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Posted 10.24.06


Properties of Gravity Altered Near Black Holes, MU Researchers Find

MU researchers found effects of gravity different for particles moving faster than 70 percent the speed of light

COLUMBIA, MO - Gravity, as Isaac Newton explained it, is one of the fundamental forces in nature: few people are surprised, for instance, that apples fall to the ground instead of flying into the sky. But research by two scientists in the College of Arts and Science at the University of Missouri-Columbia has found that gravity may not behave the same way when high speeds are involved.

Bahram Mashhoon & Carmen Chicone.Working with the modern theory of gravity, Einstein's general relativity, the research team discovered a threshold speed that causes particles near a black hole to be affected by gravity in the opposite manner than expected in Newtonian gravity. Black holes, objects with gravitational fields thought to be so strong that nothing, including light, can escape them, are some of the most mysterious objects in the universe.

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Bahram Mashhoon, professor of physics, and Carmen Chicone, professor of mathematics, worked together to study astrophysical jets, or columns of particles that shoot out at high velocities along the rotational axis of some black holes. The origin and motion of astrophysical jets have been significant unsolved problems in high-energy astrophysics, and what produces such activity around a black hole, called the central engine, has been a mystery. Mashhoon and Chicone discovered a critical speed, which is a threshold speed equal to about 70 percent the speed of light. If an object is moving faster than the critical speed, the rules of gravity are contrary to expectations based on Newtonian gravity. If an object is moving slower than the critical speed, gravity acts as expected.

"Many people have studied the properties of astrophysical jets, but we looked at the question of why they exist," Mashhoon said. "Black holes typically have what is called an accretion disk of orbiting material around them. We found that the gravitational acceleration of particles moving faster than the critical speed of 70 percent the speed of light may provide an explanation of how relativistic jets [jets of particles moving with speeds close to the speed of light] get started above and below the accretion disk around a rotating black hole."

Mashhoon and Chicone, along with the scientific community, were surprised by the findings. Mashhoon found the equations of motion used to describe the critical speed of particles approximately 30 years ago, but at that time, the problem he was working on did not lead him to the current discovery. Later, working with Chicone, the pair made their discovery of a critical speed.

"We know this finding about gravity is not the only effect at work here," Chicone said. "The next step is for astrophysicists to include these findings in their models and then compare the models with observations."

Mashhoon and Chicone have published 10 scientific papers on this subject, two of them with Brian Punsly of the International Center for Relativistic Astrophysics in Rome. Their most recent paper was in the September 2006 issue of Physical Review D.



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