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I was having a discussion with a friend and he brought up that you could float into an event horizon and not know that you had gone in. I believed this only works with a Schwarzschild black hole. He specified a very very large black hole with a large Event Horizon. My contention was that frame drag will not allow anything to enter an event horizon that has not been reduced to the most basic components. Just looking for commentary or equations that would be appropriate. Please don't leave an equation in the dark. I am old and trying to expand my horizons... pun intended. Any help would be appreciated.

It's ok. I solved it. If you leave this up I will come back to put in the answer.

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    $\begingroup$ Why would frame dragging reduce anything to its most basic components? $\endgroup$
    – TimRias
    Nov 1, 2019 at 8:13
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    $\begingroup$ Frame dragging is present only in Kerr geometry that arises due to the $g_{t\phi}$ term in the metric. In Schwarzschild geometry there is no frame dragging. $\endgroup$
    – Richard
    Nov 1, 2019 at 9:33
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    $\begingroup$ Jason, I assume that you are asking about massive black holes with spin. If so, you should make that explicit in your question. FWIW, large black holes tend to have a lot of spin, as shown in the diagram at the end of this answer on the Astronomy site. $\endgroup$
    – PM 2Ring
    Nov 1, 2019 at 12:58

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Your friend is correct, locally nothing particularly special happens when you cross an event horizon.* In fact, there is no local observation you could make that can tell you whether or not you are crossing an event horizon. Of course, as you get closer to the black hole, tidal effects will increase (gravity is pulling differently on on end of an object then on the other side). However, tidal effects will always stay finite at the horizon (also when the black hole is rotating). Moreover, the strength of the tidal effects at the horizon is smaller for bigger black holes. Hence, you can't even tell from the size of the tidal effects.

*At least as long as we stick to classical general relativity. Quantum effects could change this (and some people argue that they should).

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  • $\begingroup$ Do you have a source for the statement ' nothing particularly special happens when you cross an event horizon'? I am curious. $\endgroup$ Nov 1, 2019 at 17:05
  • $\begingroup$ @Greg.Paul, Try pretty much any textbook on general relativity. $\endgroup$
    – TimRias
    Nov 1, 2019 at 17:06
  • $\begingroup$ Are the people who argue that quantum effects are important at the horizon the same ones who insist that nothing ever falls through the horizon? $\endgroup$
    – D. Halsey
    Nov 1, 2019 at 18:06
  • $\begingroup$ @D.Halsey, No typically they are not. There are fairly reasonable arguments, that suggest quantum things may happen at the horizon. $\endgroup$
    – TimRias
    Nov 1, 2019 at 18:11
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    $\begingroup$ @PM2Ring The derivation of Hawking radiation is based on an asymptotic analysis of the spacetime, and is based on what an asymptotic observer would see. We cannot automatically infer that anything is happenning locally at the horizon for a free falling observer. (I agree that Hawking radiation is a hint that something quantum may happen at the horizon.) $\endgroup$
    – TimRias
    Nov 2, 2019 at 10:36

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