Black holes do you know them as well as you think?

[senko]

I was doing a little light reading over breakfast and got to thinking about the supermassive black holes at the centre of the galaxy so I decided to read up on them. Now this is far from a proper scientific understanding but I found out a few facts regarding black holes I didn't know and never suspected so I figured I'd share.

1) Black holes aren't stable they are constantly changing size and can become bigger or smaller. As they absorb material from the disc surrounding them the extra mass actually increases the size of the black hole. More on this in point 2. Similarly they are constanty losing a small amount of energy. Its not much and the average sized black hole would need 10 to the power of 69 million years to pass before they actually vanished but one formed with the mass of say the eifel tower would dissapear in a day. Roughly speaking there's a hole lot of complex talk about Hawking-Zel'dovich radiation and umah and trans-planking. All very handy for the setting if you look it up but a bit over my head. The basic point being the smaller the black hole the more energy it radiates and the faster it dissipates.

2) Moving on normal black holes have an estimated upper limit of 50 billion times the  mass of our son where their expansion would halt. The reason being the bigger they are the more their gravity distorts the disc surrounding them causing suns to form which are better at resisting being pulled in. At this  mass it'd reach a sort of equilibrium where it couldn't pull more in because of its own effects.

3) Of course this only applies to normal black hole growth they can still collide and merge. Yep that's right when two black holes meet they merge together with a massive release of gravity distorts forming an even bigger black hole a bit less than their combined mass as some is lost in the conversion.

4) Finally black holes can MOVE if the circumstances are right the black hole is no longer a stationary object but can be busy barreling through the galaxy. Scientists have actually detected one in a neighbouring galaxy that is trotting along at a respectable 5 million miles an hour.

Fun thoughts there and so many potential plot hooks. What happens to cultures growing up under a steadily expanding black hole or in the path of a moving one? What if its dragging its own disc and stars along with it? Entire cultures riding the gravity waves as an all consuming black hole drags them along. Time lords fighting with micro black holes to compress entire star systems to a singularity and remove them from existence. Ancient Primordial black holes travelling through the depths of space as a relic of Omega's attempts to create a stable power source and even time lords don't get it right first time.

[Eryx]

Very interesting. Point 2 made me wonder whether that's where all our stars originally came from?

[Stormcrow]

Jun 11, 2017 7:38:40 GMT senko said:

2) Moving on normal black holes have an estimated upper limit of 50 billion times the  mass of our son where their expansion would halt. The reason being the bigger they are the more their gravity distorts the disc surrounding them causing suns to form which are better at resisting being pulled in. At this  mass it'd reach a sort of equilibrium where it couldn't pull more in because of its own effects.

3) Of course this only applies to normal black hole growth they can still collide and merge. Yep that's right when two black holes meet they merge together with a massive release of gravity distorts forming an even bigger black hole a bit less than their combined mass as some is lost in the conversion.

More specifically, 2) only applies to the creation of black holes from collapsing stars. A black hole is free to get as big as it wants just from pulling in matter and energy, including other black holes.

4) Finally black holes can MOVE if the circumstances are right the black hole is no longer a stationary object but can be busy barreling through the galaxy. Scientists have actually detected one in a neighbouring galaxy that is trotting along at a respectable 5 million miles an hour.

There is no absolute reference frame in the universe, so EVERY black hole is moving. Maybe you were reading about a black hole that was moving THROUGH other objects. Most black holes will be orbiting something else, just as its original star was.

Fun thoughts there and so many potential plot hooks. What happens to cultures growing up under a steadily expanding black hole or in the path of a moving one? What if its dragging its own disc and stars along with it? Entire cultures riding the gravity waves as an all consuming black hole drags them along. Time lords fighting with micro black holes to compress entire star systems to a singularity and remove them from existence. Ancient Primordial black holes travelling through the depths of space as a relic of Omega's attempts to create a stable power source and even time lords don't get it right first time.

From the distance of planetary orbits, a black hole is no different, gravitationally, than the star it forms from. If the sun were to collapse into a black hole (which it won't), the Earth would continue to orbit it. The whole strong-gravity-pulls-you-in only applies when you get very close. The big problem would be the lack of light that the sun emits.

Most science-fiction, and Doctor Who in particular, does a lot of hand-waving regarding black holes. The latest episode got the time dilation surprisingly right, but I suspect that much of a time gradient in a single ship would mean that everyone aboard would have been crushed by the gravity long ago, and the ship is really, really close to the event horizon. Ships don't usually crash into stars; how did this one fail to notice a black hole? Other episodes have done very silly things with black holes. To get started with any of the story ideas you proposed you'd probably have to ignore a lot of physics and just make the black hole the "evil object that'll suck you up."

[senko]

I got it from here www.nasa.gov/feature/goddard/2017/gravitational-wave-kicks-monster-black-hole-out-of-galactic-core which has a nice little bit of explanation about how two galaxy's may have collided and theorises that the two supermassive black holes at their center merged then started moving away from the galactic core. I may have gotten the wrong end of the stick, not a physicist but it does sound in their video as if the quasar is definately moving under its own power not just the normal galactic movement.

[Stormcrow]

That's not "under its own power," that's just the momentum of the combined black holes going in a different direction than the rest of the galaxies surrounding them. The two black holes are the massive parts of their galaxies, and when they hit each other they go careening off in a new direction, and their orbiting stars get yanked along by gravity, or may break away.

Quasars are black holes at the centers of galaxies that are surrounded by incredibly bright accretion disks that emit a lot of electromagnetic radiation. A quasar could undergo the same shift in momentum as any other supermassive black hole if it were to collide and merge with another supermassive black hole.

[senko]

So how is that different to say a rogue planetoid travelling through space?

[Stormcrow]

The main difference is gravity. Two colliding black holes will merge into each other and have even stronger gravity, while two planetoids colliding will probably break one or both apart and scatter their debris, eventually dissipating their mass and gravity. Rogue planetoids won't have stars or accretion disks orbiting them either.

[senko]

Hmmmm I have to admit I don't really understand the difference to me the black hole is moving if it started at point A and is now at point B even if its technically not.

[Stormcrow]

Every object in the universe is moving relative to other objects. There's no such thing as "not moving." That's called Relativity.

[senko]

Oh that part I get its just when your "average" black hole is always in the same place relative to the rest of the galaxy and this one is moving around it I call that moving just like I call the earth moving in regard to its rotation around the sun.

[Stormcrow]

Oh, sure, if one black hole is much more massive than another, one can orbit the other. But mechanically speaking, the sun does NOT remain stationary at the center of Earth's orbit. The Earth does exert a pull on the sun, though a small one, which causes it to wobble a bit. In a sense, the Earth and the sun orbit each other. The other planets also do this to the sun. To put it more accurately, every body in the solar system, including the sun, orbits around the center of mass of the solar system. Because the sun is so massive, that center of mass is reeeeeeally close to the sun, so it looks like it's stationary and everything else is moving around it.

That's the case where two black holes are orbiting each other and one is much more massive than the other. There can also be the case where two black holes are not orbiting each other. Either, one, or both could be at the center of a galaxy. In all cases, the mechanics are the same.