A black hole in orbit: what would it be like?

[Sociotard]

Say you pulled out the "Beer Can Crusher of the Gods" and started to compress the moon, keeping it's mass constant. Eventually, it would get so dense that, even though it doesn't have that much mass in astronomical terms, it will collapse into a black hole. According to Pierre Simone de Laplace (circa 1795) You'd need to squeeze it down to a radius given by

r

_S=2*m*G/c²

Where M is the mass, G is the graviational constant, and c is the speed of light. Using that we'd need to get our moon (7.3483*10²² kg) down to a radius of 0.109 millimeters.

 

What I started to wonder is: what would it be like to have a miniature black hole in orbit around a planet? As far as I know, once you got a reasonable distance away from the event horizon the orbital hole would have the same tidal effects on a planet as a regular moon of the same mass. It would put out gamma ray's as cosmic flotsam inevitably got sucked into it, but probably not very much.

 

I just thought visiting a planet with a black hole in orbit would be a cool place to visit in a star hero game.

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

That seems about right. The moon is already approximately a sphere with density depending only on distance, so its gravitational field is already almost exactly equivalent to a point mass of equal mass located at its centre.

 

A lunar-mass black hole would be tiny (approximately as you calculated, though Pierre de Laplace is not perhaps the person to consult concerning an intrinsically relativistic phenomenon) and almost invisible. It would be visible only as a result of gravitational lensing in a tiny patch of sky. There would be a little hard radiation from infalling matter, but given the minute cross-section, not very much. I think the object would be too large for Hawking radiation to be detectable, but I'm not certain about that.

 

So the main points of interest for this system would be lack of moonlight and eclipses despite the presence of tides.

[Hyper-Man]

Re: A black hole in orbit: what would it be like?

 

Well it would be extremely difficult to notice that it was there in the first place.

 

I would highly recomend the short story Neutron Star by Larry Niven (it is part of a collection of stories also titled Neutron Star as well) which deals with a very similar situation where a very small silvery object is found in orbit around a planet by explorers who do not automatically figure out the orbital tides caused by this 'small moon'. To add to the confussion the travelers are also part time ancient civilization treasure hunters and the neutron star looks very much like a 'slaver statis field' (slavers being one of the ancient powerful races in this universe).

 

The story might be found in other collections by Niven as well.

[Guest Worldmaker]

Re: A black hole in orbit: what would it be like?

 

A black hole with the mass of the moon would not have any greater gravitational pull than the moon has. Thus, everything would be the same except nights would be dark and knowing where the "moon" was would be very, very difficult.

[Guest Worldmaker]

Re: A black hole in orbit: what would it be like?

 

Well it would be extremely difficult to notice that it was there in the first place.

 

I would highly recomend the short story Neutron Star by Larry Niven (it is part of a collection of stories also titled Neutron Star as well) which deals with a very similar situation where a very small silvery object is found in orbit around a planet by explorers who do not automatically figure out the orbital tides caused by this 'small moon'. To add to the confussion the travelers are also part time ancient civilization treasure hunters and the neutron star looks very much like a 'slaver statis field' (slavers being one of the ancient powerful races in this universe).

 

The story might be found in other collections by Niven as well.

 

 

Didn't it turn out to be a neutron star that was only 2 feet or so across?

[Bill_CCHKK]

Re: A black hole in orbit: what would it be like?

 

Didn't it turn out to be a neutron star that was only 2 feet or so across?

You (and Hyper-Man) are thinking of the Niven short story "There is a Tide". Louis Wu finds a planet with a 'moon' that is a five feet or so diameter sphere of neutronium. Meets up with an alien race (the Trinocs) who make the mistake of getting too close. From a distance, the 'moon' produces tides just like Luna does here, but the gravitational gradient is *incredibly* steep, so don't get too close.

 

"Neutron Star", OTOH, is Beowulf Shafer's orbit of an actual neutron star in a GP hull packed with, among other things, a bunch of explosives courtesy of Sigmund Ausfaller.

 

This ends my "Niven Nerd" fact of the day

 

- Bill

[Hyper-Man]

Re: A black hole in orbit: what would it be like?

 

Thanks for the correction Bill_CCHKK.

 

I should have checked my bookshelf.

[Outsider]

Re: A black hole in orbit: what would it be like?

 

It would change earth's energy balance a tiny, inconsequential bit, in that we wouldnt be getting the infintessimal increase in incoming photons from the moon (reflected), and we would be getting a little tiny bit more energy from the sun, as lunar eclipses would no longer block incoming solar energy for a few minutes a year, as they used to.

 

The main things I'd be worried about would be (assuming the hard radiation emission is negligible... someone do the math!) is the effect on the ecosystemof having no visible moon. Do some species use the moon as a visible cue as to when to do things, or rely on its light (faint though it is) to aid them in the hunting/other nocturnal persuits?

[Captain Obvious]

Re: A black hole in orbit: what would it be like?

 

I would guess that as a tiny black hole/moon crossed through the solar eclipse position that something strange would happen with the image of the sun as seen from the planet's surface, but I'm not nearly physicist enough to say what. Either a noticible black spot crossing the solar disc, or a weird shifting and distortion of the solar disc, or maybe even both.

 

Once someone nerdier than I comes along with a more educated guess, then you have an interesting little tidbit that would figure into the mythology of any intelligent life on the planet.

 

Also, as far as navigation goes, it would be real easy to run into that black hole unless you monitor your gravimeters. If it's that small, you won't get much in the way of a visible vortex outside the event horizon, and unless it's backlit by the sun or the planet, gravity will be the only way to detect it, that I can think of.

[Intrope]

Re: A black hole in orbit: what would it be like?

 

I'd expect the Moon-hole to be basically invisible: it's so small that you'd never see it, even if backlit (0.1 mm == small!) and any accretion-disk like effects would probably be too small to see at any reasonable distance.

 

On the other hand, this would be an incredible boon to the human race: as I understood it, you could use such a thing to turn matter into energy with about 30% efficiency! Then, just beam it home as microwaves.

 

(my understanding is that a high-speed particle beam that is shot just close enough to be captured by the hole will spiral in, gaining immense energy as it goes--much of which will be radiated off before it enters the hole. This has the effect of releasing a large percentage of the beam's mass as energy)

 

Anyway, if it's a tech civilization that's got this, they may have tapped it for power, which probably requires putting a power station around the hole.

[keithcurtis]

Re: A black hole in orbit: what would it be like?

 

Visibility might depend on how the moon was compressed. If it was spectacularly violent, there might be debris in orbit. Such debris would form an accretion disk around the black hole, which would be very energetic as particles spiral in, and thus very visible.

It really annoyed me that reviewers of Disney's "The Black Hole" took them to task that their black hole was visible. An accretion disk would be bery visible. I assume it would be red, due to the wavelength shift of particles receding at near light speeds, but I ain't no physicist.

 

Keith "I ain't no physicist" Curtis

[Sociotard]

Re: A black hole in orbit: what would it be like?

 

There'd be an awful lot of engineering problems with a black hole power plant. How do you contain the thing so the plant itself doesn't touch it? not to mention building equipment that can handle g force strain. even a mile or two away from the event horizon it'd be significant. I think that's how I'll treat this: Currently surrounded by a dozen warning satellites orbiting at high velocities (beeping "don't get to close, quantum singularity" on all radio frequencies), along with a group of old engineers trying to figure out how to harness the darned thing.

[laxrulz777]

Re: A black hole in orbit: what would it be like?

 

captain obvious is right... You wouldn't have eclipses you would have a wierd solar morphing that would make the sun grow larger and a black dot of emptiness appear in the middle. The size of the dot would depend on several factors (event horizon, distance from the black hole to the planet, size of the sun, etc.) A very large black hole could conceivably make the sun grow to a fairly large doughnut in the sky (wouldn't that look cool). It could also lead to a period of intense bombardment of solar radiation if the amount of sun absorbed by the hole was exceeded by the amount of redirected energy that went around the edge of the event horizon.

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

Well it would be extremely difficult to notice that it was there in the first place.

 

It would be very hard to see, but you'd notice it was there pretty easily because it would produce very nearly the same tides as the Moon.

 

I would highly recomend the short story Neutron Star by Larry Niven (it is part of a collection of stories also titled Neutron Star as well) which deals with a very similar situation where a very small silvery object is found in orbit around a planet by explorers who do not automatically figure out the orbital tides caused by this 'small moon'.

 

That isn't the way I recall that story. As I recall it, Beowulf Shaeffer is hired by General Products to take a ship with a General Products impenetrable hull on a close-approach orbit of a neutron star in an attempt to work out what reached through the hull and killed the scientists who last went on such an orbit. (Answer: tides.)

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

I would guess that as a tiny black hole/moon crossed through the solar eclipse position that something strange would happen with the image of the sun as seen from the planet's surface' date=' but I'm not nearly physicist enough to say what.[/quote']

 

I am physicist enough to say what. I am also physicist enough to tell you that the lensing distortion would be much too small to see, even with telescopes. The gravitational lensing effects of the Sun are just barely measurable. The gravitational lensing of an entire galaxy produces visible arcs, but only when viewed from billions of light-years away. The gravitational lensing of a tiny black hole of lunar mass are going to be detectable over only an extremely tiny angular distance. There will be "wierd solar morphing", but it will be too small to see even with a [ground-based] telescope.

[keithcurtis]

Re: A black hole in orbit: what would it be like?

 

It would be very hard to see, but you'd notice it was there pretty easily because it would produce very nearly the same tides as the Moon.

 

 

 

That isn't the way I recall that story. As I recall it, Beowulf Shaeffer is hired by General Products to take a ship with a General Products impenetrable hull on a close-approach orbit of a neutron star in an attempt to work out what reached through the hull and killed the scientists who last went on such an orbit. (Answer: tides.)

 

I enjoyed that story, but never bought into the idea that the Puppeteers would have no knowledge of tides. Beyond the fact that their sun would produce tides (they did have a sun at that point), it's very simple mathematics. Puppeteers is wayyy smart, y'know.

 

Keith "Niven fan nonetheless" Curtis

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

I enjoyed that story' date=' but never bought into the idea that the Puppeteers would have no knowledge of tides. Beyond the fact that their sun would produce tides (they did have a sun at that point), it's very simple mathematics. Puppeteers is wayyy smart, y'know.[/quote']

 

I agree on all points. And I add that Puppeteers would also be familiar witht eh tidal forces of planets on objects in orbit. The will use them to stabilise the orientation of satellites, and they will have to compensate for them in zero-gee manufacturing.

 

The blackmail/counter-blackmail thread didn't really stand up to scrutiny. Neither did the supposition that the effect would be mysterious, and neither did the supposition that the human astronomers would have plotted such a course in ignorance. So in short I enjoyed Neutron Star, but it is really a pretty silly story.

 

Regards,

[DRThrush]

Re: A black hole in orbit: what would it be like?

 

Squeezed into black hole density wouldn't the gravitational force of the moon be the same as it is now? Mass is mass is mass. It's density would be intense but it's G-force would remain the same wouldn't it?

Critters that are evolved to react to the moon do so because they are from an environment with moonlight. If they evolved without it, I suppose it's absence wouldn't change their behavior.

Another little thing the moon seems to do for us it to intercept some small percentage of incoming meteors. I imagine that the loss of it's shielding would cause the Earth to recieve more energy, by way of kinetics and heat, then the effects of photon bombardment over time.

 

-Dave

[Sociotard]

Re: A black hole in orbit: what would it be like?

 

The lunar gravity well would remain mostly the same (once you got a reasonable distance away, anyhow), so wouldn't it get nearly as many meteors?

[DRThrush]

Re: A black hole in orbit: what would it be like?

 

The lunar gravity well would remain mostly the same (once you got a reasonable distance away' date=' anyhow), so wouldn't it get nearly as many meteors?[/quote']

 

The gravity of the moon is really very low, somthing like .14G if IIRC. It might just alter the course of an 'incoming' by a little bit rather than taking the impact. I will admit that I cannot back this up with any math.

 

-Dave

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

Squeezed into black hole density wouldn't the gravitational force of the moon be the same as it is now? Mass is mass is mass. It's density would be intense but it's G-force would remain the same wouldn't it?

 

At any given distance above the present surface, gravity would be the same. But at points below what is now the surface, gravity would be higher.

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

The lunar gravity well would remain mostly the same (once you got a reasonable distance away' date=' anyhow), so wouldn't it get nearly as many meteors?[/quote']

 

No.

 

The Moon intercepts meteors because their orbits intersect its surface. If it were replaced by a black hole of the same mass the orbits would be the same because the gravitational field would be the same (at least outside the present surface of the Moon), but the surface wouldn't be there to collide with. Most of the meteroroids that impact the Moon would instead swing through its gravitational field on open orbits (approximating hyperbolas), collide with nothing, and proceed; their orbits altered but not intercepted. Only those meteoroids on dead-centre courses would impact the mini black hole. And those might not be intercepted. A tiny black hole like that might tunnel through a meteroroid and perhaps tear it to pieces with tidal stresses rather than swallow it up.

[AlHazred]

Re: A black hole in orbit: what would it be like?

 

The main thing to worry about in this scenario isn't the gravity gradient (which is pretty steep, but manageable). It's the radiation.

 

If the Moon were turned into a black hole, it would begin producing a lot more hard radiation, not to menion antimatter. Having all that so close to Earth could possibly make for spectacular, uh, "Equatorial Lights" as the high energy particles impacted the upper atmosphere. I'm not certain how powerful the Van Allen belts are, off hand, but I imagine they might be hard pressed to protect us from the additional radiation. Not to mention the antimatter impacting the upper ozone.

 

Not that it would be that much of a bother. I remember reading somewhere about black holes and their "hair." The idea is that electron tunneling (or some such) causes black holes to lose their energy. They can regain it by gobbling matter, but there's a mass at which they emit energy faster than they can gobble it up. I think the Moon has far too small a mass to stay for long; as a black hole, it's event horizon would be something like 0.1 mm, right? I imagine it would radiate away to nothing within a few days or weeks, I think.

[Agemegos]

Re: A black hole in orbit: what would it be like?

 

The main thing to worry about in this scenario isn't the gravity gradient (which is pretty steep' date=' but manageable). It's the radiation.[/quote']

 

This turns out not to be the case. See http://www.physics.hmc.edu/student_projects/astro62/hawking_radiation/gammaray.html.

 

If the Moon were turned into a black hole, it would begin producing a lot more hard radiation, not to menion antimatter.

 

Nope. A black hole the mass of the Moon would be much too massive and cool to produce antimatter. Hawking radiation of a black hole of Lunar mass would be equivalent to the thermal radiation of an object at 2.7 Kelvins. The black hole would not only be tiny, but very, very dark. In fact, it would be pretty close to the same colour-temperature as the cosmic microwave background, ie. the black part of the sky.

 

I think the Moon has far too small a mass to stay for long; as a black hole, it's event horizon would be something like 0.1 mm, right? I imagine it would radiate away to nothing within a few days or weeks, I think.

 

Well, in the first place, a black hole that size and temperature would absorb more energy from the background microwave radiation than it would lose by Hawking radiation. Given that it is in the Moon's orbit is going to soak up sunlight as well. So it will (very, very gradually) get bigger and cooler as the aeons pass, not smaller and brighter. I plugged the figures into the formula derived on the site I linked above, and estimated that if it were not absorbing any mass or energy from any source, a black hole the mass of the Moon would take about 3*10^43 years to evaporate. That is 2 * 10^33 times the current age of the Universe. More than a few days or weeks.

[Outsider]

Re: A black hole in orbit: what would it be like?

 

Critters that are evolved to react to the moon do so because they are from an environment with moonlight. If they evolved without it, I suppose it's absence wouldn't change their behavior.

-Dave

 

Yeah, but the proposition is that we use the beer can crusher of the gods to squeeze it down, not that it started that way. Animals might have some problems.