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6
Fundamental Physics and Chemistry:
Relativistic Gravitation and Microgravity
Science
OVERVI1:W
A common link among traditional space sciences such as
space astronomy and astrophysics, planetary exploration, and so-
lar plasma physics is their use of spacecraft for their observations.
One of the objectives of this study was to determine whether
there is likewise a potential to use space vehicles as laborato-
ries in which fundamental physical and chemical laws might be
investigated. The answer is decidedly positive. Spacecraft can
provide a unique environment for at least two kinds of studies:
those that would further our knowledge of relativistic gravitation
and those exploring fundamental processes that require very small
gravitational forces or very small gravitational gradients. The im-
plications of using space vehicles for the study of general relativity
have been understood for some time, and a specific strategy for
investigations of relativistic gravitation from spacecraft after 1995
has been set forth here. On the other hand, the implications of
exploiting the nearly gravity-free environment of space to study
basic properties of matter have not been wed delineated before,
51
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and the identification of opportunities In this reahn ~ an impor-
tant new achievement one of the most exciting to emerge from
this study.
A.
RELATIVISTIC GRAVITATION
BACKGROUND
General relativity relates the geometry of space and time to
the distribution of matter In the universe. Gravitation is the consul
quence of the way this spac~tune geometry affects the movement
of matter in space. As a theory, general relativity is well devel-
oped; it has important consequences that can be tested. There
are three classical tests of general relativity in weak fields such
as those near the Sun or Earth. The first involves the precession
of the perihelion of a sol" system object such as the planet Mer-
cury. The second utilizes the deflection of light passing close to
the Sun. The third involves the gravitational red shift of spectral
Imes, which attests to the effect of a gravitational field on the rate
of clods. All of these ejects can be measured with much greater
precision ~ space than on the surface of the Earth, permitting
more accurate predictions of the gravitational field strength.
TESTS OF GE:N1:RAI 1~:[ATIVITY THEORY IN WEAK
1lIE:[DS
Defection of Light
Currently, we can verify the predicted deflection of a light ray
grazing the limit of the Sun with about 2 percent uncertainty.
But we could improve this by 2 orders of magnitude if we could
make the measurement with an optical interferometer flown on the
Shuttle. This instrument would consist of an articulated pair of
stellar interferometers, having their viewing axes approximately 90
degrees apart. It would have two pairs of mirrors 25 cm in diameter
and an interferometer length of 2 m. A free-flying spacecraft could
improve even on this precision by probing longer exposure and
more stable pointing.
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Gravitational Red Shift
The gravitational red shift is a consequence of the difference
in the rate at which identical clocks measure tune at different
depths in a gravitational well. This effect has already been found
to agree with the prediction of the theory of general relativity
to within ~ part in i04. The experiment consisted of measuring
the rate ot a hydrogen maser clock as it wan carried to a height
of 10,000 km on a rocket. However, a qualitatively different test
of general relativity theory could be performed by carrying an
improved hydrogen maser close to the Sun, where the red shift
will be more pronounced since the clock will be deeper in the
gravity wed. Significant variance of the measurements made there
from the predictions of general relativity would cause a major
rethinking of the theory.
. ~ ~ ~ ~ ~
Relativistic Hame Dragging
There is another prediction of the general theory that has
never been tested. This is a nonstatic effect, and it states that
rotating bodies drag nearby inertial Dames. Although the effect is
exceedingly small in weak fields near solar system bodies, it might
be enormous and astrophysically important near a rotating black
hole. The relativity gyroscope experiment called Gravity Probe
B has been devised to search specifically for the frame-dragging
effect produced by the rotating Earth. It will use the most precise
gyroscopes yet devised. The ssion has been likened In impor-
tance to the classical Michelson-Morley ether drift experiment of
1887. The proof that there was no ether drift buttresses Einste~n's
special theory of relativity and has changed fundamental concepts
of space ~d tune. Although many times more sophisticated than
any experiment yet attempted in space, there is considerable confi-
dence that the Gravity Probe B mission wall be successful. Gravity
Probe B should be flown before 1995 unless the consequences of
the Challenger accident delay it.
PRINCIPLE OF EQUIVALENCE
General relativity is based on a fundamental principle caller}
the principle of equivalence. The principle asserts that the grav-
itational mass of an object, that is, the quantity that measures
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the gravitational force it produces, is identical to the mass that
responds inertially to any force. In short, it states that gravita-
tional and inertial masses are equal. The validity of the principle
has been demonstrated to a level of one part In 10~ in the famous
Eotvos experiment. Shuttle flight of an experiment to test this
equivalence at the level of one part in 10~4 is proposed for the near
term (before 1995~. During the period covered by this study, a
similar experiment flown in a free-fly~ng spacecraft would provide
a test to the level of one part in 10~7
.
SECULAR ClIANGE IN THE GRAVITATIONAL CONSTANT
Another important physical principle is called Mach's princi-
ple. It suggests that the expansion of the universe might cause
the elective local value of the gravitational constant G to decrease
with time as a consequence of the effect of distant mass on the in-
ertial properties of local matter. Microwave ranging to a Mercury
orbiter could improve our knowledge of the time rate of change
of G by 3 orders of magnitude. A by-product of this exper~rnent
would establish the extent to which gravity Is itself a source of
gravitation.
G}lAVITATIONAI WAVEf;
~ Newtonian theory gravitation propagates instantaneously
over infinite space. The concept of waves is not applicable. In
contrast, Einstein's general relativity requires gravitation to prom
agate with the speed of light, just as does electromagnetic ra~dia-
tion. Electromagnetic waves jiggle charged particles; gravitational
waves accelerate ma". When traversing a large object, a grav-
itational wave will deform it. In the language of relativity, a
gravitational wave ripples the curvature of space-time, deforming
any mass that sits in space.
The detection of gravitational waves is one of the most chal-
leng~g problems ~ experimental gravitation today. Observation
of gravitational waves would open new astronomical windows. It
would provide information about exotic sources of gravitational
radiation: collapsing stellar cores, colliding neutron stars or black
holes, decaying binary star systems, and rotating or vibrating
neutron stars. In the meantime, the discovery of a radio pulsar
in a binary system containing, most likely, another neutron star
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has provided very convincing evidence that gravitational waves do
indeed exist. The orbit of this system is decaying almost exactly
as expected if such waves were being emitted.
~ the explosive events In quasars and other active galaxies
are generated by black holes or sup ermassive black holes, each
explosion must generate a great gravitational wave that rattles
everything in the universe. On the other hand, the radiation pro-
duced by many astronorn~c~ interactions, such as that of a black
hole with neighboring matter, ~ of a very low frequency below
10 Hze Its detection requires an observatory in space, free from
interference by seismic noise. A gravitational wave detector con-
sisting of three spacecraft orbiting the Sun, each one a million
kilometers from the next and possessing a precise system for mon-
itoring their separation by laser ranging, would allow a detection
of gravitational waves from astronomical sources in the range of
periods from 0.3 s to 10 days. Gravitations waves would cause
the distance between these spacecraft to oscillate. The estunated
sensitivity achievable with such a system is one part ~ 1022 for
narrow-band periodic signals and as much as one part in 102° for
transient pulses at megahertz frequencies. Such a detection shy
tem offers us our best chance of directly observing the radiation
produced by distant matter accelerating In strong gravitational
fields such as those produced by black holes.
Pulsars spinning with periods close to a millisecond approach
relativistic instability; their surfaces move at close to the speed of
light. The discovery of such objects could provide the frequency
key to ground-based gravitational wave detectors ~ their search
for gravitational wave radiation. The steering group recommends
building a very large proportional-counter x-ray detector with a
receiving area of about 100 m2 that could be attached to the
Space Station or orbit as a free flyer. This very large detector
would search the sky for very fast x-ray pulsars.
PR¢1995 PROGRAM FOR RELATIVISTIC GRAVITATION
In summary, the steering group anticipates that several space
experunents prior to 1995 will advance our understanding of gen-
eral relativity in weak fields and offer a possibility of detecting
gravitational radiation. These are:
1. The flight of Gravity Probe B
-
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2. Microwave ranging to the Galile>Jupiter mission to search
for low-frequency gravitational waves;
3. Microwave ranging to the Mars Observer spacecraft to
improve the accuracy of measurements of the gravitational red
shift, and variation of G with time;
4. Shuttle flight of a cryogenic experiment to test the weak
principle of equivalence to one part in 10~4
.
RECOMMENDED PROGRAM FOR RELATIVISTIC
GRAVITATION: POST-1995
The major elements in the program recommended for the years
1995 to 2015 are:
I. Laser Grau?tational-wave Observatory in Space (LAGOS).
This mission will attempt to detect gravitational radiation at fre-
quencies below 10 Hz from space. The mission, as proposed,
consists of an optical heterodyne interferometer system accurately
measuring the separation of three spacecraft in orbit.
2. Mercury Relativity Satellite. An improved measurement of
the tune rate of change of the gravitational coupling constant such
as could be obtained by microwave ranging to a spacecraft orbiting
Mercury.
3. Precision Optical Interferometer in Space (POINTS). This
instrument wait provide a second-order test of the effect of the Sun
on electromagnetic radiation.
4. STARPROBE. This experiment involves the flight of an
accurate clock (hydrogen maser) on a spacecraft close to the Sun,
allowing the measurement of the gravitational red shift to the
second order.
5. Principle of Equivalence Experiment. This experiment will
be mounted on a free-flying spacecraft and will test this principle
to one part in 10~7.
6. Large-Area X-ray Detector. The flight of such a detector
with rn~crosecond timing capability will allow detection of x-ray
pulsars.
The successful implementation of this strategy should leave us
with a very good understanding of the validity of the genera theory
of relativity in weak fields. It would also advance our knowledge
of the behavior of matter in the neighborhood of objects such as
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57
black holes, where gravitational effects occur in fields far stronger
than those hitherto observed.
B.
MICROGRAVITY SCIENCE
BACKGROUND
The microgravity environment of a space platform may pros
vice a useful arena for testing basic theories of matter and observ-
ing new processes and new states ~ matter. Gravitational fields
cause nonuniform ties in the distribution of matter in a given sam-
ple and can cause fragile structures to collapse. The spacecraft en-
vironment can provide a very low effective gravitational field that
might provide protection from these effects. Under conditions of
low gravity, we may enhance our understanding of nonequilibrium
phenomena in fluid flow, and in condensation, combustion, and
similar dynamic processes. Low-gravity conditions may also allow
the development of static or dynamic states of matter that cannot
exist in normal gravitational fields.
OBSERVATION OF STATES IN EQUIIIBRIUM
Three categories of investigations have been considered in
these studies of states of equilibrium. The first deals with the
case in which gravitational effects induce nonuniformity in the
equilibrium state of a system, ~d thus prevent the observation
of particular states of equilibrium, such as phase transitions near
critical points. These states involve correlation lengths that are
long compared with the distance over which uniformity ir a sys-
tem can be maintained in normal gravitational fields. The most
weD-known example is the continuous phase transition in liquid
helium at its lambda point. Plans are well advanced to carry
out an experiment mvestigat~ng this phenomenon In space, where
gravitational effects will be small enough to allow uniform tem-
perature in an extended sample of liquid helium. This experiment
should be completed before 1995.
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OBSERVATION OF STATES DESTROYED BY GRAVITY
Another category of investigation involves the study of sta-
tionary states of matter that gravity destroys rather than distorts.
For example, there is the possibility that in low gravity, objects
can develop smcaDed fractal aggregates. These structures may be
so fragile that they can exist only in a m~crogravity environment.
In another case, gravitational effects can interfere with the evm
lution of a precipitate because of flows induced by buoyancy or
because of sedimentation. In a microgravity environment these
ejects could be avoided, and precipitation solely under the control
of diffusion could be observed.
STATES FAR FROM EQUIIIBRIUM
A third class of phenomena that can be observed only under
conditions of low gravity are those that exhibit complex dynamical
behavior as they are driven far from equilibrium. Plans have been
formulated for studying examples of this sort of behavior on the
Space Shuttle, including the combustion of clouds of particulates,
or surface-tension-driven hydrodynamical flows. But the steering
group believes that the possibilities for research in this field are
greater than we now realize and that they may have important
implications for biology. Many questions beckon for answers: Will
a given process will be chaotic or not? WiD spatial patterns formed
be stable? What is the role of the gravitationally induced breaking
of underlying symmetries, such as the front-t~back symmetry in
flames?
CONCLUSIONS AND RECOMMENDATIONS FOR
MICROGRAVITY SCIENCE
~ its treatment of m~crogravity science here, the steering
group has concerned itself solely with basic scientific questions.
Until these are answered, there does not seem to be any way to
structure a rational program of material processing in space. A
basic research program of this sort Is a necessary precondition to
the development of an applied program. As a branch of space
science, microgravity science is in its infancy. Thus, before we can
gauge the prospects of the field over the next 20 years, use must
know the results of preliminary experiments now being `developed.
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If the nation hopes to attract expert scientific talent into the field
flight of these experiments merits very high priority.
The following are specific recommendations regarding micrm
gravity studies:
~ ~ scheduling experiments for flight, NASA should make
every effort to fly the best of the m~crogravity physics and chem-
istry experiments as soon as possible, and see to it that the results
are rapidly published.
~ Spacecraft gravity levels and vibration spectra should be
precisely measured, characterized, and displayed on those space-
craft carrying chemistry and physics experiments.
~ Strategies for producing the lowest possible gravity condi-
tions should be considered at this tune, since experiments dealing
with long-range order are open-ended in their low-gravity needs.
Representative terms from entire chapter:
red shift
