Let me tell you a story about the time when the general theory of relativity is published …
In General Theory of Relativity, published in 1915, Albert Einstein proposed that gravity bends the path of light.
And in 1936, at the urging of an amateur scientist, he wrote a brief paper about an optical illusion due to this bending (path of light) that means…multiple images of one astronomical source located behind another. With near-perfect alignment, a full “ring” (like structure) should appear around the intermediate, lensing object.
“Of course, there is no hope of observing this phenomenon directly,” Einstein wrote. In his cover letter, he thanked the editor of Science for his “cooperation with the little publication, which Mister Mandl squeezed out of me. It is of little value, but it makes the poor guy happy.”
Today, the elegant phenomenon of “strong” gravitational lensing—the case when multiple images can occur—makes many astronomers happy. Even though such gravitational lenses are uncommon, many have been found and studied, with Hubble playing an important role.
So let’s dive into this amazing concept of Einstein Ring…
One of the main examples of the gravitational lens effect is the Einstein ring phenomenon illustrated at right. According to general relativity, gravity causes a deflection of light by the gravitational field of a massive body. In this case, a galaxy bends the light emanating from a galaxy that is directly behind it, focusing the otherwise divergent light into a visible ring.
The angular size of the Einstein ring is determined by the amount of mass—both stars and dark matter—enclosed within it.Under their stronger gravity, more massive foreground galaxies produce larger Einstein-ring images of galaxies in the background. Measuring the mass of galaxies is a difficult, but fundamental, task of astronomy. Lensing, when it occurs, is perhaps the most direct method.
The more-distant galaxy is the “source,” and the intermediate galaxy is the “lens.” If the source, lens, and observer are closely aligned, the gravity of the lens will bend some light rays from the source onto new paths towards the observer.
To the observer, these bent rays appear as if they originate at points on the sky displaced from the location of the lens. Indeed, rays from multiple paths around the lens and at different clock angles may arrive at the observer, creating multiple images of the source.
In the case of a near-perfect alignment, light rays can be bent towards the observer all around the lens, creating the appearance of a full Einstein ring,
Now you think how scientists will find out capturing these types of galaxies?
Any two objects that are found close together in the sky, but are located at vastly different distances, are prime candidates for gravitational lensing.
By sifting through all the Sloan spectra of large, luminous galaxies, astronomers have discovered hundreds of new, bright candidates, which are usually massive elliptical galaxies in front of more distant, faint, star-forming galaxies.
Another question why we observe this ?
An Einstein ring does not unambiguously tell how mass is arranged within the lensing galaxy. Nevertheless, the arrangement of mass is important for understanding the physical structure of the galaxy and its evolutionary history.
We can solve this problem by measuring the distribution, or “spread,” of stellar velocities within the lensing galaxy: the faster the stars move, the more the mass must be concentrated towards the center of the galaxy, for gravity to balance centrifugal force.
In the future, Scientists expect that spectra from large ground-based telescopes will provide improved measurements of the velocity distributions in individual lensing galaxies, which will improve the accuracy of the measured mass distributions. This will lead to a better understanding of the relationships between the mass distribution and other properties of galaxies, such as the luminosity and the history of star formation.
Sometimes an illusion is much more than it appears, as with Einstein’s rings.