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Here at SciShow Space, we sometimes talk about gravitational lensing when we talk about the discovery of new celestial objects. It's good and science-y sounding - "Gravitational Lensing". But what does it actually mean? And how is it helping us find stuff?

First, let's talk about what a lens is. Usually, it's a curved piece of glass or plastic that you find in a microscope, a telescope, or eyeglasses. It's curved so that the scattered light from whatever you're looking at gets focused onto a smaller area. Depending on the type of lens you're using, it can make things look bigger, or make them look clearer and sharper.

So now, let's talk about gravity. Yes, it's the force that makes objects that have mass attract each other, but let's be a little more relative-istic. 

Gravity is also the force that bends spacetime. Massive objects create distortions in space the same way a paper weight creates a distortion if you put it on a bed sheet. Your mass is creating a dimple in spacetime right now! And light which has no mass follows those distortions, if space is curved, light curves with it.

So, if a curved piece of glass can make a lens by bending the light that goes through it, why not a curved piece of space? Einstein first predicted gravitational lensing in 1912, three years before he published his work on the Theory of Relativity. It occurred to Einstein that if gravity bends space, it would also bend the light that was traveling through space.

All you'd need to observe that phenomenon is a distant object that emits light, an observer--us, and something between us and that light source that's massive enough to act as a lens.

Twenty-four years went by before Einstein bothered to publish his prediction; he didn't think this was something we'd actually ever observe, but in 1979, telescopes were finally powerful enough to prove him right.

The first effect of gravitational lensing we ever directly observed was Q0957-561, a double quasar that was actually only one quasar but had two images in the sky. Which seems crazy, but it's not!

Think of normal glass lenses again: yes, they make things look bigger and clearer, but what happens when they're out of focus? You start seeing double, and that's just because the light passing through the lens isn't converging perfectly on the other side. Instead of coming back together to create one, nicely focused image, it creates two separate images of the same thing.

Those two quasars were almost in focus; they looked like they were about six seconds of arc apart, which is about the width of a CD or DVD viewed from four kilometers away.

But, between us and the quasar was a big, elliptical galaxy that was serving as a lens, distorting the light just enough to create that double vision effect.

In 1985, another type of gravitational lensing effect was seen for the first time. This was what astronomers named an "Einstein Cross," or four images of the same object arranged perpendicular to each other around the lens. 

The lens in this case was a galaxy that was distorting the light that passed around it and became known as Huchra's Lens.

And in 1988, astronomers observed their first Einstein Ring. This is the beautiful effect you get when the light from an object is distorted into a halo, like what happens when you look at something through the base of a wine glass.

So, can gravitational lensing help us find something as small as a planet? You bet!

But, when scientists find a planet with gravitational lensing, the planet's not in orbit around the source of light, it's in orbit around the lens.

Stars make great lenses because they have lots of mass, but star systems can be even more powerful lenses since any planet orbiting the star has mass too. And the shape of a star system's gravitational field changes as its planets go around it.

So, if the star system's lensing effect briefly becomes more powerful, that means the lens has briefly more mass. And since stars don't just randomly fluctuate in mass, we can infer that what we just saw was the effect of a planet passing by its star.

So say that we're observing the a star system that's lensing the light of a star way behind it, but then suddenly we see a blip in its brightness, and for a few hours or days it looks a tiny bit brighter. That tells us that the lens briefly got stronger which means our lens star has at least one planet orbiting it.

I'm really glad you asked about this, all of you who did, because it's a good question with a complicated, but really fascinating answer, and the effects can be beautiful.

And thanks for joining me for SciShow Space. If you wanna learn how you can help us keep exploring the universe together, just go to Subbable.com/SciShow.

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