XPost: sci.electronics.design   
   From: PointedEars@web.de   
      
   Stefan Ram wrote:   
   > 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).   
      
   In a sense.  A photon is NOT a little ball of energy.   
      
   >   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.   
      
   Correct, in a sense.   
      
   >   Observations confirm photon deflection by the Sun (Eddington 1919)   
   >   and gravitational lensing, measured in Earth-frame time.   
      
   The latter is word salad.   
      
   >   So, there /is/ time, viz. the duration t1-t0, in which the   
   >   photons can be affected by gravity.   
      
   They are not "affected by gravity".  This has nothing to do with gravitation:   
      
   Gravitation is a consequence of the curvature of spacetime (according to GR).   
      
   The deflection ("bending") of light near other objects is another   
   consequence of the latter.   
      
   "Gravitational" redshift and "gravitational time dilation" are yet other   
   consequences of the latter.  Notice that I put those terms in "scare quotes"   
   here because they are *convenient misnomers*; they do not mean exactly what   
   they say, but we are somehow stuck with them, either for lack of a better   
   term or intellectual laziness.   
      
   >   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,   
      
   There is not, at least not an *inertial* reference frame; the existence of   
   such a rest frame would contradict the theory that it is based on.   
      
   > although a very young Einstein is reported to have wondered   
   >   what the world would look like to a photon.   
      
   He did not because the concept of a photon did not exist yet.  What he   
   wondered about, reportedly, was how it would be to ride alongside a light   
   *beam*, given that at the time Maxwell's electrodynamics was the best   
   available theory to describe light.   
      
   >   Photons lack a valid reference frame since Lorentz transformations   
   >   are undefined at v = c.   
      
   They are lacking a proper (i.e. own rest) frame that is an *inertial* one   
   because of that.   
      
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   PointedEars   
      
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