Albert Einstein predicted that the suns gravity should bend the light of background stars, so they would appear to move outward during a solar eclipse.
This deflection -- about 1/1000th the width of a full moon -- was first measured by the famous British astronomer, Sir Arthur Eddington in 1919. Several years later a group from the Lick Observatory proved that Einstein was correct, by measuring to the required accuracy.
As a result, we now believe gravity is not some "magical" force that pulls on a mass -- as Newton theorized -- but simply the bending of spacetime, and this, in turn, led to the acceptance of relativity, upon which our contemporary cosmological ideas are built.
It's a good thing that the moon, which is 1/400th the size of the sun, is also 1/400th the distance. If Einstein had been born about 20,000 years sooner or later, astronomers could not test his theory with a solar eclipse, because then the moon would appear either too large or too big to exactly "fit" over the sun. Nice of the universe to have been so accommodating!
Einstein also theorized that stars and other masses such as galaxies should also bend light to form points, arcs, and even halos around the intervening masses. Dozens of such "gravitational lenses," mostly images of quasars deflected by the gravitational fields of whole galaxies, have been discovered to date.
Using gravity to find planets
These lenses also provide us with a one-in-a-million opportunity to see evidence of planets orbiting stars.
If a planet deflects light precisely in our direction, we would see its star brighten as the deflected light is concentrated toward us. This brightening can last from 15 minutes to a month, depending on the mass of the planet and how far it is from its star.
The brightening can be about one magnitude in extent (i.e. the star can become almost 3 times brighter). To date, finding this "flash" is the only technique (other than photometric transits, which I'll discuss in a future article) by which we can detect terrestrial-type planets around Sun-like stars or close double star systems.
(A pulsar timing method can also detect terrestrial-mass objects, but that's also a topic for another future article.)
Gravitational lens experiments have been very successful in determining how many small stars there are vis-à-vis larger ones in our own and other galaxies.
Unfortunately, determining the same thing for planets is much more difficult. The main snag is that the star system must be distant: on the order of at least 5,000 light years away.
Also, since the alignment will essentially never repeat itself again, all the data have to be obtained at once by several observatories. Luckily, various projects such as MACHO and OGLE are already looking at crowded star fields to determine the mass distribution of our own Milky Way galaxy as well as some of our "next door" neighbors: the Large Magellanic Cloud, Small Magellanic Cloud, and the globular cluster Omega Centauri.
When these projects spot a stellar brightening, a group known as the PLANET collaboration leaps into action and starts sampling at a higher time resolution --the key to detecting smaller masses.
Essentially, this "flash" method uses planets themselves -- with their stars in the background -- as telescopes that focus light towards us.
I bet Einstein himself would have said "Wahoo!"
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