What Are Black Holes and How Do They Curved Space?

[AcEY]

I want to start a discussion about black holes and curved space. From what i know (not including that article posted in another thread) black holes are created when stars can no longer burn and compress so much that it creates a gravitational pull that is so big even its own light can't evade it, correct me if i have made any mistakes etc... But from what i have read it doesn't actually mention a hole in space, i mean if it is a star compressing then surely there would just be a really dense point where everything would be crushed. Sorry if I am being naive and I am actually pointing out the obvious or something, I am only a 18 year old student in the uk

 

[SpaceTiger]

I want to start a discussion about black holes and curved space.

This is probably slightly better for the relativity section, but it's a pretty general concept, so maybe it's fine here as well.

From what i know...blackholes are created when stars can no longer burn

Yes, but only for massive stars. Less massive stars will end their lives as a white dwarf or neutron star.

and compress so much that it creates a gravitational pull that is so big even its own light can't evade it,

That's basically right.

But from what i have read it doesn't actually mention a hole in space, i mean if it is a star compressing then surely there would just be a really dense point where everything would be crushed.

It's best not to take astronomy terms too literally, as they usually only offer vague descriptions. Really, it's only a "hole" in the sense that anything that passes the "event horizon" is permanently stuck inside of it. Nobody knows what happens to the matter and light once it passes beyond the event horizon because nothing can escape to tell us about it.

 

[AcEY]

yea i know its only big stars i was only rushing through it, the post would have been huge if i was to discuss pulsars and white dwarfs. I wasnt taking astronomy too literally, but i keep hearing people discussing where a black hole leads. i don't think it leads anywhere except a very dense point where everything, including light is crushed. And your last point, i know its impossible to ever know what is at the center because nothing escapes it but surely going by what it was created from a dense point is the most obvious?

 

[SpaceTiger]

And your last point, i know its impossible to ever know what is at the center because nothing escapes it but surely going by what it was created from a dense point is the most obvious?

Perhaps, but lacking a theory of quantum gravity, I'm hesitant to say anything about that. In general, physicists try to avoid infinities.

 

[Chronos]

Indeed, infinities tell us where our mathematical models cease to provide a meaningful description of physical reality. This is what happens to general relativity when a black hole forms. But this is no big surprise, general relativity is a classical theory. It only yields an approximate answer [albeit a very good one most of the time]. A quantized version of GR [i.e., quantum gravity] is needed to explain what happens after GR shoots craps. It has proven to be a difficult challenge. We have thus far been able to quantify the theories of electromagnetism and the nuclear weak force, resulting in their unification [electroweak force], using some clever mathematical tricks [renormalization]. Unfortunately, no one has figured out how to renormalize gravity [or the strong nuclear force for that matter]. It is a very complex problem and holds the key to many [and perhaps all] current mysteries.

 

[misskitty]

Chronos, do you believe it might be possible to one day travel inside a black hole and discover what is inside it?

 

[turbo]

Chronos, do you believe it might be possible to one day travel inside a black hole and discover what is inside it?

Our current physics tells us that anything more extended than a particle will be torn apart by the the extreme tidal forces exerted by a black hole. Things will be torn asunder and rendered into an infinitely dense repository existing "somewhere else", since we cannot model it as existing in our own universe. In General Relativity, an object falling into a BH will be projected on the event horizon of that BH for an infinite amount of time, and its image cannot ever leave our universe.

 

[misskitty]

This is all theory right? Or does this actually have experimental proof?

 

[turbo]

This is all theory right? Or does this actually have experimental proof?

AFIAK, there is no experimental evidence to support the claim that objects entering a black hole will be tidally disentegrated, nor is there any observational proof (beloved by astronomers, not mathemeticians) that objects approaching a black hole will appear to slow and freeze on the event horizon of that "singularity".

 

[misskitty]

Alright. I can see that.

 

[chroot]

This is all theory right? Or does this actually have experimental proof?

There is a lot of very strong evidence of the existence and behavior of black holes, but it's all "circumstantial," since no one has ever visited one up-close. Strictly speaking, there is also really no such thing as "experimental proof" -- the best you can hope for is a preponderance of evidence.

- Warren

 

[misskitty]

Strong evidence in this case seems to be the best they can get. This is going to sound ignorant; is there any point to a balck hole or is it just there?

 

[chroot]

Strong evidence in this case seems to be the best they can get. This is going to sound ignorant; is there any point to a balck hole or is it just there?

I don't see the point in ascribing "purposes" to any inanimate object. What's the purpose of stars?

- Warren

 

[AcEY]

In General Relativity, an object falling into a BH will be projected on the event horizon of that BH for an infinite amount of time, and its image cannot ever leave our universe.

Actually the image of the person will become invisible to us after a certain amount of time, but the imprint will still be there. And about traveling into a black hole, as soon as you pass the event horizon you can't escape, because to get out you would have to go faster than the speed of light. Some big black holes won't crush/ rip you for a bit after the event horizon so you may get a nice view. If you are looking for a reason for black holes, the reason i tend to use is to hold galaxys together.

 

[matt.o]

If you are looking for a reason for black holes, the reason i tend to use is to hold galaxys together.

This is not correct. Dark matter is needed to hold a galaxy together. The gravitational field due to a black hole is not enough.

 

[SpaceTiger]

This is not correct. Dark matter is needed to hold a galaxy together. The gravitational field due to a black hole is not enough.

matt.o is right. See the end of this thread for a more detailed discussion of that:

https://www.physicsforums.com/showthread.php?t=64709&page=2&pp=15&highlight=MOND

 

[AcEY]

ok i understand that, why is there a super black hole at the middle of every galaxy then, is the huge amount of DM and a super black hole that make a galaxy form?