Science desk
< June 3	<< May \| June \| Jul >>	June 5 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

June 4

Black holes.

I subscribe for Astronomy magazine. In July 2014 issue on page 34 there is a picture of three types of black holes. The type 2 in the middle does not rotate but the accretion disk (AD) rotates. The picture shows a counterclockwise AD rotation which is of course irrelevant. In the picture above it (picture #1) the black hole has a spin, it rotates clockwise but the AD rotates in the opposite direction. The lowest picture (#3) shows the black hole rotating counterclockwise and the AD likewise rotating counterclockwise. They both spin in the same direction.

My questions are:

(1)How come we have a no-spin black hole if as many times has been stated in this website, everything in the universe has angular momentum. More than that, since the AD for this black hole rotates (counterclockwise), it has a mass and angular velocity, therefore it has angular momentum. When this mass ends up in the black hole it MUST transfer the angular momentum to it. Where has it gone?

(2) When the AD and black hole have opposite spins then the mass falling into the black hole should subtract the existing angular momentum of the black hole and eventually stop it. Then at the next stage the black hole should begin rotating in the direction of the accretion disk (synergistically). Is it correct?

(3) When we have AD and the black hole spinning in the same direction and the accretion process keeps feeding the black hole continuously the transfer of angular momentum should continue indefinitely and the black hole should accelerate? Is it correct? This may not be true though. The momentum increase may occur because of the increase in the mass of the black hole and not because of the speed.

Anybody could bring some clarity to all this?

Thanks, --AboutFace 22 (talk) 01:02, 4 June 2014 (UTC)[reply]

I don't really have a good understanding of angular momentum in the context of black holes. But I do know that when a black hole eats an accretion disk, huge amounts of angular momentum can be eliminated from the system via jets. See also Blandford–Znajek process and Penrose process. Also, in general you have to be careful with your assumptions about angular momentum when dealing with general relativity (as you must when dealing with black holes), because in general a global angular momentum isn't even defined in a curved spacetime, unless the spacetime happens to be asymptotically rotationally invariant;[1] see Angular momentum#Angular momentum (modern definition). Kerr black hole may also be useful. Red Act (talk) 04:30, 4 June 2014 (UTC)[reply]

Alternative theoretical formulations

As found here [2], has this ever been used at any page in wikipedia? DVMt (talk) 00:43, 4 June 2014 (UTC)[reply]

If you are asking whether alternative scientific theories have been described in Wikipedia, yes of course. For example Black hole#Alternatives. If you are asking something else, please explain. Please do not ask the same question on alternative desks.--Shantavira|^{feed me} 07:49, 4 June 2014 (UTC)[reply]

Thank you for getting back to me. Has it been used in a health care context? DVMt (talk) 18:40, 4 June 2014 (UTC)[reply]

Other examples of Wikipedia articles discussing "alternative theoretical formulations which have a following within the scientific community" would include any of the theories mentioned in Alternatives to general relativity#Modern theories 1980s to present. Red Act (talk) 08:06, 4 June 2014 (UTC)[reply]

What that principle is saying, is that if an idea or approach has significant support among mainstream scientists, even if they are a minority, then a Wikipedia article should not state that the idea or approach is pseudoscience. Since this is a negative principle -- defined by what an article does not say -- it's hard to point to specific examples in a useful way. One article that I think is an example, though, is cold fusion -- it is not described as pseudoscience in the article even though the majority of scientists think it is bogus. Looie496 (talk) 14:49, 4 June 2014 (UTC)[reply]

Thanks Looie496. Has it been used in a health care context? DVMt (talk) 18:40, 4 June 2014 (UTC)[reply]

You mean like using almonds as a treatment for cancer? ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:07, 4 June 2014 (UTC)[reply]

This doesn't strictly apply to health care, since in biology alternative theory properly exists only as an afterthought to alternative practice. If you find that a root helps an ailment, you don't really need an explanation for it, only data.

Several examples can be found that originated from the Soviet Union around the 1940s: phage therapy, an effective method of dealing with antibiotic resistant bacteria; low dose application of interferon alpha for flu and other infections diseases; and some practices based on the ideas of Michurinism, most notably graft hybridization. I would suggest that in some of these areas the U.S. and other Western culture not merely hasn't caught up to Soviet practice that has been ongoing for much of a century, but quite possibly may never catch up, either in theory or in practice, because there is some aspect or other about them that is incompatible with the present bureaucratic order. On Wikipedia, we cover the first of these pretty well, the second not at all, and the third is represented to be solely a form of chimerism which is theoretically more palatable but discards other literature.[3], [4], [5] Wnt (talk) 22:11, 7 June 2014 (UTC)[reply]

Distant times are closer than more recent times?

Hi - a physicist was talking on the radio about a year ago about how, in terms of time travel, were it possible, it would require less energy to travel into the distant past than the recent past. His explanation was counterintuitive but elegant and simple but my recollection of it is very dodgy. Maybe I should wait until it's in the distant past before I try to recover it...

Can anyone help with an explanation of this principle?

Thanks

Adambrowne666 (talk) 02:01, 4 June 2014 (UTC)[reply]

There really doesn't appear to be a way to time travel to the past. The various ways to do that that have been investigated and are at least consistent with general relativity all wind up requiring one thing or another which isn't known to exist, and for which there's really no evidence at all that would suggest that it might exist. Travelling faster than the speed of light would require particles which propogate faster than the speed of light in a vacuum. Closed timelike curves would require something like a pre-existing infinitely long Tipler cylinder or cosmic string. An Alcubierre drive or traversable wormhole would require a kind of matter that has negative mass. How much energy it would hypothetically take to travel to some time in the past would depend on what kind of probably non-existent thing you're presuming actually exists, and how you go about using it. Red Act (talk) 07:48, 4 June 2014 (UTC)[reply]

I've never heard anyone say that, it appears to make no sense, and I couldn't find anything relevant in a web search. This may be a permanent mystery unless you can remember enough to track down the radio show. Speculating wildly, it could have been about the idea that looking at distant objects is "looking back in time", and the fact that it's easier to see quasars or the cosmic microwave background than some more recent but less energetic events... -- BenRG (talk) 16:02, 4 June 2014 (UTC)[reply]

You wren't watching Doctor Who? The Weeping Angels method of feeding seems to be based on this premise. μηδείς (talk) 18:18, 4 June 2014 (UTC)[reply]

String Theory and the various interpretations of quantum mechanics

What is the relationship between String Theory and the various interpretations of quantum mechanics? ST seems also pretty interpretative to me, and covers QM too, so is it compatible with all? From what I understood, ST combines QM and General Relativity into one single mathematical model. To me, this seems to imply that you're looking for an underlying framework that explains both models... Doesn't that require "hidden variables"? Thanks very much for any clarifications you can provide. --Jules.LT (talk) 03:01, 4 June 2014 (UTC)[reply]

As far as I can tell, string theory gives no insight into the basic rules of quantum mechanics. It just assumes them. It is probably incompatible with some interpretations (e.g. Bohmian mechanics) because they require point particles or a fixed spacetime background or something of that sort. -- BenRG (talk) 07:10, 4 June 2014 (UTC)[reply]

Naively, I thought that ST assumed a more fundamental (or at least more abstract) set of properties of these putative strings, and then derived rules of QM (among other things) as a consequence of these more basic assumptions about strings. Is that a fair understanding for an educated non-physicist? It does mesh well with the notion that ST is very difficult to empirically test directly, and that it might just all be "made up". SemanticMantis (talk) 15:50, 4 June 2014 (UTC)[reply]

No, no one has ever found a more basic principle underlying quantum mechanics, i.e., the wave-like interference between different quasi-classical histories of a system that leads to quantized energy levels and entanglement. String theory's quasi-classical part has a lot of unusual features, but the quantum part is the same as ever (as far as I can tell).

Quantum gravity is hard to test simply because gravity is so weak. Any approach to quantum gravity has that problem. -- BenRG (talk) 01:59, 5 June 2014 (UTC)[reply]

Likewise, this is my understanding. Generally, the various interpretations do not change the mathematics; they are about the phenomenon of wavefunction collapse, which has nothing to do with mathematics, as far as anyone knows. The GRW theory does change the maths, but this is highly speculative at this stage. So if string theory became the dominant theory, interpretations of quantum mechanics would become effectively interpretations of string theory as well, although you would not expect anyone to adopt this terminology. IBE (talk) 11:41, 4 June 2014 (UTC)[reply]

Thanks. For Reference, I posted the question in a sciency forum where I'm starting to get interesting answers too [6] --Jules.LT (talk) 13:23, 5 June 2014 (UTC)[reply]

The Mortality Rate for Late-Term Abortions

Out of curiosity--what exactly is the mortality rate (for the pregnant individuals) for late-term abortions? Futurist110 (talk) 04:27, 4 June 2014 (UTC)[reply]

There's obviously no known 'exact' figure because

1) The mortality rate will depend on many different factors like the maternal age, maternal health, abortion method and competency of the person performing it, access to after care etc. These will vary quite significantly from country to country.

2) Many countries severely limit 'late term abortions' or even all abortions. Many such abortions will therefore be carried out illegally. Obtaining reliable statistics for such abortions is rather difficult, even more so if the government makes it difficult. Further the number of such abortions will likely vary significantly depending on a host of local factors.

3) Note that this also means that many reported abortions will probably be in the lower end of the fetus age if we're talking about a wide range (e.g. second trimester). On the converse side, it will often be the case that you're more likely to have reports of problematic abortions at a later fetal age and that many reported abortions (i.e. legal ones) will be in problematic pregnancies anyway. Although back on the earlier side, some reported abortions may be less problematic pregnancies where the mother or her family is sufficiently wealthy that they can obtain a more above board abortion and associated competency and after care, even in cases where it's questionable if they qualify under law. However whether these are reported will again depend significantly on a whole host of local factors. (And back to the converse side, depending on local factors it's possible even if the abortion is legal, a wealthy mother may be more likely to have it unreported.)

4) You didn't define late-term abortion, as our article says there's no universal definition.

Therefore there's no way anyone can hope to come up with an 'exact' figure, even if by that you mean an average figure for all abortions performed throughout the world in one defined year (or whatever time period) according to some defined meaning of late term abortion (e.g. anything over 20 years).

But to give one figure, [7] says ~10 times 0.8-1.5 per 100000 abortions is the maternal mortality rate for second trimester mifepristone–misoprostol in the US. I'm not sure the source itself is reliable, but hopefully their source is. Of course bearing in mind what I said earlier, many of these second trimester abortions may not fit in to whatever definition of 'later term abortions' you're using.

Nil Einne (talk) 08:11, 4 June 2014 (UTC)[reply]

For the fetus, the mortality rate is by definition 100%. 24.5.122.13 (talk) 08:17, 4 June 2014 (UTC)[reply]

Not true. Some fetuses survive their abortion, e.g. the Oldenburg Baby. --Roentgenium111 (talk) 15:21, 5 June 2014 (UTC)[reply]

The case you mentioned was a failed abortion. However there is real mortality rate for staff of clinics that perform abortions and are targetted by pro-life activists that have killed at least 8 people, see Anti-abortion violence#Murders. 84.209.89.214 (talk) 16:08, 5 June 2014 (UTC)[reply]

It depends on the definition of "abortion". As I understand it, an abortion is the intentional termination of a pregnancy; the death of the aborted fetus is a usual but not necessary consequence. "Pro-life" murderers are another matter still... --Roentgenium111 (talk) 16:03, 6 June 2014 (UTC)[reply]

This is going to be extremely tough to answer with any reliability. The problem is, in many places by law, and in most others by custom, a late term abortion is only going to be given to a mother whose life is already in danger. As we usually think of defining it, the risk of the abortion would need to exclude, say, the risk of death from eclampsia, even though the abortion may be given due to eclampsia being underway. In addition to potential arguments over the cause of death (and the philosophy of cause of death in general) blurring the statistics, there's going to be a lack of controlled experiments, because there won't be many situations in which a doctor is willing to randomly abort or not abort any given fetus to get data. Wnt (talk) 12:07, 4 June 2014 (UTC)[reply]

Does a black hole die?

Does a black hole die? — Preceding unsigned comment added by 37.238.106.10 (talk) 11:55, 4 June 2014 (UTC)[reply]

See Hawking radiation. It takes a really long time. Wnt (talk) 11:58, 4 June 2014 (UTC)[reply]

They can die faster in case the Universe is to come to an end in a Big Rip scenario, see here. Count Iblis (talk) 17:21, 4 June 2014 (UTC)[reply]

Yes, Compte Diable. But that's like saying human life expectancy will be drastically shortened if the sun mysteriously goes supernova. μηδείς (talk) 18:16, 4 June 2014 (UTC)[reply]

No, because a person's life span is negligible when compared with the time until the Sun will go supernova (which is never, although it will eventually go nova), while the life span of a (non-microscopic) black hole may be significant compared with the life span of the universe, depending on which end-of-the-universe theory you subscribe to.

A better analogy for the lifespan of a black hole might be the lifespan of trees, which could be significantly affected by forest-wide events, such as forest fires. StuRat (talk) 18:41, 4 June 2014 (UTC)[reply]

Pay better attention StuRat, I said if it were to mysteriously go supernova (i.e., now) not eventually go nova. μηδείς (talk) 17:17, 5 June 2014 (UTC)[reply]

Again, not a good analogy, because you are comparing something no scientists expect to happen (the Sun going supernova for no apparent reason) with something many do expect to happen (the end of the universe). StuRat (talk) 12:44, 7 June 2014 (UTC)[reply]

And of course, death by Hawking radiation is as yet entirely theoretical; although we're now pretty sure the black holes exist (see Sagittarius A* for one particularly good candidate), the precise details of their physics have not been observed, but only inferred or extrapolated from physical laws based on observations of other things. -- The Anome (talk) 17:24, 5 June 2014 (UTC)[reply]