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Thread: Speed C, relative to what?

  1. #1 Speed C, relative to what? 
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    If someone could kindly point out where I have gone wrong here or which of the following is true, that would be appreciated.

    My understanding is that every time speed C is quoted, we're referring to the speed relative to the observer making the measurement.

    If so, then the statement that "massive objects can not be accelerated to faster than C" surely means that massive objects can not be accelerated to faster than C relative to its starting point when the acceleration began.

    Then I don't see why you can't accelerate a massive object to 3/4 of speed C in one direction, stop for a tea party and forget what your relative motion may be, then continue to accelerate another 3/4 of speed C in the same direction, ending up 1 and a half of speed C from your initial frame of reference. Indeed I don't see any reason why you wouldn't be able to keep doing this forever, eventually getting to speeds of 1000 C's.

    If this is not the case, then the notion of not being able to accelerate beyond speed C must be relative to some sort of ether and then the following would be true.

    You could accelerate a massive object to 3/4 of speed C in one direction relative to the ether, stop and then accelerate to 3/4 of speed C in the opposite direction, taking you to 1 and a half C along the way. (the first 3/4 would be deceleration, and the following 3/4 would be acceleration)


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  3. #2 Re: Speed C, relative to what? 
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    Quote Originally Posted by Monkey.Man
    If someone could kindly point out where I have gone wrong here or which of the following is true, that would be appreciated.

    My understanding is that every time speed C is quoted, we're referring to the speed relative to the observer making the measurement.

    If so, then the statement that "massive objects can not be accelerated to faster than C" surely means that massive objects can not be accelerated to faster than C relative to its starting point when the acceleration began.

    Then I don't see why you can't accelerate a massive object to 3/4 of speed C in one direction, stop for a tea party and forget what your relative motion may be, then continue to accelerate another 3/4 of speed C in the same direction, ending up 1 and a half of speed C from your initial frame of reference. Indeed I don't see any reason why you wouldn't be able to keep doing this forever, eventually getting to speeds of 1000 C's.
    The reason this does not work is that velocities do not add linearly (by the rule of w+u+v). Instead, velocities add by the relationship:



    under these rules, you would only be moving at 0.923c relative to your initial reference frame after your second acceleration as measured from either frame.

    IOW, let's say that as you take your tea break you release a buoy that shares your ship's velocity.(3/4 c relative to your starting frame. ) You now accelerate until you are moving at 3/4 c relative to this buoy. You will measure the buoy as moving at 3/4 relative to you, and your relative velocity to the initial frame as .923c (meaning the difference between the buoy and initial frame will be .173c.)

    However, someone in the initial frame will still measure the buoy's relative speed as 3/4 c while the difference between you and the buoy's speed will be .173c.

    Someone sitting at the buoy will measure its relative speed with respect to you and the initial frame as both being 3/4c.


    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  4. #3  
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    But there is a thea party in between where the engine stops and the spaceship moves without acceleration. No matter what the speed is at that moment for the people in the spaceship counts that c has not changed in comparison with before the first accelaration. So the next acceleration (the same) would cost the same amount of energy where relative intuition says that the next accelaration would need more energy. As c is not related to anything but the (ever changing) subjektive it leaves a freedom which I think is a mistake.

    :The reason this does not work is that velocities do not add linearly (by the rule of w+u+v). Instead, velocities add by the relationship.
    This idea for instance the speed relative to observation from earth comes in. But for other observers - for instant on mars - What is an acceleration of the spaceship viewed from earth can be just as well an opposite acceleration (that observer would see the spaceship go slower, a negative acceeration). So which observer has the favor or truth for the spaceship, I would say none as both views are subjektive to the observer its not possible to determine - other then subjektive - if the spaceship accelerates or it must be related to earth's gravity maybe. But after the first acceleration the spaceship might have come close to another planet or the moon and the direction of gravity can be the opposite of when it started the first acceleration.
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    The reason this does not work is that velocities do not add linearly (by the rule of w+u+v). Instead, velocities add by the relationship.
    Ok this adds up, thanks.

    However can we not use a different approach to measure velocity relative to the initial reference frame in this instance? rather than supposedly measuring the speed of photons being emitted from the receding reference frame.

    Just say hypothetically that we have applied this type of acceleration with tea parties hundreds of times since we departed, and are travelling at 1000c for a week (if velocities did add linearly) Would we not then find that the time it would take to travel back to our initial reference frame on the return journey, at a constant speed of half C, reflected that we had been travelling away from it at speeds of 1000C rather than never actually reaching more than 1C.

    It would be really valuable to my insight into all this if someone could verify that such measurements have been made to confirm this, rather than always relying on measuring the relative speed of light travelling.




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    Quote Originally Posted by Monkey.Man
    Just say hypothetically that we have applied this type of acceleration with tea parties hundreds of times since we departed, and are travelling at 1000c for a week (if velocities did add linearly) Would we not then find that the time it would take to travel back to our initial reference frame on the return journey, at a constant speed of half C, reflected that we had been travelling away from it at speeds of 1000C rather than never actually reaching more than 1C.
    Depends on whose time you're counting. If your ship left planet earth, went over to alpha centaury and came back, people on earth would see you come back 10 years later and say "Alpha centaury is 4.4 light years away, nope, sorry mate, you didn't go over light speed".

    And the guys on the ship would go "Are you kidding mate? The round trip took a couple of weeks, max! Hey, how come my girlfriend's hair is white?"
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  7. #6  
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    Quote Originally Posted by Monkey.Man
    The reason this does not work is that velocities do not add linearly (by the rule of w+u+v). Instead, velocities add by the relationship.
    Ok this adds up, thanks.

    However can we not use a different approach to measure velocity relative to the initial reference frame in this instance? rather than supposedly measuring the speed of photons being emitted from the receding reference frame.

    Just say hypothetically that we have applied this type of acceleration with tea parties hundreds of times since we departed, and are travelling at 1000c for a week (if velocities did add linearly) Would we not then find that the time it would take to travel back to our initial reference frame on the return journey, at a constant speed of half C, reflected that we had been travelling away from it at speeds of 1000C rather than never actually reaching more than 1C.

    It would be really valuable to my insight into all this if someone could verify that such measurements have been made to confirm this, rather than always relying on measuring the relative speed of light travelling.



    [/quote]

    You need to read a good book on relativity. I think that Rindler's Essential Relativity, Special, General and Cosmological would be suitable. It is available on the used book market at reasonable prices. Try Amazon, Alsibris or Abebooks on the internet.
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  8. #7  
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    Quote Originally Posted by DrRocket
    You need to read a good book on relativity. I think that Rindler's Essential Relativity, Special, General and Cosmological would be suitable. It is available on the used book market at reasonable prices. Try Amazon, Alsibris or Abebooks on the internet.
    Is this a good book for the "average" interested layperson, to read and understand, or is it pitched at a higher (science undergraduate for example) technical level.
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  9. #8  
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    Quote Originally Posted by Halliday
    Quote Originally Posted by DrRocket
    You need to read a good book on relativity. I think that Rindler's Essential Relativity, Special, General and Cosmological would be suitable. It is available on the used book market at reasonable prices. Try Amazon, Alsibris or Abebooks on the internet.
    Is this a good book for the "average" interested layperson, to read and understand, or is it pitched at a higher (science undergraduate for example) technical level.
    It is a book written for someone who wants to understand relativity. The first parts are fairly undemanding mathematically.

    If you are looking for understanding at the level of an "average" layperson, literally a guy off the street, then forget it. The average man on the street is pretty hopeless. The fact that you have even asked the question puts you at least a cut above that.

    It is the best book on the subject that I know of without getting very sophisticated in terms of the mathematics. I think it is suitable for science undergraduates and also interested non-technical people since they can skip some of the harder parts if they wish.
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