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.
10.03 – Gravitational Radiation
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來自 University of Alberta 的課程
Astro 101: Black Holes
44 個評分
從本節課中
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.
與講師見面
Sharon MorsinkAssociate Professor
Department of Physics, University of Alberta
[MUSIC]
One of the foundational principles of special relativity is that light
always travels at the same constant speed.
The speed of light is fast, but it still takes 8.3 minutes to travel
the enormous distance from the Sun to the Earth.
This means that if the sun were just suddenly change brightness, there would
be a delay of 8.3 minutes before we would see the change here on Earth.
Suppose that powerful aliens with advanced technology managed to
change the Sun's gravity, maybe by removing a big blob of hot gas.
Removing the gas also causes the Sun to dim at the same time.
If we calculate the gravitational traction between the Sun and the Earth.
Newton's equations had no time dependents.
If the sun's mass suddenly changes as it were,
if aliens removed a large portion of gas.
Newton would have predicted that the earth would feel a different
force due to gravity instantaneously, with no delay.
If gravity changes instantaneously, as Newton predicted,
it would still take the photons admitted by the Sun 8.3 minutes to reach the Earth.
Revealing the alien's actions as the Sun becomes dimmer.
This means that we could receive information about changes to the Sun at
a speed faster than light by using gravity.
This instantaneous transmission of information is sometimes
called action at a distance.
Einstein realized that Newton's theory of gravity was wrong because it
implies that action at a distance takes place and
that gravity could transmit information faster than the speed of light.
Although there were no experiments Einstein could conduct that show
that action at a distance is incorrect Einstein knew that it would imply that
there could be ways to transmit information at speeds faster than light
which would contradict the principles of special relativity.
Einstein theorized that gravity could be made compatible with special
relativity if changes in gravitational fields are transmitted by
gravitational waves that obey the speed of light limit in the universe.
Gravitational waves also called gravitational radiation are an important
part of Einstein's general theory of relativity.
Let's return to the aliens who are changing the Sun's mass and brightness.
Einstein theories predicts that it takes 8.3 minutes for
the gravitation force felt by the Earth to change, so
changes in gravity travel at the same speed as light.
The relationship between the force of gravity and
gravitational radiation is similar to the relation between electrostatic forces and
the emission of electromagnetic radiation or light.
An electron or a proton are just two examples of charged particles,
which create an electrostatic field.
Electrostatic fields can be attractive if the force is between opposite charges,
or repulsive if the force is between same charges.
By grabbing a balloon against your head you cause negative charges or electrons
from your hair to migrate to the balloon, leaving your head positive charges.
If you then hold the negatively charge balloon near your positively
charge head your hairs would stand up.
Following electrostatic field lines.
The word static, means that the system doesn't change with time.
Positive charge, like some hair, will feel and attractive electrostatic
force that will feel a force toward the negatively charged balloon.
The electric field of the balloon influences nearby objects, but
in a static system, there is no electromagnetic radiation or
light emitted in this situation.
If we want to produce electromagnetic radiation,
we need to create periodic changes in the static charges.
This can be accomplished by changing the position of a charged balloon,
say by waving it back and forth rapidly.
Positively charged hair will be attracted first in one direction, then the other.
And so on, this is situation which produces a time changing electric and
magnetic field that causes a disturbance called electromagnetic
radiation also know as light For example,
if the balloon is moved back and forth once per second, it generates a changing
electric field that moves away from the source at the speed of light.
One complete cycle of the balloon corresponds to one cycle of
the light wave.
So if the balloon moved at one hertz, it produces a photon with
a wave length equal to the distance light travels in one second.
That's 299,792 kilometers of photon at the very
long end of the wave length spectrum in the radio frequencies.
This electromagnetic wave travels at the speed of light.
Perpendicular to the motion that the balloon is being waved.
Information, about the changing balloons position reaches your hair
after a time equal the distance divided by the speed of light.
When the wave passes by a charged particle,
the particle will feel a time changing force.
That will cause it to oscillate back and
forth in a direction perpendicular to the direction the wave travels in.
Normally, we don't use balloons to create electromagnetic waves.
Instead, we might have an alternating current traveling through
a dipole antenna to create radio waves, a long wavelength form of light.
The important thing to know is that light is only emitted when there
are time-changing electric charges or magnets.
A star or planet has a gravitational field that doesn't change with time,
and unlike electrostatic forces, gravity is only attractive.
A small mass place near a heavy planet will feel an attractive
gravitational force that will pull it towards the surface.
If there is no motion of the masses,
there will be no gravitational radiation emitted.
Just like the electrostatic example, in order to excite gravitational waves,
we're going to need a gravitational field that changes with time.
When gravitational waves are created,
they move away from their source at the speed of light and, as the wave moves.
It distorts space time by stretching and
compressing spacial dimensions periodically in the two
directions perpendicular to the direction that the wave travels.
These waves are called transverse waves and
can be understood in a simplified sense.
By the analogy of a transverse wave on this rope that Curtis and
Ross are playing with.
In this video, we see Ross waving the rope up and down,
which is sort of like a gravitational wave source.
The wave travels along the rope horizontally from left to right and
then from right to left.
At each point between Ross and Curtis, the rope is forced to move up and
down at the same frequency as Ross's hand.
In a transverse wave, the movement of the medium is in
a different direction than the direction that the wave travels in.
A gravitational wave is a wave in space time,
which means that instead of moving objects.
A gravity wave compresses and decompresses space-time.
Suppose that a gravitational wave travels from
the ceiling to the floor through my body.
Focus on my arms.
The affect of the gravitational wave will be to stretch one arm out outwards and
compress the other arm inwards.
Then the second part of the wave's periodic motion will cause the opposite
changes in each arm.
Periodic motions of my arm will then occur when the wave travels through my body.
If a gravitational wave were to pass by your ear,
it would cause your eardrum to start vibrating, so that you could in principle
hear a gravitational wave if you had sensitive enough ears.
For this reason, scientists often convert
gravitational waves into equivalent sound waves,
which is how we got the chirp sound of two merging black holes.
[SOUND] In reality the wave are very weak and
changing arm lengths are microscopic.
And the Star Trek Next Generation episode called Hero Worship
the Enterprise is violently rocked by gravitational waves.
This is rather difficult to understand since in reality the waves are very weak,
and the Enterprise would have had to have been right next to a ridiculously
strong explosive event, in order to get such a strong rocking event.
Scientists on Earth have been working hard for decades to build detectors,
sensitive enough to detect gravitational waves, and
directly detected them for the first time in 2015.
