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There's a Hole in the Middle of It All 693

Posted by michael
from the bang-caused-the-hole-in-the-middle-of-it-all dept.
Apparition writes "CNN is reporting that the star at the center of our galaxy is actually a super-massive black hole. The article then claims that it occupies a volume of space about 3 times that of our solar system. If my math is correct, about 230 million suns could fit into that same volume, so it doesn't impress me that the claimed mass of the black hole is only between 2.6 and 3.7 million times that of the sun. So what is up here? Since when do black holes occupy so much space (I thought they were points)? And how can something with a density only 1/100 of our Sun be called super-massive?" I think the article is talking about a maximum possible size of the object, due to limitations on the resolution of our instruments. Nature has a no-registration story about the research. Update: 10/16 23:44 GMT by M : There's an article with more information on space.com, and a press release from the European Southern Observatory.
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There's a Hole in the Middle of It All

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  • Event Horizon (Score:5, Insightful)

    by redbaron7 (577469) on Wednesday October 16, 2002 @05:20PM (#4464602) Homepage
    Usually when people talk about the size (as in diameter & volume) they are talking about the Event Horizon, NOT the singularity.

    RB

    • Re:Event Horizon (Score:5, Informative)

      by Transient0 (175617) on Wednesday October 16, 2002 @05:28PM (#4464679) Homepage
      It's true that often when the size of a black hole is mentioned, it is the Swartzchild radius or "Event Horizon" that is being mentioned, being it's apparent size to our instruments.

      It is not however true that black holes are points. A black hole that became a point gravity source is what is referred to as a singularity. It was a singularity that became the big bang and if the "big crunch" theory is correct, it will probably be a singularity that the universe ends as, but under any other circumstances the creation of a singulairty would require a set of events so astronomically unlikely that it is not believed that any do have or will come into existence during the lifetime of the universe. So in fact black holes DO have a radius, but considering the tremendous size quoted here, I imagine they are in fact referring to the Swartzchild radius.
      • Re:Event Horizon (Score:5, Interesting)

        by benwb (96829) on Wednesday October 16, 2002 @05:38PM (#4464751)
        Current theories in no way preclude the formation of a singularity. In fact it is pretty much the required outcome when a sufficiently massive start reaches the end of it's life. There is some discussion that when quantum theory and gravity are unified quantum effects may smear the singularity out of existence, but at this point it is all hand waving. Perhaps what you're thinking of is a naked singularity. A naked singularity is a singularity that is not cloaked by an event horizon, and is extraordinarily unlikely to occur.
        • Re:Event Horizon (Score:5, Informative)

          by Zack (44) <`zedoriah' `at' `gmail.com'> on Wednesday October 16, 2002 @05:45PM (#4464803) Journal
          Current theories in no way preclude the formation of a singularity.

          True, but current theories also haven't proven that inside a black hole _is_ a singularity. Although it's been a while, I remember from an Astronomy class I took that due to the rate of spin outside the black hole, and that conservation of momentum would mean it would spin faster inside means that the odds of a true point singularity are relatively low.

          But what do I know? ;-)

          • String Theory... (Score:5, Informative)

            by BlackGriffen (521856) on Wednesday October 16, 2002 @06:34PM (#4465164)
            Some of the more avant guard sting theorists are advancing the notion that black holes are simply really really big (as in high energy) elementary particles (i.e. strings). It'll be interesting to see if this particular theory holds any water, because it might mean high energy physicists may one day be trying to sling black holes at each other ;).

            BlackGriffen
          • Re:Event Horizon (Score:3, Informative)

            by ChazeFroy (51595)
            How is this story ground-breaking news? I posted this story [slashdot.org] over a year ago on Sept 6, 2001. The original post's article [washingtonpost.com] even states that they estimate the mass to be 2.6 million times that of the sun. Nice repeat.
          • Re:Event Horizon (Score:5, Interesting)

            by tgibbs (83782) on Wednesday October 16, 2002 @07:48PM (#4465584)
            True, but current theories also haven't proven that inside a black hole _is_ a singularity

            Since we have no unified theory, it is not possible to prove anything mathematically with confidence. The current theory of gravitation, Einstein's general relativity, requires a singularity. But GR is presumed not to be valid at quantum scales of distance, and since a singularity is infinitely small in GR, all bets are off.

          • Re:Event Horizon (Score:5, Insightful)

            by andr0meda (167375) on Thursday October 17, 2002 @04:45AM (#4467601) Homepage Journal
            I agree.

            An event horizon is actually just the boundary between light escaping and light being attracted by mass. It has nothing to do with the star being a singularity or not, only by the attraction force of the mass. That's obvious, right, so if all elements including photons (which have no mass) can no longer escape from the surface of the star, this means that the attraction force is higher than maximum speed of light, c. But does this have to mean that the volume of the mass is close to or equals 0? No. The star can only do 1 thing under extreme pressure: react it's core elements into heavier elements, untill they no longer react or destabilize the star enough to break the cycle, which probably can no longer occur. As the elements react, the star becomes heavier and the density of the volume rises, moving towards a singularity, but there is no reason to assume it _is_ a perfect singularity.

            In fact, the black hole is known to radiate Hwaking radiation, which means that the hypothetical perfect singularity black hole model, which can only absorb matter, does not exist. If the said conditions are not perfectly valid for a black hole, then why would it be a perfect singularity, even if this Hawking radiation exists only on a quantum probabilistic level?

        • Naked singularities (Score:5, Interesting)

          by Jon Erikson (198204) on Wednesday October 16, 2002 @05:51PM (#4464864)
          It depends on the angular momentum of the black hole (one of the three properties a black hole can have - size, charge and angular momentum). If it is spinning fast enough (and admittedly this is faster than is likely through natural causes) then the event horizon becomes flattened, and at fast enough speeds it becomes flat enough that a naked singularity may become visible.

          Of course this is all based upon classical arguments, and without a theory of quantum gravity we can't be sure. However it hasn't stopped Hawking and Penrose arguing about "cosmic censorship principles" :)

        • Re:Event Horizon (Score:5, Informative)

          by Transient0 (175617) on Wednesday October 16, 2002 @05:52PM (#4464875) Homepage
          The general theory of relativity predicts the formation of singularities, but when taken into consideration along with quantum theory as both Stephen Hawking and Roger Penrose have, they become astronomically unlikely(but not impossible). The formation of a black hole would require a mass at least as large as the one in the centre of our galaxy to form a true point singularity and it would have to compress in a mathematically exact symmetrical fashion. Most black holes should have a radius according to modern theories which use both relativity and quantum mechanics rather than ignoring one in favor of the other. Mind you, that radius should by phenomenally tiny.

          The discussion you refer to is the one about Hawking radiation. Stephen Hawking has demonstrated that Black Holes do actually(counter to intuition) radiate an extroardinarily small amount of energy. There is considerable debate as to whether it is possible for this radiation to ever cause the black hole to dissipate.
          • How so? (Score:5, Interesting)

            by Jon Erikson (198204) on Wednesday October 16, 2002 @06:09PM (#4464994)
            The general theory of relativity predicts the formation of singularities, but when taken into consideration along with quantum theory as both Stephen Hawking and Roger Penrose have, they become astronomically unlikely(but not impossible). The formation of a black hole would require a mass at least as large as the one in the centre of our galaxy to form a true point singularity and it would have to compress in a mathematically exact symmetrical fashion.

            Eh? Could you explain what you're talking about here? Because as far as I know, Hawking and Penrose's work has nothing to do with the likelihood of black holes forming. Indeed, one of the things about black hole formation in that no matter how unsymmetrical the initial state the end result is highly symmetrical, possessing no distinguishing features other than mass, charge and angular momentum... the "black holes have no hair" theorem.

            Or are you talking about the recent results in M-theory proving Berkentstein's semi-classical formula for black hole entropy? If so, I'm still not sure what that's got to do with black hole formation... it strikes me you've got things confused...

          • Re:Event Horizon (Score:3, Interesting)

            by benwb (96829)
            I'm assuming the modern theories that your referring to are the string theories and more recent m-theory. These look promising, and would result in the behavior that you describe. Their predictions about how the formation of singularities are affected by quantum gravity is the discussion that I'm referring to. But unfortunately they have not been able to make a single prediction that can be tested as of yet.
            General Relativity on the other hand, has been extensively verified, and has been correct in every test we've set for it. General Relativity predicts that singularities will form from a collapsing star.
            I still think that m-theory is handwaving until some testable predictions come out of it. BTW, I think that m-theory or one of it's derivatives will provide us a better description of the universe, but not today.
      • by The_Shadows (255371) <thelureofshadows&hotmail,com> on Wednesday October 16, 2002 @05:49PM (#4464847) Homepage
        if the "big crunch" theory is correct, it will probably be a singularity that the universe ends as.

        I think you mean the "gnab gib." You, know, a Big Bang backwards? I've seen it before, and it's quite a sight. It plays every night at the Restaurant at the End of the Universe.
      • Re:Event Horizon (Score:5, Interesting)

        by Mt._Honkey (514673) on Wednesday October 16, 2002 @06:05PM (#4464971)
        It was a singularity that became the big bang and if the "big crunch" theory is correct, it will probably be a singularity that the universe ends as...
        This is evidently a common misconception among many people. I was just told the currently accepted theory a couple of weeks ago by a physicist at U of I.

        We haven't the foggiest idea what the universe was all the way back to time=0, but starting at at least time = 10^-43 seconds, the universe was a very large, quite possibly infinite, distribution of matter. It was not an explosion away from a point, but an expansion of matter "away". Space time expanded like a rubber sheet, with every point moving away from every other point.

        Neat, eh?
        • Re:Event Horizon (Score:5, Insightful)

          by afidel (530433) on Wednesday October 16, 2002 @06:16PM (#4465031)
          You speak of high physics like there is any one theory of anything that everyone believes in. Trust me there are many, many smart people in physics and for any given area (birth of the universe, death of the universe, basis of gravity etc) there are ususally two to three main competing theories along with a half dozen or more fringe theories.
      • Re:Event Horizon (Score:5, Informative)

        by UnknownSoldier (67820) on Wednesday October 16, 2002 @07:02PM (#4465343)
        > if the "big crunch" theory is correct,

        It's not. Astronomers have known for a while [faqs.org] that the universe was expanding, but didn't know the rate. They recently discovered that the rate was accelerating [faqs.org]!

        Cheers
      • by acgetchell (143901) on Wednesday October 16, 2002 @09:21PM (#4466100) Homepage
        The Schwartzchild radius is the radius, for a given mass, that will form a singularity. For a ten solar mass star, that is about 30 kilometers.

        The Chandrasekhar limit gives the size limit for a star to collapse and produce a white dwarf. Most stars end their lives with a gravitational collapse, but electron degeneracy pressure (from the Pauli exclusion principle) prevents further collapse. However, for stars above ~1.2 solar masses, the gravitational collapse will overcome fermion repulsion, and the collapse will continue. Once the star's density has reached a certain point, it will collapse into a singularity. That density times the star's mass determines the Schwartzchild radius.

        The event horizon is delineated by those light rays that will neither fall in nor escape from, the black hole. However, just because you cross the event horizon does not necessarily mean you will strike the singularity. Instead, it depends upon the type of black hole you've encountered.

        In actual reality, you'll be fried by the blue shifted radiation coming from the accretion disk around the hole, but let's ignore that quibble.

        Black holes have mass, spin, and charge. No other properties are discernable behind the event horizon. The fact that the above properties can be determined without a world-line (that is, information also does not propagate faster than light, and hence cannot escape) says something fundamental about those properties.

        An uncharged, unspinning black hole is called a Schwartzchild hole. Once you cross the event horizon, you will unavoidably strike the singularity and perish.

        In the other types of black holes, such as the Kerr black hole (uncharged, spinning), Reisnner-Nordstrom (charged, zero angular momentum), and the Kerr-Newman black hole (charged, spinning) it is possible to cross the event horizon without striking the singularity. Instead, you can pass into another universe.

        Indeed, it's theoretically possible that you will pass through many universes. This is a one-way trip, however. If you try to get back to where you were, you will encounter the singularity and die.

        Actual solution of the Einstein field equations for the holes listed above, however, produce perturbations. These perturbations, so far, cancel out the ability to miss the singularity and enter another universe.

        Moving on, Hawking demonstrated that black holes evaporate. Hawking radiation is produced when half of a virtual particle pair appears inside the event horizon. Since both particles are no longer available to disappear under the Heisenberg time limit, the remaining particle acquires real energy. This energy comes from the black hole.

        Since the rate of evaporation is proportional to surface area/mass, smaller black holes evaporate explosively. Indeed, no black holes smaller than a proton could exist from the big bang.

        Finally, recent research shows that the universe is inflating, due to Einstein's cosmological constant (which, he ironically labelled as his "worst mistake"). That is, Hubble's constant is increasing. There will be no Big Crunch. The universe will expand at a faster and faster rate into nothingness.

        There are a lot of good books on cosmology. General Relativity is undergoing a renaissance right now because of all of this important, new information.

    • Re:Event Horizon (Score:3, Interesting)

      by s20451 (410424)
      There is a solution to the relevant equations such that a spinning singularity with charge actually forms a ring, not a point. I wish I could remember where I read that.
    • >>Usually when people talk about the size...

      ...they're lying.
  • size (Score:5, Informative)

    by Satai (111172) on Wednesday October 16, 2002 @05:20PM (#4464605)
    The "size" of a black hole is, in fact, the size of the Schwarzchild Radius, which is the distance at which neither light nor matter can escape. The black hole itself, the singularity, is indeed a point of infinite density.
    • Re:size (Score:5, Informative)

      by u19925 (613350) on Wednesday October 16, 2002 @05:37PM (#4464735)
      The "size" as used in the article is the observed radius within which a massive object (need not be a BH) is located. The mass is inferred from orbital time and distance of a nearby star. The physicists argue that there is no model for any object to be so massive (2-3 million times the Sun) and so compact (radius less than few times solar system) and yet it can prevent self induced gravitational collapse. Therefor the object must be a BH.

      The black hole (of the mass of several million times that of sun) at the center of the Milky Way galaxy, has been suspected for decades. However, as observations keep on shrinking the confinement radius, it keeps on ruling out other potential models.

    • Re:size (Score:3, Insightful)

      by frovingslosh (582462)
      The "size" of a black hole is, in fact, the size of the Schwarzchild Radius, which is the distance at which neither light nor matter can escape. The black hole itself, the singularity, is indeed a point of infinite density.

      Actually, current theories including string theory prevent the infinite point claims, but get to the next best thing (something in the order of 10 to -37 meters if I recall right).

      The size reported makes no sense though for a Schwartzchild radius of a black hole with the indicated mass, it's way way too large.

  • by Corporate Troll (537873) on Wednesday October 16, 2002 @05:20PM (#4464606) Homepage Journal
    ....now really, do they do this on purpose? I mean, I don't even want to see all the Troll links to goase.cx pointing out what to find in the hole.
    It's just disgusting!
  • From the article: (Score:3, Insightful)

    by Cyclopedian (163375) on Wednesday October 16, 2002 @05:22PM (#4464624) Journal
    The astronomers found "unambiguously" that the central star is moving around Sagittarius A "like the Earth orbits the sun," the ESO consortium said in a statement.

    So, does that mean that in time, the blackhole will swallow up the star?

    -Cyc

    • Re:From the article: (Score:5, Informative)

      by delta407 (518868) <slashdot@ler[ ]ax.com ['fjh' in gap]> on Wednesday October 16, 2002 @05:35PM (#4464721) Homepage
      So, does that mean that in time, the blackhole will swallow up the star?
      Maybe, maybe not.

      Comets can orbit the sun for a really long time; some smack into an object (like the sun, for instance), some escape their orbit, and some just keep orbiting. There's nothing that guarantees the star will get sucked in; it all depends on the orbital path, really. It may experience a slingshot effect and leave the black hole altogether.
      • by dvk (118711) on Wednesday October 16, 2002 @06:13PM (#4465018) Homepage
        You know you've been spending too much time on /. when you read the last sentence of the above reply as "It may experience a slashdot effect".

        -DVK
      • I found this cool earth orbit physics toy and demonstration [colorado.edu] while reading one of my favorite web logs, Sensible Erection. [sensibleerection.com] (I, uh... Read it for the articles.)

        "This is the coolest this i have seen all week, click /drag the screen to put a satelite in orbit.. see how long it lasts...
        pull off a moon only orbit for maximum kudos"


        The physics for object orbits are incredible. This is a great demonstration of the exact effects you describe, and should apply to the questions and comments about orbits around a black hole.

        Enjoy!

        P.S.: You have no idea what a breath of fresh air it is to be able to visit cool links that aren't being slashdotted to hell and back.
    • by jaaron (551839) on Wednesday October 16, 2002 @05:39PM (#4464754) Homepage
      An object can orbit a black hole just like a planet can orbit the Sun (or a star). The Sun will not swallow or pull in the Earth any time soon. Black Holes are not cosmic vacuum cleaners that "suck" up everything around them. If you're in a stable orbit, it would be just like orbiting a Sun.

      That said, there is evidence from general relativity that due to graviton radiation (gravity particles), large orbiting bodies slowly move closer to each other. The gravitons leaving such a system take energy out of the system slowly bringing the orbiting bodies together. This effect is (AFAIK) theoretical, although many people are currently working on ways to detect this graviton radiation and show that it is coming from systems like this. So in this case, yes, eventually (think eons) the star and the black hole would slowly move towards each other (the star would move more since it the least mass of the two) and in this type of collision, the black hole wins.
      • There is a pair of pulsars orbiting each others that has been observed to have shrinking orbits like you write due to GR effects. In fact this pair is the most precise test of general relativity that we know.

        Roger Penrose talks about it in his book `the emperor's new mind', and here is an excellent link [cornell.edu]

  • Now we know (Score:4, Funny)

    by m_chan (95943) on Wednesday October 16, 2002 @05:22PM (#4464626) Homepage
    where Enron's accountants found work.
  • by Spicy_Italian (224301) on Wednesday October 16, 2002 @05:24PM (#4464639)
    According to my Astronomy course, Super-Massive black holes are less "violent" than their smaller brothers because most of the mass is concentrated at the center in a very very small space. Their event-horizons are very large because of this mass, which makes them seem not as dense as we would assume. With a small black hole, the event horizon is very small, and thus the effects near the point are much more drastic because mass that passes the event horizon is "consumed" immediately. I realize I am simplifying quite a bit, but hopefully you get the point.
    • by Jon Erikson (198204) on Wednesday October 16, 2002 @05:58PM (#4464922)
      Yes, because gravitational effects are proportional to M/r^2 and so drop off over distance and increase with mass... but because the radius term is squared it plays a more important role in the strength of the effect.

      As such with a larger black hole (large M, smaller 1/r^2) the difference in gravitational effects over the size of say a person is fairly small because r^2 doesn't change an awful lot. However with a small hole (small M, large 1/r^2) the difference in strength of the gravitational field over the size of a person is a lot larger and so there are tidal forces which tend to cause things to be ripped apart.

  • by smd4985 (203677) on Wednesday October 16, 2002 @05:25PM (#4464648) Homepage
    the scientists in the article seem to assert that this is CONCLUSIVE proof of a black hole's existence. but i remember reading a few months ago about a schism in the physics community - a sizable segment of the community is disputing the theoretical existence of black holes! i wonder how this discovery will affect that debate....
    • Kip Thorne has a subscription to penthouse. They exist.
    • by pagsz (450343)
      OK, then, it's a gravastar [cnn.com].

      This is not conclusive proof of black hole theory, only conclusive proof of a supermassive object at the center of our galaxy. It does not answer the theoretical question as to whether black holes or gravastars best fit the observations.

      Obviously, the scientists making this announcement would be in the black hole segment of the physics community.

      Trying to think of something witty and clever to end this post with . . . . ah, screw it . . .

  • To clarify... (Score:5, Informative)

    by pq (42856) <rfc2324&yahoo,com> on Wednesday October 16, 2002 @05:25PM (#4464649) Homepage
    Since when do black holes occupy so much space (I thought they were points)? And how can something with a density only 1/100 of our Sun be called super-massive?

    The "size" of the black hole refers to the size of its event horizon (a.k.a the Schwarzschild Radius), which is R = GM/2c^2. For a huge value of M ("supermassive"), the event horizon is very large: once you cross this, there's no coming back, and our physics stops at the edge. But since R is so large, the tidal forces are small at the event horizon - much smaller than the tidal forces at the event horizon of a smaller black hole. (Chew on it for a second and it makes sense).

    The "actual" naked singularity is in fact a point, but we have no way of probing anything inside the event horizon. So calculating the density of a black hole is misleading...

    • Re:To clarify... (Score:5, Insightful)

      by Pedrito (94783) on Wednesday October 16, 2002 @05:40PM (#4464757) Homepage
      ...once you cross this, there's no coming back, and our physics stops at the edge.

      I'm not picking on you, others have been saying things like this too. They talk about "there's no coming back", "can't communicate to the outside", and "physics stops at the edge" and such. These are all theories, not facts. I wish people would just be a little more careful in their phrasing, as indeed, black holes themselves are still theories.

      Even relativity is only a theory. But I digress.

      No, physics doesn't stop at the edge, our understanding of physics breaks down at the edge. We don't know what happens because our physics deals in infinities that make no sense once you cross the event horizon. Physics still exists, it's just undefined to us.

      In the same vain, communication from within a blackhole to the outside is impossible, assuming our basic theories of black holes are correct, and assuming that there's no way to communicate faster than the speed of light. Again, relativity is a theory, not a law. It's a theory that has come into question recently as well.

      I'm not putting down Einstein or relativity. Amazing stuff, to be sure, but it may not be entirely correct.
      • Re:To clarify... (Score:5, Insightful)

        by joto (134244) on Wednesday October 16, 2002 @06:16PM (#4465035)
        These are all theories, not facts. I wish people would just be a little more careful in their phrasing, as indeed, black holes themselves are still theories.

        I wish people had a little bit of training in theory of science, before they started worrying about phrasing in discussions about science.

        In day-to-day communication, we use the word "theory" to denote something we are not sure of. Thus in day-to-day communication "just a theory" makes sense.

        However, in science, a "theory" is basically what the majority of scientists believe to be the truth. There is no difference between a "natural law" and a theory (In fact, "natural law" is most often viewed as a misnomer, and is simply something we use for historical reasons). And there is no "higher level" something can escape to, when people think it's worthy of a higher status than "just a theory".

        If you want a word for what scientists use for the day-to-day usage of "theory", their word is "hypothesis". A hypothesis is nothing but an idea. Most theories start as a hypothesis, and then, after a sufficient number of supporting facts have been found, and experiments have been done, people will then speak of it as a "theory". Sometimes, scientists will also use the word "model" as something in-between, but most often it is used by engineers using well-known theories to model complex phenomena.

        As for black holes being "only a theory" (in the meaning of "just a hypothesis". Yes and no! It would be very hard to come up with a cosmological model that fitted our universe, that would not predict the existence of black holes. And it would be very hard to explain some observed phenomena as something else than a black hole. On the other hand, the theories of what goes on inside the hole, how it was created, and how it dies (if ever) is very much up to discussion. As for doubting their existence, well it's possible, but not easy...

        As for relativity being "only a theory", again assuming you mean "just a hypothesis". In a word, no! The basic ideas of relativity has predicted a lot of observable things in the universe better than any other model. And it has been verified again, and again through experiments. Is it entirely correct? No, it doesn't fit in with quantum mechanics, and therefore can't explain everything (just like Newtons laws can't explain everything). So it's reasonable to believe that there exists an even more complex theory of everything, that will incorporate both quantum mechanics and relativity. Unfortunately, there haven't been too much success in this area yet.

    • Re:To clarify... (Score:4, Interesting)

      by zmooc (33175) <zmooc@@@zmooc...net> on Wednesday October 16, 2002 @06:35PM (#4465170) Homepage
      once you cross this, there's no coming back

      Is this true? Could you/someone explain to me what would prevent me from building a huge strong ring around the event horizon and lowering a probe from that ring through the event horizon? The ring could be stabilized by the gravity of the black hole itself and a counter-weight on the side oppossite to the probe. Would the force on the probe be so strong that no force is strong enough to pull it back? Or is it theoretically impossible to build a probe strong enough to withstand the gravity?

      • Re:To clarify... (Score:3, Informative)

        by bravehamster (44836)
        Is this true? Could you/someone explain to me what would prevent me from building a huge strong ring around the event horizon and lowering a probe from that ring through the event horizon? The ring could be stabilized by the gravity of the black hole itself and a counter-weight on the side oppossite to the probe. Would the force on the probe be so strong that no force is strong enough to pull it back? Or is it theoretically impossible to build a probe strong enough to withstand the gravity?



        You could do that, but it would be useless, and for this reason: The force you are applying to the probe counteracts the force of gravity on the probe caused by the black hole, and the *total* force on the probe drops below the amount necessary for it be within the schwarzchild radius. However, you wouldn't be able to probe anything inside the radius. It would just be as if you pushed the event horizon back. Sort of like pushing your hand into a waterbed: your hand is now where the waterbed *used* to be, but you still aren't inside the waterbed. But once you do enter the event horizon we don't know of any way get back.

      • Re:To clarify... (Score:5, Informative)

        by Tackhead (54550) on Wednesday October 16, 2002 @08:16PM (#4465766)
        > Could you/someone explain to me what would prevent me from building a huge strong ring around the event horizon and lowering a probe from that ring through the event horizon?

        The short answer is "relativistic effects".

        Near the event horizon, gravity warps space - the conventional notions of "distance" and "time" get fscked up.

        What you propose is equivalent to saying "If I'm at the front of a train travelling at 99.999999% the speed of light, and I shoot a bullet forward at 2% of the speed of light, isn't the bullet going to be going faster than light?"

        And the answer is, "Well, no. Because space and time are fscked up when you're going very quickly."

        From the point of view of a guy standing at the end of the tracks, he'll shine a light down the track, see some X-rays bouncing back from the bullet and the train, before being flattened by both the bullet and the train almost simultaneously.

        From the point of view of you (on the train), looking forward, you'll see the entire universe running at about 10000 times normal speed - stars evolving in minutes - and the bullet flying away from you at 2% of the speed of light.

        Back to your original question - lowering a probe into the black hole and pulling it out again. Gravity will have a similarly-weird effects.

        From the point of view of the guy lowering the probe, the probe will fall towards - but never through - the event horizon. It'll just fall more and more slowly, and if he shines a light at it to observe it, he'll see it get redder and redder, until it vanishes into the infrared. And since the probe never makes it past the event horizon, he never gets any data back from beyond it.

        From the point of view of the probe, and looking up, time speeds up dramatically - in a few minutes, he sees the guy lowering him get change shifts, coming back, growing older, dying, the space station being abandoned, stars evolving, billions of years passing, whole galaxies fading into the infrared, and then when he hits the event horizon, he sees nothing avove him, and if he looks down, then it gets real weird. It's quite literally anybody's guess what he sees. But it's quite certain he can't tell anybody above him a word of it.

        Relativity's weird like that. The freaky stuff - time dilation and what-not - has all been demonstrated by experiments involving clocks and airplanes and satellites. (The relativistic corrections made to account for a satellite's motion, for instance, are part of why GPS is so accurate.)

        • Argh, I fscked up! (Like I said, relativity's weird ;)

          > From the point of view of you (on the train), looking forward, you'll see the entire universe running about 10000 times normal speed - stars evolving in minutes - and the bullet flying away from you at 2% of the speed of light.

          Argh. The sped-up universe is what a guy on the back of the train looking backwards (and the guy on the black hole probe looking up) sees.

          The guy on the front of the train (and you, lowering the probe and observing the probe) sees a universe running at 1/10000th speed - a 2.0 GHz Athlon will look like it's running at 0.2 kilohertz and what-not.

    • Re:To clarify... (Score:5, Informative)

      by RobertFisher (21116) on Wednesday October 16, 2002 @08:20PM (#4465789) Homepage Journal
      This is a key point.(Although you got a factor of 2 wrong. :-) )

      Moreover, your clarification contains the essential answer to one of the original poster's comments. The mean density within the horizon, assuming the region is spherical, is

      M / (4 / 3 pi R^3) = M / [4 / 3 pi (2G M / c^2)^3]
      = 3 c^6 / (32 pi G^3 M^2)

      The key point being that the mean density within the horizon is inversely proportional to the square of the mass of the black hole. For a black hole of 1 solar mass, the mean density within the horizon works out to be amazingly high : of order 10^16 gm/cm^3! On the other hand, for a billion solar mass black hole, this mean density is much, much smaller : of order .01 gm/cm^3.

      Another key point is that the masses are not directly detected -- the must be inferred by their gravitational influence on surrounding stars and gas. Observers currently do not have the resolution to probe down to the scale of the horizon, so the argument for a black hole is a compelling one, though not absolutely certain. The masses are not directly detected -- the must be inferred by their gravitational influence on surrounding stars and gas. The primary argument in favor of a black hole is the lack of other possible alternatives. One can prove a strict limit on the mass of a neutron star (which is the most compact stable object known to astrophysics) assuming only causality (ie, that whatever is holding up the neutron star has a sound speed less than the speed of light), is around 5 solar masses. Hence, the most tightly packed situation one could possibly imagine, with the same mass as observed, would be a cluster of several hundred thousand to millions of neutron stars. However, even such a situation is dynamically unstable over many orbits : the neutron stars will tend to form tighter and tighter binaries at the core of the cluster until they merge. Even a single merger would likely create a small seed black hole, which swallow up all the surrounding stars until no matter is left to accrete. So even in this extreme situation, the outcome would eventually be a supermassive black hole. For this reason, the argument for a black hole at the center of our galaxy and others is a very strong one -- if it were a legal case, it would likely hold up in a court of law. However, the absolute proof will require a "smoking gun". Perhaps this will consist of a detection of gas emitting from the accretion disk right around the black hole horizon, carrying with it an absolutely unambiguous signature of the horizon. Or perhaps it may come from gravitational waves radiating at very low frequencies (millihertz or below) -- a telltale sign of the slowly oscillating hole. Such waves will be undetectable from the Earth's surface due to ground noise, and will require a spaceborne mission such as ESA's LISA.

      Bob
  • So... (Score:5, Funny)

    by sirgoran (221190) on Wednesday October 16, 2002 @05:26PM (#4464659) Homepage Journal
    Would this be the proverbial drain that we're all swirling around to our eventual demise?

    Just wondering.

    -Goran
  • by Anonymous Coward on Wednesday October 16, 2002 @05:26PM (#4464662)
    From the very center, this galaxy sucks.
  • by Crispin Cowan (20238) <crispin.crispincowan@com> on Wednesday October 16, 2002 @05:27PM (#4464672) Homepage
    So what is up here? Since when do black holes occupy so much space (I thought they were points)?
    Black holes are not points. The edge of a black hole is the point at which the escape velocity (velocity required to escape the gravitational field of the object) exceeds the speed of light, and thus light can no longer escape from the object. This is called the "event horizon." [uiuc.edu]

    This would seem to imply that, in theory, a very large black hole could have rather low density inside the event horizon. It seems to me that a black hole could spontaneously form around a particularly dense cluster of stars if it was large enough and they all happened to clump together.

    But my head starts to hurt thinking about what happens to physics when a region of normal space suddenly finds itself inside a black hole like that. I am definitely not a physicist, so I can't explain what goes on inside a black hole, or if my globular cluster black hole is even possible.

    Crispin
    ----
    Crispin Cowan, Ph.D.
    Chief Scientist, WireX Communications, Inc. [wirex.com]
    Immunix: [immunix.org] Security Hardened Linux Distribution
    Available for purchase [wirex.com]

    • If my memory serves me, I believe that there is a nearly confirmed black hole at the center of M15 (a globular cluster). However the conditions for it's creation are probably still up for debate. A bunch of semi-simultaneous stellar collisions at the core is not out of the question though.
    • There probably isn't much settled ground in black hole theory. If such a globular cluster were possible I don't think it would be a matter of a quick change from 'normal' to beyond the event horizon space. In some sense, the space inside is negative, so some things that are normally always false would be true (what? flow of time, speed of light, ??? I'm not a physicist). Can there be multiple singularities inside? Does everything quickly get sucked into the singularity once it crosses the event horizon (still quite some time for galaxy BHs)? Does everything end up at the speed of light at some point (at, or after crossing EH)?

      At some level it will probably always be a mystery. It's a 'world' boundary since information can't get out (can it get in or is information crushed out at some point?). Ultimately it is a physical phenominon, not a mathematical model, so the reality may be quite a bit different than any mathematical model. If you could fly about the galaxy SF style you would probably learn a lot more about the actual structure of the universe from experiments related to this and other black holes.

      It's pretty amazing what can be learned this far out. I thought I heard a mention on the NPR report on this about a star headed for the EH. The universe is always running experiments for us if we have the instraments in place to watch closely. Try following the link to the natural nuclear reactors and follow the link under the picture about the constancy of cosmological constants. Very cool instraments ... High res. spectroscopy allows them to look back in time and try to figure out why/how these constants might adjust. The Hubble is cool, but we are going to need an array of flexible instraments above the atmosphere to get at the really interesting questions.

  • by vondo (303621) on Wednesday October 16, 2002 @05:28PM (#4464677)
    What they have found is a star that orbits the center of our galaxy at an average radius of 17 light hours and does it in 15 years. That radius is what is 3 times the size of our solar system. The event horizon of the black hole does *not* occupy all of that space.
  • by Anonymous Coward on Wednesday October 16, 2002 @05:32PM (#4464708)
    Jesus people! Of the 41 posts so far, 13 of them have said the same thing... event horizon.

    I can imagine the first few stepping on eachother, but doesn't anyone else bother to see what others have written before posting the same thing... over and over and over...?
  • Better article (Score:5, Informative)

    by Anonymous Coward on Wednesday October 16, 2002 @05:33PM (#4464711)
    Space.com is carrying a more informative article [space.com] about it.
  • by GMFTatsujin (239569) on Wednesday October 16, 2002 @05:37PM (#4464733) Homepage
    There was a show called (oddly) Supermassive Blackholes on a few months ago.

    Interesting stuff -- once they were discovered to be in just about every galaxy, people smarter than me started thinking about how they formed. Conventional wisdom says that they formed after the galaxy took shape, and that stellar matter near the center collided and merged into these monsters. Another theory, however, posits that the SMBH actually triggered stellar formation in a cloud of otherwise unremarkable hydrogen.

    The idea is that as the hydrogen gas fell inward and collapsed, the gas in the nearby area would heat up and glow. This is, of course, what we see. However, it goes further to say that this surrounding energetic gas could cause a sort of super-shockwave of energetic particles travelling back out through the surrounding gas, pushing it around and raising the density, causing the whispy bits to compress together to the point of fusion.

    Poof! Stars born by black holes at the center of a gas cloud.

    Pretty neat, I thought.
    GMFTatsujin
  • by guttentag (313541) on Wednesday October 16, 2002 @05:40PM (#4464764) Journal
    We already know there's a powerful telepath living on a planet there and he needs a space ship. If there had been a black hole in the center of the galaxy, you'd think someone would have mentioned it.
  • Black hole size (Score:5, Insightful)

    by Anonymous Coward on Wednesday October 16, 2002 @05:41PM (#4464769)
    The article is referring to a determination of the orbit of the star closest to the galactic center. The periasteron (closest point in the orbit) is 17 light hours from the galactic center. This implies that the mass necessary to create that orbit is concentrated within that radius. The only thing in our current cosmic zoo that fits 3 million solar masses inside of 17 light hours is a black hole. The event horizon itself should be smaller than that, but not by much.
    What is an interesting question is where the Roche limit is for these parameters, and how close this star is to that limit. (In other words, how much closer can the star get before it is ripped apart.) I seem to remember that it is possible to set up conditions so that the Roche limit is inside the event horizon. Obviously, the physics around there are very strange.
  • by Ichoran (106539) on Wednesday October 16, 2002 @05:43PM (#4464782)
    For anyone who wants equations to go along with the descriptive posts on event horizons and Schwarzschild radius, said radius is given by
    • r = 2GM/c^2
    where G = 6.67e-11 m^3/s^2*kg (the gravitational constant) and c = 3e8 m/s (the speed of light, of course). Plug in 3 million sun-masses (the sun weighs 2e30 kg), and you have
    • r = 8.9e9 m = 5.5 million miles = 0.06AU
    So unfortunately, the event horizon isn't three times as big as the solar system. The earth's orbit is 1AU (that's how the unit is defined). The event horizon barely stretches past the surface of the sun (7e8 meters)!

    So much for that idea!
  • by JHromadka (88188) on Wednesday October 16, 2002 @05:43PM (#4464788) Homepage
    at the middle of the galaxy was some calm looking planet with a grey-haired guy that Sybok is looking for. Thanks for bringing up horrible memories of ST:V!
  • by Rui del-Negro (531098) on Wednesday October 16, 2002 @05:48PM (#4464832) Homepage
    Since when do black holes occupy so much space (I thought they were points)?

    They're big points.

    RMN
    ~~~
  • by Anonymous Coward on Wednesday October 16, 2002 @05:58PM (#4464927)
    "I think the article is talking about a maximum possible size of the object, due to limitations on the resolution of our instruments."

    I'm sure this editorial comment was well-intentioned, but the article would have been much better off without it. What the article refers to corresponds closely quite nicely to the Schwarzschild radius of a supermassive black hole.

    A very massive black hole will necessarily be much less dense than the Sun, and can even be less dense than the Earth.

    The simple reason is that (assuming a static, spherically symmetric mass distribution) the mass of an object is directly proportional to its Schwarzschild radius. But density is proportional to mass divided by radius cubed.

    So if you double the mass of a black hole, you must necessarily double its radius. By definition this increases its volume eight-fold, and so its density is decreased by a factor of four.

    So as you consider larger and larger black holes, you must see that their densities are smaller and smaller.

    If you are in the market for a comparatively easy textbook that will teach you more about general relativity, I recommend Exploring Black Holes by Taylor and Wheeler. If you have a firm grasp of calculus and freshman physics, you will be able to handle it. It is more expensive than a normal book, but cheaper than the average textbook.
  • Discrepancy..... (Score:3, Informative)

    by deego (587575) on Wednesday October 16, 2002 @05:59PM (#4464934)
    > Since when do black holes occupy so much space (I > thought they were points)?

    Black holes are not points. They are almost indeed like holes.. A "black hole" is just a particular configuration of space, and saying whether there's an "object" in there or not is a matter of, well, semantics.

    Take a flat plastic sheet grab it at a point on it and pinch it a bit ---that represents a lowly object , like the earth --- and completely accounts for earth's gravity. But since earth has nongravitational effects too (albeit small), you do need an object in addition to the spacetime pinch.

    Now take the pinch and keep pulling on it and run away with that pinch to a very large distance.. When this pinch extends to infinity, we call it a black hole---so, like, no "object" there at all...

    The "size" of this non-object for us mortals is any reasonable definition of the radius of the pinch. Physicists use a more formal event-horizon for an indication of size.. Thus, black holes do have "size"... (And the weight is also determined by the radius of this pinch..)

    Now comes the matter of the puzzle you posted: Your calculation would indicate >> 230 million M (M being the mass of the sun) whereas the article says 3 million M. That is a discrepancy of a factor of, >> 100 in the mass, or say, in the volume of the solar system.

    Which amounts to a discrepancy in the "radius" of the solar system of >> 7.

    But I would guess that the solar system's "radius" easily has a discrepancy of a factor of > 1000 opr more depending on who you ask, depending on whether you define it to extend to Pluto, or the farthest of the farthest end of the (huge!) Oort cloud. (And 1000 >> 7 indeed..)

    So, that would explain it, i think.

  • by avgjoe62 (558860) on Wednesday October 16, 2002 @06:01PM (#4464949)
    The galaxy is a large, circular object with a hole in the middle.

    Very much like those things you find at a Krispy Kreme shop, but with a lot less frosting...

    Does this mean that the voice we will hear at The End of Time will be saying "OOOhhh... donuts..."

    • by IntelliTubbie (29947) on Wednesday October 16, 2002 @08:09PM (#4465717)
      The galaxy is a large, circular object with a hole in the middle.
      Very much like those things you find at a Krispy Kreme shop, but with a lot less frosting...

      Does this mean that the voice we will hear at The End of Time will be saying "OOOhhh... donuts..."


      Stephen Hawking: "I am intrigued by your theory of a donut-shaped universe, Homer. I may have to steal it."

      Cheers,
      IT
  • by dh003i (203189) <dh003i@nOspaM.gmail.com> on Wednesday October 16, 2002 @06:05PM (#4464973) Homepage Journal
    The author's confusion here seems to be regarding the differences between a blackhole and its singularity.

    A black hole is just that -- a black hole. It is a region of space from which nothing can escape (approximately; black holes do very slowly radiate heat). In other words, the volume a black hole occupies is defined by the Schwartzchild radius: the point beyond which the escape velocity exceeds c.

    A singularity is the "center" of a black hole; it is an infinitely dense point in space, of enormous mass.

    Interestingly, black holes may have some useful properties for astronomers. Light heading towards a black hole will be refracted around it and bent; in essence, the black hole acts like a magnifying glass.
  • by waldoj (8229) <waldo AT jaquith DOT org> on Wednesday October 16, 2002 @06:17PM (#4465042) Homepage Journal
    I can't believe how Milky-Way-centric that Slashdot still is. The bias is incredible. Nowhere in this story does it identify which galaxy, as if we all live in the same galaxy. For chrissake, people, it's the Internet.

    Jeez.

    -Waldo Jaquith
    • by elandal (9242) on Wednesday October 16, 2002 @07:26PM (#4465469) Homepage
      I think it's a reasonable assumption that the Internet has a maximum radius of max TTL of an IP packet, which excludes such close objects as Mars, let alone another galaxy.

      So please, even if You were just a visitor, considering You're posting on slashdot You might as well forget the idea of returning and start living on slashdot. You're confined to the small space of 400 seconds from slashdot.
  • by lars-o-matic (533381) <slashlars@ecentr ... minus physicist> on Wednesday October 16, 2002 @06:19PM (#4465051) Homepage

    The size issue: the companion star's orbit tells us the maximum possible size of the central object. If the orbit is 17 light hours across, the primary is at most that large. It can be smaller, just as our Sun's diameter is smaller than the orbit of Mercury.

    The proof the central object is a black hole is that nothing else can fit millions of solar masses into a sphere 17 light-hours across. The black hole need not fill that volume. More precisely, the event horizon need not fill that volume.

    Singularities, point masses, event horizons: the size of a black hole depends what you mean. The singularity is the postulated point of infinite density: outside observers can't see it because it's inside the event horizon. The event horizon is the point of no return; in classical terms, the escape velocity equals the speed of light at the event horizon. The gravitational force is finite at the event horizon, and need not be extreme if the black hole is very, very large. If the universe is closed, we are all inside a black hole now, and will experience singularity at the Big Crunch.

    But it isn't useful to think about the inside of a black hole. Different physics might apply -- lots of smart people think so. From the outside, as another poster wrote, all you get to observe is the black hole's total mass, total charge and total angular momentum -- that's plenty to work with in astronomical observations.

    As to matter 'spiralling in', or the entire galaxy being sucked in by 'infinite gravity': Earth isn't being sucked into our Sun, is it? Unless you're quite close to one, the gravitational field of a black hole essentially (asymptotically) follows an inverse square law, like the gravity from any object. (When you get close, in units of the Schwarzchild radius, you do indeed 'spiral in' because the field strength increases faster than inverse square. The precession of Mercury's orbit is used to measure the deviation from inverse-square near our Sun, and is one of the 'proofs' of Einstein's General Relativity.)

    The other mechanism for 'spiralling in' is loss of orbital energy due to friction, as in the accretion disk around neutron stars, for example.

    That is all. Return to your homes and families. :-)

  • by lamontg (121211) on Wednesday October 16, 2002 @06:23PM (#4465073)
    Please mod down all the people who are currently at +5 claiming that the size of the object is really the event horizon, which is very large due to it being a supermassive black hole. This is a true statement, but it still doesn't explain the claimed size of the black hole in the article.

    If you work out the schwartzchild radius of the sun using r=2GM/c^2 it comes out to around 3000 m. For the upper limit of 3.7 million solar masses that would mean that the black hole had a schwartzchild radius of around 1 x 10^10 m. This is about a factor of 14 larger than the radius of the sun which is 7 x 10^8 m.

    This is no where near as large as the "volume of space around 3 times larger than the solar system" which is in the article. The poster of the article was also correct that the density was way too low. It is correct that supermassive black holes have large event horizons which are larger than the radii of typical stars like the sun. However, the average density inside of that event horizon is still denser than a neutron star.

    I wish I had the 5 moderator points I had last week, I'd go to town on this story...
    • journalists aren't always very carefull about the numbers in their stories.

      wouldn't 3 times the solar system be about 17 light hours ?

      Which happen to be the size ot the orbit of the star they were tracking.

      not the size of the black hole.

  • by dpp (585742) on Wednesday October 16, 2002 @06:46PM (#4465241)
    The article then claims that it occupies a volume of space about 3 times that of our solar system.

    This might be a misinterpretation. In the ESO press release [eso.org] they say:

    ...the star approached the central Black Hole to within 17 light-hours - only three times the distance between the Sun and planet Pluto.

    So that puts an upper limit on the scale of the thing, but doesn't imply it takes up all of that space.

  • by wilgamesh (308197) on Wednesday October 16, 2002 @06:49PM (#4465269) Homepage
    Regarding discussions about whether the "volume" of the article implied the Event Horizon, that's what I thought it was at first also. But then I came up with some numbers that don't seem to correspond to those of the CNN article. I then checked out the original paper. The paper is formally on the observation of a star that seems to be orbiting the galaxial center, and this radius of orbiting is what they are pinning down as the a putative upper limit of the size of the supermassive object.

    It would seem that the original poster's comment was correct in that this was the _Upper Limit_ of the radius of the supermassive object, and not the Event Horizon radius.

    Let me clarify,

    The Schwarzschild radius (Or Event Horizon) is given by

    r_SCH = 2 G M / c^2

    where G is gravitational constant, M is mass of object, and c is speed of light. If we use, as per CNN article (yeah, I know, good source)

    M = 3 x 10 ^ 6 * mass of sun
    mass of sun = 2 x 10 ^ 30 kg
    s.t. M = 6 x 10 ^ 36 kg
    and G = 6.67 x 10^ -11 Nm^2/kg^2
    and c = 3 x 10^8 m/s^2

    then r_SCH = 12 x 10 ^ 36 * 6.67 x 10 ^-11/9 x 10^16

    r_SCH ~ 1 x 10^10 meters.

    I looked up some values of Pluto's radius, and got about 3000 million miles, or 5 x 10^9 km, or about 5 x 10^12 m.

    So this galaxial blackhole seems to have a radius 100-1000 times less than the solar system radius.

    And indeed, in the final page of the Schodel paper, there is a mention that the observed radius of the orbiting star is ~ 2000 times the Schwarzschild radius, and not the actual Schwarzschild of the star. i.e. the observed radius of orbit is much much larger than the putative Schwarzchild radius.

  • by q2a (519813) on Wednesday October 16, 2002 @06:50PM (#4465273)
    These researchers are popular here on campus [ucla.edu] at UCLA. Also, check out some nifty pictures here [nasa.gov].
  • by DjMd (541962) on Wednesday October 16, 2002 @09:45PM (#4466204) Journal
    From Gsu.edu Astrophysics [gsu.edu]:
    Any mass can become a black hole if it collapses down to the Schwarzschild radius ... The Schwarzschild radius (event horizon) just marks the radius of a sphere past which we can get no particles, no light, no information.
    R= 2(MG)/ c^2

    Therefore at 3.7 million solar masses...
    the Schwarzschild radius is
    1.0919401548997975x10^10 M
    Which is much smaller than our solar system (the earth orbits at 150,000,000 KM).
    But I imgine that they would measure the Acreation Disk.....
    The Schwarzschild radius calulation is fun. One can plot density verses radius and it becomes clear that something the size of our galaxy with density of water would be a black hole...

    Space is an empty place!

  • by Stalyn (662) on Thursday October 17, 2002 @12:02AM (#4466753) Homepage Journal
    our galaxy does suck.
  • Some calculations... (Score:3, Interesting)

    by TheSHAD0W (258774) on Thursday October 17, 2002 @03:17AM (#4467378) Homepage
    A friend and I worked out a few calculations on the black hole...

    Assuming it was 3 million solar masses, the diameter of its Schwartzchild limit (effectively the diameter of the black hole) would be 8.8 million kilometers, or about 6-1/3 times the diameter of our sun.

    If the Earth were in orbit around this black hole at the same distance we are from the sun (assuming it wouldn't be torn to shreds by tidal stresses), a year would be 5 hours long.

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