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this post was submitted on 25 Jun 2026
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It starts with a paradox - if light has no rest mass, how can it be affected by gravity? Gravity does not exert a force on massless objects.
The answer is that mass warps the fabric of spacetime around it. As the fabric is warped/bent by massive objects, light travels along these bends. This is why gravitational lensing occurs for example - distant galaxies warp spacetime, so light traveling around them gets bent back toward itself. We can use this to see distant objects in the cosmos that would otherwise be too faint to detect.
We don't feel the effect because it's very small, however we can detect it. Some atomic clocks are so incredibly sensitive that they can detect just a few meters' difference in altitude from the surface of the earth.
https://en.wikipedia.org/wiki/Gravitational_time_dilation?useskin=vector
https://www.science.org/doi/10.1126/science.1192720
Gravity can affect light. If gravity didn't affect light, it would be possible to violate the conservation of energy.
A high energy photon can turn into an electron/positron pair. A massless particle can convert into massive particles. Imagine I got really good at managing the back-and-forth transition between high energy photons and electron/positron pairs. Enough that I could control it with very high efficiency and control. (Unlikely, but we're talking physics violations here. Assuming difficult engineering is fine.)
So let's say I'm really good at converting back and forth between photons and electron/positron pairs. Good enough that I can build a machine that can do this in bulk with high efficiency. (We'll also assume mirrors of unreasonable reflective efficiency.)
If gravity did not affect light, I could use such a conversion machine as a free energy device.
Convert into photons and store in a mirrored box.
Lift the mirorred box to a great height.
Once at a height, convert the photons back to electrons/positrons.
Lower the box down, using the falling weight to spin an electric generator.
Once on the ground, convert back to photons and repeat.
Conservation of energy demands that gravity be able to affect light.
Is this why light cannot pass through a black hole? Probably a dumb question but doesn't the light go into a singularity? And is that singularity like a fixed point in time?