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Author Topic: Can multiple black holes orbit one another?  (Read 1668 times)

Offline Rocmistro

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Can multiple black holes orbit one another?
« on: 18/05/2016 19:32:10 »
Hello!

I'm a Role-Playing gamer and my group are currently playing an RPG.

I'm entertaining a scenario, and curious as to how feasible it is within the laws of physics and what we know about the universe (I realize we are talking about a mythological universe that allows for Space Wizards, Death Stars, Hyperspace travel, etc., but it's important to me that I obey the laws of physics as much as possible, and only break them when utterly needed.)

So within our game, I'm considering hiding something important (perhaps the REAL secret Rebel Base??) within a shared gravitational epi-center of 8 black holes called "The Crazy Eight".

I recently came across Castor Gemini, a sixtuplet star system of 3 binary pairs orbiting a common center of mass.

If such a thing is possible, is it also possible that...

1. We could expand this out to 8?
2. That these could be black holes (instead of stars) that are orbiting a common center of mass?

Any other hypothetical suggestions you could give me would be awesome. (ie, is it even possible for black holes to orbit one another (or anything at all for that matter))  I imagined that if it *were* possible for multiple black holes to be orbiting one another, that they would be moving tremendously fast, and that they would all be unstable, erratic orbital paths.  (I imagine something like electrons moving around a nucleus)

Getting back to the game, the idea is there is only one person (one of my players), who's character is intelligent enough to have calculated what the path of these 8 black holes is at any given time, and thus is the only one that can figure out a safe way to the secret hidden Rebel base that doesn't involve them, to quote Han, of "flying right through a star or bouncing too close to a supernova" (and that would end their trip real fast, wouldn't it?)

Thanks and my apologies in advance for the inherent silliness of my question.


« Last Edit: 31/05/2016 22:27:35 by Rocmistro »


 

Offline RD

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Re: Crazy 8, RPG hypothetical
« Reply #1 on: 18/05/2016 19:42:37 »
... is it even possible for black holes to orbit one another ...

Yes ... https://en.wikipedia.org/wiki/Binary_black_hole
 

Offline evan_au

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Re: Can multiple black holes orbit one another?
« Reply #2 on: 18/05/2016 23:14:11 »
 
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is it even possible for black holes to orbit one another?
The gravitational waves detected late in 2015 were from the merger of two black holes that were orbiting each other.

If they are very close (almost touching), they will radiate gravitational waves and approach each other more closely.

But if they are a fair way apart (like the Sun and Mercury), they will remain in stable orbits for much longer than human timescales.

Quote
2. That these could be black holes (instead of stars) that are orbiting a common center of mass?
If you replaced every star in the Castor Gemini system by a black hole with the same mass and velocity, the orbits of each of them will be undisturbed. It will just be a lot darker in the vicinity.

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1. We could expand this out to 8?
People have studied multiple star systems. There are some configurations that are semi-stable, if one object has far more mass than the others (like the Solar system), or the Trojan orbits inside the Solar system.

If you have two stars in a close orbit (2 stars), from a distance it will act like a single star with the sum of the masses.
If you have a pair of pairs, (4 stars) from a distance it will act like a single star with the sum of the masses.
If you have a pair of pairs of pairs (8 stars), from a distance it will act like a single star with the sum of the masses.

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they would be moving tremendously fast
A black hole at the distance of Mercury from the Sun would orbit much faster than Mercury (because the mass is much greater).
You can work out the orbital period here: https://en.wikipedia.org/wiki/Orbital_period#Two_bodies_orbiting_each_other

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they would all be unstable, erratic orbital paths
In general, any system with more than 2 orbiting objects is somewhat unstable and chaotic. They are likely to (eventually) throw one object out of the system, or have two objects collide.

The best you can hope for is something like the Solar system where the orbits seem to have settled down into a formation where they could remain stable for hundreds of millions of years (barring outside influences).

For more extreme configurations (like you are considering here), you could hope for something that continues for human timescales (thousands of years). Collisions are relatively unlikely, as a black hole is a very small target in the vastness of a solar system. But throwing one out of the system is a real possibility.

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calculated what the path of these 8 black holes is at any given time, and thus is the only one that can figure out a safe way...

Travel too close to a black hole, and your spaceship (and astronaut) will be torn apart by gravitational forces.
Relativistic effects like time dilation in close orbits can change the orbit slightly.
Navigation would be more complex if the black holes have different rates of spin, as this produces a nearby effect called "frame dragging", which makes orbits diverge significantly from what Newton's laws would predict.
 
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Offline PmbPhy

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Re: Can multiple black holes orbit one another?
« Reply #3 on: 19/05/2016 02:23:35 »
Rocmistro - The only time that you need to think of a black hole as being any different than an ordinary star, besides the fact they can't be seen by any other means except by looking for Hawking radiation, is when you're considering events very close to the event horizon. When you're speaking of orbits you can simply think of them as stars, unless, of course, the orbits are very small.
 

Offline chris

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Re: Can multiple black holes orbit one another?
« Reply #4 on: 19/05/2016 07:53:17 »
besides the fact they can't be seen by any other means except by looking for Hawking radiation...

What about gravitational waves?
 

Offline agyejy

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Re: Can multiple black holes orbit one another?
« Reply #5 on: 19/05/2016 08:06:53 »
Or gravitational lensing. Or accretion disks. Or scattering from the photon sphere. Hawking radiation might be the only radiation coming directly from the Event Horizon but there are many other ways in which something approximating the outline of the Event Horizon can be observed. I mean heck if you get close enough you could potentially see it transiting stars though again you'd have to account for the gravitational lensing effect.
 

Offline PmbPhy

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Re: Can multiple black holes orbit one another?
« Reply #6 on: 19/05/2016 15:17:33 »
Or gravitational lensing. Or accretion disks. Or scattering from the photon sphere. Hawking radiation might be the only radiation coming directly from the Event Horizon but there are many other ways in which something approximating the outline of the Event Horizon can be observed. I mean heck if you get close enough you could potentially see it transiting stars though again you'd have to account for the gravitational lensing effect.
Those things require special conditions. I'm referring to how a black hole can be seen under all circumstances and at all times. Let's consider each of your examples:

1) gravitational lensing: That can only be used to detect a black hole when there is a something such as a star behind it which the black hole is passing, and then under very special circumstances. You can't depend on that under all circumstances and at all times.

2) accretion disks: Not all black holes have accretion disks. An accretion disk is a structure (often a circumstellar disk) formed by diffused material in orbital motion around a massive central body. Not all black holes satisfy that criterion. In fact I'd wager that their in the minority. So again, this is not a general trait of a black hole and can't be depended on for seeing a black hole.

3) scattering from the photon sphere: Same problem as with gravitational lensing.

4) I mean heck if you get close enough you could potentially see it transiting stars: You're not making the very important distinction between "seeing" a black hole and "detecting" a black hole. An analogy might be something like people can't "see" electrons by any means but we can detect them.
 

Offline agyejy

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Re: Can multiple black holes orbit one another?
« Reply #7 on: 19/05/2016 18:43:43 »
Those things require special conditions. I'm referring to how a black hole can be seen under all circumstances and at all times.

You like to present yourself as scientifically minded so you should understand the need for precision and accuracy in any statements you make. In fact you basically just used that point against me here:

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You're not making the very important distinction between "seeing" a black hole and "detecting" a black hole.

Now given that we've established that you seem to be overly concerned when pedanticism we should exam your previous statement:

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besides the fact they can't be seen by any other means except by looking for Hawking radiation,

There are no qualifying statements. There was nothing about all circumstances or all times (which is a joke anyway because gravitational lensing will basically always work). Your statement is misleading precisely because it excludes all the other possible ways to "see" a black hole that on occasion may not be operative without giving any reason for their exclusion. Anyone reading that statement would be easily misled into thinking Hawking radiation was the only way ever. So if we're going to argue about the difference between "see" and "detect" (which honestly in terms of a layman doesn't really exist anyway) then your first statement was quite wrong due to the lack of qualifiers.
 

Offline evan_au

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Re: Can multiple black holes orbit one another?
« Reply #8 on: 20/05/2016 13:22:17 »
Quote from: agyejy
the difference between "see" and "detect"
Perhaps the easiest way to detect if your path is taking you near a black hole is to have a few spacecraft spread out over a few thousand miles, continually measuring the distance between them (eg with lasers). This should be able to detect gradients in the local gravitational field. But it won't easily tell you about distant black holes (eg 100 times the distance of the farthest measuring point, or 10 times farther than the closest black hole.)

Quote from: PmbPhy
Hawking Radiation
Emission of Hawking radiation would work for "small" black holes.

For stellar-mass black holes, the Hawking radiation is cooler than the CMBR.
So I guess you could go looking for "holes" in the CMBR.

But you need a very large aperture to resolve a black hole of < 20km radius, at a distance large enough to navigate safely.

Quote from: Chris
Gravitational Waves
There are suggestions that we could produce a sensitive gravitational wave detector by having several space probes accurately measuring the distance between them.
If the black holes had different orbital periods, and you measured the gravitational waves using enough laser paths, you may be able to determine the direction of any closely-orbiting black holes. But black holes in more distant orbits generate a very small amount of gravitational waves.

"Delta-V"
It takes enormously large rockets to navigate in the fairly gentle gravitational field of our solar system - the critical factor is the "Delta-Velocity" or change in velocity which your rocket can produce. As mentioned in this week's podcast, in our solar system, most chemical rockets don't have enough Delta-V to launch a sizable space probe to outer planets and comets, so a planetary slingshot is often used.

But in a system of orbiting black holes, the Delta-V required is enormous, and would require antimatter drives, or similar. Your very clever pilot should be able to plot a course around the black holes using repeated slingshots to reach his destination (while not turning himself into human spaghetti, or a burst of X-rays).
 

Offline Prophet12

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Re: Can multiple black holes orbit one another?
« Reply #9 on: 20/05/2016 18:10:14 »
Yes, then they combined to form super black holes eventually leading to consumption of all galaxies....this is a contracting universe and contracting at an ever increasing rate....

Hope that helps...
DS
 

Offline evan_au

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Re: Can multiple black holes orbit one another?
« Reply #10 on: 20/05/2016 23:18:46 »
Another constraint is the maximum acceleration that your pilot could withstand while maneuvering between these black holes.

After a slingshot around one black hole, the pilot must reorient the spacecraft with the right velocity and correct position to approach the next slingshot, and this may require dramatic acceleration, perhaps over extended periods (days to weeks).
  • Earth's surface gravitational acceleration is g=9.8m/s2
  • Aircraft passengers without seatbelts suffer broken bones when the acceleration changes by around g
  • Untrained people without a g-suit black out at around 3-5g (and are more likely to suffer strokes and heart attacks)
  • A trained fighter pilot wearing a g-suit can withstand around 9g for perhaps 30s before losing consciousness
So this very tricky course must not subject the pilot or passengers to excessive acceleration, but reach the destination without running out of fuel.

See: https://en.wikipedia.org/wiki/G-suit

The alternative is an antigravity field (which we don't know how to build at this point in time).
 

Offline jeffreyH

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Re: Can multiple black holes orbit one another?
« Reply #11 on: 21/05/2016 15:19:58 »
The problem with an anti gravity field is limiting the extent so that it does not cancel the external gravitational field and therefore nullify the sling shot effect.
 

Offline agyejy

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Re: Can multiple black holes orbit one another?
« Reply #12 on: 21/05/2016 17:42:33 »
The problem with an anti gravity field is limiting the extent so that it does not cancel the external gravitational field and therefore nullify the sling shot effect.

Just for the sake of clarity the term gravitational slingshot is actually a bit of a misnomer. The extra energy you get doesn't really come from the gravity of the body you use for the slingshot. After all no matter what path you take when passing through a gravitational field it takes exactly as much energy to get out of the field as you gain going into the field and in reality you can actually lose energy due to any drag if there were no other source of energy. The energy the spacecraft gains from a gravitational slingshot maneuver actually comes from the orbital kinetic energy of the larger body used for the slingshot. Luckily planets have so much that they don't notice the very tiny amounts our spacecraft steal. The equations for the speed boost you get from a gravitational slingshot don't actually contain any mention of the gravitational field of the body used for the slingshot. The only thing you really have to worry about is making sure there is enough attraction between you and the large body to make it possible to due the precise orbit you need to do to gain the delta-V you want. This does generally put practical limits on achievable speeds because planets are not points and you can only get so close to their centers (thereby increasing the gravitational attraction) before you hit their atmospheres or surfaces. Thus there is a limit on the orbits and thus the top speed that is possible which is controlled by the mass and size of the object you are using to slingshot.

Further reading including a bit about black hole slingshots at the end:
http://www.mathpages.com/home/kmath114/kmath114.htm
 
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Offline evan_au

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Re: Can multiple black holes orbit one another?
« Reply #13 on: 22/05/2016 00:31:04 »
Quote from: agyejy
you can actually lose energy
After falling towards a cluster of black holes, you would achieve quite a high velocity; you must lose much of this velocity if you are to rendezvous with your rebel base (as distinct from waving as it flashes by at a faction of the speed of light).

So using a gravitational slingshot to lose velocity is necessary.

Quote
planets are not points and you can only get so close to their centers
Fortunately, black holes are very compact, and very massive, so you can get very close to them.

But not too close - even the tidal effects between the ends of a spaceship would be significant, and the angular momentum you would pick up from a close encounter may prove fatal to the crew. Perhaps a spherically symmetric spaceship may be safer than Flash-Gordon's long, thin spaceship?
 

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Re: Can multiple black holes orbit one another?
« Reply #13 on: 22/05/2016 00:31:04 »

 

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