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Gravitational fields bend light. Albert Einstein realized this in 1912 as he labored on his theory of general relativity. A century later, astronomers routinely take advantage of this effect to better understand the universe. Because lenses curve light paths in a similar fashion, scientists refer to this phenomenon as gravitational lensing.

This interactive is based on a 2013 discovery by NASA's Fermi Gamma-ray Space Telescope. In the main pane, Fermi is at right, the lensing spiral galaxy is in the middle, and an active galaxy called a blazar is at left.

Use the slider controls to adjust both the mass of the spiral galaxy and its alignment with the blazar. Click the Flare button, watch what happens, and check out the results in the box at upper right.

When astronomers look at this galaxy with optical and radio telescopes, they see two images of the blazar. That's because the blazar's light takes different paths through the galaxy's gravitational field. Blazars often undergo outbursts that greatly increase their brightness, but its lensed images may not brighten at the same time. Light taking the shortest path through the lens reaches us first, followed later by light from the other image.

Astronomers using radio and optical telescopes can monitor how the individual images change. Fermi sees the highest-energy form of light – gamma rays – but its view of the sky is not as sharp and it cannot separate the images. Because Fermi continuously monitors the whole sky, however, it can detect the time delay between a flare in one blazar image and its playback in the other.

In the real-life example, Fermi detected a time delay of 11.46 days. For the same lens system, radio astronomers detect delays of 10.5 days. Can you replicate these results by adjusting the sliders?

Now, more than a century after Einstein first recognized how gravity could influence light, astronomers use lensing to explore the universe on a variety of scales: mapping the distribution of mysterious dark matter, studying galaxies too faint and far away to explore any other way, and searching for new planetary systems.