5.61 Fall 2007 Lecture #1 page 1
Quantum Mechanics – Historical Background
Physics in the Late 19th Century (prior to quantum mechanics (QM))
• Atoms are basic constituents of matter
• Newton’s Laws apply universally
• The world is deterministic
According to classical mechanics (CM):
!
! !
Given initial positions r0 and velocities v 0 , and given all forces F t () ⇒
all the future can be predicted!
! !
!
F ⎛! dv ⎞
!
() ∫ ! !
v t
v t = !
v0
dv ′ = ∫ t0 m
dt′ ⎜⎝ F = ma = m dt ⎠⎟
!
!
⎛ dr ! ⎞
!
() ∫ ! !
r t
r t = !
r0
dr′ = ∫
t0
vdt′ ⎜⎝ dt = v ⎟⎠
Physics was complete except for a few decimal places !
• Newtonian mechanics explained macroscopic behavior of
matter -- planetary motion, fluid flow, elasticity, etc.
• Thermodynamics had its first two laws and most of their
consequences
• Basic statistical mechanics had been applied to chemical
systems
• Light was explained as an electromagnetic wave
, 5.61 Fall 2007 Lecture #1 page 2
⎯ However there were several experiments that could not be explained
by classical physics and the accepted dogma !
• Blackbody radiation
• Photoelectric effect
• Discrete atomic spectra
• The electron as a subatomic particle
⎯ Inescapable conclusions would result from these problems
• Atoms are not the most microscopic objects
• Newton’s laws do not apply to the microscopic world of the
electron
OUTCOME ⇒ New Rules!!!
Quantum Mechanics!
• Describes rules that apply to electrons in atoms and molecules
• Non-deterministic, probabilistic ! A new philosophy of nature
⎯ Explains unsolved problems of late 19th century physics
⎯ Explains bonding, structure, and reactivity in chemistry
Quantum Mechanics – Historical Background
Physics in the Late 19th Century (prior to quantum mechanics (QM))
• Atoms are basic constituents of matter
• Newton’s Laws apply universally
• The world is deterministic
According to classical mechanics (CM):
!
! !
Given initial positions r0 and velocities v 0 , and given all forces F t () ⇒
all the future can be predicted!
! !
!
F ⎛! dv ⎞
!
() ∫ ! !
v t
v t = !
v0
dv ′ = ∫ t0 m
dt′ ⎜⎝ F = ma = m dt ⎠⎟
!
!
⎛ dr ! ⎞
!
() ∫ ! !
r t
r t = !
r0
dr′ = ∫
t0
vdt′ ⎜⎝ dt = v ⎟⎠
Physics was complete except for a few decimal places !
• Newtonian mechanics explained macroscopic behavior of
matter -- planetary motion, fluid flow, elasticity, etc.
• Thermodynamics had its first two laws and most of their
consequences
• Basic statistical mechanics had been applied to chemical
systems
• Light was explained as an electromagnetic wave
, 5.61 Fall 2007 Lecture #1 page 2
⎯ However there were several experiments that could not be explained
by classical physics and the accepted dogma !
• Blackbody radiation
• Photoelectric effect
• Discrete atomic spectra
• The electron as a subatomic particle
⎯ Inescapable conclusions would result from these problems
• Atoms are not the most microscopic objects
• Newton’s laws do not apply to the microscopic world of the
electron
OUTCOME ⇒ New Rules!!!
Quantum Mechanics!
• Describes rules that apply to electrons in atoms and molecules
• Non-deterministic, probabilistic ! A new philosophy of nature
⎯ Explains unsolved problems of late 19th century physics
⎯ Explains bonding, structure, and reactivity in chemistry
