[Detrus]

Most of their science paper is in layman English http://arxiv.org/pdf/0908.1803v1 and explains such questions.

The black holes they would build are very very tiny, about the size of a proton. They are relatively light, about 600,000 tons. Such a body doesn't have noticeable gravity, it's too light to attract anything and too small to absorb it. The big danger from these are explosions at the end of their life. The explosions are serious if they happened near Earth, but safe at the distance of Earth to Sun. They're built near the host star anyway.

[troels]

? I thought it was the mass that made a black hole - not the density?

[Confusion]

There is a minimum amount of mass necessary for an object to be able to collapse upon itself and form a black hole, such as a large star does when the outward pressure due to the internal fusion reaction stalls.

However, any amount of mass can (in classical theory) be compressed far enough to obtain a Schwarzschild radius, from which light cannot escape. This has only to do with the density, not the total mass: a very small mass can still cause a large curvature of space, though only in a very small region of space.

[troels]

OK, now that we're already discussing this topic. I just read this quote from Wikipedia:

"""If one accumulates matter at nuclear density (the density of the nucleus of an atom, about 1018 kg/m3; neutron stars also reach this density), such an accumulation would fall within its own Schwarzschild radius at about 3 solar masses and thus would be a stellar black hole."""

I take that to mean that if I wanted to create a black hole of something with less mass than 3 suns, I would have to compress it beyond the density of an atom nucleus? Is this - even in theory - possible to do? Wouldn't you need some kind of "magic wand" (to stick with the articles authors choice of words?)

[pavel_lishin]

I have strep throat and may not be at my best right now, but iirc the chandrasekhar limit is 1.5 solar masses - it's enough to form a black hole because not only is there a lot of mass, but it's also falling into the center, compressing everything further. So one of your "magic wand" options is acceleration, I think.

[Confusion]

Well, that is why I said 'classical theory' :). I have no clue whether QM allows it and I don't think anyone does: that would amount to knowing the true nature of the 'singularity' inside a black hole.

[jules]

So you could say that all point masses (e.g. electrons) are black holes?

What happens if you take a large black hole, and you throw a lot of electrons into it? Does it get an electric field measurable from the outside? If not, how come an electron does have this field?

[lutorm]

An electron is only a point mass in classical electrodynamics, which leads to all kinds of inconsistencies (the self-energy of its field would be infinite, for example.) In quantum mechanics, there is no such thing as a point mass.

But in any case, the answer to your question is yes. Black holes have 3 quantities: mass, angular momentum, and charge. So, yeah, you can charge up a black hole by dropping charges into it and the charge would be visible to the outside.

[adrianN]

No, you just require a sufficient pressure to overcome the repulsive forces between the particles to collapse them to a singularity. That's why people were afraid of the LHC.

[lutorm]

"just"...

Nuclear matter is very stiff. It can make a mass of the order of the Sun's, falling down with a significant fraction of the speed of light, bounce without becoming a black hole. So the pressure you'd need to make a black hole with higher-than-nuclear density would be, well, astronomical.

[troels]

Ah .. And normally (If that is a word which makes sense in this context), the mass of a giant object (Like a star) would be the way to create this pressure? But theoretically you could do it by other means?

[weavejester]

You essentially need to cram a lot of mass into a very small space, which is equivalent to cramming a lot of energy into a very small space.

So, theoretically, you can get a bunch of very, very large lasers, focus them all upon a very small point, and if you pump enough energy into the system you wind up with a black hole.

[billswift]

You need sufficient mass for a black hole to be stable, otherwise it "evaporates". A very small black hole would evaporate so fast that "explodes" would be a better term.