Assuming your space ship survives engaging with the accretion disk and any spaghettification issues:

(1) I’ve read that the crew can’t tell if they’ve already crossed the event horizon. There wouldn’t be a point where some crew member would analyze the recent path and what they could see what’s outside the windows on the way and go “oh sh#t”?

(2) Black holes lose mass through Hawking radiation. Once you cross the event horizon, you seem to be part of the black hole. So couldn’t the part of the black hole that goes away (to balance the outgoing HR) be part of you? Assuming you had super space sensors, could you detect missing parts and conclude you’re past the event horizon?

  • $\begingroup$ This diagram i.sstatic.net/677y2.jpg shows coordinate timeslices. The red line is when the black hole evaporates and no longer exists anywhere above this line. So nothing can cross the horizon before the black hole completely evaporates and disappears. (Technically, the black hole evaporation event is in the past light cone of all falling observers while they are still outside.) Also, you become a part of a black hole not when you cross, but while approaching the horizon from outside. So you too evaporate before crossing the horizon. Thus your question is based on a wrong premise. $\endgroup$
    – safesphere
    Sep 16, 2023 at 6:24
  • $\begingroup$ If your question is not about the Hawking radiation, but how to know you’ve crossed the horizon, then there is a simple way. If you are in a free fall, just look back and measure the redshift of the starlight exactly behind you. When this redshift is exactly 1 (the wavelengths are doubled), you are at the horizon. See this for details: physics.stackexchange.com/questions/420721 $\endgroup$
    – safesphere
    Sep 17, 2023 at 5:10
  • $\begingroup$ I'm more interested in the Hawking radiation part. Your second comment gives a way to detect if you're at the horizon. When relative to that detection does the evaporation (as mentioned in your first comment) happen? $\endgroup$
    – CTMacUser
    Oct 22, 2023 at 2:20
  • $\begingroup$ Per classical GR, all outside events happen before anything reaches the horizon. Evaporation is an outside process, except it is not classical, but quantum. As such it breaks GR at the horizon by the Information Paradox (e.g. how can you evaporate before crossing the horizon?). Thus, if the Hawking radiation exists, then GR is no linger predictable starting from the quantum scale distances to the horizon and we have no alternative validated theory (Quantum Gravity) to replace GR there. One way or another you are destroyed (by the firewall or something else) just before you reach the horizon. $\endgroup$
    – safesphere
    Oct 22, 2023 at 18:51

1 Answer 1


One might think that the outgoing radiation would hold information about what fell into the black hole. However, this information is only partial and not very useful. This is because, irrespective of what went into the black hole, from outside the radiation would appear very close to black body radiation (Hawking radiation).

Another way to look at this is - every outgoing particle should still be entangled with the rest of the interior of the black hole. Although we can measure the outgoing particle but one side of these quantum entangled states are inaccessible to us (the ones that are behind the horizon). So, merely measuring the outgoing particles will not give us complete information about what really fell in.

  • $\begingroup$ A real astrophysical black hole has no interior, because it completely evaporates before the inner singular spacetime can form (see the diagram in my comment above). Your answer also cannot apply to hypothetical (non-existing) eternal black holes, because, if the they radiated, they would already have completely evaporated in the infinite past. I understand you are just repeating what you’ve read in textbooks or from physicists who would claim anything to keep receiving research grants, but none of this has any physical meaning. $\endgroup$
    – safesphere
    Sep 16, 2023 at 6:36
  • $\begingroup$ Your second paragraph is also incorrect. The Hawking pairs are produced outside the horizon. One particle files away, the other falls toward the black hole. In our external coordinates, the second particle never crosses the horizon, but always remains outside conceptually accessible to us. However, even if we measure both particles, they carry no information about the interior, because none of them have ever been inside. And if you use the raindrop coordinates, then the external universe no longer exists once the particle crosses the horizon, except the black hole evaporates long before that. $\endgroup$
    – safesphere
    Sep 16, 2023 at 6:50
  • $\begingroup$ @safesphere I get it that from the outside things are always outside the horizon, but that does not change the nature of the Hawking radiation. You can still not recover the information of what became part of the horizon by using the information of the past line cone in some far future. $\endgroup$
    – S.G
    Sep 16, 2023 at 6:50

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