Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does a black hole have shape?

  1. May 11, 2009 #1
    Does a black hole have "shape?"

    Today I was watching the Science Channel and a program on the Hubble telescope was on. It was said that pictures of black holes have been taken not by the Hubble, but by another similar device that takes pictures of Gamma and/or X rays.

    This sparked an interest in attempting to learn more about black holes... and every thing I've read has had my head swimming in theory and terms I cannot ever hope to understand. I am fascinated by celestial bodies and the universe as we know it, but I severely lack the cognitive capacity to discuss or understand this topic at the level that would satisfy my curiosity. So I'm looking for "dumbed down" answers, if that's possible.

    While trying to comprehend a black hole I asked myself: "If a black hole could be observed from a distance beyond the event horizon, what would it look like? Would it be 2 dimensional? Would it be spherical?" For example, picture a light bulb as you flip the switch and the bulb turns on. The light emitted bursts outward from all direction to bathe the room in white light. Would a black hole be the "opposite" of this happening, where all matter and light is pulled into the center of the bulb?

    And also, if all matter approaching the black hole gets "sucked in" where does it go? I was always under the impression that matter cannot "disappear," all matter goes somewhere. So if this statement (and my school teacher) is correct, then by my limited understanding a black hole would never stop growing until the universe collapses into itself. This is where my comprehension overloads and I reach a logical paradox. To further this quandary, what happens if 2 black holes grow so immense that they "touch?"
     
  2. jcsd
  3. May 11, 2009 #2

    DaveC426913

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    It gets crushed to zero size. But it's still there at the centre; it still has the same gravitational pull.

    Black holes do not suck matter any better than any other body of the same mass.

    If the sun were magically and instantly replaced with a BH of the same mass, all the planets would contiue to orbit it with very little change. And just as a BH can acquire matter and grow more massive, so can the sun. But just as the sun does not wreak havoc on everything in its vicinity, neither does a black hole.
     
  4. May 11, 2009 #3
    Re: Does a black hole have "shape?"

    So if a black hole had the same mass as the sun it would be the same size, but the density would be exponentially larger... correct? Also, does the black hole's gravity pull matter towards the event horizon, or is the "outside" of the event horizon the distance from when escape velocity is "possible?"
     
  5. May 11, 2009 #4

    DaveC426913

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    No. A BH as massive as the sun would be tiny. A few kilometers.

    That's why BHs can tear things apart. Gravitational force falls off as the square of the distance from the centre. With the sun you cannot get any closer to it than the surface, which is 400,000 miles from the centre. But with a BH, you can get within a few miles, you don't have that falling off of distance.

    Matter is pulled to the centre of the BH. The event horizon isn't a physical object of any sort; it is nothing more than a distance at which absolutely nothing, including light, can escape.
     
    Last edited: May 11, 2009
  6. May 11, 2009 #5
    Re: Does a black hole have "shape?"

    So the EH is like the edge of a cliff.. cross it and you can only go one way. But I'm a little confused as to what my understanding of mass is. Take 2 balloons. Fill one with air and one with water to the exact same size. Attach a weight and string of identical proportions to each balloon and drop each one in an identical bucket with the same amount of water... they should each displace the same amount of water, and (to my understanding) have the same amount of mass. Different weights obviously, but each balloon takes up the same amount of space. From what I recall, that was how to determine an objects mass. Likewise, lead is very dense and aluminum is not. A baseball sized piece of each would have the same mass but different weight and density... Please forgive me if this is completely wrong, I understand trying to educate ignorance can be extremely tedious
     
  7. May 11, 2009 #6

    DaveC426913

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    Why would they have the same mass?
     
  8. May 11, 2009 #7
    Re: Does a black hole have "shape?"

    It's been a long time since I have been in any kind of classroom and it's definitely showing, ha ha! I was under the impression that the amount of space an object occupies is its mass. After submitting that last post I did some research and found this from a NYU web page:

    1) Mass is a measurement of the amount of matter something contains, while Weight is the measurement of the pull of gravity on an object.

    2) Mass is measured by using a balance comparing a known amount of matter to an unknown amount of matter. Weight is measured on a scale.

    3) The Mass of an object doesn't change when an object's location changes. Weight, on the otherhand does change with location.


    Still not entirely sure how kilograms differ from pounds (I just thought it was metric vs imperial) but I'm less confused now.
     
  9. May 11, 2009 #8

    DaveC426913

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    Kilograms measure mass. A 10kg rock masses 10kg on Earth, 10kg on the Moon and 10kg in interstellar space. But a 10kg rock weighs 9.8Newtons on Earth, about 3.2Newtons on the Moon and 0 in space.



    But that's not the issue here. Weight wasn't something we were talking about. What was confusing you was the relationship between mass, volume and density.

    If two objects mass the same, but one has a much smaller volume, then it must have a higher density. Density is nothing more than mass divided by volume.

    d=m/v

    A balloon filled with air has a low mass (a few grams). One filled with water has a high mass (a kilogram). If they have the same volume, then the water balloon must have a higher density.

    A BH with a density a million times that of the sun needs to have only by one millionth the volume in order to have the same mass (and thus the same gravitational pull (at a distance)).
     
  10. May 11, 2009 #9
    Re: Does a black hole have "shape?"

    Good question. A black hole viewed from beyond the event horizon never appears completely black, in fact. The reason for this is the following: As a star collapses to form a black hole, an observer far away sees the collapsing motion of the star slowing down exponentially as the radius of the star approaches the EH radius (called the Schwarzschild radius). In addition, the light from the star appears increasingly red as the star collapses. In theory, then, an outside observer would never see the star's radius shrink beyond the Schwarzschild radius, only approach it increasingly slowly. If the star is non-rotating, however, it would appear to be spherical in the limit.

    Additional weirdness ensues if you consider the point of view of an astronaut falling into the black hole. For such an observer, the event horizon always appears to be lying ahead; the astronaut would probably not know when he had passed the Schwarzschild radius. But an outside observer would see the astronaut getting redder and redder, and slowing down exponentially, as he approached the EH. Again, such an outside observer would never see the astronaut cross the black hole. The falling astronaut, however, experiences only a finite amount of time before hitting the singularity. Interestingly, to him, the singularity doesn't look like a single point, but rather a whole plane. (!)

    If the black hole is charged (electrically, that is), some really weird crap can happen to the astronaut. In particular, there is a point at which the astronaut falling into such a hole is in the future light-cone of a whole universe of events, meaning that he can see (theoretically) the entire history of the "universe" in one flash ("universe" is in quotes because there is some ambiguity here as to what this actually means).

    To answer your next question, it's not really known what "happens" to the matter falling into a black hole in any real physical sense. Like DaveC426913 said, classical GR seems to predict that the matter falling into a hole gets compressed to zero volume; however, it is expected that quantum effects should help sort out some of these apparent paradoxes and breakdowns that happen at the singularity. In fact, if the black hole is rotating, it is theoretically possible to exit through the singularity into another asymptotically flat region of spacetime (sometimes interpreted as another "universe," whatever that means). The reason for this is that in a rotating black hole, the singularity becomes "smeared out" into a doughnut, and you can go through the middle of the doughnut. (In addition, certain forms of time travel appear to be allowed in the region surrounding the doughnut.) Physicists have been able to determine some of the features a quantum theory of gravity must have; for example, Stephen Hawking showed that black holes must emit x-rays, and that the smaller the black hole, the "hotter" it is. Thus, given an infinite amount of time left unmolested, every black hole would eventually "evaporate," at least in theory. The exiting x-rays have been interpreted as a reincarnation of the matter-energy that fell into the hole, with one important difference: Any information aside from the total mass and angular momentum of the original star appears to be lost. This "information loss paradox" is a central problem in black-hole physics; like I've said, there really isn't a good solution to it yet.
     
  11. May 12, 2009 #10
    Re: Does a black hole have "shape?"

    How can a black hole have a mass if everything (including itself?) is crushed to nothing?

    How can it stop itself from being crushed?
     
  12. May 12, 2009 #11

    DaveC426913

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    Not "nothing", just zero size (or almost zero size. I'm not sure what the prevailing idea is on the size of the singularity. I don't know if it is considered zero-size or just near.)

    Because "it" is not a physical thing. "It" (i.e. the black hole) is nothing but what we call an area where lots of matter is compressed to near zero size. Gravity bends space-time from this mass. And that's all there is. There is no object "black hole".

    What happens to matter collasped to near-zero size?

    The short answer is: we don't really know. what we do know is that it retains its gravitational attraction.
     
  13. May 12, 2009 #12
    Re: Does a black hole have "shape?"

    Wow, thanks, very interesting.

    So does the event horizon increase in size as the BH takes in more matter?

    It's bizarre how a point of zero space can have any gravitational attraction whatsoever, let alone the monstrous gravitation of a black hole...

    In your opinion do you think we will ever find out such answers within our life-times?
     
  14. May 12, 2009 #13

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Does a black hole have "shape?"

    I want to make an addendum so that the OP isn't confused if he continues his studies: we've essentially just been talking about Newtonian black holes. But that's fine, because that's correct in the ways relevant for this discussion.

    However, general relativity is far more complex -- it's difficult even to find meaning in terms like "shape" and "density" (but it is very easy to make physically meaningless statements about them).


    It depends on what you mean by "answer" -- many people have unrealistic expectations about how knowledge works.


    We do have answers. But in the future, we may find even better answers.
     
  15. May 12, 2009 #14

    DaveC426913

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    It does, yes.
     
  16. May 12, 2009 #15

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Does a black hole have "shape?"

    No, that does not measure mass, it measures volume. You may be thinking of Archimede's method for determining density. He was tasked with determining if a crown, a complicated shape, had been made from pure gold or had had silver swapped for some of the gold. It would be possible to remove some of the gold and replace it with an equal weight of silver. But silver has a lower density than gold so if you could divide weight by volume, if the density is less than that of gold, you know that it is not pure gold. Archimede's problem was to determine the volume of this complicated shape. He did that by immersing it in a bucket full of water and measuring the volume of the water that spilled out.

    No, no, no. They have the same volume, not mass. Their is a technical distinction between "mass density" (mass divided by volume) and "weight density" (weight divided by volume) but on the surface of the earth weight is just mass times g (acceleration due to gravity, a constant, so one is just a multiple of the other.

    You are forgiven!
     
    Last edited: May 12, 2009
  17. May 13, 2009 #16

    madmike159

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    A BH has whats called a Schwarzschild radius (beyond this is the point of no return), the radius implies that it is spherical. However it also warps time (so really its 4d, but we can only see 3). Lets pretend we could see it against a white background, if you moved around it (assuming you weren’t pulled in) there would be a sphere beyond which nothing could escape.
    I think this is right, some on correct me if I'm wrong.
     
  18. May 13, 2009 #17

    Borek

    User Avatar

    Staff: Mentor

    Re: Does a black hole have "shape?"

    Intuition (which is not the best tool in this case) tells me that even horizont doesn't have to be spherical. If two BH with spherical EH merge process should be continuous, two EH merging will at first look like dumbbell.
     
  19. May 13, 2009 #18

    madmike159

    User Avatar
    Gold Member

    Re: Does a black hole have "shape?"

    I don't know if the event horrizon can be deformed like that. It doesn't sound right.
     
  20. May 13, 2009 #19
    Re: Does a black hole have "shape?"

    Are you talking about gravitational redshift? Could you tell me why the light becomes redshifted?

    It sound like you are saying that that black hole never form. Is that right?
    Can light escape the collapsing star?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does a black hole have shape?
Loading...