Not a supermassive black hole like in the center of galaxies, and not also a microscopic one. Just a spaceship sized one.
Assuming that for some reason the sensors are down (hostile alien attack) and you’re flying blindly, would a black hole be visible at all? I know that it emits all kinds of radiation, but would there perhaps be some kind of halo around it as it sucks up matter and light? Would it swirl like in artists’ conceptions?
On a side note, in all drawings I’ve seen of black holes, the artist makes it look like a flat, almost two dimensional circle. But they’re actually three dimensional aren’t they? It would be more like a sphere than a circle?
You wouldn’t ever be able to observe the singularity itself. The event horizon is larger than the theoretical physical size of the black hole. I’m not sure about your sizes, though – I believe an Earth mass collapsed object would be about baseball size. A spaceship sized black hole would be substantially more massive.
The “usual” observable types are stellar masses. There would be quite a bit of radiation produced as gasses and debris are drawn in. Some of it would be in the visible spectrum because of heating effects, xray absorbtion, etc etc. A black hole out in interstellar space might not have an accretion disk, though.
Given the proper background, you could also see gravitational lensing as light bends around the black hole.
As for the artists depictions, an accretion disk is generally flat, and there will often be jets of matter expelled “up” and “down” from the center. This is theoretically due to the spin of the black hole, in much the same way as our solar system is generally flat (not counting the halo or the oort cloud).
You won’t ever see the black hole itself, but there are things you can see and detect.
First, the disk-shape (the accretion disk) you always see in artist drawings would be the result of material being sucked into the black hole, and you would expect to see it as a disk. The process tends to heat the gas being drawn in, and it produces all manner of EM radiation, from infra-red to X-rays. Some of those would be visible even without sensors. Cygnus X-1 is a good example. However, this depends on enough gas or other material to be available.
Second, you might detect Hawking radiation even (or especially) if there was no accretion disk. I’m not sure how obvious the radiation would be with a black hole of any given size, but any source of X-rays in the middle of nowhere would be of interest. Sensors would probably be necessary for this.
Third, you might detect gravitational effects, either by monitoring your own velocity or by looking at the orbits or trajectories of other moving objects. There’s even a lensing effect on objects behind the black hole. The smaller the black hole, the less obvious this would be. Whether or not you could use this method without computer assistance would depend on a lot of factors, like how much stuff was around and how much attention you were paying.
Only for very small black holes. A stellar-mass hole has a radius of perhaps 10 km and a temperature of about a millionth of a Kelvin. Radius is directly proportional to mass, and temperature is inversely proportional to mass, so a black hole would have to have a radius of a few millimeters or less (mass comparable to the Earth) before it’d even be warmer than the cosmic background radiation.
There was a recent thread here in which somebody linked to a page containing some really nifty visualizations of what the gravitational lensing would look like. (Or, in many cases, what it would look like if you could actually get sufficiently close without being torn apart by the tidal gradients. :p)
I presume you are referring to the black hole itself, i.e., the actual hunk of incredibly dense neutrons and such.
The event horizon would be much larger, wouldn’t it? And the event horizon would be spherical, right? And even if we discount gravitational lightwave bending, there’s a lot of light that would be blocked because the event horizon is between my eye and some star which is far beyond the black hole.
So, if I got all that correctly: How large is the event horizon for a star that’s currently only 10 km in radius?
Our own sun, if it were to become a black hole (which it can’t), would have a diameter of about 6km. So, as noted in some book I read, could comfortably fit within Manhattan.
Note that is the event horizon diameter. The mass itself is squished into point in the middle of the black hole.
EDIT: Yes, it would be a sphere with a 6km diameter although if it was spinning it be a ring (like a doughnut) I believe.
Well, I think QM has it as a point. However, I think part of the problem with getting QM and General Relativity to agree is that GR does not handle points well…the geometry gets all messed up and you get a lot of infinities mucking up the works. This is all way past me but I think String Theory would replace the point with a Planck Length object. So, from our perspective a point but would actually have a size associated. Just a very, very, very small size.
So, not an infinite number but a helluva lot…forgetting for the moment that a naked singularity is impossible (I believe).
My mind breaks down trying to imagine “real world” one dimensional objects. I blame my lack of college education.
As I understand it, matter has a lot of “space” within it. There is a measurable distance between atoms, etc.
I just imagined black holes as objects where every teenie bit of space is packed “solid” [sic] with some form of sub-atomic goo (that is, all the matter was converted into some type of basic particle under the stresses occuring during the collapse & formation of the black hole).
You just have to think of volume as a field property. You can stand on the Earth because it supports you. That support comes from electromagnetic and weak nuclear bonds holding molecule to molecule, atom to atom and electrons to nucleus. In other words, 99.99% of the volume of the Earth is due to the same effect that keeps two magnets apart when you line up the two poles. Eliminate those field forces and the Earth collapses into a neutron star, basically.
But neutrons are bound to each other thanks to the strong nuclear force - it’s just another field. Neutrons themselves are made up of quarks bound together by the same strong nuclear force. If you take an anti-neutron and hit a regular neutron with it, you get no volume at all, but a whole lot of energy. This goes back to the whole E=MC2 equivalence between matter and energy. Either way, if you strip out the forces holding subatomic particles together, you strip out their volume as well.
Eventually, with black holes, we get to a point we don’t fully understand, but (if singularities really are a one dimensional point) I like to think of it as simply a point at which you’ve stripped out all of the fields that generate the illusion of volume at every level.
Matter does have a lot of space in it. In fact normal matter as we experience is by far mostly empty space.
As an example a step towards a black hole that can be stable is called a Neutron Star. Ordinarily electrons resist being packed in via electron degeneracy pressure. This is pretty significant. However, if gravity is strong enough it overcomes this pressure and the electrons get packed into the nucleus of its atom. To get an idea of the size difference a teaspoon of neutron star material has the mass of Mt. Everest.
There are other degeneracy pressures at work which resist further collapse. It is hypothesized there could be a Quark Star. Put under enough pressure the Neutron Star overcomes neutron degeneracy pressure and packs in even closer and the atoms break down into their constituent quarks.
A black hole is the ultimate end to all this. You now have everything packed in so unimaginably close I do not think scientists have a clue of how to describe what is happening there. Near infinite density and I would presume temperatures on the order of the Planck Temperature (highest temperature believed possible). If not that hot then getting there. At such high temperatures the four fundamental forces (weak/strong nuclear, electromagnetic and gravity) are believed to unify into a single force. Again, how such a thing behaves no one knows. You really are off the charts in weird territory here. It is probably a good thing there are not naked singularities (cosmic censorship hypothesis and reading the link above looks like they may exist but for my money I am hard pressed how that could be even after reading about it).
In order to collapse into a black hole you need lots and lots of gravity. As noted above electron degeneracy pressure (among other kinds of degeneracy pressure) resist getting everything all smushed together. If the star is not big enough there is not enough gravity to overcome this so the star will not collapse.
In our sun’s case it will go through some various stages at the end of its life but ultimately settle down as a White Dwarf star (no fusion occurring but cooling off over unimaginably long times).
IIRC you need a star around 7-8 solar masses to make it to a black hole (the resulting black hole will be around 4 solar masses…a lot of the star gets blown off before the core collapses).
Presumably you can make other things into a black hole but you’d have to work at doing it. They will not form from lower masses of their own accord.
Isn’t the usual line that the event horizon diameter/radius is the smallest relevant measurement of the ‘hole’, because any density breakdown beyond that is only theoretical?
As in, it takes you an infinite amount of subjective time to fall past the event horizon, and you can never report back what you find what when you get there anyway.
Smallest relevant measure inasmuch as it affects us. Everything past the event horizon can almost be considered to be out of our universe. Whatever happens in there can have no effect on us (I suppose gravity waves can have some effect and perhaps Hawking Radiation).
I am not sure about the time thing. I think I asked here once before that someone falling in to the black hole ever actually reaches the singularity. Time seems to stop at the even horizon (which would seem to make it a shell or ring). I was going on the premise that once in the event horizon the singularity is described as always in your future.
I forget the explanation but I think I was told I was mistaken. “Always in the future” just meant there was no path to avoid it and one way or another you will get up close and personal to it.
Neither GR nor quantum mechanics has any particular problem with geometric points, and quantum mechanics as currently formulated has nothing at all to say about black holes. It’s widely suspected that a true theory of quantum gravity will reveal that the “singularity” of a black hole is not a true point, but that’s just an educated hunch, and is not the motivation for developing a new theory for quantum gravity.
All discussions of the size of a black hole are always referring to the size of the event horizon, since that’s the only thing about a black hole that can be described in terms of size (at least, until we get a theory of quantum gravity). In the simplest case, for a non-rotating black hole, the horizon is perfectly spherical, and that’s the case that’s discussed most often (precisely because it’s simple, of course). In the real world, most black holes are expected to have a significant rotation (in fact, probably close to the maximum possible rotation), which causes the horizon to have a slightly flattened ellipsoid shape.
In principle, black holes can also have electric or magnetic charge, but it’s expected that a typical stellar black hole will have a charge of only a few times the charge of the electron (positive or negative), and magnetic charges are completely unheard-of in our Universe (except for that one that was detected at Stanford in 1982, but nobody’s sure what to make of that). Beyond mass, angular momentum, electric charge, and magnetic charge, black holes have no other properties whatsoever, making them the simplest objects known in the Universe.
Thanks, dracoi & Whack-a-Mole. I have no doubt that you are correct in what you explain as the current theories of what a black hole is like on the inside.
These are what I meant by “solid” (although I am not as skilled as y’all at communicating clearly). Never-the-less, it seems counter-intuitive to me that the entire matter-mass of something 8+ solar masses in… errr, mass… can all compress into a one dimensional point (essentially zero distance-units across), not counting it’s event horizon (it’s “area of effect”, if you will).
Even if you take away all the space between Quarks (or whatever) that make up each sub-atomic particle, there should be a point where you reach the smallest (in volume) component of matter/energy in the universe, something that cannot be further divided or shrunken. Even with the distance between each of these items is zero, each item itself should take up a non-zero amount of volume. (Planck length is the smallest, an non-dividible unit of measurement, right? A discreet object should not take up less than 1 Planck unit in size, should it?)
That energy, whichever form it takes, must still take up some kind of volume, doesn’t it? How big is a photon? An X-ray particle? A Neutrino?
Would this energy release you describe not violate the Law of Conservation of Energy if, in some way similar to matter-antimatter annihilation, not return the same mass in the form of (a whole lot of) energy particles?
(I know we haven’t figured out what makes gravity or magnetism work quite yet…heh.)