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Author Topic: Can a black hole die?  (Read 5693 times)

Offline Bree

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Can a black hole die?
« on: 14/07/2010 04:58:01 »
Stars are born and die. Does the same apply to a black hole?


 

Offline Soul Surfer

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Can a black hole die?
« Reply #1 on: 14/07/2010 08:52:31 »
Yes a black hole can die.  If it is in a totally empty universe a black hole will slowly evaporate by radiating Hawking radiation.  However the larger a black hole is, the lower is its effective temperature and the temperature of the background radiation in the universe must be lower than the temperature of the black hole before it starts to loose energy.   

The only processes that can create black holes in our universe (at least since the origin of the cosmic microwave background radiation) only create black holes that are several times the mass of the sun or larger. 

From       http://xaonon.dyndns.org/hawking/

A black hole with ten times the mass of the sun in isolation has the following properties

A diameter of about 36 miles   (60 Km)

A temperature of  6 x e -9 degrees kelvin  (compared with the 2-3 deg K for the background radiation)

A lifetime in an empty universe of   2 x e 61 Thousand million years.

so you see that our universe will have to get a lot older and colder before (normal sized) black holes start to evaporate. and even then they take an unimaginably long time to evaporate.

Most people just do not appreciate the scales of the numbers involved in black holes.
« Last Edit: 14/07/2010 08:55:56 by Soul Surfer »
 

Offline Soul Surfer

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Can a black hole die?
« Reply #2 on: 14/07/2010 09:12:16 »
It is interesting to note that a black hole with a life of approximately the time since the big bang i.e. 13 thousand million years has the following properties

Diameter 0.5 femtometer  (about the size of a proton)

A mass of 150 Million metric tons  (a modest mountain!) 

A temperature of around a million million degrees K

and a radiated power of 15,000 megawatts  ( about 20% of the UK electricity generating capacity)
 

Offline Soul Surfer

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Can a black hole die?
« Reply #3 on: 14/07/2010 09:23:48 »
There has already been some speculation about the LHC creating tiny black holes during its collisions.

It is easy to see from the reference above that a black hole of one microgram  (many orders of magnitude greater than the LHC could ever think of creating) lasts less than one plank time before evaporating. This is the shortest time that it is possible to conceive. and that the total energy released could not be greater than the energy put into the collision, which is easily contained by the apparatus so such fears are totally groundless.
 

Offline syhprum

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Can a black hole die?
« Reply #4 on: 14/07/2010 10:54:19 »
"Small" black holes must evaporate at an ever increasing rate as they lose mass hence their power output must continually rise, what is the end of this process ?.
 

Offline imatfaal

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Can a black hole die?
« Reply #5 on: 14/07/2010 12:52:05 »
Soul Surfer - Thanks for cool answer and link.
 

Offline Bree

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Can a black hole die?
« Reply #6 on: 16/07/2010 07:34:45 »
Soul Surfer, thanks for your reply. And thanks for having kept it simple. But let me ask you another question: From what I understand, black holes are invisible. What would happen if our solar system came near a black hole. Would our whole solar system be consumed? How and when would we know? And how long would this process take?
 

Offline CZARCAR

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Can a black hole die?
« Reply #7 on: 16/07/2010 12:43:19 »
 

Offline Soul Surfer

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Can a black hole die?
« Reply #8 on: 17/07/2010 09:15:45 »

The picture shows a black hole squrting out a powerful jet of particles with a high energy.

As everyone knows black holes have powerful gravity and will suck everything in that gets to close to them, BUT it is remarkably difficult to get sucked into a black hole. 

The reason for this is the conservation of angular momentum.

From above you can see that black holes are very small.  Around a million times smaller than the size of a typical star with the same mass.  If a particle falling towards a black hole has ANY velocity at right angles to the line between the black hole and the particle, it has angular momentum. As it falls towards the hole, by the law of conservation of angular momentum this sideways motion gets faster and faster. It's a bit like water going down a plughole, matter falling towards the hole swirls around in a vortex and has to loose angular momentum energy by ejecting some of the matter before the rest of the matter can fall into the hole.  Now if a black hole is trying to swallow a star for example,  The material swirling around prevents stuff from being thrown off at a tangent like water off a spinning bicycle wheel it can only get out at the poles and it can only punch its way out of the material around it if it is extremely energetic, that is, going at a tiny smidgin short of the velocity of light, like the perticles in the LHC or even much more energetic than that.

This means that black holes that are absorbing material have powerful jets coming out of their poles. 

The next question is why do we sometimes see only one jet and not two.  The reason for this is the doppler effect.  There is a high probability that one of the jets will be angled partially towards us and the other angled away.  The high speed of the jet means that the jet that is coming towards us is very much brighter than the one that is pointed away from us so we see one jet very much easier than the other and can miss it entirely.
« Last Edit: 17/07/2010 09:21:42 by Soul Surfer »
 

Offline Fozzie

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Can a black hole die?
« Reply #9 on: 17/07/2010 18:34:00 »
Quote
The high speed of the jet means that the jet that is coming towards us is very much brighter than the one that is pointed away from us so we see one jet very much easier than the other and can miss it entirely.
Surely the jet coming towards us will be blue shifted and the jet pointing away will be red shifted? How can it affect the brightness?
 

Offline Soul Surfer

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Can a black hole die?
« Reply #10 on: 17/07/2010 23:11:31 »
As the particles and radiation in the beam coming towards us they interact and scatter the intersteller or intergalactic particles and radiation and this happens mostly in the direction the beam is going i.e. towards us. This brightens the beam.  the beam pointed away from us produces very little backscattered radiation and this is also lower energy so it is much dimmer.
 

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Can a black hole die?
« Reply #10 on: 17/07/2010 23:11:31 »

 

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