05.07 – Spinning Through the Disc

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From the course by University of Alberta

Astro 101: Black Holes

9 ratings

University of Alberta

Astro 101: Black Holes

9 ratings

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.

From the lesson

Approaching a Black Hole

What would you see as you approached a black hole, using a black hole binary as a vehicle to explore black holes? In this module students will follow material as it is transferred from a companion star to a black hole via Roche lobe overflow or wind fed accretion. They will then follow that material down through the accretion disc to explore tidal forces to learn about the ways in which black holes can rip apart surrounding material. This material will then pass through the innermost stable orbit of the disc, before falling in. Students will also get the opportunity to look at jets - the outflow of material from the innermost regions of this structure. Module Objective: Introduce properties of black holes from the outside in, through the context of a journey into the event horizon of a black hole. What would we see as we are far away? What will we see and experience as we get closer? What is a disc? What is a jet?

05.01 – Journey to a Black Hole2:54

05.02 – Jets6:44

05.03 – Black Hole Companions1:09

05.04 – Mass Transfer in Binaries8:20

05.05 – Have a Corona!2:31

05.06 – So What is Accretion?8:32

05.07 – Spinning Through the Disc10:08

05.08 – Innermost Stable Circular Orbit4:44

05.09 – Teetering on the Edge1:05

Meet the Instructors

Sharon Morsink
Associate Professor
Department of Physics, University of Alberta

Now, let's consider what it would be like to be a little piece

of dust in the accretion disk around a black hole.

Since our piece of dust is in orbit,

along with all the other little pieces of dust,

we say that it has both angular momentum which prevents it from falling further

inward and gravitational potential which is trying to pull it further inward.

How can these particles migrate through the disk?

Well, on their own they can't.

So what do we know so far?

For one, we know that due to Kepler's laws,

that the orbital speed of a piece of dust is

related to the distance it is from a central object.

When a dust particle is far away from a central object its orbital speed is low,

but when it's close to the central object,

the orbital speed is high.

To really understand what's happening we need a second piece of dust.

Let's say that our first piece of dust, dust A,

in the accretion disk around a black hole is sitting at a distance of little a.

Now, let's consider dust A's buddy,

dust B a little further away not too far from the central black hole.

Since dust A is closer,

it'll be moving a little faster,

and since dust B is further,

dust B will be traveling a little slower.

Now, every once in a while,

dust A will catch up and bump into dust B.

And what this does is it causes dust A to slow down and dust B to speed up.

The result is that the slower dust A will

fall further into the disk gaining kinetic energy,

and dust B will use its little speed boost to find a stable orbit

further away from the black hole gaining potential energy.

In an ideal case,

we aren't losing any energy.

But reality is far from ideal,

which means that both dust A and dust B will

be slightly hotter than they were before the collision,

and that heat can be carried out of the disk

by thermal radiation, which is how we see them.

As energy is being carried away from the system,

material in the disk will be pulled further inwards.

In a real disk, there are countless dust particles participating in these collisions.

So instead of looking at individual collisions,

scientists often consider the physics of a large number of interactions.

Material moves inward through the disk by losing energy through a viscous force.

This process turns the gravitational potential energy of the disk into heat,

which is then carried away by radiation,

allowing more material to feed into the black hole.

Until now we haven't really considered what effects

the black hole has on the accretion disc other than gravity obviously.

But now we should briefly discuss how gravitational time dilation,

gravitational redshift, and the doppler effect have on the properties of the disk.

Our friendly little dust particles that were exchanging

energies were also aging at different rates.

If each of dust A and dust B were carrying tiny dust clocks with them,

they would tick at different speeds.

Clock A, being close to the black hole would appear to a distant observer to

tick more slowly compared to clock B which is further out in the disk.

Practically, what this means is that if you wanted to time travel into the future,

all you'd need to do is get very close to a black hole for a small amount of time.

In that way the clocks in the rest frame of the universe would appear to tick faster,

and you would reemerge from near the black hole into a time shifted future.

Astrophysicists need to account for

gravitational time dilation when they're

accounting for the rates that particles emit radiation.

A hot particle for example emitting radiation near

a black hole would appear to be radiating at a much slower rate.

On top of that due to gravitational redshift

the light being emitted from the accretion disc close to

the black hole will lose energy as it climbs out of the black hole's gravitational well,

thereby becoming red shifted from its original wavelength.

But wait, there's still more.

Due to the rotation of the accretion disk,

observers will also measure a difference in

the intensity and wavelength of light depending on

whether material in the accretion disk is approaching or moving away from the observer.

For a spinning disk, the material moving away will be dimmer and redder,

whereas the material moving towards will be brighter and bluer.

In order to truly understand black holes,

each of these effects along with many more need to be taken into account.

Talk about a fun puzzle for theoretical physicists.

Now that we have a good grasp on what we

would see in the environment around a black hole,

we should now ask what an observer would feel in the environment around the black hole.

Normally when we think of a spacecraft in orbit around Earth,

we imagine an astronaut experiencing the sensation of weightlessness,

with gravity only serving to keep them from being flung out into deep space.

But there is another effect that becomes significant around black holes that will be

noticeable to an astronaut near by, tidal forces.

Tidal forces are named after the tides here on Earth which we

experience as the rise and fall of sea levels that occurs periodically.

Humans have speculated about the cause of tides for millennia,

and today we know that they're caused by a combination of the gravitational forces

of the moon and the sun gently pulling on the water in the ocean or rather,

tidal forces act on everything on earth,

but it's really only the oceans that we notice.

For a simple spherical object,

gravity acts to pull the objects towards the center of mass.

However, when a second gravitational body is introduced,

the forces are now the sum of the gravitational forces due to both of the bodies.

This presents an interesting dilemma,

since the gravity from the second body changes strength with distance and thus

the tidal forces will have a different value and direction over the surface of the first.

These tiny differences in gravitational forces

are all that are needed for an object to experience tidal forces.

On earth, we observe this tiny change in

force as a major change in the height of the seawater.

In some cases, like the Bay of Fundy in Canada,

the sea level can vary by as much as 16.3 meters,

tall enough to swamp an entire five storey building.

If small forces like these can create big changes here on earth,

what do you imagine the tidal forces near a black hole might be like?

In our daily lives,

we experience one Earth gravity worth of acceleration.

It's the force that keeps us stuck to the ground.

However, there's a very slight difference in

the forces that pull on our feet compared to the forces that pull on our head,

unless of course you're laying perfectly level.

Let's calculate the difference in acceleration by

rearranging a version of Newton's formula for universal gravitation.

Here acceleration A is equal to two times G times M times h divided by r cubed.

Here, A is going to be the difference between

the acceleration of two points separated by a height h above a body

of mass m and radius r. Let's see what

the differences for a person on the surface of the earth by inputting Earth's mass,

5.97 times 10 of the 24 kilograms,

and radius, 6.3 times 10 to the six meters.

For someone my height, about 1.8 meters,

they experience a difference in acceleration of a

miniscule 5.5 times 10 to the minus six meters per second squared,

compared to one Earth gravity that's less than two parts per million,

definitely not something that we can sense.

But let's do the same thing again,

this time taking the mass and radius of the nearby black hole cygnus X-1.

It has a mass of approximately 15 solar masses or three times 10 to 31 kilograms,

and for simplicity let's just say it's

a Schwarzschild black hole with a radius of 44 kilometers.

Plugging in these numbers give us a difference in acceleration between my head and

my feet of eight million times the force of gravity.

Of course, we wouldn't survive such incredible differences and forces,

and scientists have named this effects Spaghettification,

which isn't so much delicious as it is horrifying.

Essentially, as you approach a stellar mass black hole,

you'll eventually be pulled into a thin [inaudible] that once called itself a human,

not a pretty way to go,

but that was for a small stellar mass black hole.

Let's see how it affects someone around a super massive black hole.

Sagittarius A star, is the name of the black hole at the center of our Milky Way galaxy,

weighing in at four million solar masses.

Sagittarius A has a corresponding Schwarzschild radius of 12 million kilometers,

which is more than eight times the diameter of our own Sun.

Putting these values into our equation yields a difference in

acceleration of a measly one-ten thousands the gravity on Earth.

So, what's going on here?

As it turns out the larger a black hole,

the more gradual the changes in the gravity field as one approaches.

In the movie, Interstellar,

the writers chose to use a fictional super massive black hole called Gargantua,

which is why the Crew of Endurance were able to get so

close to the event horizon without feeling tidal forces.

However, massive tidal forces were apparent on

Miller's planet when gigantic waves circulated the planet.

For super massive black holes,

the tidal forces at the event horizon are

much gentler than the forces around a smaller black hole.

In fact, if you were a space traveler,

you'd need to be extremely careful in the area around a super massive black hole,

because it's possible you could cross the event horizon and not even realize it.

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