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8/19/2019 Quantum Physics for the Terminally Confused
1/10
2/29/2016 Quantum physics for the terminally confused | Cosmos
https://cosmosmagazine.com/physical-sciences/quantum-physics-terminally-confused
FEATURE PHYSICAL SCIENCES 29 FEB 2016
With new papers on the baffling world of quantum mechanics
appearing all the time, we thought it would be helpful if our resident
physicist Cathal O'Connell provided a primer to some bewildering,
but vital concepts.
Quantum physics for the terminally confused
https://cosmosmagazine.com/physical-sciences/different-picture-quantum-surrealismhttps://cosmosmagazine.com/physical-scienceshttps://cosmosmagazine.com/
8/19/2019 Quantum Physics for the Terminally Confused
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2/29/2016 Quantum physics for the terminally confused | Cosmos
https://cosmosmagazine.com/physical-sciences/quantum-physics-terminally-confused
8/19/2019 Quantum Physics for the Terminally Confused
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2/29/2016 Quantum physics for the terminally confused | Cosmos
https://cosmosmagazine.com/physical-sciences/quantum-physics-terminally-confused
C R E D I T : B S I P / G E T T Y I M A G E S
D o e s q u a n t u m p h y s i c s m e l t y o u r b r a i n ? F i r s t , d o n ’ t p a n i c . Y o u ’ r e n o t a l o n e i n y o u r
c o n f u d d l e m e n t . A s l e g e n d a r y p h y s i c i s t R i c h a r d F e y n m a n s a i d : “ I t h i n k I c a n s a f e l y s a y
t h a t n o b o d y u n d e r s t a n d s q u a n t u m m e c h a n i c s . ”
N e v e r t h e l e s s , q u a n t u m t h e o r y i s v i t a l f o r d e s c r i b i n g h o w o u r w o r l d i s s c r e w e d t o g e t h e r .
S o w e ’ v e b r o k e n d o w n t h e i d e a s o f q u a n t u m t h e o r y t o t h e l e v e l w h e r e e v e n a f i v e - ( o r 5 5 )
y e a r - o l d c a n g e t t h e g i s t .
What is quantum theory?
A f t e r a f e w t h o u s a n d y e a r s o f a r g u m e n t w e f i n a l l y k n o w w h a t “ s t u f f ” i s m a d e o f – t i n y
p a r t i c l e s c a l l e d e l e c t r o n s a n d q u a r k s . T h e s e g u y s g a n g t o g e t h e r i n l i t t l e f a m i l i e s t o m a k e
a t o m s s u c h a s h y d r o g e n o r o x y g e n a n d m o l e c u l e s s u c h a s H O .
A t o m s a n d m o l e c u l e s a r e t h e L e g o b l o c k s o f o u r w o r l d .
T o d e s c r i b e t h e w a y t h i s t i n y w o r l d o p e r a t e s , s c i e n t i s t s u s e a c o l l e c t i o n o f i d e a s c a l l e d
q u a n t u m t h e o r y .
T h e t h e o r y m a k e s w e i r d p r e d i c t i o n s ( f o r e x a m p l e , t h a t p a r t i c l e s c a n b e i n t w o p l a c e s a t
o n c e ) , y e t i t i s a l s o t h e m o s t p r e c i s e l y v a l i d a t e d t h e o r y i n p h y s i c s .
I t u n d e r p i n s m u c h o f t h e t e c h n o l o g y a r o u n d u s , i n c l u d i n g t h e c h i p t h a t m a k e s y o u r
s m a r t p h o n e s o s m a r t .
I t ’ s w e i r d , i t ’ s r i g h t , i t ’ s i m p o r t a n t .
2
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But what does 'quantum' actually mean?
S t e p i n t o y o u r k i t c h e n w i t h a j a r o f p e a n u t b u t t e r i n y o u r h a n d . Y o u c o u l d d e c i d e t o p u t
t h e j a r o n t h e c o u n t e r - t o p , o r o n o n e o f t h e s h e l v e s a b o v e i t . B u t y o u c a n ’ t p u t t h e j a r
b e t w e e n s h e l v e s – t h a t j u s t d o e s n ’ t m a k e s e n s e .
I n p h y s i c s - s p e a k y o u ’ d s a y t h e s h e l v e s o f y o u r k i t c h e n a r e “ q u a n t i s e d ” . I t j u s t m e a n s t h e y
c o m e i n l e v e l s .
I n t h e q u a n t u m w o r l d , e v e r y t h i n g i s s p l i t i n t o l e v e l s . F o r e x a m p l e , a n e l e c t r o n i n a n a t o m
c a n s i t i n o n e o f a f e w s e t “ e n e r g y l e v e l s ” – j u s t l i k e s h e l v e s i n y o u r k i t c h e n . B u t t h e
q u a n t u m w o r l d i s w e i r d . G i v e a n e l e c t r o n a k i c k o f e n e r g y a n d i t w i l l j u m p i n s t a n t l y f r o m
o n e l e v e l t o a n o t h e r .
T h a t ’ s c a l l e d a q u a n t u m l e a p .
H e r e ’ s a n o t h e r a n a l o g y . I f y o u d r o v e a q u a n t u m c a r , y o u m i g h t b e a b l e t o t r a v e l a t 5
k m / h , 2 0 k m / h o r 8 0 k m / h , b u t a t n o s p e e d i n b e t w e e n . S h i f t g e a r s a n d y o u ’ d s u d d e n l y
j u m p f r o m 5 t o 2 0 k m / h . T h e c h a n g e i n s p e e d w o u l d b e i n s t a n t a n e o u s , s o y o u w o u l d n ’ t
e v e n f e e l t h e a c c e l e r a t i o n .
T h a t ’ s a n o t h e r q u a n t u m l e a p .
Quantum mechanics vs classical mechanics
T h e m i c r o s c o p i c w o r l d p l a y s b y v e r y d i f f e r e n t r u l e s t h a n t h e “ c l a s s i c a l ” w o r l d w e ’ r e u s e d
t o .
“ C l a s s i c a l ” i s a p h y s i c i s t ’ s w o r d f o r “ c o m m o n s e n s e ” – w h e n s o m e t h i n g b e h a v e s t h e w a y
y o u m i g h t e x p e c t i t t o f r o m e v e r y d a y e x p e r i e n c e .
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2/29/2016 Quantum physics for the terminally confused | Cosmos
https://cosmosmagazine.com/physical-sciences/quantum-physics-terminally-confused
A b i l l i a r d b a l l i s a “ c l a s s i c a l o b j e c t ” ( i t r o l l s a l o n g t h e t a b l e i n a s t r a i g h t f o r w a r d w a y ) b u t
s i n g l e a t o m w i t h i n i t f o l l o w s q u a n t u m l a w s ( l i a b l e t o d i s a p p e a r t h r o u g h t h e g r e e n f e l t a t
a n y m o m e n t ) .
S o m e w h e r e b e t w e e n t h e s c a l e o f t h e a t o m a n d t h e b i l l i a r d b a l l t h e r e i s a c r o s s o v e r p o i n t
i n t h e l a w s o f p h y s i c s – a b i t l i k e a j u r i s d i c t i o n a l h a n d o v e r b e t w e e n s t a t e a n d f e d e r a l
p o l i c e .
S t i c k e n o u g h a t o m s t o g e t h e r a n d w e i r d q u a n t u m e f f e c t s f a d e a w a y , t h e b e h a v i o u r
b e c o m e s c l a s s i c a l . T h i s i s c a l l e d t h e c o r r e s p o n d e n c e p r i n c i p l e .
Heisenberg’s uncertainty principle
S o m e t h i n g s i n q u a n t u m p h y s i c s a r e l i t e r a l l y u n k n o w a b l e . F o r e x a m p l e , y o u c a n n e v e r
k n o w b o t h w h e r e t h e e l e c t r o n i s a n d w h e r e i t ’ s g o i n g .
T h e r e a s o n f o r t h i s c o m e s d o w n t o t h e s m a l l n e s s o f t h e e l e c t r o n – t o f i n d o u t w h e r e i t i s
y o u n e e d t o d e t e c t i t w i t h s o m e t h i n g ( f o r e x a m p l e a p h o t o n o f l i g h t ) b u t t h i s p r o b i n g ,
h o w e v e r g e n t l e , w i l l k n o c k t h e e l e c t r o n o f f i t s o r i g i n a l c o u r s e .
T h e e l e c t r o n t e l l s y o u w h e r e i t i s , b u t f o r g e t s w h e r e i t w a s g o i n g .
Particle/wave duality
Q u a n t u m o b j e c t s ( l i k e p h o t o n s a n d e l e c t r o n s ) h a v e s p l i t p e r s o n a l i t i e s – s o m e t i m e s t h e y
b e h a v e l i k e w a v e s , a n d s o m e t i m e s l i k e p a r t i c l e s . T h e w a y t h e y b e h a v e d e p e n d s o n t h e
k i n d s o f q u e s t i o n s y o u a s k t h e m ( s e e t h e “ t w o - s l i t e x p e r i m e n t ” b e l o w ) .
Wave-function
A b i t o f m a t h s t h a t d e s c r i b e s w h a t a w a v e l o o k s l i k e .
8/19/2019 Quantum Physics for the Terminally Confused
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2/29/2016 Quantum physics for the terminally confused | Cosmos
https://cosmosmagazine.com/physical-sciences/quantum-physics-terminally-confused
C r u c i a l l y , q u a n t u m w a v e - f u n c t i o n s c a n h a v e m a n y p o s s i b l e s o l u t i o n s – e a c h w i t h a
d i s t i n c t p r o b a b i l i t y o f b e i n g t r u e .
A m a z i n g l y , t h e d i f f e r e n t p o s s i b l e a n s w e r s s e e m t o i n t e r a c t w i t h o n e a n o t h e r i n a s o r t o f
l i m b o o f s t a t e s c a l l e d s u p e r p o s i t i o n – a s i f c o n s p i r i n g t o g e t h e r t o g i v e u s t h e r e a l i t y o f
o u r u n i v e r s e ( s e e " t w o - s l i t s " b e l o w ) .
Superposition and Schrödinger’s cat
I m a g i n e a c a t i n a b o x a l o n g w i t h a v i a l o f c y a n i d e . T h e r e i s a h a m m e r h e l d b y a s t r i n g
a b o v e t h e v i a l . T h e h a m m e r i s d e s i g n e d t o f a l l w h e n t r i p p e d b y a r a n d o m q u a n t u m e v e n
( f o r e x a m p l e , t h e d e c a y o f a n a t o m o f u r a n i u m ) .
T h i s i s t h e t h o u g h t e x p e r i m e n t d r e a m e d u p b y E r w i n S c h r ö d i n g e r t o t r y t o c o n v e y t h e
i d e a o f s u p e r p o s i t i o n .
T h e a t o m i c d e c a y f o l l o w s q u a n t u m l a w s , a n d s o i t s w a v e - f u n c t i o n h a s t w o s o l u t i o n s :
d e c a y e d o r n o t d e c a y e d .
A c c o r d i n g t o q u a n t u m t h e o r y , u n t i l y o u m a k e a m e a s u r e m e n t t h e s e t w o p o s s i b i l i t i e s a r e
e q u a l l y v a l i d . I n f a c t y o u c a n c o n s i d e r t h e a t o m a s b o t h d e c a y e d a n d n o t d e c a y e d a t t h e
s a m e t i m e .
B e c a u s e t h e c a t ’ s f a t e i s i n t i m a t e l y c o n n e c t e d t o t h e a t o m o f u r a n i u m , u n t i l y o u t a k e a
p e e k , t h e c a t i s b o t h a l i v e a n d d e a d a t t h e s a m e t i m e .
What is entanglement?
E n t a n g l e m e n t i s w h e n t w o p a r t i c l e s ( f o r e x a m p l e p h o t o n s ) a r e i n t i m a t e l y c o n n e c t e d s o
t h a t m e a s u r e m e n t o n o n e i n s t a n t l y a f f e c t s t h e o t h e r , n o m a t t e r h o w f a r a w a y i t i s .
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I t ’ s l i k e w h e n y o u w e r e a k i d a n d y o u r u n c l e s h o w e d y o u a c o l o u r e d b a l l i n e a c h h a n d ,
t h e n m i x e d t h e m u p b e h i n d h i s b a c k . F r o m y o u r p o i n t o f v i e w , t h e t w o b a l l s w e r e
“ e n t a n g l e d ” – i f t h e r e d b a l l i s i n h i s l e f t h a n d , i t m e a n s t h e b l u e b a l l i s i n h i s r i g h t .
B u t t h e q u a n t u m s i t u a t i o n i s m o r e m y s t e r i o u s b e c a u s e t h e “ b a l l s ” d o n ’ t h a v e d e f i n e d
c o l o u r s , t h e y a r e c o l o u r s h i f t i n g – a t a n y i n s t a n t t h e y c a n t u r n o u t r e d o r b l u e w i t h e q u a l
c h a n c e . I t ’ s t o t a l l y r a n d o m .
T h e w e i r d t h i n g i s , l o o k i n g a t o n e b a l l k i l l s t h e r a n d o m n e s s ( f r e e z e s t h e c o l o u r s h i f t i n g )
n o t j u s t f o r t h e o n e y o u l o o k a t , b u t f o r b o t h . I f y o u s e e a r e d b a l l , y o u k n o w t h e c o l o u r o
t h e o t h e r i s f i x e d a s b l u e .
I n t h i s w a y , o n e e n t a n g l e d p a r t i c l e c a n s e e m t o a f f e c t t h e o t h e r i n s t a n t l y , a n d n o m a t t e r
h o w f a r a p a r t t h e y a r e . A l b e r t E i n s t e i n f e l t t h i s v i o l a t e d t h e c o s m i c s p e e d l i m i t ( a k a t h e
s p e e d o f l i g h t ) i m p o s e d b y h i s r e l a t i v i t y t h e o r y , a n d s o h e g a v e e n t a n g l e m e n t t h e l a b e l
“ s p o o k y a c t i o n a t a d i s t a n c e ” .
How does a physicist entangle photons?
T h e r e a r e a c o u p l e o f t e c h n i q u e s . O n e i s t o b r e a k a h i g h - e n e r g y p h o t o n i n t o t w o
“ d a u g h t e r p h o t o n s ” o f l o w e r e n e r g y . L i k e i d e n t i c a l t w i n s i n a h o r r o r m o v i e , t h e t w o
d a u g h t e r s p o s s e s s a m y s t i c a l c o n n e c t i o n b e t w e e n t h e m .
A n o t h e r w a y i s t o p a s s t h e t w o p h o t o n s t h r o u g h a m a z e o f m i r r o r s s o t h a t y o u c a n n o t
p o s s i b l y k n o w w h i c h d i r e c t i o n e a c h t r a v e l l e d . T h i s “ u n k n o w a b i l i t y ” c r e a t e s
t h e e n t a n g l e m e n t .
Two-slit experiment (an illustration of most of the above)
T h i s i s t h e m o s t f a m o u s e x p e r i m e n t i n q u a n t u m m e c h a n i c s , i n w h i c h p a r t i c l e s ( u s u a l l y
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e l e c t r o n s o r p h o t o n s ) a r e f i r e d a t t w o s l i t s b e f o r e b e i n g d e t e c t e d o n a s c r e e n .
I t i s s o f a m o u s b e c a u s e i t d e m o n s t r a t e s m a n y o f t h e w e i r d p h e n o m e n a m e n t i o n e d a b o v e
T h e e x p e r i m e n t h i n g e s o n t h e d i f f e r e n t b e h a v i o u r o f w a v e s v s p a r t i c l e s f o r t h e s a m e s e t
u p .
F o r e x a m p l e , y o u c a n s e t u p a b a r r i e r w i t h t w o s l i t s i n i t i n a p o o l o f w a t e r a n d t h e n d i p
y o u r f i n g e r i n a n d o u t t o g e n e r a t e w a v e s . T h e r i p p l e s w i l l p a s s t h r o u g h t h e t w o s l i t s , a n d
i n t e r f e r e w i t h o n t h e o t h e r s i d e , m a k i n g a p a t t e r n .
B u t i f y o u t a k e t h e b a r r i e r o u t o f t h e w a t e r a n d f i r e a b u n c h o f m a r b l e s a t t h e t w o s l i t s ,
t h e y f l y s t r a i g h t t h r o u g h i n t w o d i r e c t l i n e s , w i t h o u t m a k i n g a n i n t e r f e r e n c e p a t t e r n .
T h e w e i r d t h i n g i s , e l e c t r o n s c a n b e h a v e l i k e b o t h .
I f y o u f i r e e l e c t r o n s a t t h e s l i t s , e v e n o n e a t a t i m e , t h e y f o r m a n i n t e r f e r e n c e p a t t e r n o n
t h e s c r e e n – a s i f e a c h e l e c t r o n p a s s e s t h r o u g h b o t h s l i t s a t t h e s a m e t i m e , a n d i n t e r f e r e s
w i t h i t s e l f . T h i s s e e m s t o t e l l u s e l e c t r o n s a r e w a v e s .
B e c a u s e t h e e l e c t r o n i s a q u a n t u m o b j e c t , w e c a n ’ t k n o w i t s l o c a t i o n ( H e i s e n b e r g
u n c e r t a i n t y p r i n c i p l e ) . T h e e l e c t r o n h a s s o m e c h a n c e o f g o i n g t h r o u g h o n e s l i t , s o m e
c h a n c e o f g o i n g t h r o u g h t h e o t h e r – b e c a u s e b o t h a r e p o s s i b l e , i t a c t u a l l y g o e s t h r o u g h
b o t h ( s u p e r p o s i t i o n o f s t a t e s ) .
N o w , t h e “ o b s e r v a t i o n ” i s w h e n t h e e l e c t r o n h i t s t h e d e t e c t o r , s h o w i n g a b r i g h t f l a s h
( w a v e - f u n c t i o n c o l l a p s e ) .
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B u t s a y y o u t r y t o t r i c k t h e e l e c t r o n b y p u t t i n g a m e c h a n i s m a t t h e s l i t s w h i c h t e l l s y o u
w h i c h o n e t h e e l e c t r o n g o e s t h r o u g h . S u d d e n l y t h e i n t e r f e r e n c e p a t t e r n d i s a p p e a r s .
B e c a u s e y o u k n o w w h i c h s l i t t h e e l e c t r o n w e n t t h r o u g h , i t i s n o l o n g e r i n a s u p e r p o s i t i o
o f s t a t e s , a n d s o o n l y t r a v e l s t h r o u g h o n e o f t h e s l i t s . T h e e l e c t r o n ’ s w a v e - l i k e b e h a v i o r
e v a p o r a t e s , a n d i t b e h a v e s j u s t l i k e a m a r b l e .
I f y o u r h e a d i s h u r t i n g , b e c o n s o l e d t h a t p h y s i c i s t s a l s o s t r u g g l e t o e x p l a i n t h i s a p p a r e n t
p a r a d o x ( s e e “ i n t e r p r e t a t i o n s ” b e l o w ) .
T h o u g h a s F e y n m a n n o t e d i n h i s L e c t u r e s o n P h y s i c s , “ t h e p a r a d o x ” i s o n l y a c o n f l i c t
b e t w e e n r e a l i t y a n d y o u r f e e l i n g o f w h a t r e a l i t y “ o u g h t t o b e ” .
Interpretations of quantum mechanics
T h e S h u t u p a n d c a l c u l a t e s c h o o l – p h y s i c i s t s o n l y i n t e r e s t e d i n t h e a n s w e r s , a n d w h o
r e f u s e t o s p e c u l a t e a b o u t w h a t i s r e a l l y g o i n g o n .
T h e M a n y w o r l d s i n t e r p r e t a t i o n – t h o s e p h y s i c i s t s w h o m a i n t a i n t h a t e v e r y q u a n t u m
m e a s u r e m e n t s p a r k s t h e c r e a t i o n o f a n i n f i n i t e n u m b e r o f p a r a l l e l u n i v e r s e s , o n e f o r
e a c h p o s s i b l e s o l u t i o n t o t h e w a v e - f u n c t i o n . T h e s o l u t i o n … i t j u s t h a p p e n s t o b e t h e o n e
t h a t a p p e a r s i n o u r U n i v e r s e .
T h e C o p e n h a g e n i n t e r p r e t a t i o n – r e a l i t y d o e s n o t e x i s t u n t i l w e m e a s u r e i t . T h e a c t o f
o b s e r v i n g c a u s e s t h e w a v e - f u n c t i o n t o “ c o l l a p s e ” .
T h e D e B r o g l i e - B o h m o r p i l o t - w a v e i n t e r p r e t a t i o n – t r e a t s q u a n t u m o b j e c t s j u s t l i k e
c l a s s i c a l p a r t i c l e s , b u t i m a g i n e s t h e m r i d i n g l i k e a s u r f e r o n t o p o f a s o - c a l l e d p i l o t w a v e .
T h e w a v e d e t e r m i n e s w h e r e t h e p a r t i c l e e n d s u p .
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