In response to an earlier post about all posts being boring I would like to pose a question that has been bothering me this last week. I’ve been thinking a lot about black holes but something just doesn’t add up…

On the one hand a black hole is defined as a quantity of matter so dense that not even light can escape. This makes sense, the denser an object of given mass the greater the gravitational force felt at its surface and therefor the greater the escape velocity. It stands to reason that an object may reach a density so great that the escape velocity will be equal to the speed of light. The dense object is completely inside it’s so called event horizon, not even light can escape and the object is quite rightly described as a black hole. Nothing can escape from the black hole so the only clue to its existence is the gravitational attraction felt by nearby objects.

All well and good, so how does a black hole form. Well this makes good sense also. When a star starts to use up all of its Hydrogen fuel the thermal energy it produces can no longer support its mass and the star begins to collapse in on itself under its own gravity. As the gravitation attraction increases atoms themselves are ripped apart to form a thick soup of elementary particles. The stable end state for the collapsing star depends on its size. A smaller star will collapse to form a white Dwarf with a diameter of a few thousand km. At this point the collapse is halted when the ever increasing force of gravity is balanced by the electron degeneracy pressure, an artefact of the Pauli Exclusion Principle which disallows two half-integer spin particles from simultaneously occupying the same quantum state. A larger star will collapse still further; gravity is so great that even electron degeneracy pressure is overcome. Electrons and protons combine to form neutrons and the collapse is only halted by neutron degeneracy pressure. This neutron star has a diameter of only a few km and barely conceivable density of up to 6×1017 kg/m3. If a star is bigger still then it will reach a density where no law of nature can halt its collapse. The star will collapse to a single point, a singularity where the laws of physics no longer apply and crazy things like worm holes may exist; also a black hole.

So back to my question…… Do all black holes contain singularities?

The description above (which may be flawed) seems to suggest 2 separate densities at work. The density of an object from which not even light can escape – a black hole; and the density at which quantum degeneracy pressures can no longer support a body at which point it collapses to form a singularity – also a black hole. So are these 2 densities the same or is it possible to have a super dense object from which light cannot escape that has yet to reach the point of ultimate gravitational collapse. If so why are these 2 densities the same? It seems to be quite a coincidence.

Just to keep things topical I’m also interested in what would happen if I rode my bike into a black hole? Would I be spaghettified or would I ride through a worm hole and discover a new universe?

What wheel size for wormholes?

Simple answer. We don’t know.

Mind you, that’s probably incorrect as someone on STW undoubtedly, definitively will 😉

If you rode through a black hole (or really, got close), you’d be crushed into a messy, very small blob long before you got there.

No. Stockport is a fine example of a life sucking black hole where there is no light and absolutely nothing going on in the middle.

I believe 29er roll over more trail obstacles such as blackholes.

Reports suggest 650b brings the black hole alive.

So I guess in summary, only on 26″.

Not all but some cause a time rift continuum in forums, dragging posts into the wrong forum.

Luckily our tech boys have come up with experimental device for mods to try to move them.

So hang on here we go.

Ahhhhhh!

Good post OP.

I think you answer it yourself though

If a star is bigger still then it will reach a density where no law of nature can halt its collapse. The star will collapse to a single point, a singularity where the laws of physics no longer apply and crazy things like worm holes may exist; also a black hole.

So once past the point of neutron degeneracy pressure it has no where left to go but become a singularity.

So I guess the question is then could a neutron star have sufficient density to form a black hole without collapsing to a singularity?

Skoda Octavia or a Ford Mondeo.

Would I be spaghettified

As far as your bike’s concerned, I would imagine titanium and steel would be spaghettified, carbon and aluminium would be fine…obviously.

I like old peculiar which is black and it goes in my face hole and if this happens today will be the singular best thing to happen all day, ermm, yep!

So I guess the question is then could a neutron star have sufficient density to form a black hole without collapsing to a singularity?

Exactly that Richmtb!
Do you know the answer??

I believe 29er roll over more trail obstacles such as blackholes.

Reports suggest 650b brings the black hole alive.

So I guess in summary, only on 26″.

Im inclined to agree
I think the inertia of the 29er wheels would be too great to keep the unstable worm hole open – unless the rims were made of antimatter.

Yet another reason to stick with 26 😀

How do gravitons escape too? I don’t understand any of this.

Someone here thinks the answer is Yes to the op’s question…
http://physics.stackexchange.com/questions/24934/do-all-black-holes-have-a-singularity

Simple answer. We don’t know.

Slightly more complicated answer: in classical GR anything with sufficient density to form a horizon will collapse to a singularity behind it. But we know that GR by itself is incomplete and it’s possible that in a full quantum theory of gravity, space itself will have some sort of quantised structure which halts the collapse.

How do gravitons escape too?

They don’t. Think of the external gravitational field of a black hole as a kind of “fossilised” field left over from all the mass and energy that’s fallen in.

What wheel size for wormholes?

Dunno, but I hear that 650b really makes 9-dimensional compactified calabi-yau manifolds come alive…

What tyres for singularity traversal?

So I guess the question is then could a neutron star have sufficient density to form a black hole without collapsing to a singularity?
Exactly that Richmtb!
Do you know the answer??

Well a typical neutron star has an escape velocity of around 100,000km/s about 1/3 the speed of light so wouldn’t be sufficiently dense to form a black hole.

So any star that isn’t sufficiently dense to break past the neutron degeneracy limit can’t become a black hole.

There are other proposed stable ultra dense stars such as quark stars that might be sufficiently dense to form black holes without having a singularity at their centre but at the moment they are purely hypothetical

So I think at this point its safe to assume all black holes contain a singularity

if light is a constant, how can it NOT escape a black hole, is it that its bending back on itself, or its just not moving out of the hole (therefore not a constant!)

Photons have a velocity, right? So if the escape velocity for the black hole/collapsed start is greater than the speed of a photon, then it can’t escape, ergo black hole.

Not sure how that works with light also being a wave and with the speed of light being a constant though.

if light is a constant, how can it NOT escape a black hole, is it that its bending back on itself, or its just not moving out of the hole (therefore not a constant!)

Space time is curved so that all future paths of the light terminate at the singularity.

Slightly more complicated answer: in classical GR anything with sufficient density to form a horizon will collapse to a singularity behind it. But we know that GR by itself is incomplete and it’s possible that in a full quantum theory of gravity, space itself will have some sort of quantised structure which halts the collapse.

Cheers
so a black hole containing a stable mass is impossible but a black hole collapsing on its way to a singularity is.
Great – this makes much better sense now.

Is it also the case that once the mass has past the point of becoming a black hole time stands still from the point of view of observers beyond the event horizon; meaning the singularity never has time to form…..
unless you’re collapsing with the star and time is still running.

A thread about black holes? Well, this is a revelation.

A thread about black holes? Well, this is a revelation.

All this wet weather has led to me staying in and reading Steven Hawkins “A brief history of time” 🙂

Highly recommended

If we all bought Apple PCs we wouldn’t have these sort of problems to have to solve.

if light is a constant, how can it NOT escape a black hole

Semi-classical explanation: light speed is always measured as a constant but the light loses energy by climbing out of the gravitational field. This means the wavelength is getting more and more stretched out (aka redshifting). Light from behind the horizon has its wavelength stretched infinitely.

GR explanation: see richmtb’s post.

Slightly over-egged mind-bending pop-sci explanation: at the horizon, space itself is falling in faster than the light is coming out.

Is it also the case that once the mass has past the point of becoming a black hole time stands still from the point of view of observers beyond the event horizon; meaning the singularity never has time to form…..

In a hand-wavy sort way that’s true, although you can’t really say anything about the inside from an external perspective. On the other hand, if you’re actually falling in, then according to GR you won’t notice anything special as you go past the horizon (assuming it’s a large enough black hole that the tidal forces haven’t ripped you apart yet) and you’ll hit the singularity in finite time.

Nothing can escape from the black hole so the only clue to its existence is the gravitational attraction felt by nearby objects.

Hmmmmm.

I thought some of those cheeky Meson & Lepton types had been found to be entering the odd Black Hole being spun quite quickly in the same axis as the relevant galaxy faster than a fast thing and by a fast thing I mean light and being spewed from the BH in a sort of Jet type stream.

So this would suggest that if the odd stuff can get out of a BH that the singularity in the centre is dumping the odd stuff out to stop itself spinning around internally faster and faster until it falls out. (bit like a dizzy dog chasing it’s tail)

Which sort of answers the question: Yes, Maybe or No

Nothing can escape from the black hole so the only clue to its existence is the gravitational attraction felt by nearby objects.

Well black holes emit Hawking radiation – black body radiation that is inversely proportional to their mass. So as a black hole nears the end of its life it becomes a source of x-rays or gamma rays which can be detected

Also if a black hole forms in a binary system – eg alongside another star it will accrete material from its partner star, this material will get pulled round the event horizon at high speed, tidal forces will start to cause masses of friction on the accreted material that will heat up and begin to radiate x-rays too

But doesn’t the radiation actually originate from just outside the event horizon? So could the radiation not conceivably have come from some other source – not necessarily a black hole?

So just like the gravitational effects felt by the presence of a black hole, the radiation is as a consequence of, rather than directly from the black hole.

But doesn’t the radiation actually originate from just outside the event horizon?

Well strictly speaking its not the black hole that is emitting the radiation its the hot gasses being drawn in to the black hole. But these gases would not be hot enough to emit x-rays without the black holes influence

Hawking radiation is not actually very practical to detect due to the inversely proportional to size. The equivalent temperature of a stellar mass black hole is far cooler than the CMBR (its measured in nanokelvins).

The accretion region will extend well outside the horizon though which is where the visible/xray/gamma emissions come from.

Hawking radiation is not actually very practical to detect due to the inversely proportional to size. The equivalent temperature of a stellar mass black hole is far cooler than the CMBR (its measured in nanokelvins).

I didn’t know that, so almost all the detectable radiation is actually from the accretion material then.

What about at the very end of the black holes life as it evaporates down to a very small size?

Sorry

What about at the very end of the black holes life as it evaporates down to a very small size

What does this mean – I thought the black hole was already a singularity?

What does this mean – I thought the black hole was already a singularity?

Hawking radiation carries energy away from the black hole (even though it doesn’t techincally originate inside) so the horizon slowly shrinks aka “evaporation”.

What about at the very end of the black holes life as it evaporates down to a very small size?

In principle you get a brief hot flash at the very end of its life but you’ll have to wait a long time 🙂 for a stellar mass BH the theoretical lifetime is many many times the age of the universe. And in practice the rate of infall from the accretion disc will be far larger than any loss through evaporation.

Of course that does leave a possible far-distant future for the universe in which *all* the matter gets recycled through black holes into a bath of Hawking radiation…

Thanks elliptic, that does make sense. I’ll not bother setting my alarm for the cosmic fireworks then!

Simple answer. We don’t know.

We don’t but my uncle Alf does or at least did. He fell into a black hole once which was why he was so microscopically small. I didn’t know him that well, we don’t really get on that well with that side of the family, so never go to ask him about the whole singularity thing. I do remember at his funeral that even though his coffin was too small to see it still took 6 burley pallbearers to carry it.

umm just had a thought,

there is one thing that escapes a black hole then – gravity? whatever that is, i believe we dont actually know….?

From a general relativity point of view….. Gravity is the curvature of spacetime so perfectly possible to feel the effect of a black hole.

In quantum mechanics forces are due to virtual partials – gravitons for gravity although no one has ever seen one 😉 Apparently these gravitons can travel faster than light but sounds like a bit of a fudge to me 😉

Essentially the theory’s of gravity and quantum mechanics are not compatible (currently) so its all a bit of finger in the wind at the moment I think.

yup recognising some of the words… we are going to need gazzilions of lumens for this one. What light for BH

so with all that in mind, do I need a negative rise stem, drop bars and 36″ wheels?

I don’t know – are you planning on joining the circus?

You said black hole. He He…black hole

black hole.