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I wrote this essay on gravitational waves during the Winter quarter 2000 school session at University of California, Santa Cruz. At the end of this document you will find additional resources on where to find out about gravitational waves and their significance. I hope you find this essay informative and interesting to read.
Gravitational Waves
In 1916, when Albert Einstein published his General Theory of Relativity,
he made some bold predictions about the nature of the universe.
The curvature of space-time and the existence of black holes have been
two of the most convincing discoveries in favor of Einstein’s theory. Yet, the
theory is being put to test to this day, since until every aspect, every
prediction made by the theory is confirmed it cannot be considered complete and
therefore valid. Gravitational waves are one of the first predictions of
Einstein’s theory but the most difficult one to validate. As elusive as they
are, the detection of gravitational waves will be the most convincing
confirmation of Einstein’s theory to date.
Gravitational waves are created due to the motion of matter through
space. They are ripples in the
curvature of space-time analogous to waves created in water where the water is
space-time. The more massive the
objects the bigger the ripples. Gravitational
waves are predicted to travel at the speed of light. Since gravity is already the weakest of the known forces, its strength
diminishing with distance makes the waves extremely complicated to detect. The strongest gravitational waves are thought to be created during the
collision of two black holes or neutron stars. Actually, the first indirect evidence of gravitational waves came in the
1970s from binary pulsar system (PSR1913+16), a pair neutron starts spiraling
towards each other. The radio
signals received from this system showed that their orbital periods around each
other were decreasing at the rate of 75 microseconds per year, exactly the rate
predicted by General Relativity if the two stars were loosing energy in the form
of gravitational waves. Before
that, American Physicist, Joseph Weber attempted to detect gravitational waves
in the 1960s by utilizing two aluminum cylinders each weighing about 4000
kilograms to isolate them from seismic tremors and other local disturbances. Gravitational waves would, however, distort space-time as they passed
through the cylinders causing it to shrink and expand. Weber fitted the two cylinders with exceptionally sensitive detectors to
measure the changes in their dimensions that would be smaller than the size of a
typical atom. Although Weber recorded some positive preliminary results, they
could not be confirmed.
Recent advancements in materials construction technology, computers and
data acquisition systems have put the detection of gravitational waves within
the grasp of experimental scientists. The
most promising method to detect gravitational waves is to use a laser
interferometer. Laser interferometers work by measuring the time it takes light
to travel between suspended mirrors with very high precision. A laser beam is shone on a beam splitter that splits the laser light into
two parts. Each part is made to
reflect back and forth the laser many times to magnify the expected effects. The beams eventually meet each other and they interfere to create an
interference pattern. Ideally, the
waves will interfere in such a way as to cancel each other exactly. If a gravitational wave passes through the mirrors that reflect the laser
beam, they will cause the space in which the mirrors hang to stretch and
contract. This deformation of the
mirrors will cause the interference pattern of the laser beams to be disturbed
and the first real evidence of gravitational waves will be detected. Depending on the interference pattern observed, the direction, speed and
strength of the gravitational wave can be calculated. Obviously, since gravitational waves are so weak, the wavelength of the
laser has to be known and controlled precisely. The experiment has to be carried out in a vacuum chamber to
minimize the scattering of the laser light due to stray gases in the room. The mirrors have to be made so that the defects on their surface is
minimized and suspended such that all local disturbances such as seismic tremors
are eliminated. To make this task easier the buildings that will house the
experiment will have to be isolated from the rest of the civilization. Because local disturbances cannot be isolated exactly, at least two
locations some distance apart having precisely the same setup will be necessary. Local disturbances will affect only the detector where it originated,
whereas a gravitational wave will affect both sites equally. The degree of precision required is already pushing the
envelope of quantum engineering technology and bringing about innovations in
precision lasers, vacuum technology and advanced optical and mechanical systems.
Two experiments using the laser interferometer technology are currently
underway. LIGO (Laser Interferometer Gravitational Observatory) in the United
Sates and VIRGO, a collaboration of French and Italian scientists in Northern
Italy both use a similar methods described above. Other very similar experiments are being planned by scientists in
Britain, Germany, Japan and Australia. LIGO
is the most complete of all the projects and is expected to start
experimentation later in 2000. It
is collaboration between scientists at California Institute of Technology and
Massachusetts Institute of Technology and funded by the National Science
Foundation. With sites in Hanford,
Washington and Livingston, Louisiana, the separation of 2000 miles between them
should be enough to isolate them from the same local disturbances. LIGO already
boasts the largest vacuum chamber and the most precision optical instruments in
the world, sufficient to detect gravitational waves once they are up and
running. An event producing
gravitational waves as far as 70 million light-years can be detected using the
gravitational observatories in Hanford and Livingston. As project sites in other countries are completed, LIGO is bound to
become an international partnership involving scientists, universities and
governments all over the world.
The pursuit and detection of gravitational waves will have profound
effect, not only on scientists and in science but the general population of the
world. The technologies it demands
will create new industries, which as well moving the experiments forward will
benefit humankind. For example,
high precision lasers that the experiments demand could be redesigned to be used
in medical processes such as laser eye surgery. The collaboration of different countries will usher in understanding of
different cultures and promote peace and unity. This can already be seen in many scientific endeavors around
the world, the most noteworthy being alliance of different nations on the
International Space Station.
Besides
being another validation of Einstein’s General Theory of Relativity,
gravitational waves will give scientists yet another means to gaze in the outer
reaches of the universe. These
waves could be used as a tool to study the birth of black holes and neutrons
stars, collision of black holes and neutron stars and other cosmological
phenomena that has not been predicted or detected. Since gravitational waves are predicted to pass through matter
unhindered, it will allow scientists to “see” further out into the universe
than any optical or radio telescopes built. The detection of gravitational waves will open new doors in the
understanding of the universe and promote innovation in other sciences and
technology to understand this new phenomenon exactly and to build instruments
that will exploit gravitational waves and their consequences to the fullest.
Bibliography
Blair,
David G. (Ed). (1991). The
Detection of Gravitational Waves. Cambridge, Great Britain: Cambridge
University Press. Davies,
P. C. W. (1980). The Search for Gravity Waves. New York, NY: Cambridge
University Press. Ronan,
Colin A. (1991). The Natural
History of the Universe: From the
Big Bang to the End of Time. London, Great Britain: BCA. (1995).
Ripples in Spacetime. [Online] Available: http://www.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html
[11 July 1995] (1995).
LIGO: A New Window on the Universe. [Online] Available: http://www.ncsa.uiuc.edu/Cyberia/NumRel/LIGO.html [29
August 1995] (1996).
LIGO: Fact Sheet. [Online] Available: http://www.ligo.caltech.edu/LIGO_web/about/factsheet.html
[13 November 1996] (1996)
LIGO: Catching the Gravitational Wave. [Online] Available: http://www.ligo.caltech.edu/LIGO_web/about/brochure.html
[6 December 1996]
(c) 1997-1999 (kamal<@>imaginearts.com)
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