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Thread: Space-Time mind bender for a cosmology novice

  1. #1 Space-Time mind bender for a cosmology novice 
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    I have developed a late-in-life interest in cosmology (extreme novice level). To help me get started, I need help in visualizing the following scenario:

    If 2 spaceships travel away from each other, each at a speed of 3/4 the speed of light (i.e. the distance between them increases at 1.5c), what would the captains see in their rear view mirror, and why?


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    They would see the other ship moving away at .96 c.

    The relativistic forumla for the addition of velocity is

    v + u / 1 + (vu/c2)

    If, for convienence we set the speed of light equal to 1, we have

    .75 + .75 / 1 + (.75 2) = 1.5 / 1.5625 = .96


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    Its the way nature is!
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    To another universe, where the rules are simpler
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  4. #3  
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    Quote Originally Posted by gordonmcl View Post
    what would the captains see in their rear view mirror, and why?
    They would see all light red-shifted, and the images distorted due to aberration and Terrell rotation. Here is a little video to give you an idea what it would look like at various speeds :

    Viajando a la velocidad de la luz (o casi) por Tbingen - YouTube
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  5. #4  
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    Physics is an enjoyable stretch for my imagination within the context of something collectivly shared. Im less than a novice, but would like to coment. I'm just wondering since space contracts completely at 1c} then 1.5} would be out of its jurisdiction Then what ?perhaps time would recede to .5} as a result. There are other options to consider, I only considered this as a possibility because space appears curved; indicating perhaps that is the motion of things. There are other possibility's to ponder , and learn while exploring: which is what makes it such a great question.
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  6. #5 Something to concider 
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    I neglected to read carefully Alex G's equation in relation to "Space-Time mind bender",by Gordonmcl: but then I would have missed the oportunity to contemplate 2 moving objects who's opposed motions realatively combined would create a speed greater than that of light.

    Something else to concider :if space contracts completely at the speed of light, a person traveling at this rate would have to arrive not in one direction ,because a body does not excist in one dimention, but rather in three, therefore the other dementions of space would have to contract with it? would they not? Because the traveler could not leave the other two behind? observed or not? He is a body traveling a space of 3 dementions: hense he arrives instantaneously and space is not just contracting for the observer but the traveler as well..?Is it not?? Would love some responce to this ,where I don't have the knowlledge, or speculation.
    Last edited by mc2; November 23rd, 2012 at 03:20 AM. Reason: IN A clearer state
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  7. #6 Please Enlighten 
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    Quote Originally Posted by AlexG View Post
    They would see the other ship moving away at .96 c.

    The relativistic forumla for the addition of velocity is

    v + u / 1 + (vu/c2)

    If, for convienence we set the speed of light equal to 1, we have

    .75 + .75 / 1 + (.75 2) = 1.5 / 1.5625 = .96
    I lack the fundamentals necessary to understand this equation: please enlighten me as to what the U represents. thanx
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  8. #7  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by mc2 View Post
    Quote Originally Posted by AlexG View Post
    They would see the other ship moving away at .96 c.

    The relativistic forumla for the addition of velocity is

    v + u / 1 + (vu/c2)

    If, for convienence we set the speed of light equal to 1, we have

    .75 + .75 / 1 + (.75 2) = 1.5 / 1.5625 = .96
    I lack the fundamentals necessary to understand this equation: please enlighten me as to what the U represents. thanx
    v and u are the two velocities.
    ei incumbit probatio qui dicit, non qui negat
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  9. #8 error 
    Forum Ph.D. merumario's Avatar
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    they should see each other traveling slower.so also will there time tend to run slower from the 2 observer and ist....
    Last edited by merumario; November 28th, 2012 at 11:05 AM. Reason: error
    "I am sorry for making this letter longer than usual.I actually lacked the time to make it shorter."###
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  10. #9  
    Forum Ph.D. merumario's Avatar
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    xowi i meant slower!
    "I am sorry for making this letter longer than usual.I actually lacked the time to make it shorter."###
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  11. #10  
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    you could consider an astronut traveling to sirus close to the speed of light c.she has a son of six,she is 25yrs....when she comes back from the mission,she learns that her son is older than her.to us on earth her trip was 30yrs,while to her at speed c.the trip was 6yrs...her son gets older than her by 5yr........this is just an example of time not been an absoute.the calculation could be done using time contraction formular.with a good enough mathematical firgues.....this example is hypothetical.
    Last edited by merumario; November 28th, 2012 at 11:14 AM. Reason: error
    "I am sorry for making this letter longer than usual.I actually lacked the time to make it shorter."###
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