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Tiny
2004-Jul-19, 12:38 AM
:) Sry if my question confuse you... :)
A black hole was form from a massive star under its own gravitational pressure. As the star&#39;s mass increase rapidly, the core it falls deeper and deepr until the singularity was form. Here&#39;s my question, how long or how many time(implode) it takes a massive star to form a singularity?

devilmech
2004-Jul-19, 01:57 AM
Originally posted by Tiny@Jul 19 2004, 12:38 AM
:) Sry if my question confuse you... :)
A black hole was form from a massive star under its own gravitational pressure. As the star&#39;s mass increase rapidly, it falls deeper and deepr until the singularity was form. Here&#39;s my question, how long or how many time(implode) it takes a massive star to form a singularity?
Erm... a star&#39;s mass increases? Pray tell us how this happens.

StarLab
2004-Jul-19, 03:22 AM
If a star is greater than three times the mass of our own sun, it will be able to do fusion on higher orders of magnitude, making elements (such as iron) that our own sun is too light to make. As the heavy star progresses, it becomes much more and more unstable, until it reproduces its elements so fast (were talking about iron being formed in the core) that it implodes and supernovas. Then we have your black hole.

Duane
2004-Jul-19, 03:44 AM
Tiny, check out this site (http://www.fact-index.com/s/su/supernova.html) which has an explanation of how supernovas form.

Sp1ke
2004-Jul-21, 01:45 PM
Going back to Tiny&#39;s original question, the link that Duane provided says "When the mass of the iron core reaches the Chandrasekhar limit (this takes only a matter of days)...". So it only takes a few days to reach this limit but how long does the rest of the process take?

When the Chandrasekhar limit is reached, is the collapse to a singularity instantaneous? Or at light speed? Or are there other stages, each taking minutes or days?

I&#39;d guess that to an outside observer, the collapse will slow down and stop because of the event horizon but there must be a speed of collapse from the star&#39;s viewpoint.

antoniseb
2004-Jul-21, 02:16 PM
Originally posted by Sp1ke@Jul 21 2004, 01:45 PM
When the Chandrasekhar limit is reached, is the collapse to a singularity instantaneous? Or at light speed? Or are there other stages, each taking minutes or days?
What happens inside the event horizon can only be speculated about.
It is also very affected by relativity, and so it matters what frame of reference you are looking at it from. If you propose how we can observe it, I&#39;ll venture a guess as to what you&#39;ll see.

bigbluestar
2004-Jul-21, 09:21 PM
I hate doing this but I have the answer. Unfortuanatly I cannot find where I read it. From what I understand from what I was reading the time it takes to collapse 5 solar mass to the size of a city is 1/10 of a second. Yeah that fast when I read it I had to swallow it in pieces. The answer is so bizaar that I did not want to post this untill I had a direct refernce as back up. Rest assured I will keep searching for it. I noticed the forum yesterday and I have been searching since then. But that was what I gather 1/10th of a second.

Please gimmie more time I promise to post the link with the actuall info on it

Sp1ke
2004-Jul-23, 02:41 PM
If you propose how we can observe it, I&#39;ll venture a guess as to what you&#39;ll see.

I think the most interesting viewpoint is from someone on the surface of the star that is collapsing. Obviously not a pleasant place to be, from a survival viewpoint, but let&#39;s assume this observer survives the trip.

So it takes a couple of days for the surface to shrink to the Chandrasekhar limit. Then things go quicker from what bigbluestar says - maybe a fraction of a second to collapse further. Or are these two timings contradicting each other?

I&#39;d guess the speed of the collapse is exponential. The star would slowly shrink over a few days then, as it approaches the Chandrasekhar limit, it&#39;s shrinking faster and faster until it passes the limit and becomes a black hole in a fraction of a second.

As you say, antoniseb, we can only speculate. But what&#39;s your guess, from the viewpoint of the doomed and uncomfortable observer :) on the star&#39;s surface?

Eclipse
2004-Jul-24, 05:10 AM
Going back to Tiny&#39;s original question, the link that Duane provided says "When the mass of the iron core reaches the Chandrasekhar limit (this takes only a matter of days)...". So it only takes a few days to reach this limit but how long does the rest of the process take?
We need to keep in mind that prior to this stage is usually a supernova.
It is the remenant of the star AFTER the supernova which eventually becomes a black hole, and this process as I understand it occurs very fast.
Like from within fractions of a second to perhaps a few minutes.

The more mass that remained, the faster this process takes place.

Erm... a star&#39;s mass increases? Pray tell us how this happens.
Quite simply, the greater the mass, the greater the gravitational properties, hence the faster the collapse.
In order for a black hole to occur, the gravity has to be strong enough that the bodies escape velocity is greater then the speed of light.
Once the collapse reaches that point, it&#39;s further development occurs outside of the laws of physics as we understand them, and you have an event horizon with supposively a singularity inside.

Guest_Tiny
2004-Jul-24, 04:53 PM
Oh by the way, what is the precise temperature rate a singularity can have? or what is the tmperature of the singularity?

Planetwatcher
2004-Jul-24, 08:43 PM
Oh by the way, what is the precise temperature rate a singularity can have? or what is the tmperature of the singularity?
Nobody knows.

DarkChapter
2004-Jul-28, 03:40 AM
The collapse would have to be exponential wouldnt it? Just before it begins its rapid collapse, the molecules would be relatively far apart from each other and from the centre of the star, as they began to collapse inward, they would not only be closer to each other but closer to the centre of the star, the closer objects are, the more effected they are by gravity, hence they would accelerate. the closer in it reaches, the greater the effect of gravity, the faster the acceleration.

ASEI
2004-Jul-28, 05:49 AM
Keep in mind the time dialation due to a gravitational field. Anything in a shallower portion of the gravitational field would see anything deeper in the field going at a slower rate in time. Actually, from any perspective outside the event horizon, time at the event horizon should be going infinitely slow.

This raises an interesting point - any observer from any point outside an event horizon cannot observe matter cross the event horizon in a finite amount of time. If, from the perspective of the outer universe, matter never manages to cross the event horizon, then, from their perspective the event horizon never grows - or forms in the first place. If you take time dialation into account, it should take an infinite amount of time for a black hole to form for any outside observer. The result of a stellar collapse would be a massive quantity of matter approaching a critical density at a limit, redshifting indefinitely, but never quite reaching the point where it would become a black hole.

DarkChapter
2004-Jul-29, 12:52 AM
I forgot about time dialation yes (for shame). and that brings up an interesting point. For the outside observer, the matter would never enter the event horizon so to any observable sense, the matter would still exist, however where the singularity is, the matter would be destroyed and no longer exist. So The matter exists and doesnt at the same time, depending on your frame of reference.

Sp1ke
2004-Jul-29, 10:00 AM
Isn&#39;t that just an extreme example of normal astronomical observation?

If we see a star just before it becomes a supernova, at the location of the star it&#39;s already gone. All we&#39;re seeing is an image of the star.

With the black hole, we see an image of the collapsing matter frozen in time. If we were to travel to the event horizon ourselves, the matter would be long gone by the time we got there.

ASEI
2004-Jul-29, 01:24 PM
An event cannot be interpreted as having happened before the light from the event reaches you. Because light speed is the fastest speed possible, no effects or information about any event can reach you before the light from the event does - otherwise all sorts of weird time travel effects would be possible. Furthermore, the time dialation is real. If you had someone hovering x feet above the event horizon, their time would actually be y times slower than yours. If they went down, waited around, and came back up there would be an actual difference in the time elapsed. So in reality the black hole doesn&#39;t ever form for the outside universe.

antoniseb
2004-Jul-29, 01:39 PM
Originally posted by ASEI@Jul 29 2004, 01:24 PM
So in reality the black hole doesn&#39;t ever form for the outside universe.
An event horizon certainly forms, and the object in question certainly maintains all of the destructive capabilities and other interesting relativistic properties attributed to a black hole as seen from the outside.

I&#39;ve always been curious as to why it matters whether a singularity forms.

BTW, two black holes radiate a lot of gravity waves to eliminate energy as they spiral into each other. I wonder if things inside the black hole can radiate gravity waves that will escape the event horizon. If so, the arguments about the light not escaping being proof that an outside observer could never observe the formation of a singularity is bogus.

Goodchild
2008-Jul-29, 03:01 AM
So while a black hole forms is a singularity ever actually formed? If from our perspective the matter falling into create a black hole can never progress past the event horizon as time slows asymtopically to 0, does that mean that the matter NEVER actually gets past there to make the Singularity? Even if from the perspective of the infalling matter time runs as regular, the blackhole will evaporate before it reaches the event horizon, let alone a singularity within it; so would that matter's experience not simply be of falling in and being spaggettied and spit out as radiation in however long it experiences the life time of the blackhole?

A follow up then on the matter inside of the event horizon, what happens to it? Does it simply stop in time and is then slowly radiated away as well? I just don't understand how a singularity can form if time stops [from our perspective] for the matter inside the event horizon when we consider that a Blackhole is not infinite in time. Hence, from out perspective, if it takes infinite time for events to happen past the event horizon how can the collapse proceed to the singularity if the black hole does not exist forever?

Help!

Tensor
2008-Jul-29, 03:25 AM
BTW, two black holes radiate a lot of gravity waves to eliminate energy as they spiral into each other. I wonder if things inside the black hole can radiate gravity waves that will escape the event horizon. If so, the arguments about the light not escaping being proof that an outside observer could never observe the formation of a singularity is bogus.

antoniseb, no, anything inside the event horizon will not contribute to energy radiated away as gravitational waves. When two black holes first merge, they form a sort of "dumbbell" shape. This shape is unstable and gravitational waves are radiated away from the combined hole. This radiation reduces the "dumbbell" shape. The radiation and reduction of the "dumbbell" shape continue untill the merged hole is again a sphere. Note that none of the energy is brom inside the holes.

Jeff Root
2008-Jul-29, 04:34 AM
Note that this thread was just ressurrected after exactly four years.

The significance of the time dilation is greatly exaggerated in my opinion.
It certainly happens, and the magnitude of the effect is exactly as predicted
by general relativity theory, but it doesn't imply that matter doesn't really
fall into a black hole. It tells you something about what is happening relative
to an observer, not what happens to the matter itself.

Once the collapse starts, the density of the core rises to the point that the
black hole forms within a few seconds. If the spacetime inside the black hole
was not stretching, the collapsing matter would all reach the center in just
a few milliseconds. But as the matter is compressed to greater density in
the circumferential direction, spacetime stretches in the radial direction,
spaghettifying the matter and making the "distance" to the center increase
forever, without limit. The very center of the gravity well keeps getting
deeper and deeper as the matter creating the gravity well gets denser and
denser. So the matter never reaches the singularity.

However, that too is probably not of much significance, since quantum
mechanical limitations on the ability to simultaneously measure the location
and momentum of matter imply that that once the matter is squashed into
a space much smaller than an atom, there is no real meaning to squashing
it any further. The collapsing core of the star would reach that size in
less than a second of its own time. It is reasonable to say that the matter
reaches the singularity in less than a second.

-- Jeff, in Minneapolis

Neverfly
2008-Jul-29, 04:37 AM
Note that this thread was just ressurrected after exactly four years.

'Twas its DESTINY!

John Mendenhall
2008-Jul-29, 05:16 PM
Chris?

antoniseb
2008-Jul-29, 05:37 PM
anything inside the event horizon will not contribute to energy radiated away as gravitational waves. ...

Thanks for correcting my four-year-old misunderstanding. As it happens I still misunderstood it.

Tensor
2008-Jul-29, 06:00 PM
Thanks for correcting my four-year-old misunderstanding. As it happens I still misunderstood it.

Doh, I didn't look the date of your post.

alainprice
2008-Jul-29, 07:25 PM
What about the fact that the EH doesn't just appear when the star reaches black hole status. In reality, it already exists below the surface and grows outward as the surface grows inward.

Hard to say what you're measuring when you talk about how long a black hole takes to form.

Goodchild
2008-Aug-03, 03:59 AM
Thank-you Jeff,

I am much more informed by your reply. You mentioned that time dilation is over-rated, which i can appreciate but I am still hazy on the point about our (external) measure of the black holes existence and its subjective time till it reachs a singularity; though i again appreciate that any description of that state is beyond our current understanding, and hence not truely measurable yet. But essentially if it takes a subjective second to collapse smaller then a run of the mill atom there is still a measurable amount of time for an external observer, is it simply that compared the presumed lifetime of blackhole that that external time is inconsequential or am simply way off base on my understanding?

Thanks for the time. ... and for revisiting this thread (I thought my question relevant enough yo it not to warrant a new thread completely)

Tim Thompson
2008-Aug-03, 05:22 AM
If from our perspective the matter falling into create a black hole can never progress past the event horizon as time slows asymtopically to 0, does that mean that the matter NEVER actually gets past there to make the Singularity?
A standard question asked & answered so often that it should probably have its own sticky page. Seen from the point of view of a distant observer, a clock approaching the the event horizon of a black hole will appear to run continually slower, until it stops altogether at the event horizon. However, seen from the point of view of an observer falling into the black hole along with the clock, the clock will continue to run at a constant speed, not more slowly, and will not appear to stop at the event horizon. So, as seen from the point of view of the distant observer, matter falling into the black hole will never quite reach the event horizon, let alone fall through it, since time appears to stop at the event horizon. However, seen from the point of view of the matter falling into the black hole, the matter will reach the event horizon, fall through it, and reach the singularity, in a finite & measurable amount of time.

So, which one is "real"? Does the matter "really" fall into the black hole, or does it "really" not make it to the horizon? Which of the two is more "real" than the other? They are both equally "real"! From our distant outside point of view the matter never makes it past the event horizon, but since we can't see anything inside the event horizon anyway, who cares? From the point of view of the matter falling into the black hole, it gets all the way down, but since we can never see it, again who cares?

A follow up then on the matter inside of the event horizon, what happens to it?
Nobody knows. The singularity is a mathematical indication that the theory (in this case general relativity) fails to describe physics at the point of the singularity. What happens there cannot be known, at least pending the development of a more appropriate theory (i.e., quantum gravity). Between the singularity and the event horizon, space & time trade places in a peculiar trick of geometry, so what "really" happens, and how things "really" look inside a black hole remain undetermined, although there are of course numerous attempts to solve that problem (i.e., Nieuwenhuizen, 2008 (http://adsabs.harvard.edu/abs/2008arXiv0805.4169N), Doran, Lobo & Crawford, 2008 (http://adsabs.harvard.edu/abs/2008FoPh...38..160D), DiNunno & Matzner, 2008 (http://adsabs.harvard.edu/abs/2008arXiv0801.1734D), Hansen, Kholkov & Novikov, 2005 (http://adsabs.harvard.edu/abs/2005PhRvD..71f4013H), Artemova & Novikov, 2002 (http://adsabs.harvard.edu/abs/2002astro.ph.10545A), Magli, 1999 (http://adsabs.harvard.edu/abs/1999RpMP...44..407M)).

qraal
2008-Aug-03, 12:05 PM
Hi All

According to that hoary old text, "Gravitation", a blackhole infalling observer crosses the event horizon and hits the singularity in t = pi.M'/c, where M' is GM/c^2. Thus a 3 km radius blackhole (M'=1.5 km) means the "rush to the crush" is over in 1.5E-5 seconds... quicker than you can blink.

mugaliens
2008-Aug-03, 10:58 PM
Please gimmie more time I promise to post the link with the actuall info on it

wikipedia (http://en.wikipedia.org/wiki/Supernova#Core_collapse)...

Wikipedia (http://en.wikipedia.org/wiki/Supernova#Core_collapse).

Wikipedia (http://en.wikipedia.org/wiki/Supernova#Core_collapse)!

WIKIPEDIA (http://en.wikipedia.org/wiki/Supernova#Core_collapse)!!!

I hate doing that, too. But I hate it worse when people are scratching the hair off their heads when the answer is but a few mouse-clicks and keys-punched away.

Where did we learn to look stuff up when we were kids?

Encyclopedia.

Where do/can we look stuff up these days?

Wikipedia.

When the question seems tricky, the answer seems sticky, don't forget, just Wiki!

tah-ta-ta-tah-duh-da-da tuh-tuh-tah-tah du-dah-dah...

Noclevername
2008-Aug-05, 01:22 AM
Erm... a star's mass increases? Pray tell us how this happens.

I think he meant the core's mass; IE, the star's density increases, squeezing more of its mass into the core.