1. Einstein, Black Holes and Gravity Waves, oh my!
Society of Physics Students
Lecture
Einstein, Black Holes and Gravity Waves, oh my!
Prof. David Toback
Texas A&M University
Mitchell Institute for
Fundamental Physics and Astronomy
February 2016

2. Outline Quick overview of what happened
Backtrack and learn some of the physics, especially General Relativity
Gravity, Black holes and Gravity waves
Back to the discovery of Gravity Waves
Why it’s important in many ways, and why it’s not that new in others

3. 50 Word Summary
About a billion years ago, two 30MSun black holes spiraled in together to create a 60MSun black hole
This merger was so violent that it emitted a huge amount of gravity waves over a short amount of time that were recently discovered here on Earth by the LIGO experiment
Note: This is NOT the discovery of the century (IMHO), it is the discovery that took a century
Although it IS pretty awesome

4. 2nd year Grad School Course
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

5. This is a VERY different way of thinking about things…
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 Sun
This is a VERY different way of thinking about things…

6. 1st floor of the Mitchell Physics building (MPHY)
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)

7. 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

8. The heavier the ball, the bigger the dent in space-time!
Mass Curves Space
The heavier the ball, the bigger the dent in space-time!

9. 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-time makes the path of the small ball look like something is pushing it toward the big ball in 3-dimensions

10. 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

11. The way the Planets go Around the Sun in General Relativity

12. From Curved Space Time to Gravity Waves
If I have a star, or black hole, just sitting in space then other things know where it is because of the curvature of space-time
If its just sitting there (or moving with a constant velocity) space-time isn’t changing
If it accelerates, then this information is passed through space-time at the speed of light as gravity waves
Similar to if an electric charge was accelerated, or if I dropped a rock in water (or dragged my finger through it).

13. First Indirect Evidence of Gravity Waves
In 1974, Taylor and Hulse discovered two neutron stars (one of which was a Pulsar which is VERY bright) orbiting each other
They measured the period (how fast they orbit) and saw that they were CLEARLY slowing down
Why were they losing energy? They were emitting gravity waves exactly as predicted by General Relativity
Nobel Prize 1993
Been looking for DIRECT detection ever since

14. Two Black Holes Colliding/ Gravity Waves
What it would Look Like if you could see it (note the gravitational lensing)
From the Warp of Space-Time Point of view

15. Gravity Waves Moving Through Space
A good set of short clips that help tell the story, including how the gravity waves got to us and how they were detected

18. What the Data Looked Like
Ripples In Space-Time that reached us from two ~30MSun Black Holes as they spiral into each other about a billion light-years away

19. Conclusion
The first direct observation of gravity waves is very exciting, and many years in the making
This allows us to see things we’ve never seen before that don’t produce light
Lots of fun questions still remain to be answered
How often does this happen in the universe?
Where did 30 solar mass black holes COME FROM?
What other new and exciting things will come from this powerful new technology?
Enjoy!

20. Interested in learning more?
Physics department offers a course entitled “Big Bang & Black Holes”
(ASTR/PHYS 109)
Covers Stephen Hawking’s “Brief History of Time”
Origin and Evolution of the Universe
How do stars form?
What is Dark Matter? Dark Energy?
What are Black Holes?
More on General Relativity, Quantum Mechanics and Particle Physics
Has a lab (if you want)
There is an option to take is an Honors class

21. End of Lecture

22. 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

23. 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

24. Overview of Gravity for the Course
What’s so important about Gravity?
Newton’s Theory of Gravity
Einstein’s more-correct version
Curved Space-Time, and evidence for it with Dark Matter

25. 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.

26. Newton and Gravity Physics 201 or 218
Everything moves in a straight line unless acted upon by a force
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

27. 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

28. 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

29. 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

30. Light *ALWAYS* moves at the speed of light to all observers
Observational Fact
Light *ALWAYS* moves at the speed of light to all observers
So what?

31. What happens if I’m driving a car moving at half the speed of light and I turn the headlights on?

32. Two observers get different answers
The light is one foot ahead of that car!
The light is two feet ahead of me!

33. Einstein’s Answer
Einstein says both observers are correct Space and Time are more related than we thought

34. 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

35. Need to talk about what we mean by Curved Space-Time
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

36. 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 more Space-Time and Gravity

37. 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

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

39. 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

40. It has energy, but no mass
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

41. What happens to light as it goes past the Sun?
Path
doesn’t curve
Path
does curve
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

42. Do an Experiment 1st Experiment: Can see the star directly
Looks like its over here!
2nd Experiment: Sun in the way
If Einstein is right, the Sun curves the path of the starlight
If Newton is right, we won’t see the star
Question: Where does the star appear when you do the experiment?
Gross exaggeration of sizes

43. The Great Experiment of 1915
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

44. Another View

45. 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

46. 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

47. 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

48. The Data

49. Does this work for Stars?
Watch how fast a star rotates around the center of the galaxy…
Simulation without
Dark Matter
Simulation with lots of
Dark Matter particles in the galaxy
Data looks like this

50. Data well explained by lots of “Dark Matter” we can’t see This is where it gets its name In some sense, the name is a statement of almost all we know about it (it doesn’t interact with light, and it has mass)

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

52. Einstein Ring’s?

53. Another view

54. Einstein Ring The “near” galaxy
The light from the “far” galaxy spread out into a ring
Amount of lensing explained by extra mass of dark matter

55. 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

56. 1.3 billion years ago
Sept 14th, 2015
Binary black hole merger
Half the speed of light