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Moving Towards a Light Beam

  1. May 5, 2013 #1
    Hello everyone,

    First of all, I'd like to apologize if this question has already been discussed here at the forum. However, I've read dozens of discussions and none of them have helped to find a solution to this specific scenario I have in mind right know.

    I have been reading a lot of materials related to Einstein's principle of light constancy lately. One of examples I saw states that if ether really exists and light travels thought it, if a observer is moving towards a light source (with speed v) and through the ether, its speed would add to that of speed of light so that light would be travelling at v + c and not just c (based on the observer frame of reference).

    However, if you apply relativity to this case and remove the ether, there is no difference whether the source of light is moving towards the observer or the observer is moving towards the source of light. The speed of light is going to be constant, at C.

    However, even in this case, If I am flying a spaceship and decide to accelerate it at full power towards a flash of light I've just seen, from my perspective in the cockpit, isn't the light going to travel much more faster in my direction (since the speed of light and the speed of the spaceship add up?). Or I am going to see the light coming at C having the impression that my spaceship is actually stopped?

    Thank you!
  2. jcsd
  3. May 5, 2013 #2


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    You have talked about a lot of situations where you are observing light and drawing conclusions about its speed but you have never described how you would actually measure the speed of light. For example, your last statement about seeing light coming towards you at c? What does light look like at c or one-half c or double c? How would you know? Please tell me exactly what you are thinking.
  4. May 5, 2013 #3


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    It turns out that in relativity the formula for "adding" speeds in a situation like this isn't [itex]u + v[/itex] but[tex]
    \frac{u + v}{1 + \frac{uv}{c^2}}
    [/tex]When you put [itex]u = c[/itex] you get an answer of [itex]c[/itex]. But although the light's speed wouldn't change, its colour would: it would turn bluer. And if the light was coming from some direction other than straight ahead, its direction relative to you would change.
  5. May 5, 2013 #4
    I was thinking in a hypothetical situation where I am seated on a spaceship cockpit, actually stationary, and right in my direction, far far away, there is another military spaceship firing laser beans at the speed of light towards me. Imagine that actually I can see those beans coming before they hit my space ship so that I have a chance to evade the shots. My instruments also detect then and confirm they are traveling at the speed of light towards my stationary position. Imagine that at halfway distance I can actually see them coming and think to myself: "Ok, so that's how beans traveling at the speed of light looks like". If I decide to throttle my spaceship up and fly like a kamisake towards the beans, would I notice any change on their speed, at least visually? In other words, even though they are already traveling at the speed of light, would I see them accelerating based on my frame of reference?
  6. May 5, 2013 #5


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    You're dreaming--not thinking. Try thinking. Or else ask your questions on a sci-fi forum.

    Think about this: you see things because light from those things enter your eyes or instruments. What can you possibly mean by seeing the laser light beams before they hit you?
  7. May 5, 2013 #6
    You are absolutely right! It so obvious, do not know how I missed this simple but important detail! I will try to think a little bit more next time! Thank you very much!
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