Easy explanation of relativity theory

In the late 19th century scientists attempted to measure the absolute velocity of the earth using the equations of Maxwell and Galileo. Maxwell's equations gave the velocity of the speed of light, and Galileo's gave the way to measure differences between moving and stationery systems. The following is an example of how one would use these to find a velocity:

Imagine a spaceship that could move at a very high speed. Light from behind was shining past this spaceship. If a measurement of the speed of the light gave a result of 2x108m/s, then one can expect the speed of the ship to be 1x108m/s - the difference between the known speed of light (3x108m/s) and the speed measured from the spaceship.

This model could not be used until 1887, because the measuring apparatus was too imprecise. However, when this idea was finally used to try and determine the velocity of the earth, the experiment produced a remarkable result - the earth had zero velocity. Naturally scientists at the time were completely baffled by this and tried to discover where and how the laws of physics had failed.

Several scientists attempted to explain this anomaly before Einstein solved the problem with the Special Theory of Relativity - among them were Henri Poincare who suggested that it was impossible to determine an absolute velocity; H.A. Lorentz who came up with the transformation for motion; Michelson and Morley whose experiment it was that failed and started everyone thinking.

It was left to Einstein to solve the problem with the publication of the Special Theory of Relativity in 1905. He gave up the idea of an absolute velocity, and abolished the idea of the "ether", the mysterious substance through which scientists though light travelled.
Albert Einstein is the most well known physicist of the 20th century. Most famous for his Theory of Relativity, Einstein is also ranked high amongst the scientists responsible for the emergence of quantum mechanics for his proof of light traveling as a particle.

Albert Einstein was born on March 14 1879 to a middle class Jewish family in Ulm, Germany. In 1886 he began his school career in Munich. He disliked school because of the mindless drilling that was involved and he preferred to study at home where he gained an interest in mathematics and science. He began studying Calculus at age twelve at the Luitpold gymnasium. It was at about this time that his studies came into conflict with his deep religious feelings. His realisation that the Bible could not be literally true created his lifelong distrust of authority. He was granted Swiss citizenship a year after graduating from the Polytechnic Institute in Zurich.

He avoided compulsory military service thanks to his flat feet and varicose veins, but he was denied university assistantship. He then began moving around from post to post as a temporary teacher. Through a university contact he eventually gained a permanent job at the Swiss Patent Office as a technical expert, third class. In 1906, four years later, he was promoted to technical expert second class. During this time he wrote a fair amount of theoretical physics literature on a wide range of subjects. Many of these papers, written during his free time, were published and one thesis on a new determination of molecular dimensions earned him a doctorate from the University of Zurich.

During that same year of 1905 Einstein wrote two papers that turned the science world upside down. The first, on the photoelectric effect, contradicted previous perceptions of electromagnetic energy based on Maxwell's equations, and helped establish the nascent science of quantum mechanics. The second linked important parts of mechanics and Maxwell's electrodynamics to form The Special Theory of Relativity. The most important and famous part of this theory was his equation of energy and mass, E=mc2 , which was an undisputed display of pure genius.

He continued to work at the patent office until 1909 in which time he had extended the Special Theory of Relativity to include phenomena involving acceleration. He made significant contributions to the Quantum Theory and in 1908 became a lecturer at the University of Bern after submitting a further thesis for the constitution of radiation. In 1909 he left the patent office and his lectureship at Bern for the University of Zurich, where he was a professor for 2 years before being appointed a full professor at the Karl-Ferdinand University in Prague. By this time Einstein, at age 32, was recognised internationally as a leading scientist and physicist. A year later he began his work on the General Theory of Relativity. He moved to Zurich that same year to take up a chair at the Eidgenssische Technische Hochschule.

Late in 1915 he published the definitive version of the General Theory of Relativity. In 1919 British eclipse expeditions confirmed predictions derived from the General Theory of Relativity and Einstein was idolised by the press the world over.

In 1921 Einstein visited the U.S.A for the first time to raise funds for the planned Hebrew University of Jerusalem. He did lecture a few times on relativity and he received the Barnard Medal. That same year he was awarded the Nobel Prize for his work on the photoelectric effect in 1905, although he was not present for the award.For the next 6 years Einstein travelled around the world, receiving the Copley Medal of the Royal Society in 1925 and the Gold Medal of the Royal Astronomical Society in 1926. His schedule proved too hectic, for in 1928 Einstein experienced a physical collapse due to overwork. Although he did recover, he had to take things easy for the next two years.

He resumed his international visits in 1930 and in December of 1932, while he was in the U.S.A. the Nazis came to power, seizing his property after he had revoked his citizenship. He was granted permanent residence in America in 1935. At Princeton he resumed his quest to unify electromagnetic and gravitational phenomena in a theory he called the Unified Field Theory. He failed despite devoting the last 25 years of his life to this theory. In 1940 he was granted American citizenship and he made many contributions to world peace, he himself being a pacifist. By 1949 he was unwell and he began preparing for death by drawing up a will. He was offered the Presidency of Israel following the death of its first president in 1952 and, although it was difficult for him to do so, he declined the offer.

A week before his death Einstein signed his last letter. It was a letter to Bertrand Russell in which he agreed that his name should be placed on a manifesto urging all nations to give up nuclear weapons. It is fitting that one of his last acts was to argue, as he had done all his life, for world peace. He died peacefully on April 18 1955 at the age of 76.
Albert Einstein is the most well known physicist of the 20th century. Most famous for his Theory of Relativity, Einstein is also ranked high amongst the scientists responsible for the emergence of quantum mechanics for his proof of light traveling as a particle.

Albert Einstein was born on March 14 1879 to a middle class Jewish family in Ulm, Germany. In 1886 he began his school career in Munich. He disliked school because of the mindless drilling that was involved and he preferred to study at home where he gained an interest in mathematics and science. He began studying Calculus at age twelve at the Luitpold gymnasium. It was at about this time that his studies came into conflict with his deep religious feelings. His realisation that the Bible could not be literally true created his lifelong distrust of authority. He was granted Swiss citizenship a year after graduating from the Polytechnic Institute in Zurich.

He avoided compulsory military service thanks to his flat feet and varicose veins, but he was denied university assistantship. He then began moving around from post to post as a temporary teacher. Through a university contact he eventually gained a permanent job at the Swiss Patent Office as a technical expert, third class. In 1906, four years later, he was promoted to technical expert second class. During this time he wrote a fair amount of theoretical physics literature on a wide range of subjects. Many of these papers, written during his free time, were published and one thesis on a new determination of molecular dimensions earned him a doctorate from the University of Zurich.

During that same year of 1905 Einstein wrote two papers that turned the science world upside down. The first, on the photoelectric effect, contradicted previous perceptions of electromagnetic energy based on Maxwell's equations, and helped establish the nascent science of quantum mechanics. The second linked important parts of mechanics and Maxwell's electrodynamics to form The Special Theory of Relativity. The most important and famous part of this theory was his equation of energy and mass, E=mc2 , which was an undisputed display of pure genius.

He continued to work at the patent office until 1909 in which time he had extended the Special Theory of Relativity to include phenomena involving acceleration. He made significant contributions to the Quantum Theory and in 1908 became a lecturer at the University of Bern after submitting a further thesis for the constitution of radiation. In 1909 he left the patent office and his lectureship at Bern for the University of Zurich, where he was a professor for 2 years before being appointed a full professor at the Karl-Ferdinand University in Prague. By this time Einstein, at age 32, was recognised internationally as a leading scientist and physicist. A year later he began his work on the General Theory of Relativity. He moved to Zurich that same year to take up a chair at the Eidgenssische Technische Hochschule.

Late in 1915 he published the definitive version of the General Theory of Relativity. In 1919 British eclipse expeditions confirmed predictions derived from the General Theory of Relativity and Einstein was idolised by the press the world over.

In 1921 Einstein visited the U.S.A for the first time to raise funds for the planned Hebrew University of Jerusalem. He did lecture a few times on relativity and he received the Barnard Medal. That same year he was awarded the Nobel Prize for his work on the photoelectric effect in 1905, although he was not present for the award.For the next 6 years Einstein travelled around the world, receiving the Copley Medal of the Royal Society in 1925 and the Gold Medal of the Royal Astronomical Society in 1926. His schedule proved too hectic, for in 1928 Einstein experienced a physical collapse due to overwork. Although he did recover, he had to take things easy for the next two years.

He resumed his international visits in 1930 and in December of 1932, while he was in the U.S.A. the Nazis came to power, seizing his property after he had revoked his citizenship. He was granted permanent residence in America in 1935. At Princeton he resumed his quest to unify electromagnetic and gravitational phenomena in a theory he called the Unified Field Theory. He failed despite devoting the last 25 years of his life to this theory. In 1940 he was granted American citizenship and he made many contributions to world peace, he himself being a pacifist. By 1949 he was unwell and he began preparing for death by drawing up a will. He was offered the Presidency of Israel following the death of its first president in 1952 and, although it was difficult for him to do so, he declined the offer.

A week before his death Einstein signed his last letter. It was a letter to Bertrand Russell in which he agreed that his name should be placed on a manifesto urging all nations to give up nuclear weapons. It is fitting that one of his last acts was to argue, as he had done all his life, for world peace. He died peacefully on April 18 1955 at the age of 76.Symmetry in physics is generally defined as the ability of something to remain the same after undergoing a certain operation. A sphere for example has total symmetry because it looks exactly the same after being turned in any way. A cylinder on the other hand only has left right symmetry because it only remains the same if the rotation is around the vertical axis.

Not only does the object itself have to be considered, but any outside influences as well. For example if we have a machine that relies on gravity or oxygen then moving it to another place does not necessarily mean that symmetry will hold, so the operation, in this case displacement, would have to be performed on all the components relied upon by that machine to function. Physical phenomena remain unchanged (therefore their laws remain unchanged) after undergoing operations such as:

Displacement in space and time
Rotation around a fixed axis
Constant velocity in a straight line
Reflection in space
Reversal of time
Displacement in space is a seemingly obvious case as is displacement in time. Velocity in a straight line means that if we have an apparatus in a moving vehicle it would work in exactly the same way as it would if it wasn't moving provided the velocity and direction do not change. Reflection in space means that if we had two objects one looking exactly like the others mirror image, they would work exactly the same. It cannot as yet be proven, but it is believed that the physical laws hold true under the reversal of time.

Two operations that seem to conform to symmetry but do not are a change of scale and constant rotation at a fixed angular velocity. Symmetry does not hold under a change of scale because larger things deteriorate faster than smaller things. For example if we had a small bridge over a small space it would last longer than a larger bridge over a larger space and if gravity was increased according to the scale as well then the larger bridge would deteriorate even faster. Therefore increasing something in scale does not mean it will remain the same. An object rotating around a fixed point at a constant angular velocity will experience centrifugal forces. These forces are not around when that object is still, thus the object would have different forces on it and symmetry would not hold.

Constant velocity in a straight line is what Special Relativity is all about. However, the idea that constant velocity in a straight line is symmetrical did not come from Einstein, but was stated by Newton in one of his corollaries to the laws of motion. He stated, "the motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line". This means that if a spaceship was moving uniformly forward in a straight line, any experiments performed and any phenomena measured will give the same results as if the spaceship were not moving at all. This is why the experiment to determine the velocity of the earth gave a result of zero - zero is the result it would have given if the earth were not moving at all, so zero is the result it must give when moving uniformly in a straight line. It is when this principle is applied together with the principle that the speed of light also remains the same under all conditions that the strange consequences of relativity become apparent.

The principles of Symmetry are very nice, but we need some way to make them work. We do this by using mathematical devices known as transformations.

What is a transformation? A transformation is a formula which takes co-ordinates in one system, and gives us their corresponding co-ordinates in another. For example, there is a transformation which will give us the co-ordinates of a system in a system whose origin has been rotated relative to ours. Transformations are a vital part of physics, especially as they help us ensure that results are consistent - if we apply the standard transformations, our laws should come out the same before and after.

It was this seemingly simple problem which caused so many headaches at the end of the last century. It appeared as if James Clerk Maxwell's equations governing the speed of light did not obey these transformations - thus violating the principle of relativity. It was Lorentz who first suggested that Maxwell's laws were correct, and Newton's needed changing, and he did so by introducing his Lorentz transformations, which are at the heart of many Relativistic phenomena (these formulas are quite complex, and are given in the Advanced section). Einstein took this idea, and so first derived the Lorentz transformations and introduce the ground shaking ideas of Relativity.