Light passing by a massive object will be deflected by the object's gravity. The effect is very small, so you need something very massive, like a star, or better still, a black hole to get a noticable deflection. The usual result of such a deflection is that the background object is displaced a small amount from its true position. However, if the foreground object (the "lens") and the background object are nearly colinear with the observer, more exotic effects, such as high magnification and multiple imaging, are possible.

A number of practical uses have been found for lensing. Faint objects like brown dwarfs and white dwarfs in our galactic are too dim to detect directly, but occasionally they pass directly in front of a background star, causing it to brighten for a day or so. By monitoring a large number of stars and counting the number of microlensing events, astronomers can get an idea of how many faint compact objects there are in our galaxy's halo.

Galaxy clusters also can act as lenses. Observations of the distortion of background galaxies produced by the cluster can be used to reconstruct the details of the mass distribution in the cluster.

Some fascinating things about gravitational lensing:

It was the means by which the Theory of General Relativity proposed by Albert Einstein in 1915 was finally proved (by Arthur Eddington), in 1919. This first instance observed was very close to home, with the lensing being that which was caused by Earth's own Sun during a lunar eclipse. The proposition that entire galaxies could serve as the lenses for even more distant light was proposed in 1937, but not proved until 1979.

Notably, some scientists have proposed that gravitational lensing could be used as a means of facilitating extremely long-distance signal sending, a means of establishing interstellar or intergalactic beacons, amplifying their output by pinpointing them towards the edges of the gravitational wells of massive stellar bodies. It could in theory even allow for intergalactic communication in meaningful time by amplifying hypothetical faster-than-light signals in the same way as telegraph signals are amplified by repeaters along their paths.

And one other thing about gravitational lensing -- it's a barometer for the age of our Universe. Here's the thing, you see -- some people claim our Universe is only a few thousand years old, and consequently that either the speed of light used to be much faster, or that light appearing to come from distant stars was simply created in situ. But, like an arrow fired over the length of a field on a windy day, light is buffeted by the gravitational forces of the stars themselves. And other than the handful of stars closest to our own Solar System, all light from all stars is bent by the gravity of stars passed by. And despite the many billions of bodies involved, it is possible for examining astronomers to calculate the impact of gravity wells on every portion of the paths and passage of the light waves traveling to us from distant stars. By looking to the expected point of origin, and the bullseye which has been hit, measurements can confirm down to infinitesimal degrees that the light we see from even the farthest corners of our Universe has come to us not in a straight line or at a varying pace, but uniformly at the well-established speed of light, subtly bent many times along this steady course.

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