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

Detecting if you are moving ?

  1. Oct 28, 2011 #1
    Maybe an old topic.

    But if you reflect a light wave, and if the light wave is not coming back in exact the same location (very exact device), could you say I am not moving in an absolute way (suppose light has the same behavior in other universa as well) ?

    You have to try this in at least two directions, because you can be moving in the same direction of the light wave ...

    What is a light wave, is this a line with many fotons after each other, a light source is always moving so you would think it is more a bundle with fotons near each other ?
     
    Last edited: Oct 28, 2011
  2. jcsd
  3. Oct 28, 2011 #2

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    No matter what form the light, no matter how you were moving relative to other things (as long as you feel no acceleration) the result would be the same.

    I mean, this is basically the Michelson-Morley experiment. Given the principle of relativity, do you really think this hasn't been tested six ways from Sunday?
     
  4. Oct 29, 2011 #3
    Ok, you mean that when somebody has speed, lets say B compared to A (standing still), they see both eg. through a telescope (90 degrees) exact the same image starting from the meeting point (because of time dilation) (or they see as in my original question at the same time the returning light wave) ?
     
  5. Oct 29, 2011 #4

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Telescope is a different issue than you described. Your original situation was a source and mirror in a lab or truck. Can you detect motion by a change in where the light reflects back? No.

    Your new question is trivial - of course you can detect motion relative to some distant object. But that's all it tells you - motion relative to some chosen object, a star in this case.

    There are, in fact, multiple ways to detect motion relative to a star: doppler effect on frequency of spectral lines; aberration: change in apparent angle in the sky of the star. The former tells you motion to / from the star; the latter tells you transverse speed.
     
  6. Oct 29, 2011 #5
    To be sure I understand the behavior of light right :

    If A is standing still and B has a constant speed in space, on the meeting point a light wave is sending in an angel of 90 degrees into space, it is reflected far far away, and when it returns A as well as B see this lightwave back after years at the same moment (during this time B is also far far away) ?
     
  7. Oct 29, 2011 #6

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    It seems you change scenarios with every post. Your original post, as I understood it, was to detect motion in a 'lab' moving at some speed without reference to anything outside (by reflecting light back and forth). This is impossible. It is obvious, trivial, and consistent with relativity that you can detect motion relative to something else. That is , A sees B move at v, B sees A move at -v.
     
  8. Oct 29, 2011 #7
    Thanks PAllen, I understand the answer, that's why I asked this question (in relation to my original post to be sure, B moves, it is difficult to find some clear answers in books) :

    To be sure I understand the behavior of light right :

    If A is standing still and B has a constant speed in space, on the meeting point a light wave is sending in an angel of 90 degrees into space, it is reflected far far away, and when it returns A as well as B see this lightwave back after years at the same moment (during this time B is also far far away) ?
     
  9. Oct 29, 2011 #8

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    You need to specify more things:

    - who sends the light? A, B, or some other source (and if this, their state of motion).
    - who measures the 90 degrees, when do they measure it, where do they measure it?
    - what is the state of motion of the mirror?
     
  10. Oct 29, 2011 #9
    The distance is not important, it is only for my understanding.

    A sends the light on the moment that B passes (maybe for the answer I expect B must send the light, I am not sure, but let's say A sends the light, and give also an answer if B sends the light).

    Suppose A and B on a straight line, B is moving to left, the angel is exact 90 degrees up, and a mirror is exact straight on above the location 0 degrees, so the light is reflecting back to where it was send from. And let's say all in a very big vacuum room somewhere moving in space all with a constant speed (B too). Mirror at the top of that very big room.
     
  11. Oct 29, 2011 #10
    The goal of the Michelson Morley experiment was to determine the absolute velocity of the earth through the ether which was presumed to be at rest. The failure of the experiment resulted in the Theory of Relativity several decades later which said that all velocities are relative and there is no such thing as absolute rest. However, I understand that when they mapped the cosmic background radiation they found it slightly warmer in one direction and slightly cooler in exactly the opposite direction, indicating the direction and velocity of the earth relative to the universe - the goal of Michelson and Morley experiment a century earlier.
     
  12. Oct 29, 2011 #11
    To make it simple, only the answer is important in such a situation, do this in a vaccuum lab in thought ... and give B a speed of 0.5c compared to A.

    The distance is not important, it is only for my understanding.

    A sends the light on the moment that B passes (maybe for the answer I expect B must send the light, I am not sure, but let's say A sends the light, and give also an answer if B sends the light).

    Suppose A and B on a straight line, B is moving to left, the angel is exact 90 degrees up, and a mirror is exact straight on above the location 0 degrees, so the light is reflecting back to where it was send from. And let's say all in a very big vacuum room somewhere moving in space all with a constant speed (B too). Mirror at the top of that very big room.
     
  13. Oct 29, 2011 #12

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    I made an animation to depict the situation you are describing, except B is moving to the right, and instead of just one mirror at 0 degrees, I have placed mirrors all around. Note that each observer has their own set of mirrors that is stationary with respect to themselves:

    https://www.youtube.com/watch?v=dEhvU31YaCw

    Do you need any more explanation or can you figure out what is going on here on your own?
     
  14. Oct 30, 2011 #13
    Hi Gwh, thanks for the animations, that's a good demonstration for lessons.

    But there is no text, I suppose that there is one example it illustrates my situation (and maybe the first answer from PAllen).

    I want to be 100% sure, because I find this the most interesting subject in physics personally to think about further maybe in other dimensions (and no equipment is necessary, you can do it all in thought further). So I want to be sure, that I see this correct before thinking further. In my other topic I know this already in a horizontally position (counting waves).

    So it illustrates that B sees the same light wave back after reflecting (angel of 90 degrees, wave vertically, mirror 0 degrees) after a while (with time dilation) in an other location (all compared to A of course) ?
     
  15. Nov 3, 2011 #14
    Hi Gwh,

    You don't have to confirm because it is 100% logically, otherwise you could proof movement (when you did not see the lightwave back after reflection).
     
  16. Dec 24, 2011 #15
    hey pallen , i just want to ask if there is any threads about aberration before i add one i did some search but i couldnt find any ?? please reply and thank you
    PS if not i need some help in understanding the concept of aberration classical and relativistical , i really having problems understanding it and thank you
     
  17. Dec 24, 2011 #16

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Why not start a thread on it? I don't recall it being discussed here recently. It would be a welcome break from twins etc.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook