Lecture 2 Chap 1 continued

1
Lecture 2 Chap 1 continued

• Jan 20, 2004

2
Keeping Track of Space and Time

• Metric System
• Commonly used in science
• Basic unit of length is meter ( 1 m 39.37
  inches, 1 mile 1609.3 m )
• Basic unit of mass or weight is gram ( 1 pound
  453.6 g)
• Basic unit of time is second
• Larger or smaller units expressed using prefixes
• Centi (1/100) 1cm 0.01m
• Milli (1/1000) 1mm 0.001m
• Killo (1000) 1kg 1000 g
• Mega (1 million) 1Mg 1,000,000 g

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Keeping Track of Space and Time

• Units of Time
• The basic unit of time is second
• But larger units of time are defined by
  astronomical phenomena
• 1 day the time it takes for the Earth to rotate
  one full cycle 24 hours 24x3600 sec
• 1 month the time it takes for the Moon to
  revolve around the Earth for one full cycle 30
  days
• 1 year the time it takes for the Earth to
  revolve around the Sun for one full cycle
  365.25 days

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Keeping Track of Space and Time

• Light Year
• Since the speed of light is not infinite, we can
  also keep track of distance in the unites based
  on the length of time that it takes light to
  travel.
• We express the distance by long time it takes
  light to cover it
• The speed of light is 300,000 km per sec (300,000
  km/s, or 186,000 miles/s)
• 1 light-second is the distance that light can
  travel in a second 300,000 km 186,000 miles
• 1 light-year is the distance that light can
  travel in a year
• 1 year 365 days 365 x 24 hours 365 x 24 x
  3600 sec
• 1 light year speed of light x 1 year 300,000
  km/s x (365x24x3600 sec) 9,530,000,000 km or
  about 1 trillion km
• Units such as light-years, or thousands, or
  million, or billion light years, are used in
  astronomy to measure the distance in the universe

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Scientific Notation

• We deal with very large and very small numbers in
  science a lot, and especially in astronomy
• E.g. 1 light year 9,530,000,000,000 km
• To effectively express the very large and very
  small numbers, we use scientific notation
• Count the number of zeros
• Write the result as a superscript of number 10
• ? 100,000,000 108 ? 10 to the power of 8th, or
  ten raised to the 8th power, or simply, 10 to the
  8th
• For a number that is not an integer power of 10,
  we divide it into two parts
• A number between 1 and 10
• Times an integer power of 10
• E.g. 9,530,000,000,000 9.53 x 1012
• For small numbers, one uses negative exponents
• 0.001 10-3, 0.00025 2.5 x 10-4

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Power of 10 A Movie

• This movie shows
• How big the universe, and how small the
  micro-universe is
• And the power of using scientific notation to
  express the vastness and the fine details of the
  universe
• http//micro.magnet.fsu.edu/primer/java/scienceopt
  icsu/powersof10/

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Looking back in time

• Light, although fast, travels at a finite speed.
• It takes
• 8 minutes to reach us from the Sun
• 8 years to reach us from Sirius (8 light-years
  away)
• 1,500 years to reach us from the Orion Nebula
• The farther out we look into the Universe, the
  farther back in time we see!

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Fig 1-1, p. 2
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Sun, 8 light minutes
10
Mars, 12 light minutes
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Jupiter, 40 lt min.
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Pluto and Charon, 300 lt min
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A star with planets forming 50 lt year
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Andromeda, a Galaxy like ours, 2 million lt yrs
100 billion Stars 100,000 lt yrs across
You are here!
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Virgo cluster, 60 million lt yrs 1000 galaxies, 6
million lt yrs across
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Coma Cluster, 360 million lt yrs, 1000
galaxies 6 million lt yrs across
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Cosmic Background, 13 billion lt yrs ? Baby
picture of the universe, 300,000 years after the
big bang
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How old is the Universe?

• The Cosmic Calendar
• if the entire age of the Universe were one
  calendar year
• one month would be approximately 1 billion real
  years

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SPACE News of the Week

• Mars Rover Spirit
• http//marsrovers.jpl.nasa.gov/home/index.html

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Astronomy News of the Week

• The End of Hubble Space Telescope In Sight
• http//www.cnn.com/2004/TECH/space/01/16/hubble.te
  lescope.ap/index.html

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Ancient Greeks First astronomers

• Greeks built first cosmological models based on
  reasoning and observations
• They used these models to explain, and to predict
  planetary motions
• Most of Greek models are Geocentric, but even
  Heliocentric models (e.g. Aristarchus) was
  proposed (2000 years ahead of his time)
• In Greek Cosmology, we see the close relation
  between science and philosophy

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Plato (428 - 348 BC)

• All natural motion is circular
• Reason is more important than observation

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Aristotle (384 -322 BC)

• Physics
• elements
• earth
• water
• air
• fire
• quintessence

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Claudius Ptolemy (AD 100-170)

• star catalogue
• instruments
• motions model of planets, Sun, Moon

His model fit the data, made accurate
predictions, but was horribly contrived!
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Ptolemys Geocentric Model

• Earth is at center
• Sun orbits Earth
• Planets orbit on small circles whose centers
  orbit the Earth on larger circles
• the small circles are called epicycles

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Nicolaus Copernicus (1473-1543)

• He thought Polemys model was contrived
• Yet he believed in circular motion
• De Revolutionibus Orbium Coelestium

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Copernicus Heliocentric Model

• Sun is at center
• Earth orbits like any other planet
• Inferior planet orbits are smaller
• Explained the available observations better than
  Polemys model

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Johannes Kepler (1571-1630)

• Greatest theorist of his day
• a mystic
• forces made the planets move
• Keplers three laws (assignment learn Keplers
  laws at astronomyplace)

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Galileo Galilei (1564-1642)

• First man to point a telescope at the sky
• wanted to connect physics on earth with the
  heavens
• Galileo discovered that Jupiter had four moons of
  its own.
• Jupiter was the center of its own system.
• Heavenly bodies existed which did not orbit the
  earth.
• Dialogue Concerning the Two Chief World Systems

This book got him in trouble with the Church!
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Sir Isaac Newton (1687)

• Published The Mathematical Principles of Natural
  Philosophy
• Three laws of motion
• Low of universal gravitation
• The basic principles of modern physics
• Newton is, arguably, the greatest scientist ever
  in human history
• Physics
• Calculus
• Optics

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The most importance figure of modern astronomy
The further the galaxy, The faster it is moving
away ? The universe is expanding
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The Big Bang

• 1948 Russian-American physicist George Gamow
  published the first model about the universe
  started from a hot and dense fireball ? the hot
  big bang the predicted this creation left
  behind a faint glow that suffuses the universe
  today, the cosmic microwave background

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The discovery of Cosmic Microwave Background

• Bell lab physicists Arno Penzias and Robert
  Wilson discovered CMB by chance when building
  their low-noise communication antenna. This
  confirmed the Big Bang theory. The two key pieces
  of modern cosmology expansion of the universe
  and hot big bang, are firmly established.

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In the Beginning

• Modern astronomy is scientific
• Based on detailed observations and complicated
  theory
• But the quest of the origin and fate of the
  universe lasted as long as the human civilization
  existed
• The earliest models of the universe are based on
  human experience, and are strongly influenced by
  culture, mythology and religion
• But what makes astronomy a Science?

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Scientific Thinking

• It is a natural part of human behavior.
• We draw conclusions based on our experiences.
• Progress is made through trial and error.

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The Scientific Method

• Question
• Hypothesis
• a tentative explanation
• Prediction
• Test
• Result
• confirm, reject, or modify

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Hallmarks of Good Science

• Science seeks explanations for observed phenomena
  that rely solely on natural causes.
• Science progresses through the creation and
  testing of models of nature that explain the
  observations as simply as possible.
• Occams Razor
• A scientific model must make testable predictions
  that could force us to revise or abandon the
  model.

Theory
-- a model which survives repeated testing
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Scientific Theory Can be WRONG!

• In fact, the very nature of scientific theory is
  that it has to be testable, I.e., it has to be,
  in principle, falsifiable
• Scientific theory first tries to explain existing
  data
• Then seeks to make testable prediction and seeks
  to reach its limitation in application
• Then either the hypothesis can be improved, or it
  has to be abandoned
• All theories has its limitation, to put it in
  another way, all theories was, or will be WRONG!
• But at each step, we are getting closer to the
  truth!
• So, do you believe in the Big Bang???

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Do I believe in Big Bang ??

• Yes the currently observable universe started
  in and expanded from a very hot, dense state and
  it is still expanding the big bang theory has
  passed all the tests we have so far
• No the Big Bang theory can only take us to the
  limit where current physics theory (relativity
  and quantum theory) can be applied it is not the
  end of the story, and will be improved upon
• Irrelevant the Big Bang theory explains well
  the physical universe but it has little to do
  with belief or faith, and it provides no direct
  answer to your more personal universe
  spiritual, religious, ethnical etc.

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The Driving Force behind Astronomical Discoveries

• Technological innovations
• Cosmology is an experimental science
• The major discovery always followed closed to
  major technological breakthroughs
• First use of telescope ? Galileos discoveries
• First modern telescope ? Hubbles discoveries
• Radio telescope ? CMB
• Space telescope ? Hubble Space Telescopes
  determination of the size and fate of the
  universe
• Deeper understanding of the Law of Physics
• Cosmology is (now) a physical science
• Modern cosmology followed, and is built on
  Einsteins theory of General Relativity
• Luck
• Nature is full of surprised, be prepared

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Homework for Ch 1

• Chapter 1, Question 2 and 8, due (together
  with questions from Chapter 2) on Jan 29