“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.”
Source: A Brief History of Time (1988), Ch. 1
Context: 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.
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Stephen Hawking 122
British theoretical physicist, cosmologist, and author 1942–2018Related quotes

Ce n'est point l'observation mais la théorie qui m'a conduit à ce résultat que l'expérience a ensuite confirmé.
explaining how he was led to discover the law characterizing interference fringes, in [Œuvres complètes d'Augustin Fresnel, Imprimerie impériale, 1866, http://books.google.com/books?id=3QgAAAAAMAAJ, 61]

Youtube, Other, Reason Rally Ra Rant https://www.youtube.com/watch?v=isrST6wOUJA (March 28, 2012)

Source: A Brief History of Time (1988), Ch. 1
Context: 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.

Internet Encyclopedia of Philosophy
Context: Conceptual expressions are tentative and provisional... [because] the intellectual account... are constructed theories of experience. [And he cautions us to] distinguish between the immediate experience or intuition which might conceivably be infallible and the interpretation which is mixed up with it.

BBC radio broadcast, March 28, 1949. http://www.joh.cam.ac.uk/library/special_collections/hoyle/exhibition/radio/ Reprinted in April 1949 in The Listener, a BBC magazine.

conjecture
in his Nobel Lecture http://nobelprize.org/nobel_prizes/chemistry/laureates/1996/curl-lecture.html, December 7, 1996, Dawn of the Fullerenes: Experiment and Conjecture