Notes from A Brief History of Time
[Let’s start](http://ryskamp.org/brain/ryskampdotorg/lots-of-book-notes-coming) at the beginning: these notes are 8 years old. On my European train tour in 2000, I ran out of books to read in Nice, France. At an English-language bookstore there, I found [A Brief History of Time](http://books.google.com/books?id=A9nWaIpeXhkC) and learned about the universe for the next few days.
These are the things I flagged on the trains, in the hostels, and in the cities of Europe over the next few days, over 8 years ago.
### Notes
Hawking’s definition of a “good theory”:
> A theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations. – 10
And the “eventual goal of science”:
> The eventual goal of science is to provide a single theory that describes the whole universe. – 11
An interesting way to look at our role in the universe…
> Space and time not only affect but also are affected by everything that happens in the universe. – 36
121-126: a concise explanation of how the universe likely began; its first few hours.
Hawking gives a good explanation of the anthropic principle:
> We see the universe the way it is because we exist. – 130
> The weak anthropic principle states that in a universe that is large or infinite in space and/or time, the conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time. The intelligent beings in these regions should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are necessary for their existence. – 130-1
> [The strong anthropic principle claims that] in most…universes the conditions would not be right for the development of complicated organisms; only in the few universes that are like ours would intelligent beings develop and ask the questions: ‘Why is the universe the way we see it?’ The answer is then simple: if it had been different, we would not be here! – 131
Perhaps Hawking should read [Pilgrim by Tinker Creek](http://ryskamp.org/brain/books/notes-from-pilgrim-at-tinker-creek.html)…
> The strong anthropic principle would claim that this whole vast construction [of our universe] exists simply for our sake. This is very hard to believe…there does not seem to be any need for all those other galaxies… – 133
From Pilgrim:
> It occurs to me more and more that everything I have seen is wholly gratuitous…the sheer fringe and network of detail assumes primary importance. That there are so many details seems to be the most important and visible fact about the creation…If the world is gratuitous, then the fringe of a goldfish’s fin is a million times more so.
An interesting way to explain rapid expansion of the universe in its beginnings:
> In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero.
> Now twice zero is also zero. Thus the universe can double the amount of positive matter energy and also double the negative gravitational energy without violation of the conservation of energy. – 136
> “It is said that there’s no such thing as a free lunch. But the universe is the ultimate free lunch.” – Alan Guth – 136
How can the universe be finite and yet without end?
> Because one is using Euclidean space-times, in which the time direction is on the same footing as directions in space, it is possible for space-time to be finite in extent and yet to have no singularities that formed a boundary or edge. – 143
Similar to something [Einstein said](http://thinkexist.com/quotation/the_distinction_between_past-present-and_future/184152.html):
> The distinction between past, present, and future is only a stubbornly persistent illusion.