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Value of g near a black hole (revisited) 
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#73
Mar1212, 11:38 AM

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A free falling observer (free falling from infinity) will measure his distance to the EH to be exactly equal to r  rs (where rs is the event horizon). Contrast this with a stationary observer close to EH he will measure his distance from the EH to be more than r  rs 


#74
Mar1212, 11:54 AM

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What if the infaller stops one plank length away? They will see the event horizon as just in front of them, but also just in front of every other object that hasn't crossed the horizon yet but who are closer to the singularity than they are? This seems very paradoxical!



#75
Mar1212, 12:02 PM

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[Edit: this observation does clarify that I needn't have said anything about comparative distances, as it is not relevant to the main issues  see next post.] 


#76
Mar1212, 12:08 PM

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#77
Mar1212, 12:27 PM

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So let me clarify.
1). It is perfectly possible to observe objects crossing the event horizon of a black hole. 2). It is not possible to observe them at a time after they reached the event horizon. Think very carefully about your next answer. 


#78
Mar1212, 01:18 PM

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It is perfectly possible to see objects cross the horizon when you cross the horizon. It will be obvious (at that moment) that they crossed before you. Further, you can deduce for possible infallers ahead of you, that 'if they are still where they appear to be', and you know where the horizon is, they are inside the horizon. However, since you are seeing an 'old' image of them, you cannot tell if they made a last moment decision to escape (and thus are closer to you than they appear) unless you also make such a decision, and later see that they did. Further, as long as you remain outside the horizon, you cannot tell for sure whether they crossed or not. 


#79
Mar1212, 01:27 PM

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#80
Mar1212, 02:29 PM

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1) Your partner crossed the horizon. You will see their image fade to black, and their wristwatch will never quite reach the time they crossed the horizon. 2) Your partner stopped 1 foot from the horizon. Some time after you stop, you will see that your partner started accelerating to hover before you did, getting closer to you in the process, and are now stopped 2 feet away. The closer to the horizon your partner makes decision (2), the longer before you can distinguish it from (1). 


#81
Mar1212, 03:18 PM

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#82
Mar1212, 03:59 PM

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#83
Mar1212, 04:07 PM

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Basically an observer can detect a signal from another observer who passed the event horizon if he passes the event horizon as well in time. He will observe the signal only after he passed the horizon himself.
An observer who never passes the event horizon cannot receive a signal from an observer passed the event horizon. 


#84
Mar1212, 04:21 PM

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#85
Mar1212, 04:34 PM

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Repeating yet again: You don't see them actually cross the horizon if you remain outside. No exception. The turning black is just a matter of infinite red shift and time dilation relative to you if you are hovering further away. If they divert from crossing at the last minute, sometime before infinite redshift, you see them turn on their thrusters and (as in my tandem example) get closer to you (you having already hovered). All this is due to light delay. You never see turning fully black and reappearing[edit: you can see someone have arbitrarily close to infinite redshift, then approach you becoming less redshifted, even pass you]. Ultimately, after infinite time, you can infer they crossed if you never detect that they stopped and hovered. Finally, yes, the moment you cross you see prior infallers as of the moment they crossed. 


#86
Mar1212, 05:46 PM

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#87
Mar1212, 06:56 PM

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Classically, Planck length is irrelevant. Quantum mechanically, nobody knows. Take your pick depending on approach to a partial theory of quantum gravity: (a) there is nothing resembling a horizon (and surface of smallest visibility is smaller than EH as predicted by GR); (b) there is something that is not a horizon microscopically, but it looks a lot like it macroscopially; (c) there is a horizon, but with some difference in properties from the classical picture; (d) a horizon never forms and matter is always outside what would be the horizon radius. I don't understand your second paragraph at all. 


#88
Mar1212, 07:15 PM

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Let me be clearer. You're saying you can observe light coming out to your eye from inside the horizon as you hover just above it (because you see distance between you and objects ahead of you), but you're seeing light that hasn't reached the horizon yet?



#89
Mar1212, 07:35 PM

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#90
Mar1212, 07:56 PM

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It's old light from outside the horizon coming at you from inside the horizon of light that hasn't reached the horizon yet?



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