- #91
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Spin-Analyser said:
I give up. I write English, you twist into word soup.
Value of g near a black hole (re-visited)
- Context: Graduate
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- Black hole Hole Value
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Discussion Overview
The discussion centers around the behavior of gravitational acceleration (g) near a simple (non-rotating, uncharged) black hole, specifically in relation to its event horizon and singularity. Participants explore theoretical implications, mathematical interpretations, and the applicability of Newtonian physics in this context.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants assert that gravity approaches infinity asymptotically at the singularity rather than at the event horizon.
- Others argue that the event horizon is defined as the radius where escape velocity equals the speed of light, with implications for gravitational behavior at that radius.
- A participant questions the validity of using a purely Newtonian approach for calculations near the singularity, suggesting it leads to incorrect conclusions.
- There is a discussion about the nature of proper time experienced by objects falling into a black hole, with some stating that it is finite, while others emphasize the need for careful consideration of coordinates and frames of reference.
- One participant challenges the assertion that gravitational acceleration has a definitive value without specifying the coordinate system or frame of reference used.
- Another participant seeks clarification on reconciling the concept of non-infinite escape velocity with the idea of infinite gravity at the event horizon.
Areas of Agreement / Disagreement
Participants express multiple competing views regarding the behavior of gravity near a black hole, particularly concerning the event horizon and singularity. The discussion remains unresolved, with no consensus reached on several key points.
Contextual Notes
Participants note that the discussion involves complex mathematical concepts and varying interpretations of gravitational behavior, which may depend on specific definitions and coordinate systems.
I give up. I write English, you twist into word soup.
- #92
DrGreg
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No, it's old light from outside the horizon that has never been inside the horizon.Spin-Analyser said:
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.
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- #93
Passionflower
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Sorry but I cannot agree with this.DrGreg said:
I think you are forgetting the tidal forces between them.
- #94
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Passionflower said:
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.
Last edited:
- #95
Spin-Analyser
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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.DrGreg said:
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Spin-Analyser said:
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.
- #97
Spin-Analyser
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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.
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