questions about the cosmos

[Berzerker]

can black holes lose enough mass/spin etc to become visible?

i got more, but that one just hit me

existence is mind blowing

[loinburger]

I thought I read somewhere that a black hole will explode if it loses enough mass (at which point it ceases to be a black hole), but a google search for "exploding black hole" only pulled up bull**** results.

[Hauldren Collider]

I have read that black holes, if they are small enough, gradually lose mass through blackbody radation and eventually dissipate if they are not large enough to pull in more energy than they release. However, they will never become "visible" in the sense I think you mean.

[BlackCat]

Just don't extract too much matter from a black hole and you should be safe

[DaShi]

Tell that to the greedy corporations.

[Ben Kenobi]

Actually, they evaporate given enough time.

[OneFootInTheGrave]

Tell that to the greedy corporations.

QFT

[Berzerker]

but if gravity determines the body's "blackness", why wont the loss of gravity make it more visible?

must be other factors making it black that wont allow visibility with the loss of mass

maybe once stuff is compacted into a black hole the density stays ~the same even if mass is lost and density matters

[Ben Kenobi]

but if gravity determines the body's "blackness", why wont the loss of gravity make it more visible?

As the black hole loses mass, the event horizon shrinks. The singularity remains unchanged.

must be other factors making it black that wont allow visibility with the loss of mass

Actually, in theory, hawking radiation should be observable... A black hole emits a tiny, tiny, tiny amount of radiation though. Something on the order of 10 to the negative 30th power watts.

maybe once stuff is compacted into a black hole the density stays ~the same even if mass is lost and density matters

Yep. Where it reaches infinite density you get a singularity. infinity/2 = infinity.

[snoopy369]

Gravitational force is determined by mass, not density. Jupiter, for example, has a wicked gravitational pull, but has a density of only 1.33 g/cm^3, not much more than water (1 g/cm^3). The Earth on the other hand has a density of 5.52 g/cm^3 (all numbers from Google). Jupiter is just really, really big.

IANAP, but if I remember correctly, black holes are super-dense because of their mass - not the other way around (ie, the density is a result, not a cause). Less massive black holes (not sure how that would form, but they are theoretically possible) would be smaller; at a certain size they will tend to evaporate.

Basically, a black hole gives off hawking radiation, which is mass (a very small amount), and absorbs radiation (at minimum, the cosmic background radiation). If the former is greater than the latter, it will eventually evaporate; if the latter is greater than the former, it will grow.

"Become visible", however, is not a very useful term. Black holes warp space-time when they exist; when they cease to exist, they would cease to warp space-time. I don't know if anyone truly knows what would happen though as this occurred; would there at some point be a visible object? Nothing I've read suggests that there would, but I doubt there's been enough research to determine this for sure.

[Ben Kenobi]

Snoopy, all black holes are visible. They just are too dim to be seen. You could stand right next to a black hole, and you wouldn't be able to see it, (you could feel it though), and you'd notice it lens whatever it is in front of.

IANAP, but if I remember correctly, black holes are super-dense because of their mass - not the other way around (ie, the density is a result, not a cause). Less massive black holes (not sure how that would form, but they are theoretically possible) would be smaller; at a certain size they will tend to evaporate.

Black holes are actually the most and the least dense objects in the universe. The radius of a black hole is directly proportional to the mass, whereas for all other objects, the radius is an inverse square, meaning that for more massive objects, the radius shrinks. Massive black holes actually grow with their mass.

[snoopy369]

Black holes are not visible, Ben. Black holes are observable (both by their effects on space-time and the resultant gravitational lensing when observing objects behind a black hole, and their Hawking radiation), but 'visible' has a very specific meaning...

I don't entirely understand why you think more massive objects decrease in radius. I assume you're referring to gravitational compression; sure, but that's not a good way to describe it. If I take a ball of wax, and add some more wax, the radius increases as the mass increases... just not by quite as much as you would expect if there weren't compression factors from the increase in mass. That's rather different from saying the radius shrinks, which is patently wrong.

[Lefty Scaevola]

Hawking radiation might qulify as "visible" under some definitions, but since it is too faint to be detected at any signigfcant distance or amongst the emissions of infalling matter even for a close observer, that is mostly irrelevant.

[Dr Strangelove]

I thought the whole point of a black whole is that its gravitational field is so powerful that no form of mass or EM radiation can escape? At the periphery of the event horizon of a black hole matter that's being crushed may emit radiation that escapes because it's outside the zone where gravitational forces prevent the escape of EM radiation.

[The Mad Monk]

Hawking radiation is a rather bizarre form that involves both quantum fluctuations and quantum tunneling, IIRC. It occurs when a particle and its antiparticle abruptly appear in the vacuum just outside an event horizon, and one falls in while the other is shot away; the one that falls in has negative energy, or energy cannot be conserved -- which cannot happen. When a particle with negative energy falls in, the total energy (and mass) is reduced.

I did say it was bizarre.