This course is designed for anyone who is interested in learning more about modern astronomy. We will help you get up to date on the most recent astronomical discoveries while also providing support at an introductory level for those who have no background in science.
11. Testing Gravity
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From the course by University of Arizona
Astronomy: Exploring Time and Space
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From the lesson
Star Birth and Death
Stars are the crucibles of heavy element creation, and the chaotic regions of their birth are being understood though long wavelength observations. Gravity is the ultimate victor in the life story of any star, leaving behind the exotic end states of white dwarfs, neutron stars, and black holes.
Meet the Instructors
Chris ImpeyDistinguished Professor
Astronomy
The ultimate test of our current theory of gravity,
general relativity, will come from Ligo and other gravity wave detectors.
Gravitational waves present a rich phenomenology that can be studied,
involving all kinds of objects in the universe.
And that's still a couple of years away.
Meanwhile, an experiment recently completed in Earth orbit
has tested general relativity and gravity theory in a new regime.
The Gravity Probe B experiment was looking for distortions of space and time that
uniquely occur in general relativity and are not predictions of Newtonian theory.
It was not a completely successful mission.
It never produced results of the errors that had been planned, but
it improved on current tests of general relativity by over an order of magnitude.
And affirm several exotic phenomena that are the result of space-time distortions
caused by mass energy.
Gravity Probe B had four exquisitely accurate gyroscopes,
the most accurate made at the time, although they've since been exceeded.
And a telescope locked on a star to understand the phenomena that
the Gravity Probe B was designed to detect.
Let's look at the conceptual differences between Newton's theory and
Einstein's theory.
[MUSIC]
Although Gravity Probe B is an esoteric experiment to detect general relativity
effects that are extremely small.
It made its way into the popular culture.
[NOISE]
[MUSIC]
[APPLAUSE] Welcome back to the program.
The space time continuum is it distorted by the gravitation fields of
massive objects like the Earth as Einstein postulated?
Or is it simply.
Lazy? [LAUGH] Last week,
NASA set out to answer that question by launching its latest insult to
the nation's homeless, Gravity Probe B, a $750 million satellite
designed to test several key components of the theory of relativity.
Mostly the equals part.
[LAUGH] They got the e, the mc squared,
that they know equals we don't know what's going on with that.
[LAUGH] GPB blasted off into space aboard a Boeing Delta II rocket.
Let's listen to the historic countdown.
>> [CROSSTALK] Five, four, three, two, may the ignition start.
One.
Ignition and liftoff of the Delta rocket,
carrying Gravity Probe B, testing for, [COUGH] test, [COUGH].
>> Why was he standing so close?
>> [LAUGH].
>> That seems.
Falsely comic.
>> [LAUGH] >> GPB will precisely measure the gravity
of earth's orbit using four gyroscopes designed around golfball-sized cork
spheres, said to be the most perfect ever made by man.
>> [LAUGH] >> Believe me, I've seen better spheres.
[LAUGH] The measurements will then be used to test Einstein's theory.
Now, a lot of you are no doubt confused by the geodetic effect,
the way it alters Earth's mass, thus suddenly warping the space-time continuum.
[LAUGH] So if you would be so kind as to indulge me, here's how it works.
[LAUGH].
[SOUND] [SOUND] [LAUGH] I don't
actually know how it works.
[LAUGH] [APPLAUSE] Did you really think I knew how it works, I got nothing.
I don't even know how the damn binaca works quite frankly.
Jesus, I think I poked my eye.
>> The two distinct effects that Gravity Probe B was designed to see
occurred because of the Earth itself, not an extremely massive, nor
an extremely dense object.
So, its distortions of space are very subtle indeed.
One distortion is the curvature of space itself,
which should have manifested in a slight bending of the axis of the gyroscopes
in the satellite as it orbited the Earth.
The second effect was 20 times smaller still, called frame-dragging.
It's the subtle distortion of the contours of space time caused by a spinning object.
Essentially twisting the space time by a tiny amount.
Gravity Probe B detected both effects by their deviations imposed
on the gyroscopes while the telescope was locked in a very distant object,
at a fixed location in space.
The extreme distortions of space and
time expected near a black hole remain to be tested.
The distortion of space as you approach the black hole.
Produces a tunneling of what can be seen looking outwards should someone venture
close to the black hole.
At the event horizon of course, all view of the external world is lost forever.
The distortions of time are equally extreme.
In principle, if two astronauts had synchronized time keeping pieces, and
one stayed in orbit of a black hole at a safe distance,
while the other spiraled into the black hole, according to general relativity.
As seen from the astronaut at a safe distance,
the clock of the astronaut falling in would run slower and slower and
would asymptotically slow as they approach the event horizon.
In principle, at the point they reach the event horizon.
Their clock and
their image as seen from the outside would be frozen forever in time.
Is it possible to ever test this in real life?
Unfortunately black holes are rare.
The likely nearest example would be hundreds of light years away, too far for
us to venture in the near future, perhaps even in the next few centuries.
Also, falling into a solar mass black hole, or
the remnant of a massive star, would be an extremely unwise thing to do.
The differential gravity or stretching force between your head and
your feet if you fell in, on one side of your body and
the other, would be sufficient to stretch you apart and kill you.
And this is not a mild form of stretching, like pizza dough.
This is called spaghettification, where every aspect of your body from muscles,
down to nerve fibers, down to molecules,
are stretched as falling into the black hole, the gravity field increase.
So falling into a black hole is not recommended.
How big would a black hole have to be for this effect not to be disruptive?
Probably about 3,000 times the mass of the sun.
At that point, the stretching force, or the tidal force on a human-sized object
would not be so extreme as to kill that person or disrupt the object.
The second problem of an experiment of falling into the event horizon of a black
hole is that no information could be transmitted out on the conditions inside.
They will remain according to general relativities.
Forever sealed off from us.
There are many hollywood type simulations of black holes and
black holes are featured in TV shows and movies for decades.
But the real astrophysics of a black hole, general relativity can be used to predict
what it would look like in terms of space and time as you fall in.
The following animation showing how the clock is affected and how the contours of
light are affected, shows a spiraling passage in towards the event horizon.
[MUSIC]
How can we summarize the complex states and stages of stellar evolution,
the cycle of star birth and death that includes our story too?
Perhaps this is best done in the words of a popular song from Pink Floyd from
the mid 1970s.
Shine On You Crazy Diamond.
>> [MUSIC].
[MUSIC]
The song is a double metaphor, It was written in honor of Sid Baron, founder and
member of the group, present on their first two albums who left, and
eventually went into disillusion to mental illness and drug use.
He died a few years ago.
They wanted to honor Sid Barrett as the creative spark,
the young flame who lived hard, died young, and left a bright legacy.
But also in this song they pay a nod to most of the stages of stellar evolution.
There's an awful lot of astrophysics in this tune.
In just two verses,
we see references to the main sequences, to the strange carbon-like.
Diamond form in a white dwarf to black holes, to the end states of massive stars
and there are onion skin layering to the difficulty of measuring stellar distances
and in a wonderful metaphor sailing on the steel breeze,
the nuclear synthesis of heavy elements and then blast wave of the supernova.
General relativity has been tested in a new regime by Gravity Probe B
which sought the distortion of space time.
And the twisting of space time itself caused by the spinning earth.
Both effects were detected.
The ultimate test of general relativity,
however, is the study of black holes themselves.
The nearest example is likely to be dozens or hundreds of light years away.
So too far for us to venture with space probes or astronauts.
We will have to do clever astrophysical experiments to decide whether
event horizons are real.
And whether perhaps a singularity lurks at the center of every black hole.
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