Apologies in advance, I'm a layman with only a school-level education in physics.

If an object approaching the event horizon of a black hole has its light cone progressively bent towards the black hole's singularity, why can all observers in the future not see the light bouncing off of it?

In a recent UK TV documentary The Science of Doctor Who, Professor Brian Cox talked about the above and explained that the future light cone of an object would get bent towards a black hole's singularity. In the example he gave, he suggested that an object venturing towards the event horizon would leave a permanent image. If that's the case, why aren't all black holes masked by photos bouncing off of 'swallowed' objects to all possible future moments?

My mental model (which I'm sure is wrong) see this a bit like a laser being shined at a mirror rotating from a perpendicular angle through 90 degrees to an orthogonal one - as the mirror moves in a continuous fashion, the laser should be bounced back to all points in the future. This assumes that the flow of photos in the laser is also continuous, and not made up of discrete packets very close together such that they seem continuous.

Hopefully that made sense, and many thanks to anyone kind enough to take the time to answer.


There is a big difference between what an observer sees, who falls into the black hole and what a far away observer sees.

Inside the event horizon time and space somewhat swap roles. Us regular folks can move freely in space, but can only move forward in time. In the same sense, the infalling observer can only move towards the singularity. Some people even say, that the singularity is not a point in space, but a point in time.

However, the singularity is not in the future of a far away observer. It is actually out of reach, just like things which are so far away, that light had no time to travel to us.

For the far away observer, the event horizon is indeed covered with the wrecks of infalling objects. They are however awfully deformed and smeared all over the horizon. They can in a way still be "seen" (no information is lost so far).

You may want to google for Leonard Susskind's talk about the "black hole information paradox" to find out more.

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  • $\begingroup$ This answer contains so much qualitative information. $\endgroup$ – user597368 Jun 7 '19 at 17:14

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