General Theory of Relativity
Special theory of relativity was a breakthrough in physics but it did not
deal with gravity and this was a limitation as gravity is an important and
ever present force in the universe. The special theory, it is true, introduced
the notion of inertial and gravitational masses. Inertial mass is the permanent
resistance objects present when we want to change their state of motion
-moving an object on the floor- while gravitational mass -that only exists
in the presence of a gravitational field- is what gravitation acts on -lifting
a suitcase from the floor. However both gravitational and inertial masses
are the same. This was known for a long time but it was thought to be a
mere coincidence. Einstein showed why it was like this.
In one of his well-known thought experiment -for that he is famous- he realised that a man in free fall would not feel his own weight. This led him to discover a theory of gravitation. Let us consider two windowless elevators but you do not know in which you are. One is falling freely in its shaft on the earth, and the other in sitting in a space vehicle where there is no significant gravitation force. In both the passengers are weightless but they have no way to know in which they are, not with your senses or through experiment because both elevators are really in free fall. The gravitational field has only a relative existence depending on its inertial framework. The opposite situation is also true. This time one windowless elevator is immobile on the ground floor of a building and the second is in the space but pulled up by a spacecraft with an acceleration G (the passenger will be pressed on the floor with a force equal to that of the earth's gravitation). Here again it is not possible for the passengers to know in which elevator they are.
From these thought experiments Einstein concluded that gravitation is equivalent to a special acceleration. But it is not the acceleration we know in a three-dimensional space. Einstein showed that gravity was acceleration in a four-dimensional space-time continuum that is, of course, non-Euclidean. His General Theory of Relativity replaced the force of gravitation by the geometry of space. Matter curves space, and gravitation is only the acceleration of objects as they slide down their trajectories in time through curved space. The planets, for instance, skid on the inner walls of a depression in space created by the massive sun.
General relativity showed clearly that the universe is at the same time finite and unbounded since matter warps space, the sum total of the masses in the universe may be sufficient to warp space on itself with the result that the universe is a closed, four-dimensional spherical cosmos without no end. In other words, our universe is finite and unbounded. The universe has no edge in the same way as the two-dimensional earth that is finite and unbounded too.
The general theory of relativity is very beautiful but many people at first doubted that it truly represented the universe. Many experiments showed that it does and now most scientists believe that it is correct.
Space, Einstein found, may be seen as a boundless rubber sheet and not as an empty expanse. This sheet can be deformed in different ways: stretched, squeezed, straightened bent or, even, indented in some points. Massive stars like the sun sit on this sheet and produce deep depressions in it. As a consequence planets circle the sun because they are caught in the hollow carved by the sun. Newton taught that they were held there by gravitation forces but this, according to general relativity, is not the case. As long as big bodies continue to exist in the space, the indentation they produce will remain permanent. In conclusion what we think off as gravity -the attractive force between two bodies- is in fact the result of these indentations.
If we move a heavy body into the space it will send ripples in the sheet like dropping a stone in quiet water will also do. These gravity waves will move outward striking planets, stars and all the bodies in the cosmos. We know already that electromagnetic waves move through space but gravity waves, on the contrary, would really be undulations in the space itself. The gravity waves would then expand and contract the heavenly bodies they hit as well as the space itself. All the masses in the space can send gravity waves of various strength and frequencies depending on the mass and the motion of the bodies, all they have to do is move. Massive bodies in the space are generally stationary and then emit few waves. Lighter bodies like the earth that is moving around the sun emit in permanence weak gravitational waves. However gravity waves will only be detected when they result from a big event such as stars crashing into each other, explosions of supernovas and the formation of black holes.
As gravitational energy mowing through space disperses and grows weaker, it is difficult, if not impossible, to detect gravity waves.
Quantum Theory
Until about 1920 Einstein made fundamental contribution to the Quantum Theory
but later on he rejected it. In the early 1920s Einstein proposed the quantum
statistical mechanics of gases after the Indian physicist Bose had sent
him a note -that Einstein had published in the "Zeitschrift fur Physik"
in Bose's name. Bose had discovered a new method of doing quantum statistics
as applied to light quanta. Einstein realised that the same method could
be applied to gases. However he saw that the gas "particles" would
behave statistically as light quanta and therefore should show wave characteristics.
The French physicist Louis de Broglie arrived, at the same time, at the
same conclusion. On this base, Einstein was able to state "A beam of
gas molecules which passes through an aperture must undergo a diffraction
analogous to that of a light ray." And this was shown experimentally
to be true. Einstein had shown that light was not alone to behave at time
as wave and as other as particles; particles -like electrons for instance-
had the same dual behaviour.
Einstein and Schrodinger both rejected the next step proposed by Bohr and Pascual Jordan that the only possible interpretation of the waves -within the dual concept of waves/particles- was that from them, using the Schrodinger's equation, one can only calculate the probable behaviour of the particles.
The Bohr/Jordan theory can be best explained like this:
If we send a light ray through a small aperture a diffraction pattern will
appear on a screen placed on the other side of the aperture.
Let us now send light quanta, one at a time. Not all of them with hit the
screen at the same point.
Bohr/Jordan interpreted this result but saying that one cannot predict with
accuracy what a single light quanta will do when it goes through the aperture.
According to them, one can only calculate the probability that it will hit
a certain point of the screen using the Schrodinger wave equation.
Strict determinism that was believed to be true in the period of time between
Newton and Einstein had to be abandoned. Einstein and Schrodinger disagreed.
Later on Heisenberg and Bohr showed that the particle-wave duality was not an incidental feature of atomic physic but a basic fact of nature. Heisenberg called it the "Uncertainty Principle."
United Field Theory
Einstein's contribution to physics lasted until about 1923. It is not that
he lived on his reputation afterwards but he did not produce anything at
the same level that before. However he tried and he tried really hard but
without great success. During the next 32 years, until his death in 1955,
he devoted his time and efforts searching for a "United Field Theory",
a theory that would unify mathematically gravitation and electromagnetism
and, later on, also the strong and weak forces of nucleonic particle interactions.
He thought many times that he was succeeding but overtime he was disappointed
as quantum theory became more and more important but, as we said before,
he refused to accept it. After his death quite a few physicists took over
his research but without success until now.