We are always taught that light travels in straight path in a homogeneous medium, we even do experiments in primary school with cardboards with holes, "proofing" that light does not bend.

But on a wider perspective, let's say sending a pulse of light from our Sun to another star across the galaxy (and assuming it is not blocked by anything in the way nor goes near any massive source of gravity), and you observe far away 'above' the galaxy, would that pulse of light travel in a curved path (sort of like how a spacecraft would travel)? Or would it ignore the whirl of the galaxy and travel completely straight when observed in that perspective?

Edit: Let me rephrase my question. If light is massless, it would not carry the inertia from the light source (e.g. my flash light). Then it means if I shine a laser beam into the sky, I should be able to see a tiny curve in the laser's path (maybe not with my naked eye, but some special camera), because the laser pointer in my hand is moving with the Earth, right?

  • $\begingroup$ The light will travel straight (more or less by definition what straight is) $\endgroup$
    – nwolijin
    Jun 22, 2021 at 9:23
  • 3
    $\begingroup$ I'm not sure I understand your question completely. But if your "whirl" is referring to something similar to a gravitational slingshot, then no, because light is massless. However, if the receiver and emitter are both stars, then the stars have a massive enough size to bend the spacetime around it. In this case, the "straight lines" in the vicinity of the stars are described by geodesics which are no longer straight in the Euclidian sense $\endgroup$
    – D. Soul
    Jun 22, 2021 at 10:18
  • $\begingroup$ Let me rephrase my question (edits above) $\endgroup$
    – Jas T
    Jun 23, 2021 at 9:36
  • $\begingroup$ it would not carry the inertia from the light source. I'm not quite sure what you mean by that. Light is massless, but it still has momentum. Eg, if you're in a vehicle traveling at 1000 km/h east (relative to the ground) & you shine a light directly north, the beam will head north in your frame, but the observer on the ground will see the beam heading in a straight line in a direction slightly east of north. $\endgroup$
    – PM 2Ring
    Jun 23, 2021 at 9:54
  • $\begingroup$ Thanks PM2ring, your answer clicked for me $\endgroup$
    – Jas T
    Jun 23, 2021 at 13:29

1 Answer 1


According to the GTR, there is no such thing as a classic straight line in a universe with Gravity - they are replaced by the concept of a geodesic.

The weaker the gravity, the closer the geometry approximates Euclidean geometry with straight lines. Since gravity "curves" space-time, and light travels through space-time, light cannot ignore gravity


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.