Einstein's rings, also known as gravitational lensing, are a phenomenon predicted by Albert Einstein's theory of general relativity. They occur when light from a distant source, such as a quasar or a galaxy, is bent and magnified by the gravitational field of a massive object, such as a galaxy or a galaxy cluster, situated between the source and the observer. This gravitational lensing can create a circular or elliptical shape resembling a ring around the massive object.
Einstein himself did not predict the specific phenomenon of gravitational lensing, but his theory of general relativity laid the foundation for its understanding. In 1915, Einstein published his theory, which described gravity as the curvature of spacetime caused by mass and energy. One of the predictions of general relativity is that massive objects can bend the path of light, resulting in gravitational lensing.
Einstein's rings are relatively rare and incomplete for a few reasons:
Alignment: To observe a complete Einstein ring, the alignment between the source, the lensing object, and the observer needs to be extremely precise. The source, lensing object, and observer must be in near-perfect alignment for the complete ring to form. Such alignments are rare in the vastness of the universe, leading to the rarity of complete Einstein rings.
Lensing object properties: The mass distribution and shape of the lensing object affect the formation of an Einstein ring. In many cases, the lensing object's mass distribution is not spherically symmetric, leading to distortions and incomplete rings. Additionally, the presence of other nearby galaxies or objects can further complicate the lensing process and result in incomplete rings.
Observational challenges: Detecting and studying gravitational lensing can be challenging. It requires precise observations and sophisticated techniques to distinguish the lensed images from the background noise and other astrophysical effects. Many gravitational lensing events may go undetected or be difficult to observe due to various factors, including the faintness of the source, the angle of deflection, and the distance of the lensing object.
In terms of what Einstein would have expected us to see, it's difficult to say definitively as he did not specifically predict the existence of gravitational lensing. However, given his theory of general relativity, he would have recognized that massive objects can bend light and cause distortions in its path. So, in a broader sense, he would have expected that light could be influenced by gravity, potentially leading to phenomena like gravitational lensing.
Modern observations of gravitational lensing, including the detection of partial Einstein rings, align with the predictions of general relativity. The gravitational lensing phenomenon has been extensively observed and studied, providing strong support for Einstein's theory. Although complete and symmetric Einstein rings are rare, the observed lensing effects, such as multiple images and partial rings, are consistent with the expectations set by Einstein's theory.