9780553380163
Stephen Hawking
Notes
We must accept that time is not completely separate from and independent of space, but is combined with it to form an object called space-time…an event is something that happens in a particular space and at a particular time…it is often helpful to think of the four coordinates of an event specifying its position in a four-dimensional space called space-time.
When we are looking at the universe we are seeing as it was in the past. …general relativity predicts that light should be bent by gravitational fields…light from a distant star that happened to pass near the sun would be deflected through a small angle, causing the star to appear in a different position to an observer on earth.
Consider a pair of twins…if one of the twins went for a long trip in a spaceship at nearly the speed of light. When he returned, he would be much younger than the one who stayed on earth. This is known as the twins paradox, but is a paradox only if one has the idea of absolute time at the back of one’s mind. In the theory of relativity there is no unique absolute time, but instead each individual has his own personal measure of time that depends on where he is now and how he is moving.
According to the theory of relativity, nothing can travel fast than light. Thus if light cannot escape, neither can anything else; everything is dragged back by the gravitational field. So one has a set of events, a region of space-time, from which it is not possible to escape to reach a distant observer. The region is what we call a black hole. Its boundary is called the event horizon and it coincides with the paths of lights rays that just fail to escape from the black hole.
…we must remember things (experience them) in the order in which entropy increases.
…a strong thermodynamic arrow is necessary for intelligent life to operate. In order to survive, human beings have to consume food, which is an ordered form of energy. Thus intelligent life could not exist in the contracting phase of the universe…however there are at least three arrows of time that do not distinguish the past from the future. They are the thermodynamic arrow – the direction fo time in which disorder increases, the psychological arrow – the direction of time in which we remember the past and not the future, and the cosmological arrow – the direction of time in which the universe expands rather than contracts.
Anthropic Principle
Black Hole
Doppler Effect
Singularity
Uncertainty Principle
Wormhole