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Well, this question is missed by most or all of the people who have seen the film, but it’s been bugging me since I watched Interstellar for the first time. So here is my question and I hope someone can give me a valid explanation.

As the theory regarding black holes suggests, for an outside observer, nothing can cross the event horizon of a black hole. It would take an infinite amount of time for Cooper to reach the event horizon if seen from the perspective of Dr. Brand or anyone on earth. By that I mean, Cooper will keep falling and slowing down as he approaches the horizon but he will never cross it. In that time, the humans on earth would have been extinct.

So how did Cooper manage to save people on earth; whereas, for the people on earth, Cooper never crossed the event horizon, and never will. Now as for Dr. Brand, she would have started new colonies on Edmund's planet and the new colonized humans would have evolved to 5d beings, whereas, Cooper is still falling. It's too ambiguous. Or it’s too flawed.

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It's true that in principle if a falling observer was emitting light in a completely continuous way as in classical electromagnetism (rather than in discrete photons as in quantum electrodynamics), and outside observers could detect light waves that had been redshifted to arbitrarily large wavelengths (even larger than the diameter of the observable universe, say), then an outside observer could still see the image of a falling observer suspended above the horizon forever. In practice, because light is emitted in discrete photons, and because radio waves of very large wavelengths are too difficult to detect, an outside observer will cease to see any light from an infalling observer after they've gotten close enough to the horizon, as explained in the "Won't it take forever for you to fall in?" section in this answer from the Usenet Physics FAQ. Also, if you read the "Will you see the universe end?" section in that answer, you'll see that if Cooper were watching the outside universe as he approached the horizon, he would not see an infinite number of years pass on outside clocks as he approached the horizon, and he'd still see everyone outside aging at a finite rate after crossing the (outer) event horizon (a rotating black hole is predicted in relativity to have a second inner Cauchy horizon where an observer falling through would see the entire history of the universe before crossing it, but as described in The Science of Interstellar, Cooper was rescued by the Tesseract before reaching this horizon).

But even if they could still see the image of Cooper very close to the horizon, I don't think there would be anything paradoxical about them also seeing a Cooper who had escaped from the black hole. The reason is that the Tesseract seems to have scooped him out of one region of spacetime and deposited him in another region that would have been impossible for him to reach from the first region if not for the Tesseract (there's no normal way to escape from the interior of a black hole back to the outside). As an analogy, suppose there was a traversable wormhole with one mouth 1 light-year from Earth, and another mouth 5 light-years away. Suppose that in 2015 an alien who has been parked near the mouth 5 light years away decides to dive into it, emerges from the mouth 1 light year away on the same date in 2015, then travels at half the speed of light to Earth, arriving in 2017. Then we will meet the alien in 2017, but if we point sufficiently powerful telescopes at the mouth that's 5 light-years away, then since light signals from that distance take 5 years to reach us, we'll be seeing that mouth as it was in 2012, and so we should still be able to see the alien in his ship parked next to it. There's nothing really paradoxical about this, it's just a consequence of the fact that the light we're seeing from the alien 5 light-years away traveled to us the "long way", while the alien we see on Earth took a shortcut through spacetime.

Similarly, if someone dived into a black hole, and once inside found a mouth of wormhole whose other mouth was outside the black hole, so that the falling observer could escape and meet up with another observer orbiting outside the black hole, the outside observer might still be able to see a delayed image of the falling observer near the horizon* while also seeing a flesh-and-blood version of the same person next to them. The Tesseract probably provides a similar "shortcut" linking different regions of spacetime that wouldn't be reachable from one another otherwise.

*Technically the existence of the wormhole would change the location of the absolute horizon, which is defined as the region of spacetime from which it's impossible to escape and avoid the singularity, but there would still be an apparent horizon that would have the same measurable properties as the horizon of a similar black hole with no wormhole inside it.

  • Someone falling into a black hole would not see the entire history of the universe. – John Rennie Mar 14 '15 at 10:40
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    @John Rennie - Yes, that's what I said--I think you might have missed the "not" in my statement: "Also, if you read the "Will you see the universe end?" section in that answer, you'll see that if Cooper were watching the outside universe as he approached the horizon, he would not see an infinite number of years pass on outside clocks as he approached the horizon, and he'd still see everyone outside aging at a finite rate after crossing the (outer) event horizon". Though as I said, this only applies to the outer event horizon, a Kerr BH has a second inner "Cauchy horizon" where you would. – Hypnosifl Mar 14 '15 at 12:48
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As mentioned by @Hypnosifl, from the outside observers perspective things never do fall into the black hole. I wrote a whole blog entry about what that might look like and I wrote it specifically because of what appears in the movie Interstellar.

I wrote another post describing what Cooper might have seen as he fell into the blackhole.

From this and the fact that as far as our knowledge of the physical universe extends nothing escapes from a black hole; we can deduce that Cooper's interaction with the alien tesseract must have occurred above the event horizon.

  • As I said though, if you drop some kind of portal like a wormhole towards a black hole, it shifts the absolute horizon so that any escape you make must have been above that horizon, but there is still an apparent horizon that has all the observable features of an event horizon. The absolute horizon isn't really measurable at any given moment--it's defined "teleologically" as the boundary between the region where light can eventually escape and where it can't, which means the position of the absolute horizon at any given moment can depend on what happens to the BH in the future. – Hypnosifl Mar 14 '15 at 4:07
  • I agree. As you fall towards the Event Horizon, there is always a region below you from which light can reach you - even after you have fallen through the absolute Horizon. But an outside observer never sees the faller enter the absolute Horizon and the faller sets a marker at the Horizon, he also never sees himself pass that marker (this is contrary to several popular science books - see my blog on it). – Jim2B Mar 14 '15 at 13:44
  • If there is something like a light ray remaining permanently on the horizon, so it can act as a "marker", then the falling observer does in fact see himself pass the marker in finite proper time. The fact that a falling observer crosses the absolute horizon in finite proper time is a standard result in general relativity--are you claiming otherwise, or are you saying you think there is a flaw in GR itself? I didn't see anything on your blog link above which really explained why you think the falling observer never experiences crossing the horizon. – Hypnosifl Mar 14 '15 at 13:58

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