotation and orbit are irrelevant.
  16. Dec 12, 2009 #15


    Staff: Mentor

    I agree. The second postulate is that light travels at c in vacuum in any inertial frame regardless of any other factors.
  17. Dec 12, 2009 #16


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    Staff Emeritus
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    Gold Member

    According to what frame?

    In the frame of the Earth, no. The light takes a time of d/c to reach the target.

    In the frame of the Sun, then yes. The light proceeds towards the target at c, but the target moves away at the speed of 18.55 miles/sec. Thus the time it takes the light to reach the target is d/(c-18.55mps)
  18. Dec 12, 2009 #17
    I think you mean the speed of light does not depend on the speed of the light source.

    This has been verified by experiments that the speed of light cannot be altered by the motion of the light source.

    Now, that being said, from the light source, light emits at c period regardless of the motion of the light source.

    Is this correct?
  19. Dec 12, 2009 #18


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    Staff: Mentor

    "Motion" is a little ambiguous. It is acceleration that can cause problems. The speed of light is constant in all inertial (non-accelerating) frames - ie, when the source and observer are not accelerating with respect to each other.

    Also, I don't like the way "speed of the light source" is worded. Speeds are measured between two points. So hopefully, when you say "speed of the light source", you mean speed of the light source with respect to the target.

    So for a concise answer to the OP:
    It appears you are talking about a source and target both fixed to the earth. That means the speed of the target with respect to the source is zero, not 18.55 mi/s. That the speed of the target relative to some arbitrary point hanging in outer space is 18.55 mi/s is completely irrelevant.
    Last edited: Dec 12, 2009
  20. Dec 12, 2009 #19
    Well, let's see if we can clear the light source thing up.

    If a rocket rode by the earth and when "parallel", the earth and the rocket happen to shoot light in the same direction and parallel, would the light beams be located at the same x-axis distance in space at any time t in whichever frame looked at them?
  21. Dec 12, 2009 #20
    Acceleration is a problem.

    The metrics in an accelerated frame after acceleration is complete are expanded to
    x/sqrt( 1 - (v/c)^2) compared to the originating frame.
  22. Dec 12, 2009 #21
    Is the 18.55 number a pheudo absolute motion number or simply the speed around the sun.

    I cannot find anything to validate this.

    Of course, the milky way is doing something also.
    Last edited: Dec 12, 2009
  23. Dec 12, 2009 #22

    Yes, I know.

    But the geocentric model of the universe is similar to SR, no?

    We are encouraged to consider the frame as the center of the inertial frame universe?
  24. Dec 12, 2009 #23
    That is what it says, regardless of the motion of the light source.

    Thus, for all possible light source motions relative to a frame, light will move at the same speed c in a vacuum of course.
  25. Dec 12, 2009 #24
    I don't fully understand the OP's question, but I do know that if you shoot light across a playground roundabout to say some person sitting directly opposite to you, it will miss its target because of the coriolis effect. And since earth rotates 1/day, you'll definitely miss on any experiment carried out on the surface of Earth.

    So you can't just assume that because your target and source are stationary with respect to each other that your 'shoot light' will definitely hit the target.
  26. Dec 12, 2009 #25


    Staff: Mentor

    Exactly (relative to an inertial frame).

    So your initial concern or question is resolved?
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