Comment by WantonQuantum | Hacker News Reader

Comment by WantonQuantum | Hacker News Reader

Parent Full thread

The time dilation at the event horizon is infinite for an external observer. It appears that the person falling into the black hole slows down and never passes the event horizon. They redshift until you can't see them anymore.

For the unfortunate person falling into the black hole, there is nothing special about the event horizon. The spacetime they experience is rotated (with respect to the external observer) in such a way that their "future" points toward the black hole.

In a very real sense, for external observers there isn't really an interior of the black hole. That "inside" spacetime is warped so much that it exists more in "the future" than the present.

Professor Brian Cox also says that from a string theory perspective there isn't really an inside of a black hole, it's just missing spacetime. I tried to find a reference for this but I couldn't find one. Perhaps in his book about black holes.

I'm no physicist so happy to be corrected on any of the above!

> For the unfortunate person falling into the black hole, there is nothing special about the event horizon.

This is from a simplified model using black holes with infinite lifetime, which is non-physical. Almost all textbook Penrose diagrams use this invalid assumption and shouldn't be relied upon..

Fundamentally, external observers and infalling observers can't disagree on "what happens", just the timing of events. If external observers never see someone falling in, then they didn't fall in.

> Fundamentally, external observers and infalling observers can't disagree on "what happens", just the timing of events. If external observers never see someone falling in, then they didn't fall in.

This isn't true. As long as the two observers can't communicate with each other, they absolutely can have different results. To put it in simpler terms, the requirement of physics is that an experiment has a unique result according to some rule, but different experiments can have different results even if they break our intuitions.

So, if you measure the position of a particle falling towards a blackhole, you will see it disappear at the event horizon, and perhaps be radiated out later as Hawking radiation from that same event horizon. If you measure the position of the same particle while you yourself are passing through the event horizon, you will it will record no special interaction and see the particle moving completely normally. Since you can't perform both experiments at once, and you can't relay any data from one to the other, there is no contradiction.

This is just another case of a duality in physics, similar to how some experiments measure electrons as point-like particles completely localized to a certain place, and others measure them as waves spread out over a very large area.

> you will see it disappear at the event horizon

I don't believe this is the case -- the particle just becomes ever more redshifted.

If that were true, then black holes would appear as extremely bright balls of red-shifted radiation, as all particles whose trajectory ever moved towards the center of the black hole would still be visible. This is obviously not true, black holes appear as completely black objects that might have an extremely bright ring or halo of matter orbiting.

For a region of space with time dilation approaching infinity, red shift to temperatures below even the microwave background would be nothing.

Sure, timing of the events, but infinity kind of breaks it - if external observers don't see someone falling in, then they didn't fall in yet, and if external observers see that falling in takes an infinite time (as in this case), then that is on some sense just a difference in the timing of events - however, from the observer perspective where that thing takes a finite time, they will also get to observe what happens afterwards.

Black holes don’t live forever. In principle, an external observer could watch you until the black hole evaporates. As mentioned above, if they never saw you fall in, then you never fell in. GR allows for disagreement on durations of events but not the events themselves.

You know string theory is now generally considered a large scale fraud perpetrated by cox to maintain funding right?

Got a source for either part of that?

* generally considered a large scale fraud,

* perpetrated by (UK's Professor Brian) cox

Most that I know would say that it was disapointingly too big and too general to make specific predictions tied to this specific universe we occupy, although it had early promise.

Brian Cox didn't even make the wikipedia page so its difficult to claim he had any major role in perpertaring it as a large scale fraud.

https://en.wikipedia.org/wiki/String_theory

Found Sabine Hossenfelder's HN account!

I am, of course, joking but she posts this sort of easy and empty clickbait.

Laughing at someone who says “hey an idea that isn’t falsifiable isn’t a good theory and certainly not something that any other ideas or theories should be constructed upon is I think more serious than not.

Yup, her content is not very appealing these days.

Reply on news.ycombinator.com