Laws of Motion
Roemer discovers that the speed of light is a constant
Albert Einstein's Fundamental idea of relativety
Demonstration the effect of "Free Fall"
See how light rays follow curves in spacetime
Einstein imagines riding on a beam of light

For many years Aristotels was seen as the great philosopher , with a great understanding of the universe , however , as science became more popular and people got a better understanding of space and time mathematicians started to explore many of known science by applying mathematical formulas. In this respect Isaac Newton played an important role.

Laws of Motion - featuring Isaac Newton and Galileo Galilei

The Aristotelian tradition held that one could work out all the laws that govern the universe by pure thought : It was not necessary to check by observation so no one until Galileo Galilei bothered to see weather bodies of different weight did in fact fall at different speeds. Galileo Galilei rolled balls of different weights down a smooth slope. His measurements indicated that each body increased its speed at the same rate no matter what its weight. Galileo Galilei measurements were used by Newton as the basis of his laws of motion. In Galileo Galilei experiments proved that as a body rolled down a slope it was always acted on by the same force (its weight). This showed that the real effect or weight is a force always to change the speed of the body , rather than just set it moving, as was previously thought. It also meant that whenever a body is not acted on by any force it will keep on moving in a straight line at the same speed. This observation is known as Newton's first law.

What happens to a body when a force does act on it is given by Newton's second law. This states that the body will accelerate , or change its speed , at a rate that is proportional to the force. You can see this with cars. The more powerful the engine ,the greater the acceleration , but the heavier the car , the smaller the acceleration .Newton's law of gravity tells us that the further apart the bodies, the smaller the gravitational force. If the law were that gravitational attraction of a star went down faster with distance, the orbits of the planets , would not be elliptical , they would spiral into the sun ...

Roemer discovers that the speed of light is a constant

The fact that light travels at a finite , but very high speed was first discovered in 1676 by the Danish astronomer Ole Christensen Roemer . He observed that the times at which the moons of jupiter appeared to pass behind Jupiter were not evenly spaced, as one would expect if the moons went around Jupiter at a constant rate. Roemer noticed that eclipses of Jupiter's moons appeared later the further they were from Jupiter. He argued that this was because the light from the moons took longer to reach us when we were further away from the moons.

Roemer's achievement is not only proving that light travels at a finite speed , but also in measuring that speed , was remarkable coming as it did 11 years before Newton's publication of "Princepia Mathematica"

Albert Einstein's Fundamental idea of relativety

In 1905 a clerk in Swiss patent office , called Albert Einstein , put forward a much more elegant idea which he called the "Theory of Relativity". The fundamental postulate of the theory was that :
The law of science should be the same for all freely moving observers , no matter what their speed.

This was true for Newton's laws of motion , but now the idea extended to include Maxwell's theory , and the speed of light : all observers should measure the same speed of light , no matter how fast they are moving , this simple idea , has some remarkable consequences. Perhaps the best known are the equivalence of mass and energy , summed up , in Einstein's famous equation E = mc2 ( where "E" is energy , "m" is mass , and "c" is the speed of light) , and the law that nothing may travel faster than the speed of light.

This special theory of relativity , was inconsistent with the Newtonian theory of gravity , which said that objects attracted each other with a force that depended on the distance between them.

In 1915 Einstein made the revolutionary suggestion that gravity is not a force like other forces , but is a consequence , of the fact that space time is not flat , as had been previously assumed. It is curved or "warped" by the distribution of mass and energy in it.

Demonstration the effect of "Free Fall"

The effect of free fall can be demonstrated by imagining Einstein playing Ping - Pong in a tram suspended on a railway track on the edge of a cliff. A ping - pong ball appears to the observer to move differently , when it is not in free fall - When the tram is still on the rails. The Ping – Pong ball appears to move in an arc. When the tram go off the rails in its free fall , the ball appears to move in a straight line. Yet in the two situations , the ball arrives at the same time , in the same location in space time. It is just that the motion of the ball appears different to an observer in free fall.

In Newton's theory , position in space time had been relative , but time had been absolute. However in Einstein's theory , both space and time are relative.

To demonstrate that time is relative , consider two observers , moving relative towards each other. Each observer send out a pulse of light. Part of the pulse is reflected back at the event and the observer measures the time in which he receivers the echo.

The time of the event is half way between when the pulse was send and the time the reflection is received. The distance of the event is half the time taken for this round trip , multiplied by the speed of light. Using this procedure , observers who are moving relative to each other will assign different times and positions to the same event. No particular observer's measurements , are any more correct , than any other observer's , but all the measurements are related.

Any observer can workout precisely what time and position any other observer will assign to an event , provided he knows the other observer's relative velocity.

If a flash of light is send out at a particular event , it will spread out at later times. This means it will form a cone in space-time , called the future light cone of the event. Similarly the past light cone of an event is formed by a flash of light that focuses down on the event. The absolute future of an event is the part of space-time within the future light cone. It is the region that can be affected by what happens at the event. Events outside the light cone can not be influenced by what happens in the present , because nothing can travel faster than light. The absolute past is the set of all events that can possibly affect what happens in the present.( The set of all events from which signals traveling at or below the speed of light can reach the present ). If one knows what is happening at some particular time every where in the region of space that lies within the past cone one can predict what will happen in the present. The elsewhere is the region of space-time that does not ly in the future or past cones. Events elsewhere can not affect or be affected by events in the present.

See how light rays follow curves in spacetime

Objects try to move on straight lines through space-time but because it is curved , they move on paths called "geodesics" which are the nearest thing to a straight line in a curved space. In general relativity bodies always follow straight lines in four dimensional space-time, but they appear to move along curved paths in our three dimensional space. Light rays follow geodesics in spacetime . Again the fact that space is curved means that light no longer appears to travel in straight lines in space. So general relativity predicts that light should be bent by gravitational fields. It is normally very difficult to see this effect because the light from the sun makes it impossible to observe stars that appear near to the sun in the sky. However during an eclipse it is possible. Einstein's theory of light deflection was proven in 1919 by a British expedition . Observing an eclipse in West Africa when a star had a different location during the night and the day.