- #1
OSalcido
- 66
- 0
How does a black holes gravity escape its own event horizon? thanks
OSalcido, gravitationally speaking a black hole is not all that different from a star with the same mass. But in case of a star the surface of the star is outside of the event horizon so it is not a black hole.OSalcido said:I mean how are we able to measure the weight of a black hole and why is surrounding material able to feel the black holes gravity if gravity only travels at c and should be too slow to escape its own gravity.
in other words, why is the black holes gravity not limited to its own event horizon
MeJennifer said:OSalcido, gravitationally speaking a black hole is not all that different from a star with the same mass. But in case of a star the surface of the star is outside of the event horizon so it is not a black hole.
When a star collapses to a black hole there is no change in the gravitational field from the perspective of someone far away from it.
Then you should realize that gravity does not need to "escape".OSalcido said:yes, i am aware of this. but I am referring to the event horizon
OSalcido said:Yes I admit I am unclear on that. I thought gravitons had to escape, i understand now about the curvature. I am still not clear on what exactly is traveling at c in regards to gravity though and why this curvature of space should not be felt instantenously?
OSalcido said:How does a black holes gravity escape its own event horizon? thanks
OSalcido said:How does a black holes gravity escape its own event horizon? thanks
Like a bowling ball on a trampoline, if you remove the object, the curvature does not get removed instantly, it propagates at the speed of waves on the medium - in this case, the speed of light.OSalcido said:well I saw this video and it said that if the sun were to be erased from the center of the solar system, Earth would still be orbiting seemingly nothing for 8 minutes because that is how long it takes for the loss of gravity to reach Earth at the speed of light. my question is , if gravity is just curves in space, why the Earth wouldn't just instantly shoot straight off at the deletion of the sun instead of orbiting basically nothing for 8 minutes
I agree with the first part but not the second.cesiumfrog said:Because "gravity" doesn't "travel at c", only changes in gravity travel at c. So if no gravitons can escape to cause those changes, when the star collapses beneath the event horizon, the gravity outside gets left just as it was before.
No it doesn't. It only changes if something else approaches from outside! But don't take my trapped-graviton-no-hair-theorem too seriously, as the question it aimed to alleviate didn't really make sense in GR.DaveC426913 said:But if they couldn't propogate outside a BH then it would never change its mass or gravitational effect once it was created. Yet it does.
OSalcido said:How does a black holes gravity escape its own event horizon? thanks
OSalcido said:well I saw this video and it said that if the sun were to be erased from the center of the solar system, Earth would still be orbiting seemingly nothing for 8 minutes because that is how long it takes for the loss of gravity to reach Earth at the speed of light. my question is , if gravity is just curves in space, why the Earth wouldn't just instantly shoot straight off at the deletion of the sun instead of orbiting basically nothing for 8 minutes
cesiumfrog said:Nicely worded riddle. My answer is:
Because "gravity" doesn't "travel at c", only changes in gravity travel at c.
Brinx said:Consider a non-rotating black hole, with a nice spherically symmetric event horizon.
Brinx said:If one drops some mass 'into' the black hole from a specific direction, what will be the resulting gravitational field of the now slightly bigger black hole?
Brinx said:Before you say 'simply spherically symmetric again, only a bit stronger', let me elaborate.
Brinx said:I'd think that the last position from which the gravity field of the infalling mass is 'updated' for an external observer lies at the event horizon.
Brinx said:That, after all, is the place from which the changes in the gravitational field brought about by the infalling mass can still be propagated outward. After the mass passes the event horizon, its position can no longer be measured, by any means including via gravitational waves. So, then we get a slightly asymmetric gravitational field as a result: the initial spherically symmetric gravitational field of the black hole (centered, presumably, at the center/singularity of the black hole), plus the small addition of the gravitational field of the infalling mass (or rather, the 'fossil field' of the mass from the moment it passes the event horizon), of which the center resides at the point on the event horizon through which the mass fell.
The sum of these fields isn't spherically symmetric anymore, it'd seem.
Does my line of reasoning make sense?
Brinx said:Is the conclusion of a resulting non-spherically-symmetric gravitational field, if valid, problematic in any sense?
cesiumfrog said:- as the mass approaches, the "event horizon" isn't well defined (mathematically, it's a global rather than local concept).
Voltage said:I was reading something last week about off-centre galactic nuclei and quasars that were outside the main body of a galaxy. I'm at work so I can't find it, but maybe this is relevant:
http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0208215
Voltage said:I was also reading about "seat belts" in New Scientist, again I'm not sure if it's relevant:
http://space.newscientist.com/article/dn11955-seat-belts-restrain-galaxies-black-holes.html
Voltage said:On the subject of stellar black holes rather than supermassive black holes, is anybody aware of any apparent puzzles or unexpected results re the mass of black hole candidate objects?
Not always, for instance in closed universes it would not be. Strickly spreaking black holes cannot even exist in such universes.Chris Hillman said:In principle, the event horizon would generally be well defined but it is always defined in "teleological" fashion. That is, knowing the locus of the event horizon requires roughly speaking knowledge of the entire future history.
A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape from it. It is formed when a massive star dies and collapses in on itself.
In a black hole, the gravitational pull is so strong because the mass is concentrated into a very small space. This creates a deep well in space-time, causing objects to be pulled towards the center of the black hole with incredible force.
The event horizon is the point of no return for anything that gets too close to a black hole. It is the boundary where the gravitational pull becomes so strong that not even light can escape from it.
Black holes defy their own event horizon by emitting radiation in the form of Hawking radiation. This radiation is created by the quantum effects near the event horizon and causes the black hole to slowly lose mass over time.
Exploring gravity in black holes helps us understand the fundamental laws of physics and the behavior of matter in extreme conditions. It also provides insight into the origins and evolution of the universe.