Transcript Notes for Class 14, April 21

Atoms and Stars
IST 2420
Class 14, April 21
Winter 2008
Instructor: David Bowen
Course web site:
www.is.wayne.edu/drbowen/aasw08
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Agenda
• Assignments, passbacks, initial signin sheet
• Class information
o Email if much work will be late
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Review of readings
Updating the course
Emphasizing main points one more time
Review for Final
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Upcoming …
• Tonight, April 21 (last regular class)
o Lab 11 – the Orbiting Bottle
• Checking up on Newton
o Review for Final Exam
o Due: all work to count in regular grade
• Final Exam: next Monday, April 28
o Nothing that night but the Final Exam
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Course Grades
• If you are turning a bunch of work in at the
end, I may not get it graded in time for the
regular grades (see the Syllabus).
• If this is you (turning it in late), what grade
do you want for the regular grade? D, E, W,
I
• Email me to let me know – otherwise it’s
my decision.
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Your Current Status
• Grades I have for you:
o Online Grade Report, link off the course web
site (see first slide)
o Enter first name, last name, password the get
report
• Grade you are headed for:
o Grade What-If on course web site
• Ask for help with these if you are having
problems
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Opportunities for Q & A
• Tonight during the Review Session
• Day of the Final, Monday April 21, 5 – 6
PM (normal office hours) in the regular
classroom (100 Shapero)
• Call, email, set up an appointment
• IM to WSU web guy
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6
ths
16
on the Final
• Doing the math for converting 16ths
(inches, ounces) to decimal (inches, pounds)
o If this type of problem is on the Final, there will
also be a table of all divisions by 16, with a few
non-16ths extras thrown in
• 1/16 = .0625, 2/16 = .1250, 3/16 = .1875,
3/7 = .4286, 4/16 = .2500, etc.
o So the result of the division will be there, but
you will have to know what you are looking
for.
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Makeup for Final Exam
• Let me know by email that you want a
makeup, within 24 hours after the Final
(University regulation)
• Date / Time, building and room to be settled
by email.
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Experiment 9
• The technique for measuring the
circumference is valid.
o The definition of the circumference is the
distance around the outside.
o For the Circle, most groups get between 0
discrepancy and 0.2”
• The formula for the circumference of the
circle (d) is correct
• Formula for circumference of ellipse is
incorrect – actually, there is no simple
formula
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Experiment 9
•
So there are two problems this presents:
1. Recognize that there is a discrepancy for the
ellipse
•
•
•
2 inches and more discrepancy cannot be
attributed to the technique
Cut string to shorter (theoretical) length – does
not possibly go around
Can be hard to admit
2. If there is a real discrepancy, what do you do?
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Experiment 9
• Some past reports have said that the formula for
the ellipse was trusted more than the experimental
measurement
o
o
o
o
Both are actually based on measurements
The power of authority
Also, not trusting your techniques
But in this case, the authority was not trustworthy
• Many said no use in repeating measurements
o Results would be the same
• No! Every technique has a limit, will have variations when you
push that limit. Where is the limit of the string technique?
• It is never easy, but scientists will eventually come
down on the side of experiment
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Re-emphasizing Main Points
• Two pillars of science
o Experiment: makes science reliable
• Scientists led astray by logic (Aristotle) and belief
(church and geocentrism, Inquisition)
• Experiments base science on direct experience
o Theory: makes science valuable
• Once you have a reliable theory, it tells you the
answer in advance, can use it as technology
• Two quotes from Copi, Reader Pg 8
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Re-emphasizing Main Points
• I have the experiments in this course to:
o Give you direct experience
o Illustrate experiments described in class
o Illustrate social nature of science within the lab
groups
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Readings: Knowledge or
Certainty
Jacob Bronowksi
• Absolute certainty is impossible in science
o Looking at an object with infrared, then visible,
then x-rays should yield greater detail. Infrared
is very blurry, visible is pretty good, but x-rays
are too high energy to be focused. Perfect detail
of “God’s-eye” view is impossible
o Statistical uncertainty in measurements - Gauss
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Knowledge or Certainty
• 1795
• Science is discussion and argument
preceding knowledge
• Also Uncertainty Principal 1927 Werner
Heisenberg – cannot locate particle exactly
o Irreducible uncertainty or fuzzy focus
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Knowledge or Certainty
• No practical
effect at
macroscopic level, but a philosophical
problem with The Mechanical Universe and
with “The God’s eye view”
• But certainty leads to tragedy – Nazis
• (DB) Certainty and power combined
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What is Science?
Moti Nissani, What Is Science?
• Difficult or impossible to give a dictionarytype definition for science
• (DB) Working scientists rarely think about
the history or philosophy of science
• Start with philosophy of Thales – free
inquiry
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What Is Science? (cont’d)
• Then hypothesis and experiment (Torricelli)
• Falsifiability – reason and logic have not
been not sufficient to discover the truth in
science (DB: belief, either)
o But contradiction by experiment does not
always mean rejection of hypothesis – can lead
to reexamination of experiment or modification
of hypothesis
o Scientists “on the trail” have personal concerns
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What Is Science? (cont’d)
o Scientists “on the trail” have personal concerns
• Argument and community lead to progress
o Semmelweiss and deaths in maternity ward
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•
•
•
•
•
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Neighboring ward far safer
Did priest’s visit scare patients?
Washing hands – doctors did dissections beforehand
This fixed the problem
Profession slow to accept this change
Even scientists can be closed-minded, resist change
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What Is Science? (cont’d)
• Theories unify many hypotheses and
experiments
o Price is often inaccessibility to non-scientists
• Scientists usually not concerned with these
issues or with philosophical uncertainty
• Science many not be perfect, but it can still
be very good
• Many use technology but not the scientific
foundation
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Doppler Effect (Review)
• Video
• Frequency of wave higher if source is
moving towards us, lower if moving away
• Evidence that stars are moving away from
us
o Colors shifted redder (“red shift”)
o First evidence for Big Bang
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Physical Science: Current Status
• Newton’s Laws, Maxwell’s Equations and
similar classical theories (before ~ 1900)
describe world we know and see
• For things the size of molecules and
smaller, need Quantum Mechanics
• Very fast, need Special Relativity
• Very heavy, need General Relativity
• All three have weird things going on
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Relativity
• Reminder about what this is about
o Computer simulation
• Often very difficult to tell whether or not
our measurements are in a moving
coordinate system
o Earth spins on axis, moves around Sun, Sun
moves around Galaxy, is Galaxy moving?
• Theory of Relativity says we can only tell
relative motion, not absolute
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Special Relativity
• For fast-moving objects
o
o
o
o
Max speed = c (speed of light)
Objects foreshortened
Time slows down
But the traveling person says the same about
you!
o Space and time  space-time
o E = mc2
 light has mass, is bent by gravity
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General Relativity
• For very heavy objects
o Space and time warp, cause gravity
o Perihelion (closest approach to sun) of
Mercury’s ellipse not fixed as in Newton’s
Laws, but advances 43 seconds of arc per
century (observed), other effects in addition
o Says light bends twice as much as Special
Relativity says, observed 1918
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General Relativity (cont’d)
• “Einstein Halo” –
light from far
galaxy bent by near
galaxy
• Variation on
gravitational lens
• 12 found so far
•
Picture: New York Times, 12/6/05,
Pg D4 (Science)
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Quantum Mechanics
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Two different types of things
• Particle (“thing,” “object”)
o Examples: baseball, soup can, projectile, star
o One location (or center)
o Newton’s three laws govern motion
• Wave
o Examples: waves in water, sound waves, radio
waves
o Spread out, exists in many places
o “Wave Equations” governed motion (not
Newton)
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Two different types of things
Particle
Wave
Position:
Definite – one
position (center)
Spread out, no one
place
Try to catch it –
result is:
Collision with
another:
Existence:
Get all or none
Only get part, if
that
Pass through each
other
In something – the
“medium” (before
Maxwell)
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Ricochet,
bounce, shatter
All by itself
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Demonstrations
• PhET (Physics Education Technology)
http://www.colorado.edu/physics/phet/web-pages/simulations-base.html
o Particles: Gas Properties – they bounce
o Waves: Sound >> Interference by Reflection
• Interference: light  peak, dark  trough
o http://www.colorado.edu/physics/2000/schroedinger/big_interference.html
–
some areas gray (unlit)
• Light: early 1800s, Thomas Young proved light
is a wave – “double slit experiment”
o http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html
o Confine a wave – it spreads out
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Particles collide…
Particles of gas mix together, collide
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but waves pass through each other
Sound wave and its reflection
(type – sound - is unimportant here)
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Waves “interfering”
Confine a wave and it spreads out
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Waves
• Wavelength –
distance between
peaks (or troughs)
• Fixed speed
• Until 20th century,
Wave / Particle –
we thought
everything was one
or the other
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Wavelength
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Wave-Particle Duality
• In 20th century, with rise of Quantum
Mechanics, we understood that everything
was both.
o For a wave, x (position) and v (velocity)
connected
• Momentum p = m × v (m = mass, amount of matter)
o Led to “Uncertainty Principle”
• Irreducible uncertainty in our knowledge
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Uncertainty Principle
• 1795 Carl Friedrich Gauss (college student)
• Also Uncertainty Principal 1927 Werner
Heisenberg – cannot locate particle exactly
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Quantum Mechanics
• At molecular level and smaller, waves and
particles merge – everything is both
o Wave – spread out, cannot contain it
o Particle – have it or don’t
o Q.M.: wave gives chance of “catching” particle
• Cannot be made certain
• Uncertainty Principle
o Carries over to regular world, makes clockwork
universe impossible over age of universe
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Quantum Mechanics (cont’d)
• Accounts for properties of ordinary
materials
o
o
o
o
o
Theoretical: keeps matter from collapsing
Color
Solid (strength), elastic, gaseous
Solid state electronics – semiconductors
Forces – due to exchanges of particles
• No Newtonian “action at a distance”
• E.g. electrical force carried by photons – particles of
light
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Conflict!
• Heavy (G.R.) and small (Q.M.) –
mathematical conflict. Example: Black Hole
o Competing theories of gravity – “embarrassing”
• G.R.: gravity caused by masses warping space-time
• Q.M. – gravity due to exchange of “gravitons” (not
found yet)
o “String Theory” might unite these two
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• “Theory of Everything” – accelerating expansion(!)
• Matter and energy composed of elemental vibrating
strings and membranes
• Eleven dimensions, seven curled up too small to
experience directly – may have indirect experience
• Theory still developing, no unique experimental
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Issues:
• “Anthropic Principle” – physical rules seem
to favor life
o Room for God inside science?
o But “Inflationary Universe” may explain this
• Dark Matter
o Galaxies spinning fast, not enough mass to hold
them together so they should be flying apart but
this is not observed
o Must be Dark Matter at center of galaxies
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Issues (cont’d):
• Dark Energy
o Big Bang should be
slowing down
o But outer half of universe
is accelerating!
o Current hypothesis is that
dark energy at outside
fringe is attracting the inner parts.
Source: NASA
• Between these two, we see only 5%. The
universe is still surprising us!
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The end of the ride
• Strong dose of the value of science here
• One more time, about science:
o Two pillars – repeatable experiment (what makes it
reliable) and explanatory theory (what makes it valuable)
• Developed 1400 – 1800 AD: Copernicus to Dalton
o Developing hypotheses and theories is creative
o Has a boundary but expands aggressively
 not a complete basis for living
o
o
o
o
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Now drives technology
We all use it
Conflicts with some, but not all, religious beliefs
People of all ethnicities have been able to contribute
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Lab 11: Orbiting Bottle
• Swing bottle on string
o Measure distance from finger to middle of
water, convert to decimal feet (÷ inches by 12)
o Measure weight of bottle, convert to decimal
pounds
o Time ten “orbits” or circles (count from zero!)
o Measure angle down from horizontal
o Use formulae
• Large hand motion to get bottle moving,
then small hand motions to sustain motion
during measurements
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Orbiting Bottle
• String pulls in two
directions, H and V
• Two formulae for FH
1. FV (up) balances W
(down), then angle
determines FH
2. Inward force to move
bottle in circular orbit
• Two should agree,
roughly
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Lab 11: Orbiting Bottle
• If your two results (A & B) for the
horizontal (inward) force, FH, agree, then
your data are consistent with Newton’s
Laws (including Universal Law of
Gravitation).
• See Theory section for the proof of this
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Review for Final
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