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The speed of light in special relativity is a constant, 186,000 miles per second, but in general relativity, "the speed of a light wave depends on the strength of the gravitational potential along its path" (Shapiro, qtd. in Wikipedia).

How much energy or mass would be needed to have a gravitational potential strong enough to slow the speed of light down to 1 mile per second?

For context: This is to amplify gravitational waves because, based on research, if the speed of light were slowed so much and a gravitational wave is generated from black holes colliding, the gravitational waves will have a prominent effect.

Mikayla Eckel Cifrese
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Khepera
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3 Answers3

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It's not the mass that matters, it's the gravitational potential, which depends both on mass and on distance.

Also, the Wikipedia quote is a bit misleading if taken out of context. The speed of a light beam as measured by a distant observer depends on the gravitational potential, but the speed of light as measured by a local observer (next to the beam of light) is always $c$.

Finally, to answer your question more concretely, the only place in the universe where a distant observer would measure such a slow speed of light would be just outside the event horizon of a black hole, where gravitational time dilation would make all processes appear slow (again, for a distant observer, local observers would not observe anything different).

Eric Smith
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No amount of mass can change the speed of light. Mass corresponds to a change in the direction of the geodesics in which light can travel and along which light always travels at $c$, not a change in the value of $c$. Geodesics are analogous to straight lines in curved space.

g s
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For photons, $E = h\nu$ where $\nu =$ frequency. Any change in photon energy does not change the photon velocity it changes the photon frequency. Inside the sun, the intense density means photons only travel by absorption and remission. In this process, photons can take 100,000 years to reach the surface but technically it's not the same photon that started the process.

Stevan V. Saban
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