Gravitational lens

• 1. Introduction

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  Gravitational lensing is the bending of light by mass sitting between a distant source and an observer. On the 29th of May 1919, Arthur Eddington, Frank Watson Dyson, and their collaborators watched the Moon slide across the face of the Sun during a total solar eclipse. With the Sun temporarily blinded, they could photograph stars sitting close to its edge on the sky. Those stars were not where they should have been. The light coming from them had been nudged, bent by the Sun's gravity on its way to Earth. The observations were made simultaneously from Sobral in the state of Ceará in Brazil and from the island of São Tomé and Príncipe off the west coast of Africa. When the results reached the public, they made the front page of most major newspapers worldwide. What the 1919 eclipse confirmed was not just a curiosity about sunlight. It validated Albert Einstein's general theory of relativity, a framework describing gravity not as a pulling force but as the curvature of spacetime itself. That framework predicted light would bend at exactly twice the angle that Newtonian physics alone would estimate. The universe, it turned out, was full of natural lenses made of nothing but mass. How those lenses work, what they distort, and what they reveal about the cosmos is the thread running through everything that follows.

• 2. Einstein's Reluctant Paper

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  Einstein's engagement with gravitational lensing stretches across decades and carries more than a little ambivalence. He made unpublished calculations on the subject as early as 1912, but he seemed to regard the idea as a theoretical curiosity unlikely to be observed. His reasoning was practical: for a star's gravity to produce multiple images of a background light source, the alignment between source, lens, and observer would need to be almost impossibly precise. For the foreseeable future, he concluded, such alignments among ordinary stars were highly improbable. He was not alone in this pessimism. Several other physicists explored the same question and reached the same discouraging verdict. The first person to discuss the effect in print was the St. Petersburg physicist Orest Khvolson, in 1924, who described the "halo effect" that arises when source, lens, and observer fall into near-perfect alignment. Frantisek Link followed with a related discussion in 1936. That same year, the amateur scientist Rudi W. Mandl urged Einstein to publish his own treatment, and Einstein eventually complied, somewhat reluctantly. The resulting paper, "Lens-Like Action of a Star By the Deviation of Light In the Gravitational Field," appeared in the journal Science in 1936. The phenomenon of a perfect circular halo formed by this alignment is now called an Einstein ring, a name that reflects how closely the concept became attached to Einstein rather than Khvolson, partly because Khvolson had not attempted to calculate the ring's flux or radius. When asked what his reaction would have been if Eddington and Dyson had not confirmed general relativity in 1919, Einstein replied: "Then I would feel sorry for the dear Lord. The theory is correct anyway." One year after his 1936 paper, Fritz Zwicky took the concept in a new direction, arguing that the newly discovered structures then called nebulae, now known as galaxies, could act as lenses far more powerful and far more detectable than any single star.

• 3. Confirmation and the Twin QSO

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  Fritz Zwicky's 1937 prediction that galaxy clusters could serve as gravitational lenses sat unconfirmed for more than four decades. The breakthrough came in 1979, when astronomers Dennis Walsh, Bob Carswell, and Ray Weymann, working with the 2.1 meter telescope at Kitt Peak National Observatory, found an object that initially appeared to be two identical quasistellar objects sitting unusually close together in the sky. The pair came to be called the "Twin QSO" and is officially catalogued as SBS 0957+561. What Walsh, Carswell, and Weymann were actually seeing was a single quasar, its light split and redirected by a massive galaxy sitting between it and Earth. The two images were arriving along different paths bent around the same lensing mass. This discovery confirmed what Zwicky had claimed: that galaxies and galaxy clusters are massive enough to produce detectable lensing effects. The significance had also been sharpened in 1963, when Yu. G. Klimov, S. Liebes, and Sjur Refsdal each independently recognized that quasars, being extremely bright and very distant, were ideal background sources for observing gravitational lensing. Quasars provided the extreme brightness needed to survive the dimming and distortion of multiple images. The Twin QSO turned a decades-old theoretical prediction into an observational programme. By the 1980s, astronomers recognized that new CCD camera technology combined with computers could allow millions of stars to be monitored each night. Dense stellar fields such as the galactic center and the Magellanic clouds offered particularly rich ground. Projects like the Optical Gravitational Lensing Experiment, known as OGLE, emerged from that realization and went on to characterize hundreds of microlensing events, including OGLE-2016-BLG-1190Lb and OGLE-2016-BLG-1195Lb.

• 4. Three Flavors of Lensing

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  Gravitational lensing does not come in a single form. The source material identifies three distinct classes, each revealing a different slice of the universe. Strong lensing produces effects visible to the eye: Einstein rings, arcs curving around a foreground mass, and multiple separate images of the same background object. Despite carrying the word "strong," the angular separations involved are actually quite small. Even a galaxy more than 100 billion times the mass of the Sun will produce multiple images separated by only a few arcseconds. Galaxy clusters can push those separations to several arcminutes, but even then the source galaxies are many hundreds of megaparsecs away from the Milky Way. Weak lensing operates far more subtly. No individual image is obviously distorted. Instead, the shapes of large numbers of background galaxies are measured statistically. Galaxies that happen to sit behind a foreground mass appear, on average, slightly stretched perpendicular to the direction toward that mass. By analyzing those orientations across hundreds of thousands or millions of galaxies, astronomers can reconstruct how mass is distributed in a region of the sky. This includes dark matter, which emits no light of its own but still bends the light passing near it. The technique demands careful control over systematic errors: the natural elliptical shapes of galaxies, the distorting effect of a telescope's optics, and the blurring caused by atmospheric turbulence all need to be accounted for precisely. The most widely used mathematical framework for doing so, called KSB+, was developed by Kaiser, Squires, and Broadhurst in 1995, with further contributions from Luppino and Kaiser in 1997 and Hoekstra and colleagues in 1998. Microlensing is the third class. Here no distortion in shape is detectable at all. What changes is brightness. When a foreground object passes in front of a background star or quasar, the background source temporarily brightens and then fades as the alignment shifts. A star somewhere in a distant galaxy can act as a microlens and momentarily magnify another star even farther away. The first observed example of this extreme form was the star MACS J1149 Lensed Star 1, also known as Icarus. A statistical analysis of microlensing events observed between 2002 and 2007 found that most stars in the Milky Way host at least one orbiting planet within 0.5 to 10 AU.

• 5. The Sun as a Telescope

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  In 1936, Einstein calculated that rays of light grazing the Sun's edge from the same direction would converge to a focal point approximately 542 AU from the Sun. That number is important: it lies far beyond the orbit of any known planet or dwarf planet, and far beyond where any human spacecraft has reached. Voyager 1 has not come close to that distance. The space probe 90377 Sedna, on its highly elliptical orbit, will not reach it for thousands of years. Yet if a probe could be placed at 542 AU or beyond, it could use the Sun itself as a gravitational lens, magnifying objects on the far side at extraordinary scales. The high gain achievable this way prompted Frank Drake, in the early days of SETI research, to suggest that a probe sent to 542 AU could exploit signals such as microwaves at the 21-centimeter hydrogen line. A multipurpose mission concept called SETISAIL, and later a successor called FOCAL, was proposed to the European Space Agency in 1993. The concept faces serious practical difficulties. Landis identified several, including interference from the Sun's own corona, the extreme magnification that would make designing the mission's focal plane very difficult, and the inherent spherical aberration of the lens. In 2020, NASA physicist Slava Turyshev proposed a refinement called Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravitational Lens Mission. Turyshev's concept suggested the lens could reconstruct an exoplanet's surface at roughly 25-kilometer resolution, fine enough to detect surface features and potential signs of habitability.

• 6. The Next Generation of Searches

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  Most gravitational lenses known before the modern survey era were found by chance. Systematic searches have changed that. A radio-frequency programme called the Cosmic Lens All Sky Survey, or CLASS, used the Very Large Array in New Mexico to scan the northern sky and discovered 22 new lensing systems, a number the source describes as a major milestone. A complementary southern-hemisphere programme drew on the Australia Telescope 20 GHz Survey, collected with the Australia Telescope Compact Array. That survey worked at a deliberately high frequency of 20 gigahertz, partly because compact core objects such as quasars are more numerous at higher radio frequencies, making lensing events easier to detect and identify. The coming years promise a far larger harvest. The European Space Agency launched the Euclid Space Telescope in 2023. In the first 0.45 percent of its planned survey area, Euclid found around 500 strong-lens candidates. Over its six-year mission, it is expected to find around 100,000. The Vera C. Rubin Observatory, operating its Legacy Survey of Space and Time from Cerro Pachón in Chile, began collecting data in 2025 and expects to discover between 62,000 and 120,000 galaxy-scale lenses across its 10-year programme. The Nancy Grace Roman Space Telescope, scheduled for launch in 2026 or 2027, is projected to add around 160,000 more strong gravitational lenses through its High Latitude Wide Area Survey. Among individual recent discoveries, a galaxy named JWST-ER1, spotted in 2023 in James Webb Space Telescope NIRCam observations from the COSMOS-Web survey, sits approximately 17 billion light-years away and carries a complete Einstein ring. The lensing galaxy, JWST-ER1g, sits at a photometric redshift of roughly 1.94 and bends light from a background galaxy, JWST-ER1r, at a redshift of roughly 2.98. Its unusually high density carries implications for understanding how dark matter halos form and evolve.