Download Einstein, Black Holes and Gravity Waves, oh my! (x)

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
Feb 2016
David
Toback,2016
SPS Lecture
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
Feb 2016
David Toback, SPS Lecture
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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
Feb 2016
David Toback, SPS Lecture
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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”
Feb 2016
2nd year Grad
School Course
David Toback, SPS Lecture
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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 Sun
This is a VERY different way of
thinking about things…
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Curved Space?
An analogy is to
think of
curved spacetime as looking
like one of
those gravity
wells you’ve
seen
downstairs
Feb 2016
1st floor of the Mitchell
Physics building (MPHY)
David Toback, SPS Lecture
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
Feb 2016
David Toback, SPS Lecture
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-time
makes the path of
the small ball look
like something is
pushing it toward
the big ball in 3dimensions
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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
Feb 2016
David Toback, SPS Lecture
The way the Planets go Around
the Sun in General Relativity
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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).
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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
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Two Black Holes Colliding/
Gravity Waves
What it would Look Like if you could see
it (note the gravitational lensing)
https://www.youtube.com/watch?v=I_88
S8DWbcU
From the Warp of Space-Time Point of
view
https://www.youtube.com/watch?v=qfdyg
f0TL2U
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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
https://www.youtube.com/watch
?v=FlDtXIBrAYE
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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
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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!
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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
http://people.physics.tamu.edu/toback/109/
Feb 2016
David Toback, SPS Lecture
End of Lecture
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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: Chapter 6
(Gravity) Predictions
about the very large,
from sizes of a few
meters to the size of
the universe (1024 miles
across)
Chapter 7
• Quantum Mechanics:
Predictions about the
very small (atoms,
particles,<10-10 m)
Feb 2016
David Toback, SPS Lecture
Overview of Gravity for the Course
1. What’s so
important about
Gravity?
2. Newton’s Theory
of Gravity
3. Einstein’s morecorrect version
–Curved SpaceTime, and
evidence for it
with Dark
Matter David Toback, SPS Lecture
Feb 2016
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 Gravity
1. 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
Physics 201
bigger the attraction
or 218
• Light is massless  not
affected by
gravity
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David Toback, SPS Lecture
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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David Toback, SPS Lecture
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
Feb 2016
Without gravity, the
Moon would fly off into
space and ignore the
Earth completely
David Toback, SPS Lecture
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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Observational Fact
Light *ALWAYS* moves at the
speed of light to all observers
So what?
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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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Two observers get different
answers
The light
is one foot
ahead of
that car!
The light
is two feet
ahead of
me!
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Einstein’s Answer
Feb 2016
Einstein says
both
observers
are correct
Space and
Time are
more related
than we
thought 33
David Toback, SPS Lecture
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
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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
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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
Feb 2016
The rules of geometry are different in
curved space
http://en.wikipedia.org/wiki/Parallel_(geometry)
David Toback, SPS Lecture
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!!!
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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
2
E=MC
Ok… what is this equation saying?
Energy and mass may not be the
same, but in Einstein’s theory they
are equivalent
Feb 2016
David Toback, SPS Lecture
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?
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
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Do an Experiment
1st Experiment:
Looks like
Can see the
its over
star directly
here!
If Einstein is
2nd Experiment:
right, the Sun
Sun in the way
curves the path
If Newton is
of the starlight
right, we won’t
see the star Question: Where
Gross
exaggeration
of sizes
Feb 2016
does the star
appear when you do
the experiment?
David Toback, SPS Lecture
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
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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
Feb 2016
David Toback, SPS Lecture
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
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The Data
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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
http://bigbang.physics.tamu.edu/Figures/StolenAnimations/galrot_anim.gif
Feb
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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)
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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
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Einstein Ring’s?
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David Toback, SPS Lecture
Another view
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David Toback, SPS Lecture
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
Feb 2016
David Toback, SPS Lecture
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
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• 1.3 billion years ago
• Sept 14th, 2015
• Binary black hole merger
– Half the speed of light
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