Falling into a massive black hole is not necessarily noticeable

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    Black hole Falling
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Discussion Overview

The discussion revolves around the experience of falling into a massive black hole, particularly focusing on the implications of the Schwarzschild radius and the effects of acceleration and tidal forces. Participants explore the nuances of gravitational effects near black holes, including whether one would notice anything special upon crossing the event horizon.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that the fall acceleration at the Schwarzschild radius is weaker for more massive black holes, leading to the idea that one might not notice anything special when crossing the event horizon.
  • Others challenge the interpretation of fall acceleration, arguing that it is a technical concept and that proper acceleration experienced by an object differs from the force exerted by an observer at infinity.
  • One participant emphasizes that at the Schwarzschild radius, hovering is impossible, which complicates the notion of fall acceleration.
  • Another viewpoint is presented that tidal forces are more significant than fall acceleration, with larger black holes exhibiting smaller tidal forces, thus reducing the sensation of spaghettification.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of fall acceleration versus tidal forces, indicating a lack of consensus on the primary factors affecting the experience of falling into a black hole.

Contextual Notes

There are unresolved technical distinctions regarding the definitions of fall acceleration and proper acceleration, as well as the implications of tidal forces in the context of black hole physics.

Kekkuli
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I find it interesting that the more massive the black hole, the weaker the fall acceleration at the distance of the Schwarzschild radius - that's why you wouldn't necessarily notice anything special in the event horizon.

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And why do you find that in any way strange?
 
Kekkuli said:
the fall acceleration at the distance of the Schwarzschild radius
##GM / R^2## is not "the fall acceleration" except in a very technical sense: it is the "redshifted" proper acceleration of an observer "hovering" at ##R##. So, for example, if an observer at infinity were holding up an object at ##R## using a very long rope, ##GM / R^2## is the force per unit mass that the observer at infinity would have to exert on the rope. But the object at ##R## would not experience that acceleration; the object's proper acceleration would be ##GM / (R^2 \sqrt{1 - 2GM / (c^2 R)})##.

(Similar remarks apply to the coordinate acceleration of a free-falling object relative to a hovering observer at ##R##, which I suspect is what you are thinking of as "fall acceleration".)

Also, at the Schwarzschild radius, there are no "hovering" observers; it is impossible to "hover" at the Schwarzschild radius, or for an object to be held there by a very long rope, even for an instant. So even the technical sense of "fall acceleration" above is no longer meaningful at the Schwarzschild radius.

Kekkuli said:
that's why you wouldn't necessarily notice anything special in the event horizon
No, it isn't. You wouldn't notice anything special falling through the horizon because spacetime is locally Lorentzian there just like it is everywhere else. It has nothing to do with "fall acceleration".
 
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I would argue that acceleration is much less relevant than tidal forces. And big BH's have small tides.
 
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Kekkuli said:
I find it interesting that the more massive the black hole, the weaker the fall acceleration at the distance of the Schwarzschild radius - that's why you wouldn't necessarily notice anything special in the event horizon.
Saying "notice" you seem to think of tidal force or spaghettification. Yes, the larger the black hole the less you feel it, as already said in #4. The reason is tidal force goes with 1/M².
 

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