XPost: sci.electronics.design   
   From: ram@zedat.fu-berlin.de   
      
   DJ Delorie  wrote or quoted:   
   >ram@zedat.fu-berlin.de (Stefan Ram) writes:   
   >>In general relativity, photons should attract each other, but   
   >>this effect should be very weak and should hardly be observable.   
   >If the photons are travelling at the speed of light (duh) wouldn't time   
   >dilation make any effect "over time" go to zero anyway?  I.e. it doesn't   
   >matter how much photons attract each other if there's no time in which   
   >to be affected by it.   
      
     When a photon is emitted by a lamp at time t0 and later detected by   
     a camera at time t1, we assume that for times t between t0 and t1 the   
     photon's distance from the lamp is ct (the product of c and t), where   
     c is the speed of the photon (the speed of light, assuming vacuum).   
      
     This assumes that the times are measured in a coordinate system   
     where the observer is at rest, so the whole process and the   
     movement of the photon can be described without the need   
     to take time dilation into account.   
      
     Observations confirm photon deflection by the Sun (Eddington 1919)   
     and gravitational lensing, measured in Earth-frame time.   
      
     So, there /is/ time, viz. the duration t1-t0, in which the   
     photons can be affected by gravity.   
      
     There is no need to transform the whole process into the "point   
     of view" of the photon itself. There probably is no such point of   
     view, although a very young Einstein is reported to have wondered   
     what the world would look like to a photon.   
      
     Photons lack a valid reference frame since Lorentz transformations   
     are undefined at v = c.   
      
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