Can black holes explode?

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Topic 195290

I am (obviously) not a physicist, but I have had this nagging question in the back of my mind for some time now. So instead of keep pondering over it, maybe someone can it explain this to me.

I know that gravity is supposed to be so strong in black holes that nothing can escape from them, except maybe for the famous Hawking radiation. This seems to indicate that they cannot explode, because their mass is so great that every internal repulsive force within the black hole that might exist cannot lead to energy/mass escaping the gravity well. Right?

However, this doesn't seem to reconcile well with the Big Bang theory. Given that at T=0 all mass/energy of the universe was 'bundled' within a singularity, shouldn't the gravity have been similar to a black hole? However, it 'exploded' anyway. I am not sure why. Was it somehow caused by the massive expansion of space/time (or was it the other way around, did the 'explosion' cause that expansion)?

Given that mass and energy could move away from each other under the immense gravity conditions that must have existed during the beginning of the universe, doesn't that mean that at least forces must exist within the universe that could let black holes explode? Or am I just missing something?

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Can black holes explode?

Quote:
... gravity is supposed to be so strong in black holes that nothing can escape from them, except maybe for the famous Hawking radiation. This seems to indicate that they cannot explode, because their mass is so great that every internal repulsive force within the black hole that might exist cannot lead to energy/mass escaping the gravity well. Right?

Pretty much so.

Gravity is actually the weakest of all the forces. However, when you add enough mass into a small enough volume, you then also add up the gravity to something significant.

You may well get radiation and 'explosions' from material whilst it is swirling in towards the event horizon of the black hole. Aside: Due to relativistic effects, we can never see material pass over the horizon because we would have to wait an infinitely long time from our viewpoint (time appears to slow to a standstill at the event horizon).

Quote:
However, this doesn't seem to reconcile well with the Big Bang theory. Given that at T=0 all mass/energy of the universe was 'bundled' within a singularity, shouldn't the gravity have been similar to a black hole? However, it 'exploded' anyway...

The material/energy that was the "big bang" might have been a 'singularity' or something else. Regardless, the big bang didn't 'explode' so much as 'condense out' of time and space as time-and-space itself expanded. The material in the universe at that early time expanded and cooled as the universe itself expanded for the material/energy to condense into the matter that we have in our universe today.

Quote:
Given that mass and energy could move away from each other under the immense gravity conditions that must have existed during the beginning of the universe, doesn't that mean that at least forces must exist within the universe that could let black holes explode? Or am I just missing something?

You're just missing the effect of our assumptions that it is the very fabric of the universe itself that did the expanding. All our galaxies and stars are just a side effect of the condensation of matter resulting from that expansion.

It's a bit like how water vapour will condense into clouds of steam and then water droplets as you cool a volume of humid air. Except for the case of our universe, you are talking energy and subatomic particles instead of water.

Which leads onto an intriguing question:

Could a black hole be created by concentrating enough electromagnetic radiation to converge into a small enough volume?

Light is in some respects "mass-less" yet it exerts a radiation pressure, and E=mc^2 ...

As for the "big bang", we all might be just one big 'accident'!

Keep searchin',
Martin

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[DPC]Division_Brabant~Schaduwtje
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Thanks a lot for your answer

Message 99389 in response to message 99388

Thanks a lot for your answer ML1!

Okay, I understand that the fabric of space-time itself expanded. However, what I don't understand is that even though all matter was very close together at big bang time, that it didn't collapse into a gigantic black hole. Instead of collapsing, matter seems to have spread with expansion of space-time. Why?

And if it means that this is caused by expanding of space-time, does that also mean that the expansion of space-time can rip a black hole to shreds?

Quote:
It's a bit like how water vapour will condense into clouds of steam and then water droplets as you cool a volume of humid air. Except for the case of our universe, you are talking energy and subatomic particles instead of water.

I am not sure how far the analogy goes, but were there conditions in the early universe that prevented matter/energy to clog up and form black holes?

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RE: ... Instead of

Message 99390 in response to message 99389

Quote:
... Instead of collapsing, matter seems to have spread with expansion of space-time. Why?

The big bang started off so hot that gravity couldn't exist.

Quote:
And if it means that this is caused by expanding of space-time, does that also mean that the expansion of space-time can rip a black hole to shreds?

Not as far as I'm aware. The universe is now rather cool and gravity has taken hold.

It would be rather interesting if black holes due to their relativistic speed spins do actually have finite dimensions and become large enough to shed material away from their event horizon in a more spectacular way than Hawking radiation...

Quote:
Quote:
It's a bit like how water vapour will condense into clouds of steam and then water droplets as you cool a volume of humid air. Except for the case of our universe, you are talking energy and subatomic particles instead of water.

I am not sure how far the analogy goes, but were there conditions in the early universe that prevented matter/energy to clog up and form black holes?

It was all ridiculously too hot. So hot that you had a writhing soup of subatomic particles before it later became cool enough for physical effects to allow atoms and radiation to exist. All a rather different realm of physics that only cosmic rays and our big atomic colliders can approach today on Earth.

I'm sure someone must have a good link to a summary of our present theories and physics for the Big Bang.

Keep searchin',
Martin

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tullio
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I don't have any link but

Message 99391 in response to message 99390

I don't have any link but "The first three minutes" by Steven Weinberg might be a good starting point.
Tullio

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RE: The big bang started

Quote:
The big bang started off so hot that gravity couldn't exist.

Interesting. Never heard about this before. With this remark in combination I found this article on Wikipedia that explains some of this stuff.

So the basic answer to my first question is that our universe only got the properties it has now (i.e. all mass/energy + forces) after the Inflationary epoch, at which point this mass/energy was already scattered so much that it couldn't collapse into a gigantic black hole anymore. Even though this is still mostly speculation, at least it sounds reasonable. In other words, the context of the big bang differed significantly from the context of black holes. Right?

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RE: I don't have any link

Message 99393 in response to message 99391

Quote:
I don't have any link but "The first three minutes" by Steven Weinberg might be a good starting point.

Thanks, I will look into it. Even though, I am not too fond of speculative science as it is wrong too often.

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RE: Thanks, I will look

Message 99394 in response to message 99393

Quote:

Thanks, I will look into it. Even though, I am not too fond of speculative science as it is wrong too often.


There is an Italian physicist, prof.Antonino Zichichi, who says that "Galilean science" is based upon repeated experiments, like dropping weights from the Pisa Leaning Tower or smashing protons against protons like at LHC. But we cannot repeat the Big Bang nor a supernova explosion, so much of modern science is just speculation.
Tullio

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RE: But we cannot repeat

Message 99395 in response to message 99394

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But we cannot repeat the Big Bang nor a supernova explosion, so much of modern science is just speculation.
Tullio

Maybe so, but I see a big difference between a theory like the "Big Bang" theory for which we do have evidence in the form of accelerating expansion of the Universe, and verification via predictions of the CMB. Compared to stuff about which we know that we don't know much. Since we don't know the nature of black energy, dark matter, gravity on quantum scales, accelerated expansion, supersymmetry and all that kind of stuff that is related to this theorizing, these theories seem like wild semi-educated guesses on very incomplete information at best.

I would be really surprised if black holes wouldn't exist, even though we can't see one directly. There is plenty of circumstantial evidence that they do exist. I also would be really surprised if the timeline of the big bang as now proposed will hold up over time when we learn more about the nature of these things which we don't understand now.

But of course, it is possible that I don't know enough about the evidence that was used to construct the Big Bang's timeline as it is.

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RE: But of course, it is

Message 99396 in response to message 99395

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But of course, it is possible that I don't know enough...

Science is always the game of pushing back the frontiers of ignorance. You learn a little more to better appreciate all that it is you don't know.

Hypothesis and the testing of theories is a good game of leaping into the unknown to see what you might find. The testing and proving of those theories make them much more than just a 'guess'...

The first ideas in the search might indeed be nothing more than just a 'guess', but who knows!

Keep searchin',
Martin

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Mike Hewson
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Firstly : there isn't yet a

Firstly : there isn't yet a theory that deals adequately with the centre of a black hole or whatever existed at the instant of creation of the universe.

The Big Bang theory is a descriptive timeline that begins an instant after 'creation', indeed The First Three Minutes is an excellent summary of what observational evidence we have that leads us to think of that as actually occurring, compared to other propositions like the Steady State idea. Written in the 1970's by Steven Weinberg ( a subsequent Nobel winner in the area of particle physics ) it begins about 10^(-35) of a second from a point in time that, if we keep running the projector backwards, we would label as a point of infinities ( mass, density, energies etc ). Cosmic Inflation talks of even earlier fragments of time ( all this discussion assumes a meaning to the concept of time 'then' ). The CMB observations describes a surface of light scattering formed some 300,000 years later. Detail aside, the summary is : the universe is expanding, it was hotter and denser before than it is now, and we can't track using evidence available today what occurred beyond about 14 billion years ago ( give or take ). It's not a failed theory, simply a work in progress, as indeed all science is. Claims of it's demise are typically of the 'straw man' variety, ie. deliberate misrepresentations of the scientific discourse on the subject. After all : if you come into shop, the signage of which says 'Pizza' out front, then the lack of motorbike repair facilities inside ought be no surprise.

As for scientific method generally, most scientists I'm aware of don't require 'repeatability' but 'reproducibility'. There's an important difference. Nothing is repeatable in the strict sense. One can't arrange the universe, Big Bangs included, to be in any exact state ( if indeed the universe has a quality called 'exactness', see quantum mechanics ) for a re-run of results. However, for the variables declared to be of interest in some arrangement one can purport ( as a declaration of the standard of acceptable evidence ) that results be within some error tolerance if the same technique and setup are used elsewhere in time and space. Again, critiques of scientific method are all too keen to create 'straw men', by stripping experimental practice and observation of a myriad of well known pragmatic measures. So you wind up with people saying that Newton's theory of gravity is 'only a law', but unusually without provision of contrary examples in line with their beliefs, like them jumping off a cliff say ...... I would say "of course it is 'only a law'", but don't miss the practical point that you will die if you're not careful near long drops. Gravity attracts regardless and science is a practical creed. I generally leave such people alone with their thoughts, and specifically don't get into cars with them.

Now the word 'singularity' is worthy of discussion. In a strict technical sense it can be a matter of how one chooses to describe something.

Suppose I have a row of soldiers on a parade ground, all in uniform and in a neat line. This is a view from 'on high' meaning that I have a complete sense of the arrangement. Pretend that I am limited now in what I can measure : I have a long straight stick ( as long as the line of soldiers would be nice, but not required ) that I press against the chests of the soldiers. So I'm seeing them front on. Each soldier I can mark the position of, literally, using the stick. Each soldier has his own mark, not equal in position along the stick to any other soldier's mark. Now go to one or other end of the line and look along it. The soldiers now appear super-posed by eye. My stick, which I hold against the nearest soldier's arm ( as what would seem to be at a right angle to the line of soldiers when viewed from above ) cannot in this circumstance separate the soldiers. They will all share the same mark on the stick. This is the rough idea of a 'co-ordinate singularity' : a particular choice of measurement method has lead to ambiguity or overlap.

The event horizon of a black hole is a co-ordinate singularity. When seen from afar everything on it appears pasted onto a 2D surface. So the third dimension ( measured radially out from the hole's centre ) is the co-ordinate upon which stuff overlies, like the soldiers. However if one takes the viewpoint of a traveller descending inwards then nothing especial happens as the radius ( seen from afar as the event horizon ) is passed - well nothing happens on account of co-ordinate choice at least. Black holes with a relatively small radius have a fierce gravity gradient ( compare with the acceleration due to gravity being pretty constant near the Earth's surface ) that could rip you apart. Rather large black holes, with many millions of Sun's worth inside them, have large radii and the gradient at the radius of event-horizon crossing is quite mild.

The difficult singularities are those which can't be 'transformed away' by a suitable choice of co-ordinate system. The centre of a black hole is such. All choices give infinities. Ditto for the time T=0 in the Big Bang scenario.

Note that General Relativity does not prescribe what sort of universe we live in. It 'merely' determines how one state moves to the next, or how a situation over here will affect something over there. You need what are called 'initial' or 'boundary' conditions. For instance the modelling of a black hole assumes that far way from it ( at 'infinity' ) spacetime is flat. Or if you like : that the black hole is alone in the universe. Approximations. You can then compare what things appear to be from different perspectives : the far observer looking at the descending matter ever gradually going splat on the event horizon, or along for a ride with the descending matter itself heading for oblivion further in. As for the Big Bang, while gravity is attractive for sure, that doesn't exclude the effect of outwardly directed pressure or kinetic energy and whatnot. One can throw a stone upwards, and if hard enough it will orbit the Earth and perhaps leave us entirely. But there is much debate and little settled on these ( pre ) conditions for the early universe. Just beware overstatements. Stick to observables.

One line of thinking upon the Big Bang starts with the question 'where did it happen?' The answer is 'everywhere'. So what our now labelled as separate physical points some 14 billion years later, were once identical. Thus the Big Bang at T=0 is a co-ordinate singularity, but much more besides. Looking backwards in time the ruler collapses, very like the coiled-spring-retractable-metal-strip-with-a-button type builders use ( and as a child I would play with until it's destruction or confiscation by Dad ).

Cheers, Mike.

( edit ) That is : until I destroyed it, and before Dad could take it from me .... :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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