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Black Hole Questions

  1. Apr 3, 2004 #1
    Hello. I am new to this board. I hope it is acceptable to ask some foolish questions here once in a while. I am a novice when it comes to physics; but I have always been quite interested in such subjects.

    I have been looking for some reliable information regarding what is usually called "black holes." If this is not the forum to ask my questions, please let me know before I go on and on....

    I will keep it short until I see if this is the right forum for this topic. The first thing I would like to find is a definition of a black hole.

    More later... :eek:
  2. jcsd
  3. Apr 3, 2004 #2
  4. Apr 3, 2004 #3
    Thanks Chen

    I was hoping to uncover info like this here. The link you provided is very helpful; and EASY to understand! I really appreciate it. I will be back in about 2 days; and if this subject gets moved; I totally understand. I will appreciate being able to ask a few more questions here, and learning more. Now that I look things over closer; it may be that some of my questions will over-lap into your Philosophy Forum here; but for now; I am interested in exploring black holes and related topics in this thread, if that is OK.
  5. Apr 8, 2004 #4
    Gravity And Time??

    I don’t understand a lot of what I read on these topics, as I am untrained in them; but I do enjoy the physics related topics I see here for personal research. I know my questions may sound really simple to many here; but I am sincerely interested in getting at the facts with certain areas of physics.

    I once did a science project in elementary school on Einstein’s theory of relativity – drove my Dad nuts; asking him all kinds of questions about it, so I could complete my project… :eek:

    Am I correct in understanding that gravity can be seen as the result of the basic “geometry” of space time? (that geometry being curved by matter and energy). And that, therefore, certain predictions are made possible, such as black holes, and time being slowed by gravitational fields?

    From my studies of black-holes; I have been able to see the connection with gravity, I think; in my own simple way. But, as I mentioned; I am interested in not just black holes, but some other related subjects.

    Can I also ask then, what kind of “gravitational field” would slow time? Perhaps, I need to better understand the definition/s of time as I ask this question; I have always thought of time as something we measure by hours and minutes; yet as I look into the physics of black holes, and gravity, etc., I am starting to sense some other possibilities, which I would like to understand better.

    Any comments, or links that you could refer me to are appreciated. :smile:
  6. Apr 8, 2004 #5
    Gravity does indeed curve the spatial geometry of the universe. A black hole is an example where the intense gravitational concentration causes a massive indentation in the "fabric" of spacetime. A certain phenomenon regarding blackholes I find particularly fascinating is Hawking Radiation. Have you heard of this phenomenon?

    Precisely what do you mean by "what kind of gravitational field.."?

    True, what we measure by hours, minutes etc. are only accurate within particular premises on earth. Universal time on the other hand is relative.
    I would define time as the increase in entropy. That is, the increase of disorder.
  7. Apr 8, 2004 #6
    Time is simply the metering of movement or motion, nothing else non-existent....

    Gravity is 'The energy' contracting space in the Universe, rather then expanding it, like heat (energy) can also do.....

    Black Holes are exceedingly difficult to go at as so little is actually verifiable, lots of speculation though, most that has been proven is the observation of a place in Space that is empty, devoid of background light passing through that particular spot, and something has been, apparently, 'videoed' falling into one....just means it disappeared into 'apparent' emptiness.....
  8. Apr 9, 2004 #7


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    Yes to all.

    The kind that has a variable g00, off the top of my head, but I'm probably leaving out other kinds as well. You have to remember that time is relative, and it doesn't "slow down" in an absolute sense, only wrt other frames of reference. In GR, time can actually "speed up" too (again, this means in a relative manner).

    Probably. Time is traditionally defined as the parametrization of physical processes. It is also traditionally defined according to some physical standard process.

    Not "by" hours and minutes. We measure time by clocks "in" hours and minutes. Hours and minutes are different units of time, but they are not very fundamentally defined.

    Such as ...?
  9. Apr 10, 2004 #8
    I also have a question on black holes. It kind of crosses over to quantum mechanics though. I hope that is ok. Since a black hole is considered to be a singularity wouldn't that violate the uncertainty principle?
  10. Apr 10, 2004 #9


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    Yep. General relativity is incompatible with quantum mechanics. Most physicists are hopefully that a successful quantum gravity theory will remove the singularity.

    - Warren
  11. Apr 10, 2004 #10

    Is that what they call LQG? What about 5 demensional gravity theory (it's something like that)? What's the difference?
  12. Apr 10, 2004 #11


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    Both string theory and loop quantum gravity are competing theories of quantum gravity. The difference is largely that LQG is just a quantized theory of gravitation, while string theory is more ambitious and attempts to unify all four fundamental forces.

    - Warren
  13. Apr 17, 2004 #12
    Figuring it out...a bit!


    I appreciate all the responses here; and feel I am SLOWLY learning a few things that will be very worth-while. I will not always take this long to respond - but I am a Dad with 2 growing boys!! :eek:

    The question here re "gravitational field" may be a good place for me to continue here. I think, that by using this term; I was referring to anything that causes a concentration of gravity.

    I am trying to figure out as much as I can re how gravity, or concentration thereof, affects "time." And; I would like to understand "time" much better. I think that my first question may be a lot harder to grasp, if I do not correctly understand the basic concepts of time - according to modern physics. I know what it is, according to the clock on the wall; but what is it really? The black hole concept seems like a good place to understand these other two concepts I am wanting to study?

    I agree; that Hawking Radiation is a really interesting area of study; but until last week; it was a new subject to me; so it will take a little time to get it all absorbed. :smile:

    I am in the middle of formulating some questions about this; and will get back here this week end to ask them!! :confused:
    Last edited: Apr 17, 2004
  14. Apr 17, 2004 #13
    I would like to try out this one question first, if I may??

    As I read through these posts, and other, related materials I have been referred to; it is appearing to me that "time" can be assessed from more than one point of reference?

    For eg., we can assess time, by using our clock. it looks like some here have stated that "time" can also be assessed from a gravitational point of reference; which is DIFFERENT than the clock on the wall.

    Would someone be able to compile a list of all the different ways that we can view time? I would really like to look into that.

    Someone above mentioned a brief definition for time: "the amount of entropy." From what I understand; "entropy" is energy that cannot do work? Is this then, an accurate definition of "time" in consideration of the above list I have asked for? Please remember; I am a layman/novice at all this; so you have to bear with me, if some of this seems really elementary. I don't understand much of the math I have seen here; but the rest is really interesting!!
  15. Apr 17, 2004 #14
    Entropy is the enregy lost doing work, like the heat off of a car motor that cannot be used in the work process....sorta....

    Concentrating gravity is sorta a mis-conception, but if you compact mass, as gravity tends to want to do, then the gravitational field Appears as increased in strength, hence the appearance of a BH is a result of highly compacted mass (A singularity, for now) that generates the appearance of the absence of emissions of light/EMR....
  16. Apr 17, 2004 #15


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    Guy From Alberta,
    If you really want to understand blackholes, or how physicists use time, then I believe it will only confound the issue to list every possible way imaginable to consider time. Firstly, the physical processes themselves may not lend to you any strong significance, and merely serve as neat little abstractions. Secondly, the term "time" can have dramatically different meanings to different people in different contexts. What I suggest is that you learn the meaning of time in the particular physical context of interest to you (I'm assuming that to be GR), for which there are two important types: coordinate time and proper time.

    Coordinate time is an axis label, and designates one of the four space-time directions. This is the one that you have probably heard called "the fourth dimension."

    Proper time is the kind of time that is experienced and almost always serves to characterize a process in relativity. This is the time that any physical clock will tick by. The particulars of the physical mechanism that causes the clock to literally tick are somewhat inconsequential, so long as you consider the reason behind the ticking to be physically fundamental.

    All physically processes are trajectories of a physical state, and there is some parameter against which this state can be said to change. Furthermore, the effect of this parameter is shared by direct relationship with all other physical systems. Time meets these criteria. The physical processes only reflect the incrementation of this parameter. A physical process that is dedicated to just that purpose is a clock. You will find many types of these, based on many types of physical processes, but they all operate on the same fundamental principle.
  17. Apr 18, 2004 #16
    Indeed, the time measured will depend on the frame of reference. The faster you travel, the slower time will tick for you. Also, the stronger a gravitational field you are subjected to, the slower time will tick for you. These effects are known as "time dilation", and are further explained in This Link
  18. Apr 18, 2004 #17


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    There's a sub-forum
    here at PF devoted to discussion of just this topic.
  19. Apr 18, 2004 #18
    Is this place great for finding knowledge, or what!....whooohoooo!
  20. Apr 18, 2004 #19
    Hi Turin

    I really appreciated your reply in particular this time. I will need a few days to get back to this forum with some other questions/comments. :smile:
  21. Apr 18, 2004 #20
    Just like to add that there is a lot of speculation involving black-holes. Gravity is "causing" time to slow, but the other way to look at it is that the curved space-time (in the topological sense) is causing gravity. In 4d topology, the gravity is a result of the curvature. Energy present in the spacetime is what we consider causes the curvature. John Wheeler writes excellent books with great visual capacity to describe these connections.

    I'd also like to spark my personal idea that if space is stretched out (instead of contracted) in the region of a singularity, then the Planck scale is relative to the rate of aging within a given region. In other words, the Planck scale limitation of space-time could be stretched out to form event horizons or the topological surface of the "strings" or membranes in m-theory.
    Then spacetime itself is considered the underlying structure of such objects, and contains various topological properties to carry energy.

    Even if a swolen Planck area was the surface of an event horizon of a black hole, it would still not emit light other than Hawking radiation, but it would technically be "naked" and still contain no singularity. The math of an expanded planck length would eliminate the mathematical singularity within an event horizon.
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