Big Bang, Black Holes, No Math - Toback...

Big Bang, Black Holes, No Math

Physics We NeedTopic 2: Gravity

1


ASTR/PHYS 109Dr. David TobackLectures 12-14



Prep For Today (is now due) – L14• Reading:

– (Unit 2) • Warmup Quizzes:

– Read http://people.physics.tamu.edu/toback/109/WritingAssignments/Papers_and_Peerceptiv.pdf

– Warm-up Quiz Part 4 (Peerceptiv): Was due before class• Pre-Lecture Reading Questions:

– (Unit 2 Question Writing Assignment)– (Unit 2 Quiz)

• End-of-Chapter Quizzes– (Chapter 5a&b)

• Papers– Paper 1 Text and Reviews due one week after we finish Chapter

6– Feel free to use the optional Feedback Assignment in Peerceptiv

• Honors Section:– “Stage 1 Final Draft” due before next class

4



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

• Text will be due 1 week after we finish Chapter 6

5



7

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



Observational FactLight *ALWAYS* moves at the speed of light to all observers

So what?This simple observational fact

throws a monkey wrench in Newton’s theories Why?

Let’s give an example of why…18



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

and I turn the headlights on?

19



20

The light is one foot ahead of that car!

Two observers get different answers

The light is two feet ahead of

me!



Einstein’s Answer

Einstein says bothobservers are correct

Space and Time are more related than we thought 21



22

Start with Space-TimeSpace and Time are not truly 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 can happen that we’re not used to



23

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

• Also need to explain what this has to do with gravity



What are we trying to explain?

Two things:1. Why do things fall to the

surface of the Earth?2. Why do things further out

in space fall in more slowly than things close to the surface?

24



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

25



Example: 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 Sun curves the space-time around it and the Earth moves in a “straight line” through that curved space-time

This is a VERY different way of thinking about things… What is Curved

Space-Time? 26



Curved Space?

An analogy is to think of curved space-time as looking like one of those gravity wells you may have seen

1st floor of the Mitchell Physics building (MPHY)

27



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

The rules of geometry are different in curved space

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

28



29

Another Weird Thing: Mass Curves Space-Time (Analogy)Think of each of the heavy things in the universe (stars, planets etc.) as being 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



Mass Curves Space (Analogy)

The heavier the ball, the bigger the “dent” in space-

time! 30



What is a “dent in space-time” andwhat does it have to do with falling to the Earth?

• Newton has the calculations for orbits right, just for the wrong reasons– “Closer to the surface of the Earth, more attraction”

• How to understand the diagram:– Deeper part of the dent the place where there is more

attraction (like Newton)– Deeper part of the dent Space is “falling in” faster where

the dent is “deeper”• Earth: Space-time is curved here on the surface of the Earth,

but there is no “curving” to “see”– Space is just flowing downward, carrying things along with it

towards the surface of the Earth– More curvature closer to surface things speed up faster

there space “falls” faster the closer you get to the surface

31



Clicker QuizWe just found two new moons around Jupiter. They are at locations A and B. Which one feels a bigger “dent” in space time? a) Ab)Bc) Samed)Not enough info

32

A B



Clicker QuizThe two new moons have moved. Now which one feels a bigger “dent” in space time? a) Ab)Bc) Samed)Not enough info

33

A

B



Clicker QuizWe observe both to orbit in a circle around JupiterWhich one moves through space faster? a) Ab)Bc) Samed)Not enough info

34

A

B



Mass in curved space-time (Analogy)

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

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

Space is falling towards the yellow ball fasterwhere the curve is deepest 35



Back to Newton vs. Einstein: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-time around the Sun makes it go in an orbit in the three space dimensions36



37

The way the Planets go Around the Sun in General Relativity



38

Ok…• Both Newton and Einstein’s theories

accurately 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 where the predictions from the two competing theory's are very different!!!



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

39



So what?What about light?It has energy, but no mass• To Newton, there is no force on it

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

40



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

41



67

Prep For Next Time – L14• Reading:

– (Unit 2) • Warmup Quizzes:

– Warm-up Quiz Part 4 (Peerceptiv): Was due alredy• Pre-Lecture Reading Questions:

– (Unit 2 Question Writing Assignment)– (Unit 2 Quiz)

• End-of-Chapter Quizzes– (Chapter 5a&b)

• Papers– Will likely finish Chapter 6 next time, and the due date will be

one week after. The text of Paper 1 is due in Peerceptiv and TurnItIn. Do Reviews directly after

– Extra credit for all students who complete all potions within the 72 hour window (see instructions)

– Feel free to use the optional Feedback Assignment in Peerceptiv• Honors Section:

– “Stage 1 Final Draft” due before next class

Big Bang, Black Holes, No Math - Toback...

Documents

Transcript of Big Bang, Black Holes, No Math - Toback...