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Light spheres

  1. Apr 25, 2009 #1
    please see the attachment. I am on a computer usign Microsofts new crap word software and it wont allow me to copy and paste can you believe it

    In the attachment i refer to a previous post of mine .It is the post where I showed that the density of a sphere of photons will change in a moving frame as opposed to a stationary frame
     

    Attached Files:

  2. jcsd
  3. Apr 26, 2009 #2
    Hello Dread.

    This may be relevant. If you Google Headlight Effect and look at Wolfram Demonstration tou will find an animation. I copied a bit of the text:-

    -----A standard problem in special relativity asks you to consider light emitted uniformly in all directions from both the rocket frame (where the light is emitted from) and the laboratory frame (watching the rocket fly past). From the point of view of an observer in the laboratory frame, the light emitted from the rocket frame (in the forward hemisphere) is measured to be compressed into a cone in the forward direction.------

    Matheinste.
     
  4. Apr 26, 2009 #3

    JesseM

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    In the attachment you write:
    Why do you think that Dash would see an uneven photon density from the boxcar strobe, which is at rest relative to him? As long as the boxcar strobe is built the same way as the station strobe, then if Still Bill sees an even photon density from the station strobe, Dash will see an even photon density from the boxcar strobe.
     
  5. Apr 27, 2009 #4

    Dale

    Staff: Mentor

    Hi Dreads, why don't you guess my suggestion based on your other threads?

    Do you need some help with the Lorentz transforms?
     
  6. Apr 28, 2009 #5
  7. Apr 28, 2009 #6

    JesseM

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    The arguments there only suggest that if the photons are emitted at equal angles in the rest frame of the emitter, then they'll be emitted at unequal angles in the frame where the emitter is moving. I agree with that. But in your example, Dash's frame is the rest frame of the emitter on board the train, so why do you think he should see unequal angles? The "moving frame" relative to this emitter is Still Bill's frame, not Dash's.
     
  8. Apr 28, 2009 #7

    I admit on some of this this I was guessing but that bit is actually irrelevent. I am firm on the following:

    1. The boxcar pulls out of the station

    2. Still Bill may assume:
    Dash is not at his original velocity but Still Bill is at his orginal velocity (ie dash is moving)
    Still Bill is not at his original velocity, Dash is at his orginal velocity (ie Still Bill is moving)
    Dash and Still Bill are not at thier original velocity (ie both are moving)

    3.Dash may assume:
    Dash is not at his original velocity but Still Bill is at his orginal velocity
    Still Bill is not at his original velocity Dash is at his orginal velocity
    Dash and Still Bill are not at thier original velocity

    4.Still Bill's strobe goes off, he sees a sphere of photons of even density
    Dash's strobe goes off Dash see a sphere of photons UNEVEN density

    5.Dash now knows he is no longer at his original velocity

    6.Still Bill telephones Dash and they tell each other what they see

    7.Still Bill now knows he is at his original velocity.
    8. Still Bill now knows Dash is NOT at his original velocity.
    9. Dash now knows Still Bill is at his original velocity.
    10. Dash now knows he is no longer at his original velocity

    11.Dash knows he is at some velocity other than zero. Dash sees a sphere of photons from his strobe, with himsef at the centre of that sphere at all times. He must concludes either photons are ballistic or that there is an error with the theory!
     
  9. Apr 28, 2009 #8
    As I see it if Dash is at rest wrt the moving emitter he will aslo see uneven angles

    Dash is in a space ship docked at the launch pad and his strobe goes off. The striobe is at the origin os a cartesian coor system and there are 2 light intensity meters (LIM) measuring the light in each hemisphere. The reading from each LIM comes up on an LCD display. Dash reads the reading each LIM gives and they are the same.

    The space ship now takes off and attains a constant velocity. The strobe now goes off again. The LIM will give unequal readings and Dash will see that from the LCD display. That is all I am saying nothing more or less. I dont care what Still Bill sees.

    If Dash assumes his velocity at the launch pad was zero then wihtout reference to phenomena external to the space ship he will now know he is moving at a non zero velocity relative to his original velocity
     
  10. Apr 28, 2009 #9

    JesseM

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    Why do you think that? It's in direct contradiction with SR's principle of the equivalence of all inertial frames, and you haven't given any arguments as to why there is any problem with the idea that Dash will see even angles. Perhaps you are confusing yourself with the phrase "moving emitter"--remember that in SR there is no absolute truth about what's moving and what isn't, Dash's emitter is moving relative to Still Bill, but then Still Bill's emitter is also a "moving emitter" in Dash's frame.
    But do you have any actual argument for thinking this is how it will work? It's definitely not what relativity would predict.
     
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