# What is Gravitomagnetism?

Michael Anissimov

Gravitomagnetism, a theoretical idea around since 1918, is a predicted consequence of general relativity, from which it was derived. Its existence has been experimentally proven, but allegedly only once, and there are certain variants of the effect that are supported by the evidence to greater or lesser degrees. An international team claimed to discover the effect in the mid-90s, based on data from the LAGEOS I and LAGEOS II spacecraft. The measured effect was within 10% of that predicted by general relativity, although some scientists still doubt the validity of these results. In 2004, Stanford physicists launched Gravity Probe B, an extremely delicate gyroscope package, to measure gravitomagnetism in outer space with much greater precision. Its data is currently being analyzed.

After Einstein presented his theory of general relativity, it took decades to work out all of its predicted consequences. The most famous is the fundamental equivalence between matter and energy, demonstrated vividly by the atomic bomb. Lorentz contraction, the increase in mass and decrease in length seen by an outside observer looking at an object moving at relativistic (near-light) speeds, is another, and has been experimentally verified. Time is known to pass more slowly for objects moving at speeds close to that of light, or even significantly less - the effect has been observed in atomic clocks orbiting the earth.

This poorly exposed and tested consequence, gravitomagnetism, refers to the field that is supposedly created when a massive body rotates rapidly. Gravitomagnetism is misleadingly named - it is not magnetic - the force created emerges from gravity, not electromagnetism. But it is called gravitomagnetism because of the mathematical similarity between the equations that describe this effect and the creation of a magnetic field. In the same way that a magnetic field is created when a charged object rotates, a gravitomagnetic field is created when a massive body rotates. The math used to describe the two is functionally similar The effect could just as easily be called a gravitorotational field, a term which might be less misleading.

A very powerful gravitomagnetic field is expected to be observed around supermassive black holes rotating very rapidly. These black holes may have a mass millions of times greater than the sun and be rotating at a furious rate. Here in the solar system, though, the effect is predicted to be very small - on the order of a few parts per trillion in the overall scheme of gravitational interactions - making it difficult to observe without delicate sensors or proximity to massive planets or the sun.

Stanford's Gravity Probe B was extremely delicate. It contained a gyroscope with an object that was spherical to 40 atomic diameters, possessing a near-homogeneous density distribution. Designed to detect gravitomagnetism, the gyroscope was meant to measure "frame-dragging" - the source of the predicted effect is a small twist in spacetime created by the rotating mass. A spinning gyroscope in a vacuum should rotate with almost perfect uniformity, but it is predicted that gravitomagnetism disturbs this slightly. The simple way to visualize frame-dragging is to imagine a ball rotating on a stretched sheet, that creates a slight twist in the sheet at the same time as creating a major depression.

Another predicted effect is that when a satellite orbits the earth in what should be a perfect circle, it actually ends up in a slightly different place, due to the slight vortex created by the spinning earth. A difficulty in measuring gravitomagnetism is that the earth's equatorial bulge creates discrepancies in satellite/gyroscope behavior that must be correctly subtracted from other data in order to measure the magnitude of genuine frame-dragging.

Although a large amount of data has been returned from the Gravity Probe B, the analysis is ongoing. Gravitomagnetism is quite mysterious, and currently poorly understood. Whether the effect will have practical applications or not is something we will probably not know for at least a few decades.

Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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anon323132

I don't follow how there can be any uncertainty about the existence of gravitomagnetism. It is a direct result of special relativity, and is obviously required for special relativity to be correct.

Consider a planet like Saturn, with a ring system, passing an observer at near-light speeds. SR says that in the observer's frame, the rings, which are circular in the planet's rest frame, will be flattened into ellipses. Yet the only forces acting on the rings are gravitational. Simple Newtonian gravitation cannot explain such orbits. The observer can only explain what he sees as being the result of gravitomagnetic fields produced by the mass current.

anon180097

Saninassif is right. Explanation: disk galaxies once were elliptic or spheric galaxies, and the stars' orbits all swiveled into flat, prograde orbits with the same velocity, due to the central black hole's vortex. No dark matter! Look up Thierry De Mees Grav1

saninassif

I don't understand. Correct me if I'm wrong, but doesn't just about everything in our milky way and in many other galaxies, for that matter, rotate around the supermassive black hole in the center of the galaxies in the same direction?

This proves to me that the spin of the supermassive black hole is dragging space-time (gravitomagnetism) and making everything rotate around the supermassive black hole in the same direction. Am I wrong here? I mean, when you look at disk galaxies they all look like a vortex. Please give me your feedback.

anon132991

Gravitomagnetism solved a large amount of issues that remained mysteries for the General Relativity Theory.

anon126812

Gravitomagnetism looks much more natural than leading to many controversies GRT and it naturally unify with electromagnetism: e.g. these two interactions describes dynamics of rotations in 4D - spatial rotations leads to EM and small rotations of local time axis are governed by GM (5th section of arxiv:0910.2724 ).

So maybe spacetime is flat and what is curved is space alone - submanifolds of constant time (orthogonal to central axes of light cone).

anon102620

Gravitomagnetism is not the pure general relativity. Its derivation requires the formal adoption of the equation borrowed from maxwell's equation. People say it is an approximation but that is not even close. This is the biggest blunder of the gravitational physics. The gravitational dipole moment from the linearized theory of general relativity is the true magnetic gravity. Look at the papers of Dr. Eue Jin Jeong on dipole gravity.

anon22010

E=mc^2 demonstrated by the atomic bomb? Come on. Not any more than in a chemical bomb. Sure the rest mass decreases when energy is liberated, but so does it in any other process. The difference is simply one of scale.

Can we get rid of the misunderstanding that the strong force is the only force able to change the rest mass of particles? It is simply stronger.

anon1589

21 December 2012 will be a time of high probability for CMEs while Earth's magnetism is weak (already reversed in South Atlantic)and we are crossing the galactic equator. Therefore, it would be intelligent to see in the geological record what happened 25,800 years ago (when we crossed) and what happened 780,000 years ago (when we were weak magnetically). Was there a polar shift 25,800 years ago? And will a CME knock out our electronics?

anon676

There is a theory of dipole gravity (gravito magnetism) proposed by Dr. Eue Jin Jeong which predicts jets phenomena, explains dark matter problems and also the dark matter halo. It's a serious theory and you don't want to miss it.