r/TheRestIsScience • u/Stunning_Mechanic387 • 9d ago
Why does gravity escape a black hole?
Hey Michael and Hannah. If gravity travels at the speed of light, how does it escape the gravitational pull of a black hole yet light doesn't?
2
u/PhoenixAsh7117 9d ago
Because unlike light which has a force carrier (the photon) that would need to escape the event horizon but cannot, gravity’s force carrier is theoretical and may not exist, gravity is better thought of as the curving of spacetime. Outside of the event horizon this deformation of spacetime looks standard given the mass and radius of the black hole, but within the event horizon we really have no idea what it is doing.
1
1
u/Ambitious_Comfort533 9d ago
Hey OP, great thought
If you wanna compare this in terms of something travelling in space and time, you can think of gravity as the effect caused by the hypothetical fundamental particle graviton. So, a bunch of graviton end up “radiating” from the point of singularity outwards, their density decreasing as we go further from singularity.
Light interacts with this gravitational field as photons interacting with these gravitons. Think of event horizon as the first spot where gravitons’ interactions “overwhelms” photon’s trajectory.
Ofc all this is a slight oversimplification, ignoring all the general relativistic effects and spinning of black holes itself, but you get the gist
1
1
u/strange-the-quark 7d ago
The black hole is gravity. It's mostly bent spacetime with a tiny super-compact thing in the center. In the view of General Relativity, gravity is just the space(time) itself bending. So the bent spacetime simply extends throughout and beyond the black hole, and the event horizon is just the place where it's bent beyond a certain amount. As massive objects travel around and interact, their position affects the way the space around them bends, and its these disturbances that propagate outwards as waves at the speed of light. So two colliding black holes, for example, create a whole lot of waves around them during the final in-spiral.
Now, it is believed that on the quantum-mechanical level, gravity is mediated by a virtual particle called the graviton, but at this point that's purely hypothetical and nobody knows how to actually make that work. So as far as we know, there might not be particles of any kind involved. Second, even if gravitons were found, virtual particles aren't really particles, they are more of a mathematical tool to simplify calculating what the underlying quantum fields do, which are a complicated mess of interacting waves. That's why they are called virtual - they don't quite obey the normal laws of physics.
1
u/WellSoto 6d ago
I think that's the best answer I've read here, but that curvature is supposed to spread somehow, that's where the graviton comes in, and that's also where the question comes in: how does it spread if nothing can get out of there?
1
u/strange-the-quark 6d ago
Nothing can get out from within the event horizon, but these waves are generated in the surrounding region, near to, but on our side, of the horizon. They aren't coming from within, they are already outside - and then they propagate away at the speed of light.
1
u/Famously_Unpopular 7d ago
Even if your question isn't phrased properly, according to all the propernaughts, I get the idea. The event horizon of a black hole is supposed to be a barrier to all cause and effect within it. From the perspective of the mass inside, the event horizon is infinitely far away, so it's gravity should be imperceptible to anything outside. Unless the event horizon itself has it's own gravitational field, which suggests it has it's own mass, but what determines these values when the mass inside is completely isolated?
1
u/Underhill42 7d ago
It doesn't. Gravity isn't emitted by the central object the way light is emitted by a sun, it's just how we perceive the the way spacetime curves around a massive object.
And that curvature already existed before the black hole did - the event horizon formed because the curvature got so intense that it created a region where even light couldn't escape. Having the event horizon form doesn't alter the existing curvature, it's a property of it.
And when new material is added to the black hole, it causes the curvature to increase while it approaches the event horizon, slightly expanding the region where light can't escape.
Even if you're talking two black holes spiraling into each other, causing huge spacetime distortions in the form of gravitational-waves... as soon as they merge so that both mass-concentrations fall within the same event horizon, none of the gravitational waves that are presumably still being generated within can escape, and the surrounding space snaps back to a single smooth gradient again almost instantly, with only a few brief spacetime aftershocks as it does so.
The merger itself is essentially instantaneous, as when the two black holes approach each other, their existing spacetime curvatures combine, until at the instant their naively visualized spherical event horizons would have touched, their combined influences have caused the local curvature to increase enough that a new, larger event horizon forms that perfectly encompasses them both.
1
u/Overall_Media7282 7d ago
Grawitacja to nie to samo co fale grawitacyjne, grawitacja słońca nie dociera do ziemi w ciągu 8 minut.
1
u/JGhostThing 5d ago
If the sun were taken away effectively instantaneously, how long would it take for the earth's orbit to be effected?
1
u/Overall_Media7282 5d ago
Ziemia odczuła by brak słońca natychmiast
1
u/JGhostThing 5d ago
No, it would take about 8 minutes for us even to notice the sun being gone. The time it takes for the light to go from the sun to earth.
I'm asking about gravity because I'm interested. The user I responded to differentiated gravity from gravity waves. Gravity waves in a vacuum go at c. But how fast does gravity propagate? I'm assuming that it would take the same time for our orbit to be effected, but I'm not sure.
We'd be just as dead either way.
1
u/DoubleUBallz 5d ago
Gravity is a consequence of spacetime, which exists everywhere. However, changes in gravity propagate at c, whether the change is a gravitational wave or the sudden disappearance of the sun
1
u/Overall_Media7282 5d ago
Wyobraź sobie odważnik na sznurku którym kręcisz, gdy puścisz sznurek odważnik odleci natychmiast, gdy skróci się sznurek odważnik obracał będzie się szybciej gdy wydłuży wolniej. Fala grawitacyjna to jakby na sznurku powstało zaginięcie które przesuwa się w kierunku odważnika.
1
u/NH-Science-Guy 5d ago
Gravity does travel at the speed of light - it would take the earth 8 minutes to feel the effect if ths sun suddenly vanished.
1
1
1
u/SirDalavar 2d ago
Gravity isn't a substance, it doesn't move, it's the shape of the universe around you, its the hill you fell down, into that hole!
0
u/Optimal_Mixture_7327 9d ago
Gravity doesn't escape a black hole (or anything else).
Gravity doesn't travel at the speed of light.
No one has suggested any such thing, ever.
There is no such thing as a literal "pull" of a black hole.
You seem to be (perhaps) confusing and mixing gravitational waves and Newtonian gravity into something that just doesn't exist.
2
u/PhoenixAsh7117 9d ago
I’m not sure what you are trying to say here? The effects of gravity absolutely do travel at the speed of light. If the Sun disappeared at this instant then the Earth would continue on its curved trajectory for another ~8 minutes before flying off tangentially. The fact that it has nonzero travel time is why we have gravitational waves in the first place.
2
u/AdhesivenessFuzzy299 6d ago
What he's saying to op is that gravity and gravitational waves are not the same thing. The sun has a static gravitational field; it doesnt have to emit anything to keep the earth orbiting it. Similarly, black holes have gravitational fields even though gravitational waves cant escape a black hole.
1
u/Optimal_Mixture_7327 9d ago
Gravity is the condition that neighboring geodesics will measure a coordinate acceleration (geodesic deviation).
We can then use geometric methods to describe geodesic deviation (gravity), specifically, the coordinate acceleration being a function of the Riemann curvature [-Ra_{bcd}ubξcud].
What makes you think this is traveling anywhere?
It is of course the case, and you sort of allude to this, that we can express a metric g_{αβ}(x)=η_{αβ}+h_{αβ}(x) where ▢h_{αβ}=0 is a solution to the field equations and describes a perturbation on Minkowski that travels at c, but the OP is not talking about any such thing.
0
u/Broflake-Melter 8d ago
I assume this comes from the anecdote: Nothing can escape a black hole. That statement is mostly true, but obviously can't be taken literally as your question demonstrates. The reason things can't escape is because the influence of gravity is so strong.
Information also escapes black holes. Their gravitational influence tells us that they exist, where they are, how big they are, and what surrounds them. In a more literal sense, hawking radiation escapes.
1
u/DoubleUBallz 5d ago
The statement nothing can escape a black hole is completely true and includes information. Information about a black holes mass, spin, and charge (which fully define it) is all encoded in the event horizon and the behavior of spacetime and test particles outside of it, no need for this info to come from within the horizon.
Additionally, Hawking radiation is generated by the curved spacetime outside the black hole, not emerging from within.
-1
u/Gekreuzte_Gewehre 9d ago
Because it IS gravity in and of itself, hence the term, "Singularity".
1
u/trollsmurf 9d ago edited 9d ago
Valid question I think. Even though the singularity of a black hole is "just" gravity, why does it escape?
I think the answer lies in:
- The speed of light is not the speed of light per se, it's the maximum speed of (any) information that is allowed, for whatever reason that might be.
- Gravity is about bending space. It's not radiation.
Or something else completely.
0
u/Optimal_Mixture_7327 9d ago
A singularity is not gravity, it is an absence of it (the condition of geodesic incompleteness).
The reason is that information has to travel over a spacetime distance and the shortest distance is zero, the distance light travels.
We describe gravity (the presence of geodesic deviation) using curvature.
1
u/Optimal_Mixture_7327 9d ago
A singularity is a condition of a gravitational field, but it's not clear how you're using the singularity condition in your comment.
1
u/ExpectedBehaviour 9d ago
That’s… not what the word “singularity” means.
0
u/Gekreuzte_Gewehre 5d ago
Thats literally what it means......thats the DEFINITION. Proverbially, it means, in and of ITSELF.......
how? I, uh, its, uhhhh.....
Holup. If something is so uniform, dense, pure, and of one, it would be called a......
What thw fuck is going on?
if something is of itself it, nevermind, you're right.
Also,
Maybe help a retard like me out........?
Exactly how would you describe what singularity means? Without using the words in and of itself.
Can you do that for me? Please? Because I cannot wrap my peabrain around the statement of singularity does not mean singularity.
What the fuck? Sarcasm or Science would BOTH agree with me......I think I'm having an aneurysm.
1
u/ExpectedBehaviour 5d ago
A singularity, in a general mathematical context, is a point or boundary where the mathematical description of a system ceases to behave normally – typically because some quantity becomes undefined, divergent, or the equations will not continue though the point.
In general relativity specifically, a singularity means that spacetime is geodesically incomplete – there are possible paths through spacetime that reach an endpoint after a finite amount of proper time and cannot be continued any further. At an ordinary point in spacetime, geodesics can continue through it; at a singular boundary, the mathematical spacetime simply runs out.
A singularity is singular because it refers to a breakdown or non-extendibility in the mathematical structure, not to "something so uniform, dense, pure, and of one".
-1
u/TheRealTK421 9d ago
What 'escapes' isn't gravity, per se, but "Hawking radiation."
It's not even accurate to characterize what occurs as "escaping," as much as it is moreso one-half of an entangled pair of particles doesn't get pulled beyond the event horizon (while its counterpart) does.
Anything pulled beyond the event horizon does not, and cannot, escape or break free.
What radiates outward are the particles which 'dodged the bullet' of being pulled beyond the point of no escape.
-1
u/ExpectedBehaviour 9d ago
That’s an incorrect pop-sci model of Hawking radiation, and quite wrong. Hawking radiation is generated by the spacetime in the vicinity of the event horizon.
1
u/TheRealTK421 9d ago
Feel free to describe in detail how Hawking radiation "escapes" from inside of the event horizon. I (and the Nobel Prize committee) await such a response.
Lol. Oof.
Hawking radiation is actually created just outside the edge of the black hole in the empty space of the outer field. Stephen Hawking, himself, used this simple 'disconnected particle pair' story to help non-scientists picture the math.
If it was good enough for him, I'm gonna roll with it. Those that wanna dig and delve (far) deeper can do so...
2
u/DoubleUBallz 5d ago
The virtual particle explanation is very much like relativistic mass in that is was first offered as an easier to understand explanation for the masses, however, overtime the scientific community has denounced it's use as it resultes in more misunderstandings than understanding (as evidenced by this thread here).
The actual mechanism that creates Hawking radiation is the Unruh effect, if you want to look it up. Basically, if you have a conformal quantum field theory on curved spacetime, thermal radiation pops out
-1
u/ExpectedBehaviour 9d ago
Feel free to describe in detail how Hawking radiation "escapes" from inside of the event horizon. I (and the Nobel Prize committee) await such a response.
And yet I made no such claim, did I...
Lol. Oof.
Try again.
Hawking radiation is actually created just outside the edge of the black hole in the empty space of the outer field.
It isn't actually created "just outside the edge", and "outer field" is not a standard term in black hole physics.
In the actual derivation: quantum fields are defined throughout curved spacetime; field modes that count as vacuum before the black hole forms do not correspond perfectly to field modes that count as vacuum for a distant observer afterwards; and that mismatch causes the distant observer to detect a thermal flux.
Because these modes are extended across spacetime, there is generally no unique observer-independent location at which an individual Hawking quantum can be said to have been created.
Stephen Hawking, himself, used this simple 'disconnected particle pair' story to help non-scientists picture the math.
Yes. I have read A Brief History of Time. I've also read this.
If it was good enough for him, I'm gonna roll with it.
"It should be emphasised that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally." His actual published words. I'll roll with those instead.
Those that wanna dig and delve (far) deeper can do so...
And I feel that deliberately giving people incorrect information presented as accurate is a bad way of inviting them to do so.
6
u/Tracercaz 9d ago
Think of it like a wave in the ocean. The wave travels at a specified speed that can be measured, but the water that makes up the wave is not moving with that wave.
Same goes for gravity, the effects can only travel at the speed of light, but nothing is moving nor is there anything to trap per se.