In order to talk about the nature of the universe and to discuss questions such as whether it has a beginning or an end, you have to be clear about what a scientific theory is. I shall take the simpleminded view that a theory is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make. It exists only in our minds and does not have any other reality (whatever that might mean). 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. For example, Aristotle believed Empedocles’s theory that everything was made out of four elements, earth, air, fire, and water. This was simple enough, but did not make any definite predictions.
On the other hand, Newton’s theory of gravity was based on an even simpler model, in which bodies attracted each other with a force that was proportional to a quantity called their mass and inversely proportional to the square of the distance between them. Yet it predicts the motions of the sun, the moon, and the planets to a high degree of accuracy.
Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory. As philosopher of science Karl Popper has emphasized, a good theory is characterized by the fact that it makes a number of predictions that could in principle be disproved or falsified by observation. Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory.
Arbitrary 任意的
Provisional 臨時的
為了談論宇宙的性質和討論諸如它是否存在開端或終結的問題，你必須清楚什麼是科學理論。我將採用頭腦簡單的觀點，即理論只不過是宇宙或它的受限制的一部分的模型，一些聯結這模型和我們所觀察的量的規則。它只存在於我們的頭腦中，（不管在任何意義上）不再具有任何其他的實在性。如果它滿足以下兩個要求，就算是好的理論：它必須在只包含一些任意元素的一個模型的基礎上，準確地描述大批的觀測，並對未來觀測的結果作出確定的預言。例如，亞里士多德關於任何東西是由四元素，土、空氣、火和水組成的理論是足夠簡單的了，但它沒有做出任何確定的預言。
另一方面，牛頓的引力理論是基於甚至更為簡單的模型，在此模型中兩物體之間的相互吸引力和它們稱之為質量的量成正比，並和它們之間的距離的平方成反比。然而，它以很高的精確性預言了太陽、月亮和行星的運動。
在它只是假設的意義上來講，任何物理理論總是臨時性的：你永遠不可能將它證明。不管多少回實驗的結果和某一理論相一致，你永遠不可能斷定下一次結果不會和它矛盾。另一方面，哪怕你只要找到一個和理論預言不一致的觀測事實，即可證偽之。正如科學哲學家卡爾·波普所強調的，一個好的理論的特徵是，它能給出許多原則上可以被觀測所否定或證偽的預言。每回觀察到與這預言相符的新的實驗，則這理論就倖存，並且增加了我們對它的可信度；然而若有一個新的觀測與之不符，則我們只得拋棄或修正這理論。
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