[FPSPACE] FW: Gravity Probe B Update -- Spring 2008
LARRY KLAES
ljk4 at msn.com
Tue Mar 25 09:16:23 EDT 2008
>From: Bob Kahn <kahn at relgyro.stanford.edu>
>To: gpb-update at lists.stanford.edu
>Subject: Gravity Probe B Update -- Spring 2008
>Date: Mon, 24 Mar 2008 19:15:42 -0600
>
>============================================
>GRAVITY PROBE B MISSION UPDATE FOR SPRING 2008
>============================================
>
>GP-B WEB SITE GETS COMPLETE MAKEOVER
>==============================
>Over a year ago, we began the process of giving our GP-B Web site a
>complete makeover, with the intent of designing a site that will serve as a
>fitting legacy for GP-B, once the program concludes. Our goals included
>easy and intuitive navigation and browsing, visually interesting with
>easy-to-read pages, comprehensive, well-organized content, and liberal use
>of images and multimedia (animations, video, and audio clips).
>
>At long last, the makeover work is essentially complete, and we have
>brought the new site live on the Web at our long-standing URL:
>http://einstein.stanford.edu.
>
>You'll find our Spring 2008 Status Update (see below) in the Status
>tab--Current Status Update on the new site
>(http://einstein.stanford.edu/highlights/status1.html), and you can read
>more about the history of our Web site and the people who have contributed
>to it over the years in the Current Mission News section
>(http://einstein.stanford.edu/highlights/status-news.html).
>
>On behalf of everyone who has worked on the new site, I hope you enjoy
>exploring our new site. We welcome your feedback, comments and suggestions.
>
>Please Note: A few sections of the new site are still under development,
>and we are aware that some links still need to be updated or removed. The
>site is a "work-in-progress," and we will be addressing these issues in the
>coming months.
>
>
>PROGRAM STATUS
>==============
>Following is a summary of the status of the GP-B program and the progress
>we have made since our previous update, posted early in December 2007.
>
>GP-B PROGRAM EXTENDED THROUGH SEPTEMBER 2008, AND POSSIBLY MARCH 2010
>
>On November 2, 2007, we convened the 17th meeting of our external Science
>Advisory Committee (SAC) to review our progress in the refinement of the
>GP-B experimental results. The subsequent SAC report noted "the truly
>extraordinary progress that had been made in data analysis since SAC-16
>[March 23-24, 2007]" and unanimously concluded "that GP-B is on an
>accelerating path toward reaching good science results."
>
>Following a peer-reviewed bridging proposal to NASA's Science Mission
>Directorate (SMD) and actions by Stanford and a private donor, the GP-B
>program has been extended at least through September 2008. Furthermore, SMD
>opened the opportunity for GP-B to submit a proposal this month to its
>Senior Review process. This is a bi-annual event in which ongoing NASA
>science programs undergo a peer-review to determine which of those programs
>NASA should continue and/or extend in order to achieve the greatest
>scientific gain. Assuming a successful Senior Review, GP-B will be extended
>one final time, from October 2008 through March 2010.
>
>INCREASING THE PRECISION OF THE RESULTS
>
>From the outset, almost half a century ago, the goal of the GP-B experiment
>has been to make the most precise and accurate experimental measurement
>possible of the geodetic and frame-dragging gravitational effects predicted
>by Einstein's theory. With GP-B, the precision of the result improves as a
>function of the square root of the length of the data interval time cubed
>(t3/2). Theoretically, the best case would be one single, uninterrupted
>year-long data interval, and the precision resulting from such a single
>interval would be limited only by the sensitivity of the measuring
>instrument and its inherent noise level. For the frame-dragging effect,
>this would result in a precision of less than 1% and for the geodetic
>effect less than 0.01%.
>
>In reality, GP-B experienced six major or significant anomalies during the
>353-day science data collection period, and these anomalies caused the
>experimental data set to be divided into seven major segments, with a total
>of 307 days of "good" science data when all seven segments are combined.
>This segmentation reduced the best precision obtainable from the 1% goal
>down to about 2% for the frame-dragging effect and 0.02% for the geodetic
>effect. This reduced level of precision, if achieved would be
>extraordinary.
>
>Reaching this percision has been complicated by two unexpected effects in
>the data, as we reported in our announcement of first results at the
>American Physical Society meeting in April 2007:
>
>1. A time variation in the polhode motion of the gyroscopes, which creates
>complications in the gyro scale factor calibrations (conversion of
>electrical signals to angles).
>
>2. Much larger than expected classical misalignment torques on the
>gyroscopes, attributable to "patch effect" (contact potential difference)
>interactions between the gyro rotors and their housings.
>
>We have described and discussed these two effects in our past three status
>updates. After 2 _ years of data analysis, we have a very good
>understanding of physics underlying them. We now know that the interaction
>of patch effects between the gyro rotors and their housings is the root
>cause of both, and in our September 25, 2007 status update, we described
>how our team has honed and utilized a technique called trapped flux mapping
>to dramatically improve our determinations of the polhode phase and angle
>for each gyroscope throughout the entire 353-day experiment period.
>
>By necessity, our work to understand and model these two interrelated
>effects began with very small data segments. For example, the initial
>analysis was carried out using selected sets of 5-day (75-orbit) segments.
>Working at this "microscopic" level, the team was ultimately able to model
>the polhode damping and classical torques. However, stringing together a
>series of some 60 5-day segments to cover the whole experimental period in
>not the optimum method of processing the results. Rather, it is necessary
>to stretch the analysis segments to the point where the only breaks
>remaining in the data segments are those due to the six anomalies.
>
>In December, we reported that we had successfully applied our much enhanced
>knowledge and modelling of the polhode phase and angle of each gyroscope to
>a central 85-day stretch of data, from December 12, 2004 through March 4,
>2005. Since that time, we have been in the process of continuing to stretch
>the data analysis to increasingly long time intervals, so that the final
>analysis will be based on the seven segments defined by the anomalies
>experienced on orbit. This will yield the greatest precision possible in
>the results.
>
>PRECISION VS ACCURACY
>
>In addition to achieving the most precise results possible, it is necessary
>to ensure that the GP-B results are as accurate as possible. In target
>practice, if all of your shots cluster consistently at a spot on the
>outside edge of the target, your shooting would be considered precise, but
>not accurate. In order to be accurate, you need to hit the bull's eye in
>the center of the target. It is also possible to be accurate, but not
>precise. If your shots generally cluster around the bull's eye, but few are
>actually in the center ring, your shooting would be considered accurate on
>average, but imprecise due to the large variation or scatter in the
>locations of the shots.
>
>The goal of GP-B, of course, is to achieve both the most precise and most
>accurate results possible. In the GP-B experiment, the accuracy of the
>results is determined by the level of correlation between the four science
>gyroscopes, which at present is diminished due to the presence of
>systematic error sources. Until we have reached the physical limits of the
>experimental apparatus, the more we can identify, model and remove sources
>of systematic errors from the data for each gyroscope, the greater the
>correlation between all four of them.
>
>As experimentalists, we make no a priori assumptions that Einstein's
>predictions represent the bull's eye. Rather, we have collected data, we
>are doing everything humanly possible to achieve the greatest precision and
>accuracy in analyzing it. Once this has been accomplished, we will announce
>the results. If they match Einstein's predictions, then once again-to the
>achieved level of precision and accuracy-Einstein's theory will be upheld.
>If they don't, this could be evidence of a breakdown of the theory.
>
>During this extended data analysis period, we will be working closely with
>the SAC to review our progress and, if judged necessary, re-focus our
>analysis efforts. In consultation with the SAC, we may well announce
>refined interim results to keep the scientific community and the public
>apprised of our progress. Additionally, a significant portion of this
>extended data analysis phase will be dedicated to producing scientific and
>engineering papers, including details of the many exciting technologies of
>GP-B.
>
>
>===================
>PREVIOUS GP-B UPDATES
>===================
>If you wish to read any of our previous updates, our GP-B Web site includes
>a chronological archive of all the updates/highlights (with photos and
>drawings) that we have posted over the past 8 years:
>http://einstein.stanford.edu/highlights/hlindexmain.html
>
>=============================
>OTHER LINKS THAT MAY INTEREST YOU
>=============================
>
>Our NEW AND IMPROVED GP-B Web site, http://einstein.stanford.edu contains
>lots of information about the Gravity Probe B experiment, general
>relativity, and the amazing technologies that were developed to carry out
>this experiment.
>
>Video and/or audio of May 18, 2006 public lecture by Principal
>Investigator, Professor Francis Everitt, on GP-B. You can view a Flash
>video of the lecture in your Web browser:
>http://einstein.stanford.edu/Media/Everitt_Brainstorm-flash.html You can
>also download either a video or audio only copy of the lecture to an iPod
>from the Stanford University iTunes U Web site: http://itunes.stanford.edu,
>This Web page automatically launches the Apple iTunes program on both
>Macintosh and Windows computers, with a special Stanford on iTunes U "music
>store," containing free downloads of Stanford lectures, performances, and
>events. Francis Everitt's "Testing Einstein in Space" lecture is located in
>the Faculty Lectures section. People with audio-only iPods can download the
>version under the Audio tab; people with 5th generation (video) iPodfs can
>download the version under the Video tab.
>
>Visual tour of the GP-B spacecraft and payload from our GP-B Web site:
>http://einstein.stanford.edu/content/vehicle_tour/index.html
>
>PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that
>you can download print out, and assemble:
>http://einstein.stanford.edu/content/paper_model.
>
>NASA's Marshall Space Flight Center also has a series of Web pages devoted
>to GP-B: http://www.gravityprobeb.com
>
>The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York
>University (Toronto), with contributions from the Observatoire de Paris,
>have been studying the motions of the guide star, IM Pegasi for over a
>decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/
>
>In addition, you'll find information in the Guide Star FAQ on our Web site:
> http://einstein.stanford.edu/TECH/technology1.html#telescope
>
>==========================
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>==========================
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>--
>**********************************
>NASA - Stanford - Lockheed Martin
> Gravity Probe B Program
>"Testing Einstein's Universe"
> http://einstein.stanford.edu
>
>Bob Kahn
>Public Affairs Coordinator
>
>Phone: 650-723-2540
>Fax: 650-723-3494
>Email: kahn at relgyro.stanford.edu
>**********************************
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