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View Full Version : 2Qs Blackholes/legrange points

Durakken
2009-May-21, 07:11 AM
#1. Why do Legrange points exist?

#2. Would adding more matter to a blackhole eventually make it not a blackhole? Does the matter get crushed into it to make the blackhole the same size or would the matter eventually pile up to be so great that the blackhole effect would just disappear? and if that is the case, what is the maximum size of a blackhole?

korjik
2009-May-21, 07:31 AM
Lagrange points depend on having a much smaller mass orbiting with two larger masses. What happens is that the effects of the two larger masses add together so that the small mass becomes co-orbital.

The Earth-Sun L1 point is a good example. The L1 point lies between the Earth and the sun, most of the way to the Earth. Normally a mass at the L1 point would orbit faster than the Earth, but the pull of the Earth slows it down to keep it at the L1 point.

Adding mass to a black hole only makes a larger black hole. Theoretically, there is no maximum mass

Jeff Root
2009-May-21, 08:01 AM
An orbit is where the speed of a satellite balances the gravitational pull of
the body it is orbiting.

Lagrange points are the places where the speed of a satellite balances the
gravitational pull of the large body it is orbiting plus the gravitational pull of
the smaller body it is orbiting with.

Adding more matter to a black hole makes the black hole larger. There is
a supermassive black hole at the center of the Milky Way. Most galaxies
are thought to have supermassive black holes at their centers, caused by
a mass equivalent to a million Suns or more in a volume smaller than the
diameter of the Solar System. There is no limit on the mass.

-- Jeff, in Minneapolis

cbacba
2009-May-21, 01:55 PM
there are several la grange points. As mentioned, it's possible to be in one and have gravity balanced so that it is stable and any minor variations will cause forces to occur that will correct for the variation, bringing it back into alignment. An Earth satellite located in a La Grange point will stay in a fixed position relative to Earth as both orbit the Sun. A normal satellite will simply travel around the Earth every few hours to every few days as the Earth orbits the Sun. Geostationary satellites are just those whose orbits are on the plane of the equator and whose orbital periods are the same length as the time it takes for the Earth to rotate 360 degrees (about a day in length).

The supermassive bh in the milky way galaxy center appears to be a few million solar masses. It's not that impressive compared to others which have been measured at a few hundred million to a billion solar masses. That suggests there isn't really a limit to the size.

You should understand that black holes are theoretical constructs based upon possibly incomplete understanding. These monsters in the centers of galaxies are real. They have measureable effects upon their surroundings. We can tell that they are tiny and that they are tremendously massive by their effects on stars that orbit them. They are also invisible as we see those orbiting stars that appear to orbit around nothing yet the orbits show it must be tiny and must contain millions of solar masses. Since the BH theory describes rather well this circumstance, it is used. It will be interesting as we zoom in on our understanding to see how well the BH description describes these objects.

robross
2009-May-21, 07:31 PM
#1. Why do Legrange points exist?

#2. Would adding more matter to a blackhole eventually make it not a blackhole? Does the matter get crushed into it to make the blackhole the same size or would the matter eventually pile up to be so great that the blackhole effect would just disappear? and if that is the case, what is the maximum size of a blackhole?

Astronomy Cast did a whole episode on Lagrange points:

http://www.astronomycast.com/physics/ep-76-lagrange-points/

Rob

grant hutchison
2009-May-22, 10:12 AM
You should understand that black holes are theoretical constructs based upon possibly incomplete understanding. These monsters in the centers of galaxies are real. They have measureable effects upon their surroundings. We can tell that they are tiny and that they are tremendously massive by their effects on stars that orbit them. They are also invisible as we see those orbiting stars that appear to orbit around nothing yet the orbits show it must be tiny and must contain millions of solar masses. Since the BH theory describes rather well this circumstance, it is used. It will be interesting as we zoom in on our understanding to see how well the BH description describes these objects.To the reasoning you describe, we can add that we have evidence of an event horizon, rather than a material surface, bounding these supermassive objects. Accretion discs look different if they are disappearing through an event horizon at their inner edges, rather than impacting on a solid surface at relativistic velocities.

Grant Hutchison

cbacba
2009-May-22, 12:56 PM
Evidence? I was under the impression that such was still at least a few years off from being able to observe.

grant hutchison
2009-May-22, 01:35 PM
Evidence?There have been a number of papers comparing observation and theory. The following are (IIRC) courtesy of a previous post by Tim Thompson, which I've temporarily mislaid:

Grant Hutchison

Durakken
2009-May-22, 05:30 PM
So... if Blackholes don't have a max out point for mass, do they have a max out in terms of size? And/or density?

Like say you have a black hole that weights 100lbs and is made of say gold. Now what would happen if I added another 100lbs of gold? Would the black hole maintain it's size or would it double? or can't we figure that out since that would cause an increase in the size of the event horizon...

I assume the EH isn't the surface of the blackhole...

grant hutchison
2009-May-22, 05:36 PM
I assume the EH isn't the surface of the blackhole...The event horizon marks the conventional boundary for a black hole. (What do you assume is the surface of a black hole?)

Double the mass of a black hole and you double the Schwarzschild radius, which is one measure of the size of the event horizon, and therefore one measure of the size of black hole.

Grant Hutchison

alainprice
2009-May-22, 05:40 PM
The EH is an imaginary surface. It's a type of singularity, but can be solved.

The point(or ring) at the center of the EH is the matter of the blackhole itself, and it considered an unsolved singularity. It's always singular. We do not impose any strict limits on density, but there might be one.

Density makes no difference, since adding mass to a BH makes the event horizon grow. The density would never cause the matter to bulge out past the event horizon.

More complex discussions are available. See ergoshpere.

grant hutchison
2009-May-22, 05:48 PM
And in fact, if you take the volume of a sphere of Schwarzschild radius as the "volume" of the black hole (which is geometrically a bit daft, but often done), the mean density gets lower as the mass increases, until supermassive black holes have a density less than water's.

Grant Hutchison

cbacba
2009-May-22, 09:37 PM
There have been a number of papers comparing observation and theory. The following are (IIRC) courtesy of a previous post by Tim Thompson, which I've temporarily mislaid:

Grant Hutchison

tnx for the ref.s I'll try to go through them this weekend.

grant hutchison
2009-May-22, 09:43 PM
tnx for the ref.s I'll try to go through them this weekend.If you're short of time, just take a look at Sections 4 & 5 of Narayan and McClintock.

Grant Hutchison

grant hutchison
2009-May-25, 12:52 AM
There have been a number of papers comparing observation and theory. The following are (IIRC) courtesy of a previous post by Tim Thompson, which I've temporarily mislaid:
Broderick, Loeb & Narayan, 2009 (http://arxiv.org/abs/0903.1105)

Grant Hutchison

JohnD
2009-May-26, 12:23 PM
The nearest known, or suggested BH to us is the one at the centre of our galaxy, which is invisible at normal wavelengths due to intervening dust. Recent work has set up a VLBI (Very Large Baseline Interferometry) array using telscopes all around the Pacific Rim and at Hawaii. This, it is hoped, will reveal images of the event horizon around the SagA* object and show details of its shape.
Some interpretations of the data already available show a doughnut shaped accretion disc. And although the mass is enormous - 4.5 MILLION Suns - there is no evidence yet that it IS a BH. Other possibilities have been suggested.

See: http://www.newscientist.com/article/mg20227091.200-coming-soon-first-pictures-of-a-black-hole.html?full=true

John

grant hutchison
2009-May-26, 12:33 PM
The nearest known, or suggested BH to us is the one at the centre of our galaxy ...There are quite a few black hole candidates closer than the galactic centre. V4641 Sgr is only a couple of thousand light years away, and Cyg X-1 comes in about 8000 light years. Sgr A* is the closest supermassive black hole candidate.

And although the mass is enormous - 4.5 MILLION Suns - there is no evidence yet that it IS a BH.There's actually a lot of evidence that it is a black hole: it just isn't entirely conclusive yet. From Broderick, Loeb & Narayan (http://arxiv.org/abs/0903.1105):
Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of General Relativity (GR). Until recently, their compact size has prevented efforts to study them directly. Here we show that recent millimeter and infrared observations of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, all but requires the existence of a horizon. Specifically, we show that these observations limit the luminosity of any putative visible compact emitting region to below 0.4% of Sgr A*'s accretion luminosity. Equivalently, this requires the efficiency of converting the gravitational binding energy liberated during accretion into radiation and kinetic outflows to be greater than 99.6%, considerably larger than those implicated in Sgr A*, and therefore inconsistent with the existence of such a visible region. Finally, since we are able to frame this argument entirely in terms of observable quantities, our results apply to all geometric theories of gravity that admit stationary solutions, including the commonly discussed f(R) class of theories.

My bold
Grant Hutchison

antoniseb
2009-May-26, 12:58 PM
So... if Blackholes don't have a max out point for mass, do they have a max out in terms of size? And/or density? ...

You are perhaps assuming that there is a singularity. We don't actually know what happens at the center of a black hole, and we can't know until we have something like what-String-Theory-hopes-to-be that explains both GR and Quantum Theory. ... The "size" and "density" of the singularity are simply hand-waving ideas right now.

cbacba
2009-May-27, 08:17 PM
If you're short of time, just take a look at Sections 4 & 5 of Narayan and McClintock.

Grant Hutchison

you know what they say about best laid plans! The ultimate twentieth century example of bad fiction writting is the start of the novel with "It was a dark and stormy night..." . I suppose the twenty first century ultimate bad example is "I'm not really sure how it all started other than the unexpected arrival of a rather innocuous looking email. Little did I know it was going to totally change my life for the next several days or had the potential of a more substantial long term impact nor did I realize at the time I opened it just where or how far it was going to take me over the course of the next few days....." (Imagine Humphrey Bogart narrating as he reads the computer screen in the straight to DVD cinema version-not that pgp creator Zimmer wouldn't make for more realism). It's amazing how life sometime imitates bad art.

Unfortunately, I'm into my next time crunch for tonight but I may have enough time to skim the specific paper you mentioned.

cbacba
2009-May-27, 08:23 PM
So... if Blackholes don't have a max out point for mass, do they have a max out in terms of size? And/or density?

Like say you have a black hole that weights 100lbs and is made of say gold. Now what would happen if I added another 100lbs of gold? Would the black hole maintain it's size or would it double? or can't we figure that out since that would cause an increase in the size of the event horizon...

I assume the EH isn't the surface of the blackhole...

I was thinking that I've heard for a density such as about 6 H atoms per m^3 (avg density of the universe assuming the dark mass and dark energy) that the Schwartzchild radius is around 13.7 billion light years. Interesting coincidence I think.

BTW, You're dealing with densities greater than the atomic nucleus so there's no such thing as atoms like gold involved. Also, you need mass, not weight - which is the gravitational force on a mass which is different between the mass being at the Earth's surface or at the Moon's surface. The radius will increase with mass though and the average density would decrease although there is no known nuclear force capable of overcoming the force of gravity at that intensity so there is nothing to keep what is inside the radius from continuing to collapse.