Big Bang, Black Holes, No Math Physics We Need Topic 2: Gravity 1 Big Bang, Black Holes, No Math ASTR/PHYS 109 Dr. David Toback Lectures 8, 9 & 10

Big Bang, Black Holes, No Math

Physics We NeedTopic 2: Gravity

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ASTR/PHYS 109Dr. David TobackLectures 8, 9 & 10



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Prep For Today (is now due) – L9

• Reading: – (BBBHNM Unit 2)

• Pre-Lecture Reading Questions– eCampus quizzes (all 6)– Unit 1 Revision (if needed): Stage 2

•Let us know if you were misgraded on the original

– Unit 2 Revision (if desired): Stage 1•Due date move to Monday. Let us know

if you were misgraded• End-of-Chapter Quizzes

– Chapter 5, parts a & b• Paper 1:

– Not yet assigned



3

Prep For Today (is now due) – L10

• Reading: – (BBBHNM Unit 2)

• Pre-Lecture Reading Questions– eCampus quizzes (all 6)– Unit 1 Revision (if needed):

•Let us know if you were misgraded on the original

– Unit 2 Revision (if desired): Stage 1• End-of-Chapter Quizzes

– Chapter 5, parts a & b• Paper 1:

– Not yet assigned



Heads Up: Paper 1

•What is the evidence for Dark Matter?

•This will be the topic of Paper 1

• In order to understand the evidence, we next talk about gravity and Dark Matter

•Will be due 1 week after we finish Chapter 6

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Papers in CPR/ScheduleSchedule is Like PLRQ since it is in CPR:Each Paper has 4 (or 6) Steps:1.Mentioned after we start the chapter (just did

that)2.Assigned after we finish the chapter3.Submit text 1 week later (Stage 1)4.Calibrations/Reviews/Self-Assessment due the

week after that (Stage 2)5.Revision (optional): Stage 1 due one week later6.If you did a Stage 1, Calibrations/Reviews/Self-

Assessment due one week after that

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Style of the Paper

• Explain it to someone who isn’t taking the class (no jargon)

• ~600 words (This is the equivalent of both side of a sheet of paper, double spaced)

• No citations! Use your own words• Text should be professional. You

are “trusted guide” not a “buddy” or “comedian”

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Paper Format

• Must follow expected Format • Usually 5 paragraphs. Needs:– Introduction paragraph–1 paragraph per piece of

evidence (3 total?)–Conclusion paragraph that ties it

together–http://faculty.physics.tamu.edu/t

oback/109/WritingAssignments/samplepaper.shtml

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Help Available for Stage I

• Submit a draft for feedback from the TA–Submit on eCampus in “Rough

Drafts (Optional)” –Drafts due Sunday at midnight– If you submit late, we can try to

give feedback but we can’t guarantee it

• We also recommend going to the writing center 8



Biggest reason people don’t do well

1. Read ALL the instructions on CPR

– See the FAQ on papers2. Don’t forget to submit to

turnitin.com3. We are NOT giving you the

Rubrics for the papers, but lots of hints about what the Rubric questions will contain are in the Writing Prompt and Goals sections

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Best Guess Schedule

•Paper 1: –Due after we finish Ch 6, Feb 25

•Paper 2: –Due after we finish Ch 8, Mar 2?

•Paper 3: –Due after we finish Ch 12, Mar 23?

•Paper 4: –Due after we finish Ch 17, April 8?

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For People in the Honors Section

Stage 0 (not in CPR): Due Feb 25th

For details see http://people.physics.tamu.edu/toback/109/Honors/

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Outline for Unit 2: Physics We Need

Topics1. Light and Doppler Shifts Done2. Gravity, General Relativity and

Dark Matter Today3. Atomic Physics and Quantum

Mechanics After that4. Nuclear Physics and Chemistry5. Temperature and Thermal

Equilibrium



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Why Gravity Next?

•By looking at photons/light we can learn about things here on Earth as well as about inner/outer space

•What else do we need to describe/understand the Universe? Quantum Mechanics and Gravity

•To understand the answer to these questions we need to learn more



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The Two Great Theories

•General Relativity: (Gravity) Predictions about the very large, from sizes of a few meters to the size of the universe (1024 miles across)•Quantum Mechanics: Predictions about the very small (atoms, particles,<10-10 m)

Chapter 6

Chapter 7



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Overview of Gravity for the Course

1. What’s so important about Gravity?

2. Newton’s Gravity

3. Einstein’s Gravity–Curved Space-Time, and evidence for it with Dark Matter



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Gravity: Why do we care?

•Gravity: The great attraction in the Universe

•Gravity is the theory that predicts the attraction and the motion of BIG things over large distances:–Planets–Suns–Galaxies–How Galaxies form etc.



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Newton and Gravity1. Everything moves in a

straight line unless acted upon by a force

2. Gravity is a force– Every object in the

universe attracts every other object in the universe

– The further the distance between the objects, the smaller the attraction

– The bigger the mass, the bigger the attraction• Light is massless

not affected by gravity

Physics 201 or 218



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Large Number of Scales

Kinda amazing!Gravity covers the attraction between –An apple near the Earth –The Earth and the Moon–The Earth and the Sun–The Sun and our galaxy–Our galaxy and the universe–Every particle in the universe and an apple–The Earth and you–Bevo and Reveille



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Gravity continued…

The force of Gravity makes the Moon “fall” towards the Earth

Call this an orbit–Does a great job of explaining how the planets move around the Sun

Without gravity, the Moon would fly off

into space and ignore the Earth

completely



What’s next?

Tell you the different, and surprising, way that Einstein describes space, time and gravity

Then tell why his description of why the Earth goes around the Sun is better than Newton’s

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Einstein in the 1910’s• In the early 1910’s

Einstein was thinking about some recent experimental results that didn’t make any sense to him using Newton’s theories

• Decided we need new ways of thinking about space, time and Gravity

• Einstein says that Newton’s Laws aren’t really quite right…

• Einstein’s theory is known as the “General Theory of Relativity”

2nd year Grad School Course



Observational Fact

Light *ALWAYS* moves at the speed of light to all

observers

So what? 22



What happens if I’m driving a car moving at half the speed of light and I turn the

headlights on?

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The light is one foot

ahead of that car!

Two observers get different answers

The light is two feet

ahead of me!



What happens if I’m driving a car moving at half the speed

of light and I turn the headlights on?

Einstein says “it’s relative”, both observers are correct

Space and Time are more related than we thought

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Start with Space-Time

Can’t think of Space and Time as separate –Space (measured with a ruler)

–Time (measured with a clock)

Single combined entity which we call four dimensional space-time– If the four dimensions are related, unexpected things that we’re not used to can happen



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Other Weirdness

•Space and time are not only more related than we thought, Space can Curve•Need to talk about what we mean by Curved Space-Time



We’ll start by using the strange “new” words of General Relativity in an

example about why we care, and THEN explain them a bit

moreSpace-Time and

Gravity

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Newton vs. Einstein

Newton: The Earth moves around the Sun because of “the force of gravity” is pulling it

Einstein: There is no “force” of Gravity, the Earth moves in a “straight line” around the Sun in the curved space-time created by the SunThis is a VERY different way of

thinking about things…



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Curved Space?

An analogy is to think of curved space-time as looking like one of those gravity wells you’ve seen downstairs

1st floor of the Mitchell Physics building (MPHY)



Moving in Curved Space: Analogy

• Let’s say my friend and I are at the equator and we both start working due North– Exactly parallel to each

other• We will notice that we

“mysteriously” are getting closer to each other, and will eventually bump into each other

• Is there a Force drawing us together? No… moving in curved space time LOOKS like a force

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The rules of geometry are different in curved space

http://en.wikipedia.org/wiki/Parallel_(geometry)



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Another Weird Thing: Mass Curves Space-Time

Think of each of the heavy things in the universe (stars, planets etc.) like a ball in the middle of a taut rubber sheet that represents space-time

The weight of the ball will make it sink into the rubber sheet, creating a cone shaped dent around it



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Mass Curves Space

The heavier the ball the bigger the dent in space-

time!



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Mass in curved space-time

A mass moves in a “straight line” in curved space-time

In this example, this straight line in curved space makes the path of the small ball look like something is pushing it toward the big ball in 3-dimensions



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The Earth and the Sun

Newton: The Earth moves around the Sun because of “the force of gravity” is pulling it

Einstein: There is no force, the Earth moves in a straight line in four dimensions, but the curved space around the Sun makes it go in an orbit in the three space dimensions



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The way the Planets go Around the Sun in General

Relativity



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Ok…

• Both Newton and Einstein correctly predict how the planets go around the Sun–Actually mostly their predictions

for the locations of the planets at any given time are almost identical

• How do we decide if one is right and one is wrong?

• Do an experiment!!!



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When do General Relativity and Newton predict different things?

• Ok… One equation, but I’m guessing you’ve heard this one

E=MC2

Ok… what is this equation saying? Energy and mass may not be the same, but in Einstein’s theory they are equivalent



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So what?

What about light? It has energy, but no mass

To Einstein, its mass doesn’t matter, it is a particle that moves in curved space-time



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What happens to light as it goes past the Sun?

•Newton’s theory: Only things with “real” mass “feel” the force of gravity

•General Relativity: Objects move according to the curve of space-time, regardless of whether they have mass or not

Path doesn’t curve

Path does curve



Do an Experiment

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1st Experiment: Can see the star directly

If Newton is right, we

won’t see the starGross

exaggeration of sizes

If Einstein is right, the Sun

curves the path of the

starlight

2nd Experiment: Sun in the

way

Question: Where does the star

appear when you do the

experiment?

Looks like its over here!



The Great Experiment of 1915

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Look at a star’s position “behind” the sun as it “passes” in between us and the star during an eclipse to block the glare of the sunlight



Another View

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Evidence and More Evidence

• The results exactly agreed with Einstein’s predictions–Contradict the predictions of

Newton• Over time, even more compelling

evidence has come in in favor of General Relativity observations of Dark Matter in galaxies

• Next give some of the evidence for dark matter and that the story hangs together

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Evidence galaxies contain Dark Matter

• Described the Dark Matter in the Universe and in galaxies in Chapter 2

• In order to better understand dark matter in galaxies, its useful to show some evidence that there IS Dark Matter in galaxies

• Start by considering the case that there IS no Dark Matter in galaxies

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How stars move in galaxies•Laws of gravity accurately predict the orbits of planets and stars as they move around the solar system and galaxy •The brightest region of both places is the center lots of mass there•For the solar system, the data agree perfectly, but for the outer part of galaxies it should look like stars in orbit around a massive center•Problem: This isn’t what the data shows



The Data

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Does this work for Stars?

Simulation without Dark Matter

Simulation with lots of

Dark Matter particles in the galaxy

http://bigbang.physics.tamu.edu/Figures/StolenAnimations/galrot_anim.gif

Watch how fast a star rotates around the center of the galaxy…

Data looks like this



Data well explained by lots of “Dark Matter” we can’t see

This is where it gets its nameIn some sense, the name is a statement of almost all we know about it (it doesn’t

interact with light, and it has mass)

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Another Experiment

Maybe there isn’t dark matter and General Relativity is wrong?

Look at the gravitational impact on light that travels

through the Universe, through a galaxy, and

towards us



Einstein Ring’s?

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Another view

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Einstein Ring

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The “near” galaxy

The light from the “far” galaxy spread out into a ring

Amount of lensing explained by extra mass of dark matter



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Winner: General Relativity

• Many observations indicate that General Relativity correctly describes the motion of light, galaxies and stars in the Universe we live in space and time are related

• Space can curve, stretch, change etc. • Curved space-time will be very

important in later chapters when we talk about the expansion of the Universe

• Dark matter is an important part of the evolution of our universe



Paper 1

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• Abbreviated description: What is the evidence for Dark Matter? – More detail on CPR, you REALLY need to read ALL the

instructions• Explain it to someone who isn’t taking the class (no

jargon)• Follow the required Format:

– Introduction paragraph– ~1 paragraph per piece of evidence– Conclusion paragraph that ties it together

• Help:– Example of good paperhttp://faculty.physics.tamu.edu/toback/109/WritingAssignments/samplepaper.shtml

– The first 9 Rubric questionshttp://people.physics.tamu.edu/toback/109/WritingAssignments/

PaperRubrics_9Questions.pdf



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Outline for Unit 2: Physics We Need

Topics1. Light and Doppler Shifts

Done2. Gravity, General Relativity,

and Dark Matter Done3. Atomic Physics and Quantum

Mechanics Next4. Nuclear Physics and

Chemistry5. Temperature and Thermal

Equilibrium



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Prep For Next Time – L8• Reading:

– (BBBHNM Unit 2)• Pre-Lecture Reading Questions

– eCampus quizzes (all 6)– Unit 1 Revision (if needed): Stage 2

• Let us know if you were misgraded– Unit 2 Revision (if desired): Stage 1

• Let us know if you were misgraded• End-of-Chapter Quizzes

– If we finished Chapter 6 then End-of-Chapter Quiz 6 (else, just through Chapter 5, parts a & b)

• Paper 1:– If we finished Chapter 6: Stage 1 due Monday

before class– Can submit a draft for feedback on eCampus if you

like– MUST submit to turnitin on eCampus



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– (BBBHNM Unit 2)• Pre-Lecture Reading Questions

– eCampus quizzes (all 6)– Unit 1 Revision: Let us know if you were misgraded– Unit 2 Revision (if desired): Stage 1

• Let us know if you were misgraded• End-of-Chapter Quizzes

– If we finished Chapter 6 then End-of-Chapter Quiz 6 (else, just through Chapter 5, parts a & b)

• Paper 1:– If we finished Chapter 6: Stage 1 due Wednesday

before class– Can submit a draft for feedback on eCampus if you

like by Sunday night– MUST submit to turnitin on eCampus



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Clicker Quiz

Which of the following travels the fastest?

A.Radio wavesB.Gamma raysC.Red lightD.Blue lightE.They all move

at the same speed



Clicker QuizWe observe two objects in

space. From other studies, we can tell that object X is emitting infrared light and the object Y is emitting Ultraviolet light. We observe the color of the light object X to be Red and the color of Object Y to be blue.

Which statement is true?a) X and Y are moving towards

usb) X and Y are moving away

from usc) X is moving towards us and Y

is moving away from usd) X is moving away from us and

Y is moving towards us

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Clicker Question

What would happen to the Einstein ring around a galaxy who's Dark Matter suddenly wasn't there?

a)Nothingb)It would get smallerc)It would get biggerd)It would disappeare)None of the above 62



Clicker QuestionWhat would happen if you were shot by

a gun that fires "dark matter” bullets which have the same size and mass as a regular bullet?

a)Your molecules would significantly spread out from an expansion of space-time

b)A black hole would form inside youc)Objects behind you would be

gravitationally lensedd)Your body would barely noticee)You would die like you would if you

got shot with any kind of bullet

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Clicker Question

Why is dark matter "dark“?a)Because it is closely linked to dark energy

b)Because it has no massc)Because it is made of black holes

d)Because it doesn’t interact with light 64



Clicker Question

As far as we know, Dark Matter exists:

a)Everywhere in the universe

b)Only near black holesc)Mostly in or around galaxies

d)It doesn't existe)We don't know where it exists

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Clicker Question

A man shoots a red photon straight up into the air next to a very tall building. The photon rises in the air. Which statements are true:

a)The photon slows down as it rises, the same way a rock would

b)The photon loses energy, the same way a rock would

c)The photon gets blue-shifted

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Clicker QuestionInstead of throwing a rock into the air, a man shoots a red photon

straight up into the air next to a very tall building. The following question are about the photon when it reaches the top of the building. Which of the following statements are true

Question 1: Speeda) The speed of the photon stays the sameb) The speed of the photon slows down like a rock would

Question 2: Energya) The energy of the photon stays the same b) The energy of the photon gets lower (like the kinetic energy would

for a rock)c) The energy of the photon gets larger (since it is getting closer to

outer space)

Question 3: Colora) The color of the photon stays the sameb) The color of the photon is red-shifted (gets a longer wavelength)c) The color of the photon is blue-shifted (gets a shorter wavelength)

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– None• Pre-Lecture Reading Questions

– Let me know if you think you were misgraded

– Unit 1 Revision: Grades will be posted in eCampus soon, instructions to follow if you didn’t pass

– Unit 2 Revision (if desired): Stage 1 due in CPR Monday before class

• End-of-Chapter Quizzes– If we finished Chapter 6 then End-of-

Chapter Quiz 6 (else, just through Chapter 5)



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– (BBBHNM Unit 2)• Pre-Lecture Reading Questions:

– Let us know if you have been misgraded– Unit 1 Revision: New grades posted soon.– Unit 2 Revision (if desired): Stage 2

• End-of-Chapter Quizzes:– If we finished Chapter 6 then End-of-

Chapter Quiz 6 (else, just through Chapter 5)

• Paper 1: – Stage 1 due Monday before class– Can submit a draft for feedback on

eCampus if you like– MUST submit to turnitin on eCampus



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PLRQ 1 Revision

•We still have a number of people who haven’t passed all the parts yet

• I opened Stage 2 to let people finish it, thus the updated grades have not been posted on eCampus

•When they are done, I will send an email with new instructions – We will have a special way for student who need a third try



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Back to Our Example

Gravity well analogy:

• In Relativity: It’s not that there is something “pulling" towards the center, instead space is curved and the penny is just moving in a straight line in curved space



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Starting to think about “Curved Space-Time”

General Relativity talks about 4-dimensional space-time

Our eyes can only see in 3 space dimensions

To us something moving in a “straight line” in 4-dimensional curved space time “looks” like its moving in a curved path in the 3-dimensional space we’re used to

Analogy in 3 dimensions



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Prep For Next Time – L5•Note: May change depending on how far we

get in lecture•Reading:–BBBHNM: All reading through Chapter 9

•Reading Questions:–All reading questions through 9

•eLearning Quizzes–If we finished Chapter 6 then end-of-chapter quiz 6 (else just up to Chapter 5)

•Paper 1–Revisions if needed



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Prep For Next Time – L9•Reading:

– BBBHNM: Chapter 7 (already assigned)•Pre-Lecture Reading Questions:

– Chapter 7 (already assigned)•End-of-Chapter Quizzes:

– Did not finished Chapter 6, so just Chapter 5 (already due)

•Paper:– Paper 0: Submit to CPR and turnitin on

eCampus by Friday– Calibrations after that– Will assign Paper 1 next time since we will

finish Chapter 6



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get in lecture•Reading:

– BBBHNM: Chapter 7 (already assigned)•Pre-Lecture Reading Questions:

– Chapter 7 (already assigned)•End-of-Chapter Quizzes:

– If we finished Chapter 6, then Chapter 6. If not, just Chapter 5 (already due)

•Paper:– Needed to be submitted to CPR and

turnitin on eCampus– Be working on Calibrations. – Due Wednesday– Paper 1 coming up, will be due one week

after we finish Chapter 6.



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get in lecture•Reading:

– BBBHNM: Chapter 7 (already due)•Pre-Lecture Reading Questions:

– Chapter 7 (already due)•End-of-Chapter Quizzes:

– If we finished Chapter 6, then Chapter 6. If not, just Chapter 5 (already due)

•Papers:– Paper 0 due to CPR and turnitin on

eCampus on Friday– Be working on Calibrations– If we finished Chapter 6, Paper 1 will be

due next Wednesday before class



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Was Due For Today– L8•Reading–BBBHNM: None

•Pre-Lecture Reading Questions:–None

•End-of-Chapter Quizzes–Chapter 5

•Paper Stuff–Paper 0 in CPR and turnitin.com before class



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Was Due For Today– L9•Reading–BBBHNM: Chapter 7

•Pre-Lecture Reading Questions:–Chapter 7

•End-of-Chapter Quizzes–Chapter 5 was already due

•Paper Stuff–Will talk about this today



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Was Due For Today– L10•Reading–BBBHNM: Chapter 7 (already due)

•Pre-Lecture Reading Questions:–Chapter 7 (already due)

•End-of-Chapter Quizzes–Chapter 5 parts A&B (already due)

•Paper Stuff–Working on Paper 0, due Friday



Paper 0, Questions 1-51. Does the paper make it easy to understand what question is being answered, what the

answer is, and how it will be answered? If anyone of those are missing, or if you can't follow the argument, then choose No. If the argument is clear, but not said very well (or is wordy), then you should still give a Yes. If the paper is clear to you, but would not be clear to someone who hasn’t taken the course choose No. Being easy to read isn't a good enough reason to give a Yes. Having a sentence or two that are hard to understand (or poorly written) within a basically clear paper is fine, and can still deserve a Yes if it is otherwise ok.

2. Are the pieces of evidence clearly and correctly enumerated in the opening paragraph? This includes the electrons going around atoms, the Moon orbiting the Earth, and planets orbiting the Sun.

3. Does the structure of the paper follow the instructions? Specifically: Is there an introduction paragraph, separate paragraphs discussing evidence, and a conclusion paragraph? If any are missing, choose No. Does the paper directly address the question and make the argument to answer it? If the paper addresses the question and makes an argument (right or wrong) to answer it, choose Yes (we will address the wrongness in a later question). Do not choose No due to a poorly written or confusing introduction. The paper does not need to have only three paragraphs, and the pieces of evidence can be intertwined within them. Do not choose No due to missing evidence or more than three paragraphs. Do not choose No just because evidence is not constrained to these paragraphs. If the paper does not end with a conclusion paragraph, choose No. Do not choose No due to a poorly written or confusing conclusion.

4. Is the paper generally free of extraneous evidence? If there is any significant evidence that is off topic or an argument that has nothing to do with the topic, choose “No.” If there is significant evidence from outside class, choose No. Do not choose “No” due to evidence that is unnecessary if the evidence is relevant to the topic.

5. Does the paper only use language and words that can be understood by the average lay-person? If the author uses an acronym, like GR, for general relativity that is ok but only if the acronym is clearly defined. If they use an acronym that isn't defined, then select No. Similarly, just because you saw these words in a high-school science class does not mean they are understood by the average person. The average person on the street would know what an atom is, but not know what a spectral line is.

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Paper 0, Questions 6-106. Does the essay use language formal enough to give to a

statesman? If the paper uses jokes, slang or other informal language, choose No. Do not choose No due to poor grammar, spelling mistakes, incomplete sentences, or poor sentence structure (we will take off points in a different question for this). Do not choose No due to the use of an acronym, like USA, since it is common usage and not slang.

7. Is the essay generally free of instances of bad grammar, incomplete sentences or spelling mistakes? If there are more than two minor mistakes, choose No.

8. Does the paper make clear how and why the evidence presented relates to the argument? If the facts are mentioned but show no clear connection to the argument, then choose No. If the facts are not addressed throughout the paper, then choose No. If it states the data without saying why it is evidence then choose No.

9. Do the middle (evidence) paragraphs clearly tie into the overall structure?

10.Does the paper use the scientific method and evidence based decision making? It’s OK if the words “scientific method” and “evidence-based decision making” are not mentioned specifically.

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Paper 0, Questions 11-1511.Does the essay make it clear what the Scientific

Method is and why it is important in this paper?12.Does the essay clearly tie together the pieces of

evidence using the scientific method in the conclusion?13.Does this paper say that the Moon orbits the Earth?14.Does the paper mention explicitly that the Earth is

much more massive than the Moon, and that it is important to why the Moon orbits the Earth instead of the other way around? If it mentions that the Earth is more massive, and that this is what is important, then this is good enough. If it just says that it is more massive without saying why that is important in some way, then answer No. If it says "bigger" or "larger" (or that the Moon is "smaller") that is good enough.

15.Does the paper mention that the Earth or the planets orbit the Sun? Either is fine. If neither is mentioned, then give a No. 82



Paper 0, Questions 16-2016.Does the paper mention explicitly that the Sun is much more massive

than the things orbiting it, and that it is important to say why they orbit the Sun instead of the other way around? If it mentions that the Sun is more massive, and that this is what is important, then this is good enough. If it just says that it is more massive without saying why that is important in some way, then answer No. If it says "bigger" or "larger" (or that the planets are "smaller") that is good enough.

17.Does the paper mention that electrons orbit protons or that electrons orbit a nucleus? Either one is fine.

18.Does the paper mention explicitly that the proton (or nucleus) is much more massive than the electron orbiting it, and that it is important to why they orbit the proton (or nucleus) instead of the other way around? If it mentions that the proton or nucleus is more massive, and that this is what is important, then this is good enough. If it just says that it is more massive without saying why that is important in some way, then answer No. If it says "bigger" or "larger" (or that the electron is "smaller") that is good enough.

19.Does the essay spend an appropriate amount of time on each topic?20.Does the conclusion paragraph tie together all the big picture issues

discussed in the other paragraphs? If the conclusion does not summarize the evidence, then chose No. If the conclusion simply re-states the pieces of evidence, then choose No.

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Clusters of galaxies can smear out the light from a single galaxy into a long

“arc”

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Our best Understanding• Oversimplified view of a spiral galaxy

In some ways, Dark Matter

surrounds the galaxy like the

water in a fishbowl

surrounds a fish in the middle of

the bowl.Not exactly the

same… denser in the middle

because of the pull of gravity



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Lensing of Galaxies•Dark Matter “Lenses” the galaxies behind them like a prism or a pair of bad eyeglasses•Evidence that the light coming to us is passing through lots of matter we can’t see directly

Prism



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General Relativity and Light

•The large mass of the Sun can bend the path of light•Result: The apparent position of a star “moved” as the light passed from outer space, past the Sun, and to us

Light’s path in curved space time

Is here “Looks” like its here



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Looking at Light from the UniverseGalaxy

Light from a Galaxy

Looks like a Galaxy

HydrogenCloud

Looks like a Galaxy behind a

cloud of hydrogen

Looks like a Galaxy behind a cloud of heavy

“stuff” that isn’t just stars and

hydrogen

Another Galaxy



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Prep for Today (Is now due) – L8

•Reading: –BBBHNM 7

•Reading questions:–Chapter 7

•End-of-Chapter Quiz:–Chapters 5

•Paper 1:–Phase II is due Friday at 5:45PM



Phase I and Phase II in CPRPhase I: (Due 1 week after we finish

Chapter 4)• Turn it into CRP (online)• Need to also submit a copy to the

version of turnitin.com on eLearningPhase II: (Due one week after Phase I)• Three-Part Calibrations/Reviewing

1. Calibrations, learning to use the Rubrics

2. Grade three other papers3. Grade your own paper

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Prep for Next Time – P7•Reading:–BBBHNM 7–Recommended:

• SHU: 6 and 7 (up-to page 153)• BHOT: 9 and 11 (117-122)

•Reading questions–Two questions from Chapter 7 or the recommended reading

•End-of-Chapter Quizzes– If we finished Chapter 6, quiz due before the next class (else just 5)

•Paper 1–Phase II is due Friday at 5:45PM



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Prep for Next Time – P8

•Reading:–None

•Reading questions–None

•End-of-Chapter Quizzes–If we finished Chapter 6, quiz due before the next class (else just 5)

•Paper 1–Phase II is due Friday at 5:45PM



Full set of Readings So Far

•Required:–BBBHNM: Chap 1-7•Recommended:–BHOT: Chap. 1-6, 9 and 11 (117-122)–SHU: Chap. 1-3, 6 and 7 (up-to page 153)–TOE: Chap. 1

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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 Good, the Bad and the Ugly

Good:• Both make lots of predictions that are

observed in experiment– Example: What does Light do and

how does it behaveBad: • Don’t always make predictions that are

inconsistent with each other, although wean’t yet test any of those predictions– Example: Different predictions for

the center of a Black Hole



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???

•Maybe just need to modify one or the other (or both?) and combine to make a “quantum theory of gravity”?–Could be, but been working on it

for more than 60 years, no luck yet

–Maybe both are wrong? •Either way, they are our best understanding at the moment Lots of experiments are INCREDIBLY well described by them Makes us think there is SOME truth in there somewhere…



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Why do we need the world to be this complicated?

• Data says that light always moves at the speed of light.

• Einstein says that the laws of physics should be the same for all freely moving observers, no matter what their speed

• So What?• Since nothing can move faster through

space than light (it always moves at the speed of light) this has enormous implications for the way both space and time are related– Space (measured with a ruler) – Time (measured with a clock)



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The Great Experiment of 1915•Look at a star’s position “behind” the sun as it “passes” in between us and the star

•Result: The apparent position of a star is different when the sun is close to the path

•In other words, we observe that the gravity of the sun “bent” the path of the “no mass” light object coming from the star

Light’s path in curved space time



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Mass Curves Space

The heavier the ball the bigger the dent

in space-time!



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Stars

Space-time is curved or “warped” a lot by stars



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Interesting Aside

Interesting to note that if Newton’s laws were even slightly different, the orbits of the planets would not be circles or ellipses

Would spiral into the sun or escape completely…



102

Today’s Lecture

• Today we’ll talk about Gravity and General Relativity– If there are questions that you

want answers to then you should write them down and we’ll get back to you

• Only tell you the things about Gravity and General Relativity you need to know



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How this “Looks” to us in the Real World

General Relativity talks about 4-dimensional space-time

Our eyes can only see in 3 dimensions

To us something moving in a “straight line” in 4-dimensional curved space time “looks” like its moving in a curved path in the 3-dimensional space we know and love



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In-Class Writing Assignment

Write down, during the course of lecture

Two questions you want to

know the answer to

If you ask them during lecture, you can still write

them down



105

2nd Issue for General Relativity

• In General Relativity, bodies always follow a straight line in four dimensional space-time unless there is a force– Gravity is not a

force• Compare to Newton:

“Unless there is a force things move in a straight line in three dimensions; gravity provides a force”



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In General Relativity, bodies always follow a

straight line in four dimensional space-time unless there is a force Why is this important?

Better yet,Why do we care?



107

•General Relativity says that what appears to be the force of gravity pulling a mass is actually a mass moving in straight line in curved space-time



108

This is a BIG statement

This is a VERY different way of thinking. Let’s spend some on this

• In General Relativity, bodies always follow a straight line in four dimensional space-time unless there is a force– Gravity is not a force

• Compare to Newton: “Unless there is a force things move in a straight line in three dimensions; gravity provides a force”



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Einstein and General Relativity• Laws of physics should

be the same for all freely moving observers, no matter what their speed

• Changes our concept of time… – There is no absolute

time!• Space is not separate

from time, we need to combine them into 4 dimensional space-time

2nd year Grad School Course



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General Relativity

•Gravity is not a force like other forces, but rather the weird consequence that space-time is not flat•Space-time is curved or “warped” by the distribution of mass and energy in it



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Outline

1. Overview for the next set of lectures– Physics We Need– De-mystifying the

strange theories2. Overview of Today’s

lecture – Gravity and General

Relativity



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Today’s Lecture

• Today we’ll talk about Gravity and General Relativity

• I wasn’t planning on going over EVERYTHING in BHOT because I don’t think we need it for our story–Can cover extra questions in

recitation• Only tell you the things about

Gravity and General Relativity you need to know



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Next Few Lectures

• De-mystifying the strange theories

• For next time:–Reading: BHOT 4-6 (Gravity)– Lecture Prep: Create two

multiple choice questions based on the reading

• Next round of reading after that:–SHU: 3 (p55-69)–SHU: 6 and 7 (up-to page 153)–BHOT: 9 and 11 (117-122)



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Section 2The reading for the next 5 lectures:• Today: Gravity–BHOT: Mostly 4 and 6

• Next lecture –BHOT: 5 and SHU: 3 (first part,

p55-69)• After that–SHU: 6 and 7 (to page 153)–BHOT: 9



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117



118

Outline• Definitely:

– Doppler shifts– Spectral lines– Newtonian gravity

• Might need?– Quantum Mechanics– Curved space

• Other stuff– Atomic model: Electrons that surround a nucleus– Nuclear model: A nucleus is protons and neutrons

that are held together– What is the Milky Way? – Feynman diagrams here?– What is temperature?



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120

•Do we need to do atomic Physics and chemistry first (textbook?)?

•Speed of light is really freaking large

•Photons are particles, but also a wave… have a wavelength…



121

BHOT: 4: General Relativity vs. Quantum Mechanics

• General Relativity–Gravity and Theory of the very

big• Quantum Mechanics–Theory of the very small

• Aren’t consistent with each other, but both are incredibly successful in their areas

• Looking for a quantum theory of gravity

• Jonathan: Pictures from BHOT chapts 1-4



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BHOT 6 cont:

•Time runs differently at different heights in a gravitational field. Need to talk about this? Its cool, but so what…



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Documents

Big Bang, Black Holes, No Math Physics We Need Topic 2: Gravity 1 Big Bang, Black Holes, No Math ASTR/PHYS 109 Dr. David Toback Lectures 8, 9 & 10