A new scientific idea can only become a theory when it has been tested and its predictions verified. The bigger the claim, the more evidence is usually needed to prove it. Theories that redefine the nature of the universe are a rarity, but Einstein’s general theory of relativity falls into that category. It is one of the most thoroughly analyzed and verified theories in all of science as it upended many aspects of classical physics when it was put forth in the early 20th century. One of the clearest examples of general relativity in action is the gravitational lensing effect, and it’s kind of amazing.An often cited example of observable gravitational lensing is the Einstein Cross — four points of light in the constellation Pegasus that are produced by a single quasar 8 billion light years from Earth. Quasars are extremely active galactic nuclei, but how can a single quasar show up as four distinct lights? There just happens to be a very massive galaxy, known as Huchra’s lens, between the quasar and Earth which causes a gravitational lensing effect.Einstein postulated this effect when he realized that general relativity allowed for light itself to be affected by gravity. All objects with mass cause a warping of the spacetime continuum such that space ends up curved. Light rays passing through such an area of space follow the curve and can thus appear to be originating from a completely different location.So as the light from the distant quasar is passing around the core of Huchra’s lens, it’s deflected around it as space curves. Depending on the relative position of the observer (us), the lens, and the source, the single object can look like several.If you’re imagining a gravitational lens focusing like an optical one here on Earth, think again. A gravitational lens has no single focal point where the light from distant objects syncs back up. Rather, a gravitational lens has a focal line that produces a ring of distorted light around the massive object (an Einstein Ring). A true ring would only be seen if the observer, lens, and source were in direct alignment. Any misalignment results in shorter arc segments and multiplied points of light, like with the Einstein Cross.Even though the effect was supported by the equations, Einstein believed gravitational lensing would be impossible to observe. On this count, at least, he was mistaken. The first gravitationally lensed object was discovered in 1979 at Kitt Peak National Observatory. Astronomers discovered a single quasar lensed to appear as two distinct and identical objects. Dubbed Twin QSO, this quasar served as still more confirmation of relativistic gravity in action.In the years since that first discovery, larger telescopes including Hubble and the Very Large Array have found more examples of lensing from subtle arc segments, all the way up to nearly perfect Einstein Rings (as above). Recent work on gravitational lensing is also being used to improve measurements of extremely distant galaxies.Gravitational lensing can be a confusing concept, but the physics behind it are sound. Our evolved ape brains just have trouble grasping the unbelievable distances and masses involved. The universe does some awesome stuff and we’re lucky to be seeing it happen.