The world of general relativity is a full of valleys, undulations and holes. Light, as any type of matter, gets affected by the curvature of spacetime and bends around objects with a large gravitational field. We see this effect when we look at the sky, some distant galaxies become distorted and unrecognisable. This also acts like a cosmic magnifying glass, allowing us to see galaxies too far away to be observed directly with current technology. Let’s explore some examples of gravitational lensing, its applications and Einstein rings.
But wait, aren’t photons supposed to be massless? From Newton’s law of gravitation, we multiply two masses, so photons should not be affected by a gravitational force. Nonetheless, we must remember Einstein’s general relativity, which states that spacetime is curved according to the mass and energy that exert stress on it. In that picture, photons do get bent when they travel along the grooves of space. This was tested during a solar eclipse in 1919, when the light from the star Regulus was determined to be bent. It was bent so significantly that the explanation had to be from general relativity and not simply photons having mass.
An interesting phenomenon occurs when light is bent symmetrically around a center of mass and the observer is aligned with the source. The light bends in a circular shape known as an Einstein ring. The properties of an Einstein ring can be described mathematically. The size of the ring in radians is given by the equation
This is what you can see in images of black holes and when blue halos can be observed around galaxies and stars.
More interestingly, a double ring has been found, although these are exceptionally rare at 1 in 10000. These special rings provide tremendous insight into dark matter and energy distribution, as well as the curvature of space.
Analysing gravitational lensing also provides insight into the distribution of matter in galaxies. Sometimes, lensing occurs in region where there is little visible stars of gas clouds, which implies that there is dark matter present. Images of lensing taken by the Hubble Space Telescope allowed scientists to create maps of dark matter distribution in galaxy clusters.
I personally love gravitational lensing due to the beautiful halos of light it produces around objects. Check out this gallery of Einstein rings from the Hubble website. The blue auroras and the warm yellow starlight combined with the grainy quality make me feel strangely cozy. And spooked.
References
Gravitational Lensing. (2019, May 30). HubbleSite.Org. https://hubblesite.org/contents/articles/gravitational-lensing
Nave, R. (n.d.). Einstein Ring. HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/einring.html
Siegel, E. (2018, December 1). Ask Ethan: How Do Massless Particles Experience Gravity? Forbes. https://www.forbes.com/sites/startswithabang/2018/12/01/ask-ethan-how-do-massless-particles-experience-gravity/
Sutter, P. (2018, March 16). Nature’s Lens: How Gravity Can Bend Light Like a Telescope. Space.Com. https://www.space.com/39999-how-gravitational-lenses-work.html