Thus the force between two bodies would be twice as strong if one of the bodies (say, body A) had its mass doubled. In addition to his laws of motion, Newton discovered a law to describe the force of gravity, which states that every body attracts every other body with a force that is proportional to the mass of each body. And our goal is nothing less than a complete description of the universe we live in. Humanity's deepest desire for knowledge is justification enough for our continuing quest. Today we still yearn to know why we are here and where we came from. They have craved an understanding of the underlying order in the world. But ever since the dawn of civilization, people have not been content to see events as unconnected and inexplicable. The discovery of a complete unified theory, therefore, may not aid the survival of our species.
Unfortunately, however, these two theories are known to be inconsistent with each other - they cannot both be correct. Quantum mechanics, on the other hands, deals with phenomena on extremely small scales, such as a millionth of a millionth of an inch. 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. Today scientists describe the universe in terms of two basic partial theories - the general theory of relativity and quantum mechanics. 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. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.
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.
Hawking A BRIEF HISTORY OF TIMEĪny physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it.