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Gravity Probe B

Testing Einstein's Universe

Frequently Asked Questions

Questions Grouped by Category

General Information Questions:

  1. Where can I find an overview or summary that succintly describes the Gravity Probe B experiment?
  2. Is there a video or DVD describing the GP-B experiment that I can order?
  3. Do you have educational materials about GP-B that teachers or educators can order?
  4. Where can journalists and members of the press or media find information, photos and video clips to create stories about GP-B?
  5. Where can I find articles or stories that have been written about GP-B?
  6. Where can I find scientific and technical publications written about the science and technologies of GP-B by members of the GP-B team?
  7. Who do I contact to arrange for someone from the GP-B team to give a talk to our group/school/colloquium/lunch meeting etc.?

Questions about Program Status and the Experimental Results of GP-B:

  1. What is the current status of the GP-B program and the data analysis effort?
  2. How do I subscribe/unsubscribe to your mailing list to receive status updates by email?
  3. Have any preliminary results been announced, and if so where can I read about them?
  4. When will the final results be announced/published, and how can I order a copy?
  5. How can I obtain a copy of the data collected from the GP-B spacecraft & science instrument?

Questions about the History, Funding & Rationale of the GP-B Experiment:

  1. Is the GP-B experiment a NASA program or a Stanford program?
  2. Who funded GP-B and how much has it cost?
  3. Haven't Einstein's theories already been experimentally validated—why perform another test of Einstein?
  4. Where can I find information about the history and management of the GP-B program and team?
  5. Was there a Gravity Probe A (GP-A), and if so, what kind of experiment was it?
  6. Has the GP-B program or team won any awards?

Questions about the GP-B Experimental Design & Unique Technologies

  1. I've heard that GP-B had several "near-zero" requirements; what does this mean?
  2. How was the guide star, IM Pegasi, selected? What is its significance in the GP-B experiment?
  3. What were the main technological challenges that had to be solved in order to carry out the GP-B experiment?
  4. What technological or other spin-offs have resulted from GP-B?

Spacecraft and Mission Operations Questions:

  1. When and where was GP-B launched?
  2. Where can I find photos, video, or newsclips of the launch?
  3. Why did Gravity Probe B have a one-second launch window?
  4. Where was the spacecraft built, and who built it?
  5. Where is the GP-B Mission Operations Center (MOC) for controlling the spacecraft in orbit?
  6. Now that the experiment is finished, is anyone still using or controlling the spacecraft?
  7. Can I still track/view the GP-B spacecraft in the night sky?
  8. How long will the spacecraft remain in orbit and what will become of it?

Questions about Spacetime, Gravitation, Relativity, Cosmology, etc.:

  1. I have a question about spacetime or some aspect of gravity, special or general relativity, cosmology, etc.; can you explain __________?
  2. Can you please review and comment on my enclosed paper about__________?
  3. I have my own alternative theory of gravitation and/or cosmology; how can I get someone at GP-B or the physics community to take my ideas seriously?
  4. Can you recommend some references that will help me learn more about spacetime, gravitation, relativity, cosmology etc.?

Answers to General Information Questions:

  1. Where can I find an overview or summary that succintly describes the Gravity Probe B experiment?
  2. Our two-page PDF flyer entitled GP-B in a Nutshell provides a brief overview of the GP-B program, including sections on the history, technologies, science, mission and legacy of the program. If you want something more visual with less words, you can download a PDF copy of our GP-B in a Nutshell Sldieshow. In addition, the various sub-sections of the Mission Tab on this Web site provide a reasonably succint, but comprehensive overview of GP-B. Lastly, the 24-page Executive Summary from our Post-Flight Analysis—Final Report to NASA, also available as a PDF download provides a very comprehensive overview of GP-B.

  3. Is there a video or DVD describing the GP-B experiment that I can order?
  4. Yes there is. Norbert Bartel, Professor of Astrophysics and Space Sciences at York University in Toronto, Canada, has produced and directed a 26-minute documentary movie about the Gravity Probe B experiment entitled, Testing Einstein's Universe. You can view a Flash Video version of this movie from the Media Gallery, in the Resources Tab on this Web site.

    This movie, along with 80 minutes of additional video about relativity, physics, and astronomy is alaso available on a DVD. For more information about this DVD, see Norbert Bartel's Astronomy Films Web page. If you would like to purchase this DVD online, it is available through the York University Bookstore.

  5. Do you have educational materials about GP-B that teachers or educators can order?
  6. Where can journalists and members of the press or media find information, photos and video clips to create stories about GP-B?
  7. Where can I find articles or stories that have been written about GP-B?
  8. Where can I find scientific and technical publications written about the science and technologies of GP-B by members of the GP-B team?
  9. Who do I contact to arrange for someone from the GP-B team to give a talk to our group/school/colloquium/lunch meeting etc.?
  • How do I subscribe/unsubscribe to your mailing list to receive status updates by email?

    If you are interested in automatically receiving our GP-B Program Status Updates and other important GP-B mission information by email, you can subscribe to our Gravity Probe B Update email list.

    • To subscribe to the list, send an email message to "" (The subject and body of this message will be ignored, so it does not matter what you put there.)
    • To unsubscribe from this mailing list at any time, send an email message to "" (The subject and body of this message will be ignored, so it does not matter what you put there.)

    Please Note: Now that we are in the data analysis phase of the program, we typically send out new updates once a month. From time to time, we may post and email extra updates, as warranted by mission events.

  • How can I track the GP-B satellite across the night sky?

    Find the Gravity Probe B satellite in the sky at NASA's satellite tracking web site. See where GP-B is with respect to the terminator (the day-night boundary on the Earth's surface), or just enter your zip code to see if GP-B might be over your neighborhood. The best time to look for it is usually at dusk.

    Also, you can track the GP-B spacecraft on your Palm OS or Pocket PC Personal Digital Assistant (PDA), using either PocketSat or PocketSat+ from Big Fat Tail Productions. Both products are PDA Shareware, so you can try them out for free. If you decide to use them, Big Fat Tail asks that you pay a nominal shareware fee.

  • Do you have pictures and video, or news clips on the launch?

    Yes we do. Please have a look at our "news clips" section found under the News/Media section of our site.

  • Is there a DVD about this project that I can order?


  • Operations:

    1. How did you choose the guide star? What is its significance?

      The more important measurement conducted by the science instrument is that of the angle between the spin axis of our gyroscopes and the fixed reference line provided by our guide star (via our on-board telescope). Effectively, we determine the rate of change of the gyro spin directions in comparison to the direction of one particular star. By means of our specialized guiding telescope, the spacecraft is kept pointing at this "guide star" for almost the entire mission. But is this guide star an adequate reference point, fixed on the sky with respect to the galaxies lying behind it? The answer is "No!"  Like everything else in the universe, stars move, and are not fixed points on the sky. Our guide star (like all stars) is free to move slowly across the sky, and this motion cannot be ignored in the GP-B test of general relativity. In fact, the European space mission Hipparcos has shown that each year the chosen guide star moves across the sky about 35 milliarcseconds, almost as much as the entire relativistic gravitomagnetic effect in one year. Moreover, the uncertainty of this value is larger than the intended accuracy of the of the GP-B relativity tests. Some other method was needed to measure the motion of the guide star.

      The solution to this problem was to choose a star that emits not only light that can be observed with the telescope of the GP-B spacecraft, but also microwave radio noise that can be recorded by large astronomical and deep space communication radio antennas on the ground. The chosen star is IM Pegasi--in Latin "Pegasi" means "of or in Pegasus," which is a constellation of stars easily seen high over head on autumn evenings throughout North America, Europe, and Asia. The designation "IM" is just a pair of letters chosen more or less sequentially to identify certain stars. This star IM Pegasi is barely bright enough to see with the naked eye under very dark and clear skies, but when observed in the microwave range, this star is often among the brightest in the sky. The microwave noise coming from this star makes it possible to use the methods of very-long-baseline interferometry (VLBI) to measure the position of the star in comparison to several quasars that lie nearby to the star on the sky. Quasars are galaxies with a quasi-stellar (i.e., nearly unresolved) appearance found in the distant universe millions of times farther away from the Earth than the stars visible to the naked eye. The wonderful thing about these quasars is that even though they are far enough away to be ideal reference points for the relativity tests, they emit so much microwave radio energy that they appear to radio antennas to be among the brightest objects in the sky, in many cases even brighter than the guide star.

      To measure the motion on the sky of this particular star, since 1997 the GP-B program has obtained VLBI observations of this star and at least two quasars behind it about four times per year. Data is recorded with the VLA and VLBA radio antenna arrays of the National Radio Astronomy Observatory (NRAO), the 70 meter diameter dishes of NASA's Deep Space Network (DSN) and the 100 meter diameter dish of the Max-Planck-Institut für Radioastronomie (MPIfR) in sessions lasting up to 18 hours. The data from all the antennas are initially combined and processed by NRAO in Socorro, NM, before being sent to York University, in Toronto, Canada, and to the Harvard-Smithsonian Center for Astrophysics, in Cambridge, MA, for further analysis. The use of so many telescopes increases both the accuracy of the measurements and their ability to detect the the sometimes weak radio emission from IM Pegasi. The results of this analysis are a series of maps and positions from which the motions of IM Pegasi can be estimated. By the end of the GP-B mission, these observations should determine the mean (or so-called "proper") motion of IM Pegasi against the distant galaxies to an accuracy of about 0.1 milliarcseconds per year. This accuracy is comparable to the intended accuracy of the measurement of the mean rate of change of the gyro spin directions with respect to IM Pegasi. Combined together, these measurements will determine the mean rate of change of the gyro spin directions with respect to the distant universe. Together they will test, with much greater accuracy than ever before, the two predictions of general relativity for the behavior of gyroscopes near a massive body, and thus reveal something about the nature of space-time itself.

      Some readers have asked for alternative naming conventions for our guide star, as it appears listed in different catalogs. These names can include: IM Peg, HR 8703, HD 216489, SAO 108231, BD +16 4831, FK5: 3829

      Pages 18-20 of the Gravity Probe B Launch Companion contain information about the guide star and the science telescope. Furthermore, the ETH Institute of Astronomy in Zurich, Switzerland, is working with the Harvard-Smithsonian Center for Astrophysics to provide detailed optical information about the GP-B guide star, IM Pegasi. You can find out about the ETH Institute's work in monitoring magnetic activity on IM Pegasi and the Doppler Imaging Technique used for this purpose on the ETH Institute of Astronomy GP-B Web page.

      The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto) are studying the guide star to provide crucial measurements of its motion relative to far away quasars. These measurements are needed to relate the tiny changes in the gyroscopes' spin direction to the distant universe, so that general relativity can be tested. Learn more about these measurements by going to the IM Pegasi web site at York University.

    2. Where is GP-B controlled?

      The spacecraft is being controlled from the Gravity Probe B Mission Operations Center, located here at Stanford University. The Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to perform superbly.

    3. When and where was GP-B launched?

      The Gravity Probe B space vehicle was launched on Tuesday, April 20, 2004, at 9:57:24 AM Pacific Daylight Time. The space vehicle was launched using a Delta II rocket from Vandenberg Air Force Base in South-Central California. Mission life is expected to be approximately 16 months.

    4. Why did Gravity Probe B have a one-second launch window?

      The spacecraft needed to be launched exactly into an orbit plane aligned with the guide star. how close "exactly" actually needs to be is defined by the science team, but it turns out to be around +/- 0.04 degrees of longitude (in rough terms). Note the plane of the guide star (a plane defined by the center of the earth, the north pole, and the guide star) is fixed or "inertial" in space. This plane does notrotate with the earth.

      We know the earth rotates completely around (360 degrees) in 24 hrs (86400 seconds), which means the earth turns through (360 deg/86400 sec) 0.004 deg/sec. This means that you, me, and the rocket on the ground are constantly rotating around at this rate. So, in theory, it takes the earth 10 seconds to turn through 0.04 degrees, which is our maximum, theoretical "window" on any given day.

      So why was our launch window one second instead of ten seconds? The maximum accuracy of the Delta II rocket is only about +/- 0.03 degrees. This now only leaves about 2 seconds of margin in our maximum theoretical window. Build in a factor of safety here and there for other considerations, namely, whether the rocket is launched half a second early or late, and you're left with a one-second window. There are also some second- and third-order effects to consider, but they are outside the scope of this discussion.

    General Program Questions:

    1. When are you going to publish results?

      We expect to make this announcement of the results in April 2007. During the 50-week science phase of the GP-B mission and the 7-week instrument calibration phase, which lasted from August 2004 - Septermber 2005, we collected over a terabyte of experimental data. Analysis has been progressing through a 3-phase plan, each subsequent phase building on those preceding it.

      In Phase I, which lasted from the end of September 2005 through February 2006, the analysis focused on a short term—day-by-day or even orbit-by-orbit—examination of the data. The overall goals of this phase were to optimize the data analysis routines, calibrate out instrumentation effects, and produce initial "gyro spin axis orientation of the day" estimates for each gyro individually. At this stage, the focus was on individual gyro performance; there was no attempt to combine or compare the results of all four gyros, nor was there even an attempt to estimate the gyro drift rates.

      We are currently progressing through Phase II of the data analysis process, which began at the beginning of March and is scheduled to run through mid-August 2006. During Phase II, our focus is on understanding and compensating for certain long-term systematic effects in the data that span weeks or months. The primary products of this phase will be monthly spin axis drift estimates for each gyro, as well as refined daily drift estimates. In this phase, the focus remains on individual gyro performance.

      In Phase III, which is scheduled to run from late August 2006 through December 2006, data from all four gyros will be integrated over the entire experiment. The results of this phase will be both individual and correlated gyro drift rates covering the entire 50-week experimental period for all four gyros. These results will be relative to the position of our guide star, IM Pegasi, which changed continually throughout the experiment. Thus, the final step in the analysis, currently scheduled to occur in January 2007, will be to combine our gyro drift results with data mapping the proper motion of IM Pegasi relative to the unchanging position of a distant quasar. The proper motion of IM Pegasi has been mapped with unprecedented precision using a technique called Very Long Baseline Interferometry (VLBI) by Irwin Shapiro and his team at the Harvard-Smithsonian Center for Astrophysics (CfA), in collaboration with Norbert Bartel at York University in Toronto and French astronomer Jean-Francois Lestrade.

      Playing the role of our own harshest critic, our science team will then perform a careful and thorough final review of the analysis and results, checking and cross-checking each aspect to ensure the soundness of our procedures and the validity of our outcomes. We will then turn the analysis and results over to our GP-B Science Advisory Committee (SAC), that has been closely monitoring our experimental methods, data analysis procedures, and progress for 11 years, to obtain its independent review. In addition, we will seek independent reviews from a number of international experts.

      Throughout phases II and III, members of our team will be preparing scientific and engineering papers for publication in late 2006-2007. At the same time, we will be working with NASA to plan a formal public announcement of the results of this unprecedented test of General Relativity.

    2. What does "GP-B" actually stand for?

      "GP-B" stands for "Gravity Probe B".

    3. Which of Einstein's theories is the project trying to test?

      This project is attempting to test Einstein's theory of General Relativity.

    4. Is the project part of Stanford University?

      Yes. Our administration is on campus and our science instrument was built in a Stanford laboratory - the W. W. Hansen Experimental Physics Laboratory to be precise. Stanford was selected by NASA's Marshall Space Flight Center as the prime contractor responsible for the entire Gravity Probe B Program. This includes the responsibilities of building the science instrument, of overseeing deliverable components (such as our Dewar and our space vehicle) from our contractor, Lockheed Martin Missiles and Space, and Mission Operations and Data Analysis. This is the first time NASA has allowed a university to completely manage the development and operation of a major scientific satellite.

    5. Who actually built the satellite and where?

      Stanford is Prime Contractor to NASA Marshall Space Flight Center with a major subcontract to Lockheed Martin Missiles and Space. There have been in the vicinity of 500 subcontracts from Stanford and Lockheed for different individual subsystems, including, incidentally, a subcontract to the Harvard Smithsonian Astrophysical Observatory for refined measurements of the proper motion of the GP-B guide star. Essentially, there were two main groups responsible for building the entire Gravity Probe B satellite: Stanford and Lockheed. Stanford University built the Science Instrument which consists of the gyroscopes and their systems, the quartz block, the SQUIDs and the telescope. Some of the electronics for collecting and sending the science data were also built at Stanford. Many of the electronic components for GP-B as well as the Dewar, Probe and Spacecraft were built nearby by Lockheed Martin Missiles and Space. The Ground Station, which receives all data from the satellite after launch, is located on the Stanford University campus.

    6. Is GP-B funded by the National Science Foundation, NASA or other national agencies, or is it privately funded? Where does the money come from?

      The story of the funding of Gravity Probe B is a rather interesting one:

      • In March 1964, retroactive to November 1963, NASA began funding the feasibility of the mission and preliminary technology development. Low level funding continued through 1985.
      • In 1982 NASA performed a study of the program which recommended amongst other things a significant up front technology investment to develop the experiment payload.
      • After numerous discussions and negotiations, NASA in Fiscal Year 1985 initiated a program called STORE (Shuttle Test of the Relativity Experiment) to perform part of the experiment. NASA's intention at that time was to build the final flight instrument, do an experimental rehearsal with the instrument captive on Shuttle after which it would be returned to earth for refurbishing and joining with its own space vehicle. The full spacecraft then would be re-flown on Shuttle and launched from Shuttle as a free flyer, assuming shuttle launch into the required polar orbit from SLC-6 at Vandenberg Air Force Base.
      • The plans for STORE were modified in consequence of the Challenger tragedy in 1986, the subsequent cutbacks in the number of shuttle missions, and the closure of the SLC-6. STORE was, however, continued as an instrument development program without a shuttle test.
      • In 1993, following two independent studies of the Gravity Probe B spacecraft, NASA approved selection of one contractor (Lockheed Martin Missiles and Space) and the start of the full GP-B flight program.
    7. What happened to Gravity Probe "A"?

      Gravity Probe A was a joint program of NASA-Marshall Space Flight Center and the Astrophysical Observatory of the Smithsonian Institution. It was also the first test in space to explore the structure of space and time. It is known to many scientists as the "Red Shift Experiment" or the "Clock Experiment." The Gravity Probe A (GP-A) payload was launched on June 18, 1976 at 7:41 A.M. Eastern Daylight Time from the NASA-Wallops Flight Center in Virginia. Unlike Gravity Probe B, GP-A was only in space for one hour and 55 minutes in an elliptical flight trajectory over the Atlantic. It attained a maximum height of 6200 miles above the earth before impacting into the Atlantic Ocean. Why was GP-A in space for such a short time? No accidents on the launch pad - it was part of the design of the experiment (unlike Gravity Probe B, which will be in a polar orbit over the Pacific Northwest for nearly two years). To yield an accurate and inexpensive experiment, GP-A required a flight path with a large change in the gravitational potential to provide a large gravitational redshift, and it required a flight path that kept the flight Hydrogen MASER in contact with the ground Hydrogen MASER during data collection.

      The official, formal Mission Objectives of Gravity Probe A are listed by NASA as the following:

      • The primary objective of the GP-A mission is to test a fundamental postulate of gravitation and relativity theories called the "Principle of Equivalence" to an accuracy of 200 parts per million by testing whether the principle holds for extended regions of space where the gravitational acceleration has considerably different values.
      • A secondary objective is the demonstration of a hydrogen MASER clock in space.

      For more information, read The basic scope of the Gravity Probe A experiment summarized from NASA News, 1976. Or read Gravitation Research Using Atomic Clocks in Space by Dr. R. F. C. Vessot, Smithsonian Astrophysical Observatory Principal Investigator, Gravity Probe A.

      In the GP-A mission, all payload systems appeared to have functioned properly, including the newly-developed hydrogen MASER, NASA officials reported. Successful tracking was maintained throughout the mission. NASA's Post Launch Mission Operation Report of GP-A dated February 14, 1977, states the following:

      The Principal Investigator, Dr. R. F. C. Vessot began data reduction and has reported achieving 150 parts per million accuracy. The prelaunch accuracy objective of 200 parts per million from the data has thus been surpassed. Based on this report, the mission is adjudged as successful.

      To learn more about Dr. Vessot's current research, have a look at The Hydrogen Maser Clock Project home page at the Harvard-Smithsonian Center for Astrophysics.

    8. What is the purpose of Gravity Probe B? Is it worth doing?

      (Click Here for the answer)

    Questions about Relativity:

    1. What is the General Theory of Relativity?

      The short answer: According to Einstein the presence of a gravitational field alters the rules of geometry in space-time. The effect is to make it seem as it space-time is "curved."

      The explanation: Einstein realized that Newton's theory of gravitation (the classically accepted theory) did not account for events over extremely large distances. In particular, Newton's theory says that the gravitational force between two objects is proportional to the masses of the objects divided by the square of the distance between the objects. This means that two stars will have a gravitational force between them and will be attracted to each other. How strong the force is depends on how massive they each are and how much space is between them. This makes sense when the stars are not too far away from each other. But what if they were really far apart, say in different corners of the universe? And what if the mass of one of the objects suddenly changed, due perhaps to its experiencing an explosive supernova where it would convert some of its mass to energy? Newton's theory implies that the change in the gravitational force between the objects should change instantly. Einstein wondered how these immediate changes in force could be possible. The theory of Special Relativity states that nothing can travel faster than the speed of light (Special Relativity theory has been verified in many experiments all over the world), so how could gravitational force changes be "transmitted" to the rest of the universe so quickly? This implied that perhaps space was not just some empty spot where things happened instantly, but rather that space itself, combined with time, must form some kind of fabric. The shape and form of this space-time fabric would govern the flow of signal transmission and the paths of the movement of light in a way similar to the way the lay of the land determines which way water flows during a rainstorm. This fabric would react to changes in mass and energy the way any fabric responds to changes in pressure and to movement. In fact, we say that mass-energy determines the shape of the fabric of space-time. A good way to imagine how a mass-energy (such as the Earth) affects its local area of space-time might be to imagine a stretched fishing net with a basketball in it. Where the net dips and twists is dependent on how the basketball moves and where it is placed. As soon as a mass-energy is introduced into an area, space-time is warped accordingly. Because space-time is warped, and light (and other electromagnetic signals such as radio waves) has to travel in space-time, its pathways follow the warp. Electromagnetic fields should also be affected by the curvature of space-time. We at Gravity Probe B are looking for predicted changes to the local electromagnetic field in accordance with the amount of warp, or curvature, created by the presence of the mass-energy of the Earth.

      For more information on General Relativity, check out the PBS program NOVA Online "Einstein Revealed" web site.

    2. How does GP-B plan to test the General Theory of Relativity?

      The earth is a mass-energy. According to General Relativity, as a mass-energy, it should create a little dimple in the local space-time fabric. It is also theorized that the daily rotation of the earth causes a twisting of the local space-time fabric.

      "This effect is known as frame dragging and it should manifest itself as a force that pushes a gyroscope's axis out of alignment as it orbits the Earth. [GP-B will be using four small, incredibly precise gyroscopes as its main tool for detection of relativistic effects on the local space-time fabric.] Gravity Probe B will attempt to measure the force, gravitomagnetism, giving scientists an important insight into how it might affect objects that are much larger than ping pong balls, such as black holes. At the same time, the gyroscopes will experience a much bigger force - the geodetic effect - which is a result of the warping of space-time predicted by Einstein (see Diagram). This force will tend to push their axes in a direction perpendicular to the frame-dragging effect which allow it to be measured separately. The geodetic effect is hundreds of times bigger than frame dragging and the experiment should measure its size with an accuracy of 0.01 per cent the most severe test of general relativity ever undertaken.

      While the geodetic effect was first detected in 1988, gravitomagnetism has remained hidden because it is extremely weak. To get some idea of how weak it is, imagine that the axes of the spinning spheres are a kilometre long. In the course of a year, this force would move the ends of the axes by the width of a human hair, an angle of only 40 milliarcseconds. Gravity Probe B is designed to measure this effect with an accuracy of 1 per cent but it will be no easy task the slightest interference from unwanted forces will overwhelm the results." - B. Ianotta. Music of the Spheres. New Scientist, 31 August 1996, pp. 28-31.

      One instance of gravitomagnetism has been detected by a team of astronomers using recent X-ray astronomy satellite observations. For more information, see the Marshall Space Flight Center space sciences feature article: "New Observations of Black Holes Confirm General Relativity."

    3. What is space-time?

      Space-time is a four dimensional description of the universe that includes the usual three dimensions of height, width and length and a fourth dimension of time. You might be wondering how time can be considered a dimension and why it can be lumped in with space. Consider the definition of time: time as we know it is really a man-made concept and is defined by physicists to be the measurement of a series of events. If one considers what time really is, one can see that it is simply the counting or measuring of things occurring, such as the vibrations of a quartz crystal in a watch, or the movement of the earth around the sun. Really, time does not exist as its own entity; the event is the true variable we must consider when thinking about time. Without events occurring, there would not be a way to measure time. Now, getting back to our space and time link, an event must occur in a space. That point in space is particular to the observer (or measurer) of the event. Therefore each point in space is associated uniquely with an event. Thus space and time are tied intimately together. One can also see how the perception of events are "relative" to the observer depending on his vantage point. Hence the name "relativity" for this branch of physics.

      One way to think about space-time is as a large fishing net. Left unperturbed and stretched out flat, it is straight and regular. But the minute one puts a weight into the net, everything bends to support that weight. A weight that was spinning would wreak even more havoc with the net, twisting it as it spun. The mass-energy of the planet earth represents a "weight" in our net of space-time, and the daily revolutions of the earth, according to Einstein's theory, represent a twisting of local space-time. GP-B will search for this twisting effect, which has never before been measured. Note that sometimes people ask for a three dimensional analysis instead. This can be difficult to visualize, but imagining space-time as a cube of Jello instead of a net seems to provide a decent analogy.

      For a more technical definition of space-time, see a definition from the National Center for Supercomputing Multimedia Online Expo, "Science for the Millennium."

    4. What is the Equivalence Principle?

      Einstein's equivalence principle postulates that there is no way of distinguishing locally between a gravitational field and an oppositely directed acceleration. This is a fundamental postulate to general relativity, tying mass and energy together. It has been partially verified by Gravity Probe A. For more information and understanding of the equivalence principle, check out "A Cultural History of Gravity and the Equivalence Principle."