10.02.10
The second chapter of A Brief History of Time is called Space and Time. In this chapter Hawkings talks about the fact that, in zero air resistance, any 2 objects will fall at the same time. Say you have one lead ball. And then you have another lead ball that is twice as heavy. Gravity will pull on it twice as hard on it as it will on the other ball. Then how do they both land at the same time? I'm getting to that. If you have 2 cars, one 50cc and another 100cc and the 100cc one is exactly twice as big as the 50cc, they will both go at equal speeds. Same with the balls. The fact that the second ball is twice as big as the other one cancels out the fact that is being tugged by gravity twice as much as the other one giving both balls an equal acceleration.
The chapter explains how this is relevant to Newton's First Law and gravity. If the sun is twice as big, the force is twice as big. The book then explains that measuring space is not absolute. Huh, how is that possible? Say there are 2 ping-pong matches. One takes place on the sidewalk, and one takes place on a bus. Say, according to the people on the bus, the ball on this match is going at 4 miles per hour. According to the the people on the sidewalk, the ball is going 44 miles per hour. Neither of them are wrong. The ball could have moved 9 feet for the people on the bus and 39 feet for the people on the sidewalk. Therefore, space is not absolute. But time is, right?
Wrong! In 1676, a Danish astronomer Ole Christensen Roemer noticed that their seemed to be a delay in the timing of Jupiter's moon. He argued that this was because light seemed to have actually taken time to reach Earth. Until then, everyone thought light had an infinite speed. In 1865 a British physicist James Clerk Maxwell made a theory of electricity and magnetism which predicted that light should have a fixed speed. But absolute space and speed were gotten rid of because of Newton, as mentioned above. It was then suggested that a substance called "Ether" that was everywhere, even in "empty space." Light would travel through the ether like sound waves through air, and the speed should be relative to the ether. Earths was orbiting through the ether so the speed of light in the direction of our revolution should be higher than if measured in the right angle of the revolution.
In 1887, the Americans Albert Michelson and Edward Motley timed light in the direction of the revolution and of right angles and found out that the times were the same! A bunch of ether supporters tried to say why they turned up with these results, but in 1905, Einstein created the death blow to ether. He said that ether completely unnecessary if we abandon the idea of absolute time. He said that the speed of light is fixed, regardless of the viewer. This idea lead to the famous e=mc2. It means that energy is mass accelerated to the speed of light. It also means that the closer you get to the speed of light, the more mass the object gains and the more energy you need to feed in. Say when you reach 10% the speed of light the objects mass will increase by 0.5 but when you reach 90% the mass will double, including all the mass accumulated till 89%. When the object reaches the speed of light, it will have infinite mass so you will need an infinite amount of energy which is impossible. Therefore all regular objects are limited and cannot go the speed of light.
Then the book talks about the four dimensions and how you can graph the relations between them: length, width, height, and time. You can use time-space graphs and light cones. Einstein explained that gravity is a result of the fact that space-time is not flat. It can be bent if an object's mass is big in enough! It can even bend light. For an example, if a distant star's light is shining close enough the sun, Earth will see the star in a different place than it really is. This was proved during a solar eclipse in 1919 by a British team. Since light is a wave that gets weaker and weaker the farther away from it's source, Einstein predicted that time appears to be slower the closer you are to the ground. This was proven in 1967 when a tower was built with a clock on the bottom and a clock at the top. The upper one slowly became ahead of the smaller one. If time and space are not absolute, this brought up questions like, did the universe have a beginning, or will it have an end.
