Things would look…weird…
This page gives you some ideas of what is looks like around and in a black hole.
Light passing the event horizon but not pointed directly at the singularity bends around the singularity (similar to the gravitational lensing of an ordinary star), right? And in theory it can bend around the singularity and escape back out as far as infinitesimally close to the event horizon, can’t it? (Isn’t that essentially the definition of the event horizon?) It almost seems like the light would spiral down to the singularity. And so an observer passing through the event horizon would see this light.
But like Chronos said, I guess this is don’t-know-don’t-care territory. (At least until we discover a Heechee artifact).
Whack-a-Mole, thanks for the link. I don’t seem to be able to run the videos at work but will try when I get home.
Here’s a picture of what a black hole might look like in space. It displays the lensing you expect.
Again restricting myself to the simplest models of black holes here, since we can’t be certain about anything within the horizon:
Locally, there’s nothing special at all about the event horizon. In a small black hole, you’ll be spaghettified well before you pass through the horizon, but that has nothing to do with the horizon per se. The only way you can tell if you’re past the event horizon is to try to get an infinite distance away from the hole, and if you succeed, well then you weren’t past it. It is, in fact, possible to cross the event horizon of a black hole while having exactly zero evidence (not just “evidence too small to be detected”, but actually zero) that the hole even exists at all, though that would basically require a cosmic conspiracy to pull off.
Once you’re inside the event horizon, space is so strongly distorted (relative to how it is a great distance away) that t becomes a spacelike coordinate, and r becomes a timelike coordinate. You cannot avoid reaching smaller values of r (and thereby getting closer to the singularity) any more than you can avoid the ticking away of seconds, and anything you try to do to fight it will only get you to the center quicker, from your reference frame. Likewise, nothing from a smaller value of r can affect anything at a larger value of r, any more than things happening tomorrow can effect today. So no, you wouldn’t have light swinging around close to the singularity and swooping back up to just inside the horizon, or anything like that.
Were there to exist an extragalactic black hole on a trajectory normal to our galactic plane and on a collision course with our solar system, how close could it get before we notice it?
Thanks,
Rob
Correct.
The last sentence is also correct, but if you are following the beacon into the black hole, you could see it flashing. You’re just seeing the light when you’re at a smaller radius than the beacon was at when it emitted it.
We probably wouldn’t notice it until it started disrupting the planets’ orbits. It probably wouldn’t be massive enough or get close enough to any other stars to affect them noticeably. I don’t think there’s enough material in the interstellar medium for material falling into the black hole to be noticeable.
The black hole could swallow the Earth. Or, more likely, it could eject the Earth into interstellar space. In either of those two cases, pretty much all life on Earth dies (maybe some underground bacteria survive in the latter case). It could leave the Earth orbiting the Sun but in a different orbit. That almost certainly wouldn’t be good for us, but might not wipe out all life. Even a very glancing encounter wouldn’t be good- it might disrupt the orbits of comets. One comet impact can ruin your whole day.
There’s nothing we could do about it if we did know a black hole was headed our way. We don’t know of any other habitable planets, nor do we have the technology to get to any we might find outside the solar system. We can’t deflect it the way we might deflect an asteroid or comet, since all the black holes we know of are several times the mass of the Sun.
I presume we would notice it once it perturbed the orbits of the planets. We probably wouldn’t “see” it. How close it needs to be for a (say) stellar mass black hole to sufficiently mess with orbits I have no idea.
How big are we assuming this rogue black hole is, and how fast is it moving? That’ll make a big difference on how quickly we can notice it.
Black holes have infinite radius. Spherical geometry no longer applies. Black holes are specified by their diameter, because that is the same regardless. This is because inside the black hole spacetime becomes infinitely curved, giving an infinite radius (distance from Event horizon to singularity).
Only the ones between the ears of the politicians in Washington. There aren’t any besides in the Capitol Building and the White House. Some speculate that there may be 9 more in the Supreme Court building, but that could just be rumor.
Maybe, depending on the nature of gravity.
True, all theories are subject to revision by new data.
Someone earlier on this thread said that orbiting a black hole is like orbiting any point gravity source, but that’s not entirely true. When you start to get within a few event horizon diameters of the event horizon, orbital mechanics start to get weird. You can shoot a laser away from you and still hit yourself with it in certain orbits, and funky effects like that.
Right. What makes black holes do such weird things isn’t that they’re really massive, but that they’re really small. If the sun were replaced by a black hole of equal mass, as long as you’re outside the radius of where the sun used to be, things will behave just like Newton says they should. It’s once you start getting in closer, into what used to be space occupied by the sun, that things would get weird.
Once you cross the event horizon, you’ll reach the singularity in finite time, so the radius certainly can’t be infinite. In any event, “radius”, in a context of black holes, is defined in such a way that the surface area of a sphere centered at the origin (like, say, the event horizon) is 4pir^2.