The Grand Design

by Stephen Hawking

we cannot solve exactly the equations governing the gravitational interactions of every atom in a person’s body with every atom in the earth. But for all practical purposes the gravitational force between a person and the earth can be described in terms of just a few numbers, such as the person’s total mass.

we cannot solve the equations governing the behavior of complex atoms and molecules, but we have developed an effective theory called chemistry that provides an adequate explanation of how atoms and molecules behave in chemical reactions without accounting for every detail of the interactions.

Although it is not uncommon for people to say that Copernicus proved Ptolemy wrong, that is not true. As in the case of our normal view versus that of the goldfish, one can use either picture as a model of the universe, for our observations of the heavens can be explained by assuming either the earth or the sun to be at rest.

neither model can be said to be more real than the other.

A model is a good model if it: Is elegant Contains few arbitrary or adjustable elements Agrees with and explains all existing observations Makes detailed predictions about future observations that can disprove or falsify the model if they are not borne out.

The fact that the past takes no definite form means that observations you make on a system in the present affect its past.

luminiferous ether,

180 billionths of a second per circuit