9 ms·

Does this mean it's at least possible that space and time aren't linked in the way we think? That General Relativity only makes it appear so?

wwarner 7y ago
No the finding is that they tested GR near a boundary, where you might expect find issues, and found that it held up. The title and the comment Ghez makes about the interior of a black hole are a bit misleading. As far as interiors of black holes, I can only guess that she's pointing out (a) that we can't directly observe anything past the event horizon and (b) GR doesn't really make claims about what's going on in the very center at the singularity. From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center.
- contravariant 7y ago
  In the classical black hole model anything that crosses the event horizon only has a limited time before they hit the singularity. Even for particularly big black holes this wouldn't take all that long. I find it a bit tricky to say if you wouldn't encounter all the stuff that fell in before you though. I've always had trouble reconciling the fact that you won't see stuff enter the event horizon until you're close to it and the fact that black holes can spring into existence. I'm still not all that sure a true black hole isn't merely an approximation that breaks down as you get close.
  - btilly 7y ago
    It gets crazier still. Nothing ever gets into a black hole! If something is falling into a black hole, you can wait a million years and then rescue it. Or a billion. You see, time slows in a gravitational well. The bigger the gravitational well, the more it slows. From our point of view outside, time literally stops at the event horizon.
    - fspeech 7y ago
      Consider a simple linear function f(x)=ax. It maps the whole real axis bijectively to itself so long as “a” is not 0. However if a=0 everything collapses into the origin. The math that wraps some infinite process inside can be counterintuitive even as it makes things easier to compute. However it is not clear if the physical picture that involves singularity is real or just idealized approximation.
    - prewett 7y ago
      But you couldn't pull it out, either, since it's time stopped (or dramatically slowed, depending on the perspective).
- leereeves 7y ago
  The interior of black holes is one of the situations in which we need a theory that incorporates both gravity and quantum mechanics. We actually do have a couple such theories and can't verify them because the situations where they differ from GR are unobservable. It's a shame (but not surprising) this research didn't find any observable differences from GR.
  - mehrdadn 7y ago
    Are the really theories if they're known to be unfalsifiable? Or do you mean they might differ in situations we don't know about?
    - JumpCrisscross 7y ago
      > Are the really theories if they're known to be unfalsifiable? They're not unfalsifiable. (Not all of them.) Gravity is weak. Quantum mechanics are small. Measuring such small weak things is not presently possible.
- mehrdadn 7y ago
  > From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center. What does that actually mean? i.e. How would a black hole that's mostly empty inside differ from one that isn't?
  - TheOtherHobbes 7y ago
    It's not even obvious what "center" means in this context. Or for that matter "travel" and "get to."
- AgentME 7y ago
  > From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center. I can see how this could maybe be true for a black hole that never has anything fall into it after yourself, but for regular black holes that have things falling in regularly I think the situation would be pretty different. As you get closer to the event horizon, the rest of the universe appears to speed up. This means that as you get closer, the rate of objects / energy coming into the black hole past you and sometimes colliding into you will increase. You can imagine that at some point near the event horizon, every second, 100 years will pass for the rest of the universe, and 100 years worth of debris and light will enter into the black hole, some of it colliding with you. As soon as you reach the event horizon, an infinite amount of time's worth of debris and light will enter into the black hole, some of it colliding with you. From inside the black hole, it must look like everything that has ever fallen into the black hole in the history of the universe has fallen into it at the same instant. (And then I'm not entirely sure how black hole evaporation fits into this. I'd expect the perspective of someone going into a black hole must look like you immediately collide with everything that ever has and ever will fall into the black hole, and then instantly everything is obliterated into Hawking radiation.)
  - wwarner 7y ago
    I'm just a reader, but Kip Thorne doesn't agree with your picture. In his book, he claims you wouldn't notice passing through the event horizon, except for extreme tidal forces that would tear you apart. The time freezing and red shifting reverse so that looking away from the center, distant objects move faster and are bluer. His book for the layman is Black Holes and Time Warps.
    - shagie 7y ago
      The tidal forces depend on the size of the black hole. http://www.hawking.org.uk/into-a-black-hole.html http://www.hawking.org.uk/into-a-black-hole.html > ... If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is, you will be stretched out longwise, and squashed in sideways.. If the black hole has a mass of a few times our sun, you would be torn apart, and made into spaghetti, before you reached the horizon. However, if you fell into a much larger black hole, with a mass of a million times the sun, you would reach the horizon without difficulty. So, if you want to explore the inside of a black hole, choose a big one. There is a black hole of about a million solar masses, at the center of our Milky way galaxy.
jforberg 7y ago
> Our observations are consistent with Einstein’s general theory of relativity.
mhh__ 7y ago
Does this mean it's at least possible that space and time aren't linked in the way we think? Technically, we (loaded term) don't really think of anything as per se. General Relativity and the standard model - or Quantum Field Theory more specifically - are just that: Models. They are very good models, but not complete ones (Or unified, assuming they will be at some point)

wwarner 7y ago

No the finding is that they tested GR near a boundary, where you might expect find issues, and found that it held up. The title and the comment Ghez makes about the interior of a black hole are a bit misleading. As far as interiors of black holes, I can only guess that she's pointing out (a) that we can't directly observe anything past the event horizon and (b) GR doesn't really make claims about what's going on in the very center at the singularity. From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center.

contravariant 7y ago
In the classical black hole model anything that crosses the event horizon only has a limited time before they hit the singularity. Even for particularly big black holes this wouldn't take all that long. I find it a bit tricky to say if you wouldn't encounter all the stuff that fell in before you though. I've always had trouble reconciling the fact that you won't see stuff enter the event horizon until you're close to it and the fact that black holes can spring into existence. I'm still not all that sure a true black hole isn't merely an approximation that breaks down as you get close.
- btilly 7y ago
  It gets crazier still. Nothing ever gets into a black hole! If something is falling into a black hole, you can wait a million years and then rescue it. Or a billion. You see, time slows in a gravitational well. The bigger the gravitational well, the more it slows. From our point of view outside, time literally stops at the event horizon.
  - fspeech 7y ago
    Consider a simple linear function f(x)=ax. It maps the whole real axis bijectively to itself so long as “a” is not 0. However if a=0 everything collapses into the origin. The math that wraps some infinite process inside can be counterintuitive even as it makes things easier to compute. However it is not clear if the physical picture that involves singularity is real or just idealized approximation.
  - prewett 7y ago
    But you couldn't pull it out, either, since it's time stopped (or dramatically slowed, depending on the perspective).
leereeves 7y ago
The interior of black holes is one of the situations in which we need a theory that incorporates both gravity and quantum mechanics. We actually do have a couple such theories and can't verify them because the situations where they differ from GR are unobservable. It's a shame (but not surprising) this research didn't find any observable differences from GR.
- mehrdadn 7y ago
  Are the really theories if they're known to be unfalsifiable? Or do you mean they might differ in situations we don't know about?
  - JumpCrisscross 7y ago
    > Are the really theories if they're known to be unfalsifiable? They're not unfalsifiable. (Not all of them.) Gravity is weak. Quantum mechanics are small. Measuring such small weak things is not presently possible.
mehrdadn 7y ago
> From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center. What does that actually mean? i.e. How would a black hole that's mostly empty inside differ from one that isn't?
- TheOtherHobbes 7y ago
  It's not even obvious what "center" means in this context. Or for that matter "travel" and "get to."
AgentME 7y ago
> From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center. I can see how this could maybe be true for a black hole that never has anything fall into it after yourself, but for regular black holes that have things falling in regularly I think the situation would be pretty different. As you get closer to the event horizon, the rest of the universe appears to speed up. This means that as you get closer, the rate of objects / energy coming into the black hole past you and sometimes colliding into you will increase. You can imagine that at some point near the event horizon, every second, 100 years will pass for the rest of the universe, and 100 years worth of debris and light will enter into the black hole, some of it colliding with you. As soon as you reach the event horizon, an infinite amount of time's worth of debris and light will enter into the black hole, some of it colliding with you. From inside the black hole, it must look like everything that has ever fallen into the black hole in the history of the universe has fallen into it at the same instant. (And then I'm not entirely sure how black hole evaporation fits into this. I'd expect the perspective of someone going into a black hole must look like you immediately collide with everything that ever has and ever will fall into the black hole, and then instantly everything is obliterated into Hawking radiation.)
- wwarner 7y ago
  I'm just a reader, but Kip Thorne doesn't agree with your picture. In his book, he claims you wouldn't notice passing through the event horizon, except for extreme tidal forces that would tear you apart. The time freezing and red shifting reverse so that looking away from the center, distant objects move faster and are bluer. His book for the layman is Black Holes and Time Warps.
  - shagie 7y ago
    The tidal forces depend on the size of the black hole. http://www.hawking.org.uk/into-a-black-hole.html http://www.hawking.org.uk/into-a-black-hole.html > ... If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is, you will be stretched out longwise, and squashed in sideways.. If the black hole has a mass of a few times our sun, you would be torn apart, and made into spaghetti, before you reached the horizon. However, if you fell into a much larger black hole, with a mass of a million times the sun, you would reach the horizon without difficulty. So, if you want to explore the inside of a black hole, choose a big one. There is a black hole of about a million solar masses, at the center of our Milky way galaxy.

jforberg 7y ago

> Our observations are consistent with Einstein’s general theory of relativity.

mhh__ 7y ago

Does this mean it's at least possible that space and time aren't linked in the way we think? Technically, we (loaded term) don't really think of anything as per se. General Relativity and the standard model - or Quantum Field Theory more specifically - are just that: Models. They are very good models, but not complete ones (Or unified, assuming they will be at some point)