It turns out to be very difficult to devise a theory to describe the universe all in one go. Instead, we break the problem up into bits and invent a number of partial theories.
Each of these partial theories describes and predicts a certain limited class of observations, neglecting the effects of other quantities, or representing them by simple sets of numbers. It may be that this approach is completely wrong. If every-thing in the universe depends on everything else in a fundamental way, it might be impossible to get close to a full solution by investigating parts of the problem in isolation. Nevertheless, it is certainly the way that we have made progress in the past. The classic example again is the Newtonian theory of gravity, which tells us that the gravitational force between two bodies depends only on one number associated with each body, its mass, but is otherwise independent of what the bodies are made of. Thus one does not need to have a theory of the structure and constitution of the sun and the planets in order to calculate their orbits.
Today scientists describe the universe in terms of two basic partial theories - the general theory of relativity and quantum mechanics. They are the great intellectual achievements of the first half of this century. The general theory of relativity describes the force of gravity and the large-scale structure of the universe, that is, the structure on scales from only a few miles to as large as a million million million million (1 with twenty-four zeros after it) miles, the size of the observable universe. Quantum mechanics, on the other hand, deals with phenomena on extremely small scales, such as a millionth of a millionth of an inch. Unfortunately, however, these two theories are known to be inconsistent with each other - they cannot both be correct. One of the major endeavors in physics today, and the major theme of this book, is the search for a new theory that will incorporate them both - a quantum theory of gravity. We do not yet have such a theory, and we may still be a long way from having one, but we do already know many of the properties that it must have. And we shall see, in later chapters, that we already know a fair amount about the predications a quantum theory of gravity must make.
in one go 一口氣
畢全功於一役地設計一種能描述整個宇宙的理論，看來是非常困難的。反之，我們是將這問題分成許多小塊，併發明許多部分理論。
每一部分理論描述和預言一定有限範圍的觀測，同時忽略其他量的效應或用簡單的一組數來代表之。可能這方法是全錯的。如果宇宙中的每一件東西都以非常基本的方式依賴於其他的任何一件東西，很可能不能用隔離法研究問題的部分去逼近其完備的答案。儘管如此，這肯定是我們在過去取得進展所用的方法。牛頓引力理論又是一個經典的例子，它告訴我們兩個物體之間的引力只決定於與每個物體相關的一個數——它的質量；而與物體由何物組成無關。這樣，人們不需要太陽和行星結構和成份的理論就可以計算它們的軌道。
今天科學家按照兩個基本的部分理論——廣義相對論和量子力學來描述宇宙。它們是本世紀上半葉的偉大的智慧成就。廣義相對論是描述引力和宇宙的大尺度結構，也就是從只有幾英哩直到大至1億億億（1後面跟24個0）英里，即可觀測到的宇宙範圍的尺度的結構。另一方面，量子力學處理極小尺度的現象，例如萬億分之一英寸（1英寸=2.54釐米）。然而，可惜的是，這兩個理論不是互相協調的——它們不可能都對。當代物理學的一個主要的努力，以及這本的主題，即是尋求一個能將其合併在一起的理論——量子引力論。我們還沒有這樣的理論，要獲得這個理論，我們可能還有相當長的路要走，然而我們已經知道了這個理論所應具備的許多性質。在以下幾章，人們將會看到，我們已經知道了相當多的量子引力論所應有的預言。
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