9 ms·

Strangeness of Black Holes

67 points by dnetesn 8y ago

ithkuil 8y ago
I found these lectures by Leonard Susskind very insightful: - Leonard Susskind on The World As Hologram (https://www.youtube.com/watch?v=2DIl3Hfh9t https://www.youtube.com/watch?v=2DIl3Hfh9t) - Leonard Susskind | "ER = EPR" or "What's Behind the Horizons of Black Holes?" (https://www.youtube.com/watch?v=OBPpRqxY8Uw https://www.youtube.com/watch?v=OBPpRqxY8Uw) - Entanglement and Complexity: Gravity and Quantum Mechanics (https://www.youtube.com/watch?v=9crggox5rbc https://www.youtube.com/watch?v=9crggox5rbc)
- deleted 8y ago
  [deleted]
- consp 8y ago
  As a note to the poster: The first video is not available where I live (Netherlands), maybe the link is wrong, the others are however.
  - Santosh83 8y ago
    Just adding: it is unavailable for me here in India too.
  - arunix 8y ago
    This one works for me (UK): https://www.youtube.com/watch?v=2DIl3Hfh9tY https://www.youtube.com/watch?v=2DIl3Hfh9tY (Leonard Susskind on The World As Hologram)
  - ChristianBundy 8y ago
    I think they may have accidentally removed a character from the URL. The posted URL: https://www.youtube.com/watch?v=2DIl3Hfh9t https://www.youtube.com/watch?v=2DIl3Hfh9t The actual URL: https://www.youtube.com/watch?v=2DIl3Hfh9tY https://www.youtube.com/watch?v=2DIl3Hfh9tY
- speakeron 8y ago
  I would also recommend (8 lectures) Leonard Susskind Topics in String Theory https://www.youtube.com/watch?v=NZ-ElsvYKyo https://www.youtube.com/watch?v=NZ-ElsvYKyo Despite the name this is largely about general relativity and black holes and other horizons. The best detailed description of black holes for laymen (assuming you have algebra and basic calculus) I've ever seen.
- bassman9000 8y ago
  To add to those The Perimeter Institute ones The Royal Institution ones There's multiple of each devoted to cosmology and black holes in particular.
dr_dshiv 8y ago
I'm curious about the description of a black hole as a singularity. I'm going to resort to Neoplatonist philosophy to try to understand this. In the formation of the universe, the notion of a singularity points at an old idea, argued by Plotinus, that everything started with "The One". This is commensurable with the statement that the boltzmann entropy of the start of the universe, prior to inflation, would be equal to "1" -- because that is the total volume of possible phase space. That, to me, is very elegant -- although it is unclear whether there are predictions about how the 1 becomes 2 ("the indefinite dyad"), 3 and then the inflationary myriad. But still, I'm impressed that the logic of ancient philosophy could correspond reasonably with modern physics. From this perspective, I'm interpreting big bang's "singularity" as =1. What is the nature of the singularity of a blackhole? Is the event horizon itself the singularity, or does it just an effect of a singularity at the center of the blackhole? Is there a "One" at the center of the blackhole?
- robin_reala 8y ago
  I think your assumptions are off. Anyway, the event horizon isn’t the singularity itself, it’s the point of no return that surrounds the singularity.
  - platz 8y ago
    Maybe there is a One. Some physicists think that inside a black hole is a white hole, which would look like expansion. Our cosmological horizon could match up with this
    - perfmode 8y ago
      Is this the idea that our observable universe is actually inside of a larger black hole?
      - platz 8y ago
        That's right
bawana 8y ago
I find it strange that no one has characterized that boundary layer where light orbits the black hole. We know that massive objects distort space and as a result alter the path of light. At sufficiently close distances, space is so distorted that 'not even light can escape'. Everyone assumes that light 'falls into' a black hole. But for light to do that, it would need a velocity greater than light itself if it is in orbit around the black hole. Thus, any light that is captured by tangential approach to the black hole and goes into orbit around the black hole, has to stay in orbit. Any acceleration that is needed to change the path of light would add to the velocity of light and that should not be possible. Over time, the only light that is not absorbed by virtue of collision with the singularity at the center of the black hole is the light in orbit around it. This is distinct and different from the light resulting from nuclear fusion of material in the accretion disc which is just outside.
- l33tman 8y ago
  The perfect orbit geodesic is infinitely thin and therefore unstable due to quantum mechanics. So even if a photon is injected at the perfect orbit it would not stay there, it would fall in or eject. Besides, even without invoking QM, any other matter that actually falls into the hole afterwards would inflate the event horizon, causing those photons caught in the perfect orbit to now exist just infinitesimally within the orbit and fall in.
  - soVeryTired 8y ago
    Do you need quantum mechanics to see that a photon would fall in or eject? Presumably a little movement from the black hole would do the trick too.
    - Pharmakon 8y ago
      Right, but the QM view tells us that even a perfectly stationary chargeless black hole in a perfect vacuum alone in a perfectly smooth universe would still experience fluctuations in its event horizon. Unlike the Classical view, QM reveals that even in principle there exists a degree of instability in the horizon.
dpark 8y ago
I’ve never understood how infinite time dilation jives with black hole evaporation: > If we take for example the latter observation of unbounded time dilation seriously, we come to the conclusion that in the moment that we spend on the horizon all time passes for anything outside the black hole. The end of all things happens in the rest of the universe, and in fact after the moment in which we enter the black hole, there is no way back. > However, if we consider a black hole of the same mass, things look rather different. As a (partially) quantum object it radiates and evaporates within a tiny fraction of a second. All its mass converts into energy, which results in an explosion three times stronger than the bomb dropped on Hiroshima. How do these two things mix? A particle experiences asymptotic time dilation when it crosses the event horizon and the rest of the universe ends in that moment. Then the black hole evaporates and lo and behold the universe is not ended. Yeah, the particle that went in got annihilated and never actually came back out. Sure. But if the black hole does not survive until the end of the universe, then it seems fundamentally impossible that the particle in question experienced unbounded/infinite/asymptotic time dilation. The particle’s annihilation must have happened by the time the black hole evaporated. And yes, time is relative, but it doesn’t run in reverse.
- speakeron 8y ago
  > How do these two things mix? A particle experiences asymptotic time dilation when it crosses the event horizon and the rest of the universe ends in that moment. Then the black hole evaporates and lo and behold the universe is not ended. I don't know if that's necessarily correct. From the point of view of an object falling through the event horizon, nothing special happens and they can look outwards normally; whereas an observer some distance from the black hole (and maintaining that distance) sees the falling object fall slower and slower and becoming more redshifted. The falling object also become shorter in the direction of travel (according to the external observer) and end up 'smeared' on the horizon. Leonard Susskind shows this using a Penrose diagram in lecture 5 or 6 (I think, but they're all worth watching) of the Stanford Topics in String Theory lectures[1] [1] https://www.youtube.com/watch?v=NZ-ElsvYKyo https://www.youtube.com/watch?v=NZ-ElsvYKyo
  - zwkrt 8y ago

67 points by dnetesn 8y ago

I found these lectures by Leonard Susskind very insightful: - Leonard Susskind on The World As Hologram (https://www.youtube.com/watch?v=2DIl3Hfh9t https://www.youtube.com/watch?v=2DIl3Hfh9t) - Leonard Susskind | "ER = EPR" or "What's Behind the Horizons of Black Holes?" (https://www.youtube.com/watch?v=OBPpRqxY8Uw https://www.youtube.com/watch?v=OBPpRqxY8Uw) - Entanglement and Complexity: Gravity and Quantum Mechanics (https://www.youtube.com/watch?v=9crggox5rbc https://www.youtube.com/watch?v=9crggox5rbc)

deleted 8y ago
[deleted]
consp 8y ago
As a note to the poster: The first video is not available where I live (Netherlands), maybe the link is wrong, the others are however.
- Santosh83 8y ago
  Just adding: it is unavailable for me here in India too.
- arunix 8y ago
  This one works for me (UK): https://www.youtube.com/watch?v=2DIl3Hfh9tY https://www.youtube.com/watch?v=2DIl3Hfh9tY (Leonard Susskind on The World As Hologram)
- ChristianBundy 8y ago
  I think they may have accidentally removed a character from the URL. The posted URL: https://www.youtube.com/watch?v=2DIl3Hfh9t https://www.youtube.com/watch?v=2DIl3Hfh9t The actual URL: https://www.youtube.com/watch?v=2DIl3Hfh9tY https://www.youtube.com/watch?v=2DIl3Hfh9tY
speakeron 8y ago
I would also recommend (8 lectures) Leonard Susskind Topics in String Theory https://www.youtube.com/watch?v=NZ-ElsvYKyo https://www.youtube.com/watch?v=NZ-ElsvYKyo Despite the name this is largely about general relativity and black holes and other horizons. The best detailed description of black holes for laymen (assuming you have algebra and basic calculus) I've ever seen.
bassman9000 8y ago
To add to those The Perimeter Institute ones The Royal Institution ones There's multiple of each devoted to cosmology and black holes in particular.

dr_dshiv 8y ago

I'm curious about the description of a black hole as a singularity. I'm going to resort to Neoplatonist philosophy to try to understand this. In the formation of the universe, the notion of a singularity points at an old idea, argued by Plotinus, that everything started with "The One". This is commensurable with the statement that the boltzmann entropy of the start of the universe, prior to inflation, would be equal to "1" -- because that is the total volume of possible phase space. That, to me, is very elegant -- although it is unclear whether there are predictions about how the 1 becomes 2 ("the indefinite dyad"), 3 and then the inflationary myriad. But still, I'm impressed that the logic of ancient philosophy could correspond reasonably with modern physics. From this perspective, I'm interpreting big bang's "singularity" as =1. What is the nature of the singularity of a blackhole? Is the event horizon itself the singularity, or does it just an effect of a singularity at the center of the blackhole? Is there a "One" at the center of the blackhole?

robin_reala 8y ago
I think your assumptions are off. Anyway, the event horizon isn’t the singularity itself, it’s the point of no return that surrounds the singularity.
- platz 8y ago
  Maybe there is a One. Some physicists think that inside a black hole is a white hole, which would look like expansion. Our cosmological horizon could match up with this
  - perfmode 8y ago
    Is this the idea that our observable universe is actually inside of a larger black hole?
    - platz 8y ago
      That's right

bawana 8y ago

I find it strange that no one has characterized that boundary layer where light orbits the black hole. We know that massive objects distort space and as a result alter the path of light. At sufficiently close distances, space is so distorted that 'not even light can escape'. Everyone assumes that light 'falls into' a black hole. But for light to do that, it would need a velocity greater than light itself if it is in orbit around the black hole. Thus, any light that is captured by tangential approach to the black hole and goes into orbit around the black hole, has to stay in orbit. Any acceleration that is needed to change the path of light would add to the velocity of light and that should not be possible. Over time, the only light that is not absorbed by virtue of collision with the singularity at the center of the black hole is the light in orbit around it. This is distinct and different from the light resulting from nuclear fusion of material in the accretion disc which is just outside.

l33tman 8y ago
The perfect orbit geodesic is infinitely thin and therefore unstable due to quantum mechanics. So even if a photon is injected at the perfect orbit it would not stay there, it would fall in or eject. Besides, even without invoking QM, any other matter that actually falls into the hole afterwards would inflate the event horizon, causing those photons caught in the perfect orbit to now exist just infinitesimally within the orbit and fall in.
- soVeryTired 8y ago
  Do you need quantum mechanics to see that a photon would fall in or eject? Presumably a little movement from the black hole would do the trick too.
  - Pharmakon 8y ago
    Right, but the QM view tells us that even a perfectly stationary chargeless black hole in a perfect vacuum alone in a perfectly smooth universe would still experience fluctuations in its event horizon. Unlike the Classical view, QM reveals that even in principle there exists a degree of instability in the horizon.

dpark 8y ago

I’ve never understood how infinite time dilation jives with black hole evaporation: > If we take for example the latter observation of unbounded time dilation seriously, we come to the conclusion that in the moment that we spend on the horizon all time passes for anything outside the black hole. The end of all things happens in the rest of the universe, and in fact after the moment in which we enter the black hole, there is no way back. > However, if we consider a black hole of the same mass, things look rather different. As a (partially) quantum object it radiates and evaporates within a tiny fraction of a second. All its mass converts into energy, which results in an explosion three times stronger than the bomb dropped on Hiroshima. How do these two things mix? A particle experiences asymptotic time dilation when it crosses the event horizon and the rest of the universe ends in that moment. Then the black hole evaporates and lo and behold the universe is not ended. Yeah, the particle that went in got annihilated and never actually came back out. Sure. But if the black hole does not survive until the end of the universe, then it seems fundamentally impossible that the particle in question experienced unbounded/infinite/asymptotic time dilation. The particle’s annihilation must have happened by the time the black hole evaporated. And yes, time is relative, but it doesn’t run in reverse.

speakeron 8y ago
> How do these two things mix? A particle experiences asymptotic time dilation when it crosses the event horizon and the rest of the universe ends in that moment. Then the black hole evaporates and lo and behold the universe is not ended. I don't know if that's necessarily correct. From the point of view of an object falling through the event horizon, nothing special happens and they can look outwards normally; whereas an observer some distance from the black hole (and maintaining that distance) sees the falling object fall slower and slower and becoming more redshifted. The falling object also become shorter in the direction of travel (according to the external observer) and end up 'smeared' on the horizon. Leonard Susskind shows this using a Penrose diagram in lecture 5 or 6 (I think, but they're all worth watching) of the Stanford Topics in String Theory lectures[1] [1] https://www.youtube.com/watch?v=NZ-ElsvYKyo https://www.youtube.com/watch?v=NZ-ElsvYKyo
- zwkrt 8y ago