Value of g near a black hole (re-visited)


by pawprint
Tags: black, hole, revisited
Passionflower
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#73
Mar12-12, 11:38 AM
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Quote Quote by PAllen View Post
As long as an infaller is outside the horizon, they see prior infallers as they were closer to the horizon than they are. Note that distances perceived by this infaller are very different from the r coordinate value - there is "lot's of room".
You are wrong.

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
Spin-Analyser
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#74
Mar12-12, 11:54 AM
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What if the in-faller 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!
PAllen
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#75
Mar12-12, 12:02 PM
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Quote Quote by Passionflower View Post
You are wrong.

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
I wasn't actually thinking of infalling from infinity. In these scenarios of changing mind at the last second, I think in terms of observers hovering near the horizon, then shutting off fuel. However, I failed to specify this, and certainly you are right about an infaller from infinity.

[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.]
PAllen
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#76
Mar12-12, 12:08 PM
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Quote Quote by Spin-Analyser View Post
What if the in-faller 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!
No, they will see earlier infallers a 'normal' distance away, as of before they passed the horizon. Think of it this way: if they could know where the horizon was just before it hit them, they would see earlier infallers at a distance such that they could deduce they must be already inside; however, if each earlier infaller had a watch, the image they would see on the watch would be of a moment just before each earlier infaller crossed the horizon. Think more about the passing flash of light example. Further, if at this last moment, they accelerated away frantically, they would never see the earlier infaller's watches reach their infall moment.
Spin-Analyser
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#77
Mar12-12, 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.
PAllen
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#78
Mar12-12, 01:18 PM
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Quote Quote by Spin-Analyser View Post
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.
Both your statements are not quite right.

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.
Spin-Analyser
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#79
Mar12-12, 01:27 PM
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Quote Quote by PAllen View Post
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.
I'm talking about an observer who never reaches the horizon themselves. As they approach they will see objects crossing the event horizon, but they will be seeing them as they were before they reached the horizon?
PAllen
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#80
Mar12-12, 02:29 PM
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Quote Quote by Spin-Analyser View Post
I'm talking about an observer who never reaches the horizon themselves. As they approach they will see objects crossing the event horizon, but they will be seeing them as they were before they reached the horizon?
Let's talk about a supermassive BH, and let's say you are following 10 feet behind your partner, approaching the horizon. Let's say you know exactly where it is all times (by computation and knowledge of the region). Let's say you fall to 3 feet from the horizon and stop. At this moment, it is possible to still see an image of your partner that looks 10 feet from you. Now consider two cases:

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).
Spin-Analyser
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#81
Mar12-12, 03:18 PM
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Quote Quote by PAllen View Post
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.
So if you are close enough to the horizon you can observe objects crossing it?

Quote Quote by PAllen View Post
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.
They both hover just above the horizon. One of them turns off their engines. Does the other one see them disappear passed the horizon?
PAllen
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#82
Mar12-12, 03:59 PM
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Quote Quote by Spin-Analyser View Post
So if you are close enough to the horizon you can observe objects crossing it?
This is getting repetitive. You are seeing an old image from before they crossed. It is from when they were still (in my prior example) 10 feet from you but right near the horizon. This light takes a while (up to forever) to reach you.
Quote Quote by Spin-Analyser View Post
They both hover just above the horizon. One of them turns off their engines. Does the other one see them disappear passed the horizon?
The one that remains hovering sees the other one approach the horizon, effectively going black before reaching it. The image history here is quite different from tandem infallers. The image history (past world lines + light null paths) makes for the difference these cases.
Passionflower
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#83
Mar12-12, 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.
Spin-Analyser
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#84
Mar12-12, 04:21 PM
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Quote Quote by PAllen View Post
This is getting repetitive. You are seeing an old image from before they crossed. It is from when they were still (in my prior example) 10 feet from you but right near the horizon. This light takes a while (up to forever) to reach you.
I'm just trying to be absolutely clear that you're saying you can see an object turn black, effectively crossing the horizon. But at this point you're still not sure if they reached the horizon or not? If they accelerated at the last moment they would presumably reappear from the others perspective after a time. And if they cross the horizon they would suddenly see all the other objects that had fell in earlier?

Quote Quote by PAllen View Post
The one that remains hovering sees the other one approach the horizon, effectively going black before reaching it. The image history here is quite different from tandem infallers. The image history (past world lines + light null paths) makes for the difference these cases.
The one that remains hovering will see them approaching the horizon as they are falling towards the singularity, meaning the event horizon will always be falling in ahead of them?
PAllen
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#85
Mar12-12, 04:34 PM
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Quote Quote by Spin-Analyser View Post
I'm just trying to be absolutely clear that you're saying you can see an object turn black, effectively crossing the horizon. But at this point you're still not sure if they reached the horizon or not? If they accelerated at the last moment they would presumably reappear from the others perspective after a time. And if they cross the horizon they would suddenly see all the other objects that had fell in earlier?
I think I will have to let someone else answer your questions after this. Somehow, I think I'm being clear and you get something quite different from what I said out of it. Someone else may express it in a way you get it.

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.
Quote Quote by Spin-Analyser View Post
The one that remains hovering will see them approaching the horizon as they are falling towards the singularity, meaning the event horizon will always be falling in ahead of them?
I don't understand this at all. The one that is hovering simply sees the one that turns off thrusters fall towards the horizon, get redder, ultimately black, just outside the horizon. Nothing about the history from horizon to singularity can be seen by the one remaining outside.
Spin-Analyser
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#86
Mar12-12, 05:46 PM
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Quote Quote by PAllen View Post
I think I will have to let someone else answer your questions after this. Somehow, I think I'm being clear and you get something quite different from what I said out of it. Someone else may express it in a way you get it.

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.
Then you can see objects crossing the horizon (because you're one plank length away and they're in front of you), so light is escaping from inside the horizon?

Quote Quote by PAllen View Post
I don't understand this at all. The one that is hovering simply sees the one that turns off thrusters fall towards the horizon, get redder, ultimately black, just outside the horizon. Nothing about the history from horizon to singularity can be seen by the one remaining outside.
The bit I'm having trouble with is seeing distance between you (hovering a plank length above the horizon) and another object but niether of you have crossed the horizon. You could therefore move alongside the other observer and niether of you would have crossed the event horizon, so you can't have been next to the horizon in the first place?
PAllen
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#87
Mar12-12, 06:56 PM
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Quote Quote by Spin-Analyser View Post
Then you can see objects crossing the horizon (because you're one plank length away and they're in front of you), so light is escaping from inside the horizon?

The bit I'm having trouble with is seeing distance between you (hovering a plank length above the horizon) and another object but niether of you have crossed the horizon. You could therefore move alongside the other observer and niether of you would have crossed the event horizon, so you can't have been next to the horizon in the first place?
I say "you never see x" . You respond: "Then you can see x". We will never get anywhere this way.

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.
Spin-Analyser
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#88
Mar12-12, 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?
PAllen
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#89
Mar12-12, 07:35 PM
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Quote Quote by Spin-Analyser View Post
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?
Nope, never said this, said the opposite several times. I said if you are outside, you only see light that was emitted outside as well. It may look like it comes from a distance such that if the object is still that distance from you it would be inside. But the light is old, from outside the horizon.
Spin-Analyser
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#90
Mar12-12, 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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