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Thanks for linking to the video, it was the most informative handling of the subject I have seen. After watching, I have a question, please forgive if it is ri

by dsnuh 8y ago

Thanks for linking to the video, it was the most informative handling of the subject I have seen. After watching, I have a question, please forgive if it is ridiculous or displays some fundamental problem with my layman's understanding of the subject. If time becomes space-like, and space becomes time-like, can we think of black holes as "time stars"? Would every black hole within our observable universe contain information from all light cones within the observable universe? In other words, would different black holes contain different information? Does my question make sense?

philipov 8y ago
I'm not an expert either, but according to the treatments I've seen on GR, the flipping of time and space inside of black holes is just an artifact of the choice of coordinates, and can be fixed by changing your coordinate system. However, there are some ideas based on holography for solving the black hole information paradox that posit that it doesn't even make sense to talk about the inside and outside of a black hole at the same time at all (black hole complementarity), that the inside of the black hole is actually encoded on the surface of the horizon. This of course just leads to other paradoxes, and the only thing that seems certain is that GR is approximate in some way, and all these questions are the result of stretching the theory to regimes in which it is no longer valid. This is why many people are searching for fundamentally new ways of constructing theories that yield the same answers for experiments we've tested, but begin from different assumptions. [0] However, as for black holes containing the same information, our universe is expanding uniformly at an accelerating rate, and this results in cosmological horizons beyond which objects are moving faster than the speed of light (with respect to the frame of reference). Therefore, black holes in different galaxies shouldn't be able to have access to the same light cones without compromising the principle of locality. Although, if you believe that entangled black holes in different galaxies would lead to the same singularity ("ER=EPR" [1][2]), this becomes more complicated and is way out of my depth. . Nima Arkani Hamed | Unification and Fundamental Phyics: A Status Report 9-21-2018 [0]: https://www.youtube.com/watch?v=pXYvFMqwbhc https://www.youtube.com/watch?v=pXYvFMqwbhc Leonard Susskind Public Lecture | "ER = EPR" or "What's Behind the Horizons of Black Holes?" [1]: part 1: https://www.youtube.com/watch?v=OBPpRqxY8Uw https://www.youtube.com/watch?v=OBPpRqxY8Uw [2]: part 2: https://www.youtube.com/watch?v=uiG_EtVQu5o https://www.youtube.com/watch?v=uiG_EtVQu5o
- dsnuh 8y ago
  Thanks for the detailed reply! One point that was made in the video was that inside a black hole, space is collapsing faster than light. This seems like more than a change to coordinate system? Doesn't this mean that time is also moving faster than light? Could time effectively become "solid" at the singularity, with all events within the black hole's light cone at a single point in time, but in infinite space(ignoring Hawking Radiation)?
  - codethief 8y ago
    > One point that was made in the video was that inside a black hole, space is collapsing faster than light. This seems like more than a change to coordinate system? No, this refers to the fact that inside a black hole you can travel as fast as you want and do whatever you want but even if you move around at the speed of light, you still won't be able to escape the singularity. This is because the singularity is a so-called timelike singularity – it lies in your future. It's not a point in space somewhere that you can walk around and poke; while you're in the black hole, it's actually completely invisible to you until you hit it. You can basically compare the singularity to your own death: While you can't see it right here and right now but you know for sure that you'll reach it at some point in your future. A physicsy way of saying all this is that, inside the black hole, all future-directed worldlines emanating from a given spacelike surface will converge and eventually hit the singularity. (This is what Nima Arkani-Hamed means when he says that space "collapses".) Another way of saying this is that the aforementioned surface is a so-called trapped surface. Back to a more intuitive explanation: In a way, falling into a black hole and towards the singularity is like falling down a waterfall: Once you've stepped over the edge (crossed the event horizon), you certainly won't be able to stop yourself from falling anymore. You cannot fall up and escape gravity. Here's a website which explains this way of looking at black holes as "waterfalls" of space a bit further: https://jila.colorado.edu/~ajsh/insidebh/waterfall.html https://jila.colorado.edu/~ajsh/insidebh/waterfall.html > Doesn't this mean that time is also moving faster than light? As mentioned in my other comment, I recommend steering clear of the terms "time" and "space" as those are not individually well-defined and equally agreed upon by all observers. Instead, time and space are relative – if I use a different coordinate system than you, we will have very different notions of which events occur at the same point in time (and are thus part of the same spatial slice). It's called relativity for a reason. :) So, my counter question to your question would be: > Time as measured by whom? ;) But even if you define time with respect to a given observer, it still does not "move" or have a "velocity" that you could compare to the speed of light. Only massive objects actually move through spacetime and only when they're close enough, you can compare their relative velocities.
codethief 8y ago
I'm afraid I don't have the time to watch the entire video but, being a physicist, I can still try to answer your questions. Currently, however, your question > If time becomes space-like, and space becomes time-like, can we think of black holes as "time stars"? doesn't make much sense to me. Time does not become spacelike nor does space become timelike. In General Relativity, time and space are not individually defined in an observer-independent fashion in the first place. The terms we use instead are "timelike directions" and "spacelike directions" as well as timelike / spacelike hypersurfaces because they are defined in such a way that all observers will agree on whether a direction / hypersurface is timelike / spacelike. So now that we've replaced the terms "time" and "space" with "timelike" and "spacelike", I hope you'll see that your question is not exactly well-defined. But maybe I'm misunderstanding you, so please feel free to elaborate on your questions. (Or to point me to an explanation from the video in case you're referring to one.) As for your second question: > Would every black hole within our observable universe contain information from all light cones within the observable universe? In other words, would different black holes contain different information? I'm afraid I can't follow this question, either. Would you mind rephrasing it? My initial impression is that you might have a wrong idea of what a lightclone is.
- dsnuh 8y ago
  I just realized I was replying to the wrong parent comment! So sorry. I was talking about a PBS Space Time episode on YouTube I saw in (or came across via) a different comment. https://youtu.be/KePNhUJ2reI https://youtu.be/KePNhUJ2reI
  - codethief 8y ago
    No worries, I hadn't watched the video yet, anyway. :) So feel free to rephrase your questions if you like!
    - dsnuh 8y ago
      My questions were based in the video I linked above, which it sounds like may be incorrect in the way it presents the topic?

by dsnuh 8y ago

philipov 8y ago

I'm not an expert either, but according to the treatments I've seen on GR, the flipping of time and space inside of black holes is just an artifact of the choice of coordinates, and can be fixed by changing your coordinate system. However, there are some ideas based on holography for solving the black hole information paradox that posit that it doesn't even make sense to talk about the inside and outside of a black hole at the same time at all (black hole complementarity), that the inside of the black hole is actually encoded on the surface of the horizon. This of course just leads to other paradoxes, and the only thing that seems certain is that GR is approximate in some way, and all these questions are the result of stretching the theory to regimes in which it is no longer valid. This is why many people are searching for fundamentally new ways of constructing theories that yield the same answers for experiments we've tested, but begin from different assumptions. [0] However, as for black holes containing the same information, our universe is expanding uniformly at an accelerating rate, and this results in cosmological horizons beyond which objects are moving faster than the speed of light (with respect to the frame of reference). Therefore, black holes in different galaxies shouldn't be able to have access to the same light cones without compromising the principle of locality. Although, if you believe that entangled black holes in different galaxies would lead to the same singularity ("ER=EPR" [1][2]), this becomes more complicated and is way out of my depth. . Nima Arkani Hamed | Unification and Fundamental Phyics: A Status Report 9-21-2018 [0]: https://www.youtube.com/watch?v=pXYvFMqwbhc https://www.youtube.com/watch?v=pXYvFMqwbhc Leonard Susskind Public Lecture | "ER = EPR" or "What's Behind the Horizons of Black Holes?" [1]: part 1: https://www.youtube.com/watch?v=OBPpRqxY8Uw https://www.youtube.com/watch?v=OBPpRqxY8Uw [2]: part 2: https://www.youtube.com/watch?v=uiG_EtVQu5o https://www.youtube.com/watch?v=uiG_EtVQu5o

dsnuh 8y ago
Thanks for the detailed reply! One point that was made in the video was that inside a black hole, space is collapsing faster than light. This seems like more than a change to coordinate system? Doesn't this mean that time is also moving faster than light? Could time effectively become "solid" at the singularity, with all events within the black hole's light cone at a single point in time, but in infinite space(ignoring Hawking Radiation)?
- codethief 8y ago
  > One point that was made in the video was that inside a black hole, space is collapsing faster than light. This seems like more than a change to coordinate system? No, this refers to the fact that inside a black hole you can travel as fast as you want and do whatever you want but even if you move around at the speed of light, you still won't be able to escape the singularity. This is because the singularity is a so-called timelike singularity – it lies in your future. It's not a point in space somewhere that you can walk around and poke; while you're in the black hole, it's actually completely invisible to you until you hit it. You can basically compare the singularity to your own death: While you can't see it right here and right now but you know for sure that you'll reach it at some point in your future. A physicsy way of saying all this is that, inside the black hole, all future-directed worldlines emanating from a given spacelike surface will converge and eventually hit the singularity. (This is what Nima Arkani-Hamed means when he says that space "collapses".) Another way of saying this is that the aforementioned surface is a so-called trapped surface. Back to a more intuitive explanation: In a way, falling into a black hole and towards the singularity is like falling down a waterfall: Once you've stepped over the edge (crossed the event horizon), you certainly won't be able to stop yourself from falling anymore. You cannot fall up and escape gravity. Here's a website which explains this way of looking at black holes as "waterfalls" of space a bit further: https://jila.colorado.edu/~ajsh/insidebh/waterfall.html https://jila.colorado.edu/~ajsh/insidebh/waterfall.html > Doesn't this mean that time is also moving faster than light? As mentioned in my other comment, I recommend steering clear of the terms "time" and "space" as those are not individually well-defined and equally agreed upon by all observers. Instead, time and space are relative – if I use a different coordinate system than you, we will have very different notions of which events occur at the same point in time (and are thus part of the same spatial slice). It's called relativity for a reason. :) So, my counter question to your question would be: > Time as measured by whom? ;) But even if you define time with respect to a given observer, it still does not "move" or have a "velocity" that you could compare to the speed of light. Only massive objects actually move through spacetime and only when they're close enough, you can compare their relative velocities.

codethief 8y ago

I'm afraid I don't have the time to watch the entire video but, being a physicist, I can still try to answer your questions. Currently, however, your question > If time becomes space-like, and space becomes time-like, can we think of black holes as "time stars"? doesn't make much sense to me. Time does not become spacelike nor does space become timelike. In General Relativity, time and space are not individually defined in an observer-independent fashion in the first place. The terms we use instead are "timelike directions" and "spacelike directions" as well as timelike / spacelike hypersurfaces because they are defined in such a way that all observers will agree on whether a direction / hypersurface is timelike / spacelike. So now that we've replaced the terms "time" and "space" with "timelike" and "spacelike", I hope you'll see that your question is not exactly well-defined. But maybe I'm misunderstanding you, so please feel free to elaborate on your questions. (Or to point me to an explanation from the video in case you're referring to one.) As for your second question: > Would every black hole within our observable universe contain information from all light cones within the observable universe? In other words, would different black holes contain different information? I'm afraid I can't follow this question, either. Would you mind rephrasing it? My initial impression is that you might have a wrong idea of what a lightclone is.

dsnuh 8y ago
I just realized I was replying to the wrong parent comment! So sorry. I was talking about a PBS Space Time episode on YouTube I saw in (or came across via) a different comment. https://youtu.be/KePNhUJ2reI https://youtu.be/KePNhUJ2reI
- codethief 8y ago
  No worries, I hadn't watched the video yet, anyway. :) So feel free to rephrase your questions if you like!
  - dsnuh 8y ago
    My questions were based in the video I linked above, which it sounds like may be incorrect in the way it presents the topic?