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July 6[edit]

Black hole powered slingshots[edit]

Suppose there was a 10,000 solar mass black hole without an accretion disk five light years from Earth, orbiting the galactic center along with the rest of the stellar neighborhood with a typical relative motion in a random direction. How much velocity could a robot ship powered with boron-11 fusion and nitrogen propellant achieve with Gravity assist#Powered slingshots? 99.24.223.58 (talk) 00:24, 6 July 2011 (UTC)[reply]

Note that to get the most boost, you need to do multiple slingshots, so it would be useful to have two or more massive objects close to each other and sling back and forth between them. StuRat (talk) 01:45, 6 July 2011 (UTC)[reply]

It's not possible to slingshot off of a single object.
Taking the galactic center as the second object, the slingshot speed will be of the order of the orbital speed of the black hole.
The mass of the black hole is not important.

Dauto (talk) 02:15, 6 July 2011 (UTC)[reply]

[[Akbar Mohammadzade]]: somebody used to say something about black holes , I prefer to say about supernovae , the first one is death and being secretary , the second one shows celebrating and brightness , and life---------------
gravity field of black hole dosenot differ with first star in outer space than event horison .what this says us ?It says that the massive 10000 solar mas star is equal to such black hole from distance which you are monitoring , dont got afraid of black hole , it is not much more scary from far away so distance .we may near by to any black hole 1 AU and our space craft only do rotate 1.4 times more than earth speed roud sun.

--78.38.28.3 (talk) 05:51, 6 July 2011 (UTC)Akbar Mohammadzade>>M2=2M1 &d2=d1 ==> V2=1.4V1 << ;;;M2=10000M1 ==>V2=100V1--78.38.28.3 (talk) 13:43, 6 July 2011 (UTC)[reply]

This....really does not make any sense.-RunningOnBrains^(talk) 07:04, 6 July 2011 (UTC)[reply]

>>M2=1.4M1 d2=d1 V2=1.2V1 << — Preceding unsigned comment added by 78.38.28.3 (talk) 07:06, 6 July 2011 (UTC)[reply]

Note, from the section title, that the questioner is asking about a powered slingshot exploiting the Oberth effect, not just an unpowered gravitational assist. The answer will therefore depend on the capabilities of the hypothetical spacecraft - specifically, its delta-v capability, which is in turn determined by its maximum thrust and how long this can be maintained - as well as how deep it can go within the gravitational field of the black hole . Gandalf61 (talk) 08:07, 6 July 2011 (UTC)[reply]

With a supermassive black hole, frame dragging may allow for another kind of slingshot effect. The black hole would need to be rotating for that, of course. A smaller rotating black hole would also have a significant frame dragging effect, of course, but the tidal forces would probably grow too high before you got close enough. --Tango (talk) 12:30, 6 July 2011 (UTC)[reply]

Supermassive black holes would likely still have accretion disks, and would not be safe to navigate. 99.24.223.58 (talk) 18:26, 6 July 2011 (UTC)[reply]

detail about a herb[edit]

Clinathanus — Preceding unsigned comment added by 60.53.93.193 (talk) 06:32, 6 July 2011 (UTC)[reply]

Clinathanus isn't coming up with anything. Google proposes Clinanthus, Clianthus, or Clinacanthus. Of these, the last sounds most "herbal" in terms of prominent traditional medicinal uses, at first glance, but please confirm. A high-ranking blog hit describes it as "Scientific name: Clinacanthus nutans, English name: Snake plant, Chinese name: 鰐嘴花, E zui hua, 扭序花, Niu xu hua, 竹節黃, Zhu jie huang, 小接骨, Xiao jie gu, 柔刺草, Rou ci cao, 青箭, Qing jian, 鵝嘴花, E zui hua, 憂遁草, You dun cao, 沙巴蛇草, Sha ba she cao and Malay/Local name: Kitajan, Gendis." Our "snake plant" article isn't this. Clearly some research is needed to sort out if all those Chinese terms really refer to the same thing. But first, is this what you want? And what interests you about it? Wnt (talk) 06:41, 6 July 2011 (UTC)[reply]

This page [1] seems to contain reliable information about this herb. Itsmejudith (talk) 06:51, 6 July 2011 (UTC)[reply]

It is present, but not in English version. See the Chinese version 鳄嘴花 or from the interwiki to other languages available. --Tomchiukc (talk) 06:17, 3 December 2014 (UTC)[reply]

dark matter[edit]

Einstein field equation and other scientists cosmology theories resulted some mathematical and or practical results ,some of them are observed and detected in observation and practice .here we list some of theoretical and practical results :

1)dark matter ==>this seems to be any hiden dynamic ;;;;;;;;;;;;; 2)gravity wave ==>this is mathematical result of formula ;;;;;;;;;;;;;; 3)dark energy ==>equal first ;;;;;;;;; 4)world expanding ==>observed by Hubel;;;;;;;;;;;;; 5)curving light at edge of sun==>observed by Edington ;;;;;;;;;;;;;; 6)quantom gravity ==>unsolved until now ;;;;;;;;;;;;;;;;;; 7)forcasting black holes ==>foud detection way by Hawking;;;;;;;;;;;;;;;; 8)and some field equation and unite force theorem ;;;;;;;;;;;;;;;;;;

question :what do you do think about

dark matter?--78.38.28.3 (talk) 13:59, 6 July 2011 (UTC)akbarmohammadzade [reply]

copyvio from http://physics.about.com/od/physics101thebasics/a/fiveproblems.htm removed

— Preceding unsigned comment added by 78.38.28.3 (talk) 14:17, 6 July 2011 (UTC)[reply]

Your question contains a verbatim copy of content from About.com. The inclusion of this violates Wikipedia's copyright policy, so I have removed it. -- Finlay McWalter ☻ Talk 14:32, 6 July 2011 (UTC)[reply]

thanks Iwas trying to give that link — Preceding unsigned comment added by 78.38.28.3 (talk) 14:45, 6 July 2011 (UTC)[reply]

Dark matter is probably mostly intermediate mass black holes. Even if at some point in the past it wasn't, whatever it used to be would have been accreted by black holes.[2] 99.24.223.58 (talk) 03:05, 7 July 2011 (UTC)[reply]

the brightness of galaxis such as Olbros paradux make us to enter dark matter to dynamic of univers , may it be intermediate mass black holes effective on galaxies brightness , but total mass of any galaxy conatins all object of its body , although it be any black hole . Here I say again that internal properies of any black hole dos not change its interstellar charastricties . suppose any star comes to be black hole and has planets , its planets will coninue to rotate round it after its exchanging . let I want to have new very hot discuss about this subject :

akbar mohammadzade--78.38.28.3 (talk) 05:19, 7 July 2011 (UTC)[reply]

for this discussion I want any one to consider that dark matter comes from solving some equations to show dynamism of univers objects , and total univers density ,and Olbros night brightness paradox

.a. mohammadzade--78.38.28.3 (talk) 07:15, 7 July 2011 (UTC)[reply]

The reference desks are not intended to be a discussion forum. Did you have a specific factual question about dark matter that is not answered by the article to which you linked? {The poster formerly known as 87.81.230.195} 90.201.110.220 (talk) 09:54, 7 July 2011 (UTC)[reply]

Clarification: t-test and degrees of freedom[edit]

I'm referring specifically to the statistical t-test and to a worked example in Wikibooks: >http://en.wikibooks.org/wiki/Statistics/Testing_Data/t-tests> .If every parameter you compute from a sample decreases the degrees of freedom you have, why computing the standard deviation does not? In the example there is a sample of 20 measures. The t-test is worked out, using the sample mean and the sample standard deviation. Why the degrees of freedom are considered to be 19 and not 18? --82.50.23.154 (talk) 14:02, 6 July 2011 (UTC)[reply]

I'm not exactly an expert on statistics, but I believe it may be due to the fact that the standard deviation can't be used to uniquely define one of the values. In other words, if you have a sample of 20 measurements, you can calculate the mean, and then if you take any 19 of the measurements and the mean you can calculate the 20th measurement. However, if you calculate the sample standard deviation from the 20 measurements, then you the first 19 values and try to calculate the 20th, you will find that you can't, as the solution will contain a +/- from the square root.

This is only my guess though; it's a very good question.-RunningOnBrains^(talk) 18:03, 6 July 2011 (UTC)[reply]

This might be a more appropriate question for the math reference desk. I'm not sure the Wikibooks example gives a very good definition of Degrees of freedom (statistics). You might be better off with that article or Standard deviation#Estimation, which explains why you use N as a denominator for a sample, and N-1 as the same denominator for the population. 99.24.223.58 (talk) 06:34, 7 July 2011 (UTC)[reply]

solar corona[edit]

During a total solar eclipse, the solar corona can be seen with the naked eye.solar corona is most hotter than sun surface , what is the reason? has sun corona any special gravity firld , to cause special condition on it?

--78.38.28.3 (talk) 14:41, 6 July 2011 (UTC)A.mohammadzade[reply]

Using the most conventional method of temperature measurement, the corona is hotter than the interior of the sun. For the most part, this is because the plasma is so sparse. Effectively, this means there is a very low thermal conductivity. Equivalently, this means that the mean free path is very large, and the probability of a collision (or any interaction) between particles is lower in the solar corona than it is inside the interior of the sun (say, in the photosphere or in the convective zone). The energy density is lower in the corona, but the corresponding mass density is very much lower, compared to the convective region of the sun. Therefore, the energy per particle is higher in the corona. Most conventional definitions of temperature will therefore resolve a "higher" temperature in the corona. As discussed above, for a sparse plasma, temperature can be defined in more complicated ways, such as defining the energy partitions per specie. Personally, I do not think that temperature is a very helpful parameter when describing plasmas, because it encourages us to imagine the plasma behaving as a hot gas. Nonetheless, most plasma physicists use temperature (with various, specific definitions) as one of the primary defining characteristics of a plasma. I would prefer to see the energy per particle discussed and measured in electron Volts; or simply work with the plasma frequency, which tells us about the specie and energy, and is a more direct measurement anyway. Nimur (talk) 15:32, 6 July 2011 (UTC)[reply]

To sort of "dumb down" what Nimur is explaining a bit (in case it went over your head), the very simplistic way to look at temperature is as a measure of the amount of kinetic energy the particles that make up a substance has. All other things being equal (and you need to assume that all other things are really equal) that means that a higher temperature equates to particles moving at a faster speed. Keep that in mind (its a massive approximation, but it works for this purpose): Higher temperature = faster moving particles. When you get to the corona part of the sun, the particles are so spread out that they can move very far without hitting another particle; since the particles can only slow down by transfering some energy to another particle, that means that particles in the corona don't often get the chance to slow down. Since they can't be slowed down by anything, they just keep moving very fast; that means that the corona has a very high temperature. The same thing happens on Earth, see thermosphere, which is a part of the Earth's outer atmosphere which has a very high nominal temperature, measured in the thousands of degrees celsius. However, if you were floating, unprotected, in the thermosphere, with its several-thousand-degrees temperature, you would freeze to death. Why? Because you are always losing heat via radiation, and you will continue to do so in the thermosphere. The "air" in the thermospehere is so sparse, that even though the particles are moving VERY fast (high temperature) so few of them hit you that they cannot warm you up. --Jayron32 17:27, 6 July 2011 (UTC)[reply]

While the two answers above are essentially correct, they dodge the question which is "Why is the temperature so high?" Dauto (talk) 20:05, 6 July 2011 (UTC)[reply]

That's because no-one is entirely sure. See Corona#Coronal heating problem. --Tango (talk) 22:07, 6 July 2011 (UTC)[reply]

I have asked a few friends who are solar physicists. The most promising lead I have so far is this paper from UCLA: Observations of the Solar Wind... with the WIND 3D Plasma Instrument (1996), from the Berkeley SSL. "...At coronal temperatures electrons are not gravitationally bound while protons are..." - so, as the OP has suggested, there's an electromagnetic effect interacting with a gravitational effect. Electrons are able to behave as a free plasma, while protons can not; doubtless, this creates an non-equilibrium condition, where orbital protons are effectively dragging electrons in directions that they do not want to go. I would not be surprised if the result is an exchange of energy, effectively raising the electron temperature. Solar physics is hard stuff, and the data we have is limited; sorry that there are few simple answers. Nimur (talk) 01:04, 7 July 2011 (UTC)[reply]

Giant jellyfish[edit]

This picture turns up on a lot of jellyfish-related searches. Is it real? And if so, what species? I know it is not a lion's mane jellyfish, it looks more like a nomura's jellyfish. But even for a nomura's that is really huge. So what is this thing? --T H F S W (T · C · E) 18:11, 6 July 2011 (UTC)[reply]

It's a photoshopped fake. This question turned up at ref desk in May of 2010.[3] ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:21, 6 July 2011 (UTC)[reply]

Too bad! It would be neat to have a real jellyfish that big. --T H F S W (T · C · E) 01:02, 7 July 2011 (UTC)[reply]

Yeh, keep one like that in your backyard swimming pool, and it would tend to discourage neighbors from dropping by unannounced. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:24, 7 July 2011 (UTC)[reply]

Pyrite problem[edit]

I recently purchased pyrite. It has the "fool's gold" look and does not dissolve in HCl like pure FeS does. What is the problem? --Chemicalinterest (talk) 20:20, 6 July 2011 (UTC)[reply]

I thought pyrites, which is FeS₂ was insoluble in HCl (and Rutley's Elements of Mineralogy shares my opinion). DuncanHill (talk) 20:26, 6 July 2011 (UTC)[reply]

I guess there's no problem. --Chemicalinterest (talk) 22:21, 6 July 2011 (UTC)[reply]