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Spacetime Experiment
Introduction:
A large number of recent theoretical papers have developed scenarios for spatial and
temporal variations of the fine structure constant, α. Many of these papers have been
stimulated by the measurements of Webb et al [2001, 2003] showing a possible cosmological
variation of the fine structure constant. Theoretical cosmologists [Damour, 2003] predict
possible α-variations from string model efforts to achieve Grand Unification via an
Equivalence Principle (EP) violating scalar field, φ, associated with the gravitational
field. The accelerated expansion of the universe, driven by dark energy, also suggests a
nearly massless scalar field that violates EP [Parkinson et al, 2003] and would lead to
α-variations. The theoretical nature of such scalar fields has even evolved to accommodate
the tight experimental limits set by Earth-based searches for EP violations. Indeed,
in the ‘chameleon’ scenario [Khoury et al, 2003], the mass and range of this scalar field
depends upon the local matter density, so that the EP violating 5th -force mediated by
φ may be as small as 1mm on the surface of Earth and yet be several AU or more in the
interplanetary regions of the solar system, far from planetary masses. In fact,
EP violations are much larger in deep space, according to chameleon cosmology. In this
paper we describe a solar system experiment with three small high-precision atomic clocks
to test for a spatial variation of α (as predicted in all above mentioned references)
in the strong gravitational field near the Sun and in regions a few AU from planetary masses.
For many years, flight engineers at JPL have studied close solar flyby missions and
have designed space-craft that can survive to within 4 solar radii while (room temperature)
flight payload instruments carry out sensitive measurements. Variations of this scalar
field φ along the spacecraft trajectory will lead to a change in the relative frequency
of three atomic clocks, and will be used to probe for the variations of α to
the 10–16
level. Since the response of each clock to a change in α has a specific signature,
this measurement can provide unambiguous results, readily distinguishable from magnetic
field induced clock shifts that may be encountered near the sun. This deep space test of
the Equivalence Principle will be the first ever made spanning over AU distances, sampling
the low and high-density regions of the solar system to a level far exceeding present day
Earth-based experiments. This approach to EP tests is based on small ultra stable ion
clocks now under development at JPL for deep space operation. Since each of these
ultra-stable ion clocks are quite small (2-3 kg) and developed for deep space operation,
no other EP measurement technology is able to execute so sensitive an EP test in deep
space, far from Earth.
History:
Atomic clocks have traditionally been used to test the prediction of general relativity.
The first such space test was performed in 1976 by NASA’s Gravity Probe A, where the rate
of a hydrogen maser clock during a (two-hour) sub-orbital trajectory was compared to that
of a similar clock on the Earth’s surface [Vessot et al., 1980]. This measurement verified
the predictions of the EP clock shift to a part in 104, a precision that still stands
unchallenged today. More recently, we showed that variations of a would force a
corresponding change in the relative frequency of two hyperfine-based atomic clocks
[Prestage et al 1995]. The first laboratory attempt to search for a varying α set a
(temporal) upper limit of ∼4 x 10–14 per year for its variation. This technique has
recently been used in a rubidium vs cesium fountain clock comparison [Marion et al, 2003],
as well as the comparison of a cesium fountain with an optical mercury ion clock, where an
optical transition in the ion was used [Bize et al, 2003]. These more recent experiments
set the limit for a fractional time variation of α≤ 10–15/yr. This technique is the most
sensitive of the laboratory searches for changing atomic constants. Recent results are
shown in Table 1. More...
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