All his life Albert Einstein was interested in trying to understand the laws of the Universe. He was a theoretical physicist, asking probing questions and carrying out experiments in his mind.
In his Special Theory of Relativity, Einstein revolutionised the thinking about space and time. This theory provided the basis for the development of the famous equation E = mc2 which illustrates that matter and energy are interchangeable, and that a small amount of mass is made up of a large amount of energy.
The year 1905 is sometimes called Einstein's annus mirabilis (miracle year). In that year he published four outstanding scientific papers:
- An explanation of the photoelectric effect indicating that light energy came in chunks or quanta. This changed thinking on the nature of light.
- A discussion of Brownian motion demonstrating the existence of molecules.
- The nature of space and time.
- The dynamics of individual moving bodies.
These last two formed the basis of Einstein's Special Theory of Relativity and led to that famous equation, E = mc2.
In 1921, Einstein was awarded the Nobel Prize for Physics 'for his services to theoretical physics and in particular for his discovery of the law of the 'photoelectric effect'. When he made his delayed acceptance speech in 1923, he ignored the citation and spoke on his theory of relativity.
Einstein was a great intellect. He came up with explanations which at the time could not be verified experimentally and many of his theories took a long time to be accepted even within the scientific world. Greater credence was attached to his theories as science and technology advanced sufficiently to allow experimentation involving high speed travel and nuclear reactions.
It is still difficult for many people to accept or understand his theories since they require thinking beyond normal experiences.
Following is an outline of the principles that Einstein developed in a number of scientific fields.
Special Theory of Relativity
Relativity explains the way an object appears to be relative to an observer. You can understand this if you consider yourself in a car, observing other cars. Imagine you are observing a car travelling at 20 km/h. If your car is stationary, then the other car's speed relative to you is 20 km/h. However if you are travelling alongside the car also doing 20 km/h, then the other car seems to be stationary compared to you. Its speed relative to you is zero! (Ever been at the traffic lights when another car has moved away and you thought you were rolling backwards?)
Einstein developed the special theory of relativity by thinking about travelling alongside a light beam at the same speed as the light. He determined that the speed of light is constant, no matter what you are doing or how fast you are travelling, light always travels through empty space at 'the speed of light'. This means that time and length are not absolute, but depend on the relative motion of the object and the observer. If you are stationary, an object that is moving seems to get shorter and heavier, and time slows down for the object. In everyday situations, the slight changes are immeasurable, but they become obvious as the speed increases towards the speed of light. As the object travels closer to the speed of light, the length of the object appears closer to zero. Its height stays the same unless it moves up or down — the contraction only happens in the direction of movement. However if you are travelling at the same speed as the object, then everything looks normal! The 'contraction' of moving objects is the contraction of space itself not the object within space.
In moving through space, time changes. Space and time are two parts of one whole called spacetime. If you stand still, you are only moving through time. If you move at the speed of light, you move through space only and not through time — time stands still. In between, you move partly through one and partly through the other!
So the famous twin trip example: Twin A travels round the world at fast pace in a space ship while the twin B stands still. When twin A returns, (s)he will be younger than twin B. Twin B has moved only through time, twin A has moved partly through space and only partly through time — less time than the twin who stayed still.
The Special theory of relativity also led to the most famous equation which first appeared in an article by Einstein in 1907. There is a fundamental relationship between rest energy and mass according to the equation E = mc2 where E is the rest energy of an object, m is the mass of the object and c is the velocity of light. The mass of something is a measure of the energy within it; matter and energy are interchangeable, and a small amount of mass is made up of a large amount of energy.
In the 1930s nuclear fission was discovered. This gave a way to release the energy stored in the nuclei of atoms, and the possibility of nuclear weapons was realised.
The General Theory of Relativity
The Special Theory applies to objects moving in straight lines at constant speeds. Shortly after its publication, Einstein started work on generalising the theory to include curved paths and accelerating objects, considering the motion that makes planets move in orbit, and the fall of objects to the earth. This led to the equivalence principle which states that in free fall the acceleration and the force due to gravity are equivalent, and the force due to gravity is neutralised, ie in freefall you are weightless.
In 1907 Einstein developed the ideas that space and time are not constant, but they change, and that gravity was a property of spacetime rather than being an external force. As he put it: "Matter tells space how to bend and space tells matter how to move." A glimpse into the workings of Einstein's four dimensional spacetime can be gained by imagining the spacetime as a rubber sheet. Stars and planets have mass and cause the sheet close to them to change shape and curve around them. Another massive object coming close will have its motion affected by this deformation of spacetime.
The general theory predicted that a light beam passing near a massive object would actually be bent, and by how much. This prediction was supported during a total eclipse of the Sun in May 1919, and again by observations of a total eclipse in Australia in 1922.
One of the predictions of the general theory is that the Universe is expanding, giving a basis for the 'big bang' theory of the origins of the Universe. It has also been used to explain black holes and quasars.
The nature of light had been debated for many years. Was it made up of particles or waves? Isaac Newton believed light to be made up of particles, Christiaan Huygens stated it was a wave phenomenon, and this was reinforced when Thomas Young demonstrated interference. Later Maxwell suggested and Hertz proved that light was part of the electromagnetic spectrum.
In 1905 Einstein reintroduced the particle theory of light. A few years earlier, Max Planck had proposed that energy in an atom occurs in little chunks called quanta. Einstein suggested that light also existed in chunks or quanta. These quanta are now called photons. He concluded this by examining the photoelectric effect – the release of electrons from metals when light shines on them. To make this happen, the light needed to be high frequency (ultraviolet). Low frequency light (red) would not make it happen, no matter how bright the light was. Einstein explained this by thinking of light in terms of photons. Each electron is pushed out from the metal by one photon — as long as that photon has enough energy. Only high frequency light has photons with enough energy. Low frequency light has low energy photons, and no matter how many there are, none of them has enough energy to dislodge an electron.
Between 1916 and 1925, Einstein made other contributions to the study of light, including the idea of stimulated emission of radiation – a concept which led to the development of the laser.
Unified Field Theory
The quest to explain gravity and electromagnetism together as aspects of a common phenomenon occupied Einstein's scientific thoughts for much of the last thirty years of his life until he died in 1955. He wanted to provide a basis to explain the Universe in a way other than quantum mechanics which described activities in terms of probabilities. He did not succeed. Since then the weak and strong nuclear forces have been discovered.
Electromagnetism and the nuclear forces can be explained using quantum mechanics, and the search continues for a theory to explain everything!