10.06 – Pulsar Positioning System

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Astro 101: Black Holes

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University of Alberta

Astro 101: Black Holes

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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.

À partir de la leçon

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.

10.01 – Introduction2:12

10.02 – Gravitational Lensing11:12

10.03 – Gravitational Radiation10:07

10.04 – Emission of Gravitational Waves by Binary Systems19:28

10.05 – Detecting Gravitational Waves with Lasers on Earth7:39

10.06 – Pulsar Positioning System6:02

10.07 – Course Wrap-Up!11:47

Interviews with experts - Where does the gold in your ring come from?1:04

Rencontrer les enseignants

Sharon Morsink
Associate Professor
Department of Physics, University of Alberta

Since existing gravitational observatories

can only detect the strongest waves created

in collisions of massive black holes and neutron stars,

future detectors are being made with ever increasing

sensitivities to find more subtle changes in the fabric of spacetime.

One concept being developed is called a Pulsar Timing Array,

which would allow scientists to probe Einstein's general theory of relativity

and the effects of gravitational waves over thousands of light years.

Since a pulsar is a rotating neutron star which emits a jet of radiation,

if the beam of the jet points towards Earth,

we detect a short radio burst.

Those radio bursts arrive at regular intervals,

sweeping across the earth for every rotation of the pulsar.

The fastest spinning pulsar,

PSR J1748-2446AD, which lives within the globular cluster of Terzan 5,

1,800 thousand light years from earth,

rotates 716 times per second,

which would sound like an F5 tone if the radio pulses were converted to sound.

A spinning pulsar is of great interest,

because some pulsar's rotation rates are incredibly stable.

So much so, that they can arrival the precision of atomic clocks.

So, PSR J1748, the fastest spinning pulsar has been measured to rotate exactly

once every 0.01395952482 seconds,

with an error of less than 600 femtoseconds.

This incredibly precise timing,

is one of the most accurately measured observables in all of astrophysics.

By the way, this pulsar was discovered by Dr. Jason Hustle's,

who graduated with a Bachelor of Science in

Honors Physics from the University of Alberta.

The precision of a pulsars rotation rate is very much

like a clock ticking at regular intervals.

Just like the effects of gravitational Doppler shift

that redshift photons as they escape from a gravity well,

gravitational waves alter the timing of pulses from pulsars.

In order to actually do anything useful though,

you need several pulsars in an array.

Now, you know why they're called pulsar timing arrays.

It may be easier to imagine pulsar timing arrays as similar to

the technology that underpins the Global Positioning System, or GPS.

The GPS sensors in smartphones and navigation devices work by listening

carefully for radio signals from GPS satellites high in orbit above Earth.

By comparing the arrival time of the pulses from each GPS satellite,

your device can triangulate your position on the surface of the earth.

NASA's Nicer-sextant X-ray telescope,

which is on the International Space Station is observing a collection of X-ray pulsars to

test out the feasibility of using pulsar arrays as future navigational aids.

By listening to the regular pulses from several nearby pulsars,

you could triangulate your position anywhere in interstellar space around those pulsars.

This map, created by

the Jet Propulsion Laboratory was affixed to the Pioneer 10 spacecraft,

which after completing a survey of Jupiter became

the first satellite with sufficient escape velocity to leave the solar system.

The image shows the relative positions of pulsars near

Earth with their particular timings encoded on the line that joins them.

If this map were discovered,

the position of Earth could be deduced.

But our civilization hasn't reached the point of navigating with pulsar timing arrays.

Instead, we're patiently listening to them for evidence of

large scale gravitational waves passing in between Earth and the pulsars.

When a gravitational wave passes in between the Earth and a pulsar,

it causes a distortion of spacetime that affects how signals propagate.

Generally speaking, the signals from pulsars will either appear

delayed or accelerated due to the influence of a passing gravitational wave.

Just like a black hole creates a gravitational potential well

and the associated effects of gravitational redshift and time dilation,

so too can a gravitational wave create immeasurable effect as it passes by.

Imagine for example that you usually drive to work or school on a flat road.

The it time it takes you to get from point A to

point B along your route takes about the same amount of time.

What would happen if all of a sudden along the route, a hill appeared?

Or what if a depression appeared in the road instead?

In both cases, the time it takes you to go from A to B will

change ever so slightly depending on the size of the hill.

A gravitational wave in spacetime is just like this hill.

Only since it's a wave,

it will be a moving hill.

If the timing of a gravitational wave is just right,

it compresses the space time that the pulsar signals are

travelling through offsetting the arrival time by a small difference,

which is called the timing residual.

The timing residual is a measurement of the difference between

the expected arrival time of the signal and the observed arrival time.

Since gravitational waves can both stretch and

squeeze spacetime whether the signal is delayed or

accelerated will depend on the geometry of

the pulsar timing array and the incident gravitational waves.

So far, this method of monitoring pulsar timing arrays has

not resulted in any observations of gravitational waves.

However, techniques like these are

a complement to the interferometer based gravitational observatories,

and will eventually contribute to the detection of more massive binary collisions.

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