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I just stumbled upon this question.

For massive particles like an electron, it is easy to make them follow a non-geodesic path by applying an external force. In case of the electron, one can put it under some electric field so it undergoes accelerated motion.

I wonder if it is possible to make massless particles like photons follow a non-geodesic path. I think the simplest way is to send a light beam into a diamond. Will it work?

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    $\begingroup$ Use a prism? Or a lens? $\endgroup$
    – John Rennie
    Commented yesterday
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    $\begingroup$ @KDP Well, yes, but it would be another geodesic, so in the middle, it couldn't have been on one. $\endgroup$
    – ChemiCalChems
    Commented yesterday
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    $\begingroup$ @ChemiCalChems in the middle, there is no photon. $\endgroup$
    – paulina
    Commented 19 hours ago
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    $\begingroup$ In a medium with a gradient in the RI, the light will follow a non-straight path. This is, for example, what causes a mirage. $\endgroup$
    – Cris Luengo
    Commented 15 hours ago
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    $\begingroup$ The path though a lens is still "geodesic" in the sense that it locally minimizes travel time. But it is not a geodesic in the general-relativity sense. $\endgroup$
    – Kevin Kostlan
    Commented 8 hours ago

2 Answers 2

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A diamond would be very expensive. A mirror would be cheaper solution.

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  • $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$
    – Roger V.
    Commented 3 mins ago
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Your tag says GR, so the photon can travel via an atmosphere of a planet to follow a non geodesic.

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  • $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$
    – Roger V.
    Commented 3 mins ago

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