Transcript Why is this a problem?

Modern Physics: PHYS 344
Professor Michael Manfra
Office: Physics 84
Phone: 494-3016
Email: [email protected]
Lecture schedule: MWF 2:30am to 3:20am in Physics 203
Office hours: Fridays, Physics 84 from 9:00am to 10:00am or
appointments via email
Course description for PHYS344
Modern Physics will introduce you to some of the most amazing
developments in our understanding of Nature that took place in the
beginning of the 20th century
Principal among these:
Relativity theory: fundamentally changed our understanding of the
notions of space and time from those of Newtonian mechanics
Quantum theory: a completely new way to understand the behavior of
matter at small (atomic) length scales
Prerequisites
PHYS 272H, or 241
Everyone should have completed a course in
classical mechanics
Everyone should have completed a course in
electricity and magnetism
It helps to know some thermodynamics too
Examinations and Grading
A problem set will be given each week. Total points available
for all problem sets (combined) 100 points
There will be two exams given in class: tentative schedule: the
1st on Friday Oct. 3rd and the 2nd on Friday Nov. 14th. These
exams will be graded on a 100 point scale.
The final exam will occur during finals week at the normally
scheduled time and will be graded on a 200 point scale.
The final grade will be determined as follows:
Average of problem sets: 100 points max
Two in-class exams:
200 points max (total)
Final exam:
200 points max
Total:
500 points max
TA and graders
TA: Nirajan Mandal
Email: [email protected]
Office: Physics 104
Graders:
Yonggang Luo
Email: [email protected]
Office: Physics 133
Shih-Chieh Liu
Email: [email protected]
Office: Physics 175
Bedtime reading
Course required text:
Modern Physics, Kenneth Krane, 3rd Edition, Wiley & Sons
You should start by reading by reading Chapter 1: Review
of Classical Mechanics
I will also be drawing on heavily on the lectures on Modern
Physics by Prof. Rick Trebino from Georgia Institute of
Technology
Problem Sets
Problem sets will be assigned on Wednesday and due
in class the following Wednesday.
Problem sets must be turned in on time to
receive full credit.
Problem sets shouldn’t
be torture, so if you’re
having trouble, talk to
us.
We will try to return graded problem sets within one week.
Students are encouraged to work together and discuss
problems – this is a good way to learn. Nevertheless, blind
copying of another student’s work is not acceptable – Don’t do
this – we will spot it.
EMERGENCY PREPAREDNESS – A MESSAGE FROM PURDUE
To report an emergency, call 911. To obtain updates regarding an
ongoing emergency, sign up for Purdue Alert text messages, view
www.purdue.edu/ea.
There are nearly 300 Emergency Telephones outdoors across campus
and in parking garages that connect directly to the PUPD. If you feel
threatened or need help, push the button and you will be connected
immediately.
If we hear a fire alarm during class we will immediately suspend class,
evacuate the building, and proceed outdoors. Do not use the elevator.
If we are notified during class of a Shelter in Place requirement for a
tornado warning, we will suspend class and shelter in [the basement].
If we are notified during class of a Shelter in Place requirement for a
hazardous materials release, or a civil disturbance, including a shooting
or other use of weapons, we will suspend class and shelter in the
classroom, shutting the door and turning off the lights.
Please review the Emergency Preparedness website for additional
information.
http://www.purdue.edu/ehps/emergency_preparedness/index.html
Modern Physics has very some
unintuitive ideas.
In fact, this course will hit you with more than any other
course you’ll ever take. The goal is simply to expose you to
them, and later courses will cover them on more detail.
Understanding the ideas of each
lecture requires the knowledge of the
previous lectures.
If you keep
up, you won’t
end up
looking like
this the night
before the
exams!
Modern Physics is 20th century physics
(maybe not so modern….)
We’ll start with Einstein’s Special
Theory of Relativity
Then move to Quantum Theory
Before
Special
Relativity
One frame
moving at
velocity v with
respect to
another
z
x  x  vt
y  y
z  z
t  t
y
x
Basically, this seems so obvious as to not to
be necessary to say it.
Unfortunately, it’s wrong.
Why is this a problem?
In the mid-19th century, Maxwell unified
electricity and magnetism with his now
famous equations and showed that light
is an electromagnetic wave.
 E  0
 B  0
B
 E  
t
1 E
 B  2
c t
James Clerk Maxwell
(1831-1879)
where E is the electric field, B is the magnetic field, and c is the
velocity of light.
Light is an electromagnetic wave.
The electric (E) and magnetic (B) fields are in phase.
The electric field, the magnetic field, and the propagation
direction are all perpendicular.
Michelson & Morley
Waves typically occur in a medium.
So in 1887 Michelson and Morley
attempted to measure the earth's
velocity with respect to what was
then called the aether and found it
always to be zero, effectively
disproving the existence of the
aether.
Albert Michelson Edward Morley
(1852-1931)
(1838-1923)
In 1905, Einstein had a very good year.
In 1905, Einstein explained Brownian
motion and the photoelectric effect
(for which he later won the Nobel
prize).
Einstein also explained Michelson’s
and Morley’s experiment: he realized
that light didn’t need a medium and
was a property of free space.
And it traveled at the same velocity no
matter what speed you were going.
This idea forms the basis of the
Theory of Special Relativity.
Albert Einstein (1879-1955)
With Special Relativity
x 
1  v2 / c2
y  y
z  z
t 
y
x  vt
z
x
t  vx / c 2
1  v2 / c 2
The Lorentz transformations are the
correct way to transform from one frame
to the other. They yield a constant speed
of light and are NOT at all obvious!
Lorentz himself didn’t believe them.
Quantum Theory
Blackbody Radiation
When matter is heated, it not
only absorbs light, but it also
spontaneously emits it.
A blackbody is a medium that’s
black when it’s cool and so can
spontaneously emit and absorb
all colors.
Blackbodies are interesting because their optical properties are
independent of the material and only depend on the temperature.
The Ultraviolet Catastrophe
Lord Rayleigh used the classical theories of electromagnetism and
thermodynamics to show that the blackbody spectrum
should be:
Rayleigh-Jeans Formula
This worked at longer wavelengths but deviates badly at short ones.
This problem became known as the ultraviolet catastrophe and
was one of the many effects classical physics couldn’t explain.
Max Planck showed that if light is considered
as a particle, bye bye Ultraviolet Catastrophe.
Indeed, photographs taken in dimmer light look grainier.
Very very dim
Bright
Very dim
Very bright
Dim
Very very bright
When we detect very weak light, we find that it’s made up of
particles. We call them photons.
19th-century scientists could not explain
spectra.
1/Wavelength 
The Planetary model for the atom was
also a problem.
From classical E&M
theory, an accelerated
electric charge radiates
energy (electromagnetic
radiation), which means
total energy must
decrease.
And the radius r
must decrease!
Why doesn’t the electron crash into the nucleus?
If a light-wave also acted like a particle,
why shouldn’t matter-particles also act
like waves?
In his thesis in 1923, Prince Louis V. de Broglie suggested that mass
particles should have wave properties similar to light. The wavelength
of a matter wave is called
the de Broglie
where h = Planck’s constant
wavelength:
and p is the particle’s
momentum
And the mass particles would be subject to
their own Uncertainty Principle!
Quantum theory explains the Periodic Table.