A query about Black Holes

[Pyrotex]

...The collapse takes an infinite time as seen by any observer in the exterior universe, and in comparison to any other observed time in the outside universe.

Many apologies, but the blue text in your question, I do not understand. "in the outside universe" and "in the exterior universe" are essentially identical.

:hyper:

[InfiniteNow]

Infinitenow, are you sure about acceleration causing time dilation? I always understood it to be velocity.

Nope.

 

The only thing I know is that I don't know, and I'm not even sure about that. :hyper:

[sebbysteiny]

I wish I knew how to make words link to sites...

 

It's interesting to note you never *asked* for the info on how to make words link to sites. You only made a wish, which was subsequently granted.

[Farsight]

Nope. The only thing I know is that I don't know, and I'm not even sure about that. ;)

 

LOL, good one. I should learn that lesson.

 

Pyrotex, sorry, how's this:

 

The collapse takes an infinite time according to any observer or series of observers exterior to the collapse.

[sebbysteiny]

Infinitenow, are you sure about acceleration causing time dilation? I always understood it to be velocity.

 

I think they both do. But an accelarating frame will gain more time as it is no longer following a geodisic in space time. Velocity's effects will depend on which observer is observing rather than any fundamental slowing of time that all observers can agree.

[arkain101]

Does this relate somewhat?

 

Take a pop can and accelerate it to 2000m/s in the time of 3 days

versus

Take a pop can and accelerate it to 2000m/s with a cannon and lets say 0.5seconds. The can should Be flat as a pancake.

 

They both reached 2000m/s but acceleration affects the mass of things, rightly so the time of things.

[Qfwfq]

But an accelarating frame will gain more time as it is no longer following a geodisic in space time.

True, and true for all observers, but this doesn't imply that acceleration is "causing" time dilatation any more than velocity does.

 

A geodesic between two points of space-time (events) is by definition simply the shortest path between them. For timelike curves, this means the shortest proper time.

 

It is possible to have different paths which depart from and return to a same geodesic one for which the accelerations are equal but the increases in proper time differ.

[Pyrotex]

Nope.

The only thing I know is that I don't know, and I'm not even sure about that. :hihi:

InfiniteNow is mostly correct.

In Special Relativity, Einstein examines the observed effects of two frames of reference (F/R) that are traveling at some large velocity with respect to each other. The accellerations needed to provide this configuration are ignored.

 

In General Relativity, Einstein places the 'cause' of relativistic effects not just on the speed, but also on the accelleration. Gravity, which he considers as equivalent to other forms of accelleration, then also can cause relativistic effects.

[Pyrotex]

...The collapse takes an infinite time according to any observer or series of observers exterior to the collapse.

works for me.

[Farsight]

So right now, there's black holes out there sucking stuff up, but as I speak they're still collapsing at a frozen snail's pace, and as at this moment in time, there are no singularities. And never will be?

[sebbysteiny]

So right now, there's black holes out there sucking stuff up, but as I speak they're still collapsing at a frozen snail's pace, and as at this moment in time, there are no singularities. And never will be?

 

Oh so close.

 

Firstly, black holes (as with all objects with the property of mass) 'suck' stuff up. The only difference between black holes and, say, the Earth, is that if a mass can be compressed sufficiently small, then there will be a value of r in the (non relativistic simplified) equation F= m1 m2 / r^2 in which even light cannot escape. But a small object like the Earth would require too much energy to become a black hole. A larger object like an enormous but dead star, erm, how should I put this, 'does not need to overcome that energy hurdle' to reach the size necessary for a black hole.

 

Eg, if you turn an atom into a black hole, the entire gravitational force of that black hole will be no larger than that of an atom.

 

But yes, what you said here is essentially correct as far as I can see.

 

Again, yes, as you speak they are still collapsing at a frozen snail's pace.

 

But no, at this moment in time, there ARE singularities.

 

Just because you have not observed mass reach the event horizen does not mean that the event horizen is not a singularity.

 

If you imagine plotting a black hole on a graph and you view the radius of the event horizen (ie the size of the black hole) as a sphere, then everywhere on that sphere is a singularity in space time. At those co ordinates, space and time cease to exist. Inside the sphere, space and time does very wierd things.

 

Infact, the complete inablility to see anything reach the event horizen ever PROVES that there is a singularity at that point.

[HydrogenBond]

One way to understand why one would appear to slow when falling into a blackhole is that local space is highly contracted. If we assume one mile becomes contracted down to one inch, due to space contraction, and one is falling at one mile/sec, it will now take one second to go one inch.

 

For one to fall forever into a blackhole, implies infinite space contracted into a point. This is not the case since we on earth would be falling into all the blackholes. Maybe a few hundred millions miles may contract into an inch, at best, but that would allow even the space shuttle to do that in matter of days.

 

I think the math deals with ideal blackholes and not real blackholes. The observation of x-rays coming from rapidly spinning blackholes, shows not only light excaping (x-ray light), but it also shows movement faster than one rotational cycle per infinite time.

[Farsight]

... In fact, the complete inablility to see anything reach the event horizen ever PROVES that there is a singularity at that point.

PROVES? The point is, it isn't a point! Ergo no singularity! :naughty:

[Qfwfq]

In General Relativity, Einstein places the 'cause' of relativistic effects not just on the speed, but also on the accelleration.

:umno:

 

GR is an extension to general coordinate transformations. The principle of equivalence equates non-inertial (accelerating) coordinate frames to the presence of a gravitational field. This isn't quite the same as saying

Gravity, which he considers as equivalent to other forms of accelleration, then also can cause relativistic effects.

What it does predict is that a gravitational field also signifies a time dilatation in that the [math]\norm g_{00}[/math] of the metric may differ from 1; this is the square of the ratio of time intervals for different observers but it isn't equal to or proportional to acceleration. In an appropriate limit, a well known approximation relates it to the potential of the field.

 

It definitely doesn't imply that accelerating a body, by a force of arbitrary kind, will cause a time dilatation.

[sebbysteiny]

PROVES? The point is, it isn't a point! Ergo no singularity!

 

I think you are using a circular argument by trying to define a singularity as 'not a point' and then using that definition to argue 'therefore it can't be a singularity'.

 

Lets say a blackhole is a sphere of radius e (whose size is the event horizen) with co-ordinates x=y=z=0 being at the centre of the sphere.

 

At the radius e, the event horizen, there is a singularity in spacetime. Inside the sphere, spacetime becomes imaginary, but it still exists. Every object that falls into a blackhole will agree with me.

[Farsight]

I think you are using a circular argument by trying to define a singularity as 'not a point' and then using that definition to argue 'therefore it can't be a singularity'.

 

Lets say a blackhole is a sphere of radius e (whose size is the event horizen) with co-ordinates x=y=z=0 being at the centre of the sphere.

 

At the radius e, the event horizen, there is a singularity in spacetime. Inside the sphere, spacetime becomes imaginary, but it still exists. Every object that falls into a blackhole will agree with me.

 

Sebby: A singularity is usually considered to be an infinitesimal point where infinities rage at the centre of the black hole. I think we're both saying it isn't.

[sebbysteiny]

Sebby: A singularity is usually considered to be an infinitesimal point where infinities rage at the centre of the black hole. I think we're both saying it isn't.

 

Well we can definitely agree on that. It completely contradicts my understanding. Just because one has imaginary time and space at a black hole's centre does not make it a singularity.

 

The singularity, Popular, is at the event horizen, ie what some might consider 'the surface'. The centre is definately NOT a singularity.