How Does Gravitational Lensing Work in the Delayed Choice Experiment?

[bobsmith76]

This comes from Greene's Fabric of the Cosmos:

Wheeler imagines a cosmic version of the delayed choice experiment in which the light source is not a laboratory laser but instead, a powerful quasar in deep space. The beam splitter is not a laboratory variety, either, but is an intervening galaxy whose gravitational pull can act like a lens that focuses passing photons and directs them toward earth. if enough photons from the quasar are collected, they should fill out an interference pattern on a photographic plate. But if we were to put another photon detector right near the end of one route, it would provide which-path information for the photons, thereby destroying the interference pattern. the photons could have been traveling for many billions of years.

I'm confused about the words in bold. I don't see how a gravitational pull can act like a lens that focuses photons.

 

[Demystifier]

I think this question is for the Special and General Relativity subforum.

 

[tom.stoer]

One of the first successes of Einstein's theory of General Relativity was the correct prediction of light deflection in gravitational fields.

simulated gravitational lensing (black hole going past a background galaxy); source: Wikipedia

Further reading:

http://en.wikipedia.org/wiki/General_relativity#Light_deflection_and_gravitational_time_delay
http://en.wikipedia.org/wiki/General_relativity#Gravitational_lensing
http://en.wikipedia.org/wiki/Gravitational_lens