A major prediction of Einstein’s General Theory of Relativity is that light is deflected by gravity twice as much as predicted by Newton. This was confirmed in 1920 when two teams observed the positions of stars deflected by the gravity of the sun during an eclipse (Dyson, Eddington, and Davidson 1920).1 This deflection, or lensing, of light by gravity results in a magnification of the source object, has become an important tool for astronomers. It allows them to see more distant objects than they would otherwise, and also learn about the properties of the massive object doing the lensing.
Gravitational lensing can be divided into three regimes. Weak lensing results in a small change of shape (or shear) to objects fairly far (in the plane of the sky) from the lens. This shear is usually indistiguishable for individual objects, but can be detected statistically across the population. Weak lensing is essential for measuring the mass of the lens – usually the dark matter halo of a massive galaxy cluster. Microlensing results in a brief (usually somewhere between a few hours to a year) increase in brightness of the source object. It is caused by the lens moving across the face of the source. Microlensing is used to find exoplanets and is also one of the best astrophysical methods to constrain the mass of the dark matter particle. Strong lensing is the most visually dramatic form and results in large magnifications and substantial changes in shape of objects fairly close to the lens. These changes provide information both about the lens (its mass and mass distribution) and the source which can be seen more clearly due to the magnification. The most distant star and many of the most distant galaxies are only detectable because they have been magnified by a gravitational lens.
Below is an simulation of a lensing system. The background image is Abell 665, a galaxy cluster (chosen so that there are many interesting objects you can try lens). We have placed it 2200 Megaparsecs away – equivalent to a redshift of ~0.4 – a distance near the median of many modern galaxy surveys in which we might see strong lensing (DES, HSC). Play with the properties of the lens and see how the image changes; drag the lens to change its position, use the sliders to change its mass and distance from the observer, and switch the distribution of the mass from a point to the profile we expect massive dark matter halos (the primary cause of strong lensing) to have.
A nice summary of the math involved can be found in section 2.1 of (Narayan and Bartelmann 1996). The basics are very simple – basically the same as for an optical lens.
Coles, Peter. 2001. “Einstein, Eddington and the 1919 Eclipse.” In Historical Development of Modern Cosmology, edited by Vicent J. Martı́nez, Virginia Trimble, and Marı́a Jesús Pons-Borderı́a, 252:21. Astronomical Society of the Pacific Conference Series. http://arxiv.org/abs/astro-ph/0102462.
Dyson, F. W., A. S. Eddington, and C. Davidson. 1920. “A Determination of the Deflection of Light by the Sun’s Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919.” Philosophical Transactions of the Royal Society of London Series A 220 (January): 291–333. https://doi.org/10.1098/rsta.1920.0009.
Einstein, A. 1911. “Über Den Einfluß Der Schwerkraft Auf Die Ausbreitung Des Lichtes.” Annalen Der Physik 340 (10): 898–908. https://doi.org/10.1002/andp.19113401005.
Narayan, R., and M. Bartelmann. 1996. “Lectures on Gravitational Lensing.” arXiv Astrophysics E-Prints, June. http://folk.uio.no/hdahle/JeruLect.pdf.
There’s actually quite a bit more to this story. Einstein made a mistake in his prediction of the deviation of light by the sun’s gravity in a 1911 paper (Einstein 1911) that he corrected after fully nailing down GR in 1915. Between these papers, two teams set out to measure the deflection but failed: A 1912 expedition to Brazil (which included Eddington, the primary observer in 1919) was thwarted by bad weather and a planned German expedition to Ukraine in August 1914 was made impossible by the outbreak of the First World War. The war also made communication between Eddington (at Cambridge) and Einstein (in Berlin) difficult. However, Eddington was forwarded a copy of the GR paper by William De Sitter (in neutral Holland). Dyson realized that the 1919 eclipse would be a perfect test of GR as the sun would be in front of a star cluster – Hyades – at the time and so many stars would be deflected. Two observing teams were planned, to Principe in West Africa and Sobral in Brazil. Eddington was the ideal candidate to lead one of these expeditions but was eligible to be conscripted into the British army. Eddington refused on the grounds that he was a Quaker and pacifist and his appeals (and those of others on his behalf) were accepted. So, Eddington was spared a trip to Passchendaele, the war ended just in time to prepare for the 1919 eclipse, the weather was (just) good enough to make the observations, Eddington rushed home just avoiding a steamship strike, and Einstein’s predictions were confirmed. See (Coles 2001) for more history of the 1919 observations.↩︎
