Fall 2004 Table of Contents.
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 New & Now.


Battlefield Bacteria

The Last Word

Historic Disclosure

Genetically Altered

School for Scientists

Gravity Unbound

Papers and Profits


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Gravity Unbound

For more than three centuries, Isaac Newton's Law of Universal Gravitation has helped physicists and astronomers make sense of bodies in motion, whether here on Earth or in the heavens above. But no one, not even Newton himself, ever really believed the law was inviolable.

It took Einstein's General Theory of Relativity, for example, to explain why variations in the planet Mercury's orbit, known as precessions, could not be accurately calculated using Newton's formulas. Scientists following in Einstein's footsteps have since discovered other, more mysterious instances in which Newton's laws don't seem to apply.

One of the most perplexing involves the strange behavior of matter near black holes, collapsed stars whose infinite density creates gravitational forces so extreme that even light cannot escape from them. For years astronomers assumed that, if black holes did indeed exist, absolutely everything ensnared by their massive gravitational forces should be sucked right in. But this, it turns out, is not the case.

Using data obtained from international observatories, astronomers in the 1970s began describing phenomena associated with "astrophysical jets," columns of high-energy particles that blast away at near light speed from the centers of rotating, or active, black holes. Subsequent findings have yielded insights into how the jets work. But why the jets exist in the first place remains unknown.

Understanding the origins of these outflows is of particular interest to scientists who study gravity, among them Bahram Mashhoon, a professor of physics at MU, and Carmen Chicone, an MU professor of mathematics. This is chiefly because astrophysical jets are blasting through the huge gravitational fields created by black holes, fields so powerful that they would almost certainly affect the relativistic "curvature of space-time" predicted by Einstein.

And while Mashhoon and Chicone say they have yet to answer the "origins" question, they have nonetheless published some intriguing findings. The most notable involves a computation of the "threshold speed" at which the jets break free of black holes' gravitational pull.

"Black holes typically have what is called an accretion disk of orbiting material around them," says Mashhoon. "We found that the gravitational acceleration of particles moving faster than the critical speed of 70 percent of light speed may provide an explanation of how relativistic jets get started above and below the accretion disk around a rotating black hole."

Moreover, the scientists discovered, at a speed of less than 70 percent of light, particles in astrophysical jet plumes act as Newton might have expected. Above the 70 percent threshold, however, the jets slipped their Newtonian bounds, acting in ways that confound the laws of universal gravitation. This was a surprising outcome, one that could open the door to a more Einsteinian understanding of the universe.

Still, it is too early to draw overly broad conclusions, cautions Chicone. "We know this finding about gravity is not the only effect at work here. The next step is for astrophysicists to include these findings in their models and then compare the models with observations."

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Published by the Office of Research.

©2007 Curators of the University of Missouri. Click here to contact the editor.