One thing though is that our universe (presumably) has black holes in it. So for the universe to be a black hole, it would have to be possible for black holes to (however briefly) have black holes inside them. What happens when a small black hole falls into a bigger one? Do they coalesce like rain drops as soon as their event horizons touch? Or is the singularity of the smaller stripped of it’s event horizon while falling toward the other singularity; or do you have a double-event horizon, or what?
Well, what happens when black holes merge, according to NASA, is that ‘all of space jiggles like a bowl of Jell-O as gravitational waves race out from the collision at light speed’.
Generally, though, I wouldn’t think that there’s a problem with one black hole being inside the horizon of another one – if you cross the horizon of a large enough black hole, you won’t really notice anything (except some severe difficulties trying to make it back home for dinner), physics will work essentially the same way, it’s just that all of your possible trajectories eventually intersect the singularity; the same goes for an infalling small black hole.
It’s an interesting question, though. From the outside, all black holes look the same (except for the radiation). When the event horizons touch, everything’s pretty normal. When the actual singularity inside the ‘smaller’ black hole crosses the other black hole’s event horizon, though, shouldn’t the smaller black hole become undetectable? After all, it’s gotten to the point where nothing can escape from the larger black hole. The smaller black hole’s gravity well can’t just instantly vanish, though, or even instantly transfer its center of mass to the other black hole’s center of mass.
How is that reconciled? Or am I asking a question based on faulty premises?
Let’s try some (painful) formatting (data copied from joshreeves)
Schwarzschild radius:
r = 2 * G * m / c[sup]2[/sup]
r is Schwarzschild radius
6.67428 * 10[sup]-11[/sup] * m[sup]3[/sup]
G is gravitational constant = ---------------------
kg * s[sup]2[/sup]
m is mass
c is speed of light = 299,792,458 m / s
Simplified:
2 * G 2.95 km
------ = ----------
c[sup]2[/sup] solar_mass
m * 2.95 km
r = -----------
solar_mass
Mass of Milky Way galaxy:
m = 5.8 * 10[sup]11[/sup] * solar_mass
Schwarzschild radius of Milky Way galaxy:
r = 5.8 * solar_mass * 10[sup]11[/sup] * 2.95 * km / solar_mass
r = 5.8 * 10[sup]11[/sup] * 2.95 * km
r = 1.711 * 10[sup]12[/sup] km = 0.123788012 * light_year
That should be more legible. Did I get that right?
I should point out there appear to be two uses of m in that. One is for mass, but also in the speed of light it is used for meters. So there may be some confusion in the actual math. Haven’t checked.
Well “instantly” has rather two different meanings depending on whether you’re watching the black holes collide from a safe distance, or whether you’re falling into the bigger black hole along with the smaller one. For an observer falling into a black hole, it takes a finite amount of time to reach the singularity (which IIRC for a galactic-mass black hole would be “several hours”).
In practice, from a distant external viewpoint, what happens when two black holes collide is that as soon as the horizons touch, you have a single black hole with a distorted shape, which rings like a bell and quickly radiates away all of the distortion to settle down into a single smooth hole. What goes on in this time inside of the horizon(s) is a much more difficult question to answer, but it’s at least somewhat mooted by the fact that we can’t ever observe anything inside of a horizon.
Well, yeah, but consider this. From our ‘viewpoint’ outside of a given black hole nothing ever reaches the singularity. So consider Big Black Hole BBH and small black hole SBH, that are approaching each other.
From our vantage point BBH is to the left of SBH, and SBH is therefore to the right of BBH.
Also consider that we saw an apple cross beyond the event horizon of SBH from the far right.
Now the event horizons touch and there is an unobservable - in every sense *unobservable *- oval shape. At some point, the singularity that makes SBH crosses the event horizon of BBH - it is doomed, and nothing can escape from BBH that is observable about SBH - no light, obviously, but nothing else that should be detectable - for instance, gravity as distinct from BBH’s total gravity.
SBH’s event horizon should no longer be detectable - if it was, it would violate what it means to be *unobservable *within BBH’s event horizon. No gravity well should be observable around any mass that is inside BBH’s event horizon.
The apple has not yet crossed into BBH’s event horizon, but is within the space that used to be SBH’s event horizon.
Can we see it?
The result that an external observer will never observe an object to cross the event horizon depends upon the hole itself being static. Two black holes merging is about as un-static an event as you can get.
Well, I thought I’d at least ask before I told her that her apple was gone.
I don’t think it’s correct that SBH’s event horizon vanishes when its singularity crosses into the BBH’s vent horizon. The combined black hole has the distorted horizon Chronos mentions, and the apple is inside it, so it can’t be observed.
A more interesting question, I think, is whether a SBH’s EH can graze a BBH’s EH, then separate again (with neither’s singularity crossing the other’ EH). I suspect not.
If the horizons touch, they have merged. For non-rotating black holes, you can prove this just from the fact that the Schwartzschild radius is proportional to the mass: When the two holes touch, the system consisting of both holes is completely enclosed within a sphere whose radius is the Schwartzschild radius corresponding to the mass of the whole system, therefore the whole system is a black hole.
This will also happen if the holes are rotating, but it’s harder to prove in that case.
The Schwartzschild is an isolated solution, found assuming negligible mass near the horizon (or at least assuming a spherically symmetric mass distribution). I would expect that the BBH horizon would bulge out on the side near the SBH, so that the two horizons would merge when the two singularities were farther apart than the sum of their Schwartzschild radii.
I get that now. I was basing it upon the false belief that nothing inside a black hole’s event horizon can be observed in any way (therefore the SBH singularity, inside the BBH black hole’s event horizon, should no longer be detectable - even by inference from its own event horizon). That’s only true of a static black hole (which I guess doesn’t exist except in theoretical form - an outsider’s calculations would predict that nothing that falls inside is ever done falling - do I have that part right?).
I think you run into a speed-of-light speed limit problem there. Before the instant they touch, you have two separate black holes, each with its own gravity field. If they are roughly the same size, it’s easy to see that there is even a point between them where the net effect of gravity is zero, and the other points on the plane that bisects them (you know what I mean) that have net effects of gravity nothing close to inescapable.
Then at the instant they touch, every point inside the Schwarzchild radius implied by their combined mass is doomed? There *has *to be some time before that happens.
bup said:
bup, I’m no expert, but I think you have a misconception of how black holes and event horizons work. You seem to be treating black holes as if the event horizon is a big shadow projected around the singularity, such that it is a visual limit only, and has no other effects, sort of like a projected shadow outward from the center. You are trying to speculate on two black hole shadows passing through each other without the singularities touching, for instance. Or the apple sitting on the “far side” of the shadow of one black hole and that singularity going into the shadow of the other black hole, but not yet being fully sucked in (so the shadow zone is not yet fully enclosed).
I don’t think it works that way. The event horizon is a gravity shadow, not just a light shadow, and is effectively a solid object as far as outside observers are concerned. When the two shadows come near, they merge into one event horizon, even if the original singularities have not yet gotten “within each others’ event horizon”.
Think of it more like soap bubbles. When two soap bubbles touch, they don’t stay two separate circles just barely touching (like 8), but rather they clump together and try to enclose the smallest volume they can, merging into one circle. Whatever is inside the bubbles gets shifted around as necessary. I think black holes are similar.
The apple is already inside the event horizon of the smaller black hole. When the two black holes touch, it stays inside that event horizon, the event horizon just lumps and merges with the other black hole.
Oh, and Chronos, you maybe never saw this post, but I am still interested in what specific sort of mental things you pondered to help you visualize a fourth spatial dimension.
If the “Big Bang” were factual and all matter is expanding from that initial singularity and constanly moving outward, and gravity being a factor, all matter must eventually reverse course and be drawn back together essentially reversing the big bang. Back to a singularity. If the mechanics of this process are constant then black holes will eventually reach a supercritical mass and reverse action by expanding outward, either individually creating a number of medium or small “big bangs”. Unless of course all of the black holes eventually coincide and re-create a new “big bang”.
No. Nowadays, we know that the expansion actually seems to be accelerating (thus the search for “dark energy” as an explanation), but even before that, the possibility of a future “big crunch” was seen as undecided for want of sufficient knowledge.
I may be repeating someones response and excuse me if I am. This is my first time here.
A black hole has infinite density and, therefore infinite gravity.
Gravity that captures light wavicles and warps the time/space continuium around it in the ‘event threshold’. That’s serious stuff.
We’re talking about ‘things’ that don’t exist in this universe, holes in reality that aren’t here.
No, black holes have finite mass, and therefore, finite gravity. Just think about the star that collapses to create the black hole – it can’t have any more mass than went into it in that process. Hence, if we replaced the sun instantaneously with a black hole, gravity-wise, nothing would change, and the Earth would continue on along its merry way undisturbed (a bit colder, perhaps).
And while it’s true that the density of the singularity purportedly at the center of a black hole goes to infinity as the volume goes to zero, it’s thought that this is merely an artefact of describing the process using a theory not applicable in these extreme regimes, so one does not expect this to model physical reality precisely. Generally, what’s meant by ‘density’ when talking about a black hole is its mass divided by its Schwarzschild volume, or the volume circumscribed by its event horizon, which is very high for small black holes, but can get extremely low for the supersized ones.