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F a ra da y , T h o m s o n , Ma xw e ll: L in e s o f F o rc e in t h e E t h e r

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8 . 2 2 5 / S T S . 0 4 2 , P h y s i c s i n t he 2 0 t h Ce nt u r y P r ofe s s or Da v i d K a i s e r , 9 S e pt e mbe r 2 0 2 0

1. Ph ys i cs , T h en an d N o w

2. F arada y: L i n es o f F o rce i n th e E th e r

3. T h o m s o n an d M ax w el l : Ma t h a n d Me c h a n i c s

Wh a t i s t h e w o r l d m a d e o f ?

Pa r t i c l e s o f t h e “S t a n d a r d M o d e l ”

To d a y : Pa r t i c l e s e x c h a n g i n g p a r t i c l e s i n t h e v a c u u m . ( O r m a y b e s t r i n g s ? )

19t h C e n t u r y B r i t i s h P h y s i c i s t : B olloc k s ! Wr o n g , w r o n g , a n d w r o n g . . .

Di f f e r e n t i d e a s a b o ut t h e fu n dame n tal e n ti ti e s th a t m a k e u p th e w o r l d — an d y e t w e s t i l l u s e “ t h e i r ” e q u at i o n s !

Wh o p u r s u e s p h y s i c s , a n d w h e r e ?

Dur i n g t h e f i r s t h a l f o f t h e 1 9 t h cen t ur y , p eo p l e w h o w o r k ed o n p h ys i cs al s o wo r k e d o n c h e m i s t r y , m a t h e m a t i c s , a s t r o n o m y , p h y s i o l o g y , a l l g r o u p e d u n d e r t h e te r m , “ na tu r a l philosoph y . ”

Bi g s h i f t t o w a r d r e s e a r c h c o n d u c t e d i n u n i v e r s i t i e s be g inning a r ou nd 1 8 5 0 .

Mu c h o f t h i s r e s e a r c h w a s not done in un i v er s i t i es , b ut i n s e p a r a t e “ i n s t i t ut es ” o r “a c a d e m i e s , ” s e p a r a t e d f r o m t e a c h i n g . N o t o n l y we r e m o s t n a t u r a l p h i l o s o p h e r s n o t “ p r o f e s s o r s , ” som e pr om ine nt one s ha d not e v e n a tte nd e d un i v er s i t y t h ems el v es !

Mi c h a e l F a r a d a y i n h i s l a b o r a t o r y a t t h e R o y a l I n s t i t u t i o n , Lond on, 1 8 3 0 s

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Wh o p u r s u e s p h y s i c s , a n d w h e r e ?

Dur i n g t h e f i r s t h a l f o f t h e 1 9 t h cen t ur y , p eo p l e w h o w o r k ed o n p h ys i cs al s o wo r k e d o n c h e m i s t r y , m a t h e m a t i c s , a s t r o n o m y , p h y s i o l o g y , a l l g r o u p e d u n te r m , “ na tu r a l philosoph y . ”

a s not done in un i v er s i t i es , b ut i n s e p a r a t e “ i n s t i t ut es ” o r “a c a d e m i e s , ” s e p a r a t e d f r o m t e a c h i n g . N o t o n l y we r e m o s t n a t u r a l p h i l o s o p h e r s n o t “ p r o f e s s o r s , ” som e pr om ine nt one s ha d not e v e n a tte nd e d un i v er s i t y t h ems el v es !

Bi g s h i f t t o w a r d r e s e a r c h c o n d u c t e d i n u n i v e r s i t i e s be g inning a r ou nd 1 8 5 0 .

Mi c h a e l F a r a d a y i n h i s l a b o r a t o r y a t t h e R o y a l I n s t i t u t i o n , Lond on, 1 8 3 0 s

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Q u e s tion s ?

Mi c h a e l F a r a d a y becam e one of t he m ost su ccessf u l and f am ou s nat u r al phi l osopher s of t he 19t h cent u r y: a r em ar kabl e “r ags t o ric h e s ” s to r y . H e w a s b o r n in to a p o o r fa m ily a n d n e v e r a tte n d e d hi gh sc hool ( l et al one col l eg e) .

Ap p r e n t i c e d t o a book bi nder at ag e 1 3 .

He m e t t h e f a m o u s c h e m i s t Hu m p h r e y Da v y wh e n Da v y g a v e a s e r i e s o f p u b l i c le c t u re s o n n a t u ra l p h ilo s o p h y a t t h e Ro y a l I n s t i t u t i o n in L o n d o n .

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excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/

Fa r a d a y t o o k c a r e f u l n o t e s o n D a v y ’ s l e c t u r e s , b o u n d t h e m i n t o a f a n c y b o o k , a n d p r e s e n t e d th e b o o k to D a v y as a g i ft. ( Hi n t : g o o d w a y t o g e t a UR O P . . . ) D a v y to o k F arad a y o n as an as s i s tan t

— anothe r m an u al - skil l s appr ent i ceshi p .

Ev e n t ua l l y D a v y s t e p p e d d o w n a n d F a r a d a y b e c a m e di r ect or of t he R o ya l I nst i t ut i on. H e w or k ed on ch e m i s t r y , o p t i c s , e l e c t r i c i t y , m a g n e t i s m , e l e c t r o l y s i s . . . To F a r a d a y , t h e s e w e r e a l l e l e m e n t s o f a si ng l e , uni f i ed appr oac h to natu r e . H e w as fasc i nate d b y the in te r c o n v e r tib ility of f or ces .

Fa r a d a y i n h i s R o y a l I n s t i t u t i o n l a b o r a t o r y , 1 8 4 0 s

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Pa r t l y i n s p i r e d b y h i s re l i g i o u s b a c k g ro u n d : h e b e lo n g e d to a s m al l P ro te s tan t s e c t c al l e d th e Sa n d e m a n i a n s , w h o em pha si zed a n u nder l yi ng u ni t y t o a l l of na t u r e — al l peopl e and t hi ngs ar e ( som eho w ) connect ed t o al l ot her s .

Fa r a d a y d e l i v e r i n g R o y a l I n s t i t u t i o n “ C h r i s t m a s L e c t u r e , ” 1 8 5 5

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Mu c h o f F a r a d a y ’ s w o r k c e n t e r e d a r o u n d t h e et h er : a n a ll - per v asi v e m edi u m f i l l i ng space , w hi c h enabl ed di st i nct ph ysi cal pr ocesses t o af f ect one anot her ( i nt er con v er t i bi l i t y of f or ces) . W hat w as t he et her ?

Th o m a s Y o u n g , c a . 1 8 0 0 : wa v e t h e o r y o f l i g h t

If l i g h t c o n s i s te d o f w a v e s , wh a t we r e t h e s e w a v e s m a d e o f ? W h a t we r e th e y w a v e s in ?

Th e y m u s t h a v e b e e n w a v e s i n t h e

lu m in i fe r o u s e t h e r ,

lig h t ca r r yi n g

th e me d i u m in w h ic h lig h t t ra v e le d .

Wh y s h o u l d l i g h t n e e d a me d i u m ?

Fr e n c h p h y s i c i s t s h a d f o u n d c a . 1 8 0 0 t h a t t h e speed of lig h t d e p e n d s o n t h e m e d iu m in w h ic h it t ra v e ls ( s lo w e r in w a t e r t h a n a ir) .

n : in d e x o f re fra c t io n

Th e e t h e r m u s t b e t h e re fe re n c e m e d iu m , w ith re s p e c t to w h ic h a n y lo c a l ch a n g e s a r e m e a s u r e d : n 0 .

In du c t io n

So m u c h f or l i ght . W ha t a bou t el ect r i ci t y a nd m a gnet i sm ?

In 1 8 2 0 , w h i l e g i v i n g a p u b l i c l e c tu re , th e D an i s h nat u r al phi l osopher Ha n s C h r i s t i a n O e r s t e d ac c i d e ntal l y di sco v er ed t ha t el ect r i c cur r ent f l o w i ng nea r a ma g n e t i c c o mp a s s wi l l d e f l e c t t h e c o mp a s s n e e d l e .

El e ct ri c cu r r en t s co u l d in d u c e ma g n e t i c effect s !

Th i s b e c a m e h e a d l i n e n e w s t h r o u g h o u t E u r o p e . * T o Fa r a d a y , t h i s s e e m e d t o b e e x a c t l y t h e k i n d o f in te r c o n v e r s io n of for ces th at h e w as s o i n te re s te d i n . W i th i n m o n th s , F arad a y had r e pl i cat ed O er st ed ’ s f i ndi ng, and al so di sco v er ed t he in v e r s e : Mo v i n g ma g n e t s cou l d i ndu c e a n el ect ri c cu r r ent .

* t r e n d i n g : # T h e N e e d l e M o v e d

L in e s o f F o rc e

Wh i l e i n v e s t i g a t i n g i n d u c t i o n , F a r a d a y b e g a n t o s k e t c h h e l p f u l d o o d l e s : lin e s of for ce , w h ic h h e th o u g h t re p re s e n te d s tate s o f te n s io n in t h e u n d e rly in g et h er .

Th e num ber of l i nes N ra d ia tin g o u tw a rd fro m a posi t i v e c har g e ( or i nw ar d t o w ar d a neg at i v e c har g e) wa s p r o p o r t i o n a l t o t h e m a g n i t u d e o f t h e c h a r g e .

Th e densi t y of f i el d l i nes w ou l d f al l of f as 1/ r 2 : N fi e l d l i n e s w o u l d i n te rs e c t a s p h e re at an y g i v e n ra d iu s r , s o t h e num ber densi t y wo u l d s c a l e l i k e N / A , wi t h A = 4 π r 2 .

Re p r o d u c e d Co u l o mb ’ s l a w fro m e l e c tro s tati c s :

L in e s o f F o rc e

Wh i l e i n v e s t i g a t i n g i n d u c t i o n , F a r a d a y b e g a n t o s k e t c h h e l p f u l d o o d l e s : lin e s of for ce , w h ic h h e th o u g h t re p re s e n te d s tate s o f te n s io n in t h e u n d e rly in g et h er .

Th e num ber of l i nes N ra d ia tin g o u tw a rd fro m a posi t i v e c har g e ( or i nw ar d t o w ar d a neg at i v e c har g e) wa s p r o p o r t i o n a l t o t h e m a g n i t u d e o f t h e c h a r g e .

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Th e densi t y of f i el d l i nes w ou l d f al l of f as 1/ r 2 : N fi e l d l i n e s w o u l d i n te rs e c t a s p h e re at an y g i v e n ra d iu s r , s o t h e num ber densi t y wo u l d s c a l e l i k e N / A , wi t h A = 4 π r 2 .

Ma g n e t i c lin e s o f f o rc e w o u ld d i v e rg e f ro m a N o r t h pol e and con v er g e at a Sou t h pol e . A f u r t her br eak fro m N e w to n : fo rc e s d i d no t al w a y s ac t al ong shor te st - di st a nce l i nes bet w een t w o poi nt s .

13

L in e s o f F o rc e

Wh i l e i n v e s t i g a t i n g i n d u c t i o n , F a r a d a y b e g a n t o s k e t c h h e l p f u l d o o d l e s : lin e s of for ce , w h ic h h e th o u g h t re p re s e n te d s tate s o f te n s io n in t h e u n d e rly in g et h er .

Fa r a d a y i n t r o d u c e d t h e i d e a o f a fi e ld . T he e l e c t r i c an d m ag n e t i c f i e l ds de s c r i b e d t h e st a t e o f t h e et h er , a s re p re s e n t e d b y l o c a l l i n e s o f f o rc e : w h a t w e re t h e st r esses an d te n s i o n s in th e eth er a t a p a r ticu la r lo ca tio n , an d h o w di d t h o s e c h an g e o v e r t i m e ?

Image is in the public domain.

Ma g n e t i c lin e s o f f o rc e w o u ld d i v e rg e f ro m a N o r t h pol e and con v er g e at a Sou t h pol e . A f u r t her br eak fro m N e w to n : fo rc e s d i d no t al w a y s ac t al ong shor te st - di st a nce l i nes bet w een t w o poi nt s .

14

B u t W a s It R e a l?

In 1 8 4 5 , F arad a y th o u g h t o f a w a y to d e m o n s trate th at th e e th e r w as a re al , p h y s i c al su bst ance: he f ou nd t hat a m agnet i c f i el d w i l l ro t a t e th e p l an e o f p o l ari zati o n o f l i g h t. (N o w k n o w n as th e “ F arad a y e ffe c t” o r “ F arad a y ro tati o n . ” )

To F a r a d a y a n d h i s p e e r s , t h i s w a s s t r i k i n g evi dence t ha t t he et her m u st be r ea l : t he B fi e l d wa s a tw is tin g s tra in in t h e e t h e r ( a k in d o f to rs i o n ), an d i t d e fo r m e d th e m ate ri al m e d i u m wi t h i n wh i c h l i g h t t r a v e l e d .

Cf . D as c h n e r , K ai s e r , an d F o r m ag g i o , “ E x p l o i t i n g F ar ad a y r o t at i o n t o j am q u an t u m k e y d i s t r i b u t i o n vi a p o l ar i z e d p h o t o n s , ” Qua nt um I nf o r ma t i o n a nd Co mp ut a t i o n 19 ( 2019) : 1313 - 1324, ar Xi v: 1905. 01359.

Fa r a d a y S u m m a r y

Mi c h a e l F a r a d a y w o r k e d o n a v a r i e t y o f n a t u r a l p h e n o me n a ( c h e mi s t r y , o p t i c s , el ect r ol ysi s , el ect r i ci t y , m a gnet i sm . . . ) w i t h a speci a l i nt er est i n t he in te r c o n v e r tib ility o f for ce s — as hi s S and e m ani an r e l i g i on e m phasi ze d , the r e w as an e sse nti al uni t y o f nat ur e .

He w a s n e v e r t r a i n e d i n m a t h e m a t i c s , b u t w a s a n a p p r e n t i c e d t i n k e r e r wh o b e c a m e a le a d in g e x p e rim e n t a lis t , e v e n t u a lly w o rk in g a t t h e R o y a l I n s t it u t io n ( ra t h e r t h a n a t a un i v e r s i t y ) .

He i n t r o d u c e d lin e s of for ce and the n fie ld s as w a y s to c har ac te r i ze the state of the al l - per v asi v e et h er , t h e m e d iu m w it h in w h ic h n a t u ra l fo rc e s u n fo ld e d , a n d t h e re a s o n fo r an u nd e r l y i ng u ni t y .

In p l ac e o f N e w to n i an “ ac ti o n at a d i s tan c e , ” F arad a y ai m e d to e xp l ai n al l p h y s i c al phenom ena i n t er m s of l ocal fie ld s , w h ic h c o u ld c o n v e y e ffe c t s p o in t - by - poi nt th ro u g h o u t th e e th e r .

Q u e s tion s ?

Fa r a d a y u s e d v e r y l i t t l e f o r m a l m a t h e m a t i c s i n h i s st u di es — not su r pr i si ng, si nce he nev er st u di ed ma t h b e y o n d e l e me n t a r y g e o me t r y . A t r e me n d o u s shi f t beg an i n t he m i d - 19t h cent ur y: a new cul t ur e of r esear c h cent er ed ar ou nd i nt ense m at hem at i cal trai n i n g (e s p e c i al l y at C am b ri d g e U n i v e rs i ty ).

Tw o p r o d u c t s o f t h a t n e w t r a i n i n g — Wi l l i a m Tho m s o n (l ate r Lo r d K e l v i n ) an d Ja m e s C l e r k M a x w e l l

— pi c k ed u p F ar ada y ’ s pi ct or i al w or k and t r ansl at ed it in t o a p o w e rf u l m a t h e m a t ic a l f o r m a lis m . B o t h

Cam b r i d g e U n i v e r s i t y , K i n g ’ s P ar ad e , c a. 1870 we r e S c o t t i s h b o r n a n d e d u c a t e d a t C a m b r i d g e i n

Image is in the public domain. th e 1 8 4 0 s an d 1 8 5 0 s .

Bo t h T h o m s o n a n d M a x w e l l b e c a m e d e e p l y i n v o l v e d wi t h p r a c t i c a l e n g i n e e r i n g a p p l i c a t i o n s o f th e p h y s i c s o f th e e th e r , e v e n as th e y p u rs u e d fan c y m ath e m ati c al ap p ro ac h e s .

Mo s t i mp o r t a n t a p p l i c a t i o n : te le g ra p h y . B rit a in w a s a n is la n d w it h d is t a n t c o lo n ie s ; it a ls o s o u g h t to s tre n g th e n g l o b al trad i n g re l ati o n s h i p s .

Image is in the public domain.

license. For more information, see https://ocw.mit.edu/help/faq-fair-use/

Ma x w e l l , p p . v i i - vi i i : “T he i m por t a nt a ppl i ca t i ons of el ect r om agnet i sm t o te le g ra p h y ha v e al so r eact ed on pu r e sci ence b y gi vi ng a com m er ci al v alu e to ac c u rate el ect r i ca l m ea su r em ent s , a nd b y a f f or di ng t o el ect r i ci a ns t he u se of a ppa r a t u s on a scal e wh i c h

gr e a t l y t r a n s c e n d s t h a t o f a n y o r d i n a r y l a b o r a t o r y . Th e c o n s e q u e n c e s o f th is d e m a n d fo r e le ctrica l k n o w le d g e , and of the se e xpe r i m e ntal oppor tu ni ti e s for ac q u i r i ng it , h a v e b e e n a lre a d y v e r y g re a t , b o t h in s t im u la t in g th e e n e rg i e s o f ad v an c e d e l e c tri c i an s , an d i n d i ffu s i n g am ong pr ac ti c al m e n a d e g r e e of ac c u r ate k no w l e d g e wh i c h i s l i k e l y t o c o n d u c e t o t h e g e n e r a l s c i e n t i f i c pr og r ess of t he w hol e engi neer i ng pr of essi on. ”

T h o m s o n : Ma t h e m a t ic a l A n a lo g ie s

Wi l l i a m T h o m s o n re a d F a ra d a y ’ s p a p e rs a s a n u n d e rg ra d u a te . H e beg an t o f i nd t hat F ar ada y ’ s w or k on el ect r i ci t y and m agnet i sm and Ne wt o n ’ s w o r k o n me c h a n i c s c o u l d b e f o r m u l a t e d wi t h i n t h e sam e ma t h e ma t i c a l s t r u c t u r e . T h e k e y w a s t o in t ro d u c e a pot ent i al , �� .

sca l a r

(. . . i t ’ s 892 p a g e s ! )

ve c t o r ve c t o r

(. . . i t ’ s 892 p a g e s ! )

Wi l l i a m T h o m s o n re a d F a ra d a y ’ s p a p e rs a s a n u n d e rg ra d u a te . H e beg an t o f i nd t hat F ar ada y ’ s w or k on el ect r i ci t y and m agnet i sm and Ne wt o n ’ s w o r k o n me c h a n i c s c o u l d b e f o r m u l a t e d wi t h i n t h e sam e ma t h e ma t i c a l s t r u c t u r e . T h e k e y w a s t o in t ro d u c e a pot ent i al , �� .

Th e r e w a s a p r e c i s e m a t h e m a t i c a l a n a l o g y b e t w e e n el ect r i c phenom ena a nd m ec ha ni cs .

ma s s d e n s i t y  m wa s t h e s o u r c e f o r d i v e r g i n g ch a r g e d en sity  c wa s t h e s o u r c e f o r d i v e r g i n g lin e s o f th e g r a v ita tio n a l f ie ld lin e s o f th e e le c tr ic f ie ld

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“L a p l a c i a n ”:

(. . . i t ’ s 892 p a g e s ! )

Wi t h i n o n e y e a r o f F a r a d a y ’ s di sco v er y of t he “F a r a da y ef f ect , ” T hom son ha d gi v en th a t a m or e pr e c i se ma t h e ma t i c a l t r e a t me n t a s w e l l ( wh i l e s t i l l a n u n d e r g r a d u a t e ! ). H e in t ro d u c e d a vec t or pot ent i al A and a ne w ope r ator , cu rl .

Fo l l o w i n g F a r a d a y ’ s l e a d , T h o m s o n i n t e r p r e t e d h i s m a t h e m a t i c a l re s u lt me c h a n i c a l l y : t h e e t h e r w a s a n e la s t ic m e d iu m w h ic h c o u ld b e p u t un de r t e n s i o n o r s t r a i n . N e w m a t h e m a t i c a l t e c h n i que s ( l i k e cu rl ) c o u l d quant i f y th e m e d i u m ’ s re s p o n s e to p re s s u re s .

In a fam o u s s e t o f l e c tu re s i n Ba l t i m o r e , T h o m s o n e n c o u r a g e d h i s lis t e n e rs t o g e t a fe e l fo r th e n e w p h y s i c s :

(. . . i t ’ s 892 p a g e s ! )

“St i c k y ou r ha nd i n a bo w l of j el l y , a nd see ho w i t w i g gl es and vi br at es as y ou mo v e y o u r h a n d a r o u n d . ”

Tha t wa s w h a t p h y s i c s wa s a l l a b o u t !

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T h o m s o n : Ma t h e m a t ic a l A n a lo g ie s

Fo l l o w i n g F a r a d a y ’ s l e a d , T h o m s o n i n t e r p r e t e d h i s m a t h e m a t i c a l re s u lt me c h a n i c a l l y : t h e e t h e r w a s a n e la s t ic m e d iu m w h ic h c o u ld b e p u t un de r t e n s i o n o r s t r a i n . N e w m a t h e m a t i c a l t e c h n i que s ( l i k e cu rl ) c o u l d quant i f y th e m e d i u m ’ s re s p o n s e to p re s s u re s .

(. . . i t ’ s 892 p a g e s ! )

“M echa ni ca l W or l dv i ew ”: If th e ma t h wa s t h e s a m e , t h e n one cou l d m odel el ect r om agnet i sm on me c h a n i c a l mo d e l s . A n d if th o s e m o d e l s re p ro d u c e d al l th e re l e v an t p h e n o m e n a, th e n ma y b e t h e y w e r e n ’ t on ly mo d e l s . Ma y b e t h e e t h e r r e a l l y w a s ju s t a m e c h a n ic a l s y s t e m .

Ma x w e l l , “ O n F a r a d a y ’ s Li ne s of F or c e , ” 1 8 5 6

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Ja m e s C l e r k M a x w e l l wa s a f e w y e a r s y o u n g e r t h a n T h o m s o n .

He a l s o b e c a m e s t e e p e d i n t h e n e w C a m b r i d g e m a t h e m a t i c a l appr oac h, and d e v e l ope d fu r the r anal og i e s be tw e e n el ect r om a gnet i c a nd m ec ha ni ca l syst em s .

To m o d e l t h e e f f e c t s o f a m a g n e t i c f i e l d o n t h e e t h e r , h e im a g in e d t h a t t h e B fi e l d l i n e s w e re u n d e r te n s i o n , an d w e re ke p t a p a r t f r o m e a ch o t h e r by a k i n d o f h yd r o s t a t i c p r e s s u r e in a n in c o m p re s s ib le f lu id : “ m o le c u la r v o r t ic e s . ”

Be t w e e n t h e v o r t i c e s w e r e “ i d l e wh e e l s , ” wh i c h s t a y e d i n p l a c e b u t c o u l d r o t a t e a t d i f f e r e n t speeds . I f an ext er nal f or ce c hang ed t he rate at w hi c h one v or te x w as spi nni ng , that w ou l d c hang e th e rate o f s p i n o f i ts near est nei g hbo r i ng id le w h e e ls . T h o s e , in t u r n , w o u ld c h a n g e t h e ro t a t io n speed of th e ir near est nei ghbor s ( aki n t o con v ey or bel t s) . A l l loca l ef f ect s , b y w hi c h a m a gnet i c di st ur ba nce coul d pr opag at e t h r ough t h e m edi um .

Maxwe ll, p . 157: The ac t ion of one “e lec trical sy st em” on anothe r i s “e ffe ct ed, not by dire c t ac ti on at a dist anc e, bu t by me ans of a distribut ion of st re ss in a medium e xte nding continuousl y [t hroughout spac e].”

Maxwe ll, p . 164: “The distribut ion of stre ss [in the elast ic -m e diu m et he r] is pre cise ly t hat to w hich F ar aday was le d in his investigat ion o f in duc tion [ ...]. At e ve ry p oint o f t he m e dium there is a stat e of stress suc h th at t he r e is te nsion along the lines o f f orce a nd p re ssure in a ll direc t ions a t rig ht a ngles t o the se line s .”

In a re v i e w o f O l i v e r L o d g e ’ s fo l l o w - up b o o k, Mo d e r n Vi e w s o f El e c t r i c i t y (1 8 8 9 ), th e F re n c h p h y s i c i s t P i e r re D u h e m e x c l ai m e d o n th e B ri ti s h s ty l e :

“H er e i s a book i nt ended t o expou nd t he m oder n th e o ri e s o f e l e c tri c i ty [. . . ]. In i t th e re are n o th i n g b u t st r i ngs w hi c h m o v e ar ou nd pu l l eys , w hi c h r ol l ar ou nd dr um s , w hi c h g o t hr ough pea r l bea ds , w hi c h ca r r y we i g h t s ; a n d t u b e s w h i c h p u m p wa t e r w h i l e o t h e r s sw el l and cont r act ; t oot hed w heel s ar e g ear ed t o one anothe r and e ng ag e hook s . We t h o u g h t w e w e r e e n t e r i n g t h e tra n q u il a n d n e a tly o rd e r e d a b o d e o f r e a s o n , b u t w e fin d ou rselves i n a fact or y . ”

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In 1 8 6 5 , M axw e l l fi x e d s o m e p aram e te rs i n hi s m odel ( aki n t o spr i ng const ant s) , based on exper i m ent a l st u di es of ho w E and B fi e l d s beha v ed. T hen he cal cu l at ed ho w qui c k l y a me c h a n i c a l d i s t u r b a n c e w o u l d p r o p a g a t e wi t h i n hi s m odel of t he et her , and he f ou nd t hat t he fi e l d s p ro p ag ate d at t h e speed of l i gh t :

“T he v el oci t y of t r a nsv er se u ndu l a t i ons i n ou r h ypot het i ca l m edi u m [ . . . ] a g r ees so exa ct l y wi t h t h e v e l o c i t y o f l i g h t c a l c u l a t e d f r o m o p t i c a l e x p e r i me n t s [ . . . ] t h a t w e c a n s c a r c e l y a v o i d th e i n fe re n c e th at ligh t con s is ts of th e tra n s v e rs e u n d u la tion s of th e s a m e m e d iu m w h ich is th e ca u s e of el ect ri c and m agnet i c ph enom ena .”

Li g h t w a s n o t h i n g o t h e r t h a n t h e p r o p a g a t i o n o f E and B fi e l d s w i th i n th e l u m i n i fe ro u s e th e r!

Su m m a r y

Le a d i n g n a t ur a l p h i l o s o p h e r s i n t h e 1 9 t h c e n t ur y w e r e c o n v i n c e d t h a t t h e w o r l d w a s m a d e of ver y di f f er ent ent i t i es th an w h at p re s e n t - da y ph ysi ci st s bel i ev e: no t el em ent a r y pa r t i cl es i n a va c u u m , b u t me c h a n i c a l d i s t u r b a n c e s in a n a ll - per v asi v e , el ast i c me d i u m , t h e “ e t h e r . ”

Un t i l t h e m i d - 19t h cent ur y , l ea di ng na t ur a l phi l osopher s coul d ent er r esea r c h ca r eer s ba sed on an appr ent i cesh i p m odel , a n d w o rk e d in “ in s t it u t e s ” o r “ a c a d e m ie s ” ( w it h n o fo r m a l t e a c h in g fu n c ti o n ), no t at u ni v e r si ti e s .

Fa r a d a y , T h o m s o n , a n d M a x w e l l e a c h p u r s u e d anal o gi es bet w een t he l u m i ni f er ou s et her and me c h a n i c a l s y s t e ms : F a r a d a y wi t h r u d i me n t a r y g e o me t r i c a l a r g u me n t s a n d n o v e l v i s u a l a i d s , Th o m s o n a n d M a x w e l l w i t h i n c r e a s i n g l y s o p h i s t i c a t e d m a t h e m a t i c a l t o o l s . Th e y b e c a m e con vi nced of a “m ec ha ni ca l w or l dvi ew ”: el ect r om a gnet i c ef f ect s w er e at r oot me c h a n i c a l .

Mo r e o v e r , l o c a l c a u s e s y i e l d e d l o c a l e f f e c t s : n o “ a c t i o n a t a d i s t a n c e . ”

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STS.042J / 8.225J Einstein, Oppenheimer, Feynman: Physics in the 20th Century

Fall 202 0

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