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IntroductionTopic 4: Scientific Methods

Big Bang, Black Holes, No Math

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ASTR/PHYS 109Dr. David Toback

Lecture 5



Prep For Today (is now due) – L5If you just joined the class, welcome. Send me an email if you need an extension. All this is posted in the lecture notes!!• Reading:

– BBBHNM Unit 2 (Chapters 5-9): Was due before class– Recommended Reading:

• See P3 of http://people.physics.tamu.edu/toback/109/Syllabus.pdf• Warmup Quizzes:

– (Warm-up Quizzes Parts 1-3)– Warm-up Quiz Part 4 (Peerceptiv): Will assign soon– Extensions granted if needed. [email protected]

• Pre-Lecture Reading Questions:– Unit 1 Quizzes, parts A & B: Was due before class– Unit 2 Text Submission: Will assign today

• Submit to TurnItIn on eCampus. • Submit ONLY 4 questions, need all 4 to be excellent to pass• Pass/Revise assignment. If you don’t pass we will set up dates for Revisions after you get

Feedback– Unit 2 Quiz: Will assign next time

• End-of-Chapter Quizzes:– Chapter 3: Was due before class

• Honors Section:– In a heavy reading period, “Stage 0 idea Submitted” by the end of the day. Fill out the poll to

find a time to meet as a group. Need to hear from you at least once every 7 days

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Next Topic: Scientific MethodsUnit 1:1. Introduction Done2. Going Big Done3. Going Small Done4. Evidence and the Scientific

Method TodayToday we move on to the Questions

and How we go about answering them

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Plan for the Next Few Weeks1. More on the Questions

and How we go about answering them

2. Some of the history to teach us about the method

3. Need to learn some physics

Today

} Unit 2 (Chapters 5-9)



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Next few Weeks Continued… The tools in our toolbox… Learning some PhysicsTo learn Cosmology will need to learn a bit about:1. Light and Doppler Shifts 2. Gravity, General Relativity and Dark

Matter3. Atomic Physics and Quantum

Mechanics4. Nuclear Physics and Chemistry5. Temperature and Thermal Equilibrium

Won’t spend too long on these, just enough to get back to the big picture…



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Evidence and The Scientific Method• Finished: What are some of the clues at the scene of the crime?

• Next: How can we use the clues to figure out what happened? And WHY?

• Scientific Method– How do we know what we know? – What is the evidence for some of the “true things” we heard growing up?•E.g. Earth goes around the Sun



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How do we know what we know?• We have a lot of experience in the world around us

• Unfortunately, our experience is really lousy in guiding us to really understanding the bigger (and smaller) things in nature unless we’re really careful and disciplined– We can misinterpret the clues – It really does look like the Sun “rises”

• As you’ve already seen, the world is incredibly complex and much of it is different from what we experience– Good clues are hard to find, sometimes even harder to interpret



Single example to Show the Scientific Method in Action

• People have watched the sky and noticed that the stars (the dots of light in the sky) basically all move together over the course of the night and over years

• Five of them behave differently–Start this story in the 2nd century

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Fun Videos of Just the StarsStart with things you can see with your own eyes!Video of stars moving with Polaris (north star) at the centerhttps://www.youtube.com/watch?v=XTTDWhky9HY

Video of stars moving, including the Milky Way (from Chile)https://www.youtube.com/watch?v=JEHm-XUHwNw

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Ptolemy’s Universe (2nd century)The Sun, Moon and Stars go around

once a day, but a few (the planets) change their position relative to the other stars a little every night

Every so often the planets move backward through the stars for awhile

Hypothesis: They orbit the Earth in mostly circular paths, there are “mini-orbits” way out there which we call “epicycles”. It’s these the mini-orbit part that “explains” the back and forth part



Looking at Mars in the Stars

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http://bigbang.physics.tamu.edu/Figures/StolenAnimations/MarsRetrograde_top.mp4



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How Ptolemy envisioned it

http://bigbang.physics.tamu.edu/Figures/StolenAnimations/ptolemy.model.swf

•The big circle is the main orbit, and the little circle is the epicycle•This explains why the planets seem to go back and forth out there in space (retrograde motion) every so often



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Cracks in the `early cosmology’In 1514, Copernicus hypothesized that the Planets and the Earth orbit the Sun (he wasn’t really the first one to suggest this…)

Much simpler in some ways no epicycles

More complicated in others Says Earth is rotating



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Copernicus’s “Hypothesis”A more modern view of the

motion of the Earth and Mars and the stars behind them

(from the point of view of the center)

How would this explain the epicycles that people saw?

http://bigbang.physics.tamu.edu/Figures/StolenAnimations/mars_retrograde_motion.swf



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If we lived in the 1500’s, should/would we have believed Copernicus?

The Earth isn’t at REST and rotates?

• Shouldn’t we FEEL this?

• If the Earth is rotating, why don’t we fall off like an ant on a bicycle wheel?

• Why don’t we feel a wind as we rotate?

• Why doesn’t the rotating Earth move under our feet when we jump?



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Cracks in the `Early Cosmology’Should his view have just “been accepted”?

Perhaps his theory was just a “different interpretation” of the same data?–Both models are consistent with observations

Need more evidence!Need a better TOOL to test, experimentally, which is correct

Early 1600’s: Kepler and Galileo started gathering data from telescopes



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Early Data Provides EvidenceDiscover moons orbiting Jupiter! Solid evidence that not EVERYTHING orbits the Earth!



Another Piece of Evidence• Can understand the phases of the Moon because of the locations of all three

• Not eclipses

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VenusConsider the two different predictions

for how Venus moves in space

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The Phases of Venus• Venus has a full set of phases, like the moon• Sunlight shining off Venus and to our eyes

• No good way to explain this if Venus goes around the Earth

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More data• With more accurate data Kepler realizes an even better description of the data is that all planets orbit the Sun in an ellipse, not circles

• Sun-centered model now agrees with the high-quality observational data, Earth centered model does not

• No good REASON for ellipses though… then again, no good reason they should be circles (except people LIKE circles, and they are more “perfect”) 21



Need to Separate the Issues• The fights up to this point were about HOW the planets move

• There was no good explanation of WHY they move that way

• If someone could explain WHY they move that way, then maybe we can learn something close to the truth about the universe/nature

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The next generation…Newton•Newton puts forward his theory of Gravity and describes it as a Force

•So what? The same force that pulls an apple to the ground from a tree ALSO pulls the planets towards the Sun and keeps them in orbit

•This “explains” why both the orbits are ellipses AND why we don’t fall off a spinning Earth

Isaac Newton 1687



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Scientific MethodThe history is fun, but we have

a problem:• How do we separate true

stories from stories we’d like to believe, but aren’t actually true?

• Need EVIDENCE and a good Scientific THEORY– Good hypothesis testing



Lecture on Chapter 4 now complete

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Outline for Unit 2: Physics We Need

Topics1. Light and Doppler Shifts2. Gravity, General Relativity and Dark

Matter3. Atomic Physics and Quantum

Mechanics4. Nuclear Physics and Chemistry5. Temperature and Thermal Equilibrium



Prep For Next Time – L5

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If you just joined the class, welcome. Send me an email if you need an extension. All this is posted in the lecture notes!!• Reading:

– BBBHNM Unit 2 (Chapters 5-9): Was due already– Recommended Reading:

• See P3 of http://people.physics.tamu.edu/toback/109/Syllabus.pdf• Warmup Quizzes:

– (Warm-up Quizzes Parts 1-3)– Warm-up Quiz Part 4 (Peerceptiv): Will assign soon– Extensions granted if needed. [email protected]

• Pre-Lecture Reading Questions:– (Unit 1 Quizzes, parts A & B)– Unit 2 Text Submission: Submission due before next class

• Submit to TurnItIn on eCampus• Submit ONLY 4 questions, need all 4 to be excellent to pass• Pass/Revise assignment. If you don’t pass we will set up dates for Revisions after

you get Feedback– Unit 2 Quiz: Will assign next time

• End-of-Chapter Quizzes:– If we finished Chapter 4 then End-of-Chapter Quiz 4 (else just Chapter 3)

• Honors Section:– In a heavy reading period, “Stage 0 idea Submitted” by the end of the day. Fill out the

poll to find a time to meet as a group. Need to hear from you at least once every 7 days

Physics We NeedTopic 1: Light and Doppler Shifts


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Next Set of Topics: Physics We Need

Finished our introduction• Big and small things in the Universe

• Evidence and the Scientific Method

Next we move on learn some of the physics we need in order to understand why scientists have confidence in the Big Bang Theory



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Back to the Big Picture

Questions we’re trying to answer:• How did the Universe come into being?

• How did it evolve into what we know today?

• What is the evidence for all of this?



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Overview of Physics We Need

• Cosmology, the big bang and black holes are some of the most interesting things that can be understood

•However, to UNDERSTAND them better, and the EVIDENCE for them, we need to learn some physics



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The Topics• Some stuff we need to learn a little about:

1. Light and Doppler Shifts2. Gravity, General Relativity and Dark

Matter3. Atomic Physics and Quantum Mechanics4. Nuclear Physics and Chemistry5. Temperature and Thermal Equilibrium

• We won’t spend too long on each, just enough to get back to the big picture…

• Since there is no perfect way to present them (they all tie into each other) we’ll just start somewhere and get going…



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LightWhat we know about the universe comes from multiple places

One of the most important is from looking at both outer space and “inner” space

Need to know more about the “light we see”



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Outline for Light

1. Light is a wave…2.…and a particle3. Light is REALLY fast,

but does have a finite speed

4.Doppler effect



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Part 1: Light is a Wave•The light we see is a “wave”, like waves on an ocean•Best described by it’s wavelength



Why do we believe light is a wave?

• Water and Sound are also both waves–Does light behave like water or sound?

• Can “see” water waves hitting a wall and what happens to them when they hit two small openings in a wall

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Water waves moving towards a wallYou are in the sky looking down at the waves as they go by(Would see the same thing if you drop a rock in some water) 13



Why do we believe light is a wave?

Shine light from a laser at two small holes in a wall and see what comes out on the wall behind it

The waves make patterns on the wall the same way water waves do

Can measure the wavelength(Physics 202 & 208) 14



The Wavelength of Light•Have also observed that when we shine white light at a prism and it breaks up into its different colors

•Each color has a different wavelength which we can also measure

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