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Value of g near a black hole (revisited) 
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#19
Mar612, 09:30 AM

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The horizon is a very interesting boundary.....and if fully understood might reveal some underlying relationships difficult or impossible to otherwise detect. Not so long ago, black holes and horizons here virtually 'science fiction'...Einstein, for eample, did not initially believe they could exist. Here are two descriptions that reveal some of that 'character' of horizons: Kip Thorne says (Lecture in 1993 Warping Spacetime, at Stephan Hawking's 60th birthday celebration, Cambridge, England,) 


#20
Mar612, 05:53 PM

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#21
Mar612, 07:07 PM

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It may also be helpful to note that event horizons, very much like the event horizons of a black hole, form in special relativity whenever an observer accelerates  the so called Rindler horizion. A consideration of the Rindler horizon can be very helpful in understanding the event horizon of a black hole. There's some discussion of this at http://www.gregegan.net/SCIENCE/Rind...erHorizon.html, using basic calculus (no tensors). The quick explanation is that a clock stopping is not an absolute fact, but some observers might see a clock as stopped, and other observers won't. 


#22
Mar612, 08:50 PM

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Yeah, some people "would" see time as being stopped *If it was possible* to do things like travel at the speed of light or infinitely warp the fabric of space. The event horizon for a black hole seems like just an extension of Rindler logic, but as with other examples in relativity such as with relative velocity, you can't use the "Va +Vb= total velocity" for two objects moving away from each other near the speed of light, you need a completely different and more accurate equation that asymtotes at c which is specifically helpful for extremes such as when you have two objects approaching the speed of light.
However, I have yet to see any experiment where we actually see the clocks completely stopped or moving backwards as a result of the somehow infinite distortion of the fabric of space. The event horizon isn't even a physical boundary, it's just the given distance from the singularity at which you can't escape unless you accelerate faster than light. There's still infinitessimal amounts of escape velocities higher than the speed of light inside the event horizon, and there's an infinitessemal escape velocities less than the speed of light outside of it, for some reason people just happen to focus on the escape velocity that is exactly c. This reminds me of when people said time traveling to the past or distant future was possible just because they saw electrons jumping from one point to another without appearing in the intervening space, and it turns out it had nothing to do with the speed of light or even time, it's completely different math and completely different things that are happening. 


#23
Mar612, 09:02 PM

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I dispute the meaning of "escape velocity" inside the horizon. For one thing, you have to be able to escape for this to have meaning. Any tiny region inside the horizon looks just like a tiny region of interstellar space. Matter can only only travel on timelike paths, light on null paths, thus there is no escape path at all, period.
Another way of looking at this is that escape velocity is outside the horizon is defined relative to static observers (you can never talk about any velocity without specifying what it is relative to). Well, there are no static observers at or inside the horizon. So forget this nonsense about 'escape velocity' greater than c. 


#24
Mar612, 09:10 PM

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#25
Mar612, 09:10 PM

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#26
Mar612, 09:13 PM

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#27
Mar612, 09:16 PM

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If time wasn't flowing relative to an observer inside the black hole, then the couldn't possibly see that the singularity is coming closer to them because they couldn't be traveling distance over time since for them time has stopped right? So they would never reach the singularity, according to the math alone... They would have to travel distance over time in order to reach the singularity, because the singularity is x distance away from the observer, unless the event horizon somehow is the singularity. I can even read books that say something like "oh yeah, it would take a week or so for an infalling person to reach the very center of a massive black hole". It doesn't even make sense when people say "We would see that light get's frozen at the event horizon" because we wouldn't ever be able to observe if the photons are trapped at the event horizon because then they wouldn't be able to make it to our eyes if they were frozen. 


#28
Mar612, 09:28 PM

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GR predicts time flow perfectly normally for an observer crossing the horizon, and allow calculation of exactly how much time (quite short) before they reach the singularity. An outside observer sees them slow down, darken, and fairly quickly become invisible. I repeat, if you want to talk about something other than GR, you need to do that in a different forum. If you dispute what GR predicts, please explain where everyone else is misinterpreting the math of GR. 


#29
Mar612, 09:34 PM

P: 198

How does time flow so perfectly normal if the fabric of space is "infinitely" warped and matter would follow that warp or if time flows backwards or "downwards"? If time went the other way, wouldn't you travel further away from the singularity since matter will follow the path of the fabric of spacetime only to perpetually approach the event horizon to try and cross into the rest of the universe and thus not make it to the singularity? Or might you even be "stuck" at the event horizon because time flowing the other way would mean your exiting the black hole yet outside the black hole time is flowing forwards? There should just be a few wierd things that might happen, but instead there's all sorts of different things that contradict themselves. I mean, we can predict what would happen if matter went at the speed of light yet have an equation right next to it that says matter cannot travel at the speed of light because it's speed asymtotes at C. 


#30
Mar612, 09:40 PM

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#31
Mar612, 09:53 PM

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The gravitational or electric field of a black hole is suppose to be timeframe independent, so how would you measure a change in it originating from the singularity once mass added to it especially if you can't even observer an object crossing the event horizon? 


#32
Mar612, 10:09 PM

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#33
Mar612, 10:19 PM

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This is true because the gravity of the black hole (meaning, the perceived effects of gravity outside the horizon) doesn't actually come from "inside" the hole (meaning from inside the horizon). It comes from the past, from the collapsing matter that originally formed the hole. See this post (or the thread it is part of) for more: http://physicsforums.com/showpost.ph...8&postcount=24 An object of nonnegligible mass falling into the hole works similarly; it contributes to the "mass" that is measured at a particular event in the exterior of the hole if the infalling object is anywhere in the past light cone of that event. The spacetime that the hole is in is only time independent if the black hole never has anything fall into it. If something of nonnegligible mass falls into the hole, the hole changes; the spacetime the hole is in is no longer timedependent. 


#34
Mar612, 10:24 PM

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Let's take them one at a time: A clock approaching the horizon appears to approach stopping ... from the point of view of an observer further away. That is all, nothing more. This says nothing about the behavior of the clock from its own point of view. "There is no such thing as a stationary clock at the event horizon." Here, you are rather naturally confused by ambiguity in English language. Pervect is here referring to stationary in the sense of motionless relative to distant observers, not rate of time flow on a clock. The two senses of stationary juxtaposed this way lead to false perception of contradiction. Sorry about that. English is a .... <forum rules> sometimes. Pervect's statement about the horizon moving at c past any infaller is simply true. To the infaller it simply appears as the light of prior infallers reaching them. Thus the moment they cross the horizon is the moment they can see all prior infallers. I don't see the tension with any other statements. Now for Thorne. Unambiguously true is that the singularity is a point in time along an infaller's world line, not a spatial point. The infaller sees all prior infallers and all of the outside universe 'normally  except for frequency shift and lensing distortions' until they reach the singularity. They never see anyone else reach the singularity because they reach it (in time) before any light from someone else reaching it can get to them. Specifically, the last they see of any prior infaller is from some moments before that infaller reached the singularity. Thorne's comments about "a direction you would have thought was spatial" and a "downwards direction" are misguided. The only one expecting this would be someone who interpreted coordinates according the letter used to name them rather than their physical characteristics. In standard Schwarzschild coordinates, the coordinate called 'r' is spacelike outside the horizon and timelike inside the horizon. This means nothing except that 'r' is a bad label for the coordinate inside the horizon. If you instead use the local FermiNormal coordinates of a infaller, all of this nonsense disappears. 


#35
Mar612, 10:32 PM

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#36
Mar612, 10:43 PM

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http://www.blackholes.org/explore2.html search, e.g., for merging event horizons. 


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