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Review


Welcome Back!
Review
Ch 15 Special Relativity  Space and Time  1
 Ch 15 & 16 Pretest
 Ch 15 & 16 Core Ideas, Terms, and Objectives
 Background  19^{th} century physics in 15 minutes
 General state of physics in 1900
 A few nagging little problems
 If light is a wave, what does it wave?
 Why can't we detect the "a luminiferous ether?"
 Maxwell's "silly" idea
 Lorentz's "downright crazy" idea
 Postulates of Special Relativity
 Laws of physics have the same form in all inertial reference frames
 Speed of light in a vacuum is constant for all observers (all speeds are not relative)
 What's so "special" about special relativity?
 Accelerated versus nonaccelerated motion
 Inertial & Noninertial reference frames


 Video:
 "Einstein to the Rescue" (30 min.)
 "Uncommon Sense  Stretching Time


Ch 15 Special Relativity  Space and TIme  2
 Postulates (review)
 Laws of physics have the same form in all inertial reference frames
 Speed of light in a vacuum is constant for all observers (all speeds are not relative)
 Consequences
of Special Relativity
 Simultaneity
 The "train thought experiment"
 Two observers moving at different velocities will not agree on whether 2 events separated in space occur simultaneously, or which one occurs first.
 Time dilation
 The "light clock"
 (Lorentz transformation)
 Why don't we notice time slowing down?
 At rest, time moves "normally"
 At "c," time is "frozen"
 The effect is reciprocal  you see my time running slower
 Spacetime
 The "Twin Paradox"
 Relative Velocity
 Does this really work?
 Ives & Stillwell (1941)  Doppler shift in beams of positive ions
 Rossi & Hall (1941)  studied the lifetimes of muons produced by cosmic rays in the upper atmosphere
 Haefle & Keating (1971, 2005)  cesium clock flown around the world compared to stationary clock
 Confirmed daily in particle accelerator experiments
 Confirmed by accuracy of the GPS system

Answer:

Ch 15 Special Relativity  Space and TIme  3

 Finish yesterday's assignment

Ch 16 Special Relativity  Length, Momentum, & Energy  1
 Further consequences of Special Relativity
 Length contraction
 The Barn Problem
 Momentum, Inertia, and Mass
 Energy
 E_{o} = mc^{2}
 Mass is a (very concentrated) form of energy
 Correspondence Principle
 Two theories must agree in situations where they overlap.


Ch 16 Special Relativity  Length, Momentum, & Energy  2
 Go over the Ch 16 assignment
 General Theory of Relativity (NOT in the text)
 Albert Einstein  1915
 deals with motion in general (with acceleration)
 but known as a theory (explanation/model) of gravity
 Correspondence Principle revisited in a new context
 equivalence of acceleration and gravitational field
 the elevator problem (once again!)
 Some consequences
 forces are replaced by bending/warping of space time
 gravity pulls light
 time slows down where gravity is intense (black holes)
 Experimental confirmation
 the eclipse experiment  1919
 Haefle & Keating (1971, 2005)  cesium clock flown around the world compared to stationary clock
 Confirmed by accuracy of the GPS system

 Finish the assignment from yesterday

Properties of Matter
 Ch 17  The Atomic Nature of Matter
 Matter is made of atoms
 "Atoms are eternal"
 Atoms are too small to be seen (approx. 10^{10}m)
 Molecules are made of atoms
 Atoms are made of protons, electrons & neutrons
 Protons & neutrons reside in the nucleus
 Nucleus is extremely small compared to atom
 Almost all of the mass of the atom is in the nucleus
 Element determined by the number of protons
 Number of neutrons determine the isotope
 Electrons "orbit" the nucleus
 Electron mass is about 1/2000 proton or neutron mass
 Generally, number of electrons = number of protons
 Fewer or greater number of electrons determine positive/negative ion.
 Phases of Matter  solid, liquid, gas, plasma
 Ch 18 Solids
 Atoms are moreorless locked in position
 Density
 Ch 19 Liquids
 Atoms/molecules are free to slide past one another
 Ch 20 Gasses
 Atoms/molecules are free to expand to any volume
 Plasma
 "Soup" of free electrons (negative ions) and nuclei (plus inner electrons  positive ions)  outer electrons have too much energy to be held in the atom
 Excellent conductor of electricity
 Most matter is in the plasma state


Unit IV Atomic & Nuclear Physics
Ch 38 The Atom and the Quantum
 Background
 Wave terms review
 Frequency
 Wavelength
 Amplitude
 How do we know that light is a wave (and not made of particles)
 Standing waves on a spring
 Constructive interference
 Destructive interference
 Interference in 2 dimensions
 Interference in light means light is a wave
 Further evidence: the speed of light in water
Review for the quiz


Small Test/Big Quiz on Ch. 15 & 16  Relativity
Ch 38 The Atom and the Quantum


Ch 38 The Atom and the Quantum
 The Ultraviolet Catastrophe
 Theory predicts that hot objects should emit an infinite amount of energy at ultraviolet wavelengths  clearly silly
 Solution:
 Max Planck discovered a "fudge factor" in the equations implying that hot objects emit/absorb light in discrete packets called quanta
 Energy of a quantum depends on frequency: E = hf
 h is a VERY small constant (6.63 x 10^{34} J^{.}s)
 The Photoelectric Effect
 When light shines on certain metals, electrons are ejected from the metal.
 Brighter light generally ejects more electrons that dimmer light.
 Higher frequency light generally ejects faster (more energetic) electrons than lower frequency light.
 There is a "threshold frequency," which depends on the type of metal, so that light of a lower frequency will not eject any electrons, no matter how bright the light.
 Einstein's explanation (1905):
 Light is made of particles, called photons.
 A photon is a quantum of light, so the energy of a photon depends on its frequency (E = hf).
 An electron is ejected from the metal when a single photon transfers its energy to a single electron.
 Brighter light means more photons, so more electrons are ejected by brighter light.
 Higher frequency light means more energetic photons, so more energetic electrons are ejected by higher frequency light.
 For light below the threshold frequency, photons do not have enough energy to eject an electron.

 None (you've suffered enough  for now)

Ch 38 The Atom and the Quantum
 Particles as waves
 Louis De Broglie proposed that all particles have wave properties.
 The wavelength of a particle is inversely proportional to its momentum.
 The wavelengths of peoplesized particles is much too small to be noticed or detected.
 The wave nature of electrons can be demonstrated
 Electrons can produce interference patterns.
 Technology: the electron microscope.
 Waves as particles
 Compton Effect (1923)  When an xray strikes an electron, the electron recoils as if it had been struck by a particle.


Ch 38 The Atom and the Quantum
 Video:
 Lecture 18  Wave or Particle?
 Lecture 19  Quantum Uncertainty  Farewell to Determinism

 Work on the Ch 38 assignment.

Ch 38 The Atom and the Quantum
 Models of the Atom
 Lab Activity  observing line spectra
 Plum Pudding Model
 positive charge is spread out with electrons embedded in it
 Planetary Model
 a "miniature solar system" with the electric force replacing the gravitational force
 can't be true, since Maxwell showed that accelerating electric charges emit waves (and therefore lose energy)
 The Bohr Model
 electrons don't radiate energy in certain allowed orbits
 can predict the emission/absorption of specific wavelengths of light by atoms
 When an electron "jumps" between orbits, the energy difference is emitted/absorbed as a photon of the corresponding frequency (E = hf).
 Problems:
 Why don't electrons emit electromagnetic waves in certain orbits?
 Why these particular orbits (aside from the fact that they can be made to fit the data)?
 Electrons can't really "jump", since they cannot exist between the allowed orbits  so how do they get there?
 Electrons disappear from one orbit and immediately appear in another (with the emission/absorption of a photon) without ever being between the two orbits.
 The De Broglie Model
 The wave properties of electrons determine the possible orbits of electrons in the atom.
 An orbit is allowed where the electron wave interferes (with itself) constructively to produce a standing wave.

 Work on the Ch 38 assignment.

Ch 38 The Atom and the Quantum
 Quantum mechanics
 Heisenberg (German)
 Uncertainty Principle
 Position/momentum and Energy/time cannot be measured to arbitrary precision
 The end of the "Clockwork Universe"
 "Empty" space  The Law of Conservation of Energy/Mass can be violated for a (very) short time.
 Developed a quantum mechanics based on matrices
 Schroedinger (Austrian)
 Developed a quantum mechanics based on waves
 Equivalent to Heisenberg's matrices, but waves are more familiar to physicists
 Schroedinger's wave equation (state vector)
 There is a wave equation that describes the evolution of each property of an object.
 Square of the amplitude of the wave function is the probability that a measurement will give that value.
 Bohr and the Copenhagen (Standard) Interpretation of Quantum Mechanics
 The wave function is a complete description of reality.
 Collapse of the wave function occurs when a measurement is made.
 Probability of measured value becomes one, probability of every other possible value becomes zero.
 Before the measurement, the quantity was in a "superposition of states".
 Before a property is measured, its value does not exist. (What you can't measure doesn't exist.) The value is created by the measurement.
 Many people say that quantum mechanics says that "measurement affects the value being measured" but this is a classical  not quantum  idea!

 Finish the Ch 38 assignment.

Ch 38 The Atom and the Quantum
 Quantum mechanics (continued)
 The Schroedinger's Cat Thought Experiment
 Einstein's objections to quantum mechanics
 "God does not play dice..."
 It is silly to say that an object has no definite momentum (for instance) before a measurement is made. It has a value, we just don't know what it is.
 Einstein vs. Bohr (continued)
 Einstein raises objections to quantum mechanics  twice  Bohr responds  twice  Einstein is embarrassed  twice
 The EinsteinPodolskyRosen Experiment (EPR)  1935
 Principle of Locality  One object can't influence what happens to another object unless a signal (traveling at the speed of light) can be sent from one to the other.
 Principle of Reality  If you can predict the value of a measurement of some quantity before the actual measurement is made, then that quantity must actually exist.
 Entangled/twin particles
 The EPR thought experiment
 Einstein  The universe must be real and local, therefore quantum mechanics cannot be a complete description of it.
 Bohr  one particle must "influence" the other  what Einstein called (sarcastically) "spooky action at a distance."
 Bell's Theorem  suggests an actual experiment to decide who's right (1965)
 The experiment was performed and ______ (its a surprise).
 Consequences of EPR
 Alternate interpretations of quantum mechanics
 The ManyWorlds Interpretation
 When a measurement is made, the universe "splits" so that every possible value occurs in some universe.
 popular among string theorists
 HiddenVariable Interpretations
 Bohm and others  very restricted due to the results of EPR
 Quantum Theory vs. Special Relativity  black holes
 The theories violate the Correspondence Principle  at least one of them needs some serious work!

 Finish the Ch 38 assignment.














