A NEW study of Albert Einstein鈥檚 notebooks reveals that the great
physicist had realised the existence of an effect called gravitational lensing
more than two decades before he shared his breakthrough with the rest of the
world.
Gravitational lensing can produce multiple images of a distant celestial
object and is caused by the bending of light by the gravitational field of a
massive object鈥攗sually a galaxy鈥攍ying in the same line of sight.
Einstein described the effect in a paper published in 1936, and the first
example was not seen by astronomers until 1979. But science historians have now
found that Einstein described lensing in a notebook from 1912.
鈥淭his provides an important window on Einstein鈥檚 thought processes as he
struggled to develop his theory of gravity,鈥� says J眉rgen Renn of the Max
Planck Institute for the History of Science in Berlin, who led the team which
describes the notebook entries in last week鈥檚 issue of Science. This
theory of gravity, known as general relativity, was not published until
1915.
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Renn and his colleagues have been examining Einstein鈥檚 notebooks to uncover
the genesis of general relativity. The discussion of gravitational lensing was
written in April 1912. 鈥淲e know the date because Einstein was at Berlin鈥檚 Royal
Observatory visiting the astronomer Erwin Freundlich between 15 and 22 April
1912 and his calculations are interspersed with Berlin appointments and
addresses,鈥� says Renn.
Einstein imagined a situation in which a distant star was hidden behind a
nearby star. If the stars were perfectly aligned, Einstein calculated, there
would be a ring of light around the nearby star, nowadays called an 鈥淓instein
ring鈥�. If the stars were not perfectly aligned, there would be no ring but a
double image of the distant star.
Einstein calculated both the separation of such images and their
magnification. 鈥淗owever, he put the calculations aside because he thought the
effect unobservable,鈥� says Renn. 鈥淗e only published in 1936 when urged to do so
by a Czech amateur scientist.鈥�
What Renn finds most fascinating about Einstein鈥檚 work on gravitational
lensing is that it shows how much intuition he had about general relativity
before he was able to formulate the theory mathematically. Einstein had guessed
that his eventual theory must predict light bending in 1907, says Renn. 鈥淗e had
many of the pieces of the jigsaw well before he finally put them together.鈥�