What is a black hole? Do they really exist? How do they form? How are they related to stars? What would happen if you fell into one? How do you see a black hole if they emit no light? What’s the difference between a black hole and a really dark star? Could a particle accelerator create a black hole? Can a black hole also be a worm hole or a time machine? In Astro 101: Black Holes, you will explore the concepts behind black holes. Using the theme of black holes, you will learn the basic ideas of astronomy, relativity, and quantum physics. After completing this course, you will be able to: • Describe the essential properties of black holes. • Explain recent black hole research using plain language and appropriate analogies. • Compare black holes in popular culture to modern physics to distinguish science fact from science fiction. • Describe the application of fundamental physical concepts including gravity, special and general relativity, and quantum mechanics to reported scientific observations. • Recognize different types of stars and distinguish which stars can potentially become black holes. • Differentiate types of black holes and classify each type as observed or theoretical. • Characterize formation theories associated with each type of black hole. • Identify different ways of detecting black holes, and appropriate technologies associated with each detection method. • Summarize the puzzles facing black hole researchers in modern science.
10.02 – Gravitational Lensing
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From the course by University of Alberta
Astro 101: Black Holes
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From the lesson
Riding the Gravity Wave
How do you study a black hole that has no visible companion? In this module the student will be introduced to gravitational radiation. With the 2016 LIGO discovery of gravitational waves, a whole new branch of astronomy has been opened.
Meet the Instructors
Sharon MorsinkAssociate Professor
Department of Physics, University of Alberta
When light enters glass the direction that light
travels changes or bends in a process called refraction.
If the surface of glass is curved appropriately,
we produce a lens which can magnify
diffuse or distort the light rays from objects in the background.
Since gravity causes light to travel in curved space time,
any object with gravitational field appears to bend light as a gravitational lens.
The examples that we looked at earlier like this image of
the Cheshire Cat Galaxy group were cases where
the light was admitted far from the closer lensing mass.
In this image, the arcs making the smile an outline of the face are images of
galaxies lying far behind the eyes and nose galaxies whose mass is warping space time.
However, if we consider ultra dense stars with
gargantuan gravitational fields the light they emit will also travel on curved paths.
Neutron stars are examples of stars with
strong enough gravity that they exhibit gravitational lensing.
Their emitted light travels part of the way around
a star before it can escape to be observed by telescopes.
By observing how light is bent around neutron stars,
we can understand the properties of these stars better which
will help us keep from confusing neutron stars with black holes in the future.
If you look at my head,
you can only see the front of my head and you have no way
to know with the back of my head looks like, unless I turn around.
This is because in situations with weak gravity like here on earth,
light travels on straight paths from my head to the camera.
But if my head was really heavy,
so that my head had a gravitational field as strong as a neutron star,
then light originating from the back of my head will
travel on a curved path around my head and be captured by the camera.
This would give you a distorted image of my face surrounded by
all my hair and as a result my head would look larger than it really is.
This animation shows the effect of a neutron stars gravity on the light that it emits.
In these cartoon animations we show a dim star with one bright spot on it.
The left animation shows an image of the star as
though its gravity has no effect on the light that it emits.
In this case, we see the hotspot for
only half of the rotation period and it is
eclipsed when it is on the backside of the star.
Since the hotspot appears brighter than the surrounding star,
a plot of the overall brightness would increase as the hotspot
rotates into view and decreases as it becomes hidden again.
The right animation also shows another interesting effect.
When the spot is moving towards us,
the light it emits is blue shifted and when the spot is
moving away from us it is red shifted by the doppler effect.
When light is Doppler shifted,
there is another relativistic effect called Doppler boosting which makes
the blue shifted light appear brighter and the red shifted light appear dimmer.
If you pay attention to the brightness scale on the right,
you'll notice that the spot is brightest when it is moving towards
us and it is dimmest when it is moving away from us.
NASA's nicer X-ray telescope is measuring the gravitationaly
lensed and Doppler boosted light from hot spots on neutron stars.
Nicer is attached to the International Space Station and observes
x rays emitted by neutron stars with spots as well as black holes.
Black holes do not have a surface,
so we can't observe hot spots on rotating
black holes in the same way that we observe them on neutron stars.
But we do know that there are light emitting
structures like accretion disks that orbit black holes.
Images like this one drawn by artists represent accretion disks around black holes.
In most pictures the artist has drawn the accretion disk as though
the black holes gravity does not warp space time or the paths that light rays follow.
So we see the accretion disk as flat.
But if a neutron stars gravity can distort the paths of light then a black holes Can to.
This vinyl record represents a crude model of
an accretion disk and the hole at the center represents a black hole.
If we ignore the black holes strong gravity then when you look at this disk
the light rays travel from the disk to
the camera on straight lines and the disc looks flat.
A black hole with an event horizon with the same size as
this hole has a short travel radius of three point six millimeters.
This corresponds to a mass that is about half of the planet Venus squished into
this hole the strong gravity
of the black hole will distort your image of the back of the disk.
You will still see the front of the disk since the light rays don't
have to pass by the black hole in order to get to the camera.
Light emitted by the back of the disk has to
travel close to the black hole in order to get to the camera.
Light from the back is bent by gravity to
travel up and over the black hole to the camera.
As a result, this back curve will appear to look like an arch over the black hole,
but the disc also has a bottom that emits light.
Light emitted by the bottom can travel
down and below the black hole to get to the camera.
The bottom of the disc will look like a second arch below the black hole.
A more accurate drawing of an accretion disk around
a black hole will show the disk arching over and below the black hole.
The movie Interstellar features a view of
a supermassive black hole with an accretion disk.
The director Christopher Nolan wanted to have
a fairly realistic view of the disk that includes gravitational lensing.
So he consulted physicist Kip Thorne who recently shared the 2017 Nobel Prize in physics.
The resulting image produced for the movie is shown here.
In this image we can see the front of the disk the top
of the back of the disk and the bottom of the back of the disk.
However the black hole created for the movie required some simplifications.
First, they wanted a black hole they would be safe for the astronauts to visit.
So they did not include a jet.
This suggests that Gargantua is not accreating enormous amounts of matter.
They also chose a colder than usual accretion disk
which is only a few thousand degrees Kelvin,
so that it emits ultra violet visible and infrared light
but practically no harmful x rays.
When we looked at the light from a rotating neutron star,
we saw the Doppler boosting effect makes the blue
shifted side of the star appear brighter than the red shift inside.
The accretion disk orbits the black hole at high speeds,
the side coming towards you is blue
shifted and should appear brighter than the side moving away from you.
The director was worried that people watching the movie might get
confused if the Doppler boosting effect were included,
so they left it out from the rendering.
This computer animation shows the results of
a magneto hydrodynamic computer simulation of an accretion disk around a black hole.
The researchers are simulating
realistic patterns in the disk which makes it easier to see the motion of the gas.
At the start of the animation,
we are looking down on the disk and then we move downwards so that
we are viewing the disc from just above the plane of the disk.
The warping effect of gravitational lensing becomes
more apparent as we look into the equatorial plane.
When we are viewing the disc from the side,
we can see that the side that is spinning towards
us is brighter than the side spinning away from us.
Light near a rotating black hole can travel in
curves that maintain constant distance from the black hole,
but trace out a spherical shell like a photon in this animation.
These paths are unstable so a photon travelling
on this path can easily be pushed outwards or inwards.
If they are pushed inwards,
the photons can cross the event horizon and become lost inside the black hole.
If the photons are pushed outwards,
they can escape to be seen by a telescope.
Recall the black holes innermost circular orbit for photons or photons sphere.
Photons within the photon sphere travel
in spherical orbits sometimes called the Ring of fire.
Within the ring of fire there is a black region called the black hole shadow.
A region where light can no longer escape to outside observers.
The event horizon telescope is a collection of
radio telescopes scattered across many locations over the earth.
The event horizon telescope is observing
Sagittarius A Star the black hole at the center of
our galaxy and eventually it will
become sensitive enough to detect the black hole shadow.
Computer simulations suggest that
Sagittarius A star should look something like this video.
This computer simulation shows an accretion disk orbiting around
a black hole and shows a ring of fire around a central dark feature.
However the image that the event horizon telescope is
creating will not look as sharp since the picture
will be averaged over time and there will be blurring
due to light scattering off of interstellar gas and dust.
This observation will be the most detailed image of
the region directly outside of a black holes event horizon.
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