In school we have learned about black holes and that once they begin they last forever.
Does this mean that some day there will only be black holes in the univers?
If so, will they begin eating each other?
If so, will they become one, and start another singularity/big bang?
If so, is there a way to know how many times this has happend?
Hi** Myth**, until someone smarter comes along to answer your questions I pulled some relevant previous threads for you to enjoy on the subject.
How much CAN a black hole suck?
How long before we get sucked into the central black hole?
Enjoy!
My two cents:
We no longer believe that black holes can last forever. We used to, as we could see no mechanism by which mass, once part of a black hole, could leave that black hole again. But Hawking showed otherwise, see here.
With this mechanism, microscopic black holes can vanish over milliseconds, but stellar mass and heavier holes would take ages, as in, several universe-lifetimes.
If it were the case that they were ‘indestructible’, then it would indeed be the ultimate fate of the universe to become lots of black holes, which would eat each other up and eventually make one humungous hole. But since they can evaporate, we expect the universe to ultimately become a soup of photon radiation, though it might well spend a lot of the intervening time as a bunch of big black holes.
Can’t answer your qs about whether black holes can produce new big bangs… all that stuff is way past the event horizon of my understanding when I did my physics degree. 
It is now believed that as a result of “Hawking radiation” black holes eventually disintegrate. A black hole of the size of the Planck length (10^33 cm, I think) will disintegrate almost immediately in a great explosion. A black of the mass of the sun also radiates but very slowly and would take many years (probably something on the order of 10^100). At any rate, it is thought the eventual fate of the universe (as long as there is not enough matter to force it to collapse, which does not seem to be the case) is to decay into a random arrow of particles like electrons, neutrinos, and photons. This assumes that protons are not stable, which is thought to be the case based on theory. However, proton decay has not been observed despite experiments that are thought to make such decay observable. In any case, this ultimate universe is thought to take something like 10^125 years to arrive. I can hardly wait.
One important additional fact to consider is that the universe is expanding. That means space itself is expanding, driving everything in it farther and farther away from each other, except for local gravity clusters.
Black holes don’t “suck.” They are merely places with high local gravity. They’ll attract other matter to the same degree that non-black holes of equal masses would.
The expansion of the universe means that it’s unlikely that black holes will swallow up all matter or each other. More likely the universe will become a diffuse mist of matter too distant to be of significant gravitational interaction.
You may want to read up on heat death. According to it, there will be a time when the universe consists entirely of black holes and electromagnetic radiation. However, the black holes will eventually evaporate leaving the universe at maximum entropy at around 10^150 years.
Qualitatively right, but you’re off on some numbers. First, the Planck length is more like 10[sup]-33[/sup] cm, much smaller than a cm, not much larger (this was probably a typo). Second, a solar-mass black hole will only last for ten to the sixty-something years. To get a black hole which would last for a googol years, you’d need a mass of a trillion solar masses or so (which is admittedly not outside the bounds of plausibility).
Thanks for the answers and links. I’ll have to tell my teacher black holes dont last forever.
I gues our books are older than some of these studys. At least they dont say we live on a giant turtles back.
From your original post you told us that a school teacher implied that black holes last forever. Which leads me to assume that you’re in either high school or middle school. Sometimes teachers at that level will try to simplify some concepts in order for their students to try and understand them easier.
Just by asking these questions you’ve shown us that you see a flaw in these simpler concepts and would like some explanation even if it goes against what you were taught.
(warning, annoying caps ahead)
DON’T STOP ASKING THESE QUESTIONS!
I admire the fact that you’re seeking out these clarifications rather than repeat what you are taught back to the teachers and exams. Keep it up, and feel free to come back to satiate any more curiosity.
(just don’t create a new login name or you will face TD’s wrath)
Whoa, wait a minute. We don’t have any context at all for what the teacher originally said, just a paraphrase by somebody who admittedly doesn’t understand the concepts.
I can think of several ways in which a statement that black holes last forever would be perfectly correct. The numbers given here show that large black holes will last so long that they will effectively outlive the universe, at least a universe of gravitational coherence. A slowing radiating black hole with the nearest matter beyond the light horizon has outlived the universe.
Or the teacher’s context might have been that anything that goes into a black hole stays there forever and can never re-emerge. That it might be converted to Hawking radiation doesn’t negate that. Many quantum theorists are struggling to reconcile the disappearance of information into a black hole with the notion of the conservation of information.
This attitude is a general problem here on the Boards, which is why I’m making an issue of it. People nitpick general statements made to beginners and non-specialists with hyperliteral interpretations of every nuance. While extra precision is a good thing, the Boards should be seen as an endless series of footnotes to the real statement not as a substitute for it. Otherwise no general statements could ever be made and no general learning can exist if the beginner is forced to grapple with the thorniest nuances of the most advanced thinking on the subject with the very first step.
Give the general concepts first, and later, but only after the general is fully mastered, go into the finer points. Otherwise you confuse everybody and nobody learns anything. Not to mention that you can easily make false accusations of those who have done nothing wrong.
Good point. It’s perfectly reasonable that the teacher just wanted to keep things simple for his/her high school class. Hell, I know the mechanism of Hawking radiation is totally beyond me. It’s also a fairly new piece of theoretical physics.
True enough, although black holes are such extreme concentrations of gravity that they have to be understood through General Relativity rather than Newtonian mechanics. Only neutron stars are dense enough to be in the same class. To be absolutely nitpicky there is gravitational radiation, which will cause two orbiting masses to coalesque eventually. I wonder how dense/close two bodies have to be for this to take place on a human time scale (< months)?
I don’t understand this. Black holes are problematic because of the singularity at the center. That means they can’t be handled by GR. Neutron stars don’t have a singularity.
The singularity itself can’t. I meant that when you’re talking about a region of space (such as near a black hole or neutron star) that the escape velocity is a high fraction of the speed of light, then you have to use GR to calculate how things work and you have to take into consideration obscure effects like gravitational radiation.
Everyone knows that the world rests on the backs of the four great elephants. It’s the elephants who are on the giant turtle’s back.
Want to really bend your brain? The more mass concentrated in a black hole, the larger the event horizon that encloses it, and the less severe the gravitational effects of being near that horizon. This applies to both sides of the event horizon (inside and outside). So, while falling into a black hole of a few solar masses will make spaghetti of you, you could pass the event horizon of a few-galactic-mass hole without noticing it right away.
Using some currently-plausible values (though all such things are fairly speculative and hard to measure with much accuracy), the mass of the universe concentrated in a black hole would have an event horizon about the size of the known universe. In other words, the universe may already have become a giant black hole without anyone noticing. Or more precisely, it may have always been one, mathematically speaking.
After all, it’s probably true that photons inside the universe aren’t able to leave, which is one of the main properties of a black hole.
[edited to correct a typo]
It’s getting a bit more complicated than here, but I wanted to post a link to another black hole discussion going on in GQ today, so that Myth doesn’t miss out on what they’re saying there due to an inability to search.
On the one hand, I agree with you that one should address the basics first, and only hit the details later. On the other hand, though, I do think that it’s possible to avoid the details while still being consistent with them. One could, for instance, say that black holes last for an inconceivably long time, and only go away through complicated processes we won’t go into here, instead of saying that they last forever. This is at a simple enough level that the students there and then probably won’t be too confused by it, but it also means that those students who do go to a deeper level won’t need to waste as much time unlearning what they’ve already been taught. By the time you get to the introductory college level, you end up having to spend about half of your class time unlearning things from previous education. Much better to just get it right the first time.
It’s hard to disagree about getting it better the first time out, but reality states that very few of us - even teachers - choose every single word so carefully that sentences leave no room for doubt or correction. Cecil has less of an excuse in one sense - he gets to write things out ahead of time for maximum precision - but since he has no idea who he’s talking to or the level of their understanding on a given subject - plus the space restraints - he has a delicate balance to walk.
Teachers have to speak extemporaneously, which gives them a harder job, even if their audience is more uniform. And they may be thinking of making a particular point, which is not the point that is remembered later.
Responses on the boards (take that, John Mace) are usually both extemporaneous and aimed at a non-specific audience so getting the concept across in any understandable form is a significant task.
What most people who haven’t had the curse of studying professional communication don’t realize is that what I’m saying is seldom what you’re hearing. Look at the responses on practically any thread to what seem to be clear statements and this becomes painfully obvious. It’s part of what made deconstruction and “the book is not the text” philosophies so powerful before they became hopelessly tangled up in their own folds. No matter how you put it, people will read/hear/perceive/rework it in a variety of totally unforeseen ways. Perfect communication is impossible. The best that can be hoped for is consensus understanding.
And that’s at first hand. When the communication is second hand, as was the case here, or third hand or seventy-sixth hand, all bets are off.