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Lorentz Transformations or Dilation/Contraction?

  1. Dec 13, 2015 #1
    I'm doing a class on special relativity and when doing some problems, I'm never sure whether I should be using the Lorentz transformations (Eg. x' = γ(x-vt) or t'=γ(t- (v/c^2)x)) or the Time dilation and Length contraction equations to find t or x! Can anyone explain if there's any way of knowing or am I missing something really simple?
     
  2. jcsd
  3. Dec 13, 2015 #2
    You should always use the Lorentz transformation first. Time dilation and length contraction are just special cases of the transformation.
     
  4. Dec 13, 2015 #3

    Dale

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    I agree with PWiz. Always use the Lorentz transform. The length contraction and time dilation formulas will automatically fall out when appropriate.
     
  5. Dec 13, 2015 #4

    bcrowell

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  6. Dec 13, 2015 #5
    Well, for example;
    A rocket passes the earth at v=0.6c and both the rocket and earth agree it's 12:00. At t'=13:00 the rocket passes a space station. Show that the rocket passes the space station at 13:15 in the earth's frame.
    Here γ=5/4
    Which would I use here? Is it correct to say T=γTo so then if you sub To as 1 you'll get 1.25 which is an hour and a quarter (ie 13:15) or am I misunderstanding something?
     
  7. Dec 13, 2015 #6

    PeroK

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    Why not do both? Two different solutions to a problem that agree are better than one!
     
  8. Dec 13, 2015 #7

    PeroK

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    The perfect example! Do it both ways and check you get the same answer.
     
  9. Dec 14, 2015 #8
    Contrary to what the other posters say, you can perfectly go with the time dilation and length contraction formulae. I solved every single exercise that way in Physics 101, including relations with momentum and energy which transform in a similar way. If you do so, you will gain a much deeper understanding of relativity. The only caveat is that you must use proper time instead of coordinate time, which is achieved finding a clock which travels with the particle or placed baside the event taking place, and also computing the time that light uses to reach all of the observers involved. More ellaborate but will give you a better insight. With practice either method will be as easy as the other.
     
  10. Dec 14, 2015 #9
    No, not at all. The Lorentz transformation is more fundamental in SR than time dilation and length contraction. You will only run into conceptual issues if you think otherwise.
    It might seem easy to work your way through some problems without using the full Lorentz transformation, but you'll most definitely run into hot water if you try solving anything different from those "standard" relativity questions using the TD and LC formulae only.
     
    Last edited: Dec 14, 2015
  11. Dec 14, 2015 #10

    Dale

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    I completely disagree with this. Time dilation and length contraction alone cannot be used to solve problems involving the relativity of simultaneity. If your physics 101 course did not include exercises on the relativity of simultaneity then it was not enough of a curriculum to even be considered an introduction to relativity.

    That is an indication of a deficiency in the course, not an indication of the sufficiency of length contraction and time dilation.
     
  12. Dec 14, 2015 #11
    The course was relativdly good :) It was me who didn't want to use the Lorentz transf. after understanding them. The problems included the barn and pole apparent paradox, and I solved with time dilation and length contraction alone. As said, you only need to take into account the travel time of light rays, which is the faster way of communicating events, and what lies beneath the reason of the relativity of simultaneity (Einstein original paper).

    I have studied GR at the faculty, and no need of using Lorentz transform. never in my life. Why the academical books insist on this historical and rather artificial approach is beyond my understanding.
     
  13. Dec 14, 2015 #12
    I doubt it.
     
  14. Dec 14, 2015 #13
    You can take my word. As for special relativity, think of a problem and send me a message or post it here. I will use time dilation and lenght contraction formulaae alone. It is funny. Even with cuadrivector p_i , E... I solved by this method though they wefe a little harder.
     
  15. Dec 14, 2015 #14

    PeroK

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    Does your knowledge of SR extend to 4-vectors and energy/momentum transformations?
     
  16. Dec 14, 2015 #15
    Physcis 101,-introduction to. SR.
    Quantum physics - SR with attention to particle physics, matter-antimatter aniquilation, Compton effect, Thomas precession.
    General Relativity as a semester included in Astrophysics subject.
    Analytical Mechanics and Relativity.

    And yes, I solved (E,pc) problems by using time dilation and length contraction relationships. Very funny and very useful to understand relativity.
     
  17. Dec 14, 2015 #16

    Physcis 101,-introduction to. SR.
    Quantum physics - SR with attention to particle physics, matter-antimatter annihilation, Compton effect, Thomas precession.
    General Relativity as a semester included in Astrophysics subject.
    Analytical Mechanics and Relativity.

    And yes, I solved (E,pc) problems by using time dilation and length contraction relationships. Very funny and very useful to understand relativity.
     
  18. Dec 14, 2015 #17

    bcrowell

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    This just sounds wrong.
     
  19. Dec 14, 2015 #18

    PeroK

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    That would be energy dilation and momentum contraction then!
     
  20. Dec 14, 2015 #19
    Haha... Nobel prize for me, then. :)
    You can make p = mv. E = mc^2. m=m_0 * gamma.
    Now you just get the v' for a particle and you are done. For v' you could use velocity transfor. formulae, but I didn't myself to use Lorentz, can be done
    .with v=x/t alone for constant speed. Just think a little: it is just «low level Relativity».
    s
     
  21. Dec 14, 2015 #20
    I'm curious as to how you can use that formula without acknowledging the fact that you're utilizing the Lorentz transformation.
     
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