29
Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Embed Size (px)

Citation preview

Page 1: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Gravitational Wavesaka Gravity acting

Trina Cannon SMU Quark Net Workshop

6 August 2012

Page 2: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Four Fundamental Forces

Electromagnetic ForceWeak Nuclear ForceStrong Nuclear Force

Gravitational Force

Page 3: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

What will we do with this brief talk?1. Describe, compare and contrast Newton’s

concept of gravity with Einstein’s concept.2. Describe differences and similarities

between gravitational and electromagnetic forces, including theoretical descriptions and how particles interact at a distance.

3. Summarize the role of logical arguments in this study, including the motivation for new theoretical descriptions of gravity and how experiment relates to those theoretical predictions.

Page 4: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Gravity is extraordinarily weak among the fundamental forces.

Newton’s Law of gravitation states the result of empirical observations.- The force between two masses is inversely proportional to the square of the distance between their centers.- This is valid at a wide range of distances, with some problems both theoretical and experimental.

Page 5: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Einstein proposed in his theory of general relativity that gravity is actually the curvature of spacetime.- Black Holes are extreme examples of highly curved spacetime.- Gravitational waves are ripples in this spacetime fabric that have not yet been directly observed- Quantum gravity is a theory of how gravity operates in the smallest scales (yet to be completely formulated and verified).- String theory falls into this area of quantum gravity.

Page 6: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Paper clipsMagnetic postsBalloonsDish with “punched holes”

Strength of forces

Page 7: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Gravity cannot be held responsible for people falling in love.

--Albert Einstein

What is the gravitational force between two persons ? (Assume 60 kg and 90 kg)

What is the electromagnetic attraction ? (assume a 1% difference in charge)

Page 8: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

More examples Surface tension ◦Paper squares◦PepperAny other examples ?

Page 9: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Some theoretical puzzles with Newton’s theory

“Radio Waves & Electromagnetic Fields” ◦A PhET lab

http://phet.colorado.edu/simulations/sims.php?sim=Radio_Waves_and_Electromagnetic_Fields

Page 10: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

When the electron is wiggled at KPhET, how quickly is the signal received by the antenna at the house?

A. Always a the speed of light in a vacuum (c).

B. At the speed of light, which depends on the medium that it’s traveling through.

C. Anything up to the speed of light in that medium, but maybe slower.

D. Depends on how fast you wiggle the electron.E. Instantly

Page 11: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Universal Law of Gravitation

FG = GMm r2

Page 12: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

If the Sun disappeared, the Earth would fly out of its orbit. How quickly would the gravitational repercussions of the Sun’s disappearance travel to the Earth?

A. Always at the speed of light in a vacuum (c).

B. At the speed of light, which depends on the medium that it’s traveling through.

C. Anything up to the speed of light in a vacuum, but maybe slower.

D. Instantly

Page 13: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Puzzle # 1

Gravity shouldn’t travel faster than the speed of light!

Page 14: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

What do we know about free fall?1. Compare the fall of an elephant

with the fall of a kitten?2. Could you move two different

objects (with different masses) precisely the same way?

3. Could you push an elephant and a kitten across the ice with the same acceleration?

4. How does the Earth do that?

Page 15: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Gravity is unlike different forces in that the amount of “push” depends on the mass of the object.

Gravitational mass is the same asinertial mass.

Page 16: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Puzzle #2 Two objects with different masses fall at the same rate – “universality of free fall”

Page 17: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Acceleration is like gravity!

Principle of equivalence

Page 18: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Which of the following are NOT the same for the person in the rocket ship as the person standing on Earth?A. Their feeling or perceptionB. The reading on the scale that they

stand onC. What they see out of the windowD. How light will behave (bending

toward the floor)E. Something else/more than one.

Page 19: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Think-Pair-Share Questions1. What holds the person to the floor in

each situation?2. In which reference frame is the person

stationary? What is moving in the case where the person is stationary?

3. Use the equivalence principle to explain why two masses of different weight fall at the same acceleration.

4. Use the equivalence principle to construct an argument that gravity bends light

Page 20: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Some new questions!1. Why did Einstein spend time working on

gravity when Newton’s law had been measured and verified many times?

2. Why didn’t Einstein reject his original idea that nothing can travel faster than the speed of light?

3. Many of Einstein’s theories were developed through use of “thought experiments” --- how might this relate to the importance of logical arguments in his theories?

Page 21: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

No other force acts like gravity!

The 3 puzzles suggest that gravity - isn’t a force in the same way that electricity and magnetism are- Einstein realized that gravity must be related somehow to space.

Page 22: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Centripetal force, i.e. a rotating reference frame

F = maFc = mv2 ma = mv2

r r ac = v2

r

Page 23: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Perhaps - gravity is not a property of

objects themselves, - gravity is a result of the fact that

(on the Earth) we’re in a non-inertial reference frame. - That helps to explain why we can choose a coordinate system in the rocket ship where gravity disappears.

Page 24: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Gravity seems to be related in some way to a warping of space and time.

http://www.learner.org/courses/physics/unit/unit_vid.html?unit=3

Page 25: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

1. What results would lead to a Eureka! Moment in either of these experiments?

2. What are the implications of negative or null results in each of these experiments?

3. What would be the implications of positive results in each of these experiments?

Page 26: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Black Holes http://jila.colorado.edu/~ajsh/insi

debh/schw.html

Page 27: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

1. How would the rocket from Activity 1 have to move in order to replicate the gravity on Jupiter?

2. How would the rocket from Activity 1 have to move in order to replicate the gravity on a black hole?

3. What does spacetime look like in the inertial frame (the person at rest) in these cases?

4. So how does spacetime move around a black hole?

5. Why can’t light escape from a black hole?

Page 28: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

The Take-Away MessageHugh masses warp space and

time so strongly that the equivalent non-inertial reference frame (the rocket) is moving faster than the speed of light.

Thus, light cannot escape a black hole.

Page 29: Gravitational Waves aka Gravity acting Trina Cannon SMU Quark Net Workshop 6 August 2012

Cardiff University http://www.astro.cardiff.ac.uk/research/gravity/tutorial/

http://www.blackholehunter.org/

Perimeter Institute http://www.perimeterinstitute.ca/en/Outreach/Alice_and_Bob_in_Wonderland/Alice_and_Bob_in_Wonderland/.

Gravitational Waves from Imagine the Universehttp://imagine.gsfc.nasa.gov/docs/features/topics/gwaves/gwaves.html