Orbiting a black hole

[Wowbagger]

Hi all,

Forgive me if this is rudimentary, but I have just been enlightened by the fact that it IS possible for a planet (or anything) to orbit a black hole. My prior knowledge was not only informed by Science Fiction, but by documentaries such as BBC Space (with Sam Neill) which consistently imply that NOTHING can escape from a black hole. Giving it the status of some sort of monster that can eat whole Solar Systems.

Now I have learnt that (hypothetically) if the Sun were to magically turn into a black hole (minus the fireworks), the Earth's orbit would continue unchanged. :doh: So when BBC Space says something like "there are millions of black holes that could potentially swallow our planet" - they are leaving out the fact that there are billions of stars that would have exactly the same devastation if they wandered nearby.

I understand (very vaguely) that you have to pass the "event horizon" of a black hole, or in other words, be really close to it to not be able to escape. But if you were that close to a star, you would be on fire anyway, so black holes are no more dangerous than stars.

Please somebody correct me if I'm wrong, but more importantly, can anyone explain why SciFi gets this so wrong? Is this only a new development in the study of black holes, or has it always been known? As a Doctor Who fan, I am particularly dissapointed that the premise of "The Impossible Planet" episode is a complete fail.

[el_draco]

Briefly, Its all about gravity and inertia. The earth stays in orbit around the sun because the pull of gravity exactly matches the tendency of the planet to want to go in a straight line and "leave" so to speak.

The source of the gravitational field is not really relevant in this context. The "you'll never escape" concept comes from the reality that if you can not travel fast enough to atleast balance the gravitational attraction of the object, star or black hole, you will inevitably get drawn in to it.

Frizzle either way because of the radiation and temperature around either type of object. Black holes generate nasty amounts of most things that will obliterate your DNA in a flash; a star will fry you just as nicely.

Nothing in theory can escape the black hole once it passes the event horizon because the speed required would exceed the speed of light, which is probably about as fast as we can go in this universe. Hence, the hole is "black" because you cant see anything beyond the Event Horizon.

BBC is correct in that if a black hole of any appreciable mass approached our solar system, it would disrupt the gravitational balance and could happily gobble us up.

So would another star....

Now, the physics is somewhat flexible when the hole gets really big, like at a galactic core. The big buggers may be able to be "played with a bit" but rest assured, the ultimate result would be the same for a planet or entire star. Get to close and you are gonna get eaten.

Hope this, "really simplified" answer helps.

Rom

[xelasnave]

If nothing can escape from a black hole how can a black hole communicate its message of gravity to inform the rest of the Universe that it is there?:D.

alex:):):)

[el_draco]

Well, thats another question... The Event Horizon is the point at which nothing can leave the black hole, but what happens outside is more visible.

The Gravitational field is similar to any massive object in that it influences what is around it. Anything approaching will have its course altered by the pull of the black hole for example.

However, these nasties also exist in binary systems and they tend to canibalise the other party by sucking off, (In this instance Alex, "gravity does suck" :lol:), huge amounts of gas from the companion star. As the gas spirals toward the event horizon, it gets hotter than hell and emits lots of stuff like radio and X-Rays, that can be detected.

The accretion disk around the black hole will often suddenly flare resulting in a Nova, which can also be observed. Finally, a star ripping around something massive, at awful speeds, that can't be seen is a good indicator of a beastie being present.

Fun stuff. Lots of theoretical examples of how we could use Black Holes to time travel, generate energy etc. Other theories suggest they my be entrances to worm holes but I doubt it. Still others suggest black holes may naturally "evaporate". The physics around them is very poorly understood from what I gather, but we do know they are out there. Nice bed time story for the littlies huh? ;)

[that_guy]

just by being close to its high gravity but not high enough to gobble you up you can cause time dilations... TIME TRAVEL!!! Then again you can do that with a star or a dense planet... so nothing special.. just wanted to mention that

[sjastro]

The same reasons (classical and quantum mechanical) as a photon "escaping" an electromagnetic field.

Steven

[xelasnave]

From what I have read I formed the impression that all black holes are part of a binary system. If this is so I wonder why.

I dont know if this is the way it is and if there are black holes that exist as soletary units. Do you know of any exception to my perception of that rule?

Can you provide a pinch of enlightnment on this aspect:confused2:.

I do feel that to consider the question I presented may force us to discover new and interesting aspects as to gravity in general and in specific interesting characteristics of a black hole. Not withstanding all that points to their existence I am not entirely happy with the notion of black holes... the math suggests it and of consequence observations are interpreted to conclude the math was correct however we still have to see a black hole in the flesh. I csan suggest various reasons why the things we observe can be explained that does not rely upon the presence or existence of a black hole.
Remeber this. Dr A did not think different to me upon this matter and although it was his math that enabled the guy who worked up his math to produce a black hole Dr A was not convinced. I think Hollywood has more to do with the acceptance of black holes than the science by popularising the notion such that it seems to be irrefutable fact... personally I dont accept them because I can consider alternatives that are reasonable in fact more reasonable than the over the top speculations upon time travell and worm holes... mere fancy I suspect...still thinking about this cant be bad and as I said to answer my original question my lead us further.

As far as I can determine current main stream science recognises "messenger particles"... I recal Mr Greene refering to them in "The Elegant Universe" and given the seemingly mild acceptance that his movie represents valid current theoretical physics ..most seem content that "string theory" is not mere fancy such that my question could be engaged as something that needs an answer before we proceed further.

I do feel much speculation comes from entertaining the numbers when we give consideration to a black hole and this speculation will generate opportunity for the prospect of massive time dialation and worm holes which are representative of some of the aspects of a black hole I find difficult to accept.
So if we go with Mr Greene and the elegant universe I feel we are forced to engage the question I raise... firstly are/is there reasonable ground/s to consider messenger particles are the communicators of forces ..if so how can they escape the "pull" of a black hole:D..if the prospect of messenger particles does not enter the picture why is it that the power of a black hole does not corrupt the "gravity field" which I guess is a fall back position if we are to opperate without messenger particles... I guess the specualted graviton must fit the bill for a messenger particle.

So although I can go along with GR bending space time I suspect that nevertheless there will be a mechanism ..it all cant happen as if it were a magical thing we can never grasp...what communication goes on in space that sees communication between bodies happily labled at this point as forces of attraction.

I have found everything in this thread most interesting and it has refreshed things I recall from stuff I read some time ago and stimulating thinking I have not indulged for some time now:thumbsup:.

alex:):):)

[xelasnave]

But a photon cant escape a black hole as I understand it...
alex:):):)

[xelasnave]

If you have $5 and Chuck Norris had $5, He would have more money than you.
With respect a great joke but such logic is flawed.
alex

[sjastro]

I mentioned electromagnetic fields not gravitational fields.

Steven

[that_guy]

:screwy::screwy::P

[Wowbagger]

First up, thanks for the reply el_draco... you have confirmed my new found knowledge that while Black Holes are dangerous, it is unfair to treat them as a "monster" anymore than a giant star (of which there are many more)!



Well, I assume the gravitational field can "escape", as it would be illogical to think that a gravitational field is effected by its own gravity.

Off topic slightly... if we can see Black Holes through 'gravitational lensing', why would we not get the same effect from Dark Matter, which apparently takes up 90% of the universe?!? Surely it would occasionally float between us and a light source? :confused2:

[sjastro]

Yes it does.

Strong evidence for dark matter is provided by lensing of the "Bullet Cluster galaxies".

http://en.wikipedia.org/wiki/Bullet_Cluster

Steven

[Wowbagger]

Interesting link, thanks for that Steven!

[el_draco]

A black hole often forms in a binary because of differences in mass of the two stars. A big star in a binary that goes S.N. may form a neutron star that then sucks mass from the secondary in the system. As the mass is deposited, the neutron star can become to large and then collapse to form a blackhole. The dynamics are interesting and are linked to recurrent Nova'sand other interesting stuff.

However, any star big enough may form a black hole. I think the limit is about 20 solar masses but I can't remember exactly.
Summary of fates (very much simplified):
Small star = nova and white dwarf which cools after a long time
Big star = S.N. and neutron star which cools after a very long time, maybe
Massive star = Black hole which does wierd stuff

Its all related to initial mass of the star involved, and the dynamics of the fusion process. If the star is big enough, whether a binary or not, a S.N. will generate an implosion in the core but the resulting object will have to much mass to form a neutron star so the collapse under gravity continues and a black hole results.

Hope this helps

[Nesti]

To answer the original questions and points:
If sufficiently far enough from a Black Hole, ensuring that matter is not being stripped from the orbiting body, and assuming the Black Hole isn't consuming other material and getting bigger, an object should be able to orbit a Black Hole for a great period of time. This is because the orbiting body, as with our moon's relationship with earth, is simply moving through a geodesic within the gravitational field of the Black Hole. Of course the orbiting body and possibly the Black Hole are shedding mass in gravitational waves, so there is a limit to how long the orbit may be sustained (tens of billions of years perhaps?!). Now at some point close to the Black Hole, one might reason that a Photon may be able to be placed in orbit however, that's akin to standing a pencil on its lead with infinite balance; it just can't be done. So a photon cannot be in orbit around a Black Hole.
Discussed here;
Lesson 12 on General Relativity - Black Hole dynamics
http://www.youtube.com/watch?v=fVqYlSNqSQk&feature=SeriesPlayList&p=6C8BDEEBA6BDC78D

As for the Sun becoming a Black Hole and the earth's orbit should go unchanged. I don't think our sun has sufficient mass in the first place, and if it did, I believe their would certainly be a change in the gravitational field as a Black Hole's field strength is far greater, even if their was no increase in the amount of stellar material between the sun and the Black Hole. Can anyone confirm this, and if so, is the Scalar Curvature responsible for the increase expression of the field?

There's a tonne of information in the link, you should watch it to see what's happening to spacetime as it get nearer the Black Hole.

[sjastro]

The strength of the field is a function of density rather than mass. A stellar black hole will always have a smaller mass than the progenitor star (assuming of course there is no excretion matter to increase black hole mass). Yet the field strength of the black hole is far greater than the progenitor star despite having a smaller mass.

It's the metric rather than the scalar curvature that one looks at that is an indicator of field strength.

The two defining metrics for black holes are Schwarzschild metric for non rotating black holes and the Kerr metric for rotating black holes.

The Schwarzchild metric when used for low gravity field objects such as the Earth breaks down into a simple spherical metric.

Regards

Steven

[Nesti]

Thanks Steven,

So, just comparing a non-rotating black hole and just your average star or massed body; what would be the governing factor which forces you to drop the Einstein Equation and adopt the Schwarzchild Metric, that is, where does GR break down...is it that the energy density (on the right side) goes to infinity and the Einstein Tensor (left side) cannot balance the whole equation or something?

The Schwarzchild metric (http://en.wikipedia.org/wiki/Schwarzschild_metric) looks totally different from the Einstein Equation (http://en.wikipedia.org/wiki/Einstein_field_equations)...it looks like a polar coordinate system, which would explain the non-rotation part. Also, I can see how the Kerr Metric (http://en.wikipedia.org/wiki/Kerr_metric) came from he Schwarzchild Metric but it uses something I've never heard of, the Oblate Spheroid Coordinate (http://en.wikipedia.org/wiki/Oblate_spheroidal_coordinates) system.

Interesting that a Schwarzchild black hole can have any mass, but is limited only by the the conditions of its formation. SO, that means a grain of sand can become a black hole, but only if the conditions are favorable in its formation.

So who's right here??? Scientists at the LHC have been claiming that if in the unlikely event that a tiny black hole was created in a collision, it would instantly collapse, yet here we have a statement saying (subject to initial conditions for its creation) ANY size Schwarzchild black hole can exist. So I wonder how much system energy is required to sustain a Schwarzchild black hole.

"This suggests that there must be a lower limit for the mass of black holes. Theoretically this boundary is expected to lie around the Planck mass (http://en.wikipedia.org/wiki/Planck_mass) (~1019 GeV/c2 = ~2 × 10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[citation needed] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that this bound could be much lower. Some braneworld scenarios for example put the Planck mass much lower, may be even as low as 1 TeV/c2.[49] This would make it possible for micro black holes (http://en.wikipedia.org/wiki/Micro_black_hole) to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists."

[sjastro]

The solutions to Einstein equations are the metrics themselves.
In the case of the Schwarzchild and Kerr metrics, these represent gravitational fields extended into empty space from a mass source (planet, star, black hole etc). As a result the energy density tensor is equal to zero. Hence the metrics are solutions to the Ricci tensor.



For rotating black hole there are two "event horizons." One is the familiar spherical event horizon associated with the Schwarzchild (non rotating black hole), the other is a flattened horizon that extends out from the black hole. The oblate spheroid coordinate system represents this flattened horizon.
Ever wondered why accretion disks exist around black holes instead of accretion spheres? Thats because the matter is swept up into this flattened horizon.



Yes any object has a theoretical Schwarzchild radius, which is the radius required to shrink an object to a black hole. For example the Earth would need to be shrunk to the size of a ball of 0.89 cm radius to become a black hole. However there are no known physical processes that will shrink a grain of sand or the Earth to such dimensions.
That can only be achieved in the cores of massive stars when gravity eventually overcomes the energy produced by nucleur fusion.
Cosmic rays may possibly provide sufficient energy to produce mini black holes during collisions between particles.



The black hole doesn't collapse but evaporates. This is an application of Quantum Field Theory (QFT) to Black Holes.
In QFT there is no such thing as a perfect vacuum, a vacuum contains energy. As a result virtual particle/antiparticle pairs can be created in a vacuum (this has been experimentally verified through the Casimir effect).

If a virtual pair is formed near the event horizon of a black hole, the negative virtual particle is invarably "pilfered" by the black hole while the positive particle escapes. This violates the conservation of energy and black hole must therefore radiate thermal energy back into space.

A Schwarzschild event horizon is therefore required to explain the evaporation of black holes.

Regards

Steven

[AlexN]

With relation to Alex's comment regarding black holes being a binary system, Wouldn't gravity do its normal thing, with the two black holes orbiting each other untill the gravitational force eventually brought them together much like what happens with binary stars. The mass of one is usually greater than the other, and as a result, the smaller of the two ends up coming too close to the larger one, usually resulting in a pretty display of fireworks..

From what I understand two black holes would obviously not explode into a supernova like stars would, rather, one would swallow up the other, the accretion discs would merge, and the mass of the now single black hole would be equal to the sum of its parts...

If that is true, then clearly there would be single black holes out there...

Although, gravity being the way that it is, eventually you would think that 2 super massive black holes would eventually seek each other out, both drawn to each other by gravity, creating a new binary pair, and eventually a larger, single black hole...

I believe Steven is right with regards to the black holes swallowing up negative particles and spitting out the positive particles as thermal energy.. I was watching a very interesting documentary the other day "Hawkings Universe", Hawking and some of the scientists at CERN agree.