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Right: Gravity Probe B in Earth orbit. We don't feel gravitomagnetism as we go about our everyday lives on Earth, but according to Einstein's theory of General Relativity it's real. When a planet or a star or a black hole Einstein tells us that all gravitational forces correspond to a bending of spacetime; the "twist" is gravitomagnetism.
What does gravitomagnetism do? Researchers led by physicist Ignazio Ciufolini have tried to detect the gravitomagnetic precession of satellite orbits. Precise laser ranging of the pair allows their orbits to be monitored.
But there's a problem: Earth's equatorial bulge pulls on the satellites, too, and causes a precession billions of times greater than gravitomagnetism. Did Ciufolini et al. Many scientists accept their results, notes Will, while others are skeptical. The spacecraft circles Earth in a polar orbit about miles high. Onboard are four gyroscopes, each one a sphere, 1.
If Einstein's equations are correct and gravitomagnetism is real, the spinning gyroscopes should wobble as they orbit the earth. Their spin axes will shift, little by little, until a year from now they point 42 milli-arcseconds away from where they started.
Gravity Probe B can measure this angle with a precision of 0. Left: An artist's concept of twisted spacetime around Earth. Any angle measured in milli-arcseconds is tiny. One milli-arcsecond is times smaller than that. The half milli-arcsecond precision expected for Gravity Probe B corresponds to the thickness of a sheet of paper held edge-on miles away.
A National Research Council panel, among them Cliff Will, wrote in , "In the course of its design work on Gravity Probe B, the team has made brilliant and original contributions to basic physics and technology.
Its members were among the first to measure the London moment of a spinning superconductor, the first to exploit the superconducting bag method for excluding magnetic flux, and the first to use a 'porous plug' for confining superfluid helium without pressure buildup.
They invented and proved the concept of a drag-free satellite, and most recently some members of the group have pioneered differential use of the Global Positioning System GPS to create a highly reliable and precise aircraft landing system. Right: GP-B's gyroscopes are the roundest objects ever made. Irregularities must be eliminated; otherwise the gyroscopes could wobble on their own without help from gravitomagnetism.
Physicists are both anxious and excited by Gravity Probe B. They're anxious because gravitomagnetism might not be there. Einstein's theory could be wrong a possibility held unlikely by most , and that would spark a revolution in physics. They're excited for the same reason. Everyone wants to be on hand for the next great advance in science. Near Earth, gravitomagnetism is weak.
That's why the Gravity Probe B gyroscopes wobble only 42 milli-arcseconds. But gravitomagnetism could be powerful in other parts of the Universe--for instance, "near a spinning black hole or a neutron star," says Will. A typical neutron star packs more mass than the Sun into a ball only 10 km wide, and it spins a hundred thousand times faster than Earth.
The gravitomagnetic field there could be very strong. Astronomers might have already observed the effects of gravitomagnetism. Some black holes and neutron stars shoot bright jets of matter into space at nearly light speed.
These jets come in pairs, oppositely directed, as if they emerge from the poles of a rotating object. Theorists think the jets could be powered and collimated by gravitomagnetism. Left: An artist's impression of space and time twisting around a spinning black hole. Credit: Joe Bergeron of Sky and Telescope magazine.
In addition, black holes are surrounded by disks of infalling matter called "accretion disks," so hot they glow in the x-ray region of the electromagnetic spectrum. Gravitomagnetism again? Here in our solar system gravitomagnetism is, at best, feeble. This raises the question, what do we do with gravitomagnetism once we've found it? The same question was posed, many times, in the 19th century when Maxwell, Faraday and others were exploring electromagnetism.
What use could it be? Today we're surrounded by the benefits of their research. Light bulbs. Washing machines.
The Internet. The list goes on and on. What will gravitomagnetism be good for? Is it just "another milestone on the path of our natural quest to understand nature? Or something unimaginably practical? Time will tell. Editor's note: Physicist Clifford Will, whom we interviewed for this story, is not a member of the Gravity Probe B team.
He is, however, an expert in General Relativity and an independent reviewer of the Gravity Probe B project who has sat on several mission review panels at the invitation of NASA. Magnetic Gravity: Written out in full glory, the equations of General Relativity are intensely complicated. Indeed, they have been solved in only a few special cases. One of them is the case of weak gravity, like we experience here on Earth.
In the "weak field limit," Einstein's equations reduce to a form remarkably like Maxwell's equations of electromagnetism. Terms appear that are analogous to the electric field caused by charges and the magnetic field produced by the flow of charge.
The "electric terms" correspond simply to the gravity that keeps our feet on the ground. The "magnetic terms" are wholly unfamiliar; we don't sense them in everyday life.
The best place to measure gravitomagnetism is in Earth orbit. Just as a spinning ball of electric charge produces a well-defined magnetic field, a spinning mass such as Earth is expected to produce a well-defined gravitomagnetic field. There's a problem with your browser or settings. Follow this link to skip to the main content. Text Size. In Search of Gravitomagnetism.
Gravitoelectromagnetism , abbreviated GEM , refers to a set of formal analogies between the equations for electromagnetism and relativistic gravitation ; specifically: between Maxwell's field equations and an approximation, valid under certain conditions, to the Einstein field equations for general relativity. Gravitomagnetism is a widely used term referring specifically to the kinetic effects of gravity, in analogy to the magnetic effects of moving electric charge. The most common version of GEM is valid only far from isolated sources, and for slowly moving test particles. The analogy and equations differing only by some small factors were first published in , before general relativity, by Oliver Heaviside as a separate theory expanding Newton's law. This approximate reformulation of gravitation as described by general relativity in the weak field limit makes an apparent field appear in a frame of reference different from that of a freely moving inertial body. This apparent field may be described by two components that act respectively like the electric and magnetic fields of electromagnetism, and by analogy these are called the gravitoelectric and gravitomagnetic fields, since these arise in the same way around a mass that a moving electric charge is the source of electric and magnetic fields.
Gravitomagnetic Induction Predicted From Einstein's Theory