It's Official: Black Holes Have Lots Of Mass

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It's Official: Black Holes Have Lots Of Mass

Posted by chrisd on Saturday March 22, 2003 @08:08PM from the invigorating-galactic-jacuzzi dept.

KewlPC writes "Spaceflight Now reports in this article that some scientists have been able to measure the "weight" (yeah, yeah, it's actually mass, not weight) of a black hole that is (or was, 13 billion years ago) eating up the most distant known quasar, some 13 billion light years away."

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Duh. by Ayanami+Rei · 2003-03-22 20:15 · Score: 5, Funny

Even I knew that. I mean, stuff keeps falling in them. You know that last significant figure to which they measured the weight? About 10^-8 percent of that are my keys, for sure.

--
THIS THING CAN TURN ON A DIME, MACROSSZERO STYLE ALSO FUCK BETA, ~NYORON
1. Re:Duh. by vladb · 2003-03-22 20:22 · Score: 1, Offtopic
  
  Being one of the first posts, I have this sudden urge to make a perfect condidate for a -4 karma type of a post.. ehem.. to the matter however.. I'm wondering if any astronomy buffs could help me figure what does it help to know an approximate weight of a black hole? Other than an excuse to aquire more research dollars ;-)
2. Re:Duh. by Anonymous Coward · 2003-03-23 00:28 · Score: 1, Insightful
  
  You may have noticed that modern astronomy does not generally have a "goal" other than to get as much information as possible about the universe and assemble theories to explain it. We have not enough technology yet for there to be a true purpose to astronomy. (Unless you count the building of an ISS, ... but the purpose of that is still just to gather data, etc.) This is like screaming, upon seeing Fermat's Last Theorem: "AND WHAT THE HELL ARE YOU GOING TO USE THIS FOR?"
3. Re:Duh. by Alsee · 2003-03-23 02:40 · Score: 4, Funny
  
  About 10^-8 percent of that are my keys, for sure.
  
  Measurements show that 12% of the mass is single socks. Scientists are still trying to identify the other 88% of mass.
  
  -
  
  --
  - - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
4. Re:Duh. by Anonymous Coward · 2003-03-23 02:49 · Score: 0
  
  This is like screaming, upon seeing Fermat's Last Theorem: "AND WHAT THE HELL ARE YOU GOING TO USE THIS FOR?"
  
  Doesn't everyone do that?
5. Re:Duh. by robson · 2003-03-23 06:11 · Score: 4, Funny
  
  Measurements show that 12% of the mass is single socks. Scientists are still trying to identify the other 88% of mass.
  
  Scientists have informally labeled the remaining 88% dark socks. ;)
6. Re:Duh. by dimator · 2003-03-23 08:02 · Score: 1
  
  I'm interested in how many Libraries of Congress the black whole weighs.
  
  --
  python -c "x='python -c %sx=%s; print x%%(chr(34),repr(x),chr(34))%s'; print x%(chr(34),repr(x),chr(34))"
7. Re:Duh. by CvD · 2003-03-24 00:25 · Score: 1
  
  Most of the other 88% mass was found to be consisting of ball point pens.
  
  --
  The Official Steve Ballmer Webpage
Neat by Anonymous Coward · 2003-03-22 20:16 · Score: 5, Interesting

This is neat, I'd never heard of this before:

The extreme brightness of this quasar also shows that the black hole in its core is swallowing matter at the maximum rate possible. This maximum rate is called the "Eddington Limit". If the black hole were accreting matter any faster, it would shine even brighter, and the intense luminosity would actually exert enough pressure to stop any more material falling in.

So there's a limit / "max throughput" to how much matter a black hole can suck in? Very interesting.
1. Re:Neat by ndevice · 2003-03-22 20:34 · Score: 5, Informative
  
  this is how normal stars work too. The radiation pressure generated by the core keeps the core from collapsing into itself. - well not quite the same, but same idea.
  
  But I haven't heard of the eddington limit before either. Neat.
2. Re: Neat by Black+Parrot · 2003-03-23 02:13 · Score: 3, Funny
  
  > So there's a limit / "max throughput" to how much matter a black hole can suck in? Very interesting.
  
  Yep, there's bandwidth problems everywhere.
  
  --
  Sheesh, evil *and* a jerk. -- Jade
3. Re:Neat by cornjchob · 2003-03-23 03:20 · Score: 1
  
  Sounds like nature took a que from the internet and enacted something to prevent that /. effect itself.
  
  --
  We now have confirmed reports from an informed Orange County minister that Ethel is still an active communist.
4. Re:Neat by C21 · 2003-03-24 13:05 · Score: 1
  
  depending on how large it is, yes. There are massive black holes that span entire universes (theoretically) and contain much more than the limit for a small atom sized black hole.
  
  --
  this is not a sig.
5. Re:Neat by Amousha · 2003-03-29 17:46 · Score: 1
  
  the background radiation (4 degress kelvin) is also not uniform... they recently discovered an area of space which was less than 4 degrees kelvin, which remains so because of a "solar" wind (actually cold gas) pushing against the influx of new material/heat.... very interesting.
not giving people much credit by ndevice · 2003-03-22 20:22 · Score: 2, Funny

they had to write out the long way how many zeroes a quadrillion had...
interstellar dust does red shifting too by ndevice · 2003-03-22 20:30 · Score: 2, Interesting

at least that's what I remember from astronomy classes. The article doesn't say if they take that into account or not - and if it's really so far away, that would be a lot of dust that light travelled through. If they do, they would have to assume some uniform amount of dust?
Does this say anything about its size? by mcgroarty · 2003-03-22 20:31 · Score: 2, Interesting

Do we know the physical size or the particle density of black holes?
I'm curious as to whether black holes are compacted so much that most of the space between atoms (and even subatomic particles?) is gone, or whether the repulsions keeping them apart are even stronger than the force of the black hole's gravity.
Now that they have a measure of the weight, if they know anything about the density or the size, they've got the other value as well.
1. Re:Does this say anything about its size? by ndevice · 2003-03-22 20:44 · Score: 5, Informative
  
  You might be confusing neutron stars (pulars sometimes) with these quasars.
  
  Neutron stars are prevented from collapsing into black holes because of nuclear repulsion / neutron degeneracy (instead of electron repulsion). In fact, there's so much pressure that the electrons get squeezed into the protons of the atoms - hence neutron stars.
  
  Black holes have enough gravity to overcome nuclear repulsion and collapse further than neutron stars. I think there's a couple theories about just what happens inside the black hole, but the commonality is that particles don't mean much whether or not you're talking about a singularity, or the non-singularity quantum foam theories.
2. Re:Does this say anything about its size? by Flamerule · 2003-03-22 20:50 · Score: 2, Informative
  
  The other reply has some good information, but he doesn't cover
  Now that they have a measure of the weight, if they know anything about the density or the size, they've got the other value as well.
  Actually, the Schwarzschild radius of a black hole is proportionate to its mass. A black hole with mass the same as the Sun would have a 3 km radius; so just do the math if you want to find the radius of this one.
  This page has some good info.
3. Re:Does this say anything about its size? by Flamerule · 2003-03-22 21:09 · Score: 4, Informative
  
  Gack, I really need to go to sleep.
  I'm right in saying "the Schwarzschild radius of a black hole is proportionate to its mass", but more properly it's directly proportional; i.e., the proportionality constant is 1.
  Well, as long as I'm here, let's do some calculations. The article says the black hole's mass is 3 billion times that of our Sun, so multiply 3 km by 3 billion and you get 3 km * 3*10^9 = 9 billion km. To put things in perspective: the distance to Alpha Centauri is 3.8*10^16 m = 3.8*10^15 km, so this black hole's radius is only .0002% of the distance from here to the nearest star. Quite small, astronomically-speaking.
4. Re:Does this say anything about its size? by Smidge204 · 2003-03-23 01:54 · Score: 4, Interesting
  
  I'm right in saying "the Schwarzschild radius of a black hole is proportionate to its mass", but more properly it's directly proportional; i.e., the proportionality constant is 1.
  
  Something about that seems... counterintuitive?
  
  You're saying that if I have a black hole with a mass of x, it has radius y. Then you say if it has mass 2x, it has radius 2y?
  
  If a black hole is a sphere, doubling it's radius increases it's volume by a factor or about 33 1/2! Since mass only doubled, it's density just dropped by a factor of 17?
  
  I admit I'm not very experienced with black holes, but if anything it seems a black hole would condense to some maximum possible density, and it would maintain that maximum possible density regardless of how much mass you add to it... so it just seems strange that doubling it's mass would actually double it's radius.
  =Smidge=
5. Re:Does this say anything about its size? by Alsee · 2003-03-23 02:33 · Score: 4, Informative
  
  I'm curious as to whether black holes are compacted so much that most of the space between atoms (and even subatomic particles?) is gone, or whether the repulsions keeping them apart are even stronger than the force of the black hole's gravity.
  
  Ok, first you get netron stars where the space between atoms is gone. The entire star becomes one big "nucleus". Then there are quark stars (I think they may still be uncertain about whether quark stars exist). In a quark star the space between subatomic particles (neutrons protons and electons) is gone.
  
  THEN you get to black holes. Once you get within a certain distance of a black hole all laws of physics other than gravity effectively cease to exist. It isn't a question of gravity being stronger then the repulsion - the repulsion no longer exists. What happens is that the repulsive force itself gets pulled in by the gravity.
  
  Think of it this way: Imagine the repulsive force is sound and gravity is wind. A black hole is where the wind is faster than the speed of sound. No matter how strong the repulsive sound is it gets carried inwards. It can't push outwards on anything.
  
  We don't really understand what happens at this point. All known laws of physics break down within this region. We need to discover new laws of physics. The answer will probably be found in a theory of "Quantum gravity".
  
  -
  
  --
  - - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
6. Re:Does this say anything about its size? by arvindn · 2003-03-23 02:44 · Score: 1
  
  Well, as long as I'm here, let's do some calculations. The article says the black hole's mass is 3 billion times that of our Sun, so multiply 3 km by 3 billion and you get 3 km * 3*10^9 = 9 billion km.
  WTF? Mass varies as the cube of the radius, so the radius must be multiplied by the cube root of 3 billion, which is about 1440. So the size of the black hole is about 4300 km.
7. Re:Does this say anything about its size? by Anonymous Coward · 2003-03-23 02:48 · Score: 1, Informative
  
  Do we know the physical size or the particle density of black holes?
  
  We know both. Err... we know one and we know what general relativity predicts about the other, but we don't know how to merge that with quantum dynamics. The density of the black hole is infinite. Every bit of mass inside it is concentrated into a point of zero size - the singularity. So yes, all of the space between particles is gone. But of course being at such a small size, GR is outweighed by the quantum effects and we don't have a theory of quantum gravitation yet. So we're not entirely sure on that one.
  
  As for the physical size of the black hole, we have to be clear about what we mean by `black hole'. The `black' part of the hole is known as the event horizon. It is the point of no escape, after you've crossed that boundry you can't get out. No light can escape from inside the event horizon, hence the name of black hole.
  
  Now when someone says `black hole' they mean `the event horizon and everything inside it'. Fortunately there is a rather simple formula for calculating the event horizon (also known as the Schwarzchild radius after the discoverer of the formula): R = 2*G*M / c^2 where G is the gravitational constant (6.67*10^-11), c is the speed of light (2.99*10^8 m/s) and M is the mass of the black hole.
  
  From the radius we can find the `density' of the black hole, but being as all the mass is concentrated in a single point, it is a rather meaningless value.
  
  Of course it gets more complicated when the black hole is spinning (it doesn't have to be spherical then) but even in that case we can express the dimensions in terms of the mass.
8. Re:Does this say anything about its size? by taliver · 2003-03-23 02:49 · Score: 5, Interesting
  
  Actually, a quick googling found this:
  
  r0=2GM/c^2 (Eqn 10.1.5)
  
  So it is directly proportional. However, I didn't look closely at the units that they are using here, but thta shouldn't matter to the solution at hand.
  
  --
  
  I demand a million helicopters and a DOLLAR!
9. Re:Does this say anything about its size? by at_18 · 2003-03-23 03:15 · Score: 2, Informative
  
  If a black hole is a sphere, doubling it's radius increases it's volume by a factor or about 33 1/2! Since mass only doubled, it's density just dropped by a factor of 17?
  
  Correct. In your sphere example, a 2x mass increase will not yeld a 2x radius increase, since the relation between the two is not linear.
  
  But a black hole is not a physical object, it's an abstraction. The radius of a blackhole is defined as the distance where the escape velocity equals lightspeed. And that distance is directly proportional to the black hole mass, hence the 2x radius increase.
  
  but if anything it seems a black hole would condense to some maximum possible density, and it would maintain that maximum possible density regardless of how much mass you add to it
  
  That's probably true, since in theory a black hole is empty, except a point in its center where all the mass is concentrated at inifite density - so if you double the mass, you get double infinite density, which is still infinite :-)
  
  Average density continues to drop as you add more mass. The black hole of the story, with billions of solar masses, is probably less dense than water...
10. Re:Does this say anything about its size? by Anonymous Coward · 2003-03-23 04:35 · Score: 0
  
  and it would maintain that maximum possible density regardless of how much mass you add to it...
  Unless you could add all the mass in the universe to it at which point God would say 'WTF, I thought I said "Let there be light!"'.
11. Re:Does this say anything about its size? by Smidge204 · 2003-03-23 06:12 · Score: 1
  
  Ah, that seems to the be point of confusion. If the "radius" of the black hole is the radius of the event horizon instead of the actual core of mass, then at least the 1:1 mass/radius ratio isn't so counterintuitive :)
  
  Thanks for the clarification.
  =Smidge=
12. Re:Does this say anything about its size? by mmontour · 2003-03-23 06:34 · Score: 1
  
  But a black hole is not a physical object, it's an abstraction. The radius of a blackhole is defined as the distance where the escape velocity equals lightspeed. And that distance is directly proportional to the black hole mass, hence the 2x radius increase.
  
  Once a star has collapsed into a black hole, you shouldn't use "radius" to describe it. Because of the way that space is curved inside a black hole, the distance from the central singularity to the event horizon is not a well-defined quantity (and is impossible to actually measure).
  
  Instead you should talk about the circumference of the event horizon, which can be measured from outside the black hole, and which is also directly proportional to the mass.
  
  Remember that "Circumference = 2*pi*radius" is only true for flat geometries.
13. Re:Does this say anything about its size? by maraist · 2003-03-23 07:32 · Score: 2, Interesting
  
  While I acknowledge that the geometry radically alters in the presence of intense gravity (in fact, String theory suggests that the number of dimensions collapses inside a black hole to an effective singularity (though protected by minimum plank-radius)), I'm not comfortable using the phrase circumference to describe it's externally apparent dimensionality.
  
  At the least, you'd have to consider the black hole as a 2D surface (or shell). Which is still related to r^2.
  
  Only speaking as a well read lay person, I've heard black holes being described as 2D planes; all the mass exists within the event-horizon (since time slows down as you approach it, approaching the speed of light).
  
  That being said I can almost comprehend space being warped enough such that the r^2 flatens out to r, but It's still beyond me.
  
  --
  -Michael
14. Re:Does this say anything about its size? by maraist · 2003-03-23 08:24 · Score: 2, Informative
  
  Minor nit-pick with your analogy.
  
  The speed of sound in a medium is a function of the average velocity of the molecules in that medium. This is based off of the ideal-gas law, utilizing the probability of direction of particles bouncing off one another.. Collision of gas molecules against solidly compacted forces (such as the front-end of an ship (air/boat), an explosion wave-front, etc) merely causes the molecules to divert their direction, and such ping-ponging causes neighboring molecules to divert, thereby causing an effective motion of the gas. When a large body of such molecules move in the same direction (on average), we call this a current.
  
  Wind is such a current, rivers are such a current, electrons propagating along a conductor is such a current. In each of these cases, a single particle (molecule/electron) is not moving the entire length, but the net motion of all the colliding and direction-changing particles is.
  
  Thus, as with the speed of light, for net-aggregate-velocities (current velocity) much much lower than the average velocity of a given particle in the medium, you can use linear superposition. Namely, if you are in a car moving at velocity x, and you through a book forward at velocity y, then until air-resistance slows you down, the book will have velocity x + y.
  
  However, as you approach the average speed of a particle in your medium, the particles on your wave-front can't move forward fast enough to "get out of your way", so the apparent friction becomes non-linear (polynomial, in fact). You are now accelerating the particles in front of you, and apparently gaining mass (as far as propulsion is concerned). BUT, the key is that you are accelerating the particles in front of you.
  
  I believe the formula for friction in an ideal gas is something similar to:
  1 - V^2/Vavg^2
  
  Which is related to the lorenz contraction factor.
  
  Ideal gas law does not mean that no particle ever travels faster than the average.. In fact, if enough energy is released into the gas such that it's temperature (e.g. aggregate kinetic energy) rises, the it's average velocity is indeed increased.
  
  Thus my nitpick is that for the analogy where a repulsive force is sound and gravity is wind.. As your "wind" velocity increases, you are, in fact, increasing the average velocity of particles. Any perterbations (shock waves, aka sound), now can propagate faster, though in a non uniform manner (the direction of the wave will be faster in the direction of the wind (parallel direction), average in orthogonal directions and dramatically attenuated in anti-parallel directions).
  
  Further, if the auditory devices (sound-generator, and target) are carried along with the medium, then their relative velocities will be constant with respect to the medium, and sound will effectively probagate uniformly.
  
  There is one tiny wrinkle in all this. Here, the max velocity is consdered to be Vavg and is changable. In quantum physics, Vavg is the speed of light and can never be exceeded (at least on average). Thus linear superposition breaks down. Increasing the velocity of the wind means that particles ricocheting forward can't produce a net propagation velocity twice as fast relative to a stationary transmitter/reciever.
  
  Theoretically, all the quantum "forces" are effectively energy moving through space and time at different ratios. Plotting them together as a 4D space-time plot, the magnitude of the 4D coordinate should always equal c. Thus if you are traveling along the event horizon at c in any one direction, then you should have no remaining ability to travel in any other directions (including time). Thus perterbing the medium should be impossible. The event horizon would be like having a steel shell that is oblivious to outside disturbances (ignoring that steel allows for elastic collisions, including sound). I speculate (as a lay person) that the reasoning that event horizon particles are oblivious to anti-parallel disturbances is because they have
  
  --
  -Michael
15. Re:Does this say anything about its size? by KewlPC · 2003-03-23 10:10 · Score: 1
  
  The black hole of the story, with billions of solar masses, is probably less dense than water...
  
  Actually, no. From what I understand, the "size" of the black hole is actually the size of the event horizon, since the black hole itself has a size of zero and infinite density.
16. Re:Does this say anything about its size? by hplasm · 2003-03-23 23:32 · Score: 1
  
  Black holes are compressed matter which is way beyond atoms touching, or even nucleii touching, which would be the famed Netronium. The matter is squooshed down to form a mathematical singularity, where such things as distance become very minquative indeed.
  
  --
  ...and he grinned, like a fox eating shit out of a wire brush.
17. Re:Does this say anything about its size? by dadman · 2003-03-24 03:58 · Score: 1
  
  Now when someone says `black hole' they mean `the event horizon and everything inside it'.
  
  The use of the word "Inside" is interesting. The event horizon is a physical boundary of the rests of this Universe and what was that formed the black hole. So, outside is just equally appropriate, and equally meaningless (semantically, no offense).
  
  BTW, text books say the "everything" are only mass and angular momentum. But if this is true, as "all the information of the matter fall onto the black hole will be loss save the mass and angular momentum", then the history of the black hole shall went down the sink as well and the claims that stars formed black holes shall become automatically invalidated.
  
  No? Enlighten me, please.
18. Re:Does this say anything about its size? by Anonymous Coward · 2003-03-24 05:31 · Score: 0
  
  If a black hole is a sphere...
  It isn't; don't confuse yourself.
  
  IIRC, the S. radius is that spherical boundary at which the escape velocity of the mass constituting the black hole is equal to c. It is not some kind of physical surface like the one that separates the inside of the Earth from the outside. The S. radius is "special" in that once inside it, there is no escaping because Special Relativity ultimately limits your velocity to c.
  
  It is often convenient to talk about the "size" of black holes as being equivalent to its S. radius, but it's often a necessary one because it's a logically consistent way to refer to its "size." Basically, in this sense a black hole's mass and its "size" (i.e., S. radius) are trivially related through the escape velocity equation with v=c.
  
  And if that doesn't blow your mind, IIRC, the area of the S. boundary is equal to the black hole's entropy.
19. Re:Does this say anything about its size? by merlin_jim · 2003-03-24 10:39 · Score: 1
  
  If a black hole is a sphere, doubling it's radius increases it's volume by a factor or about 33 1/2! Since mass only doubled, it's density just dropped by a factor of 17?
  
  Actually, radius, circumference, and volume cease to have meaningful correlations in the vicinity of a macro relativistic object such as a black hole.
  
  Density also is a meaningless measure when it comes to a black hole. Black holes are infinitely dense, since they have a measureable mass but their size is 0. A black hole is a point source of gravity, nothing more.
  
  Now if you are measuring density as a function of space inside the Event Horizon (also known as the Schwarzschild radius) that is not at the maximum possible density. The Event Horizon is merely the line at which events that happen have a possibility of effecting the outside world. On the "inside" of the event horizon no event can possibly effect anything but events that are closer to the center of the black hole.
  
  While we're talking about black holes and radius... there's actually several competing theories about how black holes get bigger. The only mathematical model that makes sense is one where the radius smoothly grows as matter falls into it; but in this case, the radius actually starts growing BEFORE the matter hits it in some strange time-bending type action.
  
  Another model suggest discontinuous jumps; the event horizon grows in a discrete amount at the exact moment the matter falls in.
  
  That's all slightly off-topic but here's the point I was getting to: the area of the event horizon is the most meaningful spatial measure of a black hole. There is a direct correlation between the area of the event horizon, and the amount of chaos the black hole has accreted.
  
  When you measure chaos as the number of ways that a closed system can be reorganized while its macroscopic structure remains the same, then a black hole that is accreting matter REMOVES chaos from the universe, violating Newton's laws of thermodynamics. This is compensated by the fact that the area of the event horizon increases by exactly the decrease in chaos.
  
  Quite fascinating subject. Stephen Hawking's From the Big Bang to Black Holes is definitely recommended reading on this subject.
  
  --
  I am disrespectful to dirt! Can you see that I am serious?!
20. Re:Does this say anything about its size? by merlin_jim · 2003-03-24 10:56 · Score: 1
  
  Do we know the physical size or the particle density of black holes?
  
  You pose two excellent questions.
  
  The physical size of a black hole depends on how you measure it and where you are and what you're doing when you measure it. The gravity near a black hole bends space itself in such a way that rulers contract and clocks run slower. Rulers only contract when in certain directions, and if a ruler is very close to the black hole those directions change randomly, continuously.
  
  A direct consequence of this is that a black hole's circumference is not always 2 pi r. And the radius as measured by the space that was in the same place as the black hole is very different as the radius you would get if you could somehow measure it through the black hole. Which measurement is correct?
  
  Both. That's why physical size doesn't have much meaning when applied to black holes. The best, constant measurement is the area of the black hole's schwarzchild radius or event horizon. And that is actually easy to measure; measure the black hole's temperature very accurately. The area of the black hole's event horizon times its size is directly proportional to its temperature; given the temperature, one can derive the rest.
  
  As for particle density... no current theory on what happens inside a black hole states that there will be particles inside. The best model is that of a singularity... an infinitely small point-source of mass at the center. The size is theorized to be either 0 or the plank-wheeler radius, which is the smallest an object can be according to our current understanding of quantum physics, and is quite a bit smaller than an electron.
  
  Another theory contends that when the singularity in its formation reaches the plank-wheeler size, it stops being a classical object and starts being a quantum object of some sort, quite possibly a quantum probability foam, in which any region of space has a certain constantly changing probability of containing a specific amount of mass, with the totals of all of those probabilities equalling at any time the total amount of mass that has fallen into the black hole.
  
  Other more exotic theories deal with things like super strings and dimensional gateways.
  
  None of them mention anything resembling a particle.
  
  However, your basic question "Once a hole's mass is known, is its size known" I can answer that with a probably yes.
  
  There are only THREE numbers required to describe a black hole; mass, spin, and electrical charge. Spin can deform the hole's horizon (as well as swirl all the space around it) and electrical charge makes its effects felt on the size of the event horizon as well. But, for very big black holes, that is for black holes big enough to have formed themselves, these two extra quantities are very very close to zero. Any other feature of the black hole can be derived from these three numbers, so yes; we can determine the size of the black hole, however you choose to measure it.
  
  --
  I am disrespectful to dirt! Can you see that I am serious?!
21. Re:Does this say anything about its size? by osu-neko · 2003-03-24 13:08 · Score: 1
  
  Mass varies as the cube of the radius
  Err, no. Volume varies as the cube of the radius. Mass at a constant density would as well, but then we're definately not talking about black holes, degenerate matter, or indeed any mass large enough to suffer compression under its own weight.
  
  --
  "Convictions are more dangerous enemies of truth than lies."
22. Re:Does this say anything about its size? by osu-neko · 2003-03-24 13:22 · Score: 1
  
  BTW, text books say the "everything" are only mass and angular momentum. But if this is true, as "all the information of the matter fall onto the black hole will be loss save the mass and angular momentum", then the history of the black hole shall went down the sink as well and the claims that stars formed black holes shall become automatically invalidated.
  No? Enlighten me, please.
  If a great battle is fought on a beach, but over time the arrowheads are ground into sand, the shafts rot away, and at some future point, no evidence is left at the beach that the battle occured, in fact random motion of the sand particles eventually reaches a point where it might have come had no battle occured, do statements such as "A battle occured here" cease to be true? Or simply become unprovable?
  If your answer is the former, then yes, you are correct. If your answer is the latter, then no, these claims are not automatically invalidated. An inability to determine the truth of a statement does not mean that the statement is neither true nor false, just that we don't know which.
  
  --
  "Convictions are more dangerous enemies of truth than lies."
eddington limit and black hole evaporation by ndevice · 2003-03-22 21:07 · Score: 2, Interesting

Just speculating, but since black holes do evaporate, and the smaller they are the faster they evaporate, I wonder what the implications of evaporation would be in the presense of an acretion disk.

Given that in the process of evaporation, a black hole emits radiation, at some point the radiation pressure from the evaporation would balance out the force of gravity pulling matter into the black hole so then the black hole might stabilize in size.

Surely they'll have named that limit already, but I don't think it's the same as the eddington limit.

Or perhaps there won't be a limit here because the cross section area of the acretion disk would be so small compared to the surface area of the event horizon. (yes, I think that incoming matter would have to form a disk and not form an acretion shell)
1. Re:eddington limit and black hole evaporation by Flamerule · 2003-03-22 21:24 · Score: 4, Informative
  
  Given that in the process of evaporation, a black hole emits radiation, at some point the radiation pressure from the evaporation would balance out the force of gravity pulling matter into the black hole so then the black hole might stabilize in size.
  Whoa, whoa. Yeah, a very small black hole would emit enough radiation to completely counterbalance its own gravitational force, so that matter would stop coming flowing into it. But how would that make the size stable? With no more matter coming in, the black hole would just keep emitting radiation, getting smaller, and losing mass, until it evaporates.
  To put it another way, it's not a stable limit, it's an unstable limit. If a black hole is accreting mass at a rate less than or equal to this limit, the black hole will shrink and evaporate; if a black hole is accreting mass at a rate greater than the limit, it will grow.
2. Re:eddington limit and black hole evaporation by barakn · 2003-03-22 22:08 · Score: 2, Interesting
  
  Only large black holes will have accretion disks. The radiation coming from a black hole is negligible until the black hole itself is tiny. It is my guess that the radiation pressure from a black hole would never be enough to prevent a net gain of mass.
  
  --
  "I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
3. Re:eddington limit and black hole evaporation by KewlPC · 2003-03-22 23:49 · Score: 2, Interesting
  
  It is my understanding that black holes emit a very small amount of radiation, and do it very slowly. Therefore, the black hole would have to be very small (thereby having very little gravity) for this to take place.
  
  Besides, a black hole would have to swallow all the matter within its reach before it could shrink to the size necessary for the effect you describe to take place, because the more matter it takes in, the bigger it gets. And since it would have already taken in all the matter it can get ahold of, the effect you mention wouldn't happen.
4. Re:eddington limit and black hole evaporation by merlin_jim · 2003-03-24 10:45 · Score: 1
  
  Only large black holes will have accretion disks. The radiation coming from a black hole is negligible until the black hole itself is tiny. It is my guess that the radiation pressure from a black hole would never be enough to prevent a net gain of mass.
  
  Actually, very tiny blackholes can shrink. The radiation from a black hole is blackbody radiation resulting from the hole's "temperature", measured as the amount of chaos the matter that fell into the hole possessed.
  
  As a hole gets smaller, that amount gets smaller linearly while the area of the radius gets smaller exponentially... at a certain size or smaller, it becomes great enough that the black hole will completely radiate its energy away. This is an increasing reaction; the more it radiates, the faster it radiates, until once it reaches a certain very small size, it explodes.
  
  How small does it have to be for this kind of situation to occur? Pretty small... smaller than a black hole that forms in today's universe through natural means. However it may be possible to create a black hole that small and it is almost certainly possible that the early moments of the universe DID create a good number of black holes that small... which is why they're called primordial black holes.
  
  Once again I refer the reader to Stephen Hawking's From the Big Bang to Blackholes.
  
  --
  I am disrespectful to dirt! Can you see that I am serious?!
5. Re:eddington limit and black hole evaporation by Anonymous Coward · 2003-03-24 13:19 · Score: 0
  
  The first 2 sentences, in context with what barakn was replying to, seem to indicate that barakn was refering to black holes large enough to even have an accretion disk (explicitly not tiny black holes).
  
  Thanks for the Hawking reference, though. I'm currently reading Smolin but will pick up the stated book when I have money.
6. Re:eddington limit and black hole evaporation by osu-neko · 2003-03-24 13:29 · Score: 1
  
  Even worse, the smaller a black hole gets, the more quickly it loses mass, which causes to get smaller even more quickly, which causes it to loose mass even more quickly, etc. Thus, in its final moments, the "evaporation" of a tiny black hole would be at a speed that might be better characterized as "explosion".
  
  --
  "Convictions are more dangerous enemies of truth than lies."
interstellar dust reddens by barakn · 2003-03-22 21:19 · Score: 5, Informative

Atoms produce very specific patterns of absorption or emission in the light spectrum depending on species. A familiar example, is the solar spectrum, which is created by absorption of narrow bands in the spectrum by a large number of different elements in different states of ionization. Redshift causes the entire set of these lines to be moved towards the red end of the spectrum. They retain the spacing between themselves, so they can still be recognized in their new positions, and their new positions tell us how fast the object that created them is moving. Reddening caused by dust doesn't move these absorption lines. Instead it scatters light preferentially at the blue end of the spectrum, causing the entire end of that spectrum to dim, rather than creating narrow bands in it or moving narrow bands around. These two different processes are usually distinguishable.

--
"I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
1. Re:interstellar dust reddens by ndevice · 2003-03-22 21:27 · Score: 1
  
  actually, I was thinking more of the fact that they make a relatively big deal out of their redshift measurements as a measure of the distance to the object. Now you have to factor in gravitational redshift + dust + universe expanding + probably some other stuff I can't think of right now.
2. Re:interstellar dust reddens by barakn · 2003-03-23 09:21 · Score: 2, Informative
  
  And my point was that the dust is absolutely irrelevant to distance measurements using redshift. Now if we were using the object's magnitude to measure its distance (assuming it's one of those objects with a known absolute magnitude), then dust reddening would matter. QSO luminosities vary so much we'd never use a magnitude technique to guess their distances.
  
  --
  "I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
Someone's bitter memories of high-school physics? by tunah · 2003-03-22 21:37 · Score: 4, Insightful

the "weight" (yeah, yeah, it's actually mass, not weight)
What was all that about? Why not just say "the mass"? This is on a site that uses computer and physics jargon and acronyms all the time, mass isn't exactly an obscure concept.

--
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Re:Which size? by Bastian · 2003-03-22 21:39 · Score: 1

If you're talking about the radius of the event horizon, I believe that is proportional to the black hole's mass.

If you're talking about the physical size of the matter in the black hole, I don't know if that is something that can be measured. We'd have to find a way of getting data out out from below the event horizon. . .
I hope you got some sleep... by barakn · 2003-03-22 21:54 · Score: 2, Interesting

You need it. The nearest star is Proxima Centauri at 4x10^13 km (also, your numbers for Alpha Centauri are erroneous). The 9 billion solar mass black hole's radius is thus .0225% of that distance.

--
"I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
1. Re:I hope you got some sleep... by osu-neko · 2003-03-24 12:58 · Score: 1
  
  "Proxima" is not an official designation. I believe the proper name for Proxima is "Alpha Centauri C" (it being the third in that particular trinary system).
  
  --
  "Convictions are more dangerous enemies of truth than lies."
But how many silicon spheres is that? by Bradee-oh! · 2003-03-22 22:13 · Score: 1

One Quadrillion Earths is impressive and all, but let's stick to standard units people!

--
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1. Re:But how many silicon spheres is that? by Englabenny · 2003-03-23 00:21 · Score: 1
  
  If I'm thinking right, this black hole equals 6x10^39 of them silicon spheres...
2. Re:But how many silicon spheres is that? by JUSTONEMORELATTE · 2003-03-24 04:32 · Score: 1
  
  And that, my friends, takes a lot of balls.
  </I_Can_Not_Believe_I_Am_Responding_To_This>
  &rimshot;
Re:Someone's bitter memories of high-school physic by KewlPC · 2003-03-22 23:12 · Score: 1

The headline talks about "weighing" the black hole, so I used the word weight. But I knew I'd get reamed if I didn't mention that it's actually the mass of the black hole, not its weight, so I put that in there too. So much for hedging my bets.

But you're right. I should've written mass and left it at that.
Black Holes Have Lots Of Mass... by TWX_the_Linux_Zealot · 2003-03-23 07:49 · Score: 2, Funny

...but I didn't even know they were Catholic!

--

IBM had PL/1, with syntax worse than JOSS,
And everywhere the language went, it was a total loss...
1. Re:Black Holes Have Lots Of Mass... by osu-neko · 2003-03-24 13:33 · Score: 1
  
  Well, in their case, it's a black mass... :)
  
  --
  "Convictions are more dangerous enemies of truth than lies."
units by barakn · 2003-03-23 09:38 · Score: 1

The units come with the variables in the equation, the Gravitational constant G, and the constant c (the speed of light in a vacuum), and thus it is a complete equation.
mks units (favored by many physicists):
G= 6.673x10^-11 N(m/kg)^2
cgs units (those wacky astronomers!):
G= 6.673x10^-7 dyne (cm/g)^2

--
"I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
1. Re:units by taliver · 2003-03-23 09:45 · Score: 1
  
  However, you can also use systems of equatiosn where c=1 (unitless), leading to many entertaining conversions. There is also one where G=1, which leads to mass in units of meters. That's what I meant by not looking too closely.
  
  --
  
  I demand a million helicopters and a DOLLAR!
yesterday news by pmenoud · 2003-03-24 00:52 · Score: 1

That is a new low for Slashdot. An article about something which occurred 13 billions years ago?
What next? The weather forecast for the previous Big Bang?
It limits accretion rate, not mass by upper · 2003-03-24 09:29 · Score: 1

The radiation that makes black holes look bright comes from stuff falling in. I don't know what the mechanism is, but the energy comes from the mass's gravitational potential energy. So radiation, and therefore radiation pressure, increases (proportionally?) with how much mass is falling in per unit time. So the eddington limit is a limit on the black hole's accretion rate, not it's size.
Black hole evaporation is a slow process. It has essentially no effect on big black holes, and it isn't relevant to the eddington effect.
Quick timeline question... by Theaetetus · 2003-03-24 10:30 · Score: 1

This black hole is 13 billion LY away, and thus became a black hole sometime slightly longer than 13 billion years ago, and was born as a star shortly before that...
This timeline gives the birth of stars at occuring roughly 1 billion years after the big bang, which this article in January gives at between 11.2 and 20 billion years ago...
Wouldn't this hole place a lower limit of 14 billion years on the bang? And if last year's Hubble estimate of 13-14 billion years ago for the bang is right, wouldn't it pin it AT 14 billion years ago?
-T
Nevermind... by Theaetetus · 2003-03-24 10:42 · Score: 1

This has the answer.
-T
MgII by gfim · 2003-03-24 16:50 · Score: 1

The article talks about a MgII line in the spectrum. Surely that's meant to be Mg++.

--
Graham
Max limit so mass of a star. by rhfrommn · 2003-03-25 06:48 · Score: 1

I doubt anybody is still reading this thread, but in case you are here is more information.

The above posts are correct that radiation pressure balances the force of gravity. But luminosity (the amount of radiation) goes up with mass way faster than gravity, so eventually the radiation pressure "wins" and the star gently blows apart. That provides an upper limit to normal stars. When I was taking stellar structure in grad school in the early 90's it was thought the limit is about 60 solar masses. Above that a star rapidly loses mass as radiation pressure carries the outer layers away.

There is a class of stars called Wolf-Rayet stars. They are extremely luminous blue supergiants that have masses in the 80-100 solar mass range. Interestingly, they all have enormously strong stellar winds and are losing mass at very high rates. In a very short time (astronomically speaking) they will drop to 60 solar masses, at which point we assume the Wolf-Rayet mechanism shuts down and the star becomes a "normal" 60 solar mass star.

--
My motto is: Never give up - unless it's harder than you want it to be.