Naked singularity

I think that what has been observed is the action of other stuff around the black hole

(if one could get close enough one could see the silhouette of the EH of a black hole, supposing that the view was not blocked by a huge ****load of stuff falling into the hole just then, but there is no immediate prospect of "seeing" the EH of a black hole like that)

I remember last year they "observed" the BH at the center of our galaxy.
Well, they didnt get a photograph of the event horizon.
what they saw was a star orbiting it so close that they figured it was too massive and compact to be anything else.


just by looking they could tell the star is coming within a few tens of millions of miles of the center of mass---it loops in real close, and
from the orbit they can tell the central object is several million solar masses
which would make the EH radius several million miles


so they said, what kind of a physical object could have a mass that is
several million solar
but yet be smaller than ten million mile radius

and they hypothesized various things and nothing they could think of (except a BH) was concentrated enough

also another bit of evidence is that from time to time this central object makes a lot of X-rays, which makes sense because thats what happens when a bunch of stuff falls into a hole. (it whirls around first and gets hot)

so somebody else may know different, but i think that astronomers have observed a lot of black holes both in other galaxies and in this galaxy but they so far can only infer what they are from how they act----making X-rays, making relativistic jets of particles, having stars loop in close to them, and suchlike behavior

 

I think scientists calculated the size of the event horizon of a black hole... I read it in a book a few days ago. Of what I remember it was tiny. About 50km diameter. So the eh is alot smaller than the star was

One question about the black hole in the centre of our galaxy and others. Are we the remains of the supernova moving away from the bh or our we being 'realed in'? Or are we not moving at all? Possibly the expansion cancels out the pull?

 

Event horizons and black holes go together by definition (the usual one at least -- there are alternatives). It is not true, however, that every singularity is necessarily surrounded by an event horizon. There is conjecture, though, that singularities must be 'clothed' in any physical situation. Penrose called this hypothesis cosmic censorship. Nobody has proven it to be correct, and some people think it has been proven wrong. It depends on what you consider a realistic situation.

There is a joke that astrophysicists, mathematical physicists, string theorists, and numerical relativists all mean different things when they say 'black hole.'

As Marcus implied, the astrophysicists' version of a black hole is an object so dense that it doesn't seem like it can be anything else. There is no measurement of an event horizon, and in a sense, there never can be. It would require knowing the future.

For this reason and others, the more mathematically minded relativists have tried to remove the concept of event horizon from black hole definitions. The results are Ashtekar, Krishnan, et al's isolated and dynamical horizon frameworks. These give measurable ways of determining whether you're at a (dynamical or isolated) horizon. This is not the same as an event horizon, but is actually much more useful.

I think that it is in any case very difficult in practice to obtain truly strong-field measurements of black holes, at least through electromagnetic measurements. I'm not really up to date on the relevant astronomy though.

 

The event horizon is spherical, it's just a difficult to depict a spherical event on a flat display, such as a piece of paper or a viewscreen.

As for the part of the event horizon plays in keeping the black hole alive; the event horizon is a part of the definition of what the black hole is. A black hole in the region of space in which gravity is so strong that light cannot escape, right? If light gets too close, it will not escape the gravitational pull but if it does not give that close, then it will escape. The event horizon is simply the exact distance which marks the difference between these two destinations for any passing light.

 

As Lurch said, a BH is defined to be a region where nothing can escape. The event horizon denotes the boundary of that region.

I don't know what depiction of an EH that you're talking about. Perhaps you're confusing spacetime diagrams and spatial ones? The black holes that people often talk about have spherical event horizons (in general, things can be much more complicated), but its usually more interesting to look at how a horizon forms over time rather than what it looks like at each instant. You can't draw four dimensions on a sheet of paper (3 space+1 time), so one or two of the spatial dimensions are usually ignored in those diagrams.

 

I think that what you are referring to is not the event horizon, but an accretion disk around a spinning black hole. The X-ray signatures that atronomers look for in the GOODS survey are artifacts of objects being ripped up and absorbed into the accretion disk of black holes. If the radiation were emitted in the near neighborhood of the event horizon, it would be redshifted out of the X-ray into lower frequencies.

 

sometimes when there is an "artist's conception" of a black hole, the picture shows a flat spiral of material spiraling in towards the center

one name for this flat spiral of doomed matter is the hole's "accretion disk"
[edit: shucks, turbo already posted about this, I see]
a neutron star can also have an accretion disk, I think

In those pictures of black holes, the event horizon is a roughly spherical or ellipsoidal surface which is much smaller and way in the center of the picture-----sometimes it might be so small, compared with the rest of the picture, that you think it is just a point

the accretion disk is big and the EH is comparatively little

It is the accretion disk (and also possibly some jets that can come out perpendicular to it) that makes the X-rays

the accretion disk material can get to be several million Kelvin essentially because it is being crowded into a smaller and smaller space and colliding with itself----this happens when a panic-struck crowd of people are all trying to get out through one door or when cattle stampede: it comes from too much crowding and bumping----one can say that the accretion disk experiences "friction". Anyway the matter gets very hot and radiates as it is spiraling in towards the event horizon

if you could look into the core of the sun, the light being produced there would be X-rays because the core is some 10 or 15 million Kelvin and X-ray is the glow from things that hot.

why am I writing this----you Wolram know all this
what was the original question?
anyway the black hole in center of "our" galaxy from time to time makes X-rays this way

what Stingray says about "isolated horizons" is very interesting. In fact the concept of event horizon is not very operational. We humans can do better. Ashtekar is right to try to define more useful notions of horizon around black holes. try to get Stingray to talk more about this (since he is at penn state) and pay close attention.

 

You could think about the EH in terms comparable to an asteroid being gravitationally captured by a planet. Depending on its mass and velocity, there is a critical distance at which it will be gravitationally captured into orbit around the planet. At a slightly greater distance, it will do a 180 and head back parallel to the direction of approach. This critical distance corresponds to the EH of a BH. The difference being that the mass of the BH is the sole determining factor as to how close a photon may approach before being captured into orbit. Theoretically you could measure the size of an EH in this manner. Just find a sufficiently bright object in front off a BH sightline and look for a twin image. The rest would be 'easy'.

 

Something that you might find interesting to think about is that an event horizon can form in a region of space that is completely flat. It is an entirely global concept. There is no local experiment that we could do to claim that there is not an event horizon forming through the earth right now.

I'm being rather pedantic here as I'm using the formal definition of an event horizon (which doesn't really make sense in light of modern cosmology anyways). If you start making some assumptions about how stable your system is, relax the definitions somewhat, and wave your hands a bit, then you can do "practical" experiments. The problem is that all the theorems that people have spent time proving about event horizons really aren't guaranteed anymore with "heuristic event horizons." To be honest, most people just ignore that, and they're probably justified to do so in most cases.

 

A heuristic event horizon would be a discovery in and of itself. It does make sense from a quantum perspective, but, the GR implications are rather vague. I would be interested in how you would design an experiment to test it.

 

I've never heard anyone call them heuristic event horizons either. I just made up the name. It was meant in the usual english sense of "heuristic:" an intuitive guess or rule-of-thumb. So there is no real definition of what I was talking about. The rigorous definition is still in favor, but it is too difficult to apply most of the time, so some approximate, intuitive form of it is used instead. Instead of saying that something inside an event horizon can never escape to infinity, people assume it is sufficient if things seem to be trapped in a local region over whatever timescale you're interested in.

A lot of intuition in GR comes from studying exact solutions. Unfortunately, there are really only a couple which seem to have any physical relevance (e.g. its easy to generate solutions with negative energy or causality problems). If you're thinking about black holes, there's essentially only one family of solutions that is commonly thought about. It represents a black hole in equilibrium without any surrounding matter.

Because this is one of the few things that we really claim to understand in GR, people try to apply intuition gained from that particular idealization to all kinds of situations. There are good reasons to do this most of the time, but it can get dangerous to take a single example so seriously.

Getting to my point now, its common for people to just assume that whatever system they're looking at is close enough to the solution that they're familiar with that event horizons "act" similarly in both cases. So people might say that there is an event horizon if they see something like what Chronos described, but it could be formally incorrect, depending on the details of the situation.

It actually isn't too hard to construct systems where intuition can be shown to qualitatively fail.

In numerical relativity where people try to deal with cases that are far from any known solutions, they don't even try to approximate event horizons. They either use the dynamical/isolated horizons I mentioned before, or they find trapped surfaces. The latter objects are defined locally as having some particular optical properties. So you can unambiguously find trapped surfaces by looking at the behavior of light rays, but making conclusions about an event horizons from those same experiments requires several assumptions.

I unfortunately don't know of any references which discuss these things well enough without math.