reflect deeply on the course materials. please submit a journal entry on what most struck you from the readings. Thisis nota summary of the readings. Journaling about what you have read will provide an opportunity for you to critically and personally engage the material, discover what resonates with you, and construct what it means to you, using the classroom materials as a foundation. This may mean you treat each piece in turn, may mean you concentrate on concepts that cut across the readings, or may mean you focus on the one piece or idea that you found most compelling. You should pay particular attention to connecting theory with practice, and strong responses will grapple with issues on both an intellectual and practical level. Written reflections should conclude with no less than three points for in-class consideration and discussion. Pieces that are excessively short, excessively long, or fail to reflect critical thinking are not acceptable.
) Broad Points of View PERCY BRIDGMAN Whatever may be one's opinion as to our permanent acceptance of the analytical details of Einstein's restricted and general theories of relativity, there can be no doubt that through these theories physics is per- manently changed. It \vas a great shock to discover that classical concepts, accepted unquestioningly1 were inadequate to meet the actual situation, and the shock of this discovery has resulted in a critical attitude toward our whole conceptual structure which must at least in part be permanent. Reflection on the situation after the event shows that it should not have needed the new experimental facts which led to relativity to convince us of the inadequacy of our previous con- cepts, but that a sufficiently shrewd analysis should have prepared us for at least the possibility of what Einstein did. Looking now to the future, our ideas of what exter- nal nature is will always- be subject to change as we gain new experitnental knowledge, but there is a part of our attitude to nature which should not be subject to future change, namely that part which rests on the permanent basis of the Character of our minds. It is precisely here, in an itnproved understanding of our mental relations to nature, that the permanent contri- bution of relativity is to be found. We should now 1nake it our ~siness to understand so thoroughly the character of our permanent mental relations to nature that another change in our attitude, such as that due to Einstein, shall be forever impossible. It was perhaps excusable that a revolution in mental attitude should occur once, because after all physics is a young sci- 490 ence, al).d physicists have been very busy, but it would certainly be a reproach if such a revolution should ever prove necessary again. NEW KINDS OF EXPERIENCE ALWAYS POSSIBLE The first lesson of our recent experience with relativ- ity is merely an intensification and emphasis of the lesson Which all past experience has also taught, namely, that when experiment is pushed into new domains, we ffii.iStbe prei)maT~ fact-~Or .. an I Cii.tirely cnrrerent-character-rr-01ntJiose6ro1;-rTormer experience. This is taught not only by the discovery of those unsuspected properties of matter moving with high velocities, which inspired the theory of relativity, but also even more emphatically by the new facts in the q~anturn dOmain. To a certain extent, of course, the recognition of all this does not involve a change of former attitude; the fact has always been for the physicist the one ultimate thing from which there is no appeal, and in the face of which the only possible attitude is a humility almost religious. The new fea-t .: ' ture in the present situation is an. intensifie4_._s:_®Yi£. ·. tiQn that in reality new orders of 'CXperie~-~ do exist a~d that ~~rn~-~~-~-t.taJn..~iliheii:QbM1ili.11Y; W Source: From "Broad Poieo ·ew" in The Logic of Modern Phys· (pp. ~-~2) by P. Bridgma , 1927, ew York: Macmillan. Reprinted'. perm1ss1011. lections from Logical Positivism and Its Derivatives :\1,ave already encountered new phenomena in going to --)high velocities, and in going to small scales of magni- tude: we may similarly expect to find them, for exam- in dealing with relations of cosmic magnitudes, or dealing vvith the properties of matter of enonnous de,nsiitie'5, such as is supposed to exist in the stars. Implied in this recognition of the possibility of ne\V experience beyond our present range, is the recogni- tion that no element of a physical situation, no matter how apparently irrelevant or trivial, may be dismissed as without effect on the final result until proved to be without effect by actual experiment. The attitude of the physicist must therefore be one of pure empiricism. He recognizes no a priori princi- ples which determine or limit the possibilities of new experience. Experience is determined only by experi- ence. This practically means that we must give up the demand that all nature be embraced in any formula, either simple or complicated. It may perhaps turn out eventually that as a matter of fact nature can be embraced in a formula, but we must so organize our thinking as not to demand it as a necessity. THE OPERATIONAL CHARACTERS OF CONCEPTS Einstein's' Contribution in Changing Our Attitude Toward Concepts Recognizing the essential unpredictability of experi- ment beyond,. our present range, the i)hysici~t, if he is to escape continually revising his attitude, tnust use in describing and correlating nature concepts of such a character that our present experience does not exact hostages of the future. Now here it seems to me is the greatest contribution of Einstein. Although he himself does not explicitly state or emphasize it1 I believe that a study of what he has done will show that he has essentially modified our view of what the concepts useful in physics are and should be. Hitherto many of the concepts of physics have been defined- in tenns of their properties. An excellent exa1nple is afforded by Newton's concept of absolute time. The following quotation fro1n the Scholiu1n in Book I of the Prin- cipia is ilh1n1inating: I do not define Time, Space, Place or Motion, as being well known to all. Only I 1nust observe that the vulgar conceive those quafitities under no other notions but fro1n the relation they bear to sensible 491 objects. And thence arise certain prejudices, for the removing of which, it will be convenient to distin- guish them into Absolute and Relative, 'T'rue and Apparent, Mathematical and Co1nmon. (I) Absolute, True, and Mathematical 1'ime, of itself, and from its own nature flows equably with- out regard to anything external, and by another narne is called Duration. Now there is no assural}Ce whatever that there exists in nature anything with properties like those assumed in the definition, and physics, when reduced to concepts of this character, becomes as purely an abstract science and as far removed fro1n reality as the abstract geo1netry of the inathen1aticians, built on pos- tulates. It is a task for experirnent to discover whether concepts so defined correspond to anything in nature, and we rnust always be prepared to find that the con- cepts correspond to nothing or only partially corre- spond. In particular, if we examine the definition of absolute time in the light of experiment, we find noth- ing in nature with such properties. The new attitude toward a concept is entirely dif- ferent. We may illustrate by considering the concept o~iliJ what do we mean by the length of an object? We evidently know what we mean by length if we can tell what the length of any and every obj~ct is, and for the physicist nothing more is required. yfo find the length of an object, we have to perform certain physical operations. The concept of length is therefore fixed when the operations by which length is n1ea- sured are fixed: that is, the concept of length involves as much as and nothing inore than the set of opera- tions by which length is determined. In general, we mean by any concept nothing more than a set of oper- ations; the concept is syno~ous with the correspond- ing set of operations.lfthe concept is physical, as of length, the operations are actual physical operations1 na1nely1 those by which length is n1easured; or if the concept is mental, as of mathematical continuity, the operations are mental operations, namely those by which we determine whether a given aggregate of 1nagnitudes is continuous. It is not intended to itnply that there is a hard and fast division between physical and mental concepts, or that one kind of concept does not always contain an eletnent of the other; this classi- fication of concept is not itnportant for our future considerations. We 1nust detnand that the set of operations equiva- lent to any concept be a unique set, for otherwise I :i II I 492 Appendix there are possibilities of a1nbiguity in practical applica~ tions which \VC cannot adn1it. Applying this idea of "concept" to absolute ti1ne, we do not understand the 1neaning of absolute tin1e unless we can tell how to detcrn1ine the absolute tin1c of any concrete event, i.e., unless \"lC can n1easure absolute tilne. Now we n1erely have to exan1ine any of the pos~ sible operations by which we rneasure tirne to see that all such operations are relative operations. 1'herefore the previous state1nent that absolute tirne does not exist is replaced by the staternent that absolute tilne is 1neaningless. And in n1aking this staten1ent \Ve are not saying something new about nature, but are 1nerely bringing to light itnplications already contained in the physical operations used in 1ncasuring ti1ne. It is evident that if we adopt this point of view toward concepts, namely that the proper definition of a concept is not in tenns of its properties but in tenns of actual operations, we need run no danger of having to revise our attitude toward nature. For if experience is always described in tern1s of experience, there 1nust always be correspondence betv.,reen experience and our description of it, and we need never be einbar- rasscd, as we were in attetnpting to find in nature the prototype of Newton's absolute titne. Furthern1ore, if we remember that the operations to vv'hich a physical concept are equivalent are actual physical operations, the concepts can be defined only in the range of actual experiment, and are undefined and ineaningless in regions as yet untouched by experilnent. It follows that strictly speaking we cannot make statements at all about regions as yet untouched, and that when we do inake such statements, as we inevitably shall, we are making a conventionalized extrapolation, of the loose- ness of which we must be fully conScious, and the jus- tification of which is in the experiment of the future. 1'here probably is no staternent either in Einstein or other writers that the change described above in the use of "concept" has been self-consciously n1ade, but that such is the case is proved, I believe, by