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IANAA (I am not an astrophysicist) but isn't the accompanying image very different from what actually happens when a star crosses the event horizon of a black h

by portman 16y ago

IANAA (I am not an astrophysicist) but isn't the accompanying image very different from what actually happens when a star crosses the event horizon of a black hole? I thought that when an object crosses the event horizon of a black hole, to an observer outside the event horizon, it will look like the object just slowed down and sort of "suspended" there. I didn't think it would emit a burst of radiation. (This is just from reading Brian Greene and Michio Kaku, so my understanding could be way off the mark.)

lincolnq 16y ago
I think you may be confusing the event horizon with the Roche limit. The event horizon is the distance where not even light can escape. The Roche limit is the closest a body can come to another (more massive) body such that it can stay intact with itself, as opposed to being torn apart by tidal forces in the massive object's gravitational field. I suspect, but do not know for sure, that the Roche radius for a black hole and regular star is much larger than the event horizon for the same black hole.
- Retric 16y ago
  They are actually somewhat independent. The Roche limit depends on both body's mass and volume, where the event horizon relates to a single body mass and volume. EX: Cold Brown dwarfs can orbit closer together than a brown dwarf and a earth like planet. PS: Some real satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation. Jupiter's moon Metis and Saturn's moon Pan are examples of such satellites, which hold together because of their tensile strength.
  - idlewords 16y ago
    Interestingly the equation for the Roche limit has a term for the radius of the primary, but not the satellite. Only the density of the latter matters, not its size.
  - bfe 16y ago
    Basically correct (except the concept of volume doesn't hold up in a black hole). It is theoretically possible to have a black hole big enough that its Roche limit is inside its event horizon; but a black hole that big is probably at least vanishingly rare in the observable universe.
    - Retric 16y ago
      A black hole has a volume less than it's event horizon. So, it's not really undefined as much as ill defined. Also, a black hole's event horizon is not necessarily a perfect sphere ex: two black hole's orbiting each other.
- portman 16y ago
  Aha, that makes sense. And yes, some quick Googling turns up several papers that talk about the Roche limit for a black hole and a star being well outside the black hole's event horizon. http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=%22roche+limit%22+%22event+horizon%22+%22black+hole%22 http://www.google.com/search?sourceid=chrome&ie=UTF-8&#3...
InclinedPlane 16y ago
There's nothing overly special about an object disappearing across an event horizon, observationally. However, blackholes happen to have extremely severe tidal forces that border the event horizon, and objects passing through this region (even stars) will be torn apart and smashed back together and will be raised to incredible temperatures and release massive amounts of energy. It is this process which is being observed, and it is actually one of the most efficient processes for converting mass to energy in the Universe. See: http://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter http://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter
idlewords 16y ago
Objects would almost never fall straight in to a black hole. In the case where there is relative angular velocity, they spiral in, and then the physics of accretion disks takes over. This involves the release of massive amounts of energy, particularly along radial jets, as the stuff in the disk spirals inwards. The energy release happens well before the infalling matter gets close to the event horizon.

by portman 16y ago

lincolnq 16y ago

I think you may be confusing the event horizon with the Roche limit. The event horizon is the distance where not even light can escape. The Roche limit is the closest a body can come to another (more massive) body such that it can stay intact with itself, as opposed to being torn apart by tidal forces in the massive object's gravitational field. I suspect, but do not know for sure, that the Roche radius for a black hole and regular star is much larger than the event horizon for the same black hole.

Retric 16y ago
They are actually somewhat independent. The Roche limit depends on both body's mass and volume, where the event horizon relates to a single body mass and volume. EX: Cold Brown dwarfs can orbit closer together than a brown dwarf and a earth like planet. PS: Some real satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation. Jupiter's moon Metis and Saturn's moon Pan are examples of such satellites, which hold together because of their tensile strength.
- idlewords 16y ago
  Interestingly the equation for the Roche limit has a term for the radius of the primary, but not the satellite. Only the density of the latter matters, not its size.
- bfe 16y ago
  Basically correct (except the concept of volume doesn't hold up in a black hole). It is theoretically possible to have a black hole big enough that its Roche limit is inside its event horizon; but a black hole that big is probably at least vanishingly rare in the observable universe.
  - Retric 16y ago
    A black hole has a volume less than it's event horizon. So, it's not really undefined as much as ill defined. Also, a black hole's event horizon is not necessarily a perfect sphere ex: two black hole's orbiting each other.
portman 16y ago
Aha, that makes sense. And yes, some quick Googling turns up several papers that talk about the Roche limit for a black hole and a star being well outside the black hole's event horizon. http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=%22roche+limit%22+%22event+horizon%22+%22black+hole%22 http://www.google.com/search?sourceid=chrome&ie=UTF-8&#3...

InclinedPlane 16y ago

There's nothing overly special about an object disappearing across an event horizon, observationally. However, blackholes happen to have extremely severe tidal forces that border the event horizon, and objects passing through this region (even stars) will be torn apart and smashed back together and will be raised to incredible temperatures and release massive amounts of energy. It is this process which is being observed, and it is actually one of the most efficient processes for converting mass to energy in the Universe. See: http://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter http://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter

idlewords 16y ago

Objects would almost never fall straight in to a black hole. In the case where there is relative angular velocity, they spiral in, and then the physics of accretion disks takes over. This involves the release of massive amounts of energy, particularly along radial jets, as the stuff in the disk spirals inwards. The energy release happens well before the infalling matter gets close to the event horizon.