Gravity Probe B
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Gravity Probe B (GP-B) was a
satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
-based experiment whose objective was to test two previously-unverified predictions of general relativity: the geodetic effect and
frame-dragging Frame-dragging is an effect on spacetime, predicted by Albert Einstein's General relativity, general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary Field (physics), field is one that is ...
. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth-orbiting satellite at of altitude, crossing directly over the poles. The satellite was launched on 20 April 2004 on a
Delta II Delta II was an expendable launch system, originally designed and built by McDonnell Douglas, and sometimes known as the Thorad Delta 1. Delta II was part of the Delta rocket family, derived directly from the Delta 3000, and entered service in ...
rocket. The spaceflight phase lasted until ; Its aim was to measure spacetime curvature near Earth, and thereby the stress–energy tensor (which is related to the distribution and the motion of matter in space) in and near Earth. This provided a test of
general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
, gravitomagnetism and related models. The principal investigator was Francis Everitt. Initial results confirmed the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging effect was similar in magnitude to the current noise level (the noise being dominated by initially unmodeled effects due to nonuniform coatings on the gyroscopes). Work continued to model and account for these sources of error, thus permitting extraction of the frame-dragging signal. By , the frame-dragging effect had been confirmed to within 15% of the expected result, and the
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
report indicated that the geodetic effect was confirmed to be better than 0.5%. In an article published in the journal ''
Physical Review Letters ''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. The journal is considered one of the most prestigious in the field of physics ...
'' in , the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of and a frame-dragging drift rate of , in good agreement with the general relativity predictions of and , respectively.


Overview

Gravity Probe B was a relativity
gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...
experiment funded by NASA. Efforts were led by the
Stanford University Leland Stanford Junior University, commonly referred to as Stanford University, is a Private university, private research university in Stanford, California, United States. It was founded in 1885 by railroad magnate Leland Stanford (the eighth ...
physics department with
Lockheed Martin The Lockheed Martin Corporation is an American Arms industry, defense and aerospace manufacturer with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta on March 15, 1995. It is headquartered in North ...
as the primary subcontractor. Mission scientists viewed it as the second relativity experiment in space, following the successful launch of Gravity Probe A (GP-A) in . The mission plans were to test two unverified predictions of general relativity: the geodetic effect and
frame-dragging Frame-dragging is an effect on spacetime, predicted by Albert Einstein's General relativity, general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary Field (physics), field is one that is ...
. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at altitude, crossing directly over the poles. The gyroscopes were intended to be so free from disturbance that they would provide a near-perfect
spacetime In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are useful in visualiz ...
reference system. This would allow them to reveal how space and time are "warped" by the presence of the Earth, and by how much the Earth's rotation "drags" space-time around with it. The geodetic effect is an effect caused by space-time being "curved" by the mass of the Earth. A gyroscope's axis when parallel transported around the Earth in one complete revolution does not end up pointing in exactly the same direction as before. The angle "missing" may be thought of as the amount the gyroscope "leans over" into the slope of the space-time curvature. A more precise explanation for the space curvature part of the geodetic precession is obtained by using a nearly flat cone to model the space curvature of the Earth's gravitational field. Such a cone is made by cutting out a thin "pie-slice" from a circle and gluing the cut edges together. The spatial geodetic precession is a measure of the missing "pie-slice" angle. Gravity Probe B was expected to measure this effect to an accuracy of one part in , the most stringent check on general relativistic predictions to date. The much smaller frame-dragging effect is an example of gravitomagnetism. It is an analog of
magnetism Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, ...
in classical electrodynamics, but caused by rotating masses rather than rotating electric charges. Previously, only two analyses of the laser-ranging data obtained by the two LAGEOS satellites, published in and , claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively, whereas Gravity Probe B aimed to measure the frame dragging effect to a precision of 1%. A recent analysis of Mars Global Surveyor data has claimed to have confirmed the frame dragging effect to a precision of 0.5%, although the accuracy of this claim is disputed. The launch was planned for at
Vandenberg Air Force Base Vandenberg may refer to: * Vandenberg (surname), including a list of people with the name * USNS ''General Hoyt S. Vandenberg'' (T-AGM-10), transport ship in the United States Navy, sank as an artificial reef in Key West, Florida * Vandenberg S ...
but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. An unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment. On PDT ( UTC) the spacecraft was launched successfully. The satellite was placed in orbit at AM ( UTC) after a cruise period over the south pole and a short second burn. The mission lasted 16 months. Some preliminary results were presented at a special session during the
American Physical Society The American Physical Society (APS) is a not-for-profit membership organization of professionals in physics and related disciplines, comprising nearly fifty divisions, sections, and other units. Its mission is the advancement and diffusion of ...
meeting in . NASA initially requested a proposal for extending the GP-B data analysis phase through . The data analysis phase was further extended to using funding from Richard Fairbank, Stanford and NASA, and beyond that point using non-NASA funding only. Final science results were reported in .


Experimental setup

The ''Gravity Probe B'' experiment comprised four London moment gyroscopes and a reference
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
sighted on IM Pegasi, a binary star in the constellation Pegasus. In
polar orbit A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of abo ...
, with the gyro spin directions also pointing toward IM Pegasi, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both. The gyroscopes were housed in a dewar of superfluid helium, maintaining a temperature of under . Near-
absolute zero Absolute zero is the lowest possible temperature, a state at which a system's internal energy, and in ideal cases entropy, reach their minimum values. The absolute zero is defined as 0 K on the Kelvin scale, equivalent to −273.15 ° ...
temperatures were required to minimize molecular interference, and enable the
lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
and
niobium Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...
components of the gyroscope mechanisms to become superconductive. At the time of their manufacture, the gyroscopes were the most nearly spherical objects ever made (two gyroscopes still hold that record, but third place has been taken by the silicon spheres made by the Avogadro project). Approximately the size of ping pong balls, they were perfectly round to within forty atoms (less than ). If one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only high. The spheres were made of fused quartz and coated with an extremely thin layer of
niobium Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...
. A primary concern was minimizing any influence on their spin, so the gyroscopes could never touch their containing compartment. They were held suspended with electric fields, spun up using a flow of helium gas, and their spin axes were sensed by monitoring the magnetic field of the superconductive niobium layer with
SQUID A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...
s. (A spinning superconductor generates a magnetic field precisely aligned with the rotation axis; see London moment.) IM Pegasi was chosen as the guide star for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions near the celestial equator. Also important was its well-understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. In preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over several years to understand its motion as precisely as needed.


History

The conceptual design for this mission was first proposed by an MIT professor, George Pugh, who was working with the U.S. Department of Defense in and later discussed by Leonard Schiff ( Stanford) in at Pugh's suggestion, based partly on a theoretical paper about detecting frame dragging that Schiff had written in . It was proposed to NASA in , and they supported the project with funds in . This grant ended in after a long phase of
engineering Engineering is the practice of using natural science, mathematics, and the engineering design process to Problem solving#Engineering, solve problems within technology, increase efficiency and productivity, and improve Systems engineering, s ...
research into the basic requirements and tools for the satellite. In NASA changed plans for the
Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
, which forced the mission team to switch from a shuttle-based launch design to one that was based on the Delta 2, and in tests planned of a prototype on a shuttle flight were cancelled as well. Gravity Probe B marks the first time that Stanford University has been in control of the development and operations of a space satellite funded by NASA. The total cost of the project was about $750 million.


Mission timeline

This is a list of major events for the GP-B experiment. ; : Launch of GP-B from Vandenberg AFB and successful insertion into polar orbit. ; : GP-B entered its science phase. On mission day 129 all systems were configured to be ready for data collection, with the only exception being gyro 4, which needed further spin axis alignment. ; : The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode. ; : The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of liquid helium. ; : Phase I of data analysis complete ; : Analysis team realised that more error analysis was necessary (particularly around the polhode motion of the gyros) than could be done in the time to and applied to NASA for an extension of funding to the end of . ; : Completion of Phase III of data analysis ; : Announcement of best results obtained to date. Francis Everitt gave a plenary talk at the meeting of the
American Physical Society The American Physical Society (APS) is a not-for-profit membership organization of professionals in physics and related disciplines, comprising nearly fifty divisions, sections, and other units. Its mission is the advancement and diffusion of ...
announcing initial results: "The data from the GP-B gyroscopes clearly confirm Einstein's predicted geodetic effect to a precision of better than 1 percent. However, the frame-dragging effect is 170 times smaller than the geodetic effect, and Stanford scientists are still extracting its signature from the spacecraft data." ; : GP-B spacecraft decommissioned, left in its polar orbit. ; : GP-B Final experimental results were announced. In a public press and media event at NASA Headquarters, GP-B Principal Investigator, Francis Everitt presented the final results of Gravity Probe B. ; : Publication of GP-B Special Volume (Volume 32, Issue 22) in the peer-reviewed journal, '' Classical and Quantum Gravity''. On , it was announced that a number of unexpected signals had been received and that these would need to be separated out before final results could be released. In it was announced that the spin axes of the gyroscopes were affected by torque, in a manner that varied over time, requiring further analysis to allow the results to be corrected for this source of error. Consequently, the date for the final release of data was pushed back several times. In the data for the frame-dragging results presented at the meeting of the American Physical Society, the random errors were much larger than the theoretical expected value and scattered on both the positive and negative sides of a null result, therefore causing skepticism as to whether any useful data could be extracted in the future to test this effect. In , a detailed update was released explaining the cause of the problem, and the solution that was being worked on. Although electrostatic patches caused by non-uniform coating of the spheres were anticipated, and were thought to have been controlled for before the experiment, it was subsequently found that the final layer of the coating on the spheres defined two-halves of slightly different contact potential, which gave the sphere an electrostatic axis. This created a classical dipole torque on each rotor, of a magnitude similar to the expected frame dragging effect. In addition, it dissipated energy from the polhode motion by inducing currents in the housing electrodes, causing the motion to change with time. This meant that a simple time-average polhode model was insufficient, and a detailed orbit by orbit model was needed to remove the effect. As it was anticipated that "anything could go wrong", the final part of the flight mission was calibration, where amongst other activities, data was gathered with the spacecraft axis deliberately misaligned for , to exacerbate any potential problems. This data proved invaluable for identifying the effects. With the electrostatic torque modeled as a function of axis misalignment, and the polhode motion modeled at a sufficiently fine level, it was hoped to isolate the relativity torques to the originally expected resolution. Stanford agreed to release the raw data to the public at an unspecified date in the future. It is likely that this data will be examined by independent scientists and independently reported to the public well after the final release by the project scientists. Because future interpretations of the data by scientists outside GP-B may differ from the official results, it may take several more years for all of the data received by GP-B to be completely understood.


NASA review

A review by a panel of 15 experts commissioned by NASA recommended against extending the data analysis phase beyond . They warned that the required reduction in noise level (due to classical torques and breaks in data collection due to solar flares) "is so large that any effect ultimately detected by this experiment will have to overcome considerable (and in our opinion, well justified) skepticism in the scientific community".


Data analysis after NASA

NASA funding and sponsorship of the program ended on , but GP-B secured alternative funding from King Abdulaziz City for Science and Technology in Saudi Arabia that enabled the science team to continue working at least through . On , the 18th meeting of the external GP-B Science Advisory Committee was held at Stanford to report progress. The Stanford-based analysis group and NASA announced on that the data from GP-B indeed confirms the two predictions of Albert Einstein's general theory of relativity. The findings were published in the journal ''
Physical Review Letters ''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. The journal is considered one of the most prestigious in the field of physics ...
''. The prospects for further experimental measurement of frame-dragging after GP-B were commented on in the journal '' Europhysics Letters''.


See also

*
Frame-dragging Frame-dragging is an effect on spacetime, predicted by Albert Einstein's General relativity, general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary Field (physics), field is one that is ...
* Gravity Probe A * Gravitomagnetism * Modified Newtonian dynamics *
Tests of general relativity Tests of general relativity serve to establish observational evidence for the theory of general relativity. The first three tests, proposed by Albert Einstein in 1915, concerned the "anomalous" precession of the perihelion of Mercury (planet), Me ...
* Timeline of gravitational physics and relativity


References


External links


Gravity Probe B web site at NASA



Graphic explanation of how Gravity Probe B works

NASA GP-B launch site


*




IOP Classical and Quantum Gravity, Volume 32, Issue 22, Special Focus Issue on Gravity Probe B

Gravity Probe B Collection, The University of Alabama in Huntsville Archives and Special Collections
{{Orbital launches in 2004 Tests of general relativity Physics experiments Satellites orbiting Earth Spacecraft launched in 2004 Spacecraft launched by Delta II rockets