[petelamana (OP)]

Hello and thank you all for making this web site a lot of fun.   

I have read the word "singularity" used synonymously with "black hole."  I also understand that the accretion disk is largely composed of matter, gas, whatever..."circling the drain," as it were, with the BH as "the drain."  (A crude analogy of a wonderfully complex concept, forgive me.)  Here is my question...

The word "singularity" implies a single 0-dimensional "location."  Does this then mean that even the event horizon has a "spin" to it, with the "singularity" at its center?  If so, then wouldn't the immense forces of the EH gradiate to that point?

[chris]

Hello and thank you all for making this web site a lot of fun.   

Thanks - and a warm welcome to the forum.

Can I ask one favour? Please ensure that you phrase the post titles as questions; we've had this policy for over a decade and it ensures that it's much easier to organise the forum content and to help people to get their questions answered.

It's also really helpful if you can add some tags to your posts too; you do this by clicking the "be the first to tag this post" link below a post.

Thanks

Chris

[petelamana (OP)]

Thank you.  I am still learning the "hows" and "ways" of this site.  I must say, I am learning a great deal, and am having tremendous fun doing so.  As a member of less than a week, I have already recommended The Naked Scientist to several friends.

[chris]

I have already recommended The Naked Scientist to several friends.

Wonderful! Do please encourage them to come and join us here!

[evan_au]

Does this then mean that even the event horizon has a "spin" to it

Considering how black holes form from the collapse of a spinning star, or with gas spiralling into the black hole, we expect that most black holes would spin (have angular momentum).

This distorts the shape of the event horizon into an ellipsoid, just like the spinning Earth has a tidal bulge at the equator.

the event horizon ... with the "singularity" at its center?

A non-spinning black hole would be spherical, with a point singularity at its center.

A spinning black hole could theoretically have a singularity which is shaped like a donut (a torus).
See: https://en.wikipedia.org/wiki/Ring_singularity

wouldn't the immense forces of the EH gradiate to that point?

Outside the event horizon, there is a gravitational force attracting any massive object towards the black hole. Due to the inverse-square law, this gets stronger the closer you get to the center of the black hole. But someone passing near a black hole would not actually feel this force, because they would be in free-fall (often misleadingly nicknamed "zero-gravity").

Outside the event horizon, there is also a tidal force that tends to distort the shape of anything near the black hole. This follows an inverse-cube law, and would only be felt by objects passing fairly close to black hole (close in astronomical terms). This is probably survivable near a supermassive black hole, but near solar-mass black holes, the effect has been nicknamed "spaghettification".
See: https://en.wikipedia.org/wiki/Spaghettification

Inside the event horizon, there is a gradient towards the singularity, in that anything crossing the event horizon is doomed to reach the singularity. But we really can't be sure what it is like, as the nature of spacetime is twisted inside a black hole in a way that is very different from what we experience outside the black hole. And nobody crossing the event horizon can transmit a movie out past the event horizon to tell us

[petelamana (OP)]

Thank you.  I would like to pose a follow-up query...

Does the singularity at the center of a spherical BH have mass?  Or is it pure gravity?  If so, what is generating the gravitational force at the singularity?

Additionally, is it possible to have pure massless gravity?  And, if so, how would that relate to dark matter?

[Bill S]

Pop Sci books, and even experts, talk about BH singularities as though they were objects that physically exist.  For us hitch-hikers it is important to remember that this can easily be misleading.

I have quoted Chris Baird on this subject before, but I think the point is worth stressing.   

In the real universe, no black holes contain singularities. In general, singularities are the non-physical mathematical result of a flawed physical theory. When scientists talk about black hole singularities, they are talking about the errors that appear in our current theories and not about objects that actually exist. When scientists and non-scientists talk about singularities as if they really exist, they are simply displaying their ignorance.

    A singularity is a point in space where there is a mass with infinite density. This would lead to a spacetime with an infinite curvature.  Singularities are predicted to exist in black holes by Einstein's theory of general relativity, which is a theory that has done remarkably well at matching experimental results. The problem is that infinities never exist in the real world. Whenever an infinity pops out of a theory, it is simply a sign that your theory is too simple to handle extreme cases.

[jeffreyH]

At the event horizon of a black hole the escape velocity is the speed of light. This means that the velocity required to escape from the source to infinity is the speed of light. Conversely, an object freely falling from infinity will be moving at the speed of light upon reaching the horizon. The important thing to note is that IT IS NOT POSSIBLE for an object with rest mass to achieve light speed. So this means that the mathematics break down at the horizon. This cannot be what actually happens. So general relativity must need a modification for regions approaching an event horizon.

[Bill S]

  Conversely, an object freely falling from infinity will be moving at the speed of light upon reaching the horizon.

You will not be surprised that I have some reservations in this regard.  An object freely falling from infinity would never arrive.  I assume, therefore, that infinity is being used figuratively. 

Given that that might be the case; what sort of distance would be required for the speed of the falling object to approach, closely, the speed of light?

[petelamana (OP)]

Considering the proportionality of gravity relative to mass, m, and the distance to the center of mass, r, ... m/r² ... wouldn't the distance need to achieve any velocity be dependant upon the mass of the body the object is falling toward?

[Bill S]

That sounds reasonable, but would suggest that "infinity" = an unspecified distance, in this case.

[petelamana (OP)]

Most definitely.

[jeffreyH]

You can approximate infinity by choosing a point away from the source that has a gravitational potential very close to zero. The reasons that infinity is used is that the field is said to extend to infinity and the escape velocity takes an object away from the source to infinity. It just means the object escapes completely and has no likelihood of being recaptured.

[Bill S]

You can approximate infinity.......  It just means the object escapes completely and has no likelihood of being recaptured.

Which, sort of, supports what I have said on a number of occasions: "mathematically malleable infinities are approximations".  Thanks, Jeffrey.