A new spin on mysteries of gravity

Relativistic jets are among the most spectacular, and most mysterious, phenomena in the known universe.

Astrophysicists have been seeking to understand how these jets — highly energetic outflows that are thought to be moving along the rotation axes of gravitationally collapsed bodies such as neutron stars or black holes — are generated for years.

A recent discovery by MU professors Bahram Mashhoon and Carmen Chicone has led them to think they have found a novel gravitational effect implicit in Einstein’s General Theory of Relativity that might explain how these jets are formed.

According to Newton’s laws, gravity is always attractive and is inversely proportional to the square of the distance between massive objects. This means that the closer two objects are to each other, the stronger the gravitational attraction. Additionally, the gravitational attraction between two objects will be more strongly expressed on the sides that are facing each other, and thus closer, than it will on the opposite sides. The resulting tidal forces pull on objects and cause them to elongate and take on the shape of an egg or football in the direction closest to the object to which they are attracted.

So far, so good for objects with low speeds, say Mashhoon and Chicone.

However, according to their calculations, this tendency for an object to be stretched in the direction of gravitational attraction only applies to objects that are moving at less than 70 percent the speed of light. When acting on objects moving faster than this critical speed, these same tidal forces will stretch objects perpendicularly with respect to the source of the gravitational field.

Mashhoon and Chicone think that this gravitational effect, coupled with the violent collisions between particles that can cause them to be vertically oriented to the equatorial plane of massive objects, could explain how gravity might play a role in the formation and acceleration of relativistic jets.

According to their findings, the tidal force that gravity exerts on particles is contingent upon the speed and the direction that the particles are moving.

Catharinus Dijkstra, a post-doctoral associate at the MU Department of Physics and Astronomy, said that this finding was a surprise as it is contrary to conventional wisdom of the scientific community that had held since the time of Newton that gravity acted upon all objects in the same way regardless of speed.

“Now, according to Mashhoon and Chicone, it follows from the equations of the General Theory of Relativity that, under certain conditions (i.e. when particles in the disk move sufficiently fast), material in an accretion disk around a black hole or neutron star may be accelerated in directions perpendicular to that disk (due to the effects of gravity),” Dijkstra said in an e-mail.

Tidal forces will act to accelerate particles near the accretion disk, a ring of material that orbits black holes and similarly massive objects, parallel to the rotational axis when those particles are moving vertically with respect to the horizontal of the disk faster than 70 percent the speed of light. The collisions between these particles can then cause a cascading effect that could result in the formation of relativistic jets.

According to Mashhoon, this is a significant and unexpected consequence of General Relativity that, while certainly not the only factor involved in the formation and feeding of these jets, can now be included in the models of astrophysicists investigating the origins of this phenomenon.

According to Mashhoon, he and Chicone have taken steps toward the inclusion of their finding in plasma physics models of jets, and a Japanese team has also included it in their investigation into an aspect of the problem.

“However, none of these efforts is yet at the stage where our discovery would become the core of an astrophysical model for the generation of jets,” Mashhoon said. “In my experience, this would take time — possibly a decade or two.”

Chicone and Mashhoon have written a number of papers describing aspects of this effect over the last five years. Although Mashhoon developed the equations that describe this effect more than 30 years ago, he did not realize the implications of the critical speed at the time, and he credits Chicone’s expertise with differential equations in helping to explain it. Their work has been featured in the June 2004 issue of New Scientist, and their latest paper appears in the September edition of Physical Review.