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In other words, if a photon is emitted from source, is it possible to change its course en route either by introducing a gravitational lensing or some sort to change the road it travels (spacetime) before it reaches it's source?

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If only single photon is emitted from your source or if you know how to extract a single photon from a group of photons, then you can change its direction by reflection, refraction, diffraction, etc.

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  • $\begingroup$ Isn't reflection/refraction involve replacing the absorbed photon with new one with a different path? $\endgroup$ Aug 26 '12 at 5:25
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    $\begingroup$ @user8123 no reflection/refraction is "the same" photon. To "change" photons, photon should be absorbed, then re-emitted. But reflection and refraction happen without every absorbing the photon, just changing the evolution properties of its field, which is closely related to its wavefunction. That is to say, no energy from the photon is left behind in the object or material reflecting it or refracting it. (In lossy materials, some photons are absorbed and lost, but those successfully reflected or refracted retain all their original energy: their frequency is unchanged.) $\endgroup$
    – mwengler
    Aug 26 '12 at 15:56
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Is this a philosophical or practical question?

You can change the path of a photon in a variety of ways - reflection, refraction, diffraction - it's called optics. Whether it's the same photon is more a philosophy question.

Gravity will change the path of a photon, although you would need a pretty dense and massive object to have any noticable effect. We can just about detect the gravitational lensing effect of Jupiter on the light from a star.

ps. As JohnRennie point out, this is another philosophical point. Yes it is the same photon, but since it is space that curves, from the photons point of view its path didn't change.

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  • $\begingroup$ >"Whether it's the same photon is more a philosophy question." Can you elaborate? $\endgroup$ Aug 26 '12 at 5:23
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    $\begingroup$ When a photon is reflected or refracted it's energy is converted into vibrations of the atomic or electron bonds. These then emit a new photon. Diffraction is a bit less obvious but you could argue that if the photon has to interact with matter then it has been destroyed a new different photon comes out. $\endgroup$ Aug 26 '12 at 5:25
  • $\begingroup$ Thanks. So ..it is a new photon then. Does gravity change the path in same fashion? I'm under the impression that gravity does so without creating any "new" photons - hence the question: Can the path(the spacetime pathway it'll traverse till it reaches the destination) of a photon be changed while it's still hasn't reached the destination? $\endgroup$ Aug 26 '12 at 5:38
  • $\begingroup$ @user8123 - I would say that gravity DOES change the path of the same photon. $\endgroup$ Aug 26 '12 at 5:40
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    $\begingroup$ Possibly this is philosophy again, but a photon always moves in a straight line. When you see a photon's trajectory curve near a heavy object this is because you are using a different co-ordinate system to the photon and the transformation between the two co-ordinate systems is non-linear. The curvature is in your co-ordinate system not the photon's path. $\endgroup$ Aug 26 '12 at 6:26

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