Thicko's Question About Star Formation

[Mintaka]

if a black hole was visible, what would it look like?

[Mintaka]

if a black hole was visible, what would it look like?

 

I think it might look like a plughole? :blink:

[CraigD]

if a black hole was visible, what would it look like?

In the sense that we can detect radiation created by them, most black holes are visible. Stellar mass black holes attract matter to form hot, accretion disks, often with polar jets, emitting light of as low frequency as IR to as high as xray. From the linked-to wikipedia article:

 

 

Supermassive black holes can be even more spectacular – some of the brightest objects in the universe, quasars, are supermassive black holes surrounded by huge accretion disks. So it’s reasonable to say that black holes look like these objects.

 

The event horizon of a “naked” black hole – one with no matter outside its event horizon, which given how much stray gas is found everywhere is space, likely don’t exist - would in theory be very faintly visible, due to its Hawking radiation. Assuming the black hole didn’t have a lot of angular momentum – that it had “spun down” since its formation, which most theory predicts they’ll do – its event horizon would look like a nearly perfect sphere. What a very fast spinning black hole would look like is a subject of much theoretical speculation and controversy – some believe it might expose a “naked singularity”, though what that would look like isn’t clear.

[Mintaka]

In the sense that we can detect radiation created by them, most black holes are visible. Stellar mass black holes attract matter to form hot, accretion disks, often with polar jets, emitting light of as low frequency as IR to as high as xray. From the linked-to wikipedia article:

 

 

Supermassive black holes can be even more spectacular – some of the brightest objects in the universe, quasars, are supermassive black holes surrounded by huge accretion disks. So it’s reasonable to say that black holes look like these objects.

 

The event horizon of a “naked” black hole – one with no matter outside its event horizon, which given how much stray gas is found everywhere is space, likely don’t exist - would in theory be very faintly visible, due to its Hawking radiation. Assuming the black hole didn’t have a lot of angular momentum – that it had “spun down” since its formation, which most theory predicts they’ll do – its event horizon would look like a nearly perfect sphere. What a very fast spinning black hole would look like is a subject of much theoretical speculation and controversy – some believe it might expose a “naked singularity”, though what that would look like isn’t clear.

 

 

 

Thank you Craig, so we can only see them by the effect they have on what is around them, I see.

 

But my question was slightly different. It seems the only reason we can't see them is because their gravity is too strong for light to escape.

 

But what if it wasn't? What if a black 'hole' itself was visible?

 

Would it look like an ordinary star, would it be funnel shaped? Do we have any idea of what one might look like even though we can't see it?

 

After all, it IS there, isn't it? Just because we can't see it , it's there.

 

Tricky question I know...but hopefully interesting to some.

 

ps- I think the crux of it is, what SHAPE is it? Is it actually like a plughole or would it be just like an ordinary star?

[CraigD]

But my question was slightly different. It seems the only reason we can't see them is because their gravity is too strong for light to escape.

 

But what if it wasn't? What if a black 'hole' itself was visible?

 

Would it look like an ordinary star, would it be funnel shaped? Do we have any idea of what one might look like even though we can't see it?

To answer this, we’d need to have a better theory of gravity than we do now.

 

If you assume that gravity behaves as a classical mechanical force, a star-mass black hole (remember, black holes come in several very different classes, determined by mass) would look like nothing at all, because all of it would be compacted into a zero diameter “singularity”. Some sort of “light that isn’t affected by gravity”, like you propose wouldn’t be affected by it at all, rendering it completely invisible.

 

Though nobody’s yet come up with a compelling and complete gravity theory sufficient to answer your question, most physicists I’ve read speculate that gravity doesn’t behave this way, but instead get’s “quantum weird” under these conditions, in which case a black hole would be made of some variety of “quantum foam”. Its diameter wouldn’t be zero, but might be very small, perhaps on the order of the Planck length of about 10^-35 meters, or maybe much larger, approaching the diameter of a neutron star around 10 km.

 

I think the crux of it is, what SHAPE is it? Is it actually like a plughole or would it be just like an ordinary star?

If the “something there” is big enough to sensibly be said to have a shape, my guess would be that for their first few millions of years, while they still have a lot of angular momentum (that is, they’re spinning very fast, like a figure skater pulling their arms and legs in tight), they’d bulge a lot along their equators. After a few hundred million years, as their rotation slows, whatever weird stuff they’re made of would be nearly spherical.

 

The inside of a supermassive black hole might be very ordinary, perhaps even containing stars and planets. It’s possible that the entire universe is inside a universe-mass black hole, in which case we, being inside it already, can see what it looks like.

[Mintaka]

To answer this, we’d need to have a better theory of gravity than we do now.

 

If you assume that gravity behaves as a classical mechanical force, a star-mass black hole (remember, black holes come in several very different classes, determined by mass) would look like nothing at all, because all of it would be compacted into a zero diameter “singularity”. Some sort of “light that isn’t affected by gravity”, like you propose wouldn’t be affected by it at all, rendering it completely invisible.

 

Though nobody’s yet come up with a compelling and complete gravity theory sufficient to answer your question, most physicists I’ve read speculate that gravity doesn’t behave this way, but instead get’s “quantum weird” under these conditions, in which case a black hole would be made of some variety of “quantum foam”. Its diameter wouldn’t be zero, but might be very small, perhaps on the order of the Planck length of about 10^-35 meters, or maybe much larger, approaching the diameter of a neutron star around 10 km.

 

 

If the “something there” is big enough to sensibly be said to have a shape, my guess would be that for their first few millions of years, while they still have a lot of angular momentum (that is, they’re spinning very fast, like a figure skater pulling their arms and legs in tight), they’d bulge a lot along their equators. After a few hundred million years, as their rotation slows, whatever weird stuff they’re made of would be nearly spherical.

 

The inside of a supermassive black hole might be very ordinary, perhaps even containing stars and planets. It’s possible that the entire universe is inside a universe-mass black hole, in which case we, being inside it already, can see what it looks like.

 

 

hahaha I love that last line, thank you! :D