1. Presented by: Name, Affiliation
Einstein’s Lens
Presented by: Name, Affiliation
In this presentation, we will:
• Look at how Einstein’s theory of gravity explains gravitational lensing
• Look at the observations that proved gravitational lensing to be a real phenomenon
See what happens when you have different shaped “lenses”
• See how astronomers use gravitational lensing as a scientific tool
Accompanying demonstrations:
Gravitational lens lenses (using wine glasses)
Spandex, bucket, weights, and marbles (curvature of spacetime)
Magnetic marbles (deflection of light around a massive object)
(These demonstrations are available in the “Journey to a Black Hole” demonstration at Click on Resources > Educational Resources > Presentations for Informal Educators.)
This is the Title Slide. You can customize the text by double clicking on it.
Location and Date here

2. Einstein’s big idea
Mass bends space. Space tells light where to go.
Light always travels the shortest path through space. In empty space, that path is a straight line. But mass distorts space, so light follows a shortest path which, to an outside observer, looks curved.
Gravity as the curvature of spacetime (or even just space!) is a very strange concept, and this two dimensional “rubber sheet” illustration is one of the most popular ways to introduce the idea. It is of course a two-dimensional model of three dimensional space (Four-dimensional spacetime) and the presenter may want to spend a little time emphasizing this. Spacetime around an actual planet or black hole is stretched radially inwards in all directions.
Mass bends space. Light follows the shortest path through space.

3. Einstein’s big idea So a star’s position in the sky… Star here…
Mass bends space. Space tells light where to go. We only know where an object is by seeing where its light comes from. Hence the curving of space is an optical illusion on a grand scale. Almost like a mirage, a star appears to be in a place it is not.
A glass lens does sort of the same thing and is an optical illusion we are all familiar with. This presentation uses the glass lens model later on (slide 13).
So a star’s position in the sky…

4. Einstein’s big idea …appears shifted because of the bending.
Appears to be here
By comparing where a star appears to be, to where we know it should be, we can calculate the amount of bending the light path has endured. A calculation of this bending angle yields the mass of the lensing object.
…appears shifted because of the bending.

5. Testing the prediction
Stars here…
To verify this prediction, astronomers needed a big source of gravity between them and a starry background. The biggest source of gravity around was, of course, the Sun. The apparent positions of stars in the sky will shift due to the Sun’s gravitational warping of space. Einstein’s calculations predicted exactly how much of a shift would be seen.
Now, ask your audience if they see a problem with this experiment. And can they think of a solution? (Answer given on next slide)
The positions of stars in the night sky are known to high accuracy…
…but the Sun’s gravity will warp the space that the starlight travels through

6. Testing the prediction
Appear to be here
That’s right! You can’t see the stars when the Sun is shining! You need the Sun’s gravity, but not its light. Solution? An eclipse.
In the conditions were perfect to test Einstein’s prediction of gravitational lensing. A total eclipse of the Sun would occur, while the Sun was in the region of the sky containing the Hyades star cluster - lots of bright stars whose positions in the sky are very accurately known.
So when their light passes close by the Sun their positions will appear to change
…if you could see them during the day!

7. As predicted…
An expedition to observe the 1919 eclipse was lead by astronomer Arthur Stanley Eddington. The observation site was in Sobral, Brazil. The eclipse was photographed, and the positions of the background stars recorded.
Credit: National Maritime Museum, Greenwich
On May 29, 1919, the Sun passed in front of the bright Hyades star cluster
…and the Moon passed in front of the Sun.

8. As predicted…
The star’s positions were then compared to their positions in the sky without the Sun’s presence. This slide shows the apparent shift in positions of the stars around the Sun’s disc. Notice that the stellar positions are measured (along with the graph axes) in degrees, whereas the SHIFT in the star’s position, shown by the length of the red arrows, is measured in seconds of arc (1/3600 of a degree). A minute shift, but a measurable one.
The amount that the starlight was deflected matched perfectly the prediction made by Einstein’s theory of gravity. Eddington’s result ushered in the age of Einstein, and it was the result that made Einstein a superstar.
Sky map showing the amount and direction of shift of star positions.

9. Lensing goes cosmic
In some ways, the eclipse event of 1919 is where the story of gravitational lensing lay closed, until the 1970s. The next series of slides brings us into a new chapter of gravitational lensing history.
(About this image) The cloverleaf nucleus of this spiral galaxy is an illusion. The”ordinary” spiral galaxy is simply the lens - the four blobs are four images of the same quasar located many times further from us than the lensing galaxy. To create such a mirage requires a perfect, and fortuitous, alignment.
J.Rhoads, S.Malhotra, I.Dell'Antonio (NOAO)/WIYN/NOAO/NSF
Advances in telescope technology have revealed a universe of illusion!

10. Identical twins?
In some ways, the eclipse event of 1919 is where the story of gravitational lensing lay closed, until the 1970s. By that time, a big controversy had been raging for about a decade regarding quasars - star-like objects that appeared (through their spectra) to be racing away from us at incredible velocities. If their speeds were due to the expansion of the universe, then quasars were the brightest objects in the universe. Were they distant and bright, or something much more cosmologically local?
The final proof of the quasar’s great distance came from gravitational lensing. Two quasars were discovered, that were almost identical in every respect, but separated in the sky by a mere 6 seconds of arc (the width of your hand seen from a mile away). When the light from one quasar was subtracted from the light of the other, there was a small smudge of light left over. It was light from a previously unnoticed galaxy, situated half way between us and the quasars. Its gravity was acting as a lens to give us two images of the same object!
Credit: STScI (George Rhee)
The discovery of identical quasars in the 1970s took gravitational lensing to cosmological scales

11. Lensing on a cosmic scale
Here’s what was happening to the light of Quasar Exactly the same effect as Eddington noticed for starlight around the Sun. But now the lens is a galaxy of a hundred billion stars, splitting the light of a quasar billions of light years away.
Credit: NASA/CXC/M.Weiss

12. Breaking news
The discovery of lensed quasars not only showed the power of Einstein’s idea, but was the proof that the enigmatic quasars were at vast cosmological distances.
The cover of Science from August 31, 1979 shows a radio image of the double quasar Because the lensing galaxy (too faint to be seen in the radio image) is slightly off-center, the quasar’s radio jet only appears from the upper image (an extended and slightly bent feature emerging at 10 o’clock from the quasar itself).
Note 1: In this radio image from the Very Large Array telescope in New Mexico, the colors represent, from blue to red, intensity of the radio waves.
Note 2: The object in the red square on the magazine cover indicates the dimensions of a point source of light as far as the VLA is concerned. Even though the quasars appear to be blobs with a measurable width, they are actually pinpoints of light.
Historical note: Whereas quasar is now confirmed as a gravitational lens, at the time of its discovery there were conflicting theories to explain what was seen. The article in this edition of Science argued that there were two physical and very similar quasars, and not two images of the same quasar.
Quasar
from the VLA radio telescope

13. What shape is your lens? Spherical lens gives an Einstein ring
This slide can be accompanied by the gravitation lens (wine glass) demonstration.
The distortion of space depends on how the mass is distributed. For stars and some elliptical galaxies, the mass distribution is spherical. This simple geometry creates the simplest images. A perfect alignment gives a ring of light around the lensing object. Slightly off center, and the ring breaks into arcs of light, then points of light.
Graphic credit: European Space Agency
Image Credit: STScI, Imperial College (Steve Warren, Simon Dye)
Spherical lens gives an Einstein ring

14. What shape is your lens?
An elongated lens, such as the majority of elliptical galaxies and the nucleus of spiral galaxies, creates multiple images of a single source. This image of the Einstein Cross shows in close-up the nucleus of the galaxy from slide 9. The lensing galaxy is 400 million light years distant, whereas the galaxy being lensed is 20 times farther away, at a distance of 8 billion light years.
Graphic credit: European Space Agency
Image Credit: NASA and ESA
Elongated lens gives Multiple images - Einstein Cross

15. What shape is your lens? Multiple lenses gives arcs and arclets
On the grandest scale of all, galaxy clusters, not just individual galaxies, can act as gravitational lenses. Galaxies way beyond the cluster are seen as thin arclets of light. Not all the arclets are different galaxies. Many are multiple images of the same galaxy.
This image shows galaxy cluster Abell 1689, one of the most massive objects in the universe. Not only is the cluster packed with galaxies, it is held together by a huge reservoir of invisible dark matter.
Audience participation: This Abell image is a swirl of galaxies and arclets. Get the audience to try and find the gravitational arcs (maybe kids could come up to the screen or use a laser pointer). Can they count how many arcs are in the picture?
Graphic credit: European Space Agency
Image Credit: N. Benitez (JHU), T. Broadhurst (Hebrew Univ.), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick), ACS Science Team, ESA, NASA
Multiple lenses gives arcs and arclets

16. Nice pictures, but what can lensing do for us?
The next series of slides presents some ways in which gravitational lenses can be used as tools for exploring our universe.
(About this image) Like a photographer clicking random snapshots of a crowd of people, NASA's Hubble Space Telescope has taken a view of an eclectic mix of galaxies. In taking this picture, Hubble's Advanced Camera for Surveys was not looking at any particular target. The camera was taking a picture of a typical patch of sky, while Hubble's infrared camera was viewing a target in an adjacent galaxy-rich region.
Image Credit: NASA, ESA, J. Blakeslee and H. Ford (Johns Hopkins University)
Gravitational lenses have become one of the
most important tools in modern astronomy

17. 1. The biggest magnifying glass in the universe!
A galaxy cluster is the biggest concentration of mass in the universe, and so makes for the strongest gravitational lens. Here the gravity from cluster Abell 2218 magnifies and distorts the images of more distant galaxies into thin arcs. What appears to be two red galaxies are in fact two images of the same galaxy. This galaxy, whose red color is due to its enormous red shift, is perhaps the most distant object ever seen. And because looking out in space is looking back in time, the galaxy is also the youngest ever seen.
Fascinating fact: Gravitational lensing of distant galaxies effectively makes your telescope 100 times as powerful! Hubble’s 2.5-meter mirror acts like a 25-meter mirror for these distant objects.
Credit: ESA, NASA, J.-P. Kneib (Caltech/Observatoire Midi-Pyrénées) and R. Ellis (Caltech)
Seeing the most distant - and youngest - galaxies in the universe

18. 2. A new way to measure distance
Attack of the Clones! The Cloverleaf quasar, H , appears as four distinct images in this Chandra X-ray picture. The light that forms each image has to follow a slightly different path around/through the lensing object. This means that there is a time delay in light arriving from one image compared to another. How? Quasars vary in brightness over periods of hours or days. The image whose light path is shortest will brighten first, followed by the second and so on. If the geometry of the lens is known, the time delay gives an accurate distance to the lensing object. This gives a completely independent method of finding distance in the universe.
Going further: determining the expansion rate, and therefore the age, of the universe, requires measurements of recession velocity (from redshift) and an accurate distance. Lensing galaxies are generally too far away for their distance to be measured accurately in any other way, making this method highly important in determining the expansion history of the universe.
Credit: NASA/CXC/Penn State/G.Chartas et al
Different images take different paths, and have different travel times.

19. 3. Black hole hunting For animation source, see link in notes.
Black holes are found by virtue of their influence on luminous objects near by, such as hot accretion discs or stellar companions. An isolated black hole is in effect invisible except for the lensing effect of its gravity. A chance alignment of a black hole between us and a star (or galaxy) will manifest itself in a temporary brightening of the star.
Credit: Frank Summers (STScI).
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A black hole’s presence is revealed by its own gravity.

20. Just passing through…
Observations of a field of stars reveals a microlensing event. The event was recorded using a small telescope, so the Hubble Space Telescope was used (right image) to see the unlensed star is more distinctly, allowing astronomers to determine how much the star was brightened, and therefore the mass of the lensing object. From this mass calculation (roughly 6 solar masses), the lensing object has to be a black hole.
Credit: NASA and Dave Bennett (University of Notre Dame, Indiana)
A star’s brightness is magnified by a black hole lens.

21. 4. Exposing dark matter
90% of the mass of galaxies and galaxy clusters is of an unknown form that we call dark matter. But dark matter shares one property in common with normal (atomic) matter - gravity. The lensing effects of a galaxy cluster are due to both the normal luminous matter and the dark matter. In this way the amount, and distribution of dark matter can be determined, even though we haven’t a clue what the stuff actually is!
VLT image of gravitational arc caused by galaxy cluster CL
Credit: European Southern Observatory
Lensing maps both the mass we can see, and the mass we can’t.

22. 5. Ogling alien worlds
Even the gravity of a planet creates a measurable lensing effect. In similar ways to the discovery of lone black holes (in slide 16), the transit of a planet in front of a star can cause a “microlensing” event. The Optical Gravitational Lensing Experiment (OGLE) has discovered five extrasolar planets to date using microlensing. The exoplanet illustrated (2003-BLG-235) has a calculated mass of 1.5 Jupiters.
Fascinating fact: OGLE regularly monitors 170 million stars in the Galactic Bulge, 33 million stars in the Magellanic Clouds. More than 500 events detected during each Bulge season. Most objects are low mass stars and neutron stars.
Going further: A member of the audience may ask how you can distinguish between a lensing event and a star that is intrinsically variable. Gravitational lensing affects all wavelengths of light equally, whereas a star’s variability looks different at different wavelengths.
Image Credit: OGLE Collaboration; A. Udalski, B. Paczynski, M. Kubiak, M. Szymanski, M. Jaroszynski, G. Pietrzynski, I. Soszynski, K. Zebrun, O. Szewczyk and L. Wyrzykowski.
Gravitational Microlensing: A whole new way to find planets around other stars

23. Full circle
Einstein ring 4C this object represents an "Einstein Ring", a particularly symmetric case of gravitational lensing first proposed by Einstein in 1936, in which the source is imaged into a ring. In each case, the ring is a blue galaxy far beyond the yellow/orange lensing galaxy. These blue galaxies, amongst the youngest objects in the universe, would be invisibly faint without this amazing gravitational tool.
Credit: NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team
Not just proof of an amazing idea, but a cutting-edge tool of 21st century astronomy.

24. Image Credits http://www.universeforum.org/einstein/
1919 Eclipse: National Maritime Museum, Greenwich
Q : J. Rhoads, S. Malhotra, I. Dell'Antonio (NOAO) / WIYN / NOAO / NSF
Identical quasars: STScI (George Rhee)
Quasar diagram: NASA / CXC / M. Weiss
Lens shape diagrams: European Space Agency
Einstein ring: STScI, Imperial College (Steve Warren, Simon Dye)
Einstein Cross: ESA and NASA
Abell 1689: N. Benitez (JHU), et al, and the ACS Science Team, ESA, NASA
Gravitational lensing: NASA, ESA, J. Blakeslee and H. Ford (JHU))
Arcs: ESA, NASA, J.-P. Kneib (Caltech / Obs. Midi-Pyrénées) and R. Ellis (Caltech)
Cloverleaf quasar: NASA / CXC / Penn State / G. Chartas, et al
Lensing animation: Frank Summers (STScI)
Microlensing: NASA and Dave Bennett (University of Notre Dame, Indiana)
CL : European Southern Observatory
Planet search: OGLE Collaboration
Full circle: NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team
Add your own institution’s credits accordingly.
ALBERT EINSTEIN and related rights ™/© of The Hebrew University of Jerusalem, used under license. Represented by the Roger Richman Agency, Inc.,