In 1687 Isaac Newton (1642-1727) in his Principia mathematically explained how gravity curved the paths of earthly projectiles as described by Galileo and heavenly bodies as described by the laws of Kepler. In his pioneering work, Newton firmly established a scientific methodology to explain the functioning of the world. Newton was content to explain that all change in motions require forces rather than the whims of gods, spirits and ghosts used by earlier explanations. Newton left for others to ponder the mechanism by which gravity and other forces work.
170 years after the scientific revolution Newton precipitated, James Clerk Maxwell became good friends with Michael Faraday and undertook a decade long task of compiling equations that described Faraday's lines of electric and magnet forces. In the end Maxwell compiled a consistent set of equations that unified electricity and magnetism much as Newton had unified equations for heavenly and earthly motions. But unlike Newton's work, Maxwell's equations were based on a mechanism and specified a speed for the transmission of electromagnetic waves as well as electric and magnetic forces. Since visible light was one range of electromagnetic waves and light's speed had been previously measured, this speed remains known as the speed of light.
A few years later Albert Einstein (1879-1955) pondered how electric and magnetic forces could generate effects noticed by observers moving at different speeds. In 1905 Einstein wrote three papers, one of which proposed a theory of Special Relativity based on the assumption that each observer would measure the same speed of transmission for electric and magnetic effects, the same speed of light. But this assumption would required that the dimension of space in the direction of motion would be shortened by the motion, and that time would be a term in the equation as if it is a fourth dimension.
In 1908 Hermann Minkowski (1864-1909) had extended the long used theorem of Pythagoras where the speed of any object could be specified by the square root of the sum of the squares of the speed along perpendicular coordinates, x, y, and z. But time multiplied by the speed of light would have similar dimensions and could be subtracted from the sum allowing transformations in time as well as location: x2 + y2 + z2 - c2t2.
Einstein suspected that while Special Relativity was formulated on the basis of electricity and magnetism, all forces must be consistent with Special Relativity and the exchange of information at the speed of light. So the remaining force, gravity, would also be governed by similar equations. Einstein was also aware that inertial mass that resists acceleration in Newtons dynamics, F = ma, seems to be experimentally equivalent to gravitational mass which causes gravitational force in Newton's law of gravity: F = Gm1m2/d2. This seemed to imply an equivalence principle stating that there would be no experimental way to every distinguish the difference between the effects of gravity and those observed in other accelerated frames of reference. In 1915 Einstein announced his general theory of relativity which suggests that mass deforms the surrounding space so that, contrary to Euclid's geometry where the shortest distance between two points is a straight line, the shortest distance between two locations near any mass would be a curved pathway. For nearly all situations Einstein's theory makes identical predictions to Newton's theory. But Einstein proposed three distinctions:
Observations have been made for each predicted effect and the results found in agreement with Einstein's predictions. But because there are so few distinctions with Newton's theory, Einstein's theory of general relativity remains one of the least tested of scientific theories. Two years after Einstein proposed his theory, Austrian physicists Josef Lense and Hans Thurring proposed that not only would a mass distort nearby space, but a spinning mass would drag four dimension spacetime similar to how a rotating electric charge creates a magnetic field. While the spinning of the earth should create only a tiny force (gravity is much weaker than electricity), discovery of this gravitomagnetic force is the gravitational equivalent of discovering electromagnetism.
Stanford physicist Leonard Schiff and George Pugh at the Pentagon independently proposed in 1960 a satellite experiment to attempt to discover the tiny frame-dragging effect of this force. At least nine new technologies had to be invented and perfected such as spherical gyroscopes a million times better than any then available, and Super-Conducting Quantum Interference Devices (SQUIDS) to detect changes in gyroscope motions of approximately 1/40,000,000 of a degree. With nearly 100 Ph.D. dissertations written on this project, a satellite called Gravity Probe-B was designed, constructed and finally launched on April 2004.The Gravity Probe-B satellite is in an almost perfect circular polar orbit around the Earth at an altitude of 400 statute miles. The electronics compare the direction to the star pair IM Pegasus as viewed by the on board telescope with the spin axes of four 1.5 inch quartz gyroscopes which are the most perfect spheres ever made. The gyroscopes are maintained spinning frictionlessly at 1.82K in the middle of a large (9 foot) Dewar filled with liquid Helium. Experimental measurement of the gradually increasing angular deviation will continue through May 2005.