Bending Light (Part 2)
The first prediction of Einstein’s Theory of General Relativity that was put to the test was the apparent bending of light as it passes near a massive body. This effect was conclusively observed during the solar eclipse of 1919, when our Sun was silhouetted against the Hyades star cluster, for which the positions of the stars were well known. The measurements of several of the stars in the cluster showed that when the light from these stars grazed the Sun, it was bent by the exact amount of Einstein’s predictions.
During a solar eclipse our Moon blocks the light from the Sun. This photo of the 1919 solar eclipse shows the positions of some of the stars from the Hyades star cluster (pictured as white dashes to the upper right of the Sun). Credit: F. W. Dyson et al.
This so-called gravitational lensing effect is small for light passing close to the Sun. However, if light from a distant galaxy passes through a cluster of galaxies that has the mass of hundreds of trillions of Suns, it can produce distorted, funhouse copies of images of the galaxies. Astronomers use gravitational lensing as a tool to study the distribution of matter, including the mysterious dark matter, in galaxy clusters.
Since light follows the curvature of space, a massive object can act as a gravitational lens. We see the effect of gravitational lensing in this image. The light from very distant galaxies has passed through a massive cluster of galaxies that acts as a lens and bends the light. The result is that the images from the galaxies are magnified and distorted into elongated and arched shapes. Credit: NASA/STScI
They also use it to explore the process by which gas swirls into a black hole.
Black holes are the ultimate light benders. They are the densest of dense objects – cramming the mass of millions or billions of Suns into tiny little spaces. With all of that mass, black holes pack a huge gravitational punch. In fact, their pull is so strong that they can bend light rays into closed loops, and that means the light never escapes from the black hole.
This artist’s illustration shows what the area around a black hole might look like. A black hole is a dense, compact object whose gravitational pull is so strong that - within a certain distance of it - nothing can escape, not even light. Credit: NASA/CXC/A.Hobart
The Muggles Take on Harry Potter
Meanwhile, back on Earth, scientists and engineers are playing their own tricks with light in an attempt to make an invisibility cloak. We’re not just strolling through the fictional world of Harry Potter. Several different teams of researchers are hard at work trying to determine if they can use the same techniques developed for optical fibers to control how light is bent so that we could wrap ourselves – well maybe objects smaller than ourselves, but who knows? -- in an invisibility cloak, so we could disappear.