ALBERT Einstein published his Special Theory of Relativity in 1905. This was a radical advance on the concepts of classical physics.
The Special Theory describes how the world looks to an observer moving at constant speed. It contradicts our commonsense notions. Because the speed of light is constant for all such observers, moving rulers shrink, moving masses grow, and moving docks run slow.
Einstein's Special Theory was in the news recently when an Irish scientist, Dr Al Kelly, contradicted some features of the theory regarding the speed of light. In this article I will try to briefly explain the main features of Einstein's Special Theory of Relativity.
Before Einstein, the Galilean Theory of Relativity stated that observers moving at a steady velocity relative to each other each find the laws of mechanics to be the same. However, movements of the Earth affect mechanical experiments so that experimental results never perfectly match theory.
The laws of mechanics work perfectly only in a frame of reference that is at absolute rest. But what part of the universe is at absolute rest? This greatly puzzled physicists. It was postulated that absolute rest is represented by the ether, a ghostly substance that permeated the entire universe, and to which matter was as porous as a sponge is to water.
The ether also conveniently explained the movement of light through a vacuum. James Clerk Maxwell had shown that light is an electrical magnetic wave that travels through a vacuum at a speed of 186,000 miles per second.
But if light is a wave, something must be waving - presumably the ether. According to Maxwell's equations, light moves at only one velocity, - C. Maxwell assumed that this velocity was relative to the stationery ether.
In 1887 Albert Michelson and Edward Morley performed an experiment to measure the motion of the Earth through the ether. A beam of light was split into two parts, one part aimed along the direction of the Earth's path - through space and the other part aimed at right angles to this direction.
It was reasoned that the light aimed along the Earth's path would be heading straight into the "ether wind" and would be slowed relative to the speed of the light moving across the wind (every pilot knows that you travel faster across the wind than into the wind).
Surprisingly, the experiment showed that light moved at the same speed regardless of its direction of movement. In other words, there was no evidence for the existence of the ether, and the velocity of light was the same regardless of the motion of the observer.
Measurements of the speed of light always gave the same answer, regardless of the motion of the observer. This finding was puzzling and contradicted the common sense of classical physics.
For example, if you stand opposite a friend, say, at a hundred yards' distance, and he throws a ball towards you at a speed of 10 miles per hour, you will observe the ball moving, relative to you, at 10 miles per hour.
On the other hand, if you run towards your friend at a speed of 10 miles per hour, and he again propels the ball towards you with the same energy as before, you now observe the ball moving at 20 miles per hour relative to yourself. However, if you repeat this experiment and substitute a source of light for the ball you will always measure the velocity of light at 186,000 miles per second!
Einstein looked afresh at the situation and decided (a) that the ether didn't exist, and (b) he elevated the puzzle of the constant speed of light to the principle of the constant speed of light.
In abolishing the ether Einstein also abolished the primacy of the idea of absolute non motion. He updated Galileo's Theory of Relativity to say that all the laws of nature are identical in all frames of reference that move uniformly relative to each other and, therefore, there is no way to distinguish absolute uniform motion or non motion.
In a railway carriage from which you cannot see out, moving perfectly smoothly at steady speed in a straight line, there is no experiment you can perform which will tell you whether the carriage is moving or is stationary.
The other foundation stone of the Special Theory of Relativity is that the velocity of light is the same in all frames of reference moving uniformly relative to each other.
How do we explain the constant speed of light regardless of the motion of the observer? In order to measure speed one must use a ruler and a clock. Einstein reasoned that these measuring instruments change, depending on their motion, in such a way that the speed of light always appears the same.
The changes in the moving ruler and clock are apparent to an observer at relative rest, but not to an observer travelling along with them. Therefore both observers measure the same speed of light and neither detects anything unusual in the measurement or in the apparatus.
Einstein introduced the labels proper and "relative". If we are "stationary" and observe our stationary rods and clocks, we see their proper lengths and time. If we observe a rod and clock travelling very fast relative to us we see their relative length and time.
Relative length is always shorter than the proper length and relative time is always slower than the proper time. A moving object is observed to contract in its direction of motion as its velocity increases, until it disappears altogether at the speed of light.
A moving clock ticks more slowly than a clock at rest (time dilation) and this trend continues until, at the speed of light, it stops altogether.
This time dilation effect is the basis for the well known twins paradox. In this scenario, an astronaut twin leaves Earth and spends a year of Earth time travelling through space at high speed.
Because time passes more slowly for him than for his Earthbound brother, he finds when he returns to Earth that he is younger than his brother. This is not science fiction and has been confirmed experimentally.
For example, in 1972 four atomic clocks were flown around the world by aircraft. At the end of the trip they were found to be slightly behind the Earth bound twin clocks with which they were synchronised before the flight. Time dilation has also been verified by observations on high energy particles, e.g. cosmic rays.
The speed of light is the fastest speed allowed in the universe. The carriers of light energy are called photons. They can travel at the speed of light because they have no mass.
But no material body can ever be accelerated to the speed of light because this would require an infinite amount of energy. This is because mass is measured to increase with speed, rising to infinite mass at the speed of light. This effect has often been verified experimentally, e.g. noting the increased mass of the electron moving at high velocities.
The length contraction, increasing mass and time dilation consequences of moving at speed become really significant only at velocities close to the speed of light.
Measurements made by the methods of classical physics are fine for the speeds of our everyday world. However, measurements made of events moving close to the speed of light can only he made accurately by taking Einstein's theory into account.
For example, for Concorde travelling at Mach 2, the contraction is only two parts in a trillion - less than the width of an atom. However, if Concorde travelled at half the speed of light it would be seen to contract 15 per cent and its mass would increase correspondingly.
In other words, as you approach the speed of light, energy added to make an object move yet faster is largely converted into an increase in mass of the object. There is an equivalence between energy and mass. Einstein expressed this relationship in his famous formula E = MC2 This equation has been verified experimentally on countless occasions, not the least of which was the explosion of the first atomic weapon.
Classical physics considered time and space to be separate - space is continuous but time moves in a two dimensional line from the past to the present to the future.
However, arising from Special Theory of Relativity, time and space were seen to form a four dimensional (three space dimensions and a dimension of time) space time continuum. There are no breaks in a continuum. In the traditional picture, events develop with the passage of time. The space time continuum is more of a static picture where events do not develop, they just are.
If we could view the world of the space time continuum, we would see events that now seem to develop before us as time passes, already existing in total, etched on the fabric of space time.
The space time interval between two events is an absolute, but it can appear different to observers in different states of motion.
For example, imagine two observers A and B, A sitting in the centre of a steadily moving train carriage and B standing on the platform as the train passes. A light bulb is switched on in the centre of the carriage as the train passes B. Observer A sees the light hit both end walls of the carriage simultaneously, but B sees the light hit the rear wall of the carriage before it hits the front wall, because the rear wall is moving forward to meet the light.
In other words observers A and B disagree on the timing of these two events. However, if observers A and B each feeds his time and distance measurements into the formula for calculating space time interval they will both get the same result.