3 ms·

Something I was always wondering about black holes: Does there really have to be a singularity? If I understand relativity correctly, time slows down as mass i

by dukoid 3y ago

Something I was always wondering about black holes: Does there really have to be a singularity? If I understand relativity correctly, time slows down as mass increases. Couldn't this slowdown affect the contraction so that it never can actually reach an "actual" singularity point?

treeman79 3y ago
https://youtu.be/351JCOvKcYw https://youtu.be/351JCOvKcYw There are a number of theories on black holes not being singularities. PBS space time goes into a few of them
nathan_compton 3y ago
This is actually a pretty tricky question. The standard black hole solutions are highly idealized in order to make them tractable. In particular, they have a lot more symmetry in time than you would expect a black hole to have. Actually forming a black hole from realistically distributed matter undergoing gravitational collapse is a more complicated problem to talk about in GR. So you pose a genuinely interesting question. I think the singularity theorems address it however: https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singul... The basic idea is that singularities are a very general feature of general relativity, not confined to specific solutions to the field equations but sort of inevitable for a large number of initial conditions. So while the standard black hole solutions are highly idealized, we have good theoretical reason to believe that singularities exist in non-trivial solutions to the Einstein Field Equations.
- GoblinSlayer 3y ago
  You're confused by mathematical terminology. Since mathematics is ignorant about time, "inevitable" is a synonym to "exists" there, because mathematics can easily assume infinite time passed by usual induction, but in physics time isn't negligible. Even in terms of mathematics you need to believe in actual infinity to believe that formation of a black hole completes.
gizmo686 3y ago
Time slows down from the perspective of an outside observer. From the perspective of a particle falling into a black hole, the singularity will be reached in finite time.
- GoblinSlayer 3y ago
  From the perspective of a particle falling into a black hole, it will be burned by Hawking radiation even before it reaches event horizon.
- dukoid 3y ago
  That's true but from our perspective this state won't be reached anywhere in the universe -- so there can't be any singularity from our perspective?
  - friend_and_foe 3y ago
    And you don't actually see a singularity, you see only an event horizon, and more accurately, the matter trapped orbiting the event horizon.
simiones 3y ago
Well, I don't think any physicist believes that an actual singularity exists in the center of a black hole in physical reality. The mathematics of GR actually do predict a singularity, but this is typically viewed as a limitation of the theory, not an actual physical object. Instead, most physicists believe that a theory of quantum gravity (a theory which accounts for gravitational interactions between quantum particles in general) would slightly modify the equations of GR to arrive at a finite solution for the interior of the black hole. Of course, such a theory eludes us so far.
fnovd 3y ago
You're right that from an observer outside of the event horizon, a black hole hasn't happened yet (and never will). There's a fun thought experiment to be had here. We're all familiar with the concept of "going back in time". Even though the math doesn't check out and our imaginations of what that looks like are totally impossible, it's something we can (and often like to) imagine. Now, what happens when you move backwards not in time, but in space? And I don't mean changing your direction 180 degrees, but moving backwards within the confines of space. So, if your present coordinates are (X, Y, Z), you move "backwards in space" 5 meters such that no matter what your direction of motion, if you move "forwards in space" by 5 meters you end up right where you started, at (X, Y, Z). You're basically creating an inverted bubble around (X, Y, Z) and making it so that all possible paths lead "outward" toward (X, Y, Z). Any point that is A meters away from (X, Y, Z) is actually A+5 meters away from you. In a 2-D world this can be modeled with a third dimension. We all live on a flat plain but someone on a 5-meter hill is actually farther away than their 2-dimensional coordinates would imply. You're able to move away from a point in 2-D space without changing your 2-D coordinates (because you're actually moving in another dimension). For 3D space, this extra dimension is time itself. The only way to move "backwards" in a comprehensible sense within 3 dimensions is by moving backwards within time. So, I'm not actually moving 5 meters "backwards" within space but moving backwards in time such that I will appear at (X, Y, Z) in 3-D space at precisely the same time it would take me to move forward 5 meters. I'm blipped out of space until such time has passed to allow me to reach (X, Y, Z) again, so I've successfully "moved backwards in space" by 5 meters. So, if I move "backwards in space", then I'm really putting (X, Y, Z) in front of me time-wise. So, what's the center of a black hole? We can imagine it has coordinates (X, Y, Z). And let's pretend it's actually moving backwards into space, which we now conceptualize as putting its "present" coordinates forward in time. But, it's actually moving backwards in space (i.e. moving universally farther from every other point in space) at a speed faster than we can travel. The center of the black hole is always in the future and even as we move closer to the center, it's still infinitely far away. As we cross the event horizon, we ride the current and start moving faster than the universal speed limit, cutting ourselves off from ever interacting with the outside universe again. However, we're still never going to be able to reach the center, and indeed the center of the black hole has still not happened from our point of reference (and still never will).

by dukoid 3y ago

treeman79 3y ago

https://youtu.be/351JCOvKcYw https://youtu.be/351JCOvKcYw There are a number of theories on black holes not being singularities. PBS space time goes into a few of them

nathan_compton 3y ago

This is actually a pretty tricky question. The standard black hole solutions are highly idealized in order to make them tractable. In particular, they have a lot more symmetry in time than you would expect a black hole to have. Actually forming a black hole from realistically distributed matter undergoing gravitational collapse is a more complicated problem to talk about in GR. So you pose a genuinely interesting question. I think the singularity theorems address it however: https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singul... The basic idea is that singularities are a very general feature of general relativity, not confined to specific solutions to the field equations but sort of inevitable for a large number of initial conditions. So while the standard black hole solutions are highly idealized, we have good theoretical reason to believe that singularities exist in non-trivial solutions to the Einstein Field Equations.

GoblinSlayer 3y ago
You're confused by mathematical terminology. Since mathematics is ignorant about time, "inevitable" is a synonym to "exists" there, because mathematics can easily assume infinite time passed by usual induction, but in physics time isn't negligible. Even in terms of mathematics you need to believe in actual infinity to believe that formation of a black hole completes.

gizmo686 3y ago

Time slows down from the perspective of an outside observer. From the perspective of a particle falling into a black hole, the singularity will be reached in finite time.

GoblinSlayer 3y ago
From the perspective of a particle falling into a black hole, it will be burned by Hawking radiation even before it reaches event horizon.
dukoid 3y ago
That's true but from our perspective this state won't be reached anywhere in the universe -- so there can't be any singularity from our perspective?
- friend_and_foe 3y ago
  And you don't actually see a singularity, you see only an event horizon, and more accurately, the matter trapped orbiting the event horizon.

simiones 3y ago

Well, I don't think any physicist believes that an actual singularity exists in the center of a black hole in physical reality. The mathematics of GR actually do predict a singularity, but this is typically viewed as a limitation of the theory, not an actual physical object. Instead, most physicists believe that a theory of quantum gravity (a theory which accounts for gravitational interactions between quantum particles in general) would slightly modify the equations of GR to arrive at a finite solution for the interior of the black hole. Of course, such a theory eludes us so far.

fnovd 3y ago

You're right that from an observer outside of the event horizon, a black hole hasn't happened yet (and never will). There's a fun thought experiment to be had here. We're all familiar with the concept of "going back in time". Even though the math doesn't check out and our imaginations of what that looks like are totally impossible, it's something we can (and often like to) imagine. Now, what happens when you move backwards not in time, but in space? And I don't mean changing your direction 180 degrees, but moving backwards within the confines of space. So, if your present coordinates are (X, Y, Z), you move "backwards in space" 5 meters such that no matter what your direction of motion, if you move "forwards in space" by 5 meters you end up right where you started, at (X, Y, Z). You're basically creating an inverted bubble around (X, Y, Z) and making it so that all possible paths lead "outward" toward (X, Y, Z). Any point that is A meters away from (X, Y, Z) is actually A+5 meters away from you. In a 2-D world this can be modeled with a third dimension. We all live on a flat plain but someone on a 5-meter hill is actually farther away than their 2-dimensional coordinates would imply. You're able to move away from a point in 2-D space without changing your 2-D coordinates (because you're actually moving in another dimension). For 3D space, this extra dimension is time itself. The only way to move "backwards" in a comprehensible sense within 3 dimensions is by moving backwards within time. So, I'm not actually moving 5 meters "backwards" within space but moving backwards in time such that I will appear at (X, Y, Z) in 3-D space at precisely the same time it would take me to move forward 5 meters. I'm blipped out of space until such time has passed to allow me to reach (X, Y, Z) again, so I've successfully "moved backwards in space" by 5 meters. So, if I move "backwards in space", then I'm really putting (X, Y, Z) in front of me time-wise. So, what's the center of a black hole? We can imagine it has coordinates (X, Y, Z). And let's pretend it's actually moving backwards into space, which we now conceptualize as putting its "present" coordinates forward in time. But, it's actually moving backwards in space (i.e. moving universally farther from every other point in space) at a speed faster than we can travel. The center of the black hole is always in the future and even as we move closer to the center, it's still infinitely far away. As we cross the event horizon, we ride the current and start moving faster than the universal speed limit, cutting ourselves off from ever interacting with the outside universe again. However, we're still never going to be able to reach the center, and indeed the center of the black hole has still not happened from our point of reference (and still never will).