black holes

Dremmer

Are they really black? And are they really holes? I'd say they are actually neither. Would I be right?

Nabeshin

a) Classically, yes they are black. What I mean is that the obvious boundary of the object, the event horizon, is a one way barrier in that things can only go in, not out. This includes light. So an object which is absorbing all light but giving off none would indeed appear pitch black.

Quantum mechanically, the story is not so simple and it turns off that black holes do 'shine' a little. However, this effect is far too miniscule to be important for any astrophysical (read: caused by collapsing stars) black holes.

b) The word hole, in this sense, doesn't really have a precise meaning. Perhaps what is meant is that they are 'holes' in the fabric of spacetime, but this is equally imprecise. Usually this is seen visually by having an embedding diagram in which the BH literally looks like a type of funnel, but this is an artifact of the way we choose to draw these diagrams. In another sense, perhaps they are holes in that things fall in but do not come out. I don't really know what is meant by the assertion of them being just 'holes', so I can't really say.

Chronos

While some physicists suspect they may be gateways to other universes, it is definitely a one way street.

T=0

Perhaps, but you would be pulled apart by the immense gravitational pull before you got through it, which is why we don't know for sure. Also, I don't think they are "holes" so much as spheres, because, after all, they are collapsed stars.

Feodalherren

They aren't really spherical or star-like in any sense. They are infinitely small, you could argue that it's just plain weight.

Chronos

Their dimensionality is typically defined by their event horizon, which extends a measurable distance from the putative singularity.

Drakkith

The event horizon is a spherical region of space (in a non-rotating black hole) that is simply the point where the curvature of space becomes such that all paths in spacetime lead into the black hole. In more classical terms it's where the force of gravity become strong keep even light in. One inch out from this barrier the force is ALMOST as strong, but not quite, and the force is reduced with distance.

Naty1

Illustration here:

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

T=0

Surely that means that super-massive black holes could work as the cosmological constant, along with dark matter, but also could bring the end to whole solar systems, maybe even galaxies.

Chronos

Supermassive black holes are known to exist at the center of most large galaxies - some with masses on the order of 10 billion solar. They are not known to eat the entire galaxy, or corrupt neighboring galaxies.

Whovian

@T=0
Not really. For example, if the Earth suddenly condensed to the size of a BH (for whatever reason,) any observer on the Moon wouldn't notice a thing other than, of course, the Earth shrinking. Shrinking something doesn't mean it becomes superpowerful gravitationally, it just means stuff can get closer before being held up by its surface.

Alex1

Are Black Holes 2 Dimensional? And if so does that mean that when a 3 dimensional object is sucked into one that it is somehow reassembled as a 2 dimensional object and it's 3D characteristics are ripped from it and left to hover the black hole?

Alex1

Also, I read somewhere that Supermassive Black Holes less dense than water, is this true?

Nabeshin

Perhaps in some sense, but I doubt it's the sense you're thinking of. The entropy of a black hole is proportional to the area of its event horizon, a two dimensional surface, rather than the total volume of the black hole. Nevertheless, they occupy a definite three-dimensional volume in our universe (as long as we're calling anything within and including the EH the 'black hole').

And yes a SMBH can indeed be less dense on average than water. I speak on average because the calculation is done by simply dividing the total mass by the volume of the sphere enclosed in the event horizon. The radius of the the event horizon scales like M, so volume goes like M^3. Thus, density = M/Vol = M/M^3 = M^-2, so you can make the average density arbitrarily small simply by increasing the mass.

Alex1

When you use the term Entrophy in the context of a Black Hole, is that just the rate at which it's evaporating?

Nabeshin

Well since entropy is not in any way a measure of rate of evaporation no. The quantities are related in a thermodynamic context (and arose basically at the same time as Hawking and Bekenstein were figuring these things out), but the entropy is really just the standard entropy of statistical mechanics: a counting of microstates.

Alex1

Alright cool Thanks