4 ms·

It's usually referred to as a fabric because in general relativity, spacetime is dynamic, as if it were on a fabric that could stretch and contract in response

It's usually referred to as a fabric because in general relativity, spacetime is dynamic, as if it were on a fabric that could stretch and contract in response to the density of matter occupying it. This is where the rubber-sheet analogy comes from. As for what it is made of, that is one of the great open questions in theoretical physics right now, but also possibly where the analogy breaks down. Because of Lorentz Invariance (light travels at the same speed in all reference frames), it is hard to suppose that there is such a thing as "atoms of spacetime" from which it is made. However, because our world is cosmological and the big bang does actually pick out a preferred reference frame, Lorentz Invarience has to be approximate in some way. Because of cosmology and the breakdown of reductionism at plank scales, it seems that the notion of spacetime itself is approximate and needs to be replaced with something else. Some people are trying to derive spacetime from purely quantum mechanical notions, while other people are trying to find dual systems where spacetime and quantum mechanics emerge hand-in-hand. The two approaches I'm familiar involve either holography or purely geometric and combinatorial ideas where the principles of general relativity and quantum mechanics are outputs rather than assumptions. https://www.youtube.com/watch?v=WECVq2YBduY&t=0s&index=9&list=PLOchmQOEwXhEIAnI-pyHQlrCnEhmxGnQD https://www.youtube.com/watch?v=WECVq2YBduY&t=0s&index=9&lis...

aroberge 8y ago
> However, because our world is cosmological and the big bang does actually pick out a preferred reference frame, Lorentz Invarience has to be approximate in some way. A particular solution to a set of equations does not usually have all the symmetry of the equations. In fact, I would say that, any solution to GR equations with energy and/or matter (assuming homogeneity on large enough scale) you will find something that looks like a preferred frame of reference.
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)?

aroberge 8y ago

> However, because our world is cosmological and the big bang does actually pick out a preferred reference frame, Lorentz Invarience has to be approximate in some way. A particular solution to a set of equations does not usually have all the symmetry of the equations. In fact, I would say that, any solution to GR equations with energy and/or matter (assuming homogeneity on large enough scale) you will find something that looks like a preferred frame of reference.

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)?