E in s t e i n a n d E xp e rim e n t
© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/
8. 225 / ST S . 042, P h y s i c s i n t h e 20t h C e n t u r y Pr of e s s or Da v id K a is e r , 2 1 S e pte mbe r 2 0 2 0
1. Ei n s t e i n i n 1905: Ki n e ma t i c s b e f o r e Dy n a mi c s
2. R e l a t i vi t y o f S i m u l t a n e i t y *
3. Ei n s t e i n ’ s Ex p e ri m e n t a l Contex t
* S e e Le c t u r e N o t e s on “ E = m c 2 ”
W a v e s in t h e E t h e r : R e c a p
Image is in the public domain.
We s a w i n t h e l a s t l e c t u r e t h a t q u e s t i o n s a b o u t l i g h t w a v e s i n t h e e t h e r , a n d e f f o r t s t o d e t e c t th e E ar th ’ s m o ti o n th ro u g h th e e th e r , w e re at th e c e n te r o f m ai n s tre am p h y s i c s i n th e l ate 19t h cent ur y: th e e le ctr o d yn a m ics o f m o v in g b o d ie s .
v
Al b e r t M i c h e l s o n a n d Ed w a r d M o r l e y u s e d a l a r g e in te r f e r o m e te r to tr y to m e as u re c h an g e s i n th e re l ati v e s p e e d of l i ght , de pendi ng on w het her i t m o v ed di r ect l y i nt o an “et her hea d w i nd” or per pendi cu l a r t o i t . T hei r “n u l l r esu l t ” re m a in e d a life lo n g d is a p p o in tm e n t fo r (N o b e l la u re a te ) Mi c h e l s o n .
Ma t h e ma t i c a l p h y s i c i s t s l i k e H e n d r i k L o r e n t z a c c o u n t e d f o r th e n u l l re s u l t b y c o n s i d e ri n g th e for ce ex er t ed on t he a ppa r a t u s by t h e p h y s i c a l , e l a s t i c e t h e r : le n gth con tra ction . L o re n t z a n d h is peer s beg an w i t h dy nam i cs (s tu d y o f fo rc e s ) i n o rd e r to ac c o u n t fo r ki n e ma t i c s (m o ti o n o f o b j e c ts ).
Y o u n g A lb e r t
Al b e r t E i n s t e i n ( 1879 – 1955) w as b o r n i n U l m , G e r m an y . He d r e am e d o f j o i n i n g his f a the r ’ s a nd u nc le ’ s bu sine ss in e le c tr ic a l e ng ine e r ing .
El e c t r i c s t r e e t l i g h t i n g , c a . 1 9 0 0
Images © source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use /
Y o u n g A lb e r t
© ETH-Bibliothek Zurich / Science Photo Library. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https:// ocw.mit.edu/help/faq-fair-use/
He d r o p p e d o u t o f h i g h s c h o o l , t o o k t h e en t r a n ce ex a m f o r t h e E T H * i n Z ür i c h i n 1 8 9 5 – an d f ai l e d . He p as s e d t h e s e c o n d t i m e .
He t h e n p r o c e e d e d t o c u t c l as s e s t o r e ad o n h i s ow n : M a x w e l l , B o l t z m a n n , e t c .
*Ei d g e n ö s s i s c h e T e c h n i s c h e H o c h s c h ul e : F e d e r a l P o l y t e c h n i c I n s t i t ut e
Image is in the public domain.
P a t e n t O f f ic e r , 3 rd C la s s
Ei n s t e i n h a d s o a n n o y e d h i s p r o f e s s o r s t h a t u p o n gr a d u a t i o n h e c o u l d n ’ t g e t a j o b . W i t h h e l p f r o m h i s fri e n d ’ s fa t h e r , h e fi n a l l y l a n d e d a p o s i t i o n i n t h e pa te nt of f ic e in B e r n, S w itz e r la nd.
Ei n s t e i n a t t h e P a t e n t O f f i c e ,
1905 © source unknown.
Ev e n f o r Ei n s t e i n , i t ’ s n o t wh a t
yo u k n o w , b u t wh o yo u k n o w …
“O l y m p i a A c a d e m y ”: S o l o v i n e , H a b i c h t , Ei n s t e i n , c a . 1 9 0 5
Image is in the public domain.
© source unknown.
P a t e n t O f f ic e r , 3 rd C la s s
Ei n s t e i n h a d s o a n n o y e d h i s p r o f e s s o r s t h a t u p o n gr a d u a t i o n h e c o u l d n ’ t g e t a j o b . W i t h h e l p f r o m h i s fri e n d ’ s fa t h e r , h e fi n a l l y l a n d e d a p o s i t i o n i n t h e pa te nt of f ic e in B e r n, S w itz e r la nd.
“O l y m p i a A c a d e m y ”: S o l o v i n e , H a b i c h t , Ei n s t e i n , c a . 1 9 0 5
Ei n s t e i n a t t h e P a t e n t O f f i c e ,
1905 © source unknown.
Er n s t M a c h : “ p o s i t i v i s m . ” O n l y qua n t i t i e s t h a t c o ul d becom e “obj ect s of posi t i v e exper i ence” — su bj ect t o em pi r i ca l m ea su r em ent — belong ed i n ph ysi cal
th e o ri e s . A n y th i n g e l s e w o u l d l e ad to e m p ty
© source unknown.
me t a p h y s i c s a n d c o n f u s i o n . E . g . : Ne wt o n i a n “ a b s o l u t e
Image is in the public domain. space” and “absol u t e t i m e” had no m eani ng .
P a t e n t O f f ic e r , 3 rd C la s s
Ei n s t e i n h a d s o a n n o y e d h i s p r o f e s s o r s t h a t u p o n gr a d u a t i o n h e c o u l d n ’ t g e t a j o b . W i t h h e l p f r o m h i s fri e n d ’ s fa t h e r , h e fi n a l l y l a n d e d a p o s i t i o n i n t h e pa te nt of f ic e in B e r n, S w itz e r la nd.
© source unknown.
Image is in the public domain.
In s p i re d b y M ac h , E i n s te i n p u rs u e d a c ri ti c al
re - ev a l u a t i on of ki n e ma t i c s , b e fo re w o r r y in g a b o u t
dy nam i cs .
Ei n s t e i n a t t h e P a t e n t O f f i c e ,
“O l y m p i a A c a d e m y ”: S o l o v i n e , H a b i c h t , Ei n s t e i n , c a . 1 9 0 5
1905 © source unknown.
No t a B a d Y ea r …
Wh i l e w o r k i n g a t t h e p a t e n t o f f i c e , E i n s t e i n su bm itte d se v e r a l pa pe r s to the An n al e n de r P h y s i k .
Ma r c h 1 9 0 5 : l i g h t q u a n t a
Ma y 1 9 0 5 : B r o wn i a n mo t i o n
Ju n e 1 9 0 5 : s p e c i a l r e l a t i v i t y
Se pt em ber 1905: E = mc 2
“O n t he E l e c t r od y na mi c s of Mo v i ng B od i e s ”: t he t i t l e s ou nd e d c on v e nt i ona l , bu t E i ns t e i n ’ s a ppr oa c h w a s d i s t i nc t .
Q u e s tion s ?
“ A n A s y m m e t r y in t h e E xp la n a t io n ”
Wh e n t h e m a g n e t a n d c o i l w e r e i n r e l a t i v e mo t i o n , a n e l e c t r i c c u r r e n t w a s p r o d u c e d .
Ph y s i c i s t s h a d g i v e n t w o c o m p l e t e l y d i f f e r e n t expl a na t i ons f or t he ef f ect , de pendi ng on wh i c h i t e m w a s mo v i n g .
Ca s e 1 : M a g n e t m o v in g , c o il a t re s t
T he t i me - v a r y i ng B fi e l d i nd u c e s a n E fi e l d , w hi c h e x e r t s a for c e on t he c ha r g e s i n t he c oi l , pu s hi ng t he m a l ong t he c oi l a nd g e ne r a t i ng a c u r r e nt .
© www.gcse.com . All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/ help/faq-fair-use/
© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/
“ A n A s y m m e t r y in t h e E xp la n a t io n ”
Wh e n t h e m a g n e t a n d c o i l w e r e i n r e l a t i v e mo t i o n , a n e l e c t r i c c u r r e n t w a s p r o d u c e d .
Ph y s i c i s t s h a d g i v e n t w o c o m p l e t e l y d i f f e r e n t expl a na t i ons f or t he ef f ect , de pendi ng on wh i c h i t e m w a s mo v i n g .
Ca s e 2 : C o il m o v in g , m a g n e t a t re s t
T he s t a t i c ma gne t i c fi e l d v a r i e s i n s pa c e , g e ne r a t i ng a “d i s pl a c e me nt c u r r e nt . ” B y v i r t u e of t he i r mot i on v , t he c ha r g e s i n t he c oi l e x pe r i e nc e a for c e fr om B a nd a r e pu s he d a l ong t he c oi l , g e ne r a t i ng a c u r r e nt .
© www.gcse.com . All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/ help/faq-fair-use/
© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/
“ A n A s y m m e t r y in t h e E xp la n a t io n ”
Wh e n t h e m a g n e t a n d c o i l w e r e i n r e l a t i v e © www.gcse.com . All
mo t i o n , a n e l e c t r i c c u r r e n t w a s p r o d u c e d .
Ph y s i c i s t s h a d g i v e n t w o c o m p l e t e l y d i f f e r e n t
rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/
expl a na t i ons f or t he ef f ect , de pendi ng on help/faq-fair- use/
wh i c h i t e m w a s mo v i n g .
Ei n s t e i n i n s i s t e d t h a t p h y s i c i s t s h a d b e e n t o o c l e v e r b y h a l f . T h e r e w a s o n l y one p he nome non — ma g n et a n d co i l i n r el a t i v e mo t i o n , cur r en t g en er a t ed — so the r e shou ld only be one e x p l a na t i on .
1. T he l a w s of ph ysi cs a r e v a l i d i n a ny f r a m e of ref er ence m o vi ng at a const ant speed ( in e r tia l fram e s o f re fe re n c e ).
2. T he speed of l i ght c is c o n s t a n t ,
in d e p e n d e n t o f t h e m o t io n o f t h e s o u rc e .
T w o P o s t u la t e s
W h y P o s t u la t e T w o ?
Ei n s t e i n b e g a n t o w o n d e r a t a g e 1 6 : w h a t w o ul d happen i f y ou cou l d cat c h u p t o a l i ght w a v e?
Li k e a s ur f e r r i d i n g a l o n g a w a v e o n t h e o c e a n , t h e wa v e w o u l d l o o k st at i c : fro z e n in s p a c e , n o t c h a n g in g ov e r t i m e .
Bu t f r o m M a x w e l l ’ s e q u a t i o n s , i f t h e r e w e r e n o s o u r c e s n e a r b y (
= J = 0 ) , t h e r e c o u l d b e n o st at i c fi e l d c o n fi g u rati o n s , w i th E and B
fro z e n i n s p ac e .
Ho w c o u l d o n e a v o i d t h a t c o n t r a d i c t i o n ? Ma k e s u r e n o o n e c o u l d ev e r c a t c h u p w i t h a l i g h t w av e !
Kin e m a t ic s F irs t
In s p i r ed b y M a c h , E i n s t ei n be g an
wi t h k i n e m a t i c s , n o t d y n a m i c s : Wh a t w e c a n ob s e r ve ar e b o d i e s i n mo t i o n t h r o ug h s p a ce a n d t i me .
© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/ faq-fair-use/
R e la t i v it y o f Sim u lt a n e it y
If t h er e i s n o a b s o l ut e t i me , h o w ca n w e co mp a r e t i mes a s s o ci a t ed w i t h d i f f er en t ev en t s ? B y s en d i n g lig h t sig na ls , s inc e c = c o n s t a n t .
R e la t i v it y o f Sim u lt a n e it y
If t h er e i s n o a b s o l ut e t i me , h o w ca n w e co mp a r e t i mes a s s o ci a t ed w i t h d i f f er en t ev en t s ? B y s en d i n g lig h t sig na ls , s inc e c = c o n s t a n t .
Th e o b s e r v e r o n t h e t r a i n p l a t f o r m a t M wa s a n e q u a l d i s t a n c e f r o m l o c a t i o n s A and B , and she r e c e i v e d l i g ht w a v e s fr om A and B at the sam e ti m e . T hu s , she c onc l u d e s , the
fl a s h e s w e r e e m i t t e d si m ul t aneousl y fro m A and B .
R e la t i v it y o f Sim u lt a n e it y
If t h er e i s n o a b s o l ut e t i me , h o w ca n w e co mp a r e t i mes a s s o ci a t ed w i t h d i f f er en t ev en t s ? B y s en d i n g lig h t sig na ls , s inc e c = c o n s t a n t .
Th e o b s e r v e r o n t h e t r a i n a t M ’ wa s a n e q u a l d i s t a n c e f r o m l o c a t i o n s A and B , b u t s h e re c e i v e d th e lig h t w a v e fro m B bef or e she r ecei v ed t he l i ght w a v e f r om A . T h u s , s h e concl u des , t he f l a shes w er e no t em i t t ed si m u l t a neou sl y f r om A and B .
R e la t i v it y o f Sim u lt a n e it y
If t h er e i s n o a b s o l ut e t i me , h o w ca n w e co mp a r e t i mes a s s o ci a t ed w i t h d i f f er en t ev en t s ? B y s en d i n g lig h t sig na ls , s inc e c = c o n s t a n t .
Wh o w a s c o r r e c t ? Bo t h ! Re c a l l p o s t u l a t e 1 : T h e l a w s o f p h y s i c s a r e v a l i d i n any fram e o f re fe re n c e m o v in g a t a c o n s ta n t s p e e d ( in e r tia l fram e s o f re fe re n c e ).
R e la t i v it y o f Sim u lt a n e it y
If t h er e i s n o a b s o l ut e t i me , h o w ca n w e co mp a r e t i mes a s s o ci a t ed w i t h d i f f er en t ev en t s ? B y s en d i n g lig h t sig na ls , s inc e c = c o n s t a n t .
L e n g t h C o n t ra c t io n
t ra i n
tr a in a t r es t
mo v i n g
L L’
Th e p a s s e n g e r o n t h e t r a i n s a y s t h a t t h e p e r s o n o n t h e p l a t f o r m
fi r s t me a s u r e d t h e l o c a t i o n o f t h e f r o n t , t h e n wa i t e d wh i l e t h e t r a i n mo v e d , a n d o n l y la t e r me a s u r e d t h e l o c a t i o n o f t h e b a c k — so o f co u rs e t h ei r m ea s u rem en t w a s to o sh o r t .
Ho w d o w e m e a s u r e t h e le n gth of an obj ect ? At t h e s a me t i me , m e a s u re t h e lo c a t io n s o f th e fro n t an d th e b ac k o f th e o b j e c t, an d tak e th e d i ffe re n c e .
If w e di sagr ee abou t si m u lt an ei t y , t h en w e w i ll di sagr ee abou t len gt h s!
L e n g t h C o n t ra c t io n
t ra i n
tr a in a t r es t
mo v i n g
L L’
γ
10
8
6
4
2
0. 0
0. 2
0. 4
0. 6
0. 8
1. 0 v / c
L ’ = L /
To E i n s t e i n , l e n g t h c o n t r a c t i o n
A mo v i n g o b j e c t shr i n k s al o n g i t s di r e c t i o n o f m o t i o n .
had no t hi ng t o do wi t h f o r c es ex er t ed by an et h er (dy nam i cs) ; it w a s s im p ly a c onse q u e nc e of ki n e ma t i c s .
T im e D ila t io n
v
γ
10
8
6
4
2
A m o v i n g c l o c k tic k s s lo w ly .
0. 0 0. 2 0. 4 0. 6 0. 8
1. 0 v / c
Ma s s a n d E n er g y
v
E / 2
v
E / 2
© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/
v
* S e e Le c t u r e N o t e s on “ E = m c 2 ”
Ei n s t e i n a r g ue d : t h e vel oc i t y of t he bo x di d not c hang e , so t o con ser v e en er g y , t h e ma s s of t he bo x mu s t h a v e c h a n g e d : m = E / c 2 , o r E = ( m ) c 2 (fo r s m al l v ). F o r arb i trar y v , E = ( m ) c 2 .
Q u e s tion s ?
E in s t e in a n d E xp e rim e n t
Fo r a l o n g t i m e , p h y s i c i s t s , p h i l o s o p h e r s , a n d h i s t o r i a n s re a d E in s te in ’ s re la ti v ity a s a d ire c t re s p o n s e to th e Mi c h e l s o n - Mo r l e y e x p e r i me n t .
Ye t i t ’ s n o t c l e a r w h e t h e r E i n s t e i n e v e n kn e w abou t the re s u lts a t th e tim e . E ith e r w a y , th e y d o n ’ t s e e m to h a v e p la y e d mu c h r o l e i n h i s t h i n k i n g .
Doe s t ha t me a n t ha t E i ns t e i n w a s u ni nt e r e s t e d i n e x pe r i me nt s ?
T ra in T im e
Un t i l t h e l a t e 1 9 th cen t ur y , t h er e w er e n o co o r d i n a t ed t i me zo n es . E a c h t o w n k e p t l o ca l t i me , ba se d on a c loc k in its to w n sq u a r e .
Be r n t r a i n s t a t i o n , c a . 1 8 6 0
Image is in the public domain.
Pa s s e n g e r s r i d i n g f r o m B o s t o n t o N e w Y o r k Ci t y h a d t o c h a n g e t h ei r w a t c h es b y 3 7 mi n ut es af t e r t h e i r t r i p .
Fr e n c h t r a i n w r e c k , 1 8 9 5
Image is in the public domain .
“T h a t u n ity o f tim e is in d is p en s ib le fo r th e sa t i sf a c t o r y o p e r a t i n g o f r a i l w a y s i s un i v e r sa l l y r e c o g n i z e d , a n d i s n o t d i s p u t e d . B u t , me i n e H e r r e n , w e h a v e i n G e r m a n y f i v e d i f f e r e n t u n i t s o f t i m e . [ … ] W e h a v e t h u s i n G e r m a n y f i v e z o n e s , w i t h a l l t h e d r a w b a c ks a n d d i s a d v a n t a g e s w h i c h r e s u l t . T h e s e w e h a v e i n o u r o w n f a t h e r l a n d , b e s i d e s t h o s e w e d r e a d t o m e e t a t t h e F r e n c h a n d R u s s i a n b o u n d a r i e s . T h i s i s , I m a y s a y , a r u i n w h i c h h a s r e m a i n e d s t a n d i n g o u t o f t h e o n c e s p l i n t e r e d c o n d i t i o n o f G e r m a n y , b u t w h i c h , s i n c e w e h a v e b e c o m e a n e m p i r e , i t i s p r o p e r s h o u l d b e do n e a w a y w i t h . ”
Co u n t v o n M o l t k e , 1 8 9 1
T im e Z o n e s
Ge r ma n r a i l w a y s y s t e m, 1 9 1 0
Image is in the public domain.
Es p e c i a l l y r e l e v a n t a f t e r 1 8 7 0 w a r wi t h F r a n c e , a n d 1 8 7 1 u n i f i c a t i o n o f Ge r ma n y .
Al l d u r i n g E i n s t e i n ’ s c h i l d h o o d .
C o o rdin a t in g C lo c k s a t a D is t a n c e
Ma i n i d e a : i n s t a l l m a s s i v e “ m o t h e r c l o c k s ” i n c e n t r a l t r a i n sta tions , c onne c te d to othe r c loc k s v ia te le g r a ph or r a d io sig na ls .
“M o t h e r c l o c k ” i n Ne u c h a t e l , S w i t z e r l a n d , c a .
1920
Image is in the public domain.
Ei f f e l T o w e r r a d i o s t a t i o n , c a . 1 9 1 0 Ti m e s y n c h r o n i z a t i o n s c h e m e , 1 8 9 0
© Collection Tour Eiffel. All rights reserved. © source unknown . All rights reserved. This
This content is excluded from our Creative content is excluded from our Creative
Commons license. For more information, see Commons license. For more information, see
https://ocw.mit.edu/help/faq-fair- use/ https://ocw.mit.edu/help/faq-fair- use/
Pa t e n t s a n d Pa t h w a y s
Ei n s t e i n w a s i m m e r s e d i n the se de v ic e s: a t the e le c tr o - te c hnic a l de sk a t the pa te nt of f ic e , a nd e v e n on his str oll to ge t t o t h e p a t e n t o f f i c e .
Co o r d i n at e d c l o c k o n E i n s t e i n ’ s b l o c k, 1905
Co o r d i n at e d c l o c k n e t w o r k t h r o u g h o u t B e r n , 1905
Pa t e n t s o n c l o c k c o o r d i n a t i o n co m p o n en t s , 1 9 0 3 – 1905
All i mages © source s unknown. All rights reserved. This content is exclud ed from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair- use/
Re - R e a din g E in s t e in
La c k of r e f e r e nc e s: e m pha siz e pr ior it y , do w npla y pr e c e de nts;
Fo c u s o n o p e r a t i o n a l d e t a i l s o f m e a s u r i n g s p a c e , t i m e , a n d sim u lta ne it y .
Look s l i k e a p a t e nt a p p l i c a t i on …
license. For more
© source unknown. All
content is excluded from
rights reserved. This our Creative Commons information, see
https://ocw.mit.edu/help/ faq-fair-use/
MIT OpenCourseWare
https://ocw.mit.edu
STS.042J / 8.225J Einstein, Oppenheimer, Feynman: Physics in the 20th Century
Fall 202 0
For information about citing these materials or our Terms of Use, visit: http s:// ocw.mit.edu/terms .