Historians still call the year 1905 the annus mirabilis, the miracle year because
in that year Einstein published four remarkable scientific papers ranging from the
smallest scale to the largest, through fundamental problems about the nature of
energy, matter, motion, time and space.
 In March 1905 , Einstein created the quantum theory of light, the idea that light exists as
tiny packets, or particles, which he called photons. Alongside Max
Planck's work on quanta of heat Einstein proposed one of the most shocking idea in twentieth
century physics: we live in a quantum universe, one built out of tiny, discrete
chunks of energy and matter.
 Next, in April and May, Einstein published two papers. In one he invented a
new method of counting and determining the size of the atoms or molecules in a
given space and in the other he explains the phenomenon of Brownian motion.
The net result was a proof that atoms actually exist  still an issue at that
time  and the end to a millenniaold debate on the fundamental nature of the
chemical elements.
 And then, in June, Einstein completed special relativity  which added a twist
to the story: Einstein's March paper treated light as particles, but special
relativity sees light as a continuous field of waves. Such a contradiction took
a supremely confident mind to propose. Einstein, age 26, saw light as wave and particle,
picking the attribute he needed to confront each problem in turn.
 Einstein wasn't finished yet. Later in 1905 came an extension of
special relativity in which Einstein proved that energy and matter are linked
in the most famous relationship in physics:
E=mc^{2}. (The energy
content of a body is equal to the mass of the body times the speed of light squared).
This equation predicted an evolution of energy roughly a million times more
efficient than that obtained by ordinary physiochemical means.
At first, even Einstein did not grasp the full implications of his formula, but
even then he suggested that the heat produced by radium could mark the
conversion of tiny amounts of the mass of the radium salts into energy.
And after 1905, Einstein achieved what no one since has equaled:
a twenty year run at the cutting edge of physics.
For all the miracles of his miracle year, his best work was still to come:
In 1907, he confronted the problem of gravitation. Einstein began his work with one
crucial insight: gravity and acceleration are equivalent, two facets of the
same phenomenon.
Before anyone else, Einstein recognized the essential dualism in nature, the
coexistence of particles and waves at the level of quanta. In 1911 he
declared resolving the quantum issue to be the central problem of physics.
Even the minor works resonated. For example, in 1910, Einstein answered a basic
question: 'Why is the sky blue?' His paper on the phenomenon called critical
opalescence solved the problem by examining the cumulative effect of the
scattering of light by individual molecules in the atmosphere.
 Then in 1915, Einstein completed the General Theory of Relativity  the product
of eight years of work on the problem of gravity. In general relativity
Einstein shows that matter and energy actually mold the shape of space and the flow of time.
What we feel as the 'force' of gravity is simply the sensation of following the shortest
path we can through curved, fourdimensional spacetime. It is a radical
vision: space is no longer the box the universe comes in; instead, space and
time, matter and energy are, as Einstein proves, locked together in the most
intimate embrace.
( Look at a scenario designed by HHO to explain of why time varies according to general
relativity theory  see Time variations )
 In 1917, Einstein published a paper which uses general relativity to model the
behavior of an entire universe. Einstein's paper was the first in the modern field
of cosmology  the study of the behavior of the universe as a whole.
Returning to the quantum, by 1919, six years before the invention of quantum
mechanics and the uncertainty principle Einstein recognized that there might be
a problem with the classical notion of cause and effect. Given the peculiar,
dual nature of quanta as both waves and particles, it might be impossible, he
warned, to definitively tie effects to their causes.
 In 1924 and 1925 Einstein still made significant contributions
to the development of quantum theory. His last work on the theory built on
ideas developed by Satyendra Nath Bose, and predicted a new state of matter (to
add to the list of solid, liquid, and gas) called a BoseEinstein condensate.
The condensate was finally created at exceptionally low temperatures only last
year.
Einstein always had a distaste for modern quantum theory
 largely because its probabilistic nature forbids a complete description of
cause and effect. But still, he recognized many of the fundamental
implications of the idea of the quantum long before the rest of the physics
community did. (In 'Albert Einstein: Creator and Rebel' by Hoffmann, the
author describes that Max Planck himself was sceptical of his own
quantum hypothesis which was highly distasteful to him and introduced merely
as 'an act of desperation'. Between 1900 and 1905 the quantum concept
remained in limbo. In all the world there seems to have been in those years
only one man to dare take it seriously. That man was Einstein who immediately
sensed the importance of Planck's work and used the idea in his own paper about
the theory of light).
After the quantum mechanical revolution of 1925 through 1927, Einstein spent
the bulk of his remaining scientific career searching for a deeper theory to
subsume quantum mechanics and eliminate its probabilities and uncertainties.
He generated pages of equations, geometrical descriptions of fields extending
through many dimensions that could unify all the known forces of nature.
None of the theories worked out. It was a waste of time ... and yet :
 Contemporary theoretical physics is dominated by what are known as 'String
theories.' They are multidimensional. (Some versions include as many as 26
dimensions, with fifteen or sixteen curled up in a tiny ball.) They are
geometrical  the interactions of one multidimensional shape with another
produces the effects we call forces, just as the 'force' of gravity in general
relativity is what we feel as we move through the curves of fourdimensional
spacetime. And they unify, no doubt about it: in the math, at least, all of
nature from quantum mechanics to gravity emerges from the equations of string
theory.
As it stands, string theories are unproved, and perhaps unprovable, as they
involve interactions at energy levels far beyond any we can handle. But they
are beautiful, to those versed enough in the language of mathematics to follow
them. And in their beauty (and perhaps in their impenetrability) they are the
heirs to Einstein's primitive, first attempts to produce a unified field
theory.
See in the Science section:
From the Atom to String Theories
