Wikipedia:Reference desk/Archives/Science/2007 May 25

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May 25

Dark Energy

The article says the universe is expanding, if it is expanding there is each time more and more space. It also says that "a volume of space has some intrinsic, fundamental energy." More space logically means more "intrinsic, fundemantal energy" so either energy can be created out of nowhere or space is not expandng?

Also how can you say that this energy is dense? D = M/V D= g/cm2 or something like that. The energy weights? --201.253.205.216 01:02, 25 May 2007 (UTC)

See our article on Energy density of empty space — Kieff | Talk 01:17, 25 May 2007 (UTC)

With respect to your second question, there is a generalized usage of the term "density" which is "foo per unit volume", not necessarily "mass per unit volume". —Steve Summit (talk) 01:23, 25 May 2007 (UTC)

Not to mention that whole mass-energy equivalance thing, although the generic definition of density is more to the point here. Confusing Manifestation 02:14, 25 May 2007 (UTC)

The energy weights? Yes, it does.

Actually, its weight really is the only use of that dark energy, isn't it? – b_jonas 10:51, 28 May 2007 (UTC)

Deserts

Why are deserts so full of sand? What type of landscape was there before deserts? What elements made a sandy desert the way it is today? I heard deserts spread out, why? Wind and sand? PitchBlack 02:35, 25 May 2007 (UTC)

Well, without water, and without plants - there is nothing to hold the soil together. It dries out - and there is no decaying vegetation - so all there is left is rock and sand. There are rocky deserts where there isn't much sand. SteveBaker 02:52, 25 May 2007 (UTC)

You can slo get deserts full of rocks, or ice - as in Antartica. If it wasn't a desert it could be covered in vegetation. The most common chemical elements are silicon and oxygen making up quartz sand, but there would be other minerals such as feldspar in the sand too. A major factor in deserts is running water. Although rare, floods move a great deal of sediment, and leave channels that can last for thousands of years. GB 02:56, 25 May 2007 (UTC)

In fact, the desert article says that sand covers only about 20 percent of Earth's deserts. Have you read it yet? --Anonymous, May 25, 05:10 (UTC).

The following responses were posted after an anonymous editor changed "desert" to "dessert" above, as noted in Wirbelwind's link below. (PS. 213.48.15.234 has now acknowledged him/her/it self as the editor in question, and has apologised.)

Indeed. However, I think that if you just eat the 80% that contains icecream, chocolate, fruit or cake, you can safely leave the remaining sand at the bottom and nobody would be offended.  :) JackofOz 11:58, 25 May 2007 (UTC)

Well, if your dessert contains artificial sweeteners, many do indeed include powdered sand as filler, which is labeled as "silicon dioxide" to trick people into paying for and eating sand. You might also want to see our desert article, if interested in arid land. StuRat 12:14, 25 May 2007 (UTC)

For people willing to eat that chemical dreck, I believe sand is their just deserts. --TotoBaggins 14:17, 25 May 2007 (UTC)

This ends the desert/dessert anomaly

Also, sand is common in other areas, as well, but is usually not visible because it is covered with vegetation. One exception is at beaches, where the sand is also visible (because the crashing waves prevent vegetation from growing there). StuRat 12:08, 25 May 2007 (UTC)

I knew I didn't misread the first time. --Wirbelwindヴィルヴェルヴィント (talk) 17:49, 25 May 2007 (UTC)

Same here.... good find, Wirbelwind. Nimur 18:40, 25 May 2007 (UTC)

Wirbelwind may have been aided by the anon responsible giggling apologetically on a few people's talk pages. Skittle 22:16, 25 May 2007 (UTC)

Mmmm nope. I just went to the history, and it happened to be in the first page, under the last thing that had "Desert" in the edit summary. --Wirbelwindヴィルヴェルヴィント (talk) 23:05, 25 May 2007 (UTC)

Changing one atom into another

Is it possible to remove/add protons and neutrons to make different atoms? Let's say, for example, I wanted to turn hydrogen into oxygen. Is this possible? The atom page doesn't explain it, and even basic quantum mechanics confuses me. Shadowedmist 03:41, 25 May 2007 (UTC)

It is not easy, but the process of nuclear fission splits large atomic nuclei into smaller ones. This changes the "atom" because each resulting nucleus has a different number of protons (the defining characteristic of a chemical element). A roughly reverse process is called nuclear fusion, which creates larger atoms by squishing together smaller ones. Both processes are difficult to control (so you won't be doing it in a standard school laboratory), but you can read about the details at each article. Nimur 04:06, 25 May 2007 (UTC)

edit conflictGreat question! Yes, indeed it is possible, although not easy to do at will. Atomic reactions are precisely those which change the number of protons or neutrons in an atom (as opposed to chemical reactions, which only involves electrons). Atomic reactions actually occur all the time in nature: radioactive decay, for instance, is a spontaneous change of one nuclide (an atom with a particular number of neutrons and protons) into one or more other nuclides. What makes a particular nuclide stable while others unstable is a tricky question (that is where the quantum mechanics comes in).

Radioactive decay happens randomly (so randomly, in fact, no process can speed it up or slow it down--see atomic clock). It is more challenging to create such reactions. Nuclear reactors generate power by turning Uranium-235 into Krypton and Barium. Splitting an atom into two or more smaller atoms is called Nuclear fission. The sun and other stars, as well as thermonuclear weapons, generate energy using Nuclear fusion: the fusing of two (or more) atoms to form a larger atom. In both these cases, the atomic reaction gives off heat and other particles, however this is not always the case. Some reactions actually take a good deal of energy to make new atoms.

To create other reactions, researchers have built Particle accelerators, which can be used to create a wider range of atomic reactions. --TeaDrinker 04:08, 25 May 2007 (UTC)

I must comment that Wikipedia articles on chemistry are really hard to understand and confusing to people that don't already know the subject. I knew that atoms could be changed into other atoms, but I simply couldn't find this information in Wikipedia. The introduction of the article on nuclear fission is really confusing and I wouldn't understand it if I hadn't learnt about that before. A.Z. 04:10, 25 May 2007 (UTC)

Decades ago I read in a popular science book about atomic energy that an "atom smasher" could accomplish "transmutation" of lead into gold, (the alchemists' dream), but at a hideously unprofitable cost. That would be such a change. Is it really possible? Edison 04:42, 25 May 2007 (UTC)

We have an article on it: Nuclear transmutation 213.48.15.234 11:16, 25 May 2007 (UTC)

You should read some of the higher math articles, which will have you longing for the chemistry ones' clarity and accessibility. --TotoBaggins 14:21, 25 May 2007 (UTC)

Yes, it is not only possible, it is the exact way that all oxygen has been created. Stars are the foundries for elements and turn hydrogen into all the atoms that naturally occur. Atoms up to iron (Fe) I believe are formed in the fusion process of stars while heavier elements were formed during a prior supernova. Nucleosynthesis is the process by which elements are created and the CNO cycle generates the atoms you asked about. --Tbeatty 04:59, 25 May 2007 (UTC)

The article makes it sound straightforward to bombard gold with neutrons and eventually, through neutron capture and beta decay, transmute it to lead. It sounds like this could be done incidentally in the operation of, say, a nuclear reactor as an interesting but useless byproduct of power production, using the neutrons which otherwise bounce around and just make the reactor vessel brittle. But what would be the process to make the more interesting conversion of lead to gold? Our article on nuclear transmutation and beta decay do not address this. The corollary would be: approximately what cost would this be per gram, or how much energy would be required per gram. The question is less what nature achieved in ancient supernovas and more what present human technology can do. Edison 12:20, 25 May 2007 (UTC)

Energy, mass and gravity

As the topic seems to come up every day, I have done some research and found a confusing amount of contradicting views on the relation of mass and energy. The article on the subject is in an edit war, and I have even heard different answers from professional physicists.

The problem seems to be mostly a problem of definiton of terms. What I want to ask about is therefore not for another attempt at definition, but I will pose a physical situation, a thought experiment. I have designed the experiment in such a way, that it is doable in principle (no supertasks). I hope that an explanation of the correct prediction will be enlightening for me and other readers.

There are three objects, an electron (e) a positron (p) and a heavy iron block (X). The elementary particles are on a collision course and will crash together near the large object. They travel at some highly relativistic speeds. Now when the particles crash together, they annihilate and form a new heavy particle, say a neutron (N). I don't know if that exact reaction is possible, please suggest some other heavy particle in that case. The energy of the collision adds up exactly to the rest mass of a neutron and nothing else is created. As the particles had exactly the same speed in the reference frame used here, the new neutron is at rest.

     e-->           <--p        e--> <--p            N
 
 
 
              X                     X                X

         long before         shortly before        after

In the shortly before case, there is some gravitational pull by the electron and the positron on the iron block. It is measured with a spring scale and we call the result "a". We wait for the collision and then measure the pull of the neutron, call it "b".

Now for the question: Is a = b, a < b or a > b?

Silly me, I forgot what moving a particle at relativistic speeds does to its gravity. Fortunately, I do remember that the actual product of this reaction would be a pair of high energy gamma rays. You could use a proton and an electron in the same manner to make a neutron like this, and just try to ignore the stupid neutrino that flies out as well. Someguy1221 04:57, 25 May 2007 (UTC)

a < b because gravity is determined by mass only, and the neutron has more mass than the electron and positron combined. I don't see why it would be otherwise. --99.245.178.103 05:02, 25 May 2007 (UTC)

Maybe in Newtonian gravity, not in relativity. Energy and momentum contribute to gravitational fields, and particles at extremely high velocities have somewhat bizarrely shaped fields due to the special relativistic delay in the altering of the field due to the changing position of the particle. On a slightly different note, these two particles would also dissipate some of their energy before collision in the form of gravity waves, although this would surely be negligable. Someguy1221 05:12, 25 May 2007 (UTC)

Another question is what will happen if the positron is not present and the electron just races by? Half pull?

Kinetic energy warps space in the same way that matter does. It's influence as mass is m = E/c^2. In fact, as you accelerate matter to relativistic speeds, it takes more and more energy to get smaller and smaller gains in velocity. This energy goes into the mass of the object instead of it's velocity. 99.245.178.103's answer is exactly wrong. Energy and mass are different expressions of the same thing and both affect gravity in the same way. --Tbeatty 07:18, 25 May 2007 (UTC)

After I did some reading in the Feynman lectures I agree with you. I believed this from the start, but was confused by the articles here. So the problem is: Wikipedia does not agree at all with that view.

It shouldn't matter what people editing the article believe - that comes under the heading No Original Research - everything we write must be verifiable by references to books or other acceptable reference material. So - if you believe 'A' and someone else believes 'B' then there are three possibilities:

1. Neither of you can find solid references by notable scientists in peer-reviewed journals that explain the things you believe. Then neither of you should write about those beliefs - if it aint in the literature - it doesn't belong here.
2. One of you has some solid references - the other does not - then 'he who has references wins'. That's crucial. If you don't have references to back up what you are saying - and the other guy does - then graciously remove your point of view from the article and go look for better references!
3. Both of you have solid references - if those differences are irreconcilable then your article has to say something like "Scientists are divided on the subject of A and B - some scientists believe A <ref>{{cite book....}}</ref> while others believe B.<ref>{{cite book....}}</ref> ....although in the sciences, one has to take into account that a paper published 100 years ago is probably superceded by one written by in the last 10 years.

Problem solved - controversy (hopefully) ended. SteveBaker 11:25, 25 May 2007 (UTC)

Protein Shake's

Some people say that protein shakes will hurt you later on in life and could clog your heart up and u have 2 get rushed to the hospital and get it drained from you I think that is Creatine...So can i get opinons on this stuff please!

I can imagine an excessive amount of protein consumption to cause an increase in blood fat and finally causing a heart attack. But that disease would take decades to develope and would go with obesity. That was a very very wild guess. Try to link your source.

At the risk of giving medical advice, generally protein shakes would not be dangerous, especially (as with most things) if taken in moderation. For example, having a daily protein shake after training would be unlikely to be dangerous for most people. Most of the products on the market simply give a more concentrated amount of protein than say drinking milk alone. As I said though, it is best not overdo it, check out for example the information on excess protein in the protein in nutrition article. If you are talking about creatine supplements, that is a different matter. While creatine is generally considered safe, especially in moderate amounts over the short-term, it has been around and in use for less time, so long-term effects may yet be unknown. You could consult a qualified doctor or nutritionist for more specific information for your own personal needs. --jjron 08:29, 25 May 2007 (UTC)

Also, don't forget that protein is a source of calories, and, if you take in more calories than you burn, you will gain weight. If you thus become overweight or obese this will have harmful health effects, regardless of whether the calories came from protein, fat, carbs, or alcohol. Also, in Western nations protein deficiency is rare, so unless you have good reason to think you suffer from such a deficiency, you shouldn't take a protein supplement. (A lack of muscles isn't a good reason, that likely means a lack of exercise and/or lack of testosterone, most likely due to age.) A rather informal way to tell if you're getting excess protein is that your urine will smell like amino acids, which you will recognize as the smell of bacon cooking. StuRat 11:42, 25 May 2007 (UTC)

There is nothing out there that I could find that implied that there were any known long-term health risks to Creatine at this point. So whoever is talking about stories of things in your heart are probably full of crap. Now some protein shakes have relatively high levels of cholesterol, which can clog your arteries, so read the labels and remember to balance things out right. --140.247.248.93 14:39, 25 May 2007 (UTC)

So After every work out u take protein then its healthy good but what if u take it every once in a while will it have effects on the body or not and thanks for the opinons and facts cause my friends mom said she knew a guy who had to get it drained out of his heart

about horse power

What is the diefference between Brake horse power and fly wheel horse power?

Horsepower#Brake horsepower (bhp) is your friend. Cheers, Dr_Dima

Name this bird

Hi, I have a picture of a bird: he:Image:DSC00579.JPG. Can anyone tell me it's name and gender? Thanks, Yonidebest 14:58, 25 May 2007 (UTC)

For these kinds of queries, it's always helpful to say where the critter is (geographically, not "on a finger"), if possible. --TotoBaggins 15:59, 25 May 2007 (UTC)

It looks like a white budgerigar. It's certainly some sort of small parrot/parakeet; the fact that it's apparently captive suggests a budgerigar, as they're often kept as pets and can be bred to show a wide range of colours. --YFB ¿ 16:12, 25 May 2007 (UTC)

Thanks. Whats the "critter"? How can I tell what sex it is? Yonidebest 19:40, 26 May 2007 (UTC)

"Critter" is United States slang for "creature". I'm afraid I don't know anything about gender determination of birds; I suspect for cases like this where the birds don't have differentiated male/female plumage, it's very difficult. --YFB ¿ 19:48, 26 May 2007 (UTC)

Assuming that it is indeed a budgerigar, our budgerigar article describes how to tell the sexes apart: The colour of the cere (the area containing the nostrils) differs between the sexes; royal blue in males, pale-brown to white (non-breeding) or brown (breeding) in females and pink in immatures of both sexes (usually of a more even purplish-pink colour in young males). Young females can often be identified by a subtle chalky whiteness that starts around the cere nostril holes. If this individual is an albino, that could complicate the matter. 169.230.94.28 20:30, 28 May 2007 (UTC)

Units within equations

Should you include units within equations, e.g. $F=ma=52kg\times 10ms^{-2}=520N$ or just in the final answer as in $F=52\times 10=520N$.

I think you should leave them out - equations contain only operators, variables and numbers. Units are a implicit property of values. This is also how I've always been taught.

Any consensus, style guide reference etc?. Caffm8

APS style guide doesn't say anything AFAIR. The probable reason is, in all likelihood you wouldn't include the intermediate calculations in a scientific paper. In textbooks or exercises, whenever intermediate numerical calculations must be presented, it is better to specify the units. That would be $F=ma=52kg\times 10ms^{-2}=520N$ in your case. The reason is twofold. First, including the units removes ambiguity, makes it easier for the students to follow the math, and makes it easier for you to proofread. Second, in an equation A = B = C, any constituent equation (namely, A = B, B = C, and A = C) must make sense. However, $F=52\times 10=520N$ doesn't make any sense as it is. Hope this helps. Cheers, Dr_Dima.

Putting in k, m, p etc will lessen the common error of having the calculation correct except for being off by 10⁶ or some such. Edison 17:14, 25 May 2007 (UTC)

Also, it is a good idea to write $\ 10\ m\ s^{-2}\$ and not $\ 10\ ms^{-2}\$. Indeed, $\ m\ s\$ is meter*second, $\ ms\$ is millisecond. Cheers, Dr_Dima.

Definitely include them. Units aren't implicit, they are what give physical meaning to a bare number. In certain applications where the units are always the same, it seems to be common practice to leave them out, but this also results in people who are people totally unable to cope with something even slightly different from what they're used to. Keeping explicit track of all units helps make sure you are solving problems with understanding, and not just blindly throwing numbers around. --Reuben 17:33, 25 May 2007 (UTC)

There's a funny, only tangentially related story about the importance of understanding the physical meaning that numbers are attached to: Once Richard Feynman was sitting on a graduate student's thesis defense committee. The student happened to be a theorist, and Feynman asked him what the wavelength of visible light is. The student thought about it for a bit, and had no idea, so he guessed - "A centimeter?" According to the story, Feynman laughed and said "Do I look fuzzy to you?" --Reuben 17:40, 25 May 2007 (UTC)

LOL Dr_Dima.

Theorist or otherwise, how could such an advanced physics student be taken seriously with such a grave error of factual and conceptual understanding? Sigh. Nimur 18:47, 25 May 2007 (UTC)

Does it not get rather confusing when you are mixing variables? $24ms^{-1}=12s/23s$ (find s). Though I do appreciate you can use a different font. Caffm8

There are many tricks. First and foremost, avoid using the same symbol for too many things at the same time! Second, as you mentioned, different fonts. (I've never been entirely certain, but I think variables are supposed to get italicized, while units are in Roman type.) Third, you can use parentheses, like "(12 s)" vs. "(23)s". Fourth, you can use a dot to indicate multiplication, as in "23·s" as opposed to "12 s". Covering all the bases at once, you could write: $24{\frac {\mbox{m}}{\mbox{s}}}={\frac {12\cdot d}{(23{\mbox{s}})}}$! Similar problems can come up with simple numbers, like when σ² might mean sigma-squared or the second vector component of sigma. --Reuben 18:34, 25 May 2007 (UTC)

Always leave the units in when doing the calculations. This way you can check the answers. The units can be carried through the calculation just as if they are algebraic symbols. If the units don't match at the end, then you have made a mistake. This is the basis of Dimensional analysis. -Arch dude 18:25, 25 May 2007 (UTC)

I would definately put the units in too myself. In fact, in academic situations, I'll get points taken off if I leave units out. Plus, when you say 52 times 10 = 520N. 52 what? 10 what? 10 boxes that apply the force of 52 N each? 52 things that apply 10 N each? It becomes very ambiguous. Also, when you deal with more units, it'd get even more confusing. --Wirbelwindヴィルヴェルヴィント (talk) 23:02, 25 May 2007 (UTC)

Returning to Caffm8's remark that "equations contain only operators, variables and numbers.... This is also how I've always been taught": this is basically a difference between math and science usage. In science, it is normal to say that m = mass = 52 kg, a = acceleration = 10 m/s², and compute F = 520 N as per the original example. The units enter into the computation alongside the numbers; that's dimensional algebra, which I'm surprised to find no article on (maybe it's under another name, but that's the only one I know).

• Added: ah, I see, it's covered at dimensional analysis, linked above. --Anon, 23:48 (UTC).

In math, however, we would say that m is not mass, but mass in kilograms; thus m = 52 not 52 kg. Similarly a = acceleration in m/s² = 10, and hence F = force in newtons = 520. From a dimensional algebra point of view, the math custom amounts to dividing the units out first, thus enabling the equation to be written in terms of pure numbers. This is often left implicit, especially in exercises where the numbers don't relate to real-world measurements. (One exception is trigonometry, where there is tension between the mathematical convenience of angles measured in radians and the real-world convenience of degrees. People write things like "sin 45°", but the mathematical view is that this is just a funny way of writing "sin π/4".)

So Caffm8 learned about equations in math class, that's all.

--Anonymous, May 25, 2007, 23:45 (UTC).

As it is one of my pet peeves I feel compelled to add: Not only should units not be omitted, they should also be typeset upright, leaving italics to quantities. While style guides disagree on whether quantities need italics, they all agree that units are always upright. Simon A. 20:08, 26 May 2007 (UTC)

WoW Pet Name

I'm an avid player of the MMORPG World of Warcraft. When I got my first pet, this pet I named it Ursus. I wanted name it the scientific name of the the real-life animal. Some tiem after that I got this pet and named it Dasyatidae because it looks liek a sting ray. I am getting a new pet soon and want to continue the trend. My only problem is, I don't know what its real-life counterpart is. I was planning on getting this pet. Does it look like some kind of dinosaur or mythical creature? I'm really not sure. Thanks for any help. schyler 16:55, 25 May 2007 (UTC)

Looks sort of like a Komodo Dragon to me, but your monster looks more... bony? And it isn't as long as a komodo.--GTPoompt 17:20, 25 May 2007 (UTC)

reminds me of an armored dinosaur (family Ankylosauridae, considering the tail); however, AFAIK, Ankylosauridae were all herbivores. OTOH, if you can tame one, you can just as well switch it to a fish diet ;) . Have fun. Dr_Dima.

Ankylosauridae is the one I thought of, definitely. The spikes on the back and on the tail match very well. More famous depictions show a bony plated back and a club-thagomizer, but there were also ones with spikes. The body structure also matches. The colors however, maybe not :) [Mac Δαvιs] ❖ 19:37, 25 May 2007 (UTC)

Perhaps a horned lizard (aka horned toad). Check the specimen on the right side of the bottom pic in particular. How does Phrynosoma sound? Actually, the first thing that jumped to mind was the toad/dog beasties from the first Ghostbusters movie. Don't recall if they had names though. Matt Deres 20:37, 25 May 2007 (UTC)

I think I'm going to go with Matt Deres' suggestion. As the page says the pet is a caster (meaning it uses spells) and the horny toad (as we call it here in Texas) shoots blood out its eyes, if I recall correctly. Thanks all for your help. schyler 21:00, 25 May 2007 (UTC)

Molecules in a grain of salt

To within an order of magnitude, how many molecules of NaCl are in a grain of table salt? Bubba73 (talk), 18:52, 25 May 2007 (UTC)

John H. Lienhard claims: a one followed by 19 zeros. dr.ef.tymac 19:04, 25 May 2007 (UTC)

Thanks, that is a few orders of magnitude more than I guessed. Bubba73 (talk), 19:24, 25 May 2007 (UTC)

It really comes down to how large your grain of salt is. Morton Salt says it's 10 million grains per pound, which leads me to the calculation below, which sounds closer to your guess. --TotoBaggins 20:20, 25 May 2007 (UTC)

${\frac {1\ {\text{lb}}}{10^{7}\ {\text{grains NaCl}}}}\cdot {\frac {453.59237\ {\text{g}}}{1\ {\text{lb}}}}\cdot {\frac {1\ {\text{mole NaCl}}}{58.442\ {\text{g}}}}\cdot {\frac {6.022\cdot 10^{23}\ {\text{molecules}}}{1\ {\text{mole NaCl}}}}={\frac {4.67\cdot 10^{17}\ {\text{molecules}}}{\text{grain NaCl}}}$

Thank you, that's interesting, and perhaps closer to the mark. Bubba73 (talk), 20:42, 25 May 2007 (UTC)

There, I fixed your TeX for you, TotoBaggins. —Keenan Pepper 00:22, 26 May 2007 (UTC)

Thanks, I'm no TeXnician, and I'm always impressed by the brainiacs on the Math refdesk who belt it out like it ain't no thang. --TotoBaggins 01:17, 26 May 2007 (UTC)

Trick question! There are no "molecules" in a grain of salt. There are only ions. —Keenan Pepper 00:21, 26 May 2007 (UTC)

I didn't intend to make that distinction. Bubba73 (talk), 01:03, 26 May 2007 (UTC)

Dream Recall

While reading up on the subject of dreams and lucid dreaming, I began to wonder about dream recall rates. I personally, have only ever remembered 1 dream when I have woken up in the morning (I can't remember what it was now, but I remember that I remembered some parts of it because I NEVER remember ANY parts of ANY dream I have [I think it may have been Lucid, so that may be why I could remember some of it] - although I do realise that everyone does dream). Does anyone have any information about average dream recall rates and how many people only vary rarely remember any parts of their dreams. Thanks.

P.S. Has anyone else noticed how most people seem to think of it as impossible that you can't remember any parts of your dreams? This was a particular problem for me in an English lesson once, where we had to do some work on our last dream (I had to explain to the teacher that I couldn't do it, as I had absolutely no idea what my last dream was, or any other dream for that matter - she seemed to find it hard to believe) --80.229.152.246 22:09, 25 May 2007 (UTC)

This page suggests roughly 15%. Another site suggests 20-30%. I think from what I remember reading, it's about 30%, but that seems a bit high. This study is a lot more detailed, but it's beyond my understanding from a simple skimming. I'll look at it again later if no one can decipher it quickly.

As for remembering dreams, this is more original research on my part, but I have gone weeks or months without remembering dreams. It's not impossible in my mind for someone to be unable to remember a single dream, but it's certainly unlikely. I also know that I tend to forget dreams after awhile unless I write them down, because I've had several dreams where the plot was so good, I wanted to write about it. But I forgot most of the ones I didn't write down, and sometimes, even when I do write it down, I don't write enough and I can't make sense of what I wrote.

And more original research. I also tend to have dreams when I take naps instead of during the night. In fact, there have been several occassions where I dream 2-3 dreams during a nap, and at least one of the dreams seems incredibly long (over a day passes in the dream), and I wake up thinking it's been a long nap, but the nap have ranged from anywhere between a few minutes to an hour. I still get amazed at the super long dreams I have when I fall asleep for a few minutes. It's like watching the Lord of the Rings trilogy in one sitting, but it only takes 5 minutes! --Wirbelwindヴィルヴェルヴィント (talk) 22:53, 25 May 2007 (UTC)

Somewhat of a lucid-dreamer-on-hiatus, I have to say, dream recall depends entirely on your state of mind. When I MILD myself, I have about 80% dream recall, normally I have 1-3%. People who have dreams more often as children, and people who think about dreaming more often will likely have much higher rates -- Phoeba Wright^OBJECTION! 23:46, 25 May 2007 (UTC)

You must make a significant effort to recall the dream as soon as you awake. The mind has mechanisms to wash out dreams so you can hunt and kill your breakfast! --Zeizmic 02:38, 26 May 2007 (UTC)

Not true. Dream Recall is helped if you write down everything you remember as soon as you wake up, as it improves your memory of dreams, but if it takes any effort to recall it, chances are it wasn't really in your dream. -- Phoeba Wright^OBJECTION! 16:28, 26 May 2007 (UTC)

I suspect, and this is OR, that it has to do with early childhood training. Most children remember more of their dreams than adults do. Why should this be so? I suspect it's because when a kid has a nightmare, their parents tell them "Don't worry, it was only a dream". The implied message is that dreams are unimportant and not worth remembering. So the kids dutifully obey and start to forget them, and when they still do remember them, they tend to assign little or no importance to them. Some take their parents' injunctions so much to heart that they never remember any dreams again. JackofOz 07:24, 26 May 2007 (UTC)

Thanks for all the answers everyone. Unfortunately, JackfOz, I don't think your idea is necessarily correct. This is due to the fact that I am still a child, but do not remember any dreams, despite never having a nightmare. While this doesn't necessarily mean that your hypothesis is wrong, it would suggest it doesn't apply to all. Also, I would never have though that the MILD technique could be that effective at improving dream recall, Feba. Thanks again. --80.229.152.246 12:57, 26 May 2007 (UTC)

thermal noise

Could someone tell me what background thermal noise is and what it does? apparently in certain configurations it can influence things[pressure heat etc] can anyone tell me what it is and its consequences for experiments?

Thermal noise refers to random fluctuations (not permanent alterations) in a variety of measures (pressure distribution, temperature distribution, the dimensions of a solid/liquid, even voltage distributions) that are dependent on the temperature of the environment. Thermal noise results from the random motions of particles in a system, which will cause them to not always be evenly distributed in a system. Having slightly more particles in one region of space than another will cause the pressure in the first region to be slightly higher than the pressure in the second. The particles can also carry heat to uneven distributions in a room. This heat can be unevenly carried into, say, a metal block randomly through time, sometimes taking heat into it, sometimes taking heat away. This will cause the metal block to grow and shrink, although you would need a highly sensitive interferometer to actually detect this, due to how small it will be. Thermal noise, specifically Johnson noise, involves uneven distributions of electrons or other charged particles in a system. Thermal noise has a profound impact on any experiment as it sets a fundamental limit on how accurate an experiment can be. As with Johnson noise, it limits, for any given setup of electrical equipment, how accurately you can measure the current running through an electrical device. For an interferometer, it limits how accurately you can measure changes in a distance. For measurements that do not need extreme accuracy, the noise is negligable. For example, thermal noise sets a limit on how accurately you can weigh yourself on a scale, but the scale's accuracy is not nearly enough for thermal noise to make a difference. Someguy1221 23:03, 25 May 2007 (UTC)

Good answer. To add: Johnson-Nyquist noise is the term in electronics. To actually see that heat moves things around, look at Brownian motion. For an every-day effect, observe the noise on the picture taken with a digital camera at bad lightening, showing a lot of dark counts due to thermal noise. This is why better cameras cool the CCD element with a Peltier element to a freezing cold temperature. One should also add that the term "noise" comes from the fact that a thermal noise signal looks similar as the noise you hear from a radio that is not tuned to any station. Simon A. 20:15, 26 May 2007 (UTC)