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

Spin-Analyser

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?

PAllen

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

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

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

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

PAllen

I give up. I write English, you twist into word soup.

DrGreg

No, it's old light from outside the horizon that has never been inside the horizon.

If you and your partner are both falling into the black hole, then from your point of view, you and your partner are stationary and the event horizon is rushing towards you at the speed of light. You see your partner 10 feet in front of you at all times. The image you see has been delayed 10 nanoseconds; you see where your partner was 10 ns ago.

At exactly the moment you reach the event horizon (i.e. at a distance of zero, not a distance of 1 planck length) you see, 10 feet in front of you, what your partner was doing 10 ns earlier, which was crossing the event horizon. (10 ns ago the event horizon was 10 ft in front of you, as was your partner.)

This illustrated in the left-hand spacetime diagram below.



If you decide at the last minute to brake and hover at a small fixed distance outside the event horizon, you see your partner's image slow down, red-shift and darken and never actually cross the horizon. This illustrated in the right-hand spacetime diagram below.

Attached Thumbnails

 

Passionflower

Sorry but I cannot agree with this.
I think you are forgetting the tidal forces between them.

PAllen

If you go back to the post I introduced this scenario, I specified a supermassive black hole. Tidal forces can be made as small as desired by making the mass large enough, as you have noted in other discussions.

Spin-Analyser

As you approach you see objects in front of you crossing the horizon, and you're seeing light from the other side of the event horizon. When you move away the light from previous observers moves back across the event horizon. So what if you stop one plank length away from the horizon and the one in front of you never reached the horizon either? The event horizon is no longer in one place. If you moved alongside and hovered next to the one in front of you then you would still be outside the horizon. It doesn't work. The event horizon must be moving inwards at c, not outwards.

PAllen

Why do you do this? So far now, a dozen or more times, 3 of us have now told you this is false, in all different words, and clear beautiful pictures from Dr. Greg. You respond with the opposite of what everyone says. You never see light emitted from inside the horizon unless you are also at or inside the horizon. Every other part of your statement is false as well, and this has been explained multiple times.

I think this thread is dead.

Spin-Analyser

If there's distance between them and you are one plank length away from the horizon then 'As you approach you see objects in front of you crossing the horizon, and you're seeing light from the other side of the event horizon. When you move away the light from previous observers moves back across the event horizon'. I thought there was "plenty of room". The only other alternative is that they all pile up at the horizon.