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When you get enough mass together, Einstein’s theory of gravity causes space to act like a lens. Here are the results.
“The first amazing fact about gravitation is that the ratio of inertial mass to gravitational mass is constant wherever we have checked it. The second amazing thing about gravitation is how weak it is.” –Richard Feynman
In 1919, a solar eclipse proved one of Einstein’s greatest predictions: that mass curves space, and causes starlight to bend.
Positive development of the photographic plate from the solar eclipse of 1919. You can see the stars marked by vertical lines. Image credit: F. W. Dyson, A. S. Eddington, and C. Davidson, 1919.
With even more massive objects than stars — like galaxies, quasars or galaxy clusters — gravity can do more than just bend light slightly: it can act like a lens.
This image illustrates a gravitational lensing effect. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).
Just as optical lenses can focus or distort light, gravitational lenses curve space so significantly they magnify and stretch distant, background objects.
The lensing distortions from galaxy cluster Abell 2390. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).
Normally, a good alignment will distort a background galaxy into two arcs: a radial one pointing away from the foreground mass and a tangential one arcing around the mass.
Galaxy cluster Abell 2218, with many arcs characteristic of gravitational lensing. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).
Occasionally, an even better alignment will create multiple images of the same object.
The galaxy cluster Abell 68, and its many lensed and distorted background galaxies. Image credit: NASA & ESA. Acknowledgement: N. Rose.
The curvature of space forces some light paths to take longer to arrive than others, meaning we’re seeing the same background object at different times.
A quadruply-imaged supernova, thanks to gravitational lensing. Image credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI).
Most spectacularly, we’ve gotten to see a distant supernova “replay” itself due to this lensing effect.
A horseshoe-shaped Einstein ring, just short of the perfect alignment needed for a 360-degree ring. Image credit: ESA/Hubble & NASA.
In the most perfect alignment of all, a complete, 360º ring will appear due to gravitational lensing: an Einstein Ring.
The double gravitational lens system, SDSSJ0946+1006, which shows a rare near-doubly-perfect alignment. Image credit: NASA, ESA, and R. Gavazzi and T. Treu (University of California, Santa Barbara).
Although the science predicted these lenses for decades, the first one wasn’t observed until 1979′s Twin Quasar.
The Twin Quasar QSO 0957+561, as gravitationally lensed by the enormous elliptical galaxy, YGKOW G1, four billion light years away. This was the first gravitational lens ever discovered, in 1979. Image credit: ESA/Hubble & NASA.
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