Science Questions

What are jets in black holes?

Mon, 18th Apr 2016

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Francois Nel asked:

We often hear that blackholes are so massive that even light cannot escape its grasp, therefore light bends when close a blackhole. Come to close and light enters the event horizon and its lost.


Then, why do we hear that black holes spew out particles? Do the particles travel faster than light then?




Francois Nel


Kat Arney put this question to Cambridge astronomer Matt Middleton... A black hole

Matt - Jets are the most powerful events in the universe - jets from black holes.  They carry away huge amounts of material and very often theyíre moving extremely close to the speed of light - say 99.9% of the speed of light. Iím sure that we all roughly remember around our GCSEs - something like this, where you could work out the kinetic energy of a moving body from 0.5 x mv2. Take that mass - oh Katís wondering about it, alright.

Kat - Iím a biologistÖ

Matt - Youíre a biologist - you donít do equations. So if you take the mass thatís coming out of it, take the velocity, clearly thereís a huge amount of energy.  And, in fact, thereís a very nearby jet coming from a supermassive black hole called CenA, and the amount of power that is coming from that is 10 to the 12 times the power that is coming from the sun.

Chris - How do we know the jets there?

Matt - You can see it.  So these jets in particular emit everything from optical all the way through to x-rays and probably beyond.

Chris - Ah, so the stuff coming out which is radiating thisÖ

Matt - Absolutely.  It radiates acrossÖ

Chris - So is it just radiation or is it particles.  What is in the jet?

Matt - So it is particles and those particles are radiating.  So essentially for those who are on a geek out for some science you have magnetic fields and you have electrons that spiral around those magnetic fields and, because theyíre constantly changing direction, they have acceleration.

Chris - Why are they firing out of the black hole and where from the black hole are they coming from?

Matt - Okay. So, the point is these are not actually from inside the black hole.  The old adage of, you genuinely cannot get out of a black hole, is true.  You cannot escape from it.  And in fact, we should point people to the podcast. But theyíre coming from close to the black hole and, in fact, in another nearby supermassive black hole, people have been able to use radio interferometers.  Thatís when you have multiple radio dishes, and they provide a very high angular resolution view of these structures and theyíve been able to work out that itís coming from about 5 times the size of the black hole, above the black hole.  So thatís incredibly close.

Chris - Do we know what concentrates the material into a jet?  Why isn't it just sort of spinning round, getting excited, and then just radiating in all directions like our sun radiates radiation at us in all directions.

Matt - Sure.  So if you ever ask an astronomer a question they canít answer, theyíll always say magnetic fields or dust. It turns out it is magnetic fields and it genuinely is the answer.  We know there are magnetic fields there because we see what we call synchrotron emissions, so these are the electrons spiralling around magnetic field lines.  And thatís why you basically collimate all this emission thatís coming away from the black hole through these magnetic fields, so it actually does look like a jet. Itís perhaps worth mentioning that thereís aren't the only particles that we see from black holes, thereís also Hawking radiation and thatís when the black hole itself decays.  So you have a particle thatís created on the event horizon.  Itís actually is a particle-antiparticle pair. One of them goes into the black hole, the other one gets kicked out and thatís how black holes decayÖ

Chris - The black hole loses a little bit of mass and that makes it shrink?

Matt - Absolutely!

Chris - So black holes should evaporate over time?

Matt - And thatís why weíre not going to be destroyed by the Large Hadron Collider.

Kat - Hooray!

Chris - Thatís reassuring to know isnít it Kat?

Kat - It is reassuring.

Chris - Thanks Matt.



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The jets are not issuing from the black hole itself (i.e. not from beyond the event horizon) but from the region surrounding the black hole.  When there's a lot of material falling in towards the black hole, there are a lot of collisions and it gets very hot.  This heating is what's believed to produce the jets. burning, Sat, 27th Jul 2013

Astronomers do not yet have a close-up view of the accretion disk around a black hole, so there are a number of competing theories.

However, we are now getting good images of the effects that twisted magnetic fields on the Sun's surface can launch a Coronal Mass Ejection at speeds far faster than the Sun's escape velocity. The calculations to simulate these plasma/magnetism interactions are very computer-hungry, which makes it hard to resolve different theories by software modelling.

While the Sun's CME originate mostly in the wide equatorial band that supports sunspot activity, some other stars like neutron stars, protostars and T-Tauri stars can exhibit jets which exit along the poles, which exceed the escape velocity of the originating star.

The common theories suggest that an accretion disk around the star heats up until it becomes a conductive plasma. The plasma resists movement of a magnetic field through it, but the accretion disk is spinning around the central object. So the magnetic field gets twisted, which stores energy. When the magnetic field lines reconnect, it releases immense amounts of energy, which can propel gas from the accretion disk out along the poles.

To reach the enormous energies required to extract matter from a black hole's accretion disk and propel it into open space, some astrophysicists have suggested that "Frame Dragging" (a relativistic phenomenon near rotating black holes) may contribute to the twisting of the magnetic field. However, the source of material from the accretion disk lies outside the event horizon, which is the "point of no return" for matter falling into a black hole.


Speculation: The Sun's chaotic magnetic field near sunspots gets progressively more twisted over a period of days until it suddenly "snaps" in a very short time (seconds), launching a CME.
One imagines that in more active stars, with an accretion disk, the magnetic field would be twisted much more quickly - perhaps hours or minutes. When this snapped, it could release a "burp" of gas. For a neutron star, presumably the tangling could happen in milliseconds or microseconds.
Suggestion: Astronomers could look for cyclic patterns in the intensity of the polar jets from active stars. evan_au, Sun, 28th Jul 2013

  I have a theory that might suggest that light IS actually bending around the the black hole and that's what is being captured in photographs (light that hasn't been captured yet).  With refracted light waves, one can actually see the light, such as in a prism.  I figure black holes are actually what can be called as the "perfect bond", which can neither move, nor be broken by any of known principle of breaking bonds (chemical or current energy solutions), and is why light can't escape and by which I will describe why in the next section.
  I believe that in a black hole, the density of the "hole" itself has made it so that negative electrons are so tightly packed that they actually bond with their positive counterparts, the proton: leaving no room for movement within the subspace of the area surrounding the atom(s).  In other words, since the mass in a black hole is so densely packed, the basic atomic structure is disrupted into a state of "absolute zero" movement, creating an otherwise unbreakable bond.
  Just as a side noted on a similar subject, I think the only way to "rip" space time would be to place two black holes of the same density next to each other and closely enough to start pulling space away from itself.  However, they cannot be far enough away that they become obsolete in their pulling power and they can't be too close to create a gravitational anomaly that would end up sucking one into the other, and since space and time are not uniform, one WOULD eventually suck the other into itself. That's all I have to post for now and I hope this helped :-) maximrob81, Fri, 2nd Aug 2013

maximrob81 says

There is such an object: it is called a neutron star, made out of electrically neutral neutrons (strictly speaking this formation also requires another ghostly particle called a neutrino). While it is incredibly dense, the neutron star does not have the event horizon that surrounds a black hole.

However, a neutron star does not have "absolute zero" movement, as this would imply a temperature of absolute zero (about -273C). Neutron stars form from supernova explosions, and have surface temperatures of up to a million degrees - some spinning neutron stars have even been seen to flash rapidly in a telescope.

You are quite right that "one WOULD eventually suck the other into itself".

The mathematics of this scenario have been studied in some detail. It is expected that this would result in the two black holes orbiting each other, and radiating gravitational energy into space. This causes the two black holes to spiral in towards each other, spinning more rapidly around each other until they coalesce into one black hole with a mass of both black holes added together.

A number of experiments around the world are looking for this pattern of gravitational waves (eg LIGO) - but so far without success.

This process would be very disruptive for any matter or solar systems in close proximity to the spiralling black holes, but it wouldn't really qualify as a big rip in space.

In a sense, a single, stellar-mass black hole can do a pretty good job of ripping matter apart - if you tried flying too close in a spaceship, the gravitational attraction on your feet would be much greater than the gravitational attraction on your head, and you would get stretched into human spaghetti before being consumed by the black hole. evan_au, Fri, 2nd Aug 2013

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