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I'm confused as to why you can't go faster than light, if you compound velocities

  1. Mar 25, 2009 #1
    You have 2 identical spaceships, each that can go .90 the speed of light relative to Earth. One spaceship tows the other, and once reaching the maximal speed (.90c relative to earth), the second ship that is currently being towed, is released, which then proceeds to accelerate toward its maximal speed (again, .90c relative to earth). Obviously the second ship, the one that is accelerating after being towed will leave the first ship behind, so why is it not possible to go faster than light?
     
  2. jcsd
  3. Mar 25, 2009 #2
    Could the "light is relative to the observer" be wrong, and there is some hole in which you can travel faster? How do we know for certain that this is the truth, and that we are not mistaken?
     
  4. Mar 25, 2009 #3
  5. Mar 25, 2009 #4
    You are confused, because both ships are currently moving at .9c relative to earth!
    Rethink or reword your problem.
     
  6. Mar 25, 2009 #5
    I guess I should have said relative to the spaceship towing it. I just don't understand how you can have something going (say for example) .999999c, and have something else go .999999c relative to that, and have it not go over.

    What really troubles me is if someone standing still on earth and someone in a space ship going .99999c view the same beam of light it will appear the same for both viewers. Light must not be an actual thing then, but an illusion.

    It would be like watching a train go from point a to point b on earth, and watching the same thing from the ship, only on the ship more time would have passed on earth, thereby making the train late but having the ship view it in real time. How can the train be in two different positions at once?

    I think light is an illusion of some kind.
     
  7. Mar 25, 2009 #6

    DaveC426913

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    The definition of velocity is a measurement of distance over time. It's not the distance that's changing, it's the time. At .999999c, the craft experiences time dilation of a factor of 700. At that factor, another ship moving relative to it, only needs to go from .999999 up to ~.99999999 in order for them to see a doubling of their relative speeds.
     
  8. Mar 25, 2009 #7
    So theoretically you could go infinitely fast and never reach the speed of light? This is some fked up ****.
     
  9. Mar 25, 2009 #8

    russ_watters

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    That's a contradiction. You can't go infinitely fast....you can accelerate forever and not reach the speed of light, though.
     
  10. Mar 25, 2009 #9
    Infinitely fast relative to what?

    If you don't put a "relative to" or "in relation to" in your sentence, you are going to get in all sorts of trouble. And that is not some "post-Einstein" peculiarity. We always move relative to something else - we just normally make an unstated assumption about what that something else is, the Earth's surface passing below or the air through which a plane is moving, the road we are driving or walking on, the train tracks on which a train is moving, the train body in which we are walking.

    Put something into deep space and you no longer have anything obvious to move in relation to.

    Before you head down that path, no, not having anything obvious to move in relation to has no effect whatsoever on relativity. Relativity works equally well for a snail as it does for a rocket travelling at 0.9c, our inability to notice the effect notwithstanding.

    cheers,

    neopolitan
     
  11. Mar 26, 2009 #10

    DaveC426913

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    You could accelerate forever and never reach the speed of light.
     
  12. Mar 26, 2009 #11
  13. Mar 26, 2009 #12
    If the ship moves at near light speed, time dilation slows all activity on the ship, which includes any rocket propulsion, thus it takes longer to accelerate. This would be observable by earth viewers but not by the ships crew.

    They do not view the same light!

    The train path is the same for both, they will not agree on the time it took for the trip.

    Read about time dilation, if you haven't yet.
     
  14. Mar 26, 2009 #13
    I just don't understand how things can look different if physical matter is taking up space. It would mean that everything we can see is just an illusion with reference to who is seeing it.

    Does that mean that there could be infinite dimensions, since every speed can view its own light relative to it, which means that there are an infinite number of ways light can co-exist at the same time?
     
  15. Mar 26, 2009 #14

    russ_watters

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    Just because things are reference frame dependent, that does not mean they are illusions.

    Can you play billiards on a train moving at constant speed (on very smooth tracks!)? Of course. A billiards ball that is stationary on the table might be moving at 100 miles an hour relative to the ground, but it doesn't affect the game at all. That's frame dependent velocity, just like with Einstein's version of Relativity (well....almost). And Newton/Galileo would agree. That's the principle of relativity in action, it is just that those guys didn't know how to apply it correctly when having to do with the speed of light.
     
  16. Apr 9, 2009 #15
  17. Apr 14, 2009 #16
    actually, time dilation and length contraction happen at the same "time", it really depends on the way you look at it (distance = velocity * time so if time is shortened, so is the distance)
     
  18. Apr 14, 2009 #17

    JesseM

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    The distance between two markers (say, Earth and a distant star) is only shortened in a frame moving relative to those markers. So if we consider a ship moving at 0.999999c in the frame of the Earth, in this frame there is no length contraction of the distance the ship has to travel (though the ship itself will be shortened in this frame), while there is time dilation of the ship's clock. On the other hand, in the rest frame of the ship the distance from Earth to the star is contracted relative to the distance in the Earth's rest frame, but the ship's clock is not dilated (although a clock on Earth would be in this frame).
     
  19. Apr 14, 2009 #18
    I guess i should have been clearer ... whenever two observes disagree on the time measured, they will always disagree on the length as well. That's what I meant by saying that length contraction and time dilation happen at the same time - which one of the two you see depends on your frame of reference.
     
  20. Apr 14, 2009 #19

    JesseM

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    Yes, that's true, although I don't see where DaveC426913's post was contradicting this.
     
  21. Apr 30, 2009 #20
    Illusion? Consider this: The majority of today's "relativistic" scientific community is conditioned to(and obscenely comfortable with) the idea that nothing qualifies as an absolute frame of reference. This same community is also comfortable studying complex diagrams illustrating the intricacies of the various stages of imaginary twins in an imaginary experiment that proves the existence of an imaginary paradox that is in reality not a paradox at all, but a mere illusion. Although c may be constant, it takes little to distort our perception of it. If you were to take all variables into account to the smallest degree at any given time in your observation of any given event you could prove that most of what you see is indeed, an illusion. However light does not experience illusions like you do. It does exactly as it should. You just see it differently from different reference frames.
     
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