Wandering Black Holes

[Dominatus]

I had been watching How the Universe Works S1 E2, Black Holes. Wandering black holes were mentioned but not explained. A black hole has ridiculously strong gravitational forces... And this raises a question; how is it that the black hole would simply "wander" around? It would still be locked in orbit around the Supermassive black hole at the center of the galaxy, right? And if/since that's the case, if an object with stronger gravity than a star could simply orbit it's way into a new solar system, couldn't other stars do the same thing? I haven't heard of astronomers finding evidence of colliding solar systems or anything like that...

[metaphor]

A black hole would be just like any other star. Stars sometimes get close to each other while orbiting around the galaxy. It's very improbable for two stars to actually collide, but occasionally two stars can pass very close to each other. If a star or black hole passed 100 AU from the Sun, even the Earth would feel a perceptible gravitational pull towards that star, and the orbits of many Kuiper belt objects would be affected. The probability of two stars passing within 100 AU of each other is about 10,000,000 times higher than the probability of them actually colliding.

[K^2]

It doesn't matter how massive an object is. It's still going to follow the same trajectory through a galaxy. The only difference is how it's going to affect its neighborhood. A rock will just sail right through. A rogue planet might pick up some gas clouds. A wandering black hole can throw star systems it passes close to way out of wack.

[Brotoro]

I have not seen the show you mentioned.

The supermassive black hole in the center of our Galaxy contains a relatively small fraction of the Galaxy's mass, so it's best not to think of the stars and other objects in our Galaxy being "locked in orbit" around it… the stars orbit the common center of mass of the Galaxy (which does happen to be located near that supermassive black hole, but the black hole is not the main contributor to the gravitational force that keeps the stars in orbit).

When a supernova explosion produces a neutron star or black hole, the matter that gets ejected from that explosion can be blasted off asymmetrically, giving the neutron star or black hole a sizable motion in reaction, so it could end up moving relative to the stars near it. Perhaps that's what they were speaking of. Yes, it would still be in orbit around the center of mass of the Galaxy, but its orbit would be somewhat different from its neighbors (more so than it was before the supernova explosion).

[Albert VDS]

Even if the odds of stars colliding would be infinitesimally small, somewhere in the universe it's bound to be happening at this moment because it's a big place.

Also a thing to note; black holes have a strong gravitational pull, but it has the same pull as any other object with the same mass.

[rkman]

A black hole has ridiculously strong gravitational forces

Not necessarily. The gravitational force of a black hole is as strong as it would be if the same amount of mass would be a star.

Only super massive black holes in the center of galaxies can be said to have ridiculously strong gravitational force (up to many millions of solar masses). Those can end up off-center due to collision between galaxies, but that's not exactly "wandering around".

Stellar-mass black holes can wander around just like stars. Orbital mechanics (sling shot) allows for bodies be ejected from their system, sometimes at high speed.

[Dominatus]

This all lines up with the little knowledge I had previously. In addition, this is a tv show. Meaning that almost everything is explained in layman's terms. It's not surprising that they would accidently imply something is true when really the show was attempting to explain why something else behaves in a certain way. It's also a series from 2010- the way the event horizon was defined made me laugh. (Absolutely nothing can escape, huh? So much for the law of conservation of energy. Wow, I didn't know black holes literally suck everything in. Guess an orbit must be impossible)

I actually had an interesting conversation with my physics teacher on black holes today. I hypothesized that a black hole is in reality a super dense star, the elicit "black dwarf" star which marks the final die-out of a star. If neutron stars are also formed when a star supernovas/collapses, then couldn't a black hole be the neutron stars cousin? A cousin with such intense gravity that light cannot escape whatever is occurring within the star (possibly the fusion of incredibly dense matter, some of which man has yet to discover) and the only byproduct of this fusion that can escape is labeled "Hawking Radiation"?

Hey, that's just an idea I was tossing around. I'm obviously no expert. Is it possible? Or just my ignorance making me look stupid?

[K^2]

No, not possible. If you say that an object has event horizon (point from which light cannot escape) then everything bellow horizon can only be in one of two states. It's either falling directly inward, or it's already at the center. No amount of pressure can result in an object of finite size sitting bellow event horizon.

[Soda Popinski]

I actually had an interesting conversation with my physics teacher on black holes today. I hypothesized that a black hole is in reality a super dense star, the elicit "black dwarf" star which marks the final die-out of a star. If neutron stars are also formed when a star supernovas/collapses, then couldn't a black hole be the neutron stars cousin? A cousin with such intense gravity that light cannot escape whatever is occurring within the star (possibly the fusion of incredibly dense matter, some of which man has yet to discover) and the only byproduct of this fusion that can escape is labeled "Hawking Radiation"?

Since stellar black holes are what's left over after a supernova of a sufficiently massive star, then a black hole is a neutron star's cousin. Just to clarify, light isn't only trapped within the "mass" of the black hole (the point of "singularity" - where you have finite mass within infinitely small space or infinite density [wait, wouldn't infinitely small space violate Heisenberg's uncertainty principle?]). Light is trapped up to the event horizon, a sphere (or oblong sphere if the black hole is spinning). The even horizon is the demarcation line (or sphere / shell) where escape velocity is the speed of light. Since nothing can go faster than the speed of light, nothing within the even horizon can have enough velocity to escape.

At least, that's my understanding of it.

Oh, Stephen Hawking recently suggested event horizons are sort of fuzzy.

[Kerbin Dallas Multipass]

There is an interesting phenomenon called merger recoil. It's related to gravitational waves and only affects objects with extreme masses. It can give supermassive(?) black holes an intesnse kick and eject them out of their galaxies. 4000 km/s is quite a bit of speed. http://en.wikipedia.org/wiki/Gravitational_wave#Energy.2C_momentum.2C_and_angular_momentum_carried_by_gravitational_waves

The fascinating thing is that this happens because the objects are so extremely heavy.

Edited February 6, 2014 by Kerbin Dallas Multipass

[rkman]

Not meaning to stir the pot, but is it not a respected view in (theoretical) cosmology (Sean Carroll and others) that the fact that theory leads to zero size means the theory 'breaks down' there?

Doesn't that mean there is not really a good reason to conclude there actually is something of zero size there? Stupidly small, probably. Insane density, yes. Certainly not a "black dwarf". But possibly not actually zero size and infinite density.

I think the theory does support "arbitrarily close to zero". Although it is then only a small step to zero, the theory arguably does not support that.

[Nuke]

the momentum of a black hole is going to be the net momentum of everything that it consumed, correct?

[K^2]

the momentum of a black hole is going to be the net momentum of everything that it consumed, correct?

Not necessarily. As it consumes stuff, a lot of energy is radiated, which also carries away considerable amount of momentum. In case of accretion disk, the radiation is more or less symmetric. But when it swallows a single large object, radiation can be asymmetric, resulting in net momentum change. I don't know how significant this is, but I would discount it without some study.

[metaphor]

Not meaning to stir the pot, but is it not a respected view in (theoretical) cosmology (Sean Carroll and others) that the fact that theory leads to zero size means the theory 'breaks down' there?

Doesn't that mean there is not really a good reason to conclude there actually is something of zero size there? Stupidly small, probably. Insane density, yes. Certainly not a "black dwarf". But possibly not actually zero size and infinite density.

I think the theory does support "arbitrarily close to zero". Although it is then only a small step to zero, the theory arguably does not support that.

Well quantum mechanics would say that something can't be infinitely small but has to have a finite size because of the uncertainty principle. But since you can't actually measure the singularity of a black hole unless you're in it, it basically behaves like a fundamental particle anyway.

Actually, all black holes are rotating Kerr black holes, so instead of an infinitely small point you would have an infinitely thin ring as a singularity.

[K^2]

Well quantum mechanics would say that something can't be infinitely small but has to have a finite size because of the uncertainty principle. But since you can't actually measure the singularity of a black hole unless you're in it, it basically behaves like a fundamental particle anyway.

You are confusing things. Uncertainty in position and finite size are two different things. Elementary particles, such as electrons, for example, are point particles, but they are delocalized. What this means is that you can take two non-overlapping regions of space, and ask what are the odds that each region contains at least a portion of the particle in question. Delocalization says that we might find this probability to be non-zero for both regions. But if we ask what are the odds that we found particle in both of these regions, this comes out to be precisely zero.

Black hole's singularity can be delocalized and still be a true singularity. There is no contradiction there. It still leads to all sorts of interesting features. But whether or not it is so, we do not know. We do not have a field theory that works on such short scales.

Actually, all black holes are rotating Kerr black holes, so instead of an infinitely small point you would have an infinitely thin ring as a singularity.

That isn't quite so simple, either. There are some problems with Kerr solution in the interior regions^*. I'm not an expert on this, so what I know could be outdated, but as far as I know, we still don't know the exact ground state solution for the Kerr singularity. Ring is a solution, but not necessarily the only solution, and not necessarily what a rotating black hole collapses to. It might even be the case that there is a critical amount of angular momentum that a black hole must have in order for ring singularity to exist.

^* A good example of problem with Kerr metric is that if we take a black hole with enough angular momentum, a naked singularity emerges. In simple terms, a Kerr black hole with enough angular momentum does not have an event horizon, and if such a thing existed, it would allow for time travel. Of course, that by itself isn't a deal-breaker, but it is a major warning sign. And indeed, such a solution is known to be unstable.

[Seret]

And if/since that's the case, if an object with stronger gravity than a star could simply orbit it's way into a new solar system, couldn't other stars do the same thing? I haven't heard of astronomers finding evidence of colliding solar systems or anything like that...

There are thought to be a significant number of objects floating around in interstellar space. Planets, comets, etc. Gravitational interactions can perturb just about anything, there's no reason why a star or a black hole couldn't be wandering.

When Andromeda and the Milky Way collide in the future it's highly likely that some objects will be thrown clear of the collision and end up in intergalactic space.

[rkman]

The inside of black holes is a continued topic of research:

New Type of Star Emerges From Inside Black Holes

Born inside black holes, “Planck stars†could explain one of astrophysics’ biggest mysteries and may already have been observed by orbiting gamma ray telescopes, say cosmologists

...Carlo Rovelli at the University of Toulon in France, and Francesca Vidotto at Radboud University in the Netherlands. These guys say that inside every black hole is the ghostly, quantum remains of the star from which it formed. And that these stars can later emerge as the black hole evaporates.

Rovelli and Vidotto call these objects “Planck stars†and say they could solve one of the most important questions in astrophysics. What’s more, evidence for the existence of Planck stars may be readily available...

https://medium.com/p/6cf7ec0ed28b

Black hole's singularity can be delocalized and still be a true singularity.

What is a true singularity if not a purely mathematical concept? And if that is what is at the core of a black hole, how can a purely mathematical concept exist in reality?

Edited February 6, 2014 by rkman

[rkman]

selfdeleted

[metaphor]

That isn't quite so simple, either. There are some problems with Kerr solution in the interior regions^*. I'm not an expert on this, so what I know could be outdated, but as far as I know, we still don't know the exact ground state solution for the Kerr singularity. Ring is a solution, but not necessarily the only solution, and not necessarily what a rotating black hole collapses to. It might even be the case that there is a critical amount of angular momentum that a black hole must have in order for ring singularity to exist.

^* A good example of problem with Kerr metric is that if we take a black hole with enough angular momentum, a naked singularity emerges. In simple terms, a Kerr black hole with enough angular momentum does not have an event horizon, and if such a thing existed, it would allow for time travel. Of course, that by itself isn't a deal-breaker, but it is a major warning sign. And indeed, such a solution is known to be unstable.

Most black holes form in an asymmetric environment so they have some rotation at the start. As material falls into one, it tends to spin up the black hole in the same direction. But there's a limit to the amount of angular momentum a black hole can carry, and we don't know of any process that can take it beyond this limit (that's the limit where there would be a naked singularity). So most black holes in the real world are hypothesized to be very close to the angular momentum limit but not at or above it.

[K^2]

Most black holes form in an asymmetric environment so they have some rotation at the start. As material falls into one, it tends to spin up the black hole in the same direction. But there's a limit to the amount of angular momentum a black hole can carry, and we don't know of any process that can take it beyond this limit (that's the limit where there would be a naked singularity). So most black holes in the real world are hypothesized to be very close to the angular momentum limit but not at or above it.

You are just telling me why a naked singularity couldn't exist naturally, anyways. That's absolutely irrelevant. Could you make a black hole with enough angular momentum? Easily. Electron's L/m is already over this limit. Just feed it a polarized stream of electrons, and you'll have a black hole with too much angular momentum.

And this was just a demonstration of why there are obvious problems. If we assume Kerr metric to continue into interior of the black hole, there are way more problems with that. There are CTCs in the interior regions even without naked singularity. So if you've fallen into a sufficiently large rotating black hole to not be shredded by the tidal forces, there is a trajectory you can fly to go back and meet yourself from before you flew that trajectory. Then fly it again, and meet the two of previous you. And then keep doing this so that you can throw a big party for yourselves. And then, maybe you can find the you from the other universe, because interior of the Kerr metric has two exteriors, only one of which is our space.

It's crazy stuff. But like I said, the most important fact is that Kerr metric in the interior is known not to be stable. It's not the interior solution for the rotating black hole. So what's a Kerr black hole above the horizon, is not a Kerr black hole bellow the horizon. Which puts the whole question of a ring singularity rather up in the air.

What is a true singularity if not a purely mathematical concept? And if that is what is at the core of a black hole, how can a purely mathematical concept exist in reality?

What is a point object if not a purely mathematical concept? And if that is what elementary particles are, how can purely mathematical concept exist in reality?