Wikipedia:Reference desk/Archives/Science/2012 December 26#Roche limit for black holes

Name a few chemicals that are hazardous, used rarely and applied for novel research purposes (ex:certain toxins that are researched in very few laboratories against deadly unusual microbes). — Preceding unsigned comment added by 213.120.141.254 (talk) 06:07, 26 December 2012 (UTC)

: Is that what your teacher asked you to do on your homework? -- Jack of Oz ^[Talk] 06:58, 26 December 2012 (UTC)

nope. I am aware that advise and homeworks are not discussed here. — Preceding unsigned comment added by 213.120.141.254 (talk) 09:14, 26 December 2012 (UTC)

:If it looks like homework, sounds like homework, smells like homework, it probably is homework. You've asked a question in a way that probably 1000's, even millions, of chemicals, toxins, virus particles, and other substances might qualify. And it should be easy to think of a few if you give it a moment's thought - a few more moments' thought might suggest some words to google. Why don't you just run your eye over the periodic table and look up a few promising elements? Some of the high weight elements have only been synthesied in one or two labs at considerable effort just to get a few atoms. If you narrow it down a bit what you want to class as rare, and tell us what steps you have taken so far to find out, i.e., show us you are not just lazy, we might feel more inclined to do a bit of digging for you. Floda 58.164.226.231 (talk) 11:11, 26 December 2012 (UTC)

::Except that those superheavy elements are not really used for what your question seems to classify as novel research purposes – there's not much time to do much with them before they decay, given their short half-lives. Pure research is more like it. Double sharp (talk) 13:49, 26 December 2012 (UTC)

• my favorite Brevetoxin --Stone (talk) 11:27, 26 December 2012 (UTC)

:This blog[http://pipeline.corante.com/archives/things_i_wont_work_with/] is a source of wonderful stories about chemicals that no sane person would handle, usually because they're explosive, corrosive, or incredibly smelly - all backed up with references to actual research that has used them. AlexTiefling (talk) 10:14, 27 December 2012 (UTC)

::Of these chemicals, the only one I've ever had to work with is anhydrous hydrogen fluoride (we used it, and still use it, in the alkylation unit at the refinery). 24.23.196.85 (talk) 20:01, 29 December 2012 (UTC)

I know before the discovery of antibiotics, herbs were used to treat bacterial infection. Is there any information available about the success rate of this treatment? --PlanetEditor (talk) 09:24, 26 December 2012 (UTC)

:I'm not aware of any quantitative information, but the basic answer is: dismal. No herbal treatment comes anywhere close to penicillin. For superficial infections, the most effective primitive treatments are probably either licking the wound (saliva is a pretty potent antibiotic) or spreading honey on it. Looie496 (talk) 16:18, 26 December 2012 (UTC)

::Or Maggot therapy. Ruslik_Zero 18:53, 26 December 2012 (UTC)

:::Or putting a poultice of moldy bread on it, as the ancient Greeks had done (but that is actually a primitive form of antibiotic treatment, they just didn't know it at the time). 24.23.196.85 (talk) 01:08, 27 December 2012 (UTC)

:Researching it takes me to links about how we are almost back to the pre-antibiotics era (due to superbugs). Maybe we will have to use greatpa's antibiotics again. OsmanRF34 (talk) 22:09, 26 December 2012 (UTC)

1) Is it possible to increase train speed by doubling current train width?

2) Why China reduce its train speed from 350 to 300km/h, does reducing the speed by 50km/h also increase the train safety? Why?

3) What's the main propulsion inside fast train? Electric motor?

4) With current technology, is it cheaper to build a bridge or a tunnel below sea? roscoe_x (talk) 14:51, 26 December 2012 (UTC)

:I numbered your Q's for ease of response:

:1) Doubling width does not automatically allow for an increase in speed. In fact, you want to minimize cross sectional area (≈height×width) to reduce drag.

:2) Any decrease in speed increases safety, since it reduces the chances of a crash and also the severity.

:3) There are a variety of propulsion systems used, but the best choice, IMHO, is to not have any engine on-board, and instead deliver energy using the tracks, as in a mag-lev train.

:4) A bridge is cheaper in shallow water, and a tunnel in deep water (due to the height of the supports needed for a bridge). StuRat (talk) 21:42, 26 December 2012 (UTC)

::1) Widening does indeed allow for faster turning.

::2) Operating below maximum speed is standard practice, as it allows trains to make up time following delays.

::3) High Speed Trains generally use electric motors. Delivery of propulsion from the track, as is proposed above, would incur absurdly large infrastructure costs and is not a sensible proposal for a large network.

::4) A bridge is cheaper for crossing water. Building a tunnel under deep water requires a deep tunnel which is also absurdly expensive. A tunnel is used for going under mountains or the like. — Preceding unsigned comment added by 92.11.76.114 (talk) 23:38, 26 December 2012 (UTC)

:::1) Perhaps, if you mean wider and shorter. But wider and proportionally taller doesn't help.

:::3) You need to read up on maglev trains: [http://science.howstuffworks.com/transport/engines-equipment/maglev-train.htm].

:::4) The cost of building tunnels under deep water rises more slowly with additional depth than the cost of bridges does. Hence the Chunnel, as opposed to a bridge. StuRat (talk) 23:43, 26 December 2012 (UTC)

::::Indeed, "chidge" just doesn't have the same ring to it. Evanh2008 ^{(talk|contribs)} 11:12, 29 December 2012 (UTC)

:2) Per High-speed rail in China#Corruption and concerns, the official line is that "This was in response to concerns over safety, low ridership due to high ticket prices, and high energy usage." Suspicious people (like me) speculate that it is because of corruption in the construction that may compromise safety. Clarityfiend (talk) 22:21, 26 December 2012 (UTC)

:1) The main problem with doubling width is that it would necessitate a change in loading gauge, and almost certainly in track gauge. High speed trains run on regular tracks at least part of the time, if only to be taken for servicing and so on. And I understand that the Chinese authorities are trying to increase the adherence to Standard Gauge throughout Eurasia, to try and let their own network link up with other nations', and thus enable truly transcontinental trains. As Russia currently uses Russian Gauge, as do many ex-Soviet nations, that's quite a challenge. Introducing a variant gauge just for high speed would certainly over-complicate the task. AlexTiefling (talk) 00:08, 27 December 2012 (UTC)

::I remember that according to one of Albert Speer's books, Hitler had a plan for trains twice as large in every dimension which his underlings, citing similar concerns, politely convinced him to defer until after he won the war. Wnt (talk) 00:17, 27 December 2012 (UTC)

:::Stalin had no such inhibitions - see 4-14-4. Tevildo (talk) 00:43, 27 December 2012 (UTC)

::1) I recommend you read up on Brunel's seven-foot gauge.

::4) Unless built over very shallow water, a bridge is very limited in its maximum span compared to a tunnel, due to requirements for intermediate supports.

:: 24.23.196.85 (talk) 01:33, 27 December 2012 (UTC)

: One of the practical limits on the speed of passenger trains on existing track is the lateral g-forces exerted on them in corners. Hence the two ways to increase speeds are:

:# To use larger radius curves - which results in difficulties with using existing track - or even with upgrading existing track without a lot of land purchasing.

:# To attempt what British Rail did with tilting Advanced Passenger Train...which was notoriously unsuccessful.

: SteveBaker (talk) 03:55, 27 December 2012 (UTC)

::Is it the lateral forces on the passengers or train that's the problem ? StuRat (talk) 04:07, 27 December 2012 (UTC)

:::On the passengers -- they get thrown sideways. Hence the use of "pendular suspension" in many high-speed trains like the aptly-named Italian Pendolino or the Swedish X 2000. 24.23.196.85 (talk) 06:31, 27 December 2012 (UTC)

::::Seems like a much simpler fix for that is putting the backs of the seats against the windows (perhaps slanted with the feet forward and head back) and requiring people to remain seated when going through curves. StuRat (talk) 22:17, 27 December 2012 (UTC)

:::::That's supposed to be a joke, right? 24.23.196.85 (talk) 00:48, 28 December 2012 (UTC)

::::::No. Also, looking out the windows at 300 kph can cause nausea even if there's no lateral g's. StuRat (talk) 00:55, 28 December 2012 (UTC)

:::::::And just how do you stop people from getting up and walking around, or get them to reseat themselves every time the train approaches a curve? Also, not being able to look out the window on a curve can cause worse nausea than in the scenario you describe. 24.23.196.85 (talk) 01:41, 29 December 2012 (UTC)

::::::::The same way you get them to sit on airplanes when they encounter turbulence. StuRat (talk) 04:13, 29 December 2012 (UTC)

:::::::::Turbulence on an airplane happens much less frequently than curves on a railroad line. 24.23.196.85 (talk) 19:58, 29 December 2012 (UTC)

::::::::::Yes, but it's also less predictable. Being able to plan the exact times people need to remain seated is quite helpful. And airplane passengers also need to remain seated during take-off and landings. StuRat (talk) 23:27, 29 December 2012 (UTC)

:::::::::::Well, I don't think the pax would appreciate being forced to sit in their seats more than half the time, which is what would happen on most lines if this solution was adopted. 24.23.196.85 (talk) 01:08, 1 January 2013 (UTC)

::::::::::::That certainly is the case on airplanes, where they encourage people to stay seated the entire time, except for when using the bathroom. If making the train tilt correctly doubles the price, I, for one, would rather save the money. StuRat (talk) 06:54, 1 January 2013 (UTC)

:Regarding question #4, bridges need not be rigidly anchored to the sea bed. A pontoon bridge design similar to the Lacey V. Murrow Memorial Bridge may be an economic option. Likewise, a tunnel need not be embedded in the sea bed. See submerged floating tunnel. A combination of the two or a bridge-tunnel may be needed to balance shipping and cost considerations. See also transatlantic tunnel. -- Tom N (tcncv) talk/contrib 16:55, 28 December 2012 (UTC)

::You can't send a bullet train over a pontoon bridge or through a submerged floating tunnel -- it would break apart. And a bridge-tunnel crossing can only be built in shallow water -- it needs intermediate islands (natural or artificial) for the transition between bridge and tunnel or vice versa. 24.23.196.85 (talk) 01:50, 29 December 2012 (UTC)

:::Couldn't mechanical resonance be defeated by good engineering (or even by some kind of computer controlled active compensatory mechanism)? Wnt (talk) 18:51, 31 December 2012 (UTC)

::::Not for such an inherently flexible structure as a pontoon bridge or floating tunnel -- not over the full range of vibrational frequencies produced by a fast train, anyway. Hell, it's not even possible to drive a fast train over a suspension bridge, for the same reason -- so why would a pontoon bridge be any better? 24.23.196.85 (talk) 01:04, 1 January 2013 (UTC)

where is Roche limit for any black hole?Akbarmohammadzade --78.38.28.3 (talk) 16:29, 26 December 2012 (UTC)

:Black holes do not have Roche limit. They are essentially inviolable. Ruslik_Zero 18:51, 26 December 2012 (UTC)

::I think that answer is true, at least in a sense, but it is a bit unsatisfying to me. The Roche limit is a point where part of an object is going to fall away from it. How could something inside an event horizon fall away from the center? To illustrate, the Roche limit article says that a satellite can only be torn apart if its density is less than twice that of the primary, because otherwise the limit is inside the primary - so anything that could bring a black hole to the Roche limit would be darn near one already. And so my assumption is that the black hole + primary become a big rotating black hole. But note that a rotating black hole has a ring singularity, and so one can argue that you have disrupted the innards of the black hole and turned them into an "orbiting ring of debris" in some very, very loose sense. I'm not really giving you an answer with this, just making an appeal for an imaginative consideration by one of the experts. :) Wnt (talk) 20:32, 26 December 2012 (UTC)

:This is an interesting question that I've never thought about before. There are a bunch of papers/preprints about it on the arXiv. According to [http://arxiv.org/abs/0705.1570 arXiv:0705.1570] (which has pictures), the event horizon is distorted by the tidal force into a cigar shape. This actually surprises me, because the event horizon of a rotating black hole is not distorted: it's a sphere with no equatorial bulge. The singularity remains a point/line (I think), but the Planckian region, where the curvature exceeds the Planck curvature and therefore general relativity is presumably wrong, is flattened like a pancake. The paper mentions that there's a threshold tidal force beyond which the pancake extends outside the event horizon, giving you a naked singularity (more or less). This might imply the existence of a (mass-dependent) maximum tidal force analogous to the maximum rotational speed of a rotating black hole (beyond which you get a naked singularity). I'm not sure how this force could actually be achieved, though, since it seems to require another black hole essentially touching the first.

:[http://arxiv.org/abs/0910.4311 arXiv:0910.4311] mentions that a black hole orbiting another body will gain mass from tidal friction, and I assume (though it doesn't say so) that it would eventually become tidally locked to the other body. This isn't directly relevant to the question, but it's interesting. -- BenRG (talk) 01:34, 27 December 2012 (UTC)

thanks all. It might be two Roche limit for any black hole .One of them outer

than event horizon and other inner it as we know all objects elongate

infinitive there. — Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 08:52, 27 December 2012 (UTC)

:It would be useful to frame the question properly. The Roche limit for a body applies in relation to another, and depends upon properties of both (the mass of the primary, and the density and configuration of the secondary). Thus, the Roche limit of a (normal matter) body orbiting a black hole (the primary) will be the radius from the black hole where the object will disintegrate under the influence of tidal forces, and this will typically be far outside the event horizon of the black hole for smaller black holes, but inside the event horizon for supermassive black holes such as in the centre of the galaxy. The radius of a black hole's event horizon is proportional to its mass. The Roche limit is proportional to the cube root of the ratio of the black hole's mass and the orbiting body's density. The previous answers assumed that the black hole is the secondary, orbiting an even larger primary. Here the (infinite) density of a non-rotating black hole makes the Roche limit zero radius from the primary's centre, though the rotating case (a Kerr black holes as the secondary) might be interesting, since the singularity presumably differs non-trivially from a spherical configuration. — Quondum 09:35, 27 December 2012 (UTC)

When we talk about black holes we imagine some thing absorbing and destroying all around it .This is not reality about them. If it is so that any black hole has infinitive mighty and effect at all distance , this has contraction with nature laws. I want to say (as said last about planets round black hole )that the black hole with mass M and rotating properties , has as gravity field as last star .we

say if any black hole was any star which had planets , its members will rotate

round it safely .

if our sun was able to be black hole ,our earth was rotating round it ,without any difference

later and after .

why we do such mistake ?

black hole sun cannot destroy not only low density Saturn but also near mercury ,then we give some hypothetical properties to it,

of absorbing all the things?

can any black hole absorb its neighborhood star?never .

have you never calculate Roche limit of any black hole?

excuse me --Akbarmohammadzade (talk) 10:47, 27 December 2012 (UTC)<

:Hmmm, there's another problem: there are no stable orbits very close to a non-rotating black hole. See photon sphere. But black holes in such relationships might be rapidly rotating, so sometimes I suppose it is possible... (but I'm out of my depth on this) Wnt (talk) 15:38, 27 December 2012 (UTC)

Shwartz child radius for any star 10 times massive than sun is about 5kilo meters Akbarmohammadzade — Preceding unsigned comment added by 2.187.90.105 (talk) 17:08, 27 December 2012 (UTC)

1.5 times further it will be about 8 kilometers--Akbarmohammadzade (talk) 17:32, 27 December 2012 (UTC)

I require a specific article from a past Lancet journal.

The article if from 2005 volume366 pages 29-36.IT IS specifically about the ABCD criteria after a TIA (transient ischemic attack). — Preceding unsigned comment added by 24.185.248.186 (talk) 16:32, 26 December 2012 (UTC)

:The article is titled A simple score (ABCD) to identify individuals at high early risk of stroke after transient ischaemic attack. If you search for the title at scholar.google.com, you can get access to several downloadable online versions, including [http://www.sphemerg.ca/files/ABCDStrokeAfterTIA.pdf this one]. This way of finding things works pretty frequently. Looie496 (talk) 17:23, 26 December 2012 (UTC)

:Wikipedia:WikiProject Resource Exchange/Resource Request is the place to request articles that aren't available free online. -- BenRG (talk) 17:52, 26 December 2012 (UTC)

::How does it come that this is legal? OsmanRF34 (talk) 21:48, 26 December 2012 (UTC)

:::We don't give legal advice. But you might want to read fair use. 208.102.63.50 (talk) 23:04, 26 December 2012 (UTC)