376 research outputs found
A Mission to Explore the Pioneer Anomaly
The Pioneer 10 and 11 spacecraft yielded the most precise navigation in deep
space to date. These spacecraft had exceptional acceleration sensitivity.
However, analysis of their radio-metric tracking data has consistently
indicated that at heliocentric distances of astronomical units,
the orbit determinations indicated the presence of a small, anomalous, Doppler
frequency drift. The drift is a blue-shift, uniformly changing with a rate of
Hz/s, which can be interpreted as a
constant sunward acceleration of each particular spacecraft of . This signal has become known as the Pioneer
anomaly. The inability to explain the anomalous behavior of the Pioneers with
conventional physics has contributed to growing discussion about its origin.
There is now an increasing number of proposals that attempt to explain the
anomaly outside conventional physics. This progress emphasizes the need for a
new experiment to explore the detected signal. Furthermore, the recent
extensive efforts led to the conclusion that only a dedicated experiment could
ultimately determine the nature of the found signal. We discuss the Pioneer
anomaly and present the next steps towards an understanding of its origin. We
specifically focus on the development of a mission to explore the Pioneer
Anomaly in a dedicated experiment conducted in deep space.Comment: 8 pages, 9 figures; invited talk given at the 2005 ESLAB Symposium
"Trends in Space Science and Cosmic Vision 2020", 19-21 April 2005, ESTEC,
Noordwijk, The Netherland
Fundamental Physics with the Laser Astrometric Test Of Relativity
The Laser Astrometric Test Of Relativity (LATOR) is a joint European-U.S.
Michelson-Morley-type experiment designed to test the pure tensor metric nature
of gravitation - a fundamental postulate of Einstein's theory of general
relativity. By using a combination of independent time-series of highly
accurate gravitational deflection of light in the immediate proximity to the
Sun, along with measurements of the Shapiro time delay on interplanetary scales
(to a precision respectively better than 0.1 picoradians and 1 cm), LATOR will
significantly improve our knowledge of relativistic gravity. The primary
mission objective is to i) measure the key post-Newtonian Eddington parameter
\gamma with accuracy of a part in 10^9. (1-\gamma) is a direct measure for
presence of a new interaction in gravitational theory, and, in its search,
LATOR goes a factor 30,000 beyond the present best result, Cassini's 2003 test.
The mission will also provide: ii) first measurement of gravity's non-linear
effects on light to ~0.01% accuracy; including both the Eddington \beta
parameter and also the spatial metric's 2nd order potential contribution (never
measured before); iii) direct measurement of the solar quadrupole moment J2
(currently unavailable) to accuracy of a part in 200 of its expected size; iv)
direct measurement of the "frame-dragging" effect on light by the Sun's
gravitomagnetic field, to 1% accuracy. LATOR's primary measurement pushes to
unprecedented accuracy the search for cosmologically relevant scalar-tensor
theories of gravity by looking for a remnant scalar field in today's solar
system. We discuss the mission design of this proposed experiment.Comment: 8 pages, 9 figures; invited talk given at the 2005 ESLAB Symposium
"Trends in Space Science and Cosmic Vision 2020," 19-21 April 2005, ESTEC,
Noodrwijk, The Netherland
Development of a prototype superconducting radio-frequency cavity for conduction-cooled accelerators
The higher efficiency of superconducting radio-frequency (SRF) cavities
compared to normal-conducting ones enables the development of high-energy
continuous-wave linear accelerators (linacs). Recent progress in the
development of high-quality NbSn film coatings along with the availability
of cryocoolers with high cooling capacity at 4 K makes it feasible to operate
SRF cavities cooled by thermal conduction at relevant accelerating gradients
for use in accelerators. A possible use of conduction-cooled SRF linacs is for
environmental applications, requiring electron beams with energy of
MeV and 1 MW of power. We have designed a 915 MHz SRF linac for such an
application and developed a prototype single-cell cavity to prove the proposed
design by operating it with cryocoolers at the accelerating gradient required
for 1 MeV energy gain. The cavity has a m thick NbSn film on
the inner surface, deposited on a mm thick bulk Nb substrate and a bulk
mm thick Cu outer shell with three Cu attachment tabs. The cavity was
tested up to a peak surface magnetic field of 53 mT in liquid He at 4.3 K. A
horizontal test cryostat was designed and built to test the cavity cooled with
three Gifford-McMahon cryocoolers. The rf tests of the conduction-cooled
cavity, performed at General Atomics, achieved a peak surface magnetic field of
50 mT and stable operation was possible with up to 18.5 W of rf heat load. The
peak frequency shift due to microphonics was 23 Hz. These results represent the
highest peak surface magnetic field achieved in a conduction-cooled SRF cavity
to date and meet the requirements for a 1 MeV energy gain
Structured Operational Semantics for Graph Rewriting
Process calculi and graph transformation systems provide models of reactive
systems with labelled transition semantics. While the semantics for process
calculi is compositional, this is not the case for graph transformation
systems, in general. Hence, the goal of this article is to obtain a
compositional semantics for graph transformation system in analogy to the
structural operational semantics (SOS) for Milner's Calculus of Communicating
Systems (CCS).
The paper introduces an SOS style axiomatization of the standard labelled
transition semantics for graph transformation systems. The first result is its
equivalence with the so-called Borrowed Context technique. Unfortunately, the
axiomatization is not compositional in the expected manner as no rule captures
"internal" communication of sub-systems. The main result states that such a
rule is derivable if the given graph transformation system enjoys a certain
property, which we call "complementarity of actions". Archetypal examples of
such systems are interaction nets. We also discuss problems that arise if
"complementarity of actions" is violated.Comment: In Proceedings ICE 2011, arXiv:1108.014
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
Identification of Allele-Specific RNAi Effectors Targeting Genetic Forms of Parkinson's Disease
Parkinson's disease (PD) is a progressive neurological disorder affecting an estimated 5–10 million people worldwide. Recent evidence has implicated several genes that directly cause or increase susceptibility to PD. As well as advancing understanding of the genetic aetiology of PD these findings suggest new ways to modify the disease course, in some cases through genetic manipulation. Here we generated a ‘walk-through’ series of RNA Pol III-expressed shRNAs targeting both the α-synuclein A30P and LRRK2 G2019S PD-associated mutations. Allele-specific discrimination of the α-synuclein A30P mutation was achieved with alignments at position 10, 13 and 14 in two model systems, including a heterozygous model mimicking the disease setting, whilst 5′RACE was used to confirm stated alignments. Discrimination of the most common PD-linked LRRK2 G2019S mutation was assessed in hemizygous dual-luciferase assays and showed that alignment of the mutation opposite position 4 of the antisense species produced robust discrimination of alleles at all time points studied. Discrimination at this position was subsequently confirmed using siRNAs, where up to 10-fold discrimination was seen. The results suggest that RNAi-mediated silencing of PD-associated autosomal dominant genes could be a novel therapeutic approach for the treatment of the relevant clinical cases of PD in future
Nonlocal braneworld action: an alternative to Kaluza-Klein description
We construct the nonlocal braneworld action in the two-brane Randall-Sundrum
model in a holographic setup alternative to Kaluza-Klein description: the
action is written as a functional of the two metric and radion fields on the
branes. This action effectively describes the dynamics of the gravitational
field both on the branes and in the bulk in terms of the brane geometries
directly accessible for observations. Its nonlocal form factors incorporate the
cumulative effect of the bulk Kaluza-Klein modes. We also consider the reduced
version of this action obtained by integrating out the fields on the
negative-tension brane invisible from the viewpoint of the Planckian brane
observer. This effective action features a nontrivial transition (AdS flow)
between the local and nonlocal phases of the theory associated with the limits
of small and large interbrane separation. Our results confirm a recently
proposed braneworld scenario with diverging (repulsive) branes and suggest
possible new implications of this phase transition in brane cosmology.Comment: 33 pages, title changed, the focus of discussion is shifted to
nonlocality properties of Weyl-squared terms in braneworld action and phase
transitions between local and nonlocal phases of the theory, to appear in
Phys. Rev.
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