Science Questions

How can jets issue from Black Holes if even light cannot escape?

Fri, 24th May 2013

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Casper Badenhorst asked:

Hi There


I was wondering how Relativistic jets can be shot out from a black hole even though light which has no mass can't escape one?




Casper Badenhorst

Sutton - London



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Nothing can escape once it has passed the event horizon, so the jets originate outside the event horizon.

What forms the jets has been a source of a lot of conjecture.

Observations of the jet from the active black hole at the center of the M87 galaxy suggest that the jet originates at the inner edge of the accretion disk around a spinning black hole. Astronomers think that magnetic fields in the turbulent superheated plasma propel a fraction of the accretion disk out near the poles of the black hole at speeds near the speed of light.

However, we may get a closer view towards the end of 2013, as a cloud of gas is accelerating towards the hypothesised black hole in the center of our galaxy. This should light it up with some interesting fireworks. evan_au, Fri, 3rd May 2013

It might be worth considering relativistic effects in the structure of a fast spinning black hole, i.e. material at the equator may be less entropically active than material at a core more or less beween the poles. This would be due to the spin. Material at the equater would be affected by gravity plus speed while the core material would be affected by gravity only.
The difference in activity may be so serious because of the spin that the black hole may never truely form because of energy and therefore mass being dumped in polar jets.
Bracewell, Sun, 16th Jun 2013

when gravitational collapse occurs angular momentum is conserved and any residual rotation speeds up.  This will continue until the rotation become so rapid that it prevents further collapse. 

It is somewhat conterintuitive that the way a gravitationally collapsing mass manages to rid itself of excess angular momentum and continue its collapse is to loose material along jets streaming from the direction of the poles of rotation rather than the equator.

This "bipolar flow" is seen in gas clouds collapsing to form stars and in its most extreme case from neutron stars and material collapsing towards a black hole.

Another fact that is not well known is that black holes of a given mass have a limit on the maximum amount of angular momentum that they can contain and that this limit is quite low compared with the likely angular momentum in any gas collapsing towards them and that the shedding of this angular momentum makes getting into the black hole quite difficult and requiring the rlease of a great deal of energy from the polar jets.

Finally black holes are incredibly tiny things their diameter is a linear function of their mass and a several solar mass black hole is only a few miles across compared with the millions of miles of a typical star which is itself a small object compared with the typical distance between stars. 

Even the very largest billion solar mass black holes are no bigger than our solar system

I just do not know why the standard texts say that this is any sort of problem and not well known and understood.  I think that it is just that they all tend to copy earlier texts and do not really think it all out for themselves. Soul Surfer, Sun, 16th Jun 2013

As long as it happens outside a event horizon there is nothing stopping radiation from leaving a black hole. If Earth became a black hole today it would do nothing to the stability of the solar system. They would generally speaking move as usual. The compression that creates a black hole doesn't change its mass (ignoring what gets lost before it becomes a black hole), yor_on, Mon, 17th Jun 2013

Do you realise that a black hole with a mass equal to that of the earth is just under 2cm in diameter. Soul Surfer, Mon, 17th Jun 2013

It's pretty weird, and yeah. Haven't used that one in a while though :) yor_on, Mon, 17th Jun 2013

I accept the text book explanations but I think a point is missed when there is a concentration on size (diameter) rather than the criticallity of a borderline  structure. Perhaps it was a bit misleading to say 'black hole' when I really meant a structure heading to become a black hole.
What is observed are large collapsed stutures with high rates of spin, but a black hole seems to be practical only when it loses all spin, which seems to be an exceptional object.

  Bracewell, Mon, 17th Jun 2013

There are two types of black holes. The Schwarzschild solution describing a non rotating black hole. That type we haven't found yet, as far as I know. The other type is rotating black holes, some of them spinning close to the speed of light. Those we have found, indirectly evidenced. And Soulsurfers explanation to why they spin so fast, is a analogue to what you can see when a skater spin, pulling in her arms to rotate faster. As the mass 'shrinks' into a black hole, its angular momentum must increase.
= is nice.
yor_on, Mon, 17th Jun 2013

Bracewell; The non rotating or "Schwarzschild" black hole is essentially a theoretical entity devised by modellers because it is the simplest sort possible and for many years it was the only one for which the space time equations could be solved. 

The probability of one being created by gravitational collapse in our universe (and probably any other universe that has evolved) is vanishingly small because all collapsing structure have interacted with other things and contain some turbulance or angular momentum.  The only case they may occur is at or below the planck scales where tiny non rotating black holes may appear (and very rapidly disappear) as quantum entities in the basic fabric of space time.

All that then remains other than the electrically charged black holes, which are a different story, is the Kerr solution which is a rotating black hole.

However It is possible by  using the "Penrose process"  to extract rotational energy from a rotating black hole by particle interactions in the "ergosphere".  This is because the rotational energy is not all hidden inside the main event horizon because of "gravitomagnetic" or "frame dragging" effects that the very high speed rotation causes.  Look the words in quotes up for more information. 

This process has some similarities with the standard accretion disc processes in star formation that cause bipolar flows from collapsing gas clouds  (normal electromagnetic forces are considered to have a big part in driving these if you look up the latest papers)

Returning to black holes. It may be therefore possible that black holes do lose their rotational energy on the other hand the material falling in will almost certainly have vastly more angular momentum than the hole could hold anyway 

During the much better studied process of the collapse of gas clouds to form stars it is estimated and measured that during the collapse the mass in the cloud has one million times more angular momentum than the final star.  See Astronomy and Geophysics Oct 2012 page 5.19 for an interesting article on "losing spin: the angular momentum problem" for more information.

It is an interesting aside to note that the formation of planetary systems and double/multiple stars is also an important part of the process of getting rid of angular momentum to allow gravitational collapse. Soul Surfer, Tue, 18th Jun 2013

Soul Surfer - good stuff. However, I would value your opinion on the following scenario.
Imagine that two atomic clocks become caught in the rotation of a large fast spinning star. One clock orbits at the equator while the other drifts over a pole. The two clocks rapidly become de-sychronised with the polar clock running faster.
So, surely what is good for atomic clocks must be good for all other entropic processes with more energy being generated in atomic reactions at the poles. Assuming the conditions are extreme then this might also contribute energy to jets.
Bracewell, Sat, 22nd Jun 2013

Another way you could end up with (almost) non-spinning black holes is if two black holes with similar mass but opposite spin directions merged together.

In some of the images of radio galaxies, you can see a "kink" in the emission lobes, where the axis of the relativistic jets has changed suddenly, perhaps due to the galactic black hole swallowing the black hole from a satellite galaxy, which has a smaller mass and different spin axis.

The extreme case of this is in X-shaped radio galaxies, where it appears that the axis of the jets has changed by as much as 90 degrees. evan_au, Sat, 22nd Jun 2013

A random thought: If the relativistic jets were turned so they fired in or near the plane of the galaxy, could this produce the burst of star formation seen in barred spiral galaxies? evan_au, Sat, 22nd Jun 2013

Some recent discoveries show a different way that matter can escape from the vicinity of a black hole.
Astronomers have found an outward flow of relatively cool dust above and below the black hole (as distinct from relativistic jets of matter). evan_au, Wed, 26th Jun 2013

evan - very interesting about the rejection of matter from very large objects.

Question - can matter moving in orbit be considered as truely moving with orbital energy? Surely everything in orbit is moving in a straight line, that is, as far as the objects know. Bracewell, Mon, 15th Jul 2013

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