Free Republic
Browse · Search
News/Activism
Topics · Post Article

To: UCANSEE2; All
"Thank you for the response."

You're welcome. :-)

I'll ping everyone, since this may be of some general interest.

"A black hole, in and of itself, only absorbs radiation.

I hear it absorbs planets and stars."

True, but we were discussing "seeing" the black hole, and generally speaking we don't see things by reflecting planets and stars from them...

'It can be "seen" if it passes in front of a light source like a galaxy or bright nebula.'

"AND YET... if you look at the photos accompanying this article, the BRIGHT SPOTS in them are claimed to be 'black holes', are they not?"

The bright spots are indirect evidence of the black hole(s), as pointed out by my next remark:

This article isn't discussing actually "seeing" the black hole, but instead the radiation emitted from gas excited as it is compressed and heated by gravity near the black hole event horizon.
Note the "bright spots" are shining brightly in the X-ray spectrum, indicating a very energetic region. The only things that ever shine brightly in that spectral region on the Earth's surface are nuclear explosions.

"So... exactly where is the 'event horizon', and if the center is 'black' and only the 'ring' or horizon gives off radiation, then surely we would see a 'ring' shaped effect instead of a globular emanation."

There's a lot of good material out there about black holes, it'd be good for you to read some if you're interested. I'll try and hit a couple of high points here.

The event horizon is a 3D shape surrounding the singularity. If there's no rotation (unlikely) it would be spherical, otherwise it's an oblate spheroid similar to the Earth or Sun.

In a nutshell, a black hole forms when there's so much mass in a neutron star that the intrinsic "strength" of neutrons to "push each other apart" is overcome by gravity. Essentially matter itself collapses under the pressure. Then, nothing can stop the neutron star from collapsing towards a point mass, called a singularity. Current physics cannot describe the conditions at the singularity. Charge and spin are both conserved for a black hole (a black hole may have spin, which distorts the shape of the event horizon as I mentioned before).

Perhaps fortunately for physics ;-) the singularity itself can't be observed, due to the event horizon. The event horizon occurs at the distance from the singularity at which the escape velocity becomes "c" (the speed of light). As I mentioned before, it is a 3D shape surrounding the singularity. Due to some associated details of relativity, an outside observer will never witness the final collapse of matter of the original object, as intense gravitational fields slow down time from the viewpoint of such an observer. Black holes were originally called "frozen stars" for this reason.

According to current physics, nothing can escape from within the event horizon, since it would have to exceed the speed of light to do so. In the case of a rocket, which can climb away from a body without exceeding the escape velocity given enough energy, an infinite amount of energy would be required, so that that won't work either.

One thing that might have contributed to your confusion on this point is the concept of the "accretion disk". Accretion disks commonly form when material with angular momentum falls inward. Our solar system almost certainly formed from an accretion disk, for instance. They have been directly observed in many different astronomical observations.

The black holes discussed in the articles almost certainly possess accretion disks, and they are likely the source for almost all of the observed energy. However, gas surrounding the central region is also heated and excited, and will produce radiation as well. It's also not clear at what angle we're observing the accretion disk, from directly "above" or "below" it would appear circular, same as a spherical cross-section. There are also often jet phenomena associated with accretion disks that might point towards us and both directly radiate or excite gas into radiating.

"The whole concept of a 'hole' implies a two-dimensional object."

That's one of your big misconceptions... A black hole's gravity field is in fact a 3D entity, although some visualizations sure make it look 2D, for instance the one where space-time is represented as a membrane with a funnel-shaped distortion. That is to give you an idea of how gravity distorts space-time, but it does it in 3D, not 2D. A spacecraft's path will curve around a massive object regardless of the 2D plane its orbit inhabits.

"If a black hole has an event horizon, can one see it from the 'side', or from 'behind' it? Would it not have to have an event horizon no matter which direction you approached it from? If true, then the 'horizon' that we 'see' from Earth only exists to those on Earth. To someone from another far away galaxy, the 'horizon' would exist only to them."

Again, you're conflating a 2D concept with a 3D reality.

"One point made in the article is that black holes in intergalactic space (extremely little gas) would be unobservable except for the point I made first- passing in front of a light source.

"So... they would be strong enough to suck up the light, but not strong enough to suck up the source of the light?"

The gravity from a million solar mass black hole is down to a 1G (Earth surface gravity) field at only ~55 AU from the black hole. 1 AU is the distance from the Earth to the Sun, 150,000,000 km. So, again using the inverse square law of gravitational force, at a distance of 55,000 AU the gravity field from a million solar mass black hole is down to one millionth of a G. 55,000 AU may seem like a long way, but in fact a single light year is ~63,000 AU. So, you can see that the gravity from such an object is quite weak at a range of many light years (tens to billions).

The black hole not only absorbs light directly, but its gravitational field makes light bend, an effect called "gravitational lensing". Such an effect has been directly observed in astronomy, as light bends around galactic clusters. It has also been observed as starlight is bent around the Sun during a solar eclipse.

"The mass of the black hole will increase whenever either energy or mass is absorbed, as determined by E=mc2.

"I would agree. However, how do we measure the mass of a black hole, since the energy and matter 'disappear' into it?"

In astronomy, the mass of things is often determined by the gravitational effects on other things. For instance, the mass of the large black hole at the center of our own galaxy was verified using the speeds of stars orbiting near it. Nothing except a large black hole would enable the stars to attain the observed speeds, as measured by Doppler shifts of spectral lines. Here's a good explanation from here.

Data taken by astronomer Andrea Ghez (video here) was able to show the complete orbits of stars orbiting the galactic center at whiplash inducing speeds. With orbits that would fit inside our solar system, some of these stars had orbital periods of less than 10 years! The only way to get an orbit that small and that fast is to have a supermassive black hole in the center of the Milky Way. Calculations estimate its mass at 4 million times the Sun’s mass! No object other than a black hole is dense enough to fit in such a tiny space. Her work, and parallel investigations using the European Southern Observatory’s Very Large Telescope (likewise for infrared imaging), show conclusively that a massive black hole sits in the heart of our galaxy. This black hole and the environment it creates can explain all other unusual observations, from x-ray flashes to radio emission. Such supermassive black holes are proving to be common at the centers of bright galaxies, and their growth produces the spectacular phenomena of active galactic nuclei.

"(Please understand I am not 'arguing' or telling you that you are wrong, or I am right. I am only trying to learn, and the way to learn is to question those with some knowledge. Many times by asking these 'silly' questions, I learn quite a bit. Whether I 'accept' your answers or not, I do extremely appreciate your input)"

I'd suggest that "the way to learn is to question those with some knowledge" is instead a way to learn. Books are useful, and there's a ton of free information on the web.

"I do 'believe' that the entity we describe as a 'black hole' is something that is beyond our ability to understand or describe, at this time."

Actually black holes have been described in excruciating (mathematical) detail, and based on observational data either black holes exist, or something that we currently don't have a clue about exists that acts exactly as we'd expect a black hole to act. I strongly suspect black holes exist pretty much as described in the literature.

I spent a bit more time on this than I'd intended, but it was fun reviewing some of what I'd read years ago regarding black holes. I also ran across this paper today, which shows some of the interesting work going on with astronomical observations.

30 posted on 06/05/2012 6:53:22 AM PDT by PreciousLiberty (Pray for America!!!)
[ Post Reply | Private Reply | To 28 | View Replies ]


To: UCANSEE2
Darn it, I also meant to make one more point about:

"I would agree. However, how do we measure the mass of a black hole, since the energy and matter 'disappear' into it?"

The mass doesn't disappear, it is also (rather obviously) conserved. Thus, the "million solar mass" black hole. ;-)

31 posted on 06/05/2012 7:03:16 AM PDT by PreciousLiberty (Pray for America!!!)
[ Post Reply | Private Reply | To 30 | View Replies ]

To: PreciousLiberty
Thank you, again, for spending so much time providing some very informative responses. : )

Books are useful.

Yes, they are. And while reading a book on the subject(s) would help ME, it wouldn't help anyone else. (I play a dummy on the internet, but I do it for a reason)

I'll ping everyone, since this may be of some general interest.

... AND THAT is exactly the reason. I asked questions a layman might ask, and you provided detailed and informative responses (to the best of your knowledge) and now perhaps at least dozens, if not hundreds of people will get a chance to READ IT here on FR.

You also did a very good job of presenting the information in layman's terms. A person who is skilled in one particular area (like physics or astronomy), but who cannot present the information in terms that most people can understand, is not quite as 'bright' as he thinks he is.

Consider me as Tom Sawyer. I got you to paint the fence, and now the whole neighborhood is 'brighter' because of it.

P.S. There was a thread here on FR recently where 'scientists' (a term used very loosely) declared that there is no 'black hole' at the center of the Milky Way Galaxy. What do you think of that claim?

32 posted on 06/05/2012 10:00:06 AM PDT by UCANSEE2 (Lame and ill-informed post)
[ Post Reply | Private Reply | To 30 | View Replies ]

To: PreciousLiberty

thanks, enjoyed reading your post.


35 posted on 06/05/2012 10:37:44 AM PDT by jpsb
[ Post Reply | Private Reply | To 30 | View Replies ]

Free Republic
Browse · Search
News/Activism
Topics · Post Article


FreeRepublic, LLC, PO BOX 9771, FRESNO, CA 93794
FreeRepublic.com is powered by software copyright 2000-2008 John Robinson