Time to Form Black Hole? | Observer Slows Time at Event Horizon

Click For Summary

Discussion Overview

The discussion revolves around the nature of time and observation at the event horizon of a black hole, particularly whether a black hole can be said to form from the perspective of distant observers. It touches on theoretical implications of black hole formation, observational effects, and the nature of spacetime geometry.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that from the perspective of a distant observer, time slows down and ultimately stops at the event horizon, suggesting that black holes may not form in a conventional sense.
  • Others argue that a massive object collapses to a black hole in a finite time according to an observer falling in with the collapse, and that an observer falling into an already existing black hole also experiences a finite time to reach the horizon.
  • One participant clarifies that the observer they referred to was not falling in but watching from a distance, questioning whether a black hole ever forms from that perspective.
  • Another participant suggests that a distant observer would see the star becoming increasingly redshifted until it is no longer visible, but could detect the black hole indirectly if it is gravitationally bound in a binary system, potentially through an accretion disk.
  • It is noted that the concept of "from the perspective of the rest of the universe" is problematic, as the existence of a black hole is tied to the global spacetime geometry, independent of an observer's ability to see it.

Areas of Agreement / Disagreement

Participants express differing views on the implications of observation at the event horizon, with no consensus reached on whether a black hole can be said to form from the perspective of distant observers.

Contextual Notes

There are limitations in the assumptions made about observation and time at the event horizon, as well as the dependence on definitions of perspective and existence in spacetime geometry.

Cobalt101
Messages
27
Reaction score
0
Is it correct that, from the perspective of an observer, time slows down and ultimately stops at the event horizon of a black hole, implying that no black holes have had time to form in the universe ?
 
Physics news on Phys.org
Cobalt101 said:
Is it correct that, from the perspective of an observer, time slows down and ultimately stops at the event horizon of a black hole, implying that no black holes have had time to form in the universe ?

No. A massive object only takes a finite time to collapse to a black hole, according to an observer falling in with the collapse; and an observer falling into a black hole that already exists only takes a finite time by his own clock to reach the hole's horizon.
 
PeterDonis said:
No. A massive object only takes a finite time to collapse to a black hole, according to an observer falling in with the collapse; and an observer falling into a black hole that already exists only takes a finite time by his own clock to reach the hole's horizon.
Thanks - but the observer I was referring to was not falling in with the object, but rather was watching from a distance. So in particular if a black hole is formed by a collapsing star, from the perspective of the rest of the universe does the black hole ever form? (I understand that from the perspective of the collapsing star it only takes a finite time).
 
I think the observer would see what had been the star becoming increasingly red shifted until it was no longer visible.
However if it is gravitationally bound, say in a binary system, it could be detected indirectly that way, and possibly there might be a hot and visible accretion disk of remnant material surrounding the event horizon of what now is a black hole.
 
Last edited:
Cobalt101 said:
if a black hole is formed by a collapsing star, from the perspective of the rest of the universe does the black hole ever form?

There is no such thing as "from the perspective of the rest of the universe". The black hole is a feature of the global spacetime geometry; either it is there or it isn't. If the collapsing object forms a horizon in a finite time, or an infalling observer reaches the horizon in a finite time, then the black hole is there. The fact that the distant observer can't see it doesn't change that.
 

Similar threads

Undergrad You can't actually enter a blackhole?
  • · Replies 40 ·
2
Replies
40
Views
4K
Undergrad Falling through the event horizon of an evaporating black hole
  • · Replies 51 ·
2
Replies
51
Views
6K
Undergrad I don't see how a black hole's event horizon can be crossed
  • · Replies 73 ·
3
Replies
73
Views
4K
High School Is there a white hole inside every black hole?
  • · Replies 22 ·
Replies
22
Views
2K
High School How can a black hole absorb matter?
  • · Replies 31 ·
2
Replies
31
Views
2K
High School Is crossing a black hole's event horizon possible?
  • · Replies 46 ·
2
Replies
46
Views
9K
Undergrad Do any real macroscopic black holes have a singularity?
  • · Replies 11 ·
Replies
11
Views
2K
High School Entering a black hole -- I know I must be wrong (Help me understand why)
  • · Replies 29 ·
Replies
29
Views
4K
High School Theories about light and time near a black hole
  • · Replies 20 ·
Replies
20
Views
3K
High School Falling into a black hole
  • · Replies 43 ·
2
Replies
43
Views
4K