Chapter 2. Space & Time

Galileo rolled two different weight balls on a smooth surface to prove that bodies of different weight fall at different speed.

This means that effect of force is to change the speed and not just to set the body moving. This was Newton's first law which came in 1687.

In 1676, Christensen Roemer measured the speed of light by measuring the motion of Jupiter's moons. He calculated the speed as 140,000 miles per second. Actual speed is 186,000 miles per second.

James Clerk Maxwell's exeriments, in 1865, unified electricity and magnetism. According to him, light waves travel at fixed speed and they travel relative to 'ether'.

In 1887, Michelson and Morley compared light's speed in the direction of earth's motion with that at 90 degree to earth's motion. Result showed that 'c' was same. Michelson became the first American to get the Noble prize.

Between 1887-1905, Lorentz's experiments proved that objects contract and clocks slow down when they move through ether.

Then in 1905, Albert Einstein gave his theory of 'no absolute time' and so he said that idea of ether is unnecessary. According to his 'theory of realtivity', 'all laws of science are same for all freely moving observers, whatever be their speed'. Einstein also gave the world famous formula E=mc^2 which prove that nothing travels faster than light. Also, the energy which an objects has due to its motion, add to its mass. For example,
if v = 10% of c, then m = m + 0.5% of m, and
if v = 90% of c, then m = more than 2m.

Now let's see some point of differences between Newton's theory and the theory of relativity.
Newton's theory:
Suppose there are two points A and B. A light pulse starts from A for B. Different observers agree on the time that the journey of light pulse took, since time is absolute. But they'll not agree on how far light travelled, since space is not absolute. So different observers measure different speeds for light.
Einstein's theory: