by Robert W.
Bly
"…Gravity,
taken so for granted, is really something eerie, Messianic, extrasensory in
Earth's mindbody…"
--Thomas Pynchon in Gravity's Rainbow
"Everybody accepts the
notion of gravitational radiation. The controversy comes in 'How strong are
these waves?'."
--Dr. David H. Douglass, Jr., Ph.D.
Everybody living on this planet is familiar with the
effects of gravity. You drop an apple, and it falls toward the ground. We know
gravity is an attractive force. Now, if we drop this apple in a tub of water,
we see that the water is disturbed, and waves are generated along its surface.
We are all familiar with the phenomenon of
waves. But few of us ever think of gravity in terms of waves, when in
fact gravity is manifested as a wave, as well as a force.
There
is a useful analogy occurring between gravity and electricity which makes it
easier for us to accept, or at least understand, the notion of gravity waves.
One type of electricity is static electricity, which deals with stationary
charged particles. If you rub a comb through your hair, for example, it will
become charged and will pick up little bits of paper. If we are dealing with
two point charges, the force between them will be inversely proportional to the
square of the distance between them, and directly proportional to the product
of their charges.
The
gravitational force is the analog of the static electric force, the only
difference being that two masses will always attract each other, while like and
unlike charges will attract, but two like charges will repel each other. Like the electric force, the gravitational force
is inversely proportional to the square of the distance between the masses. It
is directly proportional to the product of the two masses. The electric force
is by far the stronger of the two forces.
Another aspect of electricity deals with waves. If
charged particles accelerate, electromagnetic waves are radiated from a source,
let us say from an antenna. A wave is anything which propogates with a well
defined velocity. In the case of electromagnetic waves this velocity is the
velocity of light. These waves carry energy and momentum which can be
transferred to objects in their path. If the wave from our antenna is a radio
wave, it transmits energy which is converted into sound waves when it crosses
the path of a radio.
Static electricity has its analog in gravity, so why
shouldn't electromagnetic waves? And it turns out that this is just the case.
To produce a gravity wave, you need two or more accelerating masses. Now, this
can be achieved with just one body, as long as some portion of the mass
distribution is accelerating with respect to the center of mass of the body. A
wave is then radiated, which propogates outward with a velocity equal to that
of light. The velocity of light is about one hundred and eighty six thousand
miles per second.
This
wave, like its electromagnetic counterpart, does not require a medium in which
to travel - it can propogate in a vacuum. The gravity wave carries energy,
which it can transmit to bodies in its path. This results in motion of the
bodies bombarded by these waves.
The
reader may be starting to think, "Yes, this is all very interesting, but
it's a lot of nonsense. I've never seen the effects of any gravity waves!"
And you are right, the effects of these waves have probably never been observed
by anyone on this planet. But, as you shall see in a few paragraphs, people are
trying to see them.
The
planet Earth does not generate any gravity waves itself, even though it is in
constant motion. Any sphere possesses properties of symmetry, and you need a
geometry other than spherical in order to produce gravity waves. The Earth is
very nearly transparent to gravity waves; they pass right through it as if it
weren't there at all, like sunlight through a windowpane.
The gravitational radiation that is produced by
other sources is, however, of a very small magnitude. Recently, Professor
Joseph Weber at Maryland claimed to have detected gravity waves with his
instruments. Dr. David Douglass, Professor of Physics at the University of
Rochester, repeated Weber's experiments using a similar gravity detector, and
did not find any evidence of gravity waves. Several other researches obtained
the same results as the U of R researches, and Dr. Douglass says that
"...most people believe that we have in fact disproved Weber's
claims."
Failure
to detect gravity waves does not mean the do not exist, and scientists are not
now claiming this. The experiments just tell the physicists that the wave they
are looking for are weaker than they had anticipated.
The
waves the scientists tried to detect were bursts of gravity waves from
exploding stars. In these explosions, much of the star's mass is converted to
energy and radiated as the waves.
In
order for waves of the magnitude Weber claims to have seen to have been
generated, between fifty to one hundred solar masse daily would have to be
converted to energy. A solar mass is roughly equivalent to three thousand
trillion tons of matter, and the energy equivalent of this mass can be
calculated from the well known equation E = mc2 -- energy equals
mass times the speed of light squared. Astronomers report that the amount of
mass actually converted to energy is nowhere near the amount indicated by
Weber, so observable fact contradicts Weber's claim, and supports the
conclusion of Dr. Douglass.
Just
as a mass distribution is required to generate gravity wave, so is one needed
to detect such a wave. Dr. Douglass used a large aluminum cylinder. If a wave
passes through the cylinder, the ends should oscillate with respect to the
middle.
The
wave amplitude is described in terms of the ratio of the displacement of one of
the cylinder's ends, and the length of the cylinder. This ratio is known in
physics as the strain, hence the gravitational field is a strain field.
The
detector is sensitive to a displacement of about a hundredth of the diameter of
the nucleus. In three years of research Douglass found no gravity waves. Says
Douglass, "You might think of a gravity wave detector as a very sensitive
bell just waiting to be rung…Although a gravity wave won't interact very long
with that bell, the desirable property would be to have the bell ring, or
oscillate, for hours or days.”
The
concept of a gravity wave is not at all a new one. It goes back to Einstein,
and was part of his original Theory of General Relativity. And just as
relativity -- strange as it may seem to the layman -- is accepted without
question by most modern physicists, so too is the gravity wave. Gravitational
radiation is "just part of physics that should exist," says Dr.
Douglass. Douglass has finished his series of experiments with the Weber type
apparatus, and is in the process of constructing a new gravity wave detector.
The
new detector will be about a million times more sensitive than the one used by
Weber, so hopefully it will be able to show positive evidence of gravitational
radiation. This detector involves giant artificially growl sapphires. \
These
crystals, unlike natural ones, are very large and, more importantly, are
without flaws. When and if they interact with a gravity wave, they should
oscillate for a long period of time, perhaps for days, giving scientists a
better chance to study the phenomenon. This is probably the most unusual use
found for these costly artificial gems yet.
Silicon
may also be used for such single-crystal detectors. Silicon is the material
used in integrated circuits in such devices as pocket calculators and transistor
radios. The silicon crystals are in the shape of cylinders, with diameters of
up to six inches. Researchers at this university recently had one eighteen inches
in length; lengths of up to four feet may be possible.
The
concept of gravity has been with us since Newton allegedly got beaned by a
falling apple. It is something that we take for granted, for we have all seen
the falling apple or what have you knock someone in the head.
The
concept of gravity waves has been with us since Einstein, and while physicists
accept and indeed insist upon it, they are going to have to verify the theory
experimentally before the layman accepts it. Only when the sapphire detector is
moved by a gravity wave, when that apple
hits us in the head, will we be able to appreciate the fascinating concept of
gravity waves.
Most of the information in
this article is the result of an interview with Dr. David Douglass. The article
originally appeared in Logos, December
20, 1976.
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