Astronomy

Gravitational Lensing Is A Magnifying Glass Made By Gravity

We might be surrounded by cutting-edge technology, but nature still has some tricks up her sleeve. Here's one: while engineers painstakingly grind glass into the perfect shape for a telescope, the universe is throwing us a bone by doing its own magnification. Gravitational lensing is a natural phenomenon that magnifies objects through the sheer power of gravity — and it's helping astronomers make some very big discoveries.

Gravitational lensing caused by a black hole.

Image: Urbane Legend / Wikipedia

Gravitational lensing caused by a black hole.

No Need To Adjust Your Screen

Albert Einstein's theory of general relativity says that matter curves the fabric of spacetime. As a result, any light passing through that curve will curve, too. Imagine a bowling ball sitting at the center of a trampoline, with the bowling ball acting as a massive object (like a galaxy) and the trampoline fabric acting as spacetime. If you put a golf ball in the role of light and roll that golf ball toward the bowling ball, it won't take a straight path. Instead, it'll follow the curve in the trampoline and become "bent" by the presence of the bowling ball. That's what light does around massive objects: the more massive the object, the deeper the sag in the trampoline, and the more the light will bend around it.

It just so happens that light rays do a very similar thing in a magnifying glass or a telescope: they bend with the curve in the lens, which refocuses them in a way that creates a clear picture. The bend in spacetime from a massive object, then, can act as a magnifying glass for objects behind it—and if those objects are too faint and far away to see normally, that can be a big benefit to astronomers studying them.

A Horseshoe Einstein Ring from Hubble.

Real Spacetime Has Curves

Gravitational lensing doesn't alway create such a clear picture, however. We can go back to our trampoline example and imagine rolling several golf balls toward the bowling ball. Chances are, they won't all take the same path: some might go around to the left, others to the right. It's the same way with light. Gravitational lensing can create double images, arcs, rings—all sorts of distorted images. That doesn't matter too much to astronomers, however. They can use the object's unique light signature, or spectrum, to know what they're looking at. But even more important is the fact that the lensing itself gives them a wealth of information already.

Because more massive objects create a bigger bend in light, astronomers can use the degree of lensing to measure the mass of the foreground object. Somewhere along the way, though, astronomers began to realize that some of the numbers weren't adding up: many galaxy clusters were bending light way too drastically for their actual mass. That discovery led to the idea of "dark matter," the yet-unidentified substance that doesn't emit or reflect light, but packs enough of a gravitational punch for us to know that it makes up 80 percent of the mass of the universe. To this day, gravitational lensing is the only evidence dark matter provides of its existence.

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Written By Ashley Hamer July 8, 2017