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LVII. The Common Sense of Relativity. 

 By Norman Campbell *. 



THE Principle of Relativity has been discussed so often 

 and in so many ways that it is perhaps presumptuous to 

 attempt to add anything to the discussion except by offering 

 original developments. But it appears to me that the needs 

 of " the man in the laboratory " — to paraphrase a convenient 

 modern expression — have been insufficiently considered by 

 expositors. He has been offered profound mathematical 

 investigations, which are intensely important and interesting, 

 but tend to obscure the fundamental points at issue in the 

 mind of one who thinks physically rather than mathematically. 

 And on the other hand he has been offered collections 

 of apparently paradoxical conclusions deduced from the 

 Principle, which are sometimes elegant and entertaining, 

 but more often fallacious. As a natural result he is inclined 

 to think that this new development of science, the most 

 important, in my opinion, since the days of Newton, is 

 extremely abstruse and incomprehensible. In the following 

 pages T desire to attempt to remove this misconception and 

 to show that the view of the relations of moving systems 

 adopted by the Principle of Relativity is very much simpler 

 than that which it displaces, and that all its apparent 

 difficulties are due to confusions of thought and mis- 

 apprehensions. A few of the observations offered may be of 

 interest to those who have studied the matter deeply, but it is 

 to those whose knowledge is superficial that these remarks are 

 primarily addressed. 



As a basis of the discussion the admirable authoritative 

 summary given by the original propounder of the Principle 

 himself will be used (Einstein, Jahrbuch der Radioaktivitat, 

 Bd. iv. pp. 411 &c, 1907). The notation used there will be 

 adopted without explanation. 



I. The Nature of the Principle. 



2. Let us first inquire exactly what the Principle of 

 Relativity is and what it asserts. 



The Principle is what is more often termed a " theory " — 

 that is to say, it is a set of propositions from which 

 experimental laws may be logically deduced. It can be 

 proved to be true or false in a manner convincing to 

 everybody only by comparing the laws so deduced with those 

 found experimentally ; but a theory which never conflicted 



* Communicated by the Author. 



