As far as I understood from general Relativity, gravity should be seen as the same as an accelerated motion. And that the light deviation was developed Einstein regarding to the lift model where a light particle would hit the moving lift at another point then it entered it. Shown in this image:

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But I wonder, as we are speaking about accelerated motion, this model of a lift and deviated light would also work with relative motion in the non accelerated train examples of the specific relativity. I see that I am mixing things up here, but still I don't understand this. Maybe the point is, that in the train I know that I am moving and in the lift model the trick is just, that accelerated moving and gravity feels the same?!


1 Answer 1


Yes, you said it. The accelerated observer knows that he/she is in presence of gravity or, at least, moving with some acceleration. Furthermore, another important point to take into account is that the non accelerated observer does not see any curvature of the light path, in this case the trajectory of light is a straight line going downwards (see picture below) Different motions and its corresponding light trajectory

  • $\begingroup$ ok but I dont see here a difference between a straight line of deviation and a curvature $\endgroup$
    – user276771
    Jun 27, 2021 at 14:56
  • $\begingroup$ For giving a geometrical interpretation, let's say that the following path of light rays somehow gives an idea of the underlying geometry of space (and time) where its moving. This is motivated by the Fermat Principle, which states that the light moves through the curve of minimal distance between two points. In the case of non accelerating observers, the straight lines (horizontal and descendents) are the curves of minimal distances between two points in a plane (flat spacetime). So, geometrically, that is what GR takes care, they are equal situations. $\endgroup$
    – T. ssP
    Jun 27, 2021 at 19:53
  • $\begingroup$ I propose you the little challenge of think what are the possible geometries for curved light trajectories ;) $\endgroup$
    – T. ssP
    Jun 27, 2021 at 19:55

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