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Gravity and Antimatter
Gravity and Antimatter
No one has ever dropped a single particle of antimatter. Yet most
physicists assume that it would fall to the ground just like ordinary
matter. Their arguments are based on two well-established ideas: the
equivalence principle of gravitation and the quantum-mechanical symmetry
between matter and antimatter. Today this line of reasoning is being
undermined by the possibility that the first of these ideas, the principle
of equivalence, may not be true. Indeed, all modern attempts to include
gravity with the other forces of nature in a consistent, unified quantum
theory predict the existence of new gravitational-strength forces that,
among other things, will violate the principle.
Such effects may have been seen already in recent experiments. These effects
are small in ordinary experience with matter in the earth's gravitational
field. With respect to antimatter, however, these new forces could result
in large anomalies. Hence an experiment to measure the gravitational
acceleration of antimatter could be of great importance to the understanding
of quantum gravity. We are members of an international team that has been
formed to carry out such an experiment.
If the principle of equivalence is found to be violated, it will be a
significant event in the history of physics, because the principle is the
foundation on which the gravitational theories of both Newton and Einstein
rest. The principle states that two objects fall with the same
gravitational acceleration regardless of their mass or material composition.
The idea was first expounded in this form by Galileo, who based his
conclusions on experiments with inclined planes and on mathematical
conjectures about the motion of projectiles. Einstein, recognizing that
mass and energy are equivalent, extended the principle in his general theory
of relativity to apply not only to objects with rest mass but also to all
forms of energy, including light. Einstein's bold conjecture was verified
in 1919 by Arthur Eddington, who measured the bending of light in the sun's
gravitational field during an eclipse.
With the discovery of the positron, or antielectron, in 1932, a new question
arose: Does antimatter obey physical law in the same way as ordinary matter
does? The currently accepted answer came in 1957 with the CPT theorem of
Gerhart L uders, who proved that the mathematical operations that transform
the description of a particle into a description of its antiparticle leave
the laws of physics intact. For general relativity, then, gravity does not
make any distinction between a particle and an antiparticle; all that counts
is the particle's energy. Because the energy of an isolated antiparticle is
the same as that of the corresponding particle, antimatter should fall to
the ground precisely like ordinary matter.
This conclusion is valid if one believes Einstein's general relativity is
the ultimate theory of gravitation. Recently, however, many physicists have
proposed alternatives in which gravity may interact with aspects of matter
other than energy, such as quantum number. Within these views the CPT
theorem can only state that an antiapple would fall to an antiearth in the
same way as an apple falls to the earth. It says nothing about how an
antiapple falls to an earth of ordinary matter. In other words, one should
not assume that the principle of equivalence applies to antimatter. Indeed,
this should come as no surprise, because the two fundamental constructs on
which the conventional argument was based, gravitation and quantum
mechanics, have never been successfully joined in a single theory.
The unification of quantum mechanics and gravity has become the Holy Grail
of 20th-century physics. No one has succeeded but, remarkably, the most
realistic theories today all predict new types of gravitational interactions
that might indeed cause an antiparticle to fall to the ground differently
from an ordinary particle. We therefore proposed a few years ago to measure
the gravitational acceleration of antiprotons. Such an experiment would
provide an unambiguous test, if new gravitational interactions do in fact
exist. Apparatus for the experiment is now being built, and if all goes
well, the results will be available early in 1991.
It has been only recently that investigators, guided by new experiments and
concepts, have found certain anomalies suggesting that the equivalence
principle may not hold true under certain conditions. For centuries,
however, physicists had good reason to believe in the principle: it has
survived many rigorous tests that proved its accuracy to extraordinary
precision.
Newton himself tested the equivalence principle with experiments. To
understand his experiment one needs to restate the principle somewhat.
Newton introduced the concept of mass in two contexts. In his second law of
motion, the force on an object is equal to the inertial mass times the
acceleration. In his law of gravitation, the force of attraction between
two objects is proportional to the product of their gravitational masses and
inversely proportional to the square of the distance between them (and is
therefore called an inverse-square law). The inertial mass is a kinematic
quantity having to do with motion. The gravitational mass, on the other
hand, is a "charge": an object feels a gravitational force in proportion to
its gravitational mass, just as it would feel an electromagnetic force in
proportion to its electric charge.
Although they are completely different concepts, Newton maintained that the
two types of mass, inertial and gravitational, are equivalent. To test the
idea he did experiments with pendulums. A pendulum's period of oscillation
is given by the product of two factors: one depending on the length and
another depending on the ratio of inertial mass to gravitational mass.
Nowton found that the period was always determined by the length-dependent
factor alone and that the ratio of the two types of mass was always unity;
in other words, gravitational and inertial mass are equal. In this way
Newton verified the equivalence principle to a precision of one part in
1,000. (In the late 1820's Friedrich Wilhelm Bessel carried out experiments
that increased the precision to one part in 60,000.)
The next major advance in testing the principle of equivalence was made by
the Hungarian baron Roland E otv os. In the 1880's this master geophysicist
developed a torsion balance with which to probe the earth by measuring
variations in the gravitational field. With the balance he could map local
gravitational fields and thereby infer local mass anomalies, the most
interesting being those associated with mineral deposits. The balance was
so accurate that in spite of the long time needed for each measurement, it
was a standard geophysical tool well into this century.
E otv os realized that his torsion balance could be employed to test the
principle of equivalence by placing objects of different materials at
opposite ends of the balance. The net force acting on each object is a
combination of the gravitational attraction of the earth, which is
proportional to the gravitational mass of the object, and the centrifugal
force due to the earth's rotation, which is proportional to the inertial
mass of the object. If two different materials were put on the balance and
the ratio of the gravitational mass to the inertial mass for one did not
equal that ratio for the other, the E otv os balance would rotate.
In 1890 E otv os published results showing the equality of the gravitational
and inertial masses of several substances to a precision of five parts in
10(8). He improved this work in 1909 and concluded that an appropriate
limit for the precision of his experiment was five parts in 10(9). At less
than this limit he found discrepancies between different types of material,
which he attributed to experimental error. (We shall say more about this
below.)
In the 1960's and 1970's Robert H. Dicke and Vladimir B. Braginsky performed
independent E otv os-type experiments in which they measured the ratio of
inertial mass to gravitational mass of objects in the gravitational field of
the sun rather than of the earth, and they found the principle of
equivalence was accurate to five parts in 10(11) and 10(12) respectively. A
similar precision was obtained for the equality of gravitational mass and
intertial mass of the earth and the moon from measurements, using corner
mirrors left on the moon's surface by Apollo astronauts, of the lunar orbit
around the earth as the earth-moon system itself orbits the sun. This last
experiment, incidentally, demonstrated that the sun's gravity acts on the
gravitational energy that binds matter in the earth--in short, that gravity
attracts gravity.
The idea that gravitational energy is itself subject to the force of gravity
is a consequence of the revolution in physics that occurred at about the
time of E otv os' experiments. This was the formulation of the special and
the general theories of relativity by Einstein. Because the present debate
over the existence of new gravity-related forces results from attempts to
extend Einstein's theory of gravitation, it is worth reviewing the history
of this work in some detail.
The general theory of relativity emerged from Einstein's attempt to resolve
a fundamental challenge that his earlier work on special relativity posed
for Newton's concept of gravity. For although Newtonian theory is perfectly
adequate for most practical purposes (such as sending people to the moon),
it is unacceptable to the theoretical physicist because it assumes that
gravity acts instantaneously over infinite distances and so violates special
relativity's stricture limiting the velocity of everything --objects,
energy, the propagation of a force--to the speed of light.
In classical relativistic field theory, forces are made to adhere to special
relativity by the introduction of a field, which carries energy and momentum
between interacting particles of matter (such as electrons and protons) at a
speed no faster than that of light. It is the exchange of the energy and
momentum carried by the field that produces the force experienced by the
particles. For example, oscillating electrons in a transmitting antenna
produce a field of radio waves, which propagate through space and exert a
force on electrons in the receiving antenna.
With the advent of relativistic field theory, it was natural for physicists
to study the possible forms a gravitational field could take. James Clerk
Maxwell, for example, noting that both gravitation and Coulomb's law (which
describes the force between electrically charged particles) are
inverse-square relations, wondered whether his own theory of electrodynamics
could be modified to describe gravity. Of course, some changes were
necessary because the electrodynamic force produces repulsion between like
charges, whereas gravity produces attraction. Maxwell was able to satisfy
this condition by changing the sign of the field energy and making it
negative, but he quickly realized that the resulting theory harbored a fatal
flaw: with negative field energy a system would gain energy by gravitating,
and so its energy would increase infinitely.
The first mathematically consistent, relativistic theory of gravitation was
constructed in 1913 by Gunnar Nordstr om, before Einstein's general theory
of relativity. Nordstr om's theory agreed with all gravitational
experiments of the time. Einstein and Adriaan D. Fokker discovered that
Nordstr om's gravitational field equations really described a spacetime that
was curved--it was as though one had tried to describe the surface of a
sphere in terms of a flat plane and then realized that the same surface
could be described more naturally with spherical coordinates. Nordstr om's
theory therefore unwittingly introduced the idea of a curved spacetime.
In Nordstr om's model the gravitational field acted only on objects that
have a rest mass. But according to special relativity, energy is equivalent
to mass. Therefore why should not energy also be subject to the force of
gravity? It was this concept, which Einstein described as the happiest
thought of his life, that led to the general theory of relativity.
Einstein's theory also resulted in a curvature of spacetime in the vicinity
of massive objects. Moreover, because the theory describes a gravitational
field that couples to energy and momentum rather than to mass, it predicted
that gravity should deflect massless forms of energy, such as light.
Einstein proposed that the effect could be tested by measuring an apparent
shift in the position of stars near the limb of the sun during an eclipse.
The prediction was confirmed in Eddington's 1919 expedition to Africa.
Einstein's theory also accounted for the anomalous shift of the perihelion
of Mercury and the red shift of light coming from stars or planets (later
verified in the Pound-Rebka experiment).
Theories of gravity such as Nordstr om's and Einstein's, in which the force
manifests itself through spacetime curvature, are now known as "metric
theories." In such theories force is manifested by a curvature of
spacetime, which shapes the trajectory along which an object moves.
Formulated in this way, the force is independent of the composition of the
objects on which it acts. (Note that this is another way of stating the
equivalence principle.)
The general theory of relativity, with its revolutionary and astoundingly
successful concept of the universe, stands as one of the greatest
intellectual achievements of this century. Yet its exalted status should
not blind one to the fact that it is not unique in its ability to account
for all experimental tests of gravity. Indeed, there is now evidence of
gravitational effects that appear to violate the principle of equivalence,
which, if the evidence is correct, cannot be explained by classical general
relativity. These effects may, however, be consistent with more recent
theories --work that has resulted from the effort to unify general
relativity with that other great pillar of modern physics, quantum theory.
There is, in fact, a serious incompatibility between quantum mechanics and
the principles of equivalence that underlie classical, nonquantum theories
of gravitation. For instance, according to the equivalence principle, an
object's initial position and velocity determine a definite trajectory for a
freely falling object in a gravitational field. In quantum mechanics,
however, the object's path is indeterminate and probabilistic [see "Quantum
Gravity," by Bryce S. DeWitt; SCIENTIFIC AMERICAN, December, 1983].
Therefore the equivalence principle cannot be an exact concept within a
quantum description of gravity. One should not be surprised if a quantum
theory of gravitation were to include interactions that violate the
equivalence principle.
Quantum mechanics profoundly altered classical field theory. The classical
view holds that energy and momentum are carried by a field. Quantum
mechanics asserts that this energy and momentum exist in discrete units,
called quanta, which can be described as particles. In quantum field
theory, then, forces are said to occur through an exchange of such particles
(rather the way the pitcher transfers energy and momentum to the catcher
when he hurls a baseball). Electromagnetic forces, for example, are
"mediated" through the exchange of photons, or light quanta. The strength
of the resulting force is given by the particles' "coupling strength" to the
matter that is being acted on.
The force-carrying particles have a definite rest mass (zero in the case of
photons) and an intrinsic spin, or angular momentum, which can take integer
or half-integer values. All the familiar forces--gravity, electromagnetism,
the weak force responsible for radioactive decay, the strong force that
binds atomic nuclei--are mediated by integer-spin particles, which produce
forces with distance ranges determined by the inverse of the particles'
mass. Forces mediated by massive particles, such as the weak force, act only
over a finite range. Forces mediated by massless particles, such as
electromagnetism and gravity, have an apparently infinite range, and the
force diminishes in strength inversely as the square of the distance between
the interacting particles.
It is now known that the spin of a field is related to the nature of the
force: fields with odd-integer spins can produce both attractive and
repulsive forces; those with even-integer spins, such as scalar (spin 0) and
tensor (spin 2) fields, produce a purely attractive force. Maxwell's
electrodynamics, for instance, can be described today as a spin-1 field (the
force is carried by the photon, which has a spin of 1). The force from this
field is attractive between oppositely charged particles and repulsive
between similarly charged particles.
A theory of gravity, by the same reasoning, was expected to be based solely
on scalar or tensor fields mediated by particles with even spin. Indeed, it
has been shown that when general relativity, which is based on a tensor
field, is recast as a quantum field theory, the gravitational force is
carried by a massless spin-2 particle, called the graviton. Mathematically,
however, the quantum version of general relativity is fraught with
inconsistencies. This has led physicists to consider ways to extend general
relativity in order to make gravitation more amenable to quantization.
One of the favored approaches for quantizing gravity is a class of theories
known as gauge theories. These include theories widely believed to describe
the strong and electroweak interactions, which are now the candidates for
unification in the so-called grand unified theory. Gauge theories are based
on a certain type of internal symmetry and are attractive to theorists
because only a few initial parameters will allow one to calculate all
phenomena within their purview [see "Supergravity and the Unification of the
Laws of Physics," by Daniel Z. Freedman and Peter van Nieuwenhuizen;
SCIENTIFIC AMERICAN, February, 1978].
The success of gauge theories suggests that the mathematical inconsistencies
in quantum general relativity might also be overcome by introducing what is
now called a local supersymmetry. When general relativity is augmented by
local supersymmetries, one finds (in most versions of such models) that
there is a half-integer-spin particle partner for every integer-spin
particle and vice versa, creating a kaleidoscopic cascade of new particles:
the spin-2 graviton has a spin-3/2 partner, which has a spin-1 partner (the
graviphoton), which has a spin-1/2 partner, which has a spin-0 partner (the
graviscalar). (Some models describe more than one partner of each spin.)
These new partners are like extra quantum states of the graviton, and their
existence seems to ensure that the supergravity theories have reasonable
(but perhaps still imperfect) properties for a quantum field. Indeed, all
gauge theories of quantum gravity that are now being considered contain
supersymmetric extra states.
The half-integer-spin particles in these so-called supergravity theories are
expected to be extremely massive. Their rest-mass energy is expected to be
about one trillion electron volts, or 1,000 times that of the proton. None
of these predicted particles has ever been seen. Advocates of supergravity
theories hope to produce them with new accelerators such as the Tevatron at
the Fermi National Accelerator Laboratory, LEP (Large Electron-Positron) at
the European laboratory for particle physics (CERN) and the proposed
superconducting supercollider.
The integer-spin particles, on the other hand, are, like the graviton,
mediators of forces and would generate new effects with a strength
comparable to gravity--but with some notable differences. Both the
graviscalar and the graviphoton are expected to have a rest mass and so
their range will be finite rather than infinite. Moreover, the graviscalar
will produce only attraction, whereas the graviphoton's effect will depend
on whether the interacting particles are alike or different. Between matter
and matter (or antimatter and antimatter) the graviphoton will produce
repulsion; between matter and antimatter it will produce attraction. Thus
for ordinary matter the graviscalar's attractive force will be more or less
canceled by the graviphoton's repulsive force. Between matter and
antimatter, however, both the graviscalar and the graviphoton will produce
an attractive force and so will add up. It is evidence for this additional
attraction that the antiproton experiment will seek.
Interestingly, similar effects are predicted by a separate group of theories
that have approached the quantization of gravity from a completely different
angle. Certain recent metric theories that contain more dimensions than the
conventional four of spacetime also predict the appearance of the new
particles. This work harkens back more than 60 years to the work of Theodor
Franz Eduard Kaluza and Oskar Klein, who formulated a model of gravitation
in higher-dimensional spacetime and then "projected" it into ordinary
spacetime in an effort to produce a unified theory of gravity and
electromagnetism [see "The Hidden Dimensions of Spacetime," by Daniel Z.
Freedman and Peter van Nieuwenhuizen; SCIENTIFIC AMERICAN, March, 1985].
Ultimately unsuccessful, the Kaluza-Klein model languished for half a
century, but in the past decade several theoretical physicists have
considered it anew and begun to study what would happen if the model were
extended to even higher dimensions. They found that a higherdimensional
spin-2 graviton would, when looked at in four dimensions, have several
parts: a four-dimensional spin-2 graviton, a spin-1 vector field
corresponding to the graviphoton and a one-dimensional spin-0 scalar field
corresponding to the graviscalar. The process is analogous to taking an
arrow in three dimensions and projecting it onto a plane: two dimensions
would define an arrow in the plane and the third, vertical dimension would
define a point. (As with supersymmetry, some models have many partners of
each spin.) Hence both the nonmetric supergravity theories and the
higherdimensional metric theories have strikingly similar consequences.
None of the quantum gravity theories has yet been shown to be mathematically
consistent. Even so, Joel Scherk, not long before his death in 1980,
recognized that the quantum gravity theories could have measurable physical
consquences in the world of ordinary matter. For example, the graviscalar
and the graviphoton are not massless and so do not behave according to the
Newtonian inverse-square law. Hence one might seek experimental evidence of
violations of Newton's law within the range of several hundred meters or
kilometers over which the new particles are thought to exert their effect.
Experiments could also search for possible differences in the coupling
strengths of the graviscalar and the graviphoton to different components of
matter, such as binding energy or baryon number (the total number of protons
and neutrons). The theories allow the graviscalar to couple to the binding
energy with a strength different from the one with which it couples to the
rest mass of elementary particles. For instance, the graviscalar force
might be greater on a gram of hydrogen than on a gram of iron, because for
every 56 unbound hydrogen atoms there is one iron atom containing 56 bound
protons and neutrons. The graviphoton, on the other hand, must be coupled
to some conserved quantum number of the elementary particles, such as the
total number of baryons or quarks, or the sum of baryons and leptons (such
as electrons). The graviphoton will therefore also produce a force that
depends on the composition of matter. Thus both new forces can violate the
equivalence principle.
The current theories, then, predict that two long-standing laws of physics
will be overturned: the nonzero masses of the graviscalar and the
graviphoton imply that, within their finite range, the Newtonian
inverse-square law of gravitation will not be true, and the
composition-dependent nature of the new forces means that the equivalence
principle will also be violated.
For ordinary matter in the earth's gravitational field, the violations of
the inverse-square law are expected to be minuscule. The reason is that the
forces mediated by the graviscalar and the graviphoton will approximately
cancel each other. A small residual effect may have been found by Frank D.
Stacey, Gary J. Tuck of the University of Queensland and their collaborators
in their measurements of local gravitational force made at various depths
inside Australian mine shafts. After the gravitational effect of local
geology had been accounted for, the measurements were inconsistent with the
predictions of Newtonian theory. Instead the data were roughly consistent
with the existence of a single repulsive force 100 times smaller than
ordinary gravity with a range of hundreds of meters, or of both a repulsive
and an attractive force with strengths about equal to ordinary gravity but
canceling to one part in 100, and with ranges of up to 450 kilometers.
The Australian result, and more recent anomalous results reported by Albert
T. Hsui of the University of Illinois at Urbana-Champaign and by Donald H.
Eckhardt and his colleagues at the U.S. Air Force Geophysical Laboratory,
are being tested independently by Mark E. Ander, Mark A. Zumberge and their
colleagues at the Los Alamos National Laboratory, the Scripps Institution of
Oceanography, the University of Texas at Dallas, the Amoco Corporation and
the Scott Polar Research Institute. This past summer they measured gravity
inside an existing borehole on the continental Greenland ice sheet, where
the uniform composition of the surrounding ice helped to reduce errors in
their analyses. The results should be announced within several months.
Recently there has also been much excitement over the reanalysis of the 1909
E otv os experiment, led by Ephraim Fischbach of Purdue University. The
group found a correlation between the tiny discrepancies in E otv os'
results, mentioned above, and the ratio of baryon number to inertial mass of
the various substances E otv os had measured. The Purdue group suggests
this correlation may be evidence for an entirely new "fifth force" of
nature, although we think that because it is approximately the same strength
as gravity, this force is a new aspect of gravity itself. The answer
ultimately will come from experiment and theory.
Many new experiments have been mounted to test for composition-dependent
effects. At this writing four results are in: two negative results from a
group at the University of Washington led by Eric G. Adelberger and a
National Bureau of Standards group led by James E. Faller, and two positive
results by Peter Thieberger of the Brookhaven National Laboratory and by
Paul E. Boynton and his colleagues at the University of Washington [see
"Science and the Citizen," December, 1987].
All the experiments described so far test the acceleration of ordinary
matter in the gravitational field of the earth. Recall, however, how things
change if one replaces matter with antimatter. Such an experiment would
constitute the extreme test of the equivalence principle. Here the
graviphoton effect is attractive, as is the graviscalar force. Thus instead
of canceling, the two effects would add up. Antimatter would then
experience a larger acceleration toward the earth than matter.
In 1982, motivated by earlier discussions of this possibility, two of us
proposed an experiment to measure the gravitational acceleration of
antiprotons at the Lw Energy Antiproton Ring (LEAR) at CERN. Since then
many collaborators have joined from Los Alamos, Rice University, Texas A. &
M. University, the National Aeronautics and Space Administration's Ames
Research Center, the University of Genoa, the University of Pisa and CERN.
The experiment would extract antiprotons from LEAR, cool them to just above
absolute zero and then send them, 100 at a time, into a drift tube, where
the time it takes for them to reach the top will be measured. Negative
hydrogen ions, which have the same charge and nearly the same mass as the
antiproton, will provide a time-of-flight measurement for ordinary matter.
The result will then be compared with the time of flight for antiprotons in
order to determine whether the antiprotons are undergoing a larger
gravitational acceleration. The drift-tube method stems from the pioneering
work of Fred C. Witteborn and William M. Fairbank, both then at Stanford
University, who in 1966 reported measuring the gravitational acceleration of
the electron. Fairbank now hopes to undertake a state-of-the-art positron
experiment, which would provide a complement to our antiproton experiment.
The antiproton gravity experiment is expected to achieve a precision of
better than 1 percent. If there are indeed both vector and scalar
interactions with coupling strengths close to the normal gravitational force
and ranges of about 450 kilometers, the antiproton would fall with an
acceleration 14 percent greater than that of ordinary matter. If the
coupling strengths are larger than the normal gravitational force, the
effect would be greater still.
Even if the experiment were to find no new effects, as the first measurement
of the gravitational acceleration of antimatter it would extend experiments
on gravity into new territory, just as the E otv os-Dicke and the
Pound-Rebka experiments did. These experiments got exactly the results
expected by the prevailing theory of the day. Yet they were such beautiful
and clear verifications of physics that they are classics--the kind of
experiments that go into the textbooks.
But what if the antimatter experiment were to obtain a result that violated
the classical understandign of gravitation? How would we convince ourselves
and the rest of the community of physicists that the outcome was not an
experimental error? When we discussed this difficulty with two members of
the experimental team, Ron Brown and Nelson Jarmie, their eyes lit up.
Their response says it all: "We'd love to have that problem." Whatever the
outcome, all of us eagerly await the experimental results.
Table: RANGE OF FORCE
Table: INTEGER-SPIN PARTICLES mediate the familiar forces of nature.
Particles of even spin produce only an attractive force, whereas particles
of odd-integer spin produce an attractive or a repulsive force depending on
whether the interacting matter has the same or opposite quantum numbers.
For example, electromagnetic force is carried by the spin-1 photon; thus
particles with the same charge repel each other and those with opposite
charge attract each other. Likewise the graviphoton is expected to produce
repulsion between matter and matter but attraction between matter and
antimatter.
Photo: GALILEO stands at the center of this fresco, explaining the uniform
acceleration of a sphere rolling down an inclined plane. Such experiments
led him to reason that if objects of differing mass and substance were
dropped from a height, they would strike the ground at the same instant.
This principle of equivalence, fundamental to gravitational theory, is
enshrined in popular mythology by the story in which Galileo dropped two
stones from the Leaning Tower of Pisa, seen in the distance. A modern test
of the principle will measure the gravitational acceleration of antiprotons.
According to supergravity and string theories, antiprotons should fall
faster than protons.
Photo: ELECTRON-POSITRON PAIR leaves a bifurcating spiral trace in its wake
in this cloudchamber photograph. Created by a gamma ray colliding violently
with a hydrogen nucleus, the electron and its antimatter partner have the
same mass but opposite electric charge and so curve in opposite directions
in the magnetic field of the chamber. Particles and their antiparticles
have opposite quantum numbers; hence if there is enough energy available to
conserve momentum and provide mass (in accordance with E=mc2), they can be
created in pairs because their net quantum number will be zero. A particle
and an antiparticle can also annihilate each other in a burst of energy.
Antiprotons are created in accelerators by smashing high-energy particles
into suitable targets.
Photo: THEORIES OF GRAVITY describe the force between two masses. Newton,
who related the magnitude of gravitational force to the objects' mass and
the distance between them, assumed that the force acts instantaneously over
distance (a). The special theory of relativity, however, proved that
nothing moves faster than the speed of light. Classical field theory
introduced the idea of a field that propagates force at finite speed (b).
Einstein recognized that the field equations for gravitation describe a
spacetime that is curved near massive objects. In his general theory of
relativity gravity is manifested by the motion of objects along paths that
follow the shortest possible distance in a curved spacetime ©. Quantum
mechanics asserts that the path is indeterminate (d). This inconsistency
between quantum theory and general relativity still plagues physicists.
Photo: QUANTUM FIELD THEORY introduced the idea of a particle that mediates
force. Two interacting particles exchange a third particle that transfers
energy and momentum from one to the other, rather in the way a thrown ball
transfers energy and momentum from pitcher to catcher. Particles with mass
act over a finite range. Massless particles, such as photons (light) and
gravitons (carriers of gravity), act over infinite distance.
Photo: TWO THEORETICAL APPROACHES predict the existence of new
gravity-related interactions that are remarkably similar. Supergravity
theories that apply four or more "supersymmetry" operations (arrows) to the
spin-2 graviton give rise to a series of new particles: the 3/2-spin
gravitino, the spin-1 graviphoton, the 1/2-spin goldstino and the spin-zero
graviscalar. The graviphoton and the graviscalar would mediate new forces.
A separate class of ideas called metric theories, which describe forces in
terms of spacetime curvature, make remarkably similar predictions: a spin-2
graviton in higher dimensions "decomposes" into a spin-2 graviton and one or
more spin-1 graviphotons and spin-zero graviscalars in ordinary
four-dimensional spacetime.
Photo: VIOLATIONS of the equivalence principle would be allowed by new
theories because the three particles that mediate gravity and related forces
could couple to mass an energy with different strengths, as is suggested by
varying intensities of color in this illustration. The graviton couples
with equal strength to mass (spheres) and binding energy (squiggles). The
graviscalar could couple to mass differently from the way it does to binding
energy, and so elements with many bound protons and neutrons could
experience a weaker force than those that contain only a few bound nucleons.
The graviphoton could couple to internal quantum numbers such as baryon
number (for example, protons and neutrons) rather than to mass or binding
energy. One test is to compare the gravitational force on elements that
have different proportions of protons and neutrons, because protons have a
smaller inertial mass than neutrons but the same baryon number. Both
graviscalar and graviphoton effects would be seen only over a finite range,
which is predicted by theory to be between a few hundred meters and a few
hundred kilometers.
Photo: NEW PARTICLES would cause matter to exert a force on antimatter
different from the force it would exert on ordinary matter. The graviton
and the graviscalar would produce attraction in both cases, but the
graviphoton would produce repulsion for matter and attraction for
antimatter. If the graviphoton and graviscalar effects are almost equal,
they would nearly cancel each other for interactions between ordinary
matter, but for interactions between matter and antimatter they would add
up.
Antimatter, then, could fall to the ground perhaps 14 percent or more times
faster than matter.
Photo: GRAVITY METER is carefully hoisted out of a deep borehole in the
Greenland ice sheet a few miles south of the Arctic Circle. The
half-million-dollar device measures local gravitational force at various
positions to a depth of 1,600 meters with a precision of one part in 100
million. The meter, which is normally used for petroleum prospecting, is
testing for the possible existence of new gravity-related interactions.
Seen in the photograph are Casey Rohn of the University of Nebraska's Polar
Ice Coring Office and James Wirtz of Amoco. Ted Lautzenhiser is inside the
red winch-operating cabin.
Photo: EFFECT OF GRAVITY on antiprotons will be measured at the Low Energy
Antiproton Ring (LEAR) of the European laboratory for particle physics
(CERN). Antiprotons are extracted from LEAR at an energy of two million
electron volts, decelerated to between 10,000 and 20,000 electron volts and
captured in the catching trap and storage trap, where they are cooled to 10
degrees Kelvin (-263 degrees Celsius). They are then launched, 100 at a
time, up a one-meter-high drift tube. The antiprotons most useful to the
experiment will have a starting velocity averaging four meters per second.
As they drift upward the tug of gravity will slow them down. Hence the more
energetic particles will reach the detector first and the less energetic
ones will reach it later. There eventually will be a cutoff time after
which no more particles will reach the detector because the slowest
particles will not have enough speed to reach the region of the accelerating
grid before their upward motion is overcome by gravity. The experiment will
separately measure and compare the cutoff time both for antiprotons and for
negative hydrogen ions (black curve), which have the same charge and almost
the same mass as antiprotons. If antimatter were subject to a larger
gravitational force downward than ordinary matter, the antiprotons would
have a shorter cutoff time (colored curve) than the hydrogen.
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