Gravity Probe

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Gravity Probe welcomed two space missions of NASA to test the general theory of relativity of Albert Einstein . A proposed third mission has not yet been realized.

Gravity Probe A

Gravity Probe A ( GP-A ) flew on June 16, 1976 with an extremely accurate atomic clock in a steep, ballistic orbit with a peak height of 10,000 km. During the almost two-hour flight, the clock rate was compared with two identical clocks on the ground using a microwave connection. For this purpose, the clock signal of the probe was impressed by a transponder on a signal received from the ground and sent back again. This method avoided the disruptive effect of the Doppler effect and allowed the gravitational redshift, based on the equivalence principle , to be measured with an accuracy of 0.02%. In 1965, the accuracy was still 1%, measured using the Mössbauer effect over a drop height of only 15 meters. A little later, the GPS satellite system allowed far more precise measurements.

Gravity Probe B

In the satellite Gravity Probe B ( GP-B ), four rapidly rotating silica spheres were freely suspended. The extremely precise observation of their axes of rotation provided information on the torques that were caused by two relativistic effects. The idea for this comes from the theorist Leonard Schiff , who discussed it with William Fairbank Sr. in the early 1960s . The scientific direction of the NASA mission was with a former Fairbank employee at Stanford University , Francis Everitt . The evaluation of the seriously disturbed data recorded in 2004/2005 dragged on until 2011. As expected, the result was in agreement with the theory, but not more precise than previous measurements using the geodetic satellites LAGEOS and GRACE .

Artist's impression of Gravity Probe B in space

The satellite mission Gravity Probe B was supposed to enable  an experimental verification of two statements of the general relativity theory - at the time of its planning by Fairbank for the first time :

The space-time curvature and the Lense-Thirring effect act on the gyroscope in mutually perpendicular directions.
  1. The curved spacetime : The general theory of relativity predicts that a mass in space, for example the earth, deforms the surrounding spacetime, creating a dent or curvature in it.
  2. The Lense-Thirring effect ( frame dragging effect ): A few years after Einstein published the general theory of relativity, the Austrian mathematician Josef Lense and the Austrian physicist Hans Thirring predicted in 1918 that the rotation of a mass in space would pull local space-time with it and this twisted them.

According to the predictions of the physicists, the axes of rotation of the four gyroscopes should tilt by 6606.1 milli- arcseconds per year due to the space-time curvature and, additionally, by 39.2 milli- arcseconds in a different direction due to the Lense-Thirring effect. The measurement of such small changes in the axis of rotation is an extreme challenge for experimental technology. The gyroscopes developed especially for this mission consisted of quartz spheres the size of a table tennis ball (3.8 cm), which rotated in a vacuum at 10,000 revolutions per minute. They were cooled to 1.8 K in order to make their surface coated with niobium superconducting and to generate a magnetic field in the direction of the axis of rotation through the London moment . Changes in the axis of rotation were recorded using highly sensitive superconducting quantum interference detectors, so-called SQUIDs . External magnetic fields were reduced by 240 dB by double shielding made of superconducting material  . In this way, changes of one milli-arcsecond could be measured (with 10 hours of integration time). At this angle, a pin head appears at a distance of 1,000 km.

The four spheres were located in the axis of rotation of the satellite and rotated in pairs in the same or in opposite directions. The axes of rotation were measured in relation to the satellite and related to the spectroscopic double star IM Pegasi with a small telescope in its axis of rotation . Several start trackers and gyroscopes were used to control the position of the satellite around its axis and during the earth's occultation of IM Pegasi. IM Pegasi was chosen because it lies close to the plane of the earth's equator, is bright enough for precise bearing and its strong radio emissions can be detected by VLBI , so that its movement could be related to the reference system of distant quasars .

Gravity Probe B took off on April 20, 2004 at 9:57:24 PDT aboard a Delta II

The satellite was on 20 April 2004 by the US Air Force Base Vandenberg aboard a Delta II 7920 - rocket successfully launched. The satellite's orbit passed the poles at an altitude of approx. 740 km.

On August 28, 2004, the preparations for the actual measurements were completed. However, the calibration measurements carried out previously had already shown that unforeseen effects (described later as misalignment torque and roll-polhode resonance torque ) influenced the direction of rotation of the gyroscopes. Once the cause was understood, the effects could be modeled and initially roughly calculated. This was an undesirable coupling between the ball surface and the wall due to inhomogeneous electric fields due to local variations in the work function . In addition, an interaction of this field with the activity of the electrostatic centering of the ball led to a dampening of its precession movement , which made the analysis of the measured values ​​difficult and delayed.

The coupling constants with the wall, i.e. with the rotating satellite, which could not be determined precisely enough from the actual measurements, were measured separately in a calibration phase planned towards the end of the mission by setting the axis of rotation of the satellite roughly incorrectly, which multiplied the effects of interference. Another measurement phase followed until the helium for cooling was used up at the end of 2005.

Everitt himself took on the public admission of the problems.

Shortly before the NASA mission ended in 2007, he requested an extension for one year, as a further analysis promised more precise results. This was criticized by other researchers, who were worried about funding their missions, arguing that LAGEOS had already enabled similarly accurate tests of the theory of relativity. A 15-member NASA committee approved only scanty funds, which were supplemented by a private donation from Fairbank's son and funds from the university. The mission's final report was published in December 2008, but the evaluation was continued. a. with funds from Saudi Arabia. The final report of the university appeared with renewed public attention in May 2011.

The effect of the spacetime curvature (theory: 6606.1, all in milli-arcseconds / year) should be determined with a precision of 0.01%, the weaker Lense-Thirring effect (theory: 39.2) to 1%. The result is 6601.8 ± 18.3 (0.28%), or 37.2 ± 7.2 (19%), error indications in each case 1 σ . As early as 2004, an evaluation of LAGEOS orbit data collected over eleven years had predicted the Lense Thirring Effect at 1% with an uncertainty of 5%.

Gravity Probe C (lock)

Gravity Probe C (lock) is a 1997 proposal for a third mission. The experiment would consist of two satellites orbiting the earth in equatorial orbits in opposite directions. According to general relativity, the orbital times of the satellites should differ by about 100 nanoseconds due to gravitomagnetic effects (caused by the rotation of the earth)  . In order to be able to calculate other effects, the earth's gravitational field would have to be examined more closely before the mission.

See also

Web links

Commons : Gravity Probe B  - collection of images, videos and audio files

Individual evidence

  1. ^ SC Marsden et al .: A Sun in the Spectroscopic Binary IM Pegasi, the Guide Star for the Gravity Probe B Mission , Astrophys. J. Lett., 634, 2005, pp. 173-176, doi : 10.1086 / 498941 , online (PDF; 130 kB).
  2. ↑ DE Lebach et al .: VLBI Imaging and astrometry of the RS CVn binary star IM Pegasi , in: RT Schilizzi et al. (Ed.): Galaxies and their Constituents at the Highest Angular Resolutions , IAU Symp., 205, 2001. bibcode : 2001IAUS..205..318L
  3. ↑ S. Buchman, J. P. Turneaure: The effects of patch-potentials on the gravity probe B gyroscopes , Rev. Sci. Instrum., 82, 2011, 074502, doi : 10.1063 / 1.3608615 , ( online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. ) .@1@ 2Template: Dead Link / rsi.aip.org  
  4. ↑ Francis Everitt: Testing Einstein in Space - The Gravity Probe B Mission , public lecture by the head of mission on May 18, 2006, on stanford.edu.
  5. ↑ Everitt et al., Stanford Press Department: The Gravity Probe B Experiment, Science Result - NASA Final Report . (PDF; 2.6 MB)
  6. ↑ a b nytimes.com: 52 Years and $ 750 Million Prove Einstein Was Right , May 5, 2011.
  7. ↑ C. W. F. Everitt, et al .: Gravity Probe B: Final results of a space experiment to test general relativity , Phys. Rev. Lett. 106, 2011, p. 221101, doi : 10.1103 / PhysRevLett.106.221101 ( online ; PDF; 542 kB).
  8. ↑ Der Spiegel : NASA satellite confirms Einstein theory .
  9. ↑ Krishna Ramanujan (NASA GSFC): As World Turns it Drags Time and Space , Feature, October 21, 2004.
  10. ^ Page of the Institute for Space Research of the Austrian Academy of Sciences on Gravity Probe C iwf.oeaw.ac.at, accessed on May 2, 2011
  11. ↑ Frank Gronwald et al .: Gravity Probe C (lock) - Probing the gravitomagnetic field of the Earth by means of a clock experiment. bibcode : 1997gr.qc .... 12054G