Wikipedia:Reference desk/Archives/Science/2012 March 3

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March 3

Nuclear fission catalyzed by a high energy proton

I know nuclear fission is normally catalyzed by a neutron, but can it be catalyzed instead by a high energy proton? ScienceApe (talk) 02:30, 3 March 2012 (UTC)

No. Nucear fission does not result in protons being emitted. If your reaction does not regenerate the catalyst...well then it's not a catalyst. Also the proton cannot initiate a fission reaction because it would have to smash hard into the nucleus, which it can't do due to the fact that both have positive charges. Maybe you can initiate with an antiproton though? 203.27.72.5 (talk) 02:38, 3 March 2012 (UTC)

No, I was asking if it's just theoretically possible. And actually you're wrong, some protons are emitted in fission sometimes, but I wasn't asking about a self sustaining reaction. I was thinking more along the lines of using a particle accelerator to accelerate protons to high velocities and then have them impact a fission target. ScienceApe (talk) 05:47, 3 March 2012 (UTC)

Individual fissions can be initiated readily using suitably-energetic protons and susceptible heavy nuclei. For example, this paper describes fission of uranium-238 by 20-60 MeV protons. As a semantic quibble, I'm not sure I'd call it 'catalysis', as the proton tends to be part of the fission products, rather that released as a free particle. TenOfAllTrades(talk) 05:57, 3 March 2012 (UTC)

I just mean catalyze in the general sense, meaning to cause something to happen, not in the chemistry sense. I don't think the chemistry definition of catalyst applies to nuclear reactions anyway. ScienceApe (talk) 13:18, 3 March 2012 (UTC)

Catalysis has a similar meaning as applied to nuclear reactions. See muon catalysis and magnetic catalysis. 203.27.72.5 (talk) 20:32, 3 March 2012 (UTC)

Nevermind, I actually got my answer. In the article on nuclear fission it describes it, the first fission experiment actually used a proton to split lithium. "Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium-7, to split this nucleus into two alpha particles." ScienceApe (talk) 05:54, 3 March 2012 (UTC)

I'm aware of proton emission from radioactive decay, but I've never heard of protons as a fission product. What fission reactions result in proton emission? Obviously I was wrong about protons not being able to make contact with the nucleus; but since they can why is it so hard to get fusion to occur i.e. why is ⁷Li + p → 2α so much easier than p + ⁷Li → ⁸Be? 203.27.72.5 (talk) 06:26, 3 March 2012 (UTC)

Actually, according to the Lithium burning article, ⁸Be is unstable and decays to 2α so apparently the net reaction is the same for both fusion and fission. So is this correctly a fusion reaction or a fission reaction? Is the ⁸Be unstable intermediate also present in the fission reaction pathway? 203.27.72.5 (talk) 06:51, 3 March 2012 (UTC)

In the first paragraph on the nuclear fission article, "Most fissions are binary fissions, but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced in a ternary fission. The smallest of these ranges in size from a proton to an argon nucleus." ScienceApe (talk) 13:18, 3 March 2012 (UTC)

Question about Higgs data

This is a graph showing 2011 data in the search for the Higgs particle. Random noise in data makes some data points 1 or 2 standard deviations from what is expected. But in this data, the "noise" is not scattered above and below - it is above for a while, below for a while, and can be connected in a more-or-less smooth curve. Why is the data like that rather than scattered? Bubba73 ^{You talkin' to me?} 03:12, 3 March 2012 (UTC)

I'm not sure I understand this graph, but basically the reason is that all of the candidate Higgs events are consistent with a fairly wide range of Higgs masses. They're superimposing a bunch of blurs, not a bunch of points. To put it another way, rapid variation would imply that their statistical model is able to make fine distinctions in the Higgs mass on the basis of the data, which would mean either that the data is very good or that there's something wrong with the model. -- BenRG (talk) 08:52, 3 March 2012 (UTC)

Ronni Kahn claims CO₂ reduction

Social entrepreneur Ronni Kahn won the 2010 Australian Local Hero Award for founding a charity that collects discarded food from restaurants and uses it to create meals for the needy. In her acceptance speech, she claimed that her organisation not only helps the needy, but also reduces CO₂ emissions by preventing the food from spoiling.

Since the food is saved from digestion by microbes only to be digested by people and, since the waste products of human digestion are in turn digested by microbes and, since practically all carbon in the biosphere is sourced from the atmosphere anyway, isn't the charity's net effect on carbon emissions more or less zero? 203.27.72.5 (talk) 05:43, 3 March 2012 (UTC)

Well, no, but not for the reason she gives -- the real reason is that creating food uses significant amounts of fossil fuels. But perhaps a more meaningful response is to point out that current estimates are that humans are adding around 10 billion tons of carbon to the atmosphere each year. It's hard to imagine that her effort will make a major difference. Looie496 (talk) 06:14, 3 March 2012 (UTC)

Obviously I don't have any concrete numbers, but I assumed that any fossils fuels saved by not producing new food will be consumed by driving around town to pick up left overs from restaurants, refridgerating them, turning them into new meals and reheating them. I'm pretty sure that if they just opened a soup kitchen its net carbon footprint would be the same or smaller, even if you allowed for all of the spoiled restaurant food that wouldn't be saved. 203.27.72.5 (talk) 07:28, 3 March 2012 (UTC)

An often cited approximation (e.g. here) is that each calorie in food that we eat in the developed world, requires ten calories of fossil fuel to be produced. Considering this, I would say that it is a fair assumption that any measure to decrease food waste will decrease CO₂ emissions, even if you have to drive around to collect the food. If you use one litre of fuel, so long as you collect more than 10% of the energy contained within it (not difficult) then the CO₂ emissions will be less, than if that food was wasted and the people who would have eaten the wasted food, have to eat something else instead. SmartSE (talk) 14:11, 4 March 2012 (UTC)

Doing the other half of the calculation, the energy density of gasoline is about 35 megajoules per liter, about 8000 food Calories (kilocalories). TenOfAllTrades(talk) 15:34, 4 March 2012 (UTC)

The quote from the article Smartse cited is "All together the food-processing industry in the United States uses about ten calories of fossil-fuel energy for every calorie of food energy it produces." That's a really confusing way to phrase it, since we don't know if we're using the same unit in both quantities or if we're using food calories for food energy and physical calories for energy from hydrocarbons. If the author is mixing units then you have to collect food equivalent to 10,000% of the energy contained within your fuel. So that's 800,000 food calories per litre of fuel burned. And while the equivalence might hold for the fancy processed food most of us consume daily, it would be significantly reduced for the sort of food served up at your typical soup kitchen. 203.27.72.5 (talk) 21:49, 4 March 2012 (UTC)

Which Insect is this ?

I did asked this question previously , I'm asking it again as I was recommended to do so. Here are the pictures http://www.mediafire.com/imgbnc.php/cddd07abfa9b042f748ea3294428b71476d5fba1075598ef99d7079528e1c3a26g.jpg and http://www.mediafire.com/imgbnc.php/2a135256a5246b1df1a662a4c4939f7ae23cacd5d43e04c7670dfcbfd1f77bfc6g.jpg — Preceding unsigned comment added by 182.178.154.135 (talk) 06:33, 3 March 2012 (UTC)

I wish you had something less blurry. The second picture is also inaccessible. However, I don't think they are woodlice. How large were they and where were these pictures taken? My first guess would be cockroaches (Blattodea). One of the wood-eating ones with wingless adults or nymphs and females. Compare with wood cockroaches (Cryptocercus) and trilobite cockroaches (Laxta). My second guess would be mites (Acariformes). If these were tiny and all of the eight appendages I can see are legs. See if they resemble anything here.-- OBSIDIAN†SOUL 07:36, 3 March 2012 (UTC)

The previous question OP links to points to this other photo, though that photo doesn't seem too helpful. It also says photo “was taken it in March 2011, Pakistan. It is spreading in my home garden”. – b_jonas 09:10, 4 March 2012 (UTC)

Try asking at http://www.whatsthatbug.com/ SmartSE (talk) 14:12, 4 March 2012 (UTC)

Hm. So only two pictures all in all? Two other possibilities: flat bug nymphs (Aradidae) and barklice.-- OBSIDIAN†SOUL 14:48, 4 March 2012 (UTC)

How many shades of gray can the human eye see?

In general, how many shades of gray can we see? A simple Google search turns up websites claiming anywhere from 35 to 64 to 100 to 460 to 500 shades of gray. 8-bit grayscale images only have up to 254 shades of gray (is that right? plus white and black = 256), and I was wondering how 256 shades would be enough for images to look smooth and perfect to us. I can't find a straight answer. – Kerαunoςcopia^◁_galaxies 19:58, 3 March 2012 (UTC)

It's complicated because the brain interprets what the eyes see based on context. See Checker shadow illusion for a good example of how this can mess with our perception of shades of grey. --Tango (talk) 20:33, 3 March 2012 (UTC)

I believe the source of the confusion is that it's not linear. Using a linear 256 shade gradient, you probably can't tell the difference between a 127/256 and 128/256 gray, but can between a 0/256 and 1/256 grey. How far away you are and the size of the pixels also matters. Another confusing factor is that the quantity matters. So, one pixel of 0/256 on a screen 1/256 greys wouldn't be detectable, while half a screen of 0/256 next to half a screen of 1/256 gray would be. I find the most challenging test to be a "rainbow", where the shades are arranged in adjacent bands with the minimal gradient between each line. In such a test, if you were able to adjust the shades to be nonlinear to match our perception, perhaps 256 might be enough to avoid visible banding. If you keep it linear, then more like 500 is right, especially close up. Using nonlinear grays and a dithered image (alternating pixels of various grays, no solid areas of one gray), then 35 might be enough, from a distance. StuRat (talk) 20:39, 3 March 2012 (UTC)

One place where this comes up is when watching a dark scene on streaming video. What they should do is adjust the grays available for each frame so they have 256 shades on each frame. Unfortunately, they don't seem to do this, but rather use a fixed palette. Thus, you may have frames which only use a few of the 256 shades, with no dithering, making the banding quite apparent. StuRat (talk) 20:55, 3 March 2012 (UTC)

You can easily test yourself. I tested myself just now by using Windows Paint. I opened Paint full screen, so the whole screen is full white (on my system this is 255 grey). I set up adjacent blocks of grey, each about 2 x 2 cm, using Paint's feature to define arbitary custom colours. I found I could very easily distinguish 127 grey from 128 grey, but I could only just distinguish 255 grey (actually full white)from 253 grey. Then I added a large black background, and then found I could easily distinguish 2 cm square blocks of 254 from 255. From this it is reasonable to infer that:-

a) the resolution of the human eye considerable exceeds 256 shades and can probably do ~4000.

b) the maximum resolution is not achieved if the surrounding shade is not favourable.

Note: If you try this test near full black, you'll usually find that you can't resolve at all well. This is NOT because of a non-linear response of the eye (although it is more or less logarithmic) but because the response of computer and TV screen departs from the proper curve with very dark shades near full black. The eyes is not linear but video systems are not designed to be linear - what is termed "gamma correction" is built in, but the correction is not perfect near black.

the priciple of distraction applies in video & imaging systems: The more infrmation in the scene, the more human visual processing discards information that is not relevant to what the scene is consciously portaying. This means that if you set up a highly artificial test as I descibed above, the resolution of shades of grey is superb. But in a natural scene with colour, lots of detail, and movement, the reolution of shades drops considerably. I have seen demonstartions where on certain scenes, a resolution of only 16 shades of grey was quite acceptable. Ratbone120.145.19.89 (talk) 01:38, 4 March 2012 (UTC)

19th century psychophysicists and experimental psychologists such as Ernst Heinrich Weber, Gustav Fechner and Wilhelm Wundt studied "Just-noticeable differences" (JNDs) and psychometric scaling in seeking to answer this question. The JND varies as the log of the stimulus intensity. The JND is typically a fixed proportion of the baseline stimulus. Methodology will affect the answer, such as asking whether the difference is noticed 75% of the time or some other fraction such as 50%, but [1] says that for one situation, a 500 cd/m² luminance monitor, the eye should detect "700 different shades of gray." Another paper says that in a luminance range from 0 to 4000 cd/m², there are 1000 JND, while if the luminance range is only from 0.1 to 10 cd/m², there are only 200 JND. This terminology is somewhat obsolete . See also Signal detection theory and Stevens' power law. Edison (talk) 05:15, 4 March 2012 (UTC)

The above few posts don't take into consideration that LCD displays are quite rubbish at displaying "black".. That's another factor you'd have to take into account if you were going to try to test this using an LCD screen. You could display "255 Black" on 10 different monitors and be able to distinguish each as a different shade. Vespine (talk) 21:41, 4 March 2012 (UTC)

The point is not that you can tell 255 Black on one LCD from 255 Black on another monitor beside it. You will be able to see that they vary, not because LCD montitors aren't good at displaying black, but because they are subject to adjustment. What matters is whether you can tell 255 black from (say) 254 black on the same monitor. You'll find that you can't, because as I said above, the gamma correction is not very good near black - the monitor response curve does not match the eye's logarithmic response. The same applies to CRT monitors. Ratbone58.169.234.13 (talk) —Preceding undated comment added 01:37, 5 March 2012 (UTC).

I know it's not really your point, I agree with what you're saying, but just for the practical part of the experiment, on my screen (a dell 21" LCD, nothing very fancy), I can quite clearly see the difference between 000000 and 010101 side by side, as taken from this site. Actually if I look slightly from above it above makes the difference even more obvious, while looking slightly from below does make them look the same. I don't think you'd get this effect on a CRT Vespine (talk) 02:47, 5 March 2012 (UTC)

Which large predators can't see you if you don't move?

Or is that idea outdated and discredited now - and the advice that one should stand perfectly still when faced with a large carnivore and wait for it to leave now considered bad advice? --Kurt Shaped Box (talk) 22:22, 3 March 2012 (UTC)

I think there's something to it, in that if you are distant from them and the wind isn't blowing your scent in their direction, they are less likely to spot you, if you stay still (this works against humans, too, BTW). However, this often gets exaggerated to the absurd claim that you can be standing right in front of them without them seeing you.

A second effect is that many predators won't eat anything already dead, since their digestive tract can't handle carrion. So, "playing dead" may work with these animals, particularly if you are rather foul-smelling (I wonder if defecating might help, not that you would have much choice :-) ).

Running, on the other hand, triggers the predator response. Some predators are so confused when their prey doesn't run, they don't know quite what to do. Tigers, for example, only attack from behind, so wearing a mask with another face on the back protects you.

And, finally, being still may be seen as a sign of submission, and an animal might therefore not feel the need for a dominance fight with you. This might apply, say, to a pack of dogs. StuRat (talk) 22:29, 3 March 2012 (UTC)

Another thing to consider is that, in a running race against a tiger, you ain't gonna win. Much better to stand there and prepare yourself to fight than to turn your back and try and outrun something that has been training for the 400-metre-Kurt-chase all its life. Here is a BBC article that gives some more useful advice, in this case specifically to do with dogs, but I'm sure much of it is applicable to any aggressive animal. - Cucumber Mike (talk) 22:50, 3 March 2012 (UTC)

You might win the race, if you only need to get to someplace nearby, like your truck or rifle. StuRat (talk) 02:52, 6 March 2012 (UTC)

Hmmm. I have seen the advice given that one should make unblinking eye contact, bare one's teeth and scream/snarl/roar when faced with an aggressive dog. The theory being that you are much bigger and that the dog won't actually step to you if the impression is given that you are prepared to fight. Against something like a tiger, it's hard to think what would be the best course of action if it was right on you and you were unarmed. Try to gouge its eyes or kick its groinal area? (that was the advice given vs. bear in a survival book I read once) --Kurt Shaped Box (talk) 23:15, 3 March 2012 (UTC)

Just make sure you have an unfit friend with you: "I don't have to outrun the tiger, I just have to outrun you." --Tango (talk) 23:51, 3 March 2012 (UTC)

I don't think it is ever an all-or-nothing situation. Every visual system that I know of is strongly activated by movement, so any visual predator will have increased chance of spotting prey that moves. Conversely, if you stand out sufficiently from the background, most predators will detect you even if you don't move. My guess is that movement-dependence is greatest in animals that depend on the optic tectum for visual control of behavior -- that means all vertebrates other than mammals. Mammalian predators tend to have a well-developed visual cortex, which is much better at detecting static shapes than the optic tectum. Looie496 (talk) 23:21, 3 March 2012 (UTC)