Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Light clock with problems

  1. Apr 2, 2016 #1

    DAC

    User Avatar

    Hello P.F.
    I have previously raised the idea of a light clock that ticked each time the light travelled one metre. Dale has said if I could explain how it could be done, I could open a new thread. Here goes.
    A normal light clock ticks each time the light goes mirror to mirror.
    I propose a light clock that ticks each time the light goes one metre.With the mirrors one metre apart in the stationary frame, the two are the same.
    What happens when the train is moving?
    The mirrors will still be one metre apart in both frames.
    The diagonal light path will still be the same diagonal light path.
    The sensors locations however have to change. Remove the existing sensors and replace with sensors at one metre intervals along the light's path.
    So if the perpendicular path is one metre, the clock ticks once. If the diagonal path is two metres the clock ticks twice. Both clocks tick at the same rate, once per metre, irrespective of the light paths length.
    This conflicts with S.R. can you explain.
    Thanks.
     
  2. jcsd
  3. Apr 2, 2016 #2

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    There is no common definition of travelling one metre. The light will have travelled one metre in the ground frame when it is still short of the mirror.

    And, of course, this is true in classical physics too. If someone on the train throws a ball up 1m, then someone on the ground sees that ball travel more than 1m up and across. Your confusion is not acually with light clocks and Special Relativity. Your confusion is actually in grasping the concept of two reference frames and basic trigonometry.
     
  4. Apr 2, 2016 #3

    Ibix

    User Avatar
    Science Advisor

    If the sensors are at rest in the train frame then you only need two of them - one at each mirror. If the sensors are not at rest in the train frame you need infinitely many of them, at various points at different heights along the zigzag path of the light (because in a frame moving at v with respect to the train the path length of the light on one leg is ##\sqrt {1+v/\sqrt {c^2-v^2}}##, if my mental algebra is correct, which is not 1m in general).

    Which setup were you meaning?

    To me, it still looks as if you are failing to recognise that the distance travelled to get from one end of a moving object to the other is not the same as its length. Until you grasp this fact (true in Galilean relativity as well as Einsteinian relativity) you will continue to fail to understand relativity.
     
  5. Apr 2, 2016 #4

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    PeroK's and Ibix's replies are sound. Your problem seems to be in grasping that the same physical situation must be described by all frames without changing the basic setup. You cannot have the sensors located at the mirrors in one frame and at different places in another.

    I stand by the suggestion that you should probably spend a significant amount of time to study Galilean relativity before trying to deal with special relativity.

    From your posts, it is clear that you do not have a grasp of the subject equivalent to that of a graduate student in physics. I have therefore relabeled the thread accordingly.
     
  6. Apr 2, 2016 #5

    DAC

    User Avatar

    One
    When the sensors are not at rest applies to the standard light clock as well. The sensors move with the clock/train. Yes, one metre will fall at different points along the diagonal path
     
  7. Apr 2, 2016 #6

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    But the key point is that you are then not describing the same physical situation. If you place the sensors at the mirrors, they will be at the mirrors in all frames. This is not something you can change from frame to frame.
     
  8. Apr 2, 2016 #7

    DAC

    User Avatar

    The sensors are placed at the same interval, one metre apart. Why is that less valid than sensors every mirror to mirror apart? I have not changed the basic set up of the stationary frame, I have adhered to it.
     
  9. Apr 2, 2016 #8

    DAC

    User Avatar

    The clock ticks each time light goes one metre. This is the same in both frames. I have changed nothing.
     
  10. Apr 2, 2016 #9

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    No it is not, you are describing different clocks. One where the sensors are at the mirrors and one where they are not. It just so happens that you have constructed them in such a way that they tick at the same rate when one of them is moving. This does not violate SR as much as it demonstrates a fundamental misunderstanding of relativity (Galilean and special). Before you address this misunderstanding, you are in no position to make claims about inconsistencies.

    As suggested by other posters, you first need to remedy your misconceptions before trying to implement your thoughts, or you will end up in the wrong place.
     
  11. Apr 2, 2016 #10

    Samy_A

    User Avatar
    Science Advisor
    Homework Helper

    @DAC: It's a pet peeve of me, but I would suggest you use a little more numbers and a little less words to describe the situation. That will (hopefully) help clear up what you misunderstand about the situation.
     
  12. Apr 2, 2016 #11

    DAC

    User Avatar

    In the stationary frame with the mirrors one metre apart, the clock ticks every one metre travelled. And in the moving frame it also ticks every one metre travelled. There is no difference between frames.
     
  13. Apr 2, 2016 #12

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Yes there is! It is not the same clock! You have simply constructed two different clocks based on an underlying principle in two different frames. Due to time dilation, applying this construction to the moving clock will result in a clock which is physically different from one which is constructed at rest - as evidenced by the fact that one clock has its sensors at the mirrors and the other does not.
     
  14. Apr 2, 2016 #13

    Dale

    Staff: Mentor

    You have not explained how you would build such a clock, you have only again asserted your wishful assumption that such a clock were possible. I gave you an example of how you could specify the design of a clock which could be analyzed.

    https://www.physicsforums.com/threads/light-clock-confusion.839036/page-2#post-5277568

    This thread is closed, we are done with this topic.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Light clock with problems
  1. Light Clock (Replies: 8)

  2. Light Clock Problem (Replies: 165)

  3. Light Clock Problem (Replies: 3)

  4. Light Clock problem (Replies: 28)

Loading...