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Black holes and Singularities

  1. Mar 17, 2003 #1
    Continuing on the PF2 thread about singularities... I had a thought that might give a sort of intuitive explanation of the singularity theorems -- eg the proofs that singularities exist at the center of black holes:

    Remember how photons emitted just exactly at the edge of black hole are supposed to be 'trapped' there and not moving at all? Now, how do we reconcile this with the relativity notion of light always going at c? Well, in a frame right next to the edge of the black hole, you see the event horizon itself expanding at almost the speed of light -- run as you can, you can't escape it.

    The intuitive picture this leads to is that the black hole is sort of "sucking" spacetime itself into it. The effect of curvature->pulling matter together->more curvature goes into a feedback loop, and the spacetime fabric pulls the matter into a singularity.

    A better way to put it: in a black hole, the curvature of spacetime -- the rate at which distances shrink -- becomes greater than the speed of light. Since particles can't move faster than this, it doesn't matter what forces or accelerations they feel; spacetime 'wins' and keeps pushing them closer together.

    From what I know of GR and have read of the singularity thms, this seems to be the intuitive/heuristic explanation of what the field equations actually say happens. I think.... does this help anyone?
  2. jcsd
  3. Mar 17, 2003 #2
    If the gravitational field of a black hole,or singularity stretches spacetime does'nt mean that the distance between two points in the presents of gravity changes.one would think that the object travelling through spacetime always traveles the the same distance it should anyway,because before the object traveled near the blackhole,spacetime was already stretched,so the distance should still be the same as it would anyway!
  4. Mar 17, 2003 #3
    what about naked singularities? If they're real, then does that mean that there are singularities that are quite the contrary. So, what would happen if 2 singularities actually collided with each other?
  5. Mar 17, 2003 #4


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    Is it true in general that all gravitating bodies "suck in" spacetime like that?

    I have been trying on my own to figure out what black hole geometry looks like, and I had came close to the description you gave, but I conceptually blocked myself by thinking I needed to reconsile black hole geometry with the fact the event horizon occupies a static position in space-time by demanding that there was no "sucking" at the event-horizon, and was coming up with some odd conceptual ideas like space-time being continually warped more and more in an angular direction at the event horizon

  6. Mar 17, 2003 #5
    I think it's really only applicable to black holes... regular objects give a fixed curvature, no runaway effect. I suppose I should be saying "space" since spacetime doesn't change by definition.

    > So, what would happen if 2 singularities actually collided with each other?
    I have no idea....
  7. Mar 17, 2003 #6


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    Hrm, oh, are different types of reference frames involved here? The classic schwartzchild solution for a static mass, of course, isn't flat... but an observer in his flat reference frame would see space being "sucked" towards the mass?

  8. Mar 18, 2003 #7
    Always does. :) The whole heuristic explanation was aimed at explaining (vaguely) what happens inside the horizon; I don't know how useful it would be outside. Coordinates of a particular observer there get so weird.... I think it depends on what coordinates you want to use globally. Hmm.
  9. Mar 18, 2003 #8
    "Naked singularities"?
  10. Mar 18, 2003 #9
  11. Mar 18, 2003 #10

    Note that the revolving singularities can't be naked singularities!
  12. May 24, 2003 #11


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    This is an intriguing thread and it would be great to see more of these. I would be glad if PF mentors would start threads on interesting subjects----often!

    In the Astronomy game chroot just calculated that a 1.5 trillion solar mass hole (of the usual sort) would have surface gravity of one standard earth gee. 9.8 meters/second^2.

    I am wondering what it would be like to be lowered on a long strong cable down to near (but not touching) the event horizon and experience that normal gee gravity. Must go but maybe be back later
    Last edited: May 24, 2003
  13. May 24, 2003 #12


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    Re: Re: Black holes and Singularities

    For starters the event horizon would look very flat to me because the radius of a 1.5 trillion solarmass hole horizon is 3 trillion miles.

    So I would be being lowered down to near the surface of a very large ball----3 trillion mile radius ball. And it would look like a flat plane stretching to infinity.

    Light rays are not bent very much in one earth gee so optically things would not be too surprising for ME

    The people in the Starship Enterprise up there who lowered me by the cable and who are watching me might see a lot of weird effects. Like I would be moving around and waving signs at them very slowly and I would be redshifted and all that.

    We have a long cable so they are way higher in gravitational potential than me so there are all those effects. But for me in my own neighboring space down near the surface I dont think it would be too strange.

    Only that, looking up, I would see the Ship and much of the usual starry panoply of heaven concentrated in a patch of sky directly overhead, and blueshifted, instread of being spread out as per normal. A kind of "fish-eye optics". I could probably see the stars on the other side of the black hole--with their light bent around so as to be in my periferal vision. So that's weird. But right around me probably would seem OK.

    Anyway I dont picture the event horizon rushing at me at the speed of light---as per damgo's picture of what it would be like to be near the event horizon. So this is a difference of viewpoint which is interesting.

    Damgo what do you say to this example and could you resolve the difference or say where the mistake is if any?
  14. May 24, 2003 #13
    What's a singularity?
  15. May 24, 2003 #14


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    Hello Atheist, I was first in line and my question was about this
    thing damgo said:

    "...light always going at c? Well, in a frame right next to the edge of the black hole, you see the event horizon itself expanding at almost the speed of light -- run as you can, you can't escape it."

    the singularity is way far away down in the center of this 3 trillion mile radius sphere. We are close to the surface---not actually touching it but close.

    do you think we see the surface rushing at us at the speed of light?

    What is it like, in your view, anything to report? You help me imagine the surface and I promise to help you imagine what if the string broke and you fell thru and ended up at the singularity
  16. May 24, 2003 #15
    Still confused. I'll have to read books on the subject.

    What books would you guys recommend on reading about theses areas of physics?

    I am interested in studying especially cosomoligical areas of physics, and strange things like reletivity and light...

    Currenty I'm reading THE ELEGANT UNIVERSE

    Lemme know so I can learn please....
  17. May 25, 2003 #16


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    I remember you from "current radius of the observable universe" thread. You said:

    "Marcus - It's unfortunate no one here was able to give you the answer to your question "what is the current radius of the universe".

    The answer is: 4 x 10^26 meters

    There you have it."

    And thats right! I converted it to light years and it came out to 42 billion LY, about three times the Hubble distance.

    So you have some grasp of cosmology. The Elegant U is supposed to be an excellent book---one could read it at whatver level I imagine. Havent read it myself. If someone else shows up they may have suggestions.

    I would go directly to journal articles even if they are dense and often frustrating. I am leery of popularization. I read journal articles and do thought experiments.

    I know some journal articles in arXiv that are kind of accessible if you decide you want to go that route.

    Michael Turner has an outstanding summary of the changes that have occured in cosmology in the last 5 years and where the field seems to be going. But it is written in dense academic style (with some formulas) so you have to really want to read it.

    The web is a great place, with Ned Wright's cosmology tutorial and movies of what you see falling into a black hole and all those goodies. The web is almost better than books---if its for cosmology.

    Keep me posted on your progress. I like cosmology.
  18. May 25, 2003 #17
    marcus - Wow you actually referenced me knowing a piece of information! How cool!

    You know, the article I got that from stated that that actual size was called a hubble volume, meaning whatever the size of our universe is is the current "hubble volume".

    So what's with that? Is there double usage here?
  19. May 25, 2003 #18


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    there seems to be a double usage, unfortunately.

    1.the hubble time tHubble is 1/H0

    that's for sure, at least no confusion there. and it is not equal to the age exactly (what the age is depends on various things, but no matter what the age is, tHubble is always tHubble , a definite independently defined interval of time.

    2. the hubble length is just c tHubble
    the distance light can travel in that interval of time.

    As for the rest, confusion apparently

    In Scientific American and popularized cosmology, where one can be somewhat vague and needs to have concepts immediately mean something to the reader-----one says hubble volume is
    "the volume of the observable universe" or something.
    But I do not see that in scientific journals. On the contrary there was a large computer simulation project called "Hubble volume
    project" where the volume of space simulated was a cube
    one c tHubble on a side. I double checked this and it was clear.

    The volume of the so-far observed universe is not that cube but a spherical volume and quite substantially larger----many Hubble volumes worth.

    So the difference between the professional cosmologist language and the Scientific American popular style is immediately apparent right there.

    So in answer, yes there seems to be split usage---that is a bit confusing and I cant say how this will be worked out. I suppose eventually it will be made consistent, maybe by people adopting an entirely new word or somehow else.

    thank goodness planck length, area and volume are unambiguous
  20. May 28, 2003 #19
    i have always thought that (not including naked singularities) the area of the event horizon increase to a maximum, as does the entropy of the universe. an icrease in area greater than c would suggest an increase in entropy thus.
  21. May 28, 2003 #20


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    Re: Re: Black holes and Singularities

    How?! At the EH, escape volocity is > c. At 1G, escape volocity is much less than c. The very definition of the Event Horizon is the distance at which gravitational influence becomes so strong that light cannot escape. Wouldn't that have to be the same G-force, regardless of the mass of the BH?
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