Relativistic jet. The environment a

round theAGN

 where the relativistic plasma is collimated into

jets which escape along the pole of

the supermassive black hole.

Courtesy Wikipedia

Polar jets

(This article is speculative at this stage. It may well be completely wrong)

 Polar jets are known to emanate from the poles of rapidly spinning black holes but the mechanism that drives them is as yet unknown. The most popular explanation is that they are driven by existing (interstellar) magnetic fields that are twisted by the rotating spacetime around a rotating black hole to form a tight bundle. A similar process to what is observed in the Sun, when great masses of material are sometimes ejected. Although this explanation works well for the Sun, there are differences. First, the jets with black holes are highly collimated, travelling in straight lines for hundreds of thousands of light years - larger than the width of most galaxies. Secondly, they are extremely powerful, with speeds of up to eight-tenths of the speed of light. Thirdly the contents of the jets include neutral particles. And fourthly, a length of hundreds of thousands of light years implies stability and collimation are maintained over the same period.

Now consider first, an extremal black hole. Matter in the vicinity will form an accretion disk. Such disks emit copious X-rays, and the loss of energy in generating these allows particles in the disk to slow down and move to a closer orbit. This process can continue until they reach the innermost stable circular orbit (ISCO). Once there, the existing theory requires that the particle must fall all the way to the event horizon. For the theory presented here, this presents a problem as an extremal black hole cannot absorb any more mass.

The only conclusion is that the in-falling mass must be wholly converted into radiant energy. If the mass is unchanged, this is likely to be gravitational radiation. Copious amounts of it. One kilogram of matter will generate an amazing \(8.98755179 × 10^{16} \) joules of radiation by the well-known equation \(e=mc^2\).


Elliptical galaxy M87 emitting a relativistic jet,

as seen by the Hubble Space Telescope.

Courtesy Wikipedia

Now for a little diversion into gravitational radiation. One amazing fact is that a beam of gravitational energy is self-collimating. This is so because, remembering \(e=mc^2\), any radiation has mass and attracts adjacent radiation by gravitational attraction. This is true for electromagnetic waves as well, but as gravitational waves are longitudinal waves, there is no limit to this attraction to the wavelength involved. The beam can contract all the way down until it forms an event horizon. In a normal beam, any divergence in the original beam will override the self-collimation effect, but with the massive power of the waves generated by an extremal black hole, it is argued that a string of small black holes will form. Another effect will then take over, limiting the lifetime of these black holes: Hawking radiation. The smallest black holes evaporating totally in a short distance.

This much is true for an extremal black hole but must be true for all spinning black holes, but to a lesser degree, because a defined fraction of any in-falling matter will add to the black hole's mass and spin.

A supermassive black hole is likely to attract larger lumps of matter at one time. Imagine an Earth-size body in the ISCO of such a black hole. The black hole radiated would have a substantial size and hence lifetime.

Another effect must be considered. Whenever radiation is converted into a black hole, the black hole has rest mass, and consequently can no longer travel at the speed of light. Conservation of linear momentum will then determine the resulting velocity.

The lifetime of a black hole is given by

\[ t_{ev}=\frac{5120\pi G^2M_0^3}{\hbar c^4}\]

where \(t_{ev}\) is the evaporation time, \(M_0\) the black hole mass, \(G\) is the gravitational constant. It follows that for a 100,000 light year jet, travelling at 80% of the speed of light, this would mean black holes of a size of approximately 12.2 mg.

This proposal is only one possibility, and I put it forward in the hope of raising a discussion, rather than imagining that it will be the last word. Have you a better idea?