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Can light really be stopped or attracted by a strong enough gravitational field, such as at a black hole? Or could it be explained as an optical effect of the spacetime curvature?

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The light is not attracted in the traditional sense of the word used in physics. Instead it follows geodesics (the "straightest" possible path) along curved spacetime caused by the presence of mass or energy. If the energy/mass density is great enough, a black hole is created which curves spacetime enough as to cause light to "orbit" around it. So no, light bending is not an illusion it is an actual physical phenomenon. To be honest though, what we observe with telescopes is the apparent position of objects behind large gravitational field like this picture here which demonstrates the effect.

EDIT: I did not address the first part of the question. The answer is no light cannot be stopped. It always propagates at speed $c$ in vacuuum, it will just follow curved paths.

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  • $\begingroup$ FWIW, there is only one closed orbit for light around a non-spinning black hole, and it is an unstable equilibrium orbit. So it is possible, but it is possible in the sense that it is possible to blance a pencil on its point. $\endgroup$ – Jerry Schirmer Jun 16 '14 at 23:51
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The answer to both your statements:

Can light really be stopped or attracted by a strong enough gravitational field, such as at a black hole?

and:

Or could it be explained as an optical effect of the spacetime curvature?

is yes. The former is just an extreme case of the latter.

If you analyse some series of lenses refracting light then the refraction can be explained by the changes in the speed of light caused by the glass. Really it's due to changes in the relative phase along different paths, but this can be related to the change in the velocity of light. Gravitational lensing works in the same way. The spacetime curvature changes the phase of the light and this causes lensing. You can describe this as a result of the change in the velocity of light. See my answer to Speed of light originating from a star with gravitational pull close to black-hole strength? for more, or search this site for many related Q/As.

Any mass will reduce the speed of light to some extent. A black hole is simply a mass large (and compact) enough that it reduces the speed of light to zero.

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