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A radio beacon @ the speed of light.

  1. Sep 14, 2004 #1
    I just read of an idea to send a craft outside our solar system using the various stellar bodies to sling shot it and also create a way for it to continually accelerate. In which case it would eventually near the speed of light. So in fifty years it could send us pictures and what not of faraway places. Which, of course, one would think it would take fifty to get back.

    But then it occured to me that if it is traveling close to the speed of light and it sends signals toward the direction from which it is traveling, near (or at) the speed of light, the signal would actually be traveling, relative to us, VERY slow. Is this accurate? Or am I missing something? If I'm traveling in the back of a pick-up truck at 55mph and I throw a ball behind the truck at 40mph, is not the ball traveling in the direction of the truck 15mph?

    Thank you in advance for your response.
  2. jcsd
  3. Sep 14, 2004 #2


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    Staff: Mentor

    Welcome aboard.

    Yes, you are missing something: the speed of light is constant to all observers regardless of inertial frame of reference. Speeds do not add and subtract in the same way as your truck-ball example. That only works at very low speed.

    So it doesn't matter what speed the probe is going: when it passes alpha centuari, for example (4.36 light years distant), the signal it sends will take exactly 4.36 years to get back to earth.
  4. Sep 14, 2004 #3
    Wow, ok, that's a mind boggler.
  5. Sep 14, 2004 #4
    yeah especially since from the frame of the craft the signal would have gotten to earth sooner.
  6. Sep 15, 2004 #5
    The other question is.

    If you sent a signal to the space craft that is traveling at the speed of light away from the earth will it ever get there?
  7. Sep 15, 2004 #6
    Nothing can travel the speed of light. Period.

    Even if it could, it's time would be mixed up, and everything will just crash. What they see will be both the end and the start of the universe, and everything else.

    From the reference frame of the Earth, however, we will observe that the light beam never reaches the spacecraft.
    I wish that helps.
  8. Sep 15, 2004 #7


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    Staff: Mentor

    To make it physically possible, I'll change it a little: if you sent a signal to a spacecraft traveling just under the speed of light, it would take a long time (from earth's point of view) for the signal to get there.
  9. Sep 15, 2004 #8
    Yup, it is boggling for everyone who first learns about it. And that's why Einstein is nearly worshiped (although he is sometimes overrated, others were quite close to solving the riddle at the time.). It's what relativity is all about.
  10. Sep 16, 2004 #9
    Is that so? :rolleyes:
    Think about what you just wrote.

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