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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Из курса от партнера University of Alberta
Astro 101: Black Holes
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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.
Познакомьтесь с преподавателями
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 spacetime,
any object with a 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 emitted far from the closer lensing mass.
In this image, the arcs making the smile and 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 was
strong enough gravity that they exhibit gravitational lensing.
They're emitted light travels part of the way around
the 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 or 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 what the back my head looks like, unless I turned 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 would be captured by the camera.
This would give you a distorted image of my face surrounded by all my hair.
As a result, my head would look larger than it really is.
This animation shows the effect of a neutron star's 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 a star as
though it's 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
Eclipse twin is 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 shows the same star,
but now it includes the gravitational effects.
The star strong gravity causes the light emitted by the star to travel on curved paths.
So, that parts of the star that would normally be hidden are distorted into view.
The strong gravity allows us to see
the bright spot all of the time even when it is on the backside of the star.
The stars gravitational field distorts the circular spot image,
so that it looks like a thin curve,
and it allows us to see around the spin polls.
Since the hotspot is now visible throughout the rotation of a star,
there's much less variation in the overall brightness with time
as shown on the graph below the animation.
The stars rotating so fast that
the equator is moving at 30 percent of the speed of light.
When the spot is moving towards us,
the light it emits is blueshifted.
When the spot is moving away from us,
it is redshifted by the Doppler effect.
When light is Doppler shifted,
there is another relativistic effect called Doppler boosting which makes
the blueshifted light appear brighter and the redshifted 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 gravitationally
lensed and Doppler boosted light from hotspots or 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 star's gravity can distort the paths of light,
then a black holes can too.
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 disk looks flat.
A black hole with an event horizon with
the same size as this hole has a Schwarzschild radius of 3.6 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 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 disk 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 disk will look like a second arch below the black hole.
A more accurate drawing of an accretion disk around
the 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 physicists 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 that would be safe for the astronauts to visit.
So, they did not include a jet.
This suggests that Gargantua is not accreting enormous amounts of matter.
They also chose a colder than usual accretion disk
which is only a few thousand degrees Kelvin,
so that emits ultraviolet 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 blueshifted side of the star appear brighter than the redshifted side.
The accretion disk orbit the black hole at high speeds.
The side coming towards you is blueshifted
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 the 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're looking down on the disk and then we move
downwards so that we are viewing the disk 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 disk 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 and curves that maintain a constant distance
from the black hole but trace out a spherical shell like the photon in this animation.
These paths are unstable,
so 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 circle orbit for photons or Photon Sphere.
Photons within the photon sphere travel
and spherical orbits sometimes called the Ring of Fire.
Within the Ring of Fire,
there was a black region called the black hole shadow,
a region where light can no longer escape 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 simulation suggests 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 Hole Event Horizon.
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