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Simultaneous paradox

  1. Apr 20, 2009 #1
    see the attachment

    referring to the pic 1 simultaneity was explained to me like this

    there is a moving box car. At point A there is an observer in the boxcar at rest wrt the box car his name is Dash.

    There is another observer not at rest wrt the box car at point C his name is Still Bill.

    Lightening strikes both ends of the boxcar at point X and Z at time To.

    Point C and A are equidistant from the ends of the boxcar say a distance L

    Time for photons to reach point C from point X and Z is L/c so Still Bill thinks lightening strikes are simultaneous

    As the boxcar is moving Dash, who was originally at point A, will some time later, say T1, be located at point B.

    It is assumed Dash is moving relative to the photons coming from the lightening strikes.

    Dash will see the photons from lightening strike Z before photons from lightening strike X so Dash will not think strikes are simultaneous. Simple!

    Can I emphasise that Dash is assumed to be moving relative to the photons from the lightening strikes.

    now look at picture 2
    again moving boxcar
    person in the box car at rest wrt the box car standing at point A again his name is Dash
    lightening strikes at To at point A. ie the lightening strike is at the feet of Dash.
    Again at T1 Dash, originally at point A, will be located at point B.
    The photons from the lightening strike will move out in a perfect circle. The concentric circles show the photon front moving out from the lightening strike at various times.

    I have not included the observer not at rest wrt the boxcar, Still Bill, as I don’t care what he sees.

    Theory says Dash should see a sphere of photons moving away from him in all direction, clearly this is not the case. Dash was originally located at point A . At T1 he will be located at point B. If Dash at T1 when located at point B was to take a tape measure and measure the distance to the wave front of photons clearly he would get different measurements and therefore assume he is NOT in a sphere of photons?

    Still referring to picture 2 The distance Dash measures to the photon front using his tape measure will not be a function of the velocity of the boxcar it will be a function of his absolute velocity.

    So if you are in a moving frame of reference at rest wrt a point light source and you turn the light source on at say T0. If after a short delay, at say time T1, you measure the distance to the wave front from the point source of light.

    If the distance to the wave front is the same in all directions then you can conclude you are absolutely stationary. As you are absolutely stationary the time on your clock is absolute time and the distance you measure is absolute distance.
  2. jcsd
  3. Apr 20, 2009 #2
    for some reason the upload file thing is not working ? Keeps giving upload error To get the attachment please email me, probably some firewall port microsoft goddam thing
  4. Apr 20, 2009 #3
    Pictures are below

    Picture 1
    | |
    | |
    | |
    | |
    | |
    | 0 |
    | -|- |
    | /\ |
    X ____________A_____B______Z


    Picture 2
    | |
    | |
    | |
    | |
    | |
    | 0 |
    | -|- |
    | /\ |

    Imagine there are concentric circles radiating out from point A and Point A is at the centre of the circles

    sorry for the crudeness of the pictures but the upload thingy wouldnt work for me. If you want non crude pics then email me
  5. Apr 20, 2009 #4
    sorry all when I posted the reply the pictures got completely destroyed as it must remove multiple white space
  6. Apr 20, 2009 #5


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    Try the [CODE ] [/CODE ] tags... no internal spaces in the tags.

    Code (Text):
     A  B  C           D
  7. Apr 20, 2009 #6


    Staff: Mentor

    We already went over this in your light bulb thread. It is a simple matter to show that a spherical wavefront propagating at c in one frame is a spherical wavefront propagating at c in all frames. Just take the Lorentz transform.
    Last edited: Apr 20, 2009
  8. Apr 20, 2009 #7
    with quotes :-p~
  9. Apr 20, 2009 #8
    nope quotes didnt work either

    Can I ask a big favour can someone email me


    Ill email the pics to you and you can upload them for me please.....pretty please
  10. Apr 20, 2009 #9
    Last edited by a moderator: Apr 24, 2017
  11. Apr 20, 2009 #10
    I agree with you photons do radiate in a sphere from a point light source in all frames.

    If you have an observer who is moving and who is at rest wrt a point light source, which obviously is also moving. When the light source is turned on what I am saying is that if the observer sees a sphere of photons then the explanation of simultenaity is wrong and on the flip side if the explanation of simultesity is right then the observer will NOT see a sphere of photons...which is it? it cant be both!

    See the pictures before you reply. I may be wrong but I cant see the error in my logic given the way simultenaiety is explained.
  12. Apr 20, 2009 #11


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    Gold Member

    You've misunderstood the import of relativity if you think that there is such a thing as absolute velocity. How can anything be absolutely stationary if one can always find a frame in which it moves ?

    Please go away and learn relativity properly.
  13. Apr 20, 2009 #12
    Hi Mentz

    have you seen the pics?

    I know it is not intuitive but I cant see where my logic is wrong?

    In my post is the explanation of simulteniaty wrong? or the other way round?
  14. Apr 20, 2009 #13


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    Gold Member

    Hi Dreads,
    yes, but I couldn't relate to them. Simultaneity is observer dependent. If there are two space separated events at which light is emitted in all directions, all observers who are equi-distant ( or equi-time) away from the those events will say they are simultaneous, all others will disagree. There's not much more to be said and no potential paradoxes lurking.

  15. Apr 20, 2009 #14
    if you have a sphere of photons and assuming photons travel in a striaght line out from a central point at a constant velocity C ie they form a sphere.

    Remebering photons are not ballistic ie the velocity of the frame from where the photons originate does NOT add to the velocity of the photons.

    Wont the point that represents the centre of that sphere always be at the same point in space? if not why not?

    so if you found a sphere of photons and if you measured back from the wave front of that sphere and found the centre of that sphere and then flew a space ship in the right direction and speed so that you always remained at the centre of that sphere then the space ship would be at the same point in space at all times. Evereything would probaby be moving around you but you would be theoretically stationary

    or am I worng?
  16. Apr 20, 2009 #15
    so referring to picture 2

    will the observer measure the same distance to the wavefront in all directions?
  17. Apr 20, 2009 #16


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    You would be stationary in the frame of the matter that emitted the sphere of light. The matter might not be there anymore ( as with the CMBR ).

    I think you just invented the auto-pilot.
  18. Apr 20, 2009 #17
    with post #14 I am ignoring the effects of gravity as gravity would distort a sphere of photons so that it was no longer a sphere
  19. Apr 20, 2009 #18
    I think I see where the misunderstading is coming from. If the light source is on for a period of time photons are continually streaming from the light soruce and so the centre of that stream is not stationary it will form a cone shape

    I read in a science journal that there is a single photon light source now.

    So if the single photons light source is in a moving frame of ref and it emitts a very brief flash of light so you have a sphere of single photons radiating out from the matter that emitted the photons in all directions. The centre of that sphere of photons is a stationary point in space for all time, ignoring gravity
  20. Apr 20, 2009 #19

    referring to picture 2 of my post
    changing the wording to the light source emits a sphere of photons one photon thick in all directions

    will the observer measure the same distance to the wavefront in all directions or not?
  21. Apr 20, 2009 #20
    I cant figure this out in my head I see a paradox
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