Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 July 8 
Posted on 07/07/2002 10:26:43 PM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 July 8
Explanation: Sometimes staring into empty space is useful. Pictured above is a region of sky that was picked because it had, well, nothing: no bright stars, no bright galaxies, and no picturesque nebulas. What could not be avoided, however, were a few stars in our own Galaxy, and many distant galaxies strewn across the universe. Now the more distant galaxies have their light slightly deflected by the gravity of more nearby galaxies, causing them to appear slightly distorted. By analyzing these gravitational lens distortions, nearby mass concentrations can be found, regardless of how bright they appear. Using this method, astronomers can now weigh entire clusters of galaxies and search for large groupings of relatively dark matter. Circled in the lower right of the above image is a cluster of galaxies that was found not by its light, but by its mass.
Yes indeed! The force of gravity does bend light. It's called gravitational lensing.
Here's a dramatic example, courtesy of the Hubble Space Telescope:
See the strange curved streaks and arcs in the photo?
Those are the distorted images of galaxies on the FAR SIDE of the visible cluster.
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Anyone who has had trouble with their own eyesight can recognize the blurring affect of inexact vision. While this photo may provide evidence of gravity bending light, I caution against accepting the photo (in reply 1) as proof.
The massive body (or group of bodies) acts like a lens, with an "index of refraction" that reflects the effect of spacetime curvature on the light path.
In English: Yes, it's space itself which is distorted; thus the light path is bent. As for the bending process, which is "refraction", try this. Place a pencil in a glass of water. The water's surface seems to split the pencil into adjacent halves.
This picture points out multiple images occurring in the cluster Cl 0024+1654.
Did you know that even before HST's optics were fixed (dumb oversight not to test, I know), it could image various objects as well as many Earth-based scopes? The Earth's atmospheric turbulence limits our ground-based imaging efforts.
A buddy of mine helped assure Hubble's success in nearly every other way. (I helped make GRO the success it was. You want not to hear my opinion of NASA choosing not to reboost its altitude. They spent almost a billion extra of our tax dollars just to make it fit into a shuttle -- a decision that inadvertently caused the delay of its launch due to Challenger -- and then abandoned it.)
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