Black Holes & Spacetime
Gravitational Lensing
Gravitational lensing happens when the gravity of a massive object bends the light passing near it, distorting or magnifying what lies behind.
It is a direct consequence of general relativity, first tested during a solar eclipse in 1919.
- Observe
Stretched arcs, rings or duplicate images of a distant source.
- Measure
Positions, shapes and brightness of the distorted images.
- Model
The mass distribution needed to bend light this way.
- Explain
Gravity acts as a natural telescope and a scale for mass.
What is it?
Massive objects curve spacetime, and light follows that curvature. A galaxy or cluster of galaxies between us and a more distant source can bend and focus its light, creating arcs, rings or multiple images.
Astronomers describe three regimes. Strong lensing makes obvious arcs and multiple images. Weak lensing slightly distorts many galaxies and is measured statistically. Microlensing is a temporary brightening as a compact object passes in front of a star.
How does it happen?
- Stage 1Distant source emits light
- Stage 2Massive object lies between source and observer
- Stage 3Its gravity bends the light paths
- Stage 4Observer sees arcs, rings or brightening
What do we actually observe?
- Arcs and rings
- Galaxy clusters can stretch background galaxies into arcs, and a nearly perfect alignment can produce an Einstein ring.
- Multiple images
- The same quasar or supernova can appear several times, with slight delays between images.
- Microlensing brightening
- A star briefly brightens and fades smoothly as a foreground object passes in front of it.
How do scientists detect it?
- Hubble and JWST. Deep images of massive clusters reveal lensed galaxies.
- Microlensing surveys. OGLE, MOA and KMTNet monitor millions of stars for microlensing.
In 1919 Arthur Eddington and colleagues measured starlight bending near the Sun during a total solar eclipse, an early confirmation of general relativity.
Why does it matter?
- It maps dark matter, since lensing depends on total mass, seen or unseen.
- It magnifies very distant galaxies, letting JWST see fainter, earlier ones.
- Microlensing detects rogue planets that emit no light.
The SpaceWeatherTracker connection
The 1919 measurement used a solar eclipse, one of the events SpaceWeatherTracker tracks. The same physics later grew into a tool for weighing galaxies.
Related phenomena
Primary family: Black Holes & Spacetime
Gravitational Lensing: common questions
What is gravitational lensing?
The bending of light by gravity. A massive object in the foreground can distort, magnify or duplicate the image of a background source.
Why is lensing useful for finding dark matter?
Lensing depends on total mass, so how strongly light is bent reveals mass that does not shine, including dark matter.