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Adrian Wagner
Gravity Waves
Sir Isaac Newton viewed gravity
as a force of attraction between two bodies that manifest itself
via a gravitational field. For
400 years, this description of gravity was left unchanged; it
stood solid in the minds of physicists and in the explanations
of their experimental observations. Its was not until the end of the 19th century, that astronomers began to notice a slight differences
in the calculated and observed orbit of Mercury, which could not
be explained via Newton's gravitational laws.
By 1915, this error of observation
had been mathematically rationalized, but it was only after building
an entire new theory of gravity. This theory is known as Einstein's General Theory of Relativity.
Einstein replaced Newton's
gravitational force, with the idea that mass effected space and
time itself by curving it. According
to the General Theory of relativity, it is this curvature that
causes what can be observed as the force of gravity.
In the figure above, which
represents the curvature of space-time by the large central white
mass, it is the mass itself that is causing the curvature. Any object placed near the mass will fall towards it, just
as if it was being pulled towards it by a force. Any object with perpendicular motion - relative to the
line formed between its position and the center of the sphere
- will, if moving at the right velocity, circle in orbit around
the sphere. Just as the
earth revolves around the sun, a body in such a relationship to
a central mass that is curving space and time, will orbit the
central body. According to General Relativity, it is not
a force that acts upon the body. Rather, the body in orbit feels no force at all. It is space and time that is curved, such that the body,
moving, unchanging in its movement, un-accelerated - can travel
through this curved space as if it were unaffected by any force, and appear as
if it were being accelerated by a force if it were indeed in flat
space. It is the very notion of a path that must be
revisited. A line that
is straight in flat space is not so in curved space, such that
the line formed by a body's trajectory can loop back upon itself
as in an orbit. This is the general premise of Einstein's theory.
One of the outcomes of the theory of General Relativity
is the prediction of Gravity Waves. If a body with significant mass is at rest, it curves space
and time around it as previously described. If, however, the body moves suddenly, the curvature of
space-time must also be altered around it. Yet space-time can only experience change from
the point of origin at the rate that it takes light to radiate
from that origin. In other
words, nothing in the known universe can travel faster than light. As such, the very fabric of space-time that makes up the
universe can also not be effected by change - in this case the
change of its curvature - at a speed faster than light. So if gravity curves space and the curvature of space is
altered by moving the gravitational 'source' that curves it, the
change of this curvature travels across the fabric of space time
at the speed of light, as a ripple travels across the surface
of a pond.
These gravitational waves,
Einstein predicted, "are produced when objects are accelerated,
or when strong gravitational fields interact dynamically."
Gravitational waves cause a
variable strain of space-time. What this means is that as a gravitational wave passes
a region of space, the wave changes the distance between points
in that space. The size
of these changes is proportional to the distance between the points. It is therefore possible to detect gravitational waves,
given that one could detect minute changes distance. However, the change of distance caused by a fairly large
gravitational wave would alter the distance between the sun and
the earth by less than the diameter of an atom. Clearly detecting such an effect is a great
challenge.
To date, no one has observed a gravitational wave directly
by such means. However
there is already strong evidence that they indeed do exist. One of the outcomes of their prediction is that any source that
emits gravitational waves looses energy as it produces them.
A strong candidate for the
production of significantly high-energy gravitational waves is
a pair of orbiting neutron stars and/or black holes. Because of the high densities of these objects, they produce
very powerful distortions in the curvature of space-time. The interaction of these curvatures should produce high-intensity
gravitational waves. Consequently,
the two orbiting pairs will emit energy, thus loosing the momentum
necessary to maintain their orbits. As the stars get closer together, the period of their orbit
decreases according to Kepler's third law.
This effect has been observed
in the binary pulsar system PSR 1913+16 discovered in 1974. This system consists of two massive objects,
probably neutron stars, separated by a distance only a few times
larger than the distance from the Earth to the moon. As one of the stars emits radio signals, the period of
its orbit can be determined by looking at the Doppler shifts in
the signals frequency. Indeed the pair is slowly spiraling inward,
at a rate 99.5% of that predicted by the theory of General Relativity.
This slowing of the orbital
period is the strongest evidence found for the existence of gravity
waves. However, large and costly projects are underdevelopment
to detect these waves directly.
As already noted, the presence
of gravity waves changes the distances between points every so
slightly. If these changes
in distance can be detected, the magnitude and frequency of these
changes can shed light on the very ripples of space-time that
pass through them. In fact, if multiple detectors are set up in
a three dimensional array, the shape and direction of these waves
can be detected. The value
of such a systems come from the fact that gravitational waves,
unlike electromagnetic ones, are not absorbed, deflected, not
dispersed by matter. They do not travel through space, but rather the are the fluctuations
of space-time itself. Therefore,
they may one day allow us to pear into the center of object that
are otherwise not transparent to electromagnetic waves. We may be able to see the inside of a supernova. We may be able to see the properties of a black
hole. We may even be able
to see into the far reaches of time, in the early moments of the
universe. Such a devise "may be able to peer into
the nursery of creation. The
earliest instant following the Big Bang - the Planck era - marked
its passage by emitting gravitational waves." After 300,000 years, the universe had cooled enough to
allow atoms to form, thus becoming transparent to the microwave
radiation that we can observe today. But prior to this time, the history of the universe is
hidden. The detection
of gravitational waves, which were able to stream freely as early
as 10-43 seconds after the big bang, could enable us
to hear "at last the hush of the universe's birth."
The above depicts how a gravitational
wave detector would work. The
underlying principle is based on the fact that the distance between
two bodies will change as a gravitational wave passes through
them. This change in distance, however, can be very
difficult to detect. In fact, the changed effected by a fairly
large gravitational wave on two large masses separated by 4km
can be as little as the side of the nucleus of an atom. Do be able to measure such distances, scientists propose
to use laser light to bounce a beam back and forth between the
two masses. Any change
in the distance should result in a noticeable change in the phase
of the light, and bands of dark phase cancellations should appear. Another challenge is isolating any seismic
disturbances from the earth from these masses. Elaborate spring dampening and active seismic cancellation systems
are now in development.
If the detection of gravitational
waves proves to one day be possible, it will in theory open up
an entire new spectrum into the universe. We will be able to see inside of otherwise mysterious object,
and perhaps even pear into our universe's very origin.
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