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Reversing a black hole ?

  1. Mar 18, 2013 #1
    Now what I'm about to say may sound like a sort of speculation but not in the typical bad way.

    Now we have a black hole in universe somewhere.(Doesn't matter where for my theoretical purposes) Now we know that once a big star looses all or most it's fusion elements and some of it's mass that went off as energy through that fusion , it's left with no fusion heat/power and alot of mass , now it collapses to a black hole.Ok we cannot undo this.
    Now let's imagine a huge hydrogen gas cloud in the order of mass similar to a large star approaches the black hole.Now the black hole "sucks" the matter (hydrogen gas) in.
    Black holes do evaporate via hawking radiation but if a black hole with the mass "x" would suck in hydrogen gas that is heavier than the star from which it formed (assuming the black hole hasn't been around long enough to build up it's mass from other infalling matter)
    Now wouldn't that mass of hydrogen under the immense pressures of a black hole start off a fusion reaction faster than it could evaporate via hawking?

    P.S. I know we really don't know what happens at a singularity once past event horizon as there is no "light" anymore to show us what's going on but theoretically if we believe that all the matter that goes in can't just disappear but rather builds up in the singularity wouldn't that make the hydrogen to build up and cause fusion to start and cause the singularity to expand?

    Ok maybe another theoretical approach would be asking what would happen if we would put an atomic bomb with mass higher than the mass of the black hole with a timer set in a way that it explodes after it has passed through the event horizon?

    Or maybe the gravity beyond the event horizon is so strong that it could stop the explosion of a atomic or H bomb or whatever force that is rapidly expanding , because if not even light cannot escape the horizon then i guess all other matter too exploding or not would still plummet towards the singularity once past the event horizon?
    Last edited: Mar 18, 2013
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  3. Mar 18, 2013 #2


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    No radiation ever leaves from beyond the event horizon - the escape velocity is higher than c.
    Whatever eventually happens with the infalling matter doesn't matter(duh!) as far as radiative ability of the black hole goes.

    Note that the Hawking radiation is not emitted from beyond the event horizon, so the above doesn't apply.
  4. Mar 19, 2013 #3
    Yes but this is interesting because energy has to be conserved it can't just disappear.
    I understand that the escape velocity is higher than c and that that is the reason why matter can't get out.
    I guess even at c it couldn't get out as that would require infinite energy for a particle that has rest mass.
    That's why I asked these questions.I was thinking what happens to certain matter like hydrogen which has not fused yet and still has it's potential energy that it could release under these conditions.Basically the energy can't escape but if it can't then what happens to it inside?
  5. Mar 19, 2013 #4
    Mass, energy, it is all the same. When it enters the black hole, it adds to its mass. It is preserved.
  6. Mar 19, 2013 #5
    Ok preserved but in what state? Preserved in it's original state that it entered or compressed underwent fusion and then released via hawking radiation? This is the point of my question?
    obviously an object that enters the black hole has some certain amount of energy and a state at which it is like solid, gas or liquid and a corresponding potential energy that can be released under certain conditions like fusion from hydrogen under huge pressure and the black hole is just the place with such pressure so what happens , theoretically?
  7. Mar 19, 2013 #6


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    The point is that what happens inside the event horizon doesn't matter. Take you example of a nuclear bomb falling through the event horizon and exploding after it falls in. From outside the event horizon, nothing has changed. Some of the rest mass of the nuclear bomb has been converted into kinetic energy which is flying around inside the event horizon. But the total mass-energy inside the event horizon has not changed, so an observer outside the event horizon sees no change. So whether the material inside the event horizon is liquid, solid, gas, or whatever is irrelevant. Only the total mass, total spin, and total charge inside the event horizon determine what is seen outside.
  8. Mar 19, 2013 #7
    quote """" But the total mass-energy inside the event horizon has not changed, so an observer outside the event horizon sees no change."""

    Are you telling me that when matter falls inwards a black hole it doesn't add to the mass of the hole? Then how they get bigger? By magic?
    The observer outside may not see a change just as the observer falling inwards doesn't see a change from his point of view of the condition in which he is but in overall there has to be a change.I am not speaking or asking my questions from the point of observers here but from a purely theoretical standpoint.
  9. Mar 19, 2013 #8


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    Uh ... Hawking Radiation has NOTHING to do with things that go on inside the EH, so your statement makes no sense. You could have a zillion A-bombs going off inside the EH or none at all and the amount of Hawking Radiation would be unaffected. And by the way, Hawking Radiation has such a trivial effect in anything less than billions of years that it might as well not be happening.
  10. Mar 19, 2013 #9
    Ok phinds.So basically it turns out that the black hole is the "perfect government meeting behind closed doors." People (matter) go in information goes in but nothing comes out , nobody has any clue or knowledge of what is happening inside and no clue what or who or how much will ever come out , if at all... like that?

    Ok just for a thought exercise...
    So technically if a huge black hole would have been present soon after the big bang before the inflationary process and if it would have managed to suck in all the matter at once then the universe would have collapsed to a singularity behind an event horizon of a black hole?
    And because after passing the EH nothing can come out basically all the energy of the universe present at the time before entering the EH would be lost? Now if so then that is what you would call a sudden change in entropy.

    The point about the hawking radiations was meant in a way that if a black hole has more matter falling in and the EH should get bigger which would indicate that the BH itself has gotten bigger so in this case wouldn't there be a difference in the size of the EH between a single bomb falling in or your mentioned ""zillion A-bombs"" going in ?

    Ok maybe my way of reasoning is absurdly wrong so I'll try to give a more clear question?
    Is there a difference from what we know between two black holes of the same size from one of which has swallowed mostly the remains of a collapsed star and it's surroundings and from a one which would have got the hydrogen gas mixture from my earlier examples?
    I guess the answer is No then.
  11. Mar 19, 2013 #10


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    When the bomb falls into the black hole, the total mass of the black hole goes up, so the event horizon gets larger. But when the bomb then explodes (inside the event horizon), this does not change the total mass of the black hole, so nothing changes.

    Similarly, in your earlier example of unfused hydrogen falling into the black hole, when the hydrogen falls in, the mass of the black hole goes up, but if it subsequently fuses after falling inside the event horizon, nothing changes outside the event horizon.
  12. Mar 19, 2013 #11


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    Right. The answer is no. If they have the same mass, they look the same.
  13. Mar 19, 2013 #12
    @phyzguy ok I agree to that yes.As I didn't actually thought that the mass should change from the explosions happening inside or not because I understand that if the escape velocity is bigger than c then nothing can come out no matter in what state it is radiation, or solid or shockwaves or anything.
    I was rather thinking abstractly of wouldn't something drastic happen to the black hole itself if a mass let's say two times the mass of the hole itself would fall in and explode there.
    But I guess as long as the escape velocity goes higher than c nothing would happen just the size of the EH would get bigger.
  14. Mar 19, 2013 #13


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    Yes, sounds like you are getting it.

    By the way, I like your description of a BH as the perfect government meeting behind closed doors :smile:
  15. Mar 19, 2013 #14


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    You misunderstand the early universe. It is believed that (1) The pressure was larger than the gravity so a black hole would not form and (2) the universe may well have been infinite in size in which case even if a BH could form, it would only be a trivial local event, not universe-wide (in an infinite expanse, there is no such thing as "universe-wide")
  16. Mar 19, 2013 #15
    No. He is telling you that when stuff happens inside the black hole, it does not matter. Whether a bomb explodes or not, whether an energy turns to mass or the other way around, whether flying purple demons materialize and start sorting cold and hot atoms or a tree falls in a forest or a philosopher stops thinking or starts existing, it is all irrelevant and unobservable.

    When stuff goes form outside to the inside, obviously, it matters and is noticeable and is a change. But is not what he is talking about.
  17. Mar 19, 2013 #16
    To be perfectly honest even though some misunderstandings arose I knew before that there is a event horizon beyond which the matter accelerates or is being pulled faster than light which means that not only it cannot escape and is doomed to travel further in a given direction but the observer outside the hole cannot see anymore.

    As I mentioned above I was thinking purely abstract that maybe there was a way in which a huge explosion could have disrupted the black hole but then realized that the escape velocity would forbid that so yeah anyways thanks for a discussion on this topic.

    @phinds the reason why I mentioned about a possible BH back in the early universe was that I thought that for a black hole it would have been easier to expand and grow back when matter was much closer and denser than now when distant galaxies are separated by vast distances and the gravity is too weak to pull those closer and let the black hole "eat"
    Now back in those conditions I was thinking that a black hole could have made a possible positive feedback loop in which as more matter would fall in the bigger the EH would get and because matter was dense and close it could have "eaten: it's way through the universe like this.is there any atleast theoretical chance that this might be real given the conditions?

    By pressure was larger than gravity do you mean that the matter was so hot and energized that everypoint was repelling every other point stronger than gravity could have pulled them together? Well I guess so otherwise the universe would have fallen back to a singularity after it came out from one.

    Well the infinite universal expansion (the most likely scenario) posses a problem to my hypothetical black hole feedback because the galaxies receding faster than light I guess from what I know the EH couldn't catch up and would fall behind which would also let the distant matter escape the destiny of a fall into a forever expanding black hole.
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