Black holes and event horizons

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.