A brief video introduction...
After spending some time searching for an appropriate article to match my interest, I eventually settled on one from space.com (link) that purported that one of NASA's satellites had collected empirical evidence supporting two of Einstein's theories of gravity. One of the interesting things about gravity is the discrepancy between how common our experience is with it versus our current level of understanding. It's one of many very mysterious and active areas of science. In a world where our current level of technology makes it appear that the only science left to do is to figure out how to stream movies to my cell phone, gravity allows us an area to experience wonder and beauty.
Of course, I tracked this Space.com article over to NASA's website (link) where they have announced the findings on May 4th, 2011. Attempting to find a more journal article, I am directed towards Physical Review Letters (link). Unfortunately, they are available by subscription only. In the meantime, I have located Stanford's website (link) who NASA funded throughout most of the project. The basic concept of this project was first brought up in 1959. NASA began reviewing the idea a couple years later and allocated funds to the project in 1964. It would be fifty years before the satellite actually launched. Delayed by politics, changing shuttle and rocket designs as well as engineering technicalities, the project would eventually sum an estimated
$750,000,000. Stanford University was put in charge of development and operations.
There were two ideas to be tested. The Newtonian theory of gravity as seen above is the classical notion of the inverse square law in normal three dimensional space. Einstein proposed more complicated notions about the nature of gravity and it's interactions with spacetime. Many of Einstein's ideas have been proven empirically and applied technologically. This specific experiment was designed to test the effects of frame dragging and the geodetic effect. The picture to the right above shows a rough image of both. The idea of massive bodies creating a curvature of space is known as the geodetic effect and is represented by the apparent depth of the plane. The swirling lines are representative of frame dragging as the rotating mass has a sort of friction that causes spacetime to move along with it.
A Newtonian object revolving around Earth would continue to rotate for all eternity whereas an Einsteinian object would slowly precess as the massive body of Earth rotates. Frame dragging was predicted by the general theory of relativity and derived by Josef Lense and Hans Thirring. This added the rotational variable alongside mass in attempting to calculate the gravitational field of an object. This effect would cause the precession of an orbiting satellite due to the distortion of spacetime. The Newtonian idea of rotation seems to imply that it is moving or not moving dependent on some absolute reference frame. Ernst Mach argued that without an absolute space to compare it to, rotation must only be defined relative to other objects. This implies a relationship between the two bodies which was postulated by the general theory of relativity. Less noticeably, the geodetic effect also provides predictions that diverge from Newtonian expectations. This was also predicted by Einstein's theory of general relativity and was worked on by Willem de Sitter.
In order to test this hypothesis, the satellite would take a gyro into orbit and observe it for a period of time. It would measure its precession about each axis carefully and compare its results to predictions. Even a simple task requires some elaborate engineering. Four gyroscopes were used in order to provide some redundancy. Each one was purported to be the most perfect sphere ever manufactured by man. They were created using very pure quartz and coated in a solid layer of niobium. The engineers claim that the imperfections around the edge of the sphere vary by as little as 40 atoms. To put this into scale, they said if you enlarged the sphere to the size of the Earth that the highest mountains and the deepest valleys would be no more than 8 ft in either direction.
The entire inside of the satellite takes awhile to prepare once in orbit because it must achieve a temperature very near absolute zero. This temperature is necessary because much of the experiment relies on superconductivity. The gyroscopes were each spun up to 5000 RPM using the xenon gas inside the satellite. Afterwards, the xenon gas was removed in order to create as perfect a vacuum as possible in order to prevent any frictional forces from interfering with the gyroscope. The spheres are then held in place without contact using an electric field trap. It took over 4 months to achieve the temperature, pressure and rotations necessary to begin scientific data collection.
In orbit for 17 months and 9 days, Lockheed Martin designed the capsule to keep the internal components both cool and shielded. The dewar was filled with supercooled helium to keep at -468 degrees Fahrenheit near absolute zero. Gravity Probe B engineered a new design to let the liquid and gaseous phases to interact. This allows the liquid helium to boil off keeping the supercooled fluid inside. The helium that was removed from the system was recycled as a minute corrective force to maintain a near drag-free orbit. The discharge of the helium was described as about a tenth of a human breath. Even at the great altitude of the telescope, there is still a slight atmospheric friction that interferes with the orbit of the satellite; however, this force can be counter acted. Designed to hold 4 ping pong balls, the ultimate satellite was over 24 feet tall.
The primary measurements are taken using the principles of superconductivity. The spherical gyroscopes were coated with niobium which was then supercooled to the point of achieving superconductivity. When a superconductor rotates, the effective lack of friction in the electrons of the metal produces a net movement of the nucleus versus the free electrons. As an analogy, it is as if then niobium moves underneath the electrons which results in a net motion relative to the sphere. The flow of electrons produces a magnetic moment. This magnetic moment is perfectly aligned along with the axis of rotation. This property is used to measure the axis of rotation while minimizing the observer's effect on the gyroscope. The magnitude of the magnetic moment is directly proportional to the spin. A device is placed within the field called the SQUID. This acronym describes a superconducting quantum interference device. The sensitivity if the SQUID is such that it can detect a field as weak as 5×10−18 T. The amount of precession predicted by the theory of relativity is quite small so the high degree of precision is very important. The accuracy of a milliarcsecond is the width of a hair seen at 10 miles. The types of angles being observed is also put into perspective as the angle formed between the top of Lincoln's eye to the bottom of a penny in New York as seen from Paris.
In the above image, the z-axis which is the axis of rotation is aligned down a guide star to measure against. It would be better to measure relative to quasars due to their stability but they are too difficult to see. The on board telescope separates the visual image of the star into four quadrants and is able to very accurately focus on those images. Instead of a quasar, the star IM Pegasis is tracked which has a very well analyzed motion in the sky. This motion is understood relative to other bodies in the sky such as the quasars. Thus, instead of measuring relative to the quasars directly they are measured to them indirectly. There are 3 quasars nearby which can be used. IM Pegasis' position moves due to our own annual parallax as well as IM Pegasi's own motion in the galaxy. It also has an orbital motion because of its binary interactions. Furthermore, changes shape slightly from flares and tidal forces. The motion of IM Pegasis far outweighs any motion in the gyroscope so it must be measured accurately in order to correct the data.
An amalgamation of data from several telescopes is used to accurately measure the movements of IM Pegasis which has become one of the most studied stars in the sky. They have been collecting data quarterly for years in order to maintain a stable and precise measurement. Sixteen telescopes have collected data including NASA'S Deep Space Station 43 in Australia, Natural Resources Canada's Algonquin Radio Observatory in Ontario, the Max Planck Institute's Effelsberg Telescope in Germany, the National Radio Astronomy Observatory's Very Large Baseline Array in Hawaii and their Very Large Array in New Mexico. These telescopes combined with others from across the world to form a virtual telescope as large as the Earth. The data is analyzed by the supercomputer at the National Radio Astronomy Observatory in New Mexico. The relative motion of IM Pegasi versus the three nearby quasars is mapped out in careful detail.
Gravity Probe B was lanched in April 2004. By August of 2005, it had finished data collection. The first phase of data analysis was released as early as February of 2006 but on May 4th, 2011 the final results were released and the project completed. The geodetic predictions were confirmed within 18.3 percent and frame dragging within 7.2 percent. Such extensive analysis was required because of equipment calibrations, tracking the guide star IM Pegasi, as well as taking into consideration several anomalies that happened during the data collection such as solar flares.
While the original article suggests that this experiment has proved Einstein correct, it is not quite so exhaustive. It has provided some additional empirical evidence on behalf of these two particular ideas. It has also been valuable because of all the different engineering accomplishments that were made possible due to the funding, research and the drive put behind this mission. For example, the porous valve that existed between the liquid helium and xenon gas layers was highly specific and efficient and is already finding other applications. In these areas, we have already experienced and been measuring to some degree of specificity the effects of gravitational lensing.
It has been suggested that with further speculation and experimentation, the raw data from Gravity Probe B may continue to shed light on other areas. In an opening speech introducing Albert Einstein, George Bernard Shaw announced, "There is an order of men who are makers of Universes. Ptolemy made a Universe which lasted 1,400 years. Newton also made a Universe which has lasted 300 years. Einstein has made a Universe and I can't tell you how long that would last." If the Academy Award and Nobel Prize winner was not eloquent enough, I think Monty Python aptly sums up the bulk of our empirical data. (Warning: if our professor was not indication enough, the British may be a bit odd, and this video may not be safe for work)
Digital Bibliography, accessed May 2011.
http://xkcd.com/
http://prl.aps.org/accepted/L/ea070Y8dQ491d22a28828c95f660a57ac82e7d8c0
http://www.nasa.gov/mission_pages/gpb/gpb_results.html
http://einstein.stanford.edu/
http://www.space.com/11570-nasa-gravity-probe-einstein-theory-relativity.html
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