Gravitomagnetism, a consequence of general relativity, has been experimentally proven but is poorly understood. Stanford’s Gravity Probe B is analyzing data to measure the effect, which is expected to be observed around rapidly rotating supermassive black holes. The effect is small in the solar system, making observations difficult.
Gravitomagnetism, a theoretical idea that has been around since 1918, is an expected consequence of general relativity, from which it derives. Its existence has been proven experimentally, but presumably only once, and there are some variants of the effect that are supported by the evidence to a greater or lesser extent. An international team said they discovered the effect in the mid-1990s, 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, to measure gravitomagnetism in space with much greater accuracy. Your data is currently being analysed.
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, vividly demonstrated by the atomic bomb. The Lorentz contraction, the increase in mass and decrease in length as seen by an outside observer looking at an object moving at relativistic (near light) speeds, is another, and it has been verified experimentally. It is known that time passes more slowly for objects moving at speeds close to the speed of light, or even significantly slower: the effect has been observed in atomic clocks orbiting the earth.
This underexposed and untested consequence, gravitomagnetism, refers to the field 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’s called gravitomagnetism because of the mathematical similarity between the equations describing 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 mathematics used to describe the two are functionally similar. The effect could just as easily be called a rotational gravitational field, a term that might be less misleading.
A very strong gravitomagnetic field is expected to be observed around very rapidly rotating supermassive black holes. These black holes can be millions of times the mass of the sun and rotate at a furious pace. Here in the solar system, however, the effect is expected to be very small – on the order of a few parts per trillion in the general pattern of gravitational interactions – making observations difficult without delicate sensors or proximity to massive planets or the sun. .
Stanford’s Gravity Probe B was extremely delicate. It contained a gyroscope with a spherical object at 40 atomic diameters, with an almost 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 near-perfect uniformity, but gravitomagnetism is expected to disturb it slightly. The easiest way to visualize frame dragging is to imagine a ball rolling on a taut sheet, which creates a slight twist in the sheet and at the same time creates a major depression.
Another expected 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 rotation of the earth. One difficulty in measuring gravitomagnetism is that the Earth’s equatorial bulge creates discrepancies in satellite/gyro behavior that must be properly subtracted from other data to measure the amount of true frame drag.
Although a large amount of data has been returned from Gravity Probe B, analysis is ongoing. Gravitomagnetism is quite mysterious and currently poorly understood. Whether or not the effect will have practical applications is something we probably won’t know for at least a few decades.
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