Black holes and event horizons

doctoremma

I have A-level Physics, reading popular science books about space, light, etc. I am struggling with a statement I recently read, which was (in essence): to an independent observer, an object approaching a black hole will never cross the event horizon, instead it will appear to remain motionless at the threshold. According to the book (and to other sources e.g. Wiki etc), it should be obvious why this is, so I feel quite stupid not getting it.

Conceptually, here's, rightly or wrongly, where I'm at (as it might help anyone pitching an answer at the right level!):
1. As an object approaches the event horizon, light from this object is not all sucked in by the black hole's gravity and can reach the independent observer, who therefore views the object normally.
2. When an object is past the event horizon (from the object's POV), all light is sucked into the black hole and thus, the object cannot be seen by the independent observer.

What happens to the light when the object is AT the event horizon? Why does the object appear stationary to the observer? What's happening to the light? I'm OK with gravity, time dilation, spacetime as a measurement but haven't covered spacetime bending due to gravity yet (just in case the answer involves these concepts).

Thanks!

Popoutman

Light from the entity close to the event horizon appears more and more redshifted as the entity approaches the EH, until it gets so redshifted that it is effectively no longer visible to the outside. As the entity moves towards the EH, it appears to slow down in its apparent movement, and this is to account for the redshift.

(could be very wrong, bu this is my understanding of it)

doctoremma

Popoutman wrote: »

Light from the entity close to the event horizon appears more and more redshifted as the entity approaches the EH, until it gets so redshifted that it is effectively no longer visible to the outside. As the entity moves towards the EH, it appears to slow down in its apparent movement, and this is to account for the redshift.

(could be very wrong, bu this is my understanding of it)

So as the object nears the event horizon, gravitational time dilation increases and increases until it is (conceptually) infinitely stretched i.e. the object appears not to move?

Rubecula

doctoremma wrote: »

So as the object nears the event horizon, gravitational time dilation increases and increases until it is (conceptually) infinitely stretched i.e. the object appears not to move?

In a nutshell yes that is right.

EDIT: To the person observing it would be an infinite time before the said object was "eaten" by the black hole. But to the object the fall would appear almost instantaneous.

doctoremma

Rubecula wrote: »

In a nutshell yes that is right.

EDIT: To the person observing it would be an infinite time before the said object was "eaten" by the black hole. But to the object the fall would appear almost instantaneous.

OK, I will try to cement this in my head

If the object undergoes infinite time dilation based on proximity to a gravitational source, this explains the redshift? The frequency ("timing") of the light waves decreases, thus wavelength increases? What happens when this process causes the light waves to shift from the visibility spectrum?

Rubecula

doctoremma wrote: »

OK, I will try to cement this in my head

If the object undergoes infinite time dilation based on proximity to a gravitational source, this explains the redshift? The frequency ("timing") of the light waves decreases, thus wavelength increases? What happens when this process causes the light waves to shift from the visibility spectrum?

At that point it becomes invisible. But it is still visible in it's perceived stationary position.

dbran

Hi

It is more to do with time dilation I think.

If you are close to a gravity field time travels slower then if you are away from a gravity field. This has been measured very accurately with atomic clocks on Earth. In fact in order for the GPS in a satnav to work, an additional 6 minutes each day needs to be factored into the calculation of your position due to the fact that the satalite which in geosyncronus orbit above the Earth is experiencing time traveling faster then your time on the surface of the Earth.

This effect increases the more stronger the gravity field. When you come to the event horizon in a black hole this time dilation actually reaches infinity and time appears to stop.

To the outside observer you would see the object falling towards the black hole and the event horizon but you would never see them actually cross it. As the object approached the event horizon the light you would see reflected from it would be redshifted as it is requiring more and more energy to climb out of the gravity well. But each of the photons themselves would actually be travelling at the speed of light.

It is only at the event horizon where the escape velocity is the speed of light that a photon would essentially hover. This is the eqivalent to time standing still or stopping as space time is essentially collapsing inwards at the speed of light.

You will never see light from an object when it is at the event horizon only outside it. And you will also never get to see anything happening inside the event horizon.

The actual object falling into the black hole would notice nothing as it crossed the event horizon and would continue to fall until it reached the singularity at the centre.

Thats my understanding of it anyway

Hope this helps

dbran

doctoremma

Rubecula wrote: »

At that point it becomes invisible. But it is still visible in it's perceived stationary position.

Ok, you've lost me with that one.

If light from the object is increasing redshifted as it approaches the event horizon, at some point the independent observer will no longer be able to view the light (it shifts out of the visibility spectrum). The light emitted is in microwave or radio wave territory? How can the observer still see the object (as stationary or otherwise)?

doctoremma

dbran wrote: »

It is only at the event horizon where the escape velocity is the speed of light that a photon would essentially hover. This is the eqivalent to time standing still or stopping as space time is essentially collapsing inwards at the speed of light.

The photon doesn't move?

dbran wrote: »

The actual object falling into the black hole would notice nothing as it crossed the event horizon and would continue to fall until it reached the singularity at the centre.

So, I get this (I think). What I'm missing is that - if the object falls into the black hole, how is light corresponding to its position at the event horizon still being transmitted to the observer?

dbran

Hi

The external observer will never ever see the object cross the event horizon or at the event horizon.

The closer it gets to the event horizon the slower time will appear to be passing for the object as observed by the observer. As time stops at the event horizon, it would take an infinate amount of time for an observer to see the object reach the event horizon.

From the object's point of view, they will observe time for the observer to be actually speeding up and getting faster and faster. In an instant they will notice all of the stars slowly running out of hydrogen, burning out, turning into white dwarfs and then cooling to black dwarfs. They will notice the galaxies are actually spinning, crashing into each other and merging and then going dim and dying. All this as they get closer and closer to the event horizon.

From the object's point of view there will be no difference as it reaches and then crosses the event horizon.

But it is impossible to know exactly what happens to it once the object goes past the event horizon and enters the black hole because all the laws of physics break down at that point. Beyond the event horizon space time will be falling inwards faster then light which is impossible but essentially hidden from view.

Hence the next big breakthrough in physics will be when somebody develops a theory which explains what happens in the extreme environment of a black hole. This theory would replace Einstein's theory's in the same way that Einstein's theory of relativity replaced newtons theory

dbran

doctoremma

dbran wrote: »

The external observer will never ever see the object cross the event horizon or at the event horizon.

The closer it gets to the event horizon the slower time will appear to be passing for the object as observed by the observer. As time stops at the event horizon, it would take an infinate amount of time for an observer to see the object reach the event horizon.

Ok, I think I get it now. Thanks.

dbran wrote: »

From the object's point of view, they will observe time for the observer to be actually speeding up and getting faster and faster. In an instant they will notice all of the stars slowly running out of hydrogen, burning out, turning into white dwarfs and then cooling to black dwarfs. They will notice the galaxies are actually spinning, crashing into each other and merging and then going dim and dying. All this as they get closer and closer to the event horizon.

Wow, never thought of it in this reverse POV. Does this event appear represented in a film or video somewhere?

Popoutman

I don't think that an observer on the entity at the EH would see the universe time-compress as they crossed the EH.

From the outside, the entity would apparently take forever to reach the EH, becoming darker and more red-shifted as they approach. For the entity at the EH, it would pass the EH without any local effects.

(depending on the size of the black hole, as the EH may be outside where tidal effects would cause breakup of the entity.)

Enkidu

Basically what others have said.

For the observer outside the black hole:
The observer falling toward the hole appears to move slower and slower and appear more and more red shifted. Eventually their clock would appear to almost completely stop and they would only be visible with a radio receiver.

For the observer falling into the black hole:
They would perceive the rest of the universe to speeding up gradually as they approached the hole. As they fall in, they see the entire history of the universe play out.
When they cross the horizon they are greeted by every single object ever swallowed by the black hole, even ones eaten a billion years after themselves.

Popoutman

For a stationary observer hovering above a black hole, the outside universe will appear to age faster. If it were possible to hold station close enough to a large enough black hole, one could watch the universe age very rapidly.

Note that a non-stationary observer (one falling into the black hole) sees things rather differently, as per the faq:
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

doctoremma

Enkidu wrote: »

For the observer outside the black hole:
The observer falling toward the hole appears to move slower and slower and appear more and more red shifted. Eventually their clock would appear to almost completely stop and they would only be visible with a radio receiver.

He he, I came back to this thread as I realised that I still can't think my way through it...

As per above, I understand the redshifting thing. That would mean that the light from the object at the black hole would eventually shift out of the visibility spectrum (in terms of the human eye). At that point, the object would become invisible to the human eye but obviously detectable at longer wavelengths?

As this process of redshifting proceeds, at some point can the wavelength of light become no longer? What's are the physical limitations for this? Does time continue to dilate after this boundary is reached, with no further increase in wavelength? Or is that a meaningless question if time is actually defined by some parameter of the wavelength?

Enkidu

doctoremma wrote: »

As this process of redshifting proceeds, at some point can the wavelength of light become no longer? What's are the physical limitations for this? Does time continue to dilate after this boundary is reached, with no further increase in wavelength? Or is that a meaningless question if time is actually defined by some parameter of the wavelength?

Well the light will gradually increase its wavelength with no upper limit, it will simply get longer and longer over time. However it will eventually lengthen beyond the ability of any instrument to detect.

dbran

Popoutman wrote: »

For a stationary observer hovering above a black hole, the outside universe will appear to age faster. If it were possible to hold station close enough to a large enough black hole, one could watch the universe age very rapidly.

Note that a non-stationary observer (one falling into the black hole) sees things rather differently, as per the faq:
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

This is a very interesting read, thanks for posting. So if your falling in you wont be able to see the universe play out because you will always be ahead of the light that is falling into the black hole after you. If you were stationary and turned your rockets to full thrust you would indeed see the universe play out.

One thing is he talks about a non rotating black hole which would only exist in theory as in all cases the black hole wold still retain the rotation and charge of progeniter star that created it.

dbran

Enkidu

dbran wrote: »

One thing is he talks about a non rotating black hole which would only exist in theory as in all cases the black hole wold still retain the rotation and charge of progeniter star that created it.

dbran

What happens is pretty much the same in the rotating case except there it is possible to see the history of the universe play out. Otherwise there isn't much difference.

SNAKEDOC

Enkidu wrote: »

Basically what others have said.

For the observer outside the black hole:
The observer falling toward the hole appears to move slower and slower and appear more and more red shifted. Eventually their clock would appear to almost completely stop and they would only be visible with a radio receiver.

For the observer falling into the black hole:
They would perceive the rest of the universe to speeding up gradually as they approached the hole. As they fall in, they see the entire history of the universe play out.
When they cross the horizon they are greeted by every single object ever swallowed by the black hole, even ones eaten a billion years after themselves.

i don't think that rendering is accurate. yes while viewing a black hole from the outside you would see an object been swallowed slowing down as it reached the event horizon but never crossing it
however in real tie from the perspective of the object time would have continued and the object would be obliterated as it crossed the event horizon.
what the viewer is actually seeing is time slowed down from our perspective, the object is no longer there due to light been captured by the black hole and been reflected away been suspended in a snapshot of time because of the immense gravity forces.

although i do have one question which someone might answer. if a black hole is so powerful and can eat so much how come it will not expand past its size and devour the galaxy.

Enkidu

SNAKEDOC wrote: »

i don't think that rendering is accurate. yes while viewing a black hole from the outside you would see an object been swallowed slowing down as it reached the event horizon but never crossing it
however in real time from the perspective of the object time would have continued and the object would be obliterated as it crossed the event horizon.

This is incorrect. There is no real time, as no reference frame is objectively correct. In one reference frame the object is always outside the black hole and this is not a "trick of the light". It is genuinely for all times outside the event horizon.

In the reference frame of the object itself it falls through the event horizon in finite time.

SNAKEDOC wrote: »

although i do have one question which someone might answer. if a black hole is so powerful and can eat so much how come it will not expand past its size and devour the galaxy.

Black Holes slowly grow, but only very slowly. At great distances from them, they have no more gravitational influence than anything else of their mass.

Myrddin

Enkidu wrote:

For the observer falling into the black hole:
They would perceive the rest of the universe to speeding up gradually as they approached the hole. As they fall in, they see the entire history of the universe play out.
When they cross the horizon they are greeted by every single object ever swallowed by the black hole, even ones eaten a billion years after themselves.

Wow. Normally I can visualise these things & grasp them on a basic level, but not that one. Can you give me the Anne & Barry version of it?

Enkidu wrote: »

this is not a "trick of the light". It is genuinely for all times outside the event horizon.

Gah, is that because of time dilation? Time stretches & slows to a point that the object effectively stops?

What a mind bendingly fascinating thread :cool:

Enkidu

EnterNow wrote: »

Wow. Normally I can visualise these things & grasp them on a basic level, but not that one. Can you give me the Anne & Barry version of it?

It's related to why you can never leave a black hole. The actual truth is that a black hole completely flips over the dimensions. So basically, outside the black hole you have two coordinates:

r - the distance from the centre of the black hole.
t - time on the wrist watch of a person far away from the black hole.

However at the even horizon these swap places. Before the event horizon, the centre of the black hole is in front of you at "r = 0", however when you cross the event horizon "r" becomes a time coordinate, so that the center of the black hole is no longer in front of you in space, but in front of you in time. Similarly outside the black hole is no longer behind you in space but behind you in time, i.e. in your past. So trying to leave the black hole is like trying to get back to last Teusday.

This is why everything arrives at the same time. Everybody enters the black hole at the same place, the Event horizon. However inside the black hole the Event horizon is no longer a place, but a time, a moment. So everybody enters the black hole at the same time.

Basically you can think of the black hole as a separate little universe. To outsiders the black starts in spatial sense at the event horizon. However in the mini-world inside the black hole, time began at the event horizon.

Popoutman

SNAKEDOC wrote: »

although i do have one question which someone might answer. if a black hole is so powerful and can eat so much how come it will not expand past its size and devour the galaxy.

The only effect that a black hole has on its surroundings is the effect of its gravity. It doesn't actively go out and suck in stuff that's nearby. Stuff nearby can lose orbital energy in tidal interactions and multi-body gravitational enounters, and if a body loses enough orbital energy it would have its orbit around the black hole altered as a result of that, and could then end up passing too close to the event horizon. A star 100 light years out from the black hole at the centre of our galaxy would go through its life without any direct effect from the black hole. (of course the intense x-rays and other hard radiations from the accretion disk would sterilise any planets around said star). As it is it appears that there are a very large number of stars within 400 light years of the galactic centre, and this appears to be stable enough a location for many new stars to form,

Also the fact that a black hole is incredibly concentrated in mass is a factor in why its rate of mass increase is very slow at this stage in the life of the galaxy. The galactic black hole has an event horizon that's about the size of the orbit of planet Mercury. This is pretty tiny compared to the average star to star distance throughout the galaxy, and still very tiny compared to the density of stars around the Galactic centre.

SNAKEDOC

when a place becomes time thats when my questions turns into a headache. my head hurts. maybe i should go back to the conspiracy theory forum.:D

doctoremma

A supplement to why black holes aren't consuming the universe: black holes are also predicted to lose mass via Hawking radiation....

Myrddin

Enkidu wrote:

For the observer falling into the black hole:
They would perceive the rest of the universe to speeding up gradually as they approached the hole. As they fall in, they see the entire history of the universe play out.
When they cross the horizon they are greeted by every single object ever swallowed by the black hole, even ones eaten a billion years after themselves.

Just getting back to this briefly. Does this mean that its largely accepted that 'the future' has already been established? If there's a black hole out there now, that has an object inside the EH that it won't consume for another thousand years, surely that means the future is already laid out?

Enkidu

EnterNow wrote: »

Just getting back to this briefly. Does this mean that its largely accepted that 'the future' has already been established? If there's a black hole out there now, that has an object inside the EH that it won't consume for another thousand years, surely that means the future is already laid out?

I know this will mess with your head, but....

Remember that in our time, outside the hole, objects never cross the event horizon. So from out point of view there are no objects yet inside the event horizon.

Myrddin

Enkidu wrote: »

I know this will mess with your head, but....

Remember that in our time, outside the hole, objects never cross the event horizon. So from out point of view there are no objects yet inside the event horizon.

Your killing me!

I actually can't get me head around that. So far, I with you that as an object approaches the EH, gravitational dilation will slow time down for that object more & more the closer it gets. Eventually, it'll reach the EH where time for the object effectively stops, & that's why to the outside observer, the object never seems to actually cross over. Is that right so far?

So how is there anything at all actually inside the EH?

Really sorry this is probably very basic stuff, feel free to anneandbarry.ie

Enkidu

EnterNow wrote: »

Your killing me!

I actually can't get me head around that. So far, I with you that as an object approaches the EH, gravitational dilation will slow time down for that object more & more the closer it gets. Eventually, it'll reach the EH where time for the object effectively stops, & that's why to the outside observer, the object never seems to actually cross over. Is that right so far?

So how is there anything at all actually inside the EH?

There is nothing inside the black hole, from our perspective, because nothing ever crosses the event horizon from our perspective. That's the origin of the name. As far as we are concerned no events ever occur beyond that point. Hence, for us, it is an event horizon.

Of course, for things falling into the black hole, they just fall straight through the horizon and inside the black hole is full of stuff.

Enkidu

Also wait until you find out how the singularity kills you.

Myrddin

Enkidu wrote: »

There is nothing inside the black hole, from our perspective, because nothing ever crosses the event horizon from our perspective. That's the origin of the name. As far as we are concerned no events ever occur beyond that point. Hence, for us, it is an event horizon.

Of course, for things falling into the black hole, they just fall straight through the horizon and inside the black hole is full of stuff.

Difficult to get my head around the concept of there not being anything inside a black hole, but there is if you go into it.

*I know we're not talking about actual objects, more like matter/mass.

Enkidu wrote: »

Also wait until you find out how the singularity kills you.

Spaghettification? Ouch :P

Wetbench4

Enkidu wrote: »

This is why everything arrives at the same time. Everybody enters the black hole at the same place, the Event horizon. However inside the black hole the Event horizon is no longer a place, but a time, a moment. So everybody enters the black hole at the same time.

So if you could look back after crossing the EH, would it not be an immense whiteout of all the light and energy that was ever trapped by the black hole, in its history and future??

Enkidu

EnterNow wrote: »

Difficult to get my head around the concept of there not being anything inside a black hole, but there is if you go into it.

*I know we're not talking about actual objects, more like matter/mass.

Yeah, I know it is pretty strange.

EnterNow wrote: »

Spaghettification? Ouch :P

Basically time ends at the singularity. Although Spaghettification would kill a person first.

Enkidu

Wetbench4 wrote: »

So if you could look back after crossing the EH, would it not be an immense whiteout of all the light and energy that was ever trapped by the black hole, in its history and future??

Basically that is what happens in most black holes. However in some cases, like a black hole that isn't rotating, the light just happens to not catch up with you so you don't see a total white out.

Myrddin

When they say the known laws of physics break down inside a black hole, are they referring to the likes of the conservation of energy etc? [energy cannot be created or destroyed]

Also, the idea of time & distance basically flipping over & swapping places is very interesting. Is this something based on mathematics or is there some other way of showing it? I've never heard about that before.

slade_x

EnterNow wrote: »

When they say the known laws of physics break down inside a black hole, are they referring to the likes of the conservation of energy etc? [energy cannot be created or destroyed]

.

No one knows what exactly goes on inside a black hole. Its the mathematical models that are used in the very desciptions of physics that completely break down. No one is stating exactly what can and cant happen in a black hole, its just our understanding of physics through models and mathematics cant be used to ascertain whatever goes on in a black hole. by definition all our observations and working knowledge of the laws of physics and chemistry etc. arent applicable to beyond the event horizon of a black hole.

Matter in a black hole would be in a very peculiar state. Possibly even unrecognisable. There are no mathematical models for the internal structure of black holes or their composition. I'd say the closest thing we have which could be nothing like it at all are the models for the structure of neutron stars (the most dense objects known), which is a big possibly/maybe but may be worth a look if such things interest you

http://en.wikipedia.org/wiki/Neutron_star#Structure

http://imagine.gsfc.nasa.gov/docs/science/know_l1/neutron_stars.html

http://science.nationalgeographic.com/science/space/solar-system/neutron-stars/

Myrddin

slade_x wrote: »

No one knows what exactly goes on inside a black hole. Its the mathematical models that are used in the very desciptions of physics that completely break down. No one is stating exactly what can and cant happen in a black hole, its just our understanding of physics through models and mathematics cant be used to ascertain whatever goes on in a black hole. by definition all our observations and working knowledge of the laws of physics and chemistry etc. arent applicable to beyond the event horizon of a black hole.

Matter in a black hole would be in a very peculiar state. Possibly even unrecognisable. There are no mathematical models for the internal structure of black holes or their composition. I'd say the closest thing we have which could be nothing like it at all are the models for the structure of neutron stars (the most dense objects known), which is a big possibly/maybe but may be worth a look if such things interest you

http://en.wikipedia.org/wiki/Neutron_star#Structure

http://imagine.gsfc.nasa.gov/docs/science/know_l1/neutron_stars.html

http://science.nationalgeographic.com/science/space/solar-system/neutron-stars/

Fascinating stuff in the links, thanks for the explanations too

Robert Edmonds

EnterNow wrote: »

Spaghettification? Ouch :P

Depends on the mass of the BH. Oddly enough the massive ones do not "speghettify" as much.

Myrddin

Robert Edmonds wrote: »

Depends on the mass of the BH. Oddly enough the massive ones do not "speghettify" as much.

So the denser the object, the more force is required to 'stretch' it out as such? Surely though as the object gets closer in proximity to the EH, the forces increase exponentially, eventually succeeding in 'spaghettifying' any object?

Zubeneschamali

EnterNow wrote: »

So the denser the object, the more force is required to 'stretch' it out as such?

No, Robert is saying that the more massive the Black Hole is, the less severe the tidal effects will be at the event horizon.

This is a bit like the tidal effects of the Sun and Moon on the Earth. The Suns gravity at the Earth's surface is about 200 times the gravity of the Moon, but the Sun is also further away, so the tidal effects on Earth due to the Sun are smaller than the tidal effects due to the smaller but closer Moon.

Similarly, the more massive a black hole is, the stronger its gravity but the further out its event horizon will be. Tidal effects are due to the gravity gradient, which is less severe at the event horizon of a big black hole than a small one.

Myrddin

Zubeneschamali wrote: »

No, Robert is saying that the more massive the Black Hole is, the less severe the tidal effects will be at the event horizon.

This is a bit like the tidal effects of the Sun and Moon on the Earth. The Suns gravity at the Earth's surface is about 200 times the gravity of the Moon, but the Sun is also further away, so the tidal effects on Earth due to the Sun are smaller than the tidal effects due to the smaller but closer Moon.

Similarly, the more massive a black hole is, the stronger its gravity but the further out its event horizon will be. Tidal effects are due to the gravity gradient, which is less severe at the event horizon of a big black hole than a small one.

Ah I see, I missed the 'BH' bit in his post Cheers

Fascinating stuff, really is

Enkidu

c = Speed of light
G = Newton's constant

Just thought I'd come back to this interesting thread.

First the metric for flat space
[latex]ds^2 = dt^2 - \frac{1}{c^2}dr^2[/latex]
The metric is the distance rule for a spacetime. Basically take two events in time and space, work out the difference between the time the first occurred, [latex]t_1[/latex] and the time the second occurred [latex]t_2[/latex]. This gives you:

[latex]dt = t_2 - t_1[/latex]

Similarly work out the radius at which one occurred [latex]r_1[/latex] and the radius another occurred at [latex]r_2[/latex], this gives you:

[latex]dr = r_2 - r_1[/latex]

The radius values are measured by picking some random point to be [latex]r = 0[/latex] and then working out the values.

So if you pop these into the formula above, you get the (square of the) spacetime distance between two events, called [latex]ds^2[/latex]

Really in the formula I should have extra terms like [latex]d\theta[/latex] representing the different angles at which events occurred but I'm ignoring that for simplicity.

The main two things to notice:
1. The only difference between time and space is that [latex]dt^2[/latex] has a + sign and [latex]dr^2[/latex] a - sign. According to modern physics this is the only difference between space and time. Time is a dimension that adds to distance, while space subtracts.
2. If [latex]\frac{1}{c^2}dr^2[/latex] is bigger than [latex]dt^2[/latex], then [latex]ds^2[/latex] becomes negative, so [latex]ds[/latex] is imaginary. This means that there is no sensible distance relation between the two events, they can't be connected.
An example of two such events are the location of your chair at this exact moment and the sea of tranquillity on the moon one second from now. These events have no connection. Hence you can't go from here to the moon in one second, the rules of geometry prevent it. You can't do it, any more than you can change any other theorem of geometry. This is why you can't go faster than light, it's geometrically impossible.

However a black hole distorts spacetime (or rather it is the distortion of spacetime) and the distance rule changes to:
[latex]{ds}^{2} =
\left(1 - \frac{2GM}{c^2 r} \right) dt^2 - \frac{1}{c^2}\left(1-\frac{2GM}{c^2 r}\right)^{-1} dr^2[/latex]

The main thing to notice here is that when [latex]r[/latex] is smaller than [latex]\frac{2GM}{c^2}[/latex], the factor in front of [latex]dr^2[/latex] becomes negative and so [latex]r[/latex] becomes a time dimension. [latex]r = \frac{2GM}{c^2}[/latex] is the Event Horizon.

Duff

^^^^^

Enkidu

Well hopefully somebody finds it useful.

dbran

You are hereby appointed the "Shelden Cooper" of the Astronomy forum.

mooliki

Kudos to doctoremma for starting this thread and everyone who added to it, really fascinating stuff.

Enkidu wrote: »

Black Holes slowly grow, but only very slowly. At great distances from them, they have no more gravitational influence than anything else of their mass.

I have a question. If black holes are essentially a singularity, how do they grow? Is it merely the reach of their gravitational pull that "grows"? Would this be a case that the matter they consume increases their mass nd therefore their reach, but their own physical "size" (or whatever the equivalent term may be) remains a singularity? Or have I completely the wrong idea of what a singularity is, i.e. mass compressed into an infinitely minute point.

Apologies if this is ridiculous question I am in no way educated in this area!

Zubeneschamali

mooliki wrote: »

If black holes are essentially a singularity, how do they grow? Is it merely the reach of their gravitational pull that "grows"?

What grows is the Event Horizon. This is edge of the "hole", in that anything that crosses it never comes back. The more massive the black hole is, the further out the Event Horizon is.

SuprSi

Zubeneschamali wrote: »

What grows is the Event Horizon. This is edge of the "hole", in that anything that crosses it never comes back. The more massive the black hole is, the further out the Event Horizon is.

How does it grow? Do we know if black holes are common throughout the universe? I realise they're difficult to find using current methods, but do we know how many have been detected so far?

Zubeneschamali

SuprSi wrote: »

How does it grow?

The Event Horizon is just the (imaginary) sphere at a distance beyond which nothing escapes the black hole. The more massive the hole, the further out the sphere is, as the hole's gravity is stronger.

So as mass enters the hole, it's gravity increases and the Event Horizon gets further from it, and hence grows.

As to how many there are, we don't know. Probably a lot, since once a mass gets above a certain size (say 5 times the mass of our Sun), it requires some active process to stop it collapsing into a black hole. For large stars that burn out their fuel, there is no known process to stop them collapsing into black holes.

Very energetic but small sources are often seen at galactic cores: these are believed to be matter falling into a black hole.

Kxiii

I saw this video on youtube, some interesting stuff.