QotW - 11.10.16 - Why don't black holes explode once they lose enough mass?

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Offline thedoc

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If it takes a very massive star collapsing to form a black hole, and Hawking's radiation eats it away, then why doesn't it blow up after enough matter is eaten away?

I was listening to one of your previous Naked Astronomy shows and they stated that eventually the black hole will disappear.  If after awhile it no longer has enough mass to sustain being a black hole why doesn't it just blow up?
Asked by Dennis Slone

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« Last Edit: 18/10/2011 18:36:30 by _system »


Offline nommiiss

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Why don't black holes explode once they lose enough mass?
« Reply #1 on: 11/03/2011 10:51:12 »

Hi Dennis,

Very cool question,

You need mass for an explosion. If the blackhole is "eating its own mass" there will be no mass for an explosion.

This is not to say that it may collapse when the mass is not grate enough to sustain its self.

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Offline syhprum

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Why don't black holes explode once they lose enough mass?
« Reply #2 on: 11/03/2011 13:04:22 »
As the blackhole evaporates it will eventually reach the planck mass of 22 micrograms then disappear in a 2.2*10^10J burst of radiation.
This will only happen to primordeal blackholes as those created by the collapse of massive stars cannot evaporate as the temperature of CMBR is too high.


Offline Supercryptid

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Why don't black holes explode once they lose enough mass?
« Reply #3 on: 11/03/2011 21:21:02 »
I suppose what you are trying to ask is, if there is a minimum mass for a star to become a black hole, shouldn't the black hole no longer be capable of staying a black hole once it goes under that minimum mass? The answer is that any amount of mass can be a black hole so long as that mass occupies a region within it's Schwarzchild radius. That is, it can be a black hole if it is compressed enough. It's just that it takes a very heavy star in order to sufficiently compress matter to that point. Once it reaches that point, it's "stable" (except in regard to Hawking Radiation) and can stay that way.
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Offline yor_on

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Why don't black holes explode once they lose enough mass?
« Reply #4 on: 11/03/2011 22:49:54 »
This is all theory, but there is ideas of them exploding.

"But what happens when the tiny black hole evaporates so small that it becomes so tightly wrapped around the structure of a fifth dimension (other than the “normal” three spatial dimensions and one time dimension)? Well, the black hole will explosively show itself, much like an elastic band snapping, emitting energy. These final moments will signify that the primordial black hole has died. What makes this exciting is that researchers believe they can detect these events as spikes of radio wave emissions and the hunt has already begun…"

Measuring the Emission from the Fifth Dimension.

On the other hand, what goes on inside a black hole nobody knows. You might view it as a 'opening' into 'something else'. Assuming that the universe have its own way of 'enclosing' singularities they might just 'vanish' at some point, like our expansion make new 'space' just appear without any 'explosions' as far as I know. If you imagine the gravity in a black hole to be at 'max' and then assume that gravity is 'everywhere' you could imagine that a black hole is the natural state of gravity being only 'energy'. This 'energy' have no measurable size as we expect, much in the same way as point like particles like photons, so maybe that is what we all rest on, immeasurable gravity.
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Offline Soul Surfer

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Why don't black holes explode once they lose enough mass?
« Reply #5 on: 13/03/2011 00:02:07 »
Yes Black holes will eventually evaporate. 

The smaller the black hole is the faster it will evaporate so it will go out quite quickly in its last stages but it is not an explosion in the true sense of the word

BUT  and a very big but.  You have to put things in the correct perspective and time scale

A black hole will not evaporate until the cosmic microwave background is at a lower temperature than the evaporating black hole because all the time it is higher it will be soaking up more energy than it is emitting.   

Most black holes that are being formed from the collapse of a star are greater than 5 solar masses and they can get up to many billions of solar masses.

Let us consider the properties of a very small black hole  on this scale say 1 solar mass

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

a 1 solar mass black hole 

has a temperature of 61 nanokelvin  about  one ten millionth of the temperature of the CMB at the moment so it wont start evaporating for many millions of times that the universe has been in existence today and even when it does it will take  2 e +58 years to evaporate into a totally cold universe.  This is an almost unimaginably long time.

The final demise is a very small pop only managing to reach solar luminosity for the last few nanoseconds so one could only be detected if it was quite close.

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