# Can the mass within the event horizon of a black hole interact gravitationally with the mass outside the event horizon?

If so, gravitons and their fields, unlike photons, must be able to cross the event horizon freely in both directions. If not, the observed mass of a black hole must depend only on the particles orbiting outside the event horizon. The environment inside the event horizon must be massless, in the gravitational sense.

• From an outside observer's point of view no mass ever crosses the event horizon, so there is no problem. The event horizon is observer dependent, and it will retreat in front of a freely falling observer, so that's not a problem, either. Mar 15 '16 at 3:34
• Essentially a duplicate of physics.stackexchange.com/q/937/2451 and links therein. Mar 15 '16 at 7:06
• @CuriousOne: The event horizon is not observer dependent.
– MBN
Mar 15 '16 at 13:09
• @CuriousOne: The question is about the event horizon, not about apparent horizons.
– MBN
Mar 16 '16 at 6:48
• @CuriousOne: The event horizon is a geometric property of the space-time and as such is absolute. And there is nothing relative to it. The definition is the boundary of the causal past of future null infinity. It is this horizon that the question is about. So, making a comment about other types of horizons is only going to confuse the poster.
– MBN
Mar 16 '16 at 11:15

Gravity couples to anything with a non-zero stress-energy tensor, as $G_{\mu\nu} = 8\pi T_{\mu\nu}$. So, if we imagine some universe with black holes with matter within their event horizons, they will contribute to the overall stress-energy tensor and thus must interact with gravity.

• How did the matter get inside the event horizon (from the point of view of a distant observer)? Mar 28 '16 at 8:08

Can the mass within the event horizon of a black hole interact gravitationally with the mass outside the event horizon?

From the subsequent propositions it seems you are talking about quantization of gravity. In a quantum field theory, interaction is represented by exchange of virtual particles. These are mathematical constructs, not on mass shell .

If so, gravitons and their fields, unlike photons, must be able to cross the event horizon freely in both directions.

Assuming that gravity is quantized and gravitons exist, i.e. zero mass spin two bosons, they will behave the same way as photons, i.e. they will be trapped within the horizon, on shell gravitons cannot cross it.

If not, the observed mass of a black hole must depend only on the particles orbiting outside the event horizon.

In my opinion this is a non sequitur. The recent measurement of black holes merging gave masses to the two black holes and to the merged one using general relativity equations. The mass seen by an external observer follows the rule of all masses and at a limit behaves in a newtonian fashion. The real answer of how virtual gravitons build a gravitational field, similar to virtual photons building an electric field, needs a quantization of gravity which is still a research project.

The environment inside the event horizon must be massless, in the gravitational sense.

This in my opinion is also a nonsequitur conclusion. The question needs a definitive quantization of gravity model. String theories do have quantization of gravity and there are studies within that format for the mathematically inclined.

I would say yes it interacts gravitationally with all mass in the universe. I don't agree that something like gravitons reach out and grabs things to pull them in but there is obviously a gravity well outside the event horizon. Whos to say there is no mass in a black hole? Whats wrong with the idea that that's all it is? Just a mass so large (and growing) that it's acceleration do to gravity will not allow light to escape beyond a certain point.

The mass inside doesn't affect anything outside.

But the infalling matter and energy that make up the black hole curved the spacetime outside as they passed through.

This happens even to a regular star. When the gas that formed our sun collapsed to form our sun, it curved the spacetime in the new spacetime outside of itself as the matter and energy moved inwards.

The curvature out here by Earth was caused when the gas collapsing to form our sun was the size of an Earth orbit.

The curvature out by Mars formed earlier, it formed back when the collapsing gas was the size of a Mars orbit.

The curvature in by Venus formed later, it formed back when the collapsing gas was the size of a Venus orbit.

The idea that mass sends gravitons out at the speed of light towards other objects is completely wrong for general relativity. The curvature in a region simply evolves based on the curvature nearby, and matter and energy (and momentum and their fluxes) simply makes curvature evolve differently than it does in a vacuum.

Which is what happened as the gas came past where we are now, it turned the curvature into a stronger curvature, but one that is fully capable of causing itself right there without any further intervention.

This is a tricky topic.

From my understanding of relativity,

• Nominally, apart from the whole there wouldn't be anything to interact with in the first place, since mass doesn't seem to exist within a black hole. What?! Okay, to clarify, I can't speak of the nature at the point of the singularity itself. What I am referring to are ambient levels of matter between the horizon and the center, assuming the black hole had been surrounded by a perfect vacuum--nothing nearby from the cosmos to suck in (except maybe related to Hawking radiation: virtual particles that appear out of nothing, whether you like it or not).
• Speaking of "gravitons" as if they are known particles leads to misconceptions. From what I've read they are only speculative, unlike photons and other bosons, and are meant to serve rather as analogies in language, helping us understand something more tangible in context, like gravity field transfer or gravitational wave propagating through spacetime (like the 2015-09 detected compression/expansion wave in the news lately).

For the sake of argument, let's say that sure, for a brief period of time, there does exist some fresh in-falling matter just beneath the event horizon, before it gets completely converted into the curvature(spacetime distortion) and sucked into the dimensionless center. Then I'd be inclined to say that yes--isolated from the BH itself, it absolutely would "interact gravitationally" with matter just outside the horizon, in proper time.

I can't readily offer research to support this directly, but otherwise we'd have to believe eg. that these spacetime compression waves don't ordinarily pass right through even black holes at c speed unaffected. Part of the argument ties in to how theoretically, with the right technology in the distant future, these gravitational wave pulses could be used to "see"/infer into a time even before the universe was opaque to photons (about 380k years after the BB?), a prospect which had many physicists excited, because it'd offer a way to get information older than what electromagnetism could yield. In those days, certainly environments existed with extreme distortion in our space fabric.

That new way of "seeing" into the more distant past wouldn't seem possible if your "gravitons" were impeded by the extreme spacetime curvatures typical of black holes.

To further support my YES answer**, a compelling counter-example occurred to me, to also show that "gravitational interaction" is likely for two small bodies of mass, on opposing sides very near an event horizon.

Thought experiment: Take a supermassive black hole, such as at the center of a galaxy. Typically, its average density can be less than that of water, and should your space ship get irreversibly sucked in, it could take hours (or even days?) before you'd begin to feel the tidal forces that rip you apart.

Now convince me that your massive ship of 200 megatons, in the first seconds after crossing the point of no return, isn't experiencing an infinitesimal attraction due to gravity of my massive ship too, just a few meters away, but on the other side of the horizon.

• **It's safe to argue that (B)ob's mass just inside is lightly pulled by (A)lice's just outside the border. But since A never actually detected/saw B fall in and cross the horizon in finite time, it's difficult to avoid paradoxes arguing the vice versa, i.e. that "symmetrically", Alice is attracted to Bob. Mar 16 '16 at 17:45
• Your thought experiment is flawed. The event horizon is defined as the boundary beyond which no signal or influence can emerge. In the situation you describe, neither ship would think that there was an event horizon between them. Mar 26 '16 at 14:04
• @RobJeffries Seems you misunderstand, and haven't been able to substantiate how my scenario is flawed. Merely stating that "no influence can emerge" is either too vague to serve in scientific discussion, or outright mistaken. Nothing prevents objects from being so close to each other, at least for a finite time from (B)ob's reference inside, and we both seem to agree that the EH boundary is well-established in space, objectively, regardless of where one stands, yet regardless of how each side "thinks" of it, as you phrased. Mar 31 '16 at 12:35
• Advise you read up on en.wikipedia.org/wiki/Supermassive_black_hole, specifically, how the realities at the EH of a 17 light-hours wide BH would be significantly different than at one from a collapsed star. Anyhow, aside from this (valid & arguable) thought experiment, it is just icing to my main answer. Mar 31 '16 at 12:37