This activity introduces gravitational waves and the NASA technology being developed to detect them in space. It involves building a metaphorical interferometer that demonstrates how the mission (and all interferometry) works. Educational levels: Middle school, High school

NASA JPL

English

Digital Library for Earth System Education

1Originally published in The Technology Teacher, December 2003/January 2004, by the International Technology Education Association
LISTENING FOR RINGS FROM SPACE
Did it ever occur to you how really amazing are your
senses of vision and hearing?  They allow you to understand
objects and events in the world “remotely,” without being in
direct contact with them.  While walking around, you can
avoid running into things just by processing and understand-
ing the information being carried to your brain by the light
waves entering your eyes.  Even in a room with no win-
dows, you can understand that a thunderstorm is raging
outside just by the sound waves entering your ears.
Light waves and sound waves are forms of energy.
They carry information about whatever produced (and
possibly reflected) them.  Waves of candlelight that enter
your eyes from your friend’s face carry information not only
about the candle that produced the light but also about the
face that reflected it.  Likewise, ocean waves are waves of
energy moving through the water that carry information
about the storm or the tides or Earth’s rotation from which
the energy originated.  If there is an earthquake or suddenly
erupting volcano at sea, a giant tsunami wave may form,
carrying information about the cataclysmic event that has
just occurred.
LOCKED IN A SHOCKING DEATH SPIRAL
Likewise, extremely fast motions by very massive objects in
space produce waves of energy, not in air or water, but in
the fabric of spacetime itself.  For example, black holes are
very massive and dense objects.  They have so much
matter packed into a small volume that their gravitational
fields are too powerful for even light to escape.  That’s why
they “appear” (or, actually, disappear) as black holes.
When two of them get close enough together, they begin a
spiraling dance of doom, their gravitational fields pulling
each other in closer and closer.  As they spiral through
space, they release huge amounts of their gravitational
energy.  This energy radiates away in all directions in
pulses, or waves—gravitational waves.  Finally, spiraling
faster and faster, and closer and closer, they collide and
merge.  The last few days and minutes of this dance
release huge amounts of gravitational energy.
No one has detected a gravitational wave, but they are
predicted by Einstein’s theories and are certain to exist.
Astronomers want to detect and study these waves of
gravitational energy, because it would help them understand
these very strange objects and events.  Other violent
cosmic events that produce gravitational waves are super-
nova explosions, and mergers of compact stars with each
other or with black holes.  Also, gravitational waves are still
ringing from the Big Bang that started the universe.  Those
waves could tell us about our very beginnings.
So, how do we detect gravitational waves?  No
telescope or listening device ever invented would be able to
detect gravitational waves.  They are an entirely different
form of energy from electromagnetic waves (light in all its
visible and invisible forms).
What would we have to be able to measure in order to
detect gravitational waves?  To answer this question, we
have to ask what happens when a gravitational wave
passes by us and our gravitational wave detector.
Just as a sound wave causes air molecules to be
momentarily squeezed together as it passes, a gravitational
wave actually causes space itself to be squeezed!  This
squeezing has the effect of changing the distance between
objects—for example, the distance between the spacecraft
in the LISA mission.
In this artist’s concept, two black holes spiral around one
another locked in a gravitational dance of doom.  The three
LISA spacecraft detect the waves of gravitational energy
released by the motions of these massive bodies.
2Originally published in The Technology Teacher, December 2003/January 2004, by the International Technology Education Association
Gravitational waves that reach our neighborhood of
space from millions of light years away are extremely
weak.  So in order to be able to measure them, we need a
technology that can measure distances with extreme
precision.
AN EAR FOR THE GRAVITATIONAL MUSIC
LISA (for Laser Interferometer Space Antenna) is a
mission to measure gravitational waves in space.  It will
consist of three spacecraft flying in the form of a triangle 5
million kilometers (3 million miles) on a side!  These three
spacecraft will use laser beams to keep in touch.  The laser
beams will form the sides of this virtual triangle and enable
the three spacecraft to remain a very precise distance apart
and be able to detect any squeezing or stretching of the
distance between them caused by a passing gravitational
wave.  With three spacecraft, LISA will be able to pinpoint
from which direction the gravitational wave originated.
Because the waves are so weak,  LISA must be able
to measure distance so precisely that it could detect a
change the diameter of a human hair in the distance from
here to the nearest star, Alpha Centauri.  This precision is
what the lasers will provide.
THE POWER OF LASERS
Remember that electromagnetic energy (light) travels
in waves.  The length of the wave is what determines
whether the electromagnetic energy is, for example, a radio
wave, a visible light wave, an infrared wave, or an x-ray.
Laser stands for “light amplification by stimulated
emission of radiation.”  Laser beams have three character-
istics that make them different from ordinary beams of light:
First, the light from a laser contains exactly one color
or wavelength rather than a lot of different wavelengths.
Scientists say that laser light is “monochromatic,” meaning
of one color.
Second, all the wavelengths are in phase.  That is,
they are all “waving” in unison.  All the wave crests (high
points) and troughs (low points) are lined up.  Scientists say
the laser light is “coherent.”
And third, the particles of light, or photons in the laser
light waves are all traveling in the same direction, exactly
parallel to one another.  They are not headed off in different
directions.  This means that laser light beams are very
narrow and can be concentrated on one tiny spot.  Scien-
tists say the laser light is “collimated.”
Because the laser light is monochromatic, coherent,
and collimated, all of its energy is focused to produce a
small point of intense power.  This also means that a laser
beam can shine a very long distance without losing a lot of
its power, since it doesn’t spread out much.  This character-
istic makes lasers perfect for LISA, since the beam gener-
ated by one spacecraft has to travel a 5-million-kilometer
straight line through space to “connect” with another
spacecraft.
So, how is it that LISA will be able to measure this
huge distance so precisely?
THE POWER OF INTERFEROMETRY
As we mentioned, waves have crests and troughs, and
in a laser beam they are all lined up, or coherent. The
following explanation is a bit simplified.  In principle, one
LISA spacecraft sends a laser beam toward another
spacecraft.  The other spacecraft reflects that laser beam
right back.  If the distance between them has not changed,
the returning laser beam should still be in phase (coherent)
with the sent laser beam.  If their distance has changed
even a fraction of a wavelength, then the two laser beams
will be out of phase with one another and the LISA elec-
tronics will detect and measure how much out of phase they
are.  Since about 1000 wavelengths of laser light would fit
3Originally published in The Technology Teacher, December 2003/January 2004, by the International Technology Education Association
on a speck of dust, a part of a wavelength is unimaginably
tiny.  This explains the amazing precision of LISA’s mea-
surements of very long distances.
This measurement technique is called interferometry,
because the measurement is based on the interference of
electromagnetic waves.  When waves that are in phase
encounter one another, the energy of their crests and
troughs add together.
When waves are totally (180°) out of phase, the crest
of one wave cancels out the trough of the other.
MAKE A SIMPLE “MINI-LISA”
We can mimic in a simple way LISA’s precision laser
interferometer technology using ordinary materials and
three students to stand in for the three LISA spacecraft.
You will be making three “laser beam interferometer”
models for the entire class.  Each of these models will
represent one side of the triangle formed by the three LISA
spacecraft, and the three students will each be “attached”
to two of its sides.
MATERIALS:
• 3 file folders
• 6 photocopies on transparency film of the strip on the
last page of this article
• String or twine (non-stretchable), about 30 meters
(about 99 feet),  (3 10-meter or 33-foot lengths)
• 3 paper clips
• 3 rubber bands
• Scissors
• X-acto knife
• Stapler
• Tape
• Cotton swabs, matchsticks, or toothpicks (6)
CONSTRUCTION:
Cut out, fold, and construct the interferometer model
as shown (see illustration).
4Originally published in The Technology Teacher, December 2003/January 2004, by the International Technology Education Association
The stripes on the transparent strips represent laser
beam wavelengths.  Each dark line stands for a trough and
each “bright” line stands for a crest.  In this case, your
“laser beam” has a wavelength of 1 centimeter, or about
10,000 times longer than LISA’s laser beams.
On each of the model interferometers you have made,
the “interferometer window” represents one of the space-
craft sending out a laser beam toward one of the other
spacecraft.  The clip at the other end represents a mirror
reflecting the laser beam (thus the loop in the string) back to
the originating
spacecraft.
Now, let’s put it
all together.  One
student (A) holds the
“interferometer
window” end of the
“laser beam interfer-
ometer.”  Another
student (B) clips the
other end onto his or
her clothes.  In turn,
B holds the “Interferometer window” end of the second
model and a third student (C) clips the other end onto his or
her clothes.  All that’s left to complete the triangle is for C
to hold the window end of the third model and A to clip the
end onto his or her clothes.
Now everyone moves into a position so that there is
no slack and just a bit of tension in the strings.  You now
represent the three LISA spacecraft flying in formation,
connected by laser beams.  You are only about 5 meters
apart, but the LISA spacecraft will be a billion times farther
apart!
But even as close together as you are, notice what
happens in the interferometer windows when someone
moves the slightest amount, thus changing the length of the
“laser beam.”
Look back at the drawings labelled 1, 2, and 3 of laser
light waves in and out of phase.  If the outgoing and re-
flected waves are in phase (1), the interference pattern on
your interferometer will look like the Drawing 1 below.
This could mean that the distance the laser beams had
traversed either had not changed at all, or had changed by
one or more whole wavelengths.  Similarly, if the outgoing
and reflected laser beams are 1/4 wavelength out of phase,
the bands of the interference pattern will overlap as in
Drawing 2.  If the outgoing and reflected waves are totally
(180°) out of phase, the bands will appear to cancel each
other out as in Drawing 3.  If a computer (or you, in this
demonstation) records all the shifts in the interference
patterns as they take place, it (or you) can then calculate to
the nearest 1/4 wavelength the total distance moved.
In the case of your interferometer, a “wavelength” is 1
centimeter. So, how accurate is your interferometer?
At 5 meters, you can easily see a shift of as little as
one-quarter of a wavelength.  Since 1 wavelength is 1 cm,
¼ wavelength = .25 cm
And the
Total distance = 5 m x 100 cm = 500 cm
To find the accuracy, we divide the total distance by the
smallest change in distance that we can measure, or
500 cm / .25 cm = 2000
So your interferometer is accurate to 1 part in 2000.  You
can measure a change of as little as 1/2000th of the dis-
tance between the “spacecraft.”   Not bad!
Now, how accurate is LISA’s interferometer?  For
LISA,
1 wavelength = 1 micron (one-millionth of a meter)
Total distance = 5,000,000 kilometers (multiply by
1,000 to get total meters)
Total Meters / .25 micron = ? (answer at bottom of
page)
Now, how can LISA detect a passing gravitational
wave?  And how will LISA know from which direction it
comes?  As the gravitational wave passes, it will stretch and
squeeze the space along the three arms of the interferom-
eter, depending on the direction it is moving.  LISA’s
computers will be able to calculate this direction, as well as
the strength and wavelength of the gravitational wave based
on these tiny measurement differences.
5Originally published in The Technology Teacher, December 2003/January 2004, by the International Technology Education Association
PUSHING THE TECHNOLOGY OF PRECISION
The “gravitational wave detecting antenna” you’ve made demonstrates
some fundamentals of interferometry, with no technological tricks to make it
cheaper, faster, better. Your system, with suitable software to process the
data from the three lasers, could be used to measure the presence of
gravitational waves.
But you would encounter some problems; at the enormous distances,
even laser beams become weakened. Instead of reflecting a weak signal to
the transmitting spacecraft, you might have the target spacecraft re-transmit
a return beam at full power, with waves in the same sequence as though
reflected.
With your system, data about distance change has to be collected from
laser systems on three legs, then calculated to determine what moved and in
which direction. It would be more efficient to measure two legs at once; use
one laser and shine it through a piece of special glass at 45 degrees to split
the laser beam, sending half to each of the other spacecraft. The reflected
beams return through the same glass and both can be seen from a single
point (on the other side of the glass). Waves from the two reflections come
together to create the same kind of interference patterns as shown in your
demonstration system. Changes of the interference pattern reveal move-
ments, but only in two legs. Data from all three legs is needed to accurately
measure distance and motion of the spacecraft.
LISA’s system solves that in this way:  The transmitting spacecraft (A)
splits the laser beam, sending half to spacecraft (B) and half to spacecraft
(C). Instead of returning the “reflected” laser beam to the transmitting
spacecraft (A), receiving spacecraft (B) transmits the beam to the space-
craft (C), which in turn transmits it back to (A). Similarly, (C) sends to (B)
which returns to (A).
Interference of waves in the returning beams represent movements in
all three legs of the LISA triangle.  If the two arms of the interferometer are
exactly the same length, a bright spot will appear on the detector, giving the
maximum electrical signal. This indicates no gravitational wave has passed.
If the difference in the arm lengths is one half of the wavelength of the laser
light, then the two light beams will cancel each other out and the detector
output will be a minimum.
This can indicate a passing gravitational wave. Because the wave-
length of laser light is very small, an interferometer is a very sensitive
instrument for measuring small changes in length and is thus ideal for use in
detecting gravitational waves.
Find out more about gravitational waves and do a LISA online cross-
word at spaceplace.nasa.gov/lisa_fact2.htm .
This article was written by Diane Fisher, writer and designer of
The Space Place website at spaceplace.nasa.gov.  Alex Novati drew the
illustrations.  Thanks to Gene Schugart, Space Place advisor, for
activity concept and helpful advice.   The article was provided through
the courtesy of the Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, California, under a contract with the National
Aeronautics and Space Administration.


Technology Teacher: Listening for Rings from Space

http://spaceplace.nasa.gov/review/classroom-activities/pdf/LISA_antenna_activity.pdf

Technology Teacher: Listening for Rings from Space

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