A Brief History and Philosophy of Physics

A Brief History and Philosophy of Physics, updated 2001; by Alan J. Slavin
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  A Brief History and Philosophy of Physics Trent University : by Alan J. Slavin, epartment o! hysics, #rent $niversity, August 1994 Updated 15 June 21 Introduction #his brie! history and philosophy o! physics has been written to %ive physics students some appreciation o! where their discipline has come !rom, and o! the philosophical principles underpinnin% it. &t is hoped that this will provide students with a sense o! physics as a livin%, evolvin% discipline, ando! their place in its evolution. hysics, indeed all o! science, is not a static a%%lomeration o! proven !acts and inviolable theories. 'hile there are many theories which are so well tried that they are %enerally accepted as bein% correct, all scienti!ic theories are still open to attac( !rom some new, reproducible e)periment which disa%rees with them. #he history below bears this out. *urthermore, while science  per se  may be value+less, neither %ood or bad, the teachin% or application o! science always has values attached. &! it is tau%ht by a scientist without any mention o! the need to use the learnin% responsibly, then students may assume that scientists need not be concerned about the application o! science. &! this happens, then the scientist ultimately abdicates to the politician or manu!acturer the decision on the use o! her or his own wor(, even thou%h it is unli(ely that either the  politician or manu!acturer will understand as well as the scientist the e!!ects o! its use. y this, & am not su%%estin% that scientists are the only ones -uali!ied to decide on the application o! science scientists can also be blind to the potential in what they do. &t is hoped that this paper will contribute tothe ability o! students to as( the necessary -uestions re%ardin% the science they and others participate in, both now and throu%hout their lives. #his summary is desi%ned to outline the %eneral development o! the main branches o! physics as we (now them today. &t is presented here as occurrin% in a !airly linear !ashion, and discusses only the  principal !i%ures in each area. owever, it must always be remembered that there were a %reat many more people wor(in% on these problems than mentioned here, with many o! them bein% unaware o! thewor( o! the others. As a result, many o! these areas pro%ressed in a more+or+less 0random wal(0  between theory and e)periment until about the last two hundred years, when improved communications made it much easier to (eep up to date with developments world+wide. 1iven the !act that hal! the world2s population is !emale, there is a notable absence o! women in this history. #his is lar%ely because women have been systematically e)cluded !rom science over the centuries until very recently, with !ew e)ceptions. 3ven when women did ma(e ma4or contributions as  part o! a lar%er team in relatively recent times, as was the case o! the women 0computers0 in astronomyat arvard olle%e 6bservatory in the late 78s, usually only the male team leader %ained reco%nitionossiter;. 6ne can only mourn the loss to the discipline !rom the e)clusion o! other <arie uries, andwor( towards encoura%in% the participation o! many more women in the !uture.  Earliest Beginnings, and the Greeks eople have always been acutely aware o! the re%ularities in nature: the sun rises every day the moon appears at the same place in the s(y rou%hly every twenty+seven days, about the same as a woman2s menstrual cycle the seasons always !ollow in the same order the pattern o! the 0!i)ed0 stars =all the heavenly bodies e)cept !or the planets, sun, moon and comets> repeats itsel! at the same time every year snow!la(es all have si) points a dropped stone always !alls. &n !act, the very well+bein% o! a !amily depended until recent times on (nowin% when to plant, or when to move camp !or the ne)t season2s %ame. #his obvious order be%%ed !or e)planation, and the earliest people attributed it to a ran%e o! %ods and %oddesses who controlled the world. 'ith the 1ree(s, !or e)ample, 1aea was the earth %oddess, ?eus threw li%htnin% bolts, and Apollo drove the !iery chariot o! the sun once per day across the heavens. 0Science0 is the attempt to %ive a rational, rather than reli%ious or ma%ical, e)planation !or the order in nature. eople in di!!erent parts o! the world be%an to develop science at di!!erent times, with di!!erent emphases. As one e)ample, as early as @ .. ole, p.B; the <ayan people o! what is now <e)ico and entral America used a calendar with an accuracy e-uivalent to (nowin% the len%th o! the year to within si) seconds, and plotted the movement o! the sun, moon and planets. #hey also used a 0place system0 !or numbers =li(e our decimal place system> at the time when the omans were still usin% a new symbol !or every new power o! ten they encountered, and the <ayans employed the Cero centuries be!ore 3urope. =#he Cero was used in &ndia !rom about 85 A..> Althou%h the <ayans had recorded much o! their customs and learnin% on hundreds o! boo(s made o! beaten+bar( paper, very little remains today. #heir Spanish con-uerors systematically destroyed almost all o! this 0heathen0 literature. #he !irst 3uropean attempts to provide a rational e)planation !or the wor(in%s o! nature be%an with the1ree(s, about  .. *or e)ample, ytha%oras =58D+5 ..> and his !ollowers belon%ed to a reli%ious !raternity dedicated to the study o! numbers. #hey believed that the world, li(e the whole number system, was divided into !inite elements, an early precursor to the idea o! atoms =0atom0 means 0indivisible0>. #heir discovery o! irrational numbers such as √ D, which could not be e)pressed as a ratio o! whole numbers, was a serious threat to this system, and history tells us that they (illed the ytha%orean who released this secret to the world. #he 1ree(s Eeucippus =FBB ..>, emocritus =FBD ..> and 3picurus =@BD+DG ..> put !orward the hypothesis that matter was composed o! e)tremely small atoms, with di!!erent materials bein% composed o! di!!erent combinations o! these atoms. Aristarchus o! Samos =@7+D@ ..> is the !irst  person (nown to have proposed that the earth rotates once per year around the sun, rather than the intuitive e)planation that the sun rotates around the earth. e also attempted to calculate relative siCes !or the earth, moon and sun. owever, it was not considered necessary by the 1ree(s to test such hypotheses e)perimentally all that most o! them were loo(in% !or was a sel!+consistent e)planation o! the world based on a small number o! philosophical principles. Aristotle is %enerally credited with providin% the most comprehensive o! such e)planations. e  believed that there were !our earthly elements: earth, water, air and !ire. 3ach had its natural place determined by its wei%ht. 3arth, bein% the heaviest, 0wanted0 to be at the centre o! the universe. 'ater was above the earth, with air above water, and then !ire. #his order ma(es intuitive sense. Solid =0earthy0> bodies sin( in water i! you release air under water the air bubbles to the sur!ace and !lames  leap upward durin% burnin%. ='ood could !loat even thou%h it was a solid body, because it contained  both earth and !ire the !ire was released on burnin%.> #he !arther a body was !rom the earth, the more  per!ect it became. ence the moon was the least per!ect o! the heavenly bodies, as could be seen by its uneven appearance, while the !i)ed stars were the most per!ect o! all, and were composed o! a !i!th element =the 0-uintessence0> which had no wei%ht at all. &n Aristotle2s physics, a movin% body o! any mass had to be in contact with a 0mover0, somethin% which caused its motion, or it would stop. #his mover could either be internal as !or animals, or e)ternal as in the case o! a bowstrin% pushin% on an arrow. #he arrow was (ept in !li%ht by air displaced !rom the !ront rushin% to the bac( to !ill the vacuum le!t by the arrow. Since Aristotle said that a vacuum was impossible =0nature abhors a vacuum0>, this e)planation o! an arrow2s motion was a%ain internally consistent. owever, because the stars were without mass, once they were put in motion by a 0prime mover0 they could continue to move by themselves. #he 1ree(s spent much e!!ort tryin% to e)plain the motion o! the sun, moon, planets and stars. Since this motion also played a ma4or role in the development o! modern science, it is worth discussin% in some detail. #he stars are so !ar !rom us that their relative motions cannot be observed e)cept over timescales o! a !ew centuries. #here!ore, to someone standin% on the earth the stars appear to be !i)ed in a vast sphere, concentric with the earth. #his sphere rotates at constant speed about the earth at a rateo! 4ust more than once in twenty+!our hours, returnin% to almost the same position at a %iven time o! day once every year. Similarly, the sun and moon appear to lie on spheres, which rotate about the earth once per day and once every DG days, respectively. #he motions o! the planets appear much more complicated to an earthly observer. 'e now (now that the planets are all on orbits with di!!erent avera%e distances !rom the sun, and orbital periods that increase the !arther the planet is !rom the sun. *or e)ample Henus, 3arth2s nearest and bri%htest planetary nei%hbour, has a period o! DD5 days, compared to 3arth2s @5. #his means that as Henus ma(es its annual pil%rima%e throu%h the ni%ht s(y as viewed !rom 3arth, it occasionally moves bac(wards relative to the !i)ed stars, in 0retro%rade motion0, as its orbit carries it opposite to the direction the earth is movin%. =ence the name 0planet0, meanin% 0wanderer0.> #he 1ree(s usually described this motion usin% a device invented by 3udo)us o! nidus =B9+@5 ..>, who was apparently the !irst 1ree( to use -uantitative observation to develop a mathematical description. Iotin% that the motion o! the planets was periodic, he developed a system o! spheres each o! which carried a planet, with each sphere centred on the earth but with its a)is o! rotation !i)ed in a lar%er sphere. #his e)planation !itted with the 1ree( belie! that the circle was the most per!ect %eometrical !orm. owever, this system was appro)imate at best. Apollonius o! er%a =FDD ..> su%%ested, instead, that each planet was attached to a small sphere which, in turn, rolled on a lar%e sphere centred on the earth, with the lar%er one rotatin% rou%hly once per day. #he lar%e sphere accounted !or the daily motion o! the planet, while the small one =the 0epicycle0> e)plained the retro%rade motion. A later addition was the use o! the 0eccentric0, which allowed the centre o! rotation o! the lar%e sphere !or each planet to lie away !rom the centre o! 3arth. As the accuracy o! the mathematical description increased, so did the need !or reliable observations. #his was reco%niCed by ipparchus o! Iicea =79+7D ..> who had studied the observational records o! the earlier 1ree(s and abylonians, with the latter datin% bac( to the seventh century .. &n this process, ipparchus discovered the 0precession o! the e-uino)es0 that is, that it ta(es the sun about D seconds more to return to its position at the e-uino) every year than it does to return to its  position amon% the !i)ed stars. #o satis!y the need !or accurate data, ipparchus catalo%ued the  position and bri%htness o! 78 stars. y the time o! tolemy =85+75 A..>, who observed at  Ale)andria in 3%ypt, the system o! epicycles and eccentrics re-uired ei%hty circles to describe the (nown periodicities o! the heavens. 6! course, the 1ree(s did not restrict their science to physics. *or e)ample, the ippocratic oath sworn by doctors today ta(es its name !rom ippocrates o! os =FB+@GG ..>. Aristotle2s most lastin% contribution to science was in biolo%y, where he classi!ied about 5B animal species, and carried out care!ul dissections o! at least 5 di!!erent animals. Archimedes =D8G+D7D ..>, scientist+en%ineer, has  been described as one o! the three %reatest %eniuses o! all time ramer;. e invented the Archimedean screw !or raisin% water, discovered the principle o! buoyancy o! a body in a li-uid, and calculated an accurate value !or π , amon% other accomplishments. &n li%ht o! his !uture in!luence on the course o! 3uropean science, it is o! interest to loo( at Aristotle2s attitude towards the role o! women. &n his 01eneration o! Animals0 he says, 0'herever possible and so!ar as possible the male is separate !rom the !emale, since he; is somethin% better and more divine in that he; is the principle o! movement !or %enerated thin%s, while the !emale serves as their matter ... 'e should loo( upon the !emale state as it were a de!ormity, thou%h one which occurs in the ordinary course o! nature.0 *rench p.7@;. #his attitude was not shared by all 1ree(s. *or e)ample, ytha%oras admitted women to his school e-ually with men. *rench, p.7BB.; The Dark Ages, and the Translations 'ith the !all o! the oman empire about B A.., most o! the 1ree( learnin% was lost to 3urope as itentered the ar( A%es. 3ven the (nowled%e that the 3arth was round, (nown to the 1ree(s who had a %ood estimate !or its diameter, was replaced by the conception o! a !lat 3arth. =#his does not mean thatall learnin% stopped durin% the ar( A%es important technolo%ical discoveries were made durin% this  period, such as the invention o! the plou%h and the water wheel.> #he 1ree( (nowled%e itsel!, however, was not lost. &t had mi%rated into the <iddle 3ast and 3%ypt under the 1ree( and oman empires, and was translated into Arabic by the people who lived in these re%ions. #he Arabs not only (ept 1ree( science alive, they added to it considerably. *or e)ample, the Arabs had important medical schools and !irst discovered the law o! re!raction, now (nown as Snell2s law. #hey also translated ma4or &ndian scienti!ic wor(s into Arabic, and be%an to use the numerals and al%ebra developed in &ndia. Al+attani =F858+9D9 A..> measured a value !or the precession o! the e-uino)es that was more accurate than tolemy2s. #he Arabs also transported the art o! paper+ma(in% !rom hina to the west. #heir contribution remains enshrined in Arabic words which we still use today, includin% al%ebra and al%orithm. 'hen hristians recaptured Spain in the eleventh century, the brid%e was !ormed to carry this learnin%  bac( into 3urope. A ma4or translation centre was set up in #oledo a!ter it was captured in 785, with a lesser centre in Sicily a!ter it !ell to the hristians in 797. #ranslation was done primarily into Eatin, the lan%ua%e o! learnin% in 3urope at this time. owever, most o! the translators !ocused on the 1ree( wor(s, and some Arabic and ersian wor(s remain untranslated today. The Middle Ages #he scholarly wor( in 3urope durin% the ar( A%es =rou%hly !rom the !all o! ome to the be%innin% o! the <iddle A%es, or <edieval period, about 77> had been primarily concerned with the copyin% o! church manuscripts. As a result, it was natural that as ancient learnin% be%an to reach 3urope it should
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