Questions on Black Holes - Answers Wanted

[Markrids]

Ive got a few questions hopefully someone can answer.

1) If you were to look at a black hole, would it appear flat. If so, what would you see when looking at it side on?

2) Do the dimensions of black holes change or are they always the same. If they do change size, what differences does this make?

Thanks in advance.

 

[Jorrie]

Ive got a few questions hopefully someone can answer.

1) If you were to look at a black hole, would it appear flat. If so, what would you see when looking at it side on?

I'm not sure what you mean by "appear flat". Non-rotating black holes are spherical. Rotating black holes are slightly flattened, i.e. bulging a bit at the equator. They cannot be flat like a pancake.

2) Do the dimensions of black holes change or are they always the same. If they do change size, what differences does this make?

Black holes grow if they "swallow" other matter or when two black holes becomes one. Mini black holes can also "evaporate" slowly due to Hawking radiation. Bottom line, black holes are not generally static in dimensions. The strength of the gravitational field of a black hole increases non-linearly with its mass.

 

[Markrids]

Ok. Thanks for the reply.

 

[MeJennifer]

Non-rotating black holes are spherical.

Assuming you are not talking about the event horzon, aren't non-rotating black holes the size of a point?

Bottom line, black holes are not generally static in dimensions.

Well I understand they can increase in mass, but in dimensions? How?
Or are you talking about the event horizon?

 

[Jenny]

By conservation of momentum.. how can a black hole not rotate? All this mass accrete into it carrying large angular momentum.

2) Do the dimensions of black holes change or are they always the same. If they do change size, what differences does this make?

Well I understand they can increase in mass, but in dimensions? How?
Or are you talking about the event horizon?

I ask the same question as Jennifer, black holes are very massive and change the geometry of space. When we measure things, we assume space is flat. We measure things by stretching a tape measure in a straight line (a geodesic) but around black holes, straight lines are weird...Does anybody know how black hole's dimensions can be 'measured' and what this means?

 

[Jorrie]

Assuming you are not talking about the event horzon, aren't non-rotating black holes the size of a point?


Well I understand they can increase in mass, but in dimensions? How?
Or are you talking about the event horizon?

I thought it was customary to talk about the event horizon as the 'size' of a BH. After all, it is the EH that makes it a BH!

 

[Jorrie]

I ask the same question as Jennifer, black holes are very massive and change the geometry of space. When we measure things, we assume space is flat. We measure things by stretching a tape measure in a straight line (a geodesic) but around black holes, straight lines are weird...Does anybody know how black hole's dimensions can be 'measured' and what this means?

Black hole mass/energy can be measured by means of its gravitational field, charge and rotation rate. The size and shape of the event horizon (EH) is then inferred by means of GR theory.

Although the radius of the EH is commonly used, it is advocated by many (e.g. Kip Thorne) that the circumference must rather be used. I presume it is in principle possible to measure the circumference optically, but why would one?

 

[Jorrie]

By conservation of momentum.. how can a black hole not rotate? All this mass accrete into it carrying large angular momentum.

I'm not sure of this one, but may it be possible that a black hole that forms during a stellar collapse can be non-rotating? Or perhaps the notion of non-rotating black holes is just an artifact of the way the theory developed. In 1917 there were (in theory) only Schwarzschild black holes - non-rotating and permanently at rest at the origin of the coordinate system. Later came Kerr black holes, that rotated and still later...

 

[GOD__AM]

Doesn't rotation speed depend on the observer? I can't imagine a situation where any object will appear not to rotate to every possible observer. That would be assuming some kind of preferred frame of reference which doesn't exist.

What is it not rotating compared to? Seems this is like saying an object is not moving in any reference frame, which seems impossible...

 

[MeJennifer]

Doesn't rotation speed depend on the observer? I can't imagine a situation where any object will appear not to rotate to every possible observer. That would be assuming some kind of preferred frame of reference which doesn't exist.

What is it not rotating compared to? Seems this is like saying an object is not moving in any reference frame, which seems impossible...

Relativity of motion only works for non-inertial frames.

 

[Jorrie]

What is it not rotating compared to? Seems this is like saying an object is not moving in any reference frame, which seems impossible...

Rotation can measured in an absolute sense, relative tot the universe at large (e.g. the distant galaxies/quasars). Further, a simple gyroscope or accelerometer (for centripetal force) can do the job. Remember Newton’s rotating bucket?

 

[cracker]

I don't think that a black whole is flat since its an very small dence object so it must pull from all directions...