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Black hole-event horizon

  1. Nov 4, 2005 #1
    Im new here but i need help with something my teacher cant answer...

    as you know already as you get closer to a black hole time is curved thus time slows down...eventually stoping at the singularity...so my question is how can anything enter the black hole

    i also know that black holes consume matter and grow so my conjecture must be untrue...please help
  2. jcsd
  3. Nov 4, 2005 #2
    You've encountered an issue that deals with reference frames. If you try to watch a person fall into a black hole, and are at the same time watching a clock they are holding, you will watch that clock move slower and slower as they get closer. As they reach the even horizon, you will see their clock move even slower, right up until it appears to stop completely at the event horizon. You will even see their motion stop, it will appear as if they stop altogether at the horizon, you will never actually see them pass through.

    However, the person falling in does not notice at all, and simply falls through the event horizon.
  4. Nov 5, 2005 #3
    So then all matter attracted to a black hole is gathered and stopped just outside the event horizon if someone is watching? Sounds bogus to me. It wouldn't be black then would it? Or does it fall in when no ones looking?
    Last edited: Nov 5, 2005
  5. Nov 6, 2005 #4


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    The "proper time" of an infalling observer is finite. Thus if you fall into a black hole, you will reach the event horizon in a finte amount of your time, and pass through it, without noticing anything strange (as long as the black hole is large enough so that the tidal forces at the event horizon don't tear you apart).

    The "coordinate time" increases without limit. This is the time from the point of view of an observer at infinity. If also turns out that if you try to "hold station" by acclerating away from a black hole, outside time will pass faster and faster the closer you get to the event horizon. You won't actually be able to experience infinite time dilation unless you can find a rocket with an infinite accelration however because the acceleration required to "hold station" increases without limit the closer you get to the event horizon.

    If you want some references, try the sci.physics. black hole faq and Ted Bunn's black hole faq:



    points out the same facts, that the time is finite for the infalling observer, and infinite for an observer watching at a safe distance outside.

    Basically you've already gotten a good answer from Franz.
  6. Nov 6, 2005 #5
    I believe this is partially wrong. Here is my conjecture on what happens:

    The outside observer sees the persons clock slow down as he falls into the black hole because that persons time frame is slowing relative to the outside observers. The person does not slow down in his approach velocity as the outside observer sees it, he accelerates towards the black hole normally and falls into it as you would expect. To the person falling in whose time frame is slowing, he sees himself accelerate towards the black hole much faster than the outside observer does because his time reference is slowing. Another words for example his minute is an hour to us.

    This seems to me to much better match the way the observed universe works and is logical. Is there any experimental evidence that proves any part of that conjecture wrong? Yes I am totally unhindered by being a physicist but I would bet that I am right. I believe I am right because I think that physics generally accepted concept of what time is is wrong. More, I am sure you have left the original poster just as confused as when he started.

    "Fundamental challenges to disciplines tend to come from outside. It is customary for students to be introduced to their fields of study gradually, as slowly unfolding mysteries, so that by the time they can see their subject as a whole, they have been so thoroughly imbued with conventional preconceptions and patterns of thought that they are extremely unlikely to be able to question its basic premises."-Martin Bernal Black Athena
    Last edited: Nov 6, 2005
  7. Nov 6, 2005 #6


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    Careful, lad... you're edging into Independent Research territory. Let's stick to what we know.
  8. Nov 6, 2005 #7
    So answer my question, is there any experimental evidence which says I'm wrong? It seems to me observational evidence doesn't support everything stopping when it gets near, for example, the giant black hole at the center of a galaxy. First you say it stops and then you say the person falling in falls in, that's contradictory and why I say you left the original poster just as confused as when he started. Why should it stop at the event horizon anyway? And this whole subject is beyond what we know.
  9. Nov 6, 2005 #8
    Well, there's experimental evidence for the mathematical equations which in turn suggest as franznietzsche and pervect have said. For your statement to be true, you must show a consistent, observable error in General Relativity, which also fits all available data at least as well as General Relativity and furthermore allows for what you say to happen at black holes. In the absence of this alternative, I'm inclined to go with the current theory.

    What makes intuitive sense is not really relevant here. In fact, General Relativity as a whole is pretty non-intuitive -- intuitively, we'd like to believe that simultaneity is real and time is a universal constant. It just isn't. And just wait until you see some of the results of Quantum Mechanics.

    No, it isn't. He says it "stops" relative to the guy a safe distance away (if you'd rather, you can think of it like it takes an infinite amount of outside-time), and does not "stop" relative to itself.

    Because spacetime is sharply curved in the presence of singularity. If you want something more definitive, then you look at the equations.
  10. Nov 7, 2005 #9


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    A black hole is not an objective, physical entity, such as a planet or star. It is a region of the universe where spacetime itself becomes fuzzy. The observational limit is the event horizon. This is the theoretical size of the region surrounding a black hole where even light can no longer escape. But this is not a difinitive surface, like the surface of an asteroid, it is a limit that can never actually be reached. From the point of view of an observer outside the event horizon, nothing ever reaches it... not even the object that collapsed to form the putative black hole... It's just a fuzzball of photons redshifted out to, but never quite reaching infinity.

    So what I'm trying to say is a black hole is a hugely dense smear of matter trapped on a surface whose geometry depends on various factors, like spin and charge, and is enormously difficult to describe.
  11. Nov 7, 2005 #10


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    If you look at an object infalling into a black hole, or a collapsing sphere of matter forming a black hole, there exists a point at time at which no signal from the outside world, including light, can reach the infalling object until after the object hits not only the event horizon, but the central singularity.

    For this reason, I disagree with the "frozen star" model. There exists a time at which one can effectively say "The future of the object is beyond my intervention". It is necessary to assume that the object does not accelerate for this simple analysis to hold, we assume the object is commited to following a geodesic.

    A web reference for this


    The problem is most easily solved in ingoing Eddington-Finklestein coordinates, r,V. The geodesics of infalling light-beams are lines of constant V in these coordiantes. It can be seen from the diagram that the radius of the infalling sphere has a finite value of the V coordianate, drawn in this diagram as the yellow diagonal lines.
  12. Nov 7, 2005 #11
    So if the matter hasn't reached the event horizon it isn't to the point where light is trapped, it can still reflect light and therefore would not be a 'black' hole and could be seen. Where is the observational evidence of objects like this? Never mind quasars which don't seem to worry about everything sticking to them but act like I think a black hole acts.
  13. Nov 7, 2005 #12
    No it doesn't stop there, it simply appears to from the point of view of someone wacthing from far away.
    And blackholes are not strictly black. They do radiate and have temperature, but that is opening up another whole can of worms that isn't really relevant to the question at hand.
    If you look at the Schwarzschild Metric (solution to General Relativity for a sperically perfect star) you'll see it makes perfect sense:
    g^{**} = \left [ \begin{array}{llll}
    \frac{1}{1-\frac{2M}{r}} & 0 & 0 & 0 \\
    0 & r^2 & 0 & 0 \\
    0 & 0 & r^2 \sin^2 \theta & 0 \\
    0 & 0 & 0 & -(1-\frac{2M}{R}) \\
    \end{array}\right ]
    What this means, if you take the bottom right component, is that you get:
    \Delta t' = -(1-\frac{2M}{R}) \Delta t
    Where [tex]\Delta t[/tex] is the time measured by the person falling into the whole and [tex]\Delta t'[/tex] is the time measured by a far away observer. As R (distance from the center of the black hole) approaches a value of 2M, the far away observer measures longer and longer time for each second on the falling persons clock, until he measures infinity for the point where the person reaches [tex] R = 2M[/tex], which is the event horizon (this expression is misleading, units have been chosen so that physical constants like [tex]c[/tex] are 1 for reasons of simplification). So on your watch it takes a person literally forever to fall through the event horizon. However, on their watch ([tex]\Delta t[/tex]) they pass right through the event horizon on their merry way to oblivion.
    Last edited: Nov 7, 2005
  14. Nov 7, 2005 #13


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    You should really try reading some of the FAQ's on the topic, or a good book like Thorne's book "Black holes & time warps". (The FAQ is free, however).


  15. Nov 8, 2005 #14


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    We can all agree it's a mind bending [not to mention spacetime bending] proposition no matter which particle happens to be observing the process. The far away particle thinks the infalling particle is moving in slow motion and the infalling particle thinks the far away observer is moving in fast forward. But consider this, no particle will ever actually observe any particle nearer to the event horizion reach the event horizon. Even imposing a Planck scale limit on d/t does not entirely make this go away to anyone's satisfaction [so far as I know]. This is a fascinating problem [which explains the many papers on the subject] that just won't go away unless and until you can nail down a small scale limit on relativity.
  16. Nov 8, 2005 #15
    Which means not only can't they grow to the size of the theoretical ones at the center of a galaxy given the age of the universe, they couldn't form to begin with? Or form a quasar which is REALLY old? Sounds like all this would require the universe to be several orders of magnitude older than what we think and it's not.

    You can quote mathmatical formulae all day but it doesn't make them accurately describe reality. We thought Newtons formulae described reality accurately once and people like you argued that Einstein was full of it. Especially when we are talking about time. Since we don't know what time is, any formula that describes how time is affected by intense gravity is just a guess. I still say my conjecture seems to describe the visible universe better.
  17. Nov 8, 2005 #16
    No, its more complicated than that. As someone else pointed out, due to the quantization of energy, there is a finite time at which something falling in is no longer observable, even though you do not observe it cross the event horizon.

    No, thats not how it works. You're making hand waving arguments left and right with no basis behind them.

    People like me? I never said such a thing.

    No its not just a guess. It is an empricically verified formula, well tested by experiment. Without it, GPS would not work. Frame dragging, a recently observed phenomenon would not exist. The perihelion shift of Mercury would be much smaller than it is observed to be, but its observed shift matches GR to a level more precise than we can measure.

    Honestly its beginning to sound like you don't really understand how science and the scientific method work. GR has passed every experiment designed to test it with flying colors. So we then proceed to treat it as fact, until we can find experimental conditions under which it breaks down. Scientists did the same thing with newtonian theory, until they found experimental setups where it did not work, circa 1900. All the times GR has been shown to be true inclines us to believe what it predicts about black holes. You on the other hand are making things up without any basis in reality, because it seems to make sense.
  18. Nov 8, 2005 #17


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    Let us remind you, again, that this forum is devoted to discussions about mainstream science, not your conjectures.

    If you can pass the standards, you are invited to submit your conjectures to the Independent Research forum.

    One of the standards for a sucessful submission is

    This means you are going to have to use some math yourself, and make some actual quantitative predictions, and not suggest that mathematics is irrelevant.

    Also, comparing yourself to Einstein, Newton, Galelio, the Pope, or even the Queen of England is not particularly relevant, justified, or convincing. Sorry.

    Another of the standards is

    Judging from your posts here, you will have to do a lot of reading before you come even close to meeting this standard.

    This is where your participation in the mainstream forum could be useful to you, if you can forget about pushing your own personal non-mainstream theories for a bit. If you can't avoid the temptation to keep hawking your personal theories, I would predict a rather short future for you here, howver. Before you can improve on the mainstream theories, you have to understand what they are. At the current time, this does not appear to be the case.

    Here is one specific example of where further reading could be quite beneficial to you. Scientifically, we know quite a bit about time and how it is affected by gravitational fields. While there is plenty of room for *philosophical* debate, this is a scientific forum, not a philosophical one. Scientifically, we know quite a bit about time, and about how it acts in the gravitational fields of our solar system. For instance, experiments have *already been done* to test relativistic time dilation due to gravity in the solar system, such as the Harvard tower experiment, or the Scout rocket experiment. The later has confirmed Einstein's formulas for gravitational time dilation to within .01% accuracy.

    Measurements of the Shapiro effect, the time delay of radar signals passing close to the sun, is another example of the success of Einstein's theory of gravity, and the associated prediction of gravitational time dilation.

    While it is always possible that some future theory will provide a better fit for the data when more data is gathered, it is important for the success of any theory that it match existing known experimental evidence. (This is another guideline for submission to the indpendent research forum, BTW). We have a lot of existing data about how gravity affects time already available, which you are apparently not aware of or are otherwise not taking into account.
    Last edited: Nov 8, 2005
  19. Nov 8, 2005 #18
    Hey, I am just asking questions to understand why my one conjecture is wrong. And I am not comparing myself and my intuitive conjecture to Einstein and his math.

    I just don't see how all of this fits in with what I have read about the observed universe. Like that the oldest quasar is seen as it was when the universe was less than a billion years old and the universe is around 14 billion years old. With all of this time effect around a black hole, and a quasar is supposed to be a big one?, how did it get that big if it would take a near infinite time to grow? I can maybe understand a quasar forming around the time of the big bang when the laws of physics weren't quite what they are now but the big black holes at the center of galaxies? Did they form at the beginning of the universe?

    You guys act like you have this stuff down cold, how come you don't have a grand unified theory yet then?
    Last edited: Nov 8, 2005
  20. Nov 8, 2005 #19


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    I can only speak for myself. I spend far too much time drinking and carousing to get any research done. (That, and I'm not a scientist.)
  21. Nov 8, 2005 #20
    Black holes do not take an infinite amount of time to form. What I was saying earlier is that you cannot observe something passing through the event. As for the amount of time needed for actual collapse see this http://arxiv.org/pdf/gr-qc/0403029
    paper on the collapse of rotating neutron stars. While neutron stars are already quite small and compact, the paper gives times of roughly 0.5 milliseconds for the collapse beneath the event horizon, measured in the proper time of the star (I only skimmed the paper, and GR is not my field, so hopefully one of the more experienced forum members can help out on this).

    They don't take infinite time to form. It takes infinite time to observe something pass through the horizon, this is not the same thing.

    We don't know that the laws of physics have changed over time, and it is largely speculation to say that they might have.

    I don't think you realize just how complex this stuff is. I hardly have it down cold. I do somethings, a lot more than most people, but I hardly have it down cold. This is very difficult stuff, its not simple in the least. Its fairly rare for a person getting a BS in physics to even take a class on General Relativity (not incredibly so, but the majority of students do not take a class in it).
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