I've been having a small back and forth on another website about the nature of objects that fall into black holes.

I know that they never reach the event horizon from the perspective of a distant observer. They slow down and appear to stop. The image then redshifts to infinity and disappears.

I also know that from the perspective of the object, it crosses the event horizon like it would move through normal space, and then joins the singularity.

How I reconcile these is that it's just the image of the object that remains outside the event horizon. Using simple physics, you can calculate the acceleration due to the gravity that the object would feel, and be drawn past the event horizon at a very high speed. So in my mind the object can't actually freeze at the event horizon. It's just that the high gravity warps the light coming from the object so that the distance it has to travel is stretched out, leading to a redshift.

The other person disagrees, and says that since the object is visible, and frozen, that in his perspective it is still actually there. That the object itself hasn't yet crossed the event horizon. He'd be able to shine a laser on it and get light back.

He says this is caused by gravitational time dilation. Time appears to move more slowly when close to higher gravity objects.

This doesn't sit right with me.

Say that with the simple physics calculations, you know an object would cross the event horizon at time Tn, from its own perspective. The outside observer is to shine a laser on the object at time Tn + 1 second, from the observer's perspective.

I feel like the laser would never reach the object and reflect back to the observer. At time Tn + 1, the object would be beyond the event horizon from the object's perspective.

The other guy countered that time never reaches Tn, because the time dilation goes to infinity as the object approaches the event horizon. Tn + 1 comes just fine for the observer, so they can shine the laser on the object and those photons would return to the observer.

Which one of us is incorrect? I get the feeling it might be both. I'm fairly certain about my own understanding, but I can't find anything to disprove what the other guy is saying.

If what he is saying is true, then that means that nothing can ever enter a black hole. It would take an infinite amount of time. Any objects that were pulled up to the event horizon would be frozen in time for trillions of years and they would suddenly pop into existence when the black hole evaporates.

That doesn't make sense as black hole become bigger as they consume matter. In my understanding.

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    $\begingroup$ "It would take an infinite amount of time." - whose time? $\endgroup$ – Alfred Centauri Dec 15 '15 at 1:19
  • $\begingroup$ Yeah, that's what I don't understand, it'd be from both an observer's perspective and the object entering the black hole. In some other explanations of this people have said that the object would observe falling toward the event horizon as normal, and as they get close, time speeds up on the outside. They see the entire universe's history go by, and trillions of years later the black hole dies before they would have fallen in. $\endgroup$ – G3n0c1de Dec 15 '15 at 1:37
  • $\begingroup$ There are quite a few questions and answers already here regarding such controversies; you might see a few in the column to the right. $\endgroup$ – Alfred Centauri Dec 15 '15 at 2:00
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    $\begingroup$ This explanation with it's diagram says that things take finite time to cross the event horizon and reach the singularity. Which more or lines up with my mode of thinking. I have no experience with those diagrams, however. But looking through the other answers, it seems like there isn't much of a consensus about this topic. Like this post. $\endgroup$ – G3n0c1de Dec 15 '15 at 2:49
  • $\begingroup$ That makes sense given how theoretical this all is, there's room for discussion. Looking at John Rennie's diagram, how does that line up with what the guy I'm debating with says? I feel like he'd argue that from the outsider's perspective, the infalling object will never cross into region II from their point of view, so they'd be able to shine a light on it forever. $\endgroup$ – G3n0c1de Dec 15 '15 at 2:53

Black holes are strange. The best way to explain the idea of shining a laser on an object falling into the black hole is to say time ends at the event horizon. Yes the laser light will reach the image of the object (but not the object itself) exactly as it crosses the event horizon. That will be true for anything going into the black hole for the rest of history of the universe. But, we (being outside) will never see it actually get to the first object. To say it another way, everything that enters a black hole arrives at the singularity inside at the same time. And from the point of view of the black hole's singularity that time appears to be the end of the universe and the time it became a black hole (at the "same time") because time no longer advances inside the black hole.

  • $\begingroup$ I mostly agree. $\endgroup$ – Jason Williams Nov 8 '17 at 20:36

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