Wikipedia:Reference desk/Archives/Science/2009 June 9

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June 9

Strange photometer problems

I was doing an experiment on Malus' law using a photometer, which was basically a light detector hooked up to a power supply unit and an ammeter. I turned up the sensitivity using a knob on the power supply, and starting measuring light intensity for different angles between the polarizers. For every angle that's not 85-95 degrees, I measured a current of 65.0-65.2 mA. The light falling on the photometer was obviously changing in brightness, but the ammeter reading was not! So I had to repeat the whole experiment using the lowest possible sensitivity, which gave better results.

Then, out of curiosity, I turned off my optical bench's light source. There was still a reading on the ammeter. I covered the photometer with my thumb, and the reading did not change AT ALL! I would attribute that to electronic noise, except I walked over to somebody else's photometer, which was EXACTLY THE SAME, and the ammeter read 0.0 when I covered it.

What's the explanation for my strange observations? Why didn't the ammeter readings change when I turned up the sensitivity? Why the reading when I covered the photometer? --99.237.234.104 (talk) 00:28, 9 June 2009 (UTC)

Your photometer is broken. SpinningSpark 15:56, 9 June 2009 (UTC)

Good--I wanted to make sure I wasn't missing something obvious about the operation of photometers. --99.237.234.104 (talk) 18:57, 9 June 2009 (UTC)

Plant species identification??

Could anyone help in identifying the plants in these photos. Here is the first entire plant [1] and here is a closeup of the flowers [2], the second entire plant is here [3] with a closeup of the flowers [4] The plants are growing in New Hampshire. Thanks --Captain-tucker (talk) 00:51, 9 June 2009 (UTC)

Some species of Lilac maybe? Not sure. We had lilacs in our yard growing up in New Hampshire, so I know they grow there. --Jayron32.talk.contribs 03:53, 9 June 2009 (UTC)

I'd say they are both members os the genus Weigela. The first possibly being weigela florida , the second I cannot determine. Originally from Asia but now common garden shrubs widely grown in temperate regions. Richard Avery (talk) 06:49, 9 June 2009 (UTC)

I concur but it is most likely a cultivar, bred for colour, flower size, duration of bloom and shrub size.67.193.179.241 (talk) 10:44, 9 June 2009 (UTC) Rana sylvatica

dynamics

plane kinetics of rigid body —Preceding unsigned comment added by Msohaibg (talk • contribs) 01:19, 9 June 2009 (UTC)

Do you have a question? The reference desk is not a search-engine, it is staffed by human volunteers, and we can best help you if you phrase your question in full sentences rather than keyword queries. Nimur (talk) 01:49, 9 June 2009 (UTC)

Surprising BBC statement about type 2 diabetes

This article http://news.bbc.co.uk/1/hi/health/8085762.stm says (in the info box on the right) that type 2 diabetes is a "Long-term condition caused by too much glucose in the blood". The first impression this gave me was that they are saying that eating sugary foods will give you diabetes, but that's a myth, isn't it? My understanding is that 1) being obese may aggravate a genetic predisposition to get diabetes, and 2) we don't really know what causes it.

Perhaps they mean something else, such as "characterized by" or "aggravated by" rather than "caused by"?

I am thin, with a sweet tooth, a state of affairs I enjoy, hence my interest. 213.122.54.27 (talk) 01:51, 9 June 2009 (UTC)

As I understand it, diabetes causes too much glucose in the blood, not the other way around. That seems like a mistake by the BBC - their science reporting is better than many news sources, but it still leaves a lot to be desired. --Tango (talk) 03:17, 9 June 2009 (UTC)

Actually, Diabetes mellitus type 2 IS caused by extended periods of elevated blood glucose. It is called "insulin resistant diabetes" also; and is a very different disease than Diabetes mellitus type 1. Type 1 Diabetes is basically a disease where your pancreas shut down and stops making insulin at all. No insulin means that your body stops processing glucose correctly. Type 2 Diabetes means that your body makes insulin just fine, but that your metabolism is out-of-whack and your body "resists" the insulin, and your glucose remains elevated at unsafe levels. Basically, with Type 2 Diabetes, if you constantly maintain a high blood-glucose level, your body gets used to that, and stops trying to lower it using insulin. It is caused mainly by eating too much high-glycemic index foods. Unlike Type 1 diabetes, much Type 2 Diabetes is mostly behavioral disease (except in the elderly and in a few other cases) and in the early stages is reversable with lifestyle changes. The BBC report is 100% accurate in the statement about what causes Type 2 Diabetes. --Jayron32.talk.contribs 03:49, 9 June 2009 (UTC)

I'd disagree slightly about this description of Type 2 Diabetes... While it is generally true that the pancreas "makes insulin just fine" the problem is that the pancreas eventually can't make enough insulin to overcome the peripheral insulin resistance. The problem develops slowly and insidiously, probably as a result of BOTH environmental/behavioral (poor diet, lack of exercise) and genetic components leading to impaired glucose (blood sugar) homeostasis. I agree with Tango that the high blood sugar is really an end result of a system that's out of whack. In the very early stages of the disease (what some would call "pre-diabetes" but which is really just part of the ongoing process), you might not have abnormally elevated blood sugar but rather abnormally elevated insulin levels, as the body struggles to keep the blood sugar in the normal range. Eventually, the peripheral resistance to the action of insulin becomes so great that even with maximal insulin production the body doesn't respond enough to keep the blood sugar level normal. Elevated blood sugar is what the diagnosis of "diabetes" is based on. This is why, as Jayron says, the early stages of the disease are reversible, since if you can fix the diet and increase exercise you can reduce the resistance of the peripheral tissues (mostly adipose tissue) so that they respond to insulin normally. What the BBC might have meant is that type 2 diabetes is characterized by high blood sugar, which then has a whole host of secondary effects (peripheral neuropathy, renal failure, coronary artery disease) that collectively make up the major morbidity of the condition. --- Medical geneticist (talk) 13:44, 9 June 2009 (UTC)

Sorry 213 but BBC is right, as Jayron pointed out. You should watch your sugar intake even if you are thin as a broom. Dauto (talk) 04:18, 9 June 2009 (UTC)

You need to be more specific. What exactly do you mean by "watch your sugar intake"? How much is too much? It would be a bit reactionary to tell someone that they can't enjoy a bite of chocolate or a cookie every so often. That is most certainly NOT going to "cause" diabetes. --- Medical geneticist (talk) 13:57, 9 June 2009 (UTC)

I think watch you sugar intake, while not that specific, is good enough in simple terms. If the OP wants to know more, they can ask or research themselves. I don't think it's misleading as it's not as if Dauto said you should never intake sugar or should cut your sugar intake down to zero. You can 'watch' you sugar intake fine even if you enjoy a bite of chocolate or cookie every so often. Similarly, it is fine general advice to watch you sodium intake or watch you intake of fats and oils. It doesn't mean you need to cut them completely out of your diet or that you can never eat a Pavlova or bag of chips Nil Einne (talk) 19:11, 9 June 2009 (UTC)

It IS misleading. Everyone has a different basal metabolic rate and different levels of physical activity. Some will do better with a low-fat diet, others should follow a low-carb diet. There isn't really a one-size-fits-all recommendation other than to maintain a healthy body weight and try to exercise regularly. The OP may be perfectly fine with whatever "sugar intake" s/he maintains. It isn't our place to give advice or opinions, just references. If the OP wants advice s/he should go to a nutritionist. (I'm not trying to invoke the medical advice restriction here, just pointing out that advice is complicated and "watch your sugar intake" is too vague and may not apply to everyone.) --- Medical geneticist (talk)

Calm down there. My statement was indeed prety vague, but not by accident. I gave a vague statement because I didn't know all the necessary information in order to give a less vague statement but I still wanted to convey the idea that just because someone is thin doesn't necessarily mean they can eat whatever amount of sugar they fill like without any health consequences. I think my statement was the best possible under the circunstances. I stand by what I said enougth to say it again. People should watch their sugar intake even if they are thin as a broom. Dauto (talk) 03:51, 10 June 2009 (UTC)

I'm not trying to have a cow, just asking for references... enough said. --- Medical geneticist (talk) 23:40, 10 June 2009 (UTC)

The high blood sugar in Type 2 diabetes is a symptom or result rather than a cause. The BBC is misleading. If a non-diabetic, who does not have impaired glucose tolerance eats some big sugar laden treat, it does not result in prolonged high blood sugar, thanks to the adequate release of insulin and the normal tissue response to it. See Glucose tolerance test. In a normal patient, even 75 g of sugar does not produce high blood sugar(over 7.8mmol or 140 mg/dl) 2 hours later. Edison (talk) 17:02, 9 June 2009 (UTC)

However, people who maintain high blood sugar levels over long periods of time, for example, have demonstrably higher incidences of Type 2 Diabetes. There is a correlation between the onset of Type 2 Diabetes and lifestyle... --Jayron32.talk.contribs 02:50, 10 June 2009 (UTC)

Thunder does not cause lightning. Correlation is not causation. Edison (talk) 05:22, 10 June 2009 (UTC)

The BBC soundbites are misleading at best. Medical geneticist and Edison give good answers. Dauto's comments are less helpful. Axl ¤ [Talk] 19:25, 10 June 2009 (UTC)

Reduction Potential in Pourbaix Diagrams

Hello. I am wondering how to calculate the reduction potential in Porubaix Diagrams. For example, the reduction potential between Fe3+ and FeO4 -2 in thisdiagram? Please! I have looked in my textbook, lecture notes, and online and I find nothing, and my exam is tomorrow! Thanks =) P.S The answer is NOT 0.4. P.PS Link to the diagram http://upload.wikimedia.org/wikipedia/commons/thumb/8/85/Pourbaix_Diagram_of_Iron.svg/605px-Pourbaix_Diagram_of_Iron.svg.png Cuban Cigar (talk) 10:10, 9 June 2009 (UTC)

It's not that hard. You don't give enough information to find the answer; but you also need to know the pH of the solution. At a given pH, draw a vertical line on the graph. The cell potential needed to make the transition between two states is where this vertical line crosses a region border on the graph. For example, at pH=2, the cell potential of the Fe²⁺ -> Fe reduction is E0 = ~-0.6 while the Fe³⁺ -> Fe²⁺ has E0 = ~0.79 and the FeO4^2- -> Fe³⁺ has E0 = ~1.75. The other forms in the chart are not stable at pH = 2 and could not exist. At other pH levels, there would be different values. --Jayron32.talk.contribs 12:13, 9 June 2009 (UTC)

Oh yes I forgot to include: the ph is 0. I'm still a bit confused though...is there a diagram that would explain it? I think a diagram would make it much more clear =)114.77.68.9 (talk) 12:51, 9 June 2009 (UTC)

OK, so if the pH=0 then use the vertical line at pH equals 0, and the reduction potential of each of the transitions is listed on the vertical axis. --Jayron32.talk.contribs 02:47, 10 June 2009 (UTC)

Ah ok thanks heaps =)114.77.68.9 (talk) 07:41, 11 June 2009 (UTC)

Look-and-say sequence (now asked at WP:RDMA)

This is maybe rather a question concerning language than science: When Conway introduced this sequence in his 1986 article, he directly explained the rule and stated: "I note that more usually one is given a sequence such as [example] and asked to guess the generating rule or next term." Can we conclude from that whether he invented the system himself or not? If not - is there a way to ask him (there is no public contact address)? --KnightMove (talk) 11:33, 9 June 2009 (UTC)

Are you asking whether or not Conway invented the "Look and say sequence" (which is a specific sequence of numbers that obeys certain rules) OR are you asking if Conway invented the "Here's a series of numbers, identify the next number in the series" type of question? Because the former may be true, but the latter is definately not; it has been part of IQ tests such as WAIS since well before Conway was a working as a mathematician and likely before he was born. See Integer sequence for some examples. --Jayron32.talk.contribs 12:04, 9 June 2009 (UTC)

Also, you are likely to find better answers at the Mathematics Ref Desk. --Jayron32.talk.contribs 12:06, 9 June 2009 (UTC)

Thx for the hint. I was asking for the former, but you may start with any other sequence; Conway himself gave the example 55555 ; 55 ; 25 ; 1215 ; 11121115. --KnightMove (talk) 12:23, 9 June 2009 (UTC)

In that case, if you have access to the Journal he published the original 1986 paper in you should read that paper. I would guess that if he were basing his research on earlier work, he would clearly have listed his references which contained the more seminal work. The other option is that the concept of a "Look and say" sequence is old enough to have been lost in the mists of time; and as such does not have an original discoverer. But either way your best chance is to find his original paper and look there. --Jayron32.talk.contribs 12:32, 9 June 2009 (UTC)

These sorts of tests always infuriated me. As anyone who has ever studied polynomial interpolation can attest to, providing a subsequence of n numbers defines a countably infinite number of valid polynomials - so asking "what is the next number in this sequence" is making a huge number of unstated assumptions about the behavior of the overall sequence. Most commonly, a requirement is that all members of the sequence must be integers; but even then, with sufficient mathematical maneuvering, a countably infinite number of valid polynomials will still always fit. And if you're willing to use any other method of sequence generation (besides integer-sampled polynomials), you have an uncountably infinite set of sequences. Bluntly put, the people who ask these sorts of questions profess little to no knowledge of higher mathematics - and they intend to test somebody else's "IQ" ? Nimur (talk) 17:05, 9 June 2009 (UTC)

I've heard this sentiment before, but it seems tantamount to saying that science is impossible. The sequence is raw data and the answer you give is a theory, and some theories are better than others. I think almost anyone would agree that the sequence 55555, 55, 25, 1215, 11121115 is better explained by the look-and-say rule than by 5/6 (66666 − 3473104x + 6210289x² − 3362372x³ + 558587x⁴). The latter is clearly not an explanation at all since it's longer than the data. The look-and-say rule I think is shorter than this data in whatever encoding system people use when judging the value of scientific theories. -- BenRG (talk) 18:55, 9 June 2009 (UTC)

There is another factor in science, though - the mechanism. A simple law is of limited use if you don't have a sensible theory regarding the mechanism behind that law. You can use the law to make predictions about precisely what the law is about, but nothing else, to come up with predictions about new things requires a mechanism. With these kinds of number sequences there is never any kind of mechanism, it is all completely arbitrary. --Tango (talk) 18:59, 9 June 2009 (UTC)

What do you mean by mechanism? Is that just another word for a model? Is that extra step always necessary or even useful? What is the mechanism behind quantum mechanics? Does that last question even make sense? Dauto (talk) 21:24, 9 June 2009 (UTC)

The mechanism behind something is what makes it happen. A scientific law is just a formula, for example Ohm's law is V=IR, the mechanism has to do with electrons bouncing around. Kepler's laws of planetary motion are just simple rules with no explanation, the mechanism behind them is the inverse square law of (Newtonian) gravity (Kepler didn't know that, he derived the laws empirically, and they were still useful before Newton explained them, but once you understand the mechanism behind them you can improve upon them - for example, you can introduce new factors to get more precise results). These examples aren't very good... The concept is easier to understand in something like biology. We may have results that say a particular drug increases someone's chances of surviving a disease by 10%, which is all well and good, but if we want to improve on that it is useful to know the mechanism by which the drug works - for example, it might inhibit a particular enzyme. If somebody makes a scientific claim without giving a plausible mechanism, you would be more sceptical of their results. So, if the sequence above arose in nature and someone wanted to predict what would come next, they would probably go with the polynomial extrapolation, rather than the look-and-say rule, since it is easier to think of a plausible mechanism for it. I can't think of any way natural could follow the look-and-say rule, but polynomials come up in all sorts of ways. --Tango (talk) 23:15, 9 June 2009 (UTC)

I thought the Kepler's laws vs the inverse square law was a good choice for what you are saying. Kepler's laws can only be used to describe the very planetary motions that Kepler used to obtain the laws to begin with. But once you understand that they are a consequence of Newton's gravitation you can than use that to describe other motions such as precessions and all sorts of wobbles, etc. The problem with that is that Newton's inverse square law isn't a mechanism at all. It's just another simple law described by a simple mathematical formula and would therefore need its own explanatory mechanism according to your statement. I think your statement was too general. Sometimes a working model is better than a mechanism. Coming back to the original question, clearly, as BenRG said, the look and say is a better model than the polynomial extrapolation (which doesn't explain anything). I would be very wary of any model rooted on a polynomial of fourth degree extrapolation unless, as you said, there was a good mechanism behind that polynomial. Dauto (talk) 05:03, 10 June 2009 (UTC)

The fact that gravity isn't really a mechanism is why it was a bad example - I think most examples in physics end up being long chains of theories upon theories. Biology is the same, of course, but people are more willing to stop at a reasonable point and just accept it as fact. Mechanisms certainly aren't always required - Kepler didn't have one but that didn't make him doubt his results. They are nice to have, though, and give us a good idea of how much confidence to have in a claim for which we don't have particularly good empirical evidence (and a sequence of 5 numbers isn't very good empirical evidence for anything). --Tango (talk) 05:18, 10 June 2009 (UTC)

Dead arm while sleeping

I'm not sure where to find info on this, and in fact, I'm not even sure how to really look for it (is there no different term for limb numbness [i.e. that which isn't caused by some sort of condition] than parasthesia?), but I've always been rather prone to sleeping on my arm and then waking up finding it numb; as far as I can remember it would happen every few weeks or so, definitely nothing of concern. Lately it's been interrupting my sleep more though, yesterday I woke up three times! I sleep on my front (I can't sleep on my back -- so much so that I often turn on my back in the morning to prevent myself from falling back asleep!) and I'm pretty tall and skinny, which I think might make it easier for me to get caught up in my arms while I'm asleep, but I was wondering if anybody had any suggestions for how I stop my arm from getting squashed so much? I alternate between sides (front-ish orientation) and sleeping on my front, but my arm always seems to be able to find a position to get caught in! Thanks! 210.254.117.186 (talk) 12:17, 9 June 2009 (UTC)

Not that I have any recommendations, but the medical term for "limbs falling asleep", or rather going numb due to laying on them is called Paresthesia and is usually caused by compressing a nerve in the limb. The compressed nerve stops sending signals to the Central Nervous System, so your brain basically changes the way it interprets the signals from that limb. When the compression stops, the nerve begins to transmit signals to the CNS normally, but the CNS takes some time to "catch up" and re-adjust its processing to correctly interpret the signal. This "readjustment" period is what causes the numbness and "Pins and needles" feeling as the limb comes "back on line". Its exactly like walking from a dark room to a light room; your body does not adjust instantly so there is temporary (but harmless) discomfort during the adjustment phase. --Jayron32.talk.contribs 12:28, 9 June 2009 (UTC)

Anything that is an ongoing problem with sleeping is worth talking to a doctor about. I don't mean to sound alarmist, but there is always a chance that what's changed is something you aren't thinking of. Of course the Reference Desk does not allow medical advice, let alone speculation. --Anonymous, 12:47 UTC, June 9, 2009.

If you don't think that the pressing of your arm is the entire reason, read this. I'm not diagnosing, of course, but this one is getting more and more common as we go on. Mxvxnyxvxn (talk) 23:23, 15 June 2009 (UTC)

Assuming it is just a quirk of your nature and nothing medical a body pillow or memory foam mattress topper might be worth considering. But do check with your doc. 71.236.26.74 (talk) 14:33, 9 June 2009 (UTC)

Perhaps sleep with your arm under the pillow? ~AH1^(TCU) 01:40, 11 June 2009 (UTC)

Logic Gate Amplifiers.

I built that on silicon in 2005, measured it, and it's one of my first contributions to Wikipedia! Nimur (talk) 17:08, 9 June 2009 (UTC)

1. Where can I find articles and analysis on using NOT gates in their linear region as amplifiers?(TTL, CMOS, LS TTL, high speed CMOS)
2. Also, up to what frequency can they be used?
3. I want to see if I can use them to amplify signals in 10-50MHz range. —Preceding unsigned comment added by 59.93.15.239 (talk) 15:03, 9 June 2009 (UTC)

That's a blast from the past, haven't seen that trick used in a long while. Not sure where to find articles, don't think I've seen one since the internet started. Basically you need lots of negative feedback to keep it in the linear region. Probably won't work with most modern gates because they are heavily buffered to keep the device out of the linear region. It worked with older unbuffered CMOS because basically, you only had one transistor stage in there and it was an amplifier, just not being used as one. The frequency bandwidth it will work on depends on the speed of the device, rough rule of thumb, halve the maximum bit rate and call it Hz. SpinningSpark 15:23, 9 June 2009 (UTC)

You can check the Inverter (logic gate) article - and take a look at that fine image of a voltage transfer curve for a NOT gate operating in its linear region! (Un)fortunately, most commercial inverters designed for CMOS or TTL logic have much much higher gain, so you'll have to carefully select your operating point. Why wouldn't you just use a high-gain amplifier, which is designed for tunable bias points? Take a look at the datasheet of any amplifier you are interested in using. 50MHz is not at all unreasonable, but you're really going to have to watch your operating point, because unless you are right on the dead-center of the linear region, you'll likely trip the circuit to high- or low- output only, (after all, they are intended for digital circuits). Nimur (talk) 17:08, 9 June 2009 (UTC)

Also, read through the Application Notes for the 7404 inverter at Texas Instruments, especially Designing with Logic and Input and Output Characteristics of Digital Integrated Circuits (data). You may find the App Notes discussions a bit more high-level than the data-sheet (which is, of course, just the specifications). Nimur (talk) 17:48, 9 June 2009 (UTC)

A resistor connected from output to input of the NOT gate (inverter) suffices to bias it in its linear region. If the resistance is low enough for the voltage drop due to gate input current to be insignificant, the working point can be found simply by drawing a line from origin (0,0) to (2,2) on the transfer curve shown. The working point is where the line crosses the curve. Connect the input signal to the gate input via a capacitor (ac-only amplifier) or a resistor (dc and ac amplifier). The inverter shown has only about 2x voltage gain which limits its usefulness as an amplifier. Actual high gain CMOS and TTL inverters are easy to use this self-biased way and can provide the bandwidth you want. However you cannot use Schmitt trigger inverters that have no linear region. A useful application is to amplify a weak ac signal to a full logic swing. A clock oscillator can be constructed from one or two inverters. Cuddlyable3 (talk) 19:46, 9 June 2009 (UTC)

The reason I asked is that on micro chip's site, in one of the app notes on power line communication, there is a 2stage amp using not gates. and my friend who is building that schematic for his project wanted to know how they designed values of those biasing resistors. 59.93.26.185 (talk) 08:23, 10 June 2009 (UTC)

Biased in its linear region the NOT gate is like an operational amplifier that has only its - input accessible. The same rules apply for defining gain by external components as for an opamp e.g. A 1k ohm feedback resistor and a 330 ohm input resistor define an amplifier of gain -3. Higher resistor values can be used with CMOS inverters that have small input current. Cuddlyable3 (talk) 19:12, 12 June 2009 (UTC)

I read the links @ the bottom. One reason I don't want to use rf amp chip is that most are surface mount and I cant solder them for my home projects and that they are harder to obtain in nearby shops. Thanks for the links. 59.93.26.185 (talk) 08:23, 10 June 2009 (UTC)

How about a simple operational amplifier? 741 amp chips still come in DIP package. When you do you search, most manufacturers and distributors allow you to restrict to a particular package type - you probably want DIP for basic hobbyist stuff (it will fit into perforated board and prototyping kits, etc). Nimur (talk) 14:04, 10 June 2009 (UTC)

Personally, I think you'd be better off using vacuum tubes instead of NOT gates. FWIW 76.21.37.87 (talk) 05:46, 14 June 2009 (UTC)

Help Me Preparing My Work

There is going to be held a competition in our medical school in which students have been asked to present their research works about any scientific topic. I am going to present my work about the relation that exists between structure of an object and the function realized by it, especially focused on human body going from atomic level to organism as a whole. I need answers of the questions like , if the protons are inside the nucleus then what importance it may have on the functioning of an atom and its role in biological systems, if glycogen has a cyclic structure then how it is important in its role in human body, if renal cortex has a vertical arterial system then why it is so, the functional importance of the cells participating in the structure of brain. Can someone guide me knowing answers of such questions and getting interesting information , Something precise PLZ. I searched on Pub Med but there is cluster of scattered things. Some web addresses, some opinions, some guidelines. Thanks —Preceding unsigned comment added by 200.55.135.211 (talk) 16:35, 9 June 2009 (UTC)

The topic "structure and function" sounds far too general for a presentation of less than encyclopedic length. For a med school presentation, your idea on the structure of glycogen, or the value of the arrangement of arterial structure of the kidneys are possibilities, but the function of the proton in the atom or the "function of cells in the brain" sound far too general. Maybe start with an interesting new development in one of these areas and make that the conclusion of your presentation. Edison (talk) 16:55, 9 June 2009 (UTC)

Are you looking to do a "report" on a topic (i.e. summarize a body of work) or are you really planning to prepare an independent "research project" where you identify a question and study it in a laboratory setting? It sounds like what you want to do is summarize what others have discovered about "structure and function" which, as pointed out by Edison, is far too broad-reaching a question to be summarized in a precise fashion. Perhaps you should pick an area to focus on. It doesn't matter what RefDesk contributors think is interesting, you should pick something that is deeply interesting to YOU so that you'll be motivated to do the best job. There are so many possible examples that it doesn't even make sense for us to suggest articles or books for you to start from. I'm sure you'll get better answers here (not to mention a better final result) if you focus your question first. By the way, glycogen is a branched polymer (of glucose molecules), not a cyclic structure. Glucose is cyclic. --- Medical geneticist (talk) 17:29, 9 June 2009 (UTC)

Careful with micro- vs macromolecular description: flycogen's structure discussion says it's a branched structure of glucose units, each of which is cyclic. DMacks (talk) 17:43, 9 June 2009 (UTC)

Isn't that what I said? Clarification added. BTW, I'd like to see what "flycogen looks like!  ;-) --- Medical geneticist (talk) 17:51, 9 June 2009 (UTC)

d'oh. google/googlescholar find a bunch of that typo in the literature too:) DMacks (talk) 18:04, 9 June 2009 (UTC)

Can someone please show me a structure or at least a molecular formula for flycogen? I've never heard of it before, is it something that they discovered / synthesized just recently? :-) 76.21.37.87 (talk) 05:41, 14 June 2009 (UTC)

Internet over radio (not internet radio)

Is it possible to connect everything through radio? Would it be much slower than fiber optic? Is it in use somewhere in the world? --Mr.K. (talk) 16:52, 9 June 2009 (UTC)

Many people access the internet via a wireless (radio) connection. At what point do you contemplate having it work by radio? There might be bandwidth problems if every home communicated with every other home at will by radio connected internet. It could be done via cellphone-like technology, but the charges might be extremely high. Edison (talk) 16:58, 9 June 2009 (UTC)

Take a look at packet radio, which was a precursor to the modern 802.11 family of wireless protocalls. It is indeed possible to connect things by radio, but open air is a shared-channel by "unknown number" of users (unlike a wire, which is shared by N users, a maximum number pre-determined by the engineering specifications of the protocol). As per my earlier discussion about air-to-ground radio, available bandwidth is proportional to operating frequency (by physical limitation); and unfortunately, on earth (with atmosphere), higher frequencies have limited range. This means you need a hierarchical, hub-based "repeater" system - with all of the associated troubles of an untrusted network and a peer-to-peer network routing protocol (at the network switching level, not at the software-level as a modern file-sharing program uses "p2p"). In short, if you could control every radio in the world, and force it to comply with your protocol, then "wireless-only" internet would probably be a very efficient, low-infrastructure-cost alternative, and it would certainly solve the "last mile problem" that plagues wire-based network infrastructures. But, radio is a shared channel, and despite government regulations, you can't assure that your signals won't be interfered with by other users - so it's best to keep wireless transmissions short-range, high-bandwidth, and use lots of error-checking along the way. Nimur (talk) 17:23, 9 June 2009 (UTC)

endangered animals

OK, I have a question on how the endangered labels work. An example using data from our articles shows that the black rhino is critically endangered (the highest level of scarcity) with 3,600 animals left. The White Rhino is only near threatened (1 level lower then common) and it has 17,500 animals left. That does not seem like a very big difference. Can someone explain how this works? 65.121.141.34 (talk) 19:11, 9 June 2009 (UTC)

It may be worth reading IUCN_Red_List#Categories and look into the rules. I think this is the standard framework used for grouping species in terms of their endangered. ny156uk (talk) 20:57, 9 June 2009 (UTC)

It doesn't really explain why two very similar animals would have such a different label for a relatively small difference in numbers. 65.121.141.34 (talk) 18:47, 10 June 2009 (UTC)

There isn't a relatively small difference in numbers....there are nearly 5 times as many of one as there are of the other. Alaphent (talk) 10:44, 11 June 2009 (UTC)

If you look at the links from there Near Threatened (white rhino) and Critically endangered species (black rhino) you'll see that to be considered critically endangered it needs to be "species numbers have decreased, or will decrease, by 80% within three generations." From here it is logically quite easy to see that one species is considered in greater danger because of either it's fast-reduction in numbers, or an anticipated fast reduction in numbers. Near threatened is not quite as 'bad' a scenario. Volumes alone is not really a good comparison factor since the 'normal' population of animal will be very different depending on its size, location and habitat. ny156uk (talk) 21:19, 10 June 2009 (UTC)

Speed of light...

I was under the impression that it is impossible for anything with mass to move at the speed of light. Yet earlier when I asked about an object falling for infinity long (imagine it falling in a vacuum or something close to a vacuum such that its terminal velocity is greater then c), someone told me the relativistic mass would increase. Shouldn't the relativistic mass go to zero as v goes to c? —Preceding unsigned comment added by 24.171.145.63 (talk) 21:44, 9 June 2009 (UTC)

The rest mass of something traveling at c must be zero. The relativistic mass of something traveling at c need not be zero. However, to examine the condition you ask about, the mass (rest or relativistic) of something approaching c can be anything. — Lomn 22:02, 9 June 2009 (UTC)

(edit conflict) Interesting question. As I understand it, no object with mass can go faster than the speed of light, even if it is falling (and therefore accelerating) an infinitely long ways. Your friend is right that the object's mass would increase—as the object's velocity approaches c, some of the energy that is added to it with the intent of speeding it up is instead converted into mass through E=mc². Interestingly enough, this would only increase the gravitational pull in your scenario, compounding the effect...I'm no physicist, but I imagine this scenario would violate a law of thermodynamics because energy is being constantly created. Also, gravitational pull decreases exponentially as you move farther out, so there would theoretically be no gravitational force making the object fall if the larger object is infinitely far away. Just tossing ideas out there—hopefully I'm correct.  :) —Pie4all88 ^{T C} 22:06, 9 June 2009 (UTC)

Definitely not. As an object speeds up, its relativistic mass increases, making it harder for it to go faster and impossible for it to reach the speed of light. You may be thinking along the lines that as matter approaches c, it becomes more like light, but that's not the case. Clarityfiend (talk) 22:26, 9 June 2009 (UTC)

It's an odd thing. The relativistic mass increase is a factor that gets larger and larger as you approach the speed of light - and AT the speed of light, it's infinite. So an object that has any mass at all at 'normal' speeds would have infinite mass at the speed of light. That's why things with mass can't go that fast - to give it infinite mass requires infinite energy. But for photons - which conveniently have a zero rest-mass - their mass at the speed of light is (in a sense) zero times infinity...which could be any number you could imagine. Hence they can have non-zero relativistic mass - but if they were to slow down by even the tiniest amount - their mass would drop to zero. SteveBaker (talk) 03:10, 10 June 2009 (UTC)

Say a photon is traveling away from a black hole that it neared. The light is moving slower than c, correct? Does this mean, then, that there is a fundemental difference between light that is traveling in a vacuum without gravitational interference and light that is still in a vacuum but not moving at its maximum speed? Would the photoelectric effect still work if light slowed down at all? Or do I have this all wrong? —Pie4all88 ^{T C} 16:58, 10 June 2009 (UTC)

No, light always travels at c. Light moving away from a black hole is redshifted, so it loses energy but due to having a lower frequency, rather than a lower speed. --Tango (talk) 17:26, 10 June 2009 (UTC)

Ah, thanks for the link! I was under the impression that redshift only occurred with acceleration or the expansion of the universe...but I suppose that an object escaping a large gravitational field is decelerating, so this would make sense. One more thing, though—say a friend of yours falls into a black hole. I remember being told that an outside observer would never actually see him reach the event horizon; he would seem to get closer and closer while gradually fading away (I imagine he would also get blueshifted). Why would he fade away if light always travels at c? I assumed that the light reflecting off of him and barely escaping from falling into the event horizon would be moving exponentially more slowly toward the observer, which is why you would see him fade away rather than fall in. Can someone please enlighten me as to where I am going wrong? —Pie4all88 ^{T C} 18:35, 10 June 2009 (UTC)

You're going wrong at the same point you went wrong before. the light doesn't move more slowly toward the observer. It redshifts, and that's what's meant by fading away. Dauto (talk) 18:42, 10 June 2009 (UTC)

Ah, I may understand what you guys are saying now. So the person wouldn't "fade away", per se; they would seem more and more redshifted as they fell in and sometime before entering the event horizon, the light would stop being in the visible spectrum. This would happen in real time, then (er, as instantaneous as it can be given c), right, rather than residual light slowly fading away? —Pie4all88 ^{T C} 20:04, 10 June 2009 (UTC)

Getting closer, but still not quite there yet. As the person approaches the black hole's horizon any light being emitted would suffer an ever increasing redshift that would tend to infinite. That has two important consequences. First, the light would not only shift until it was no longer visible, but would actually shift until its frequency would go to zero and therefore become unobservable, hence we say the person would fade away. Second, redshift means a decrease in frequency which is the same as a increase in time periods. So if the person is sending a beep every second (as measured by their watch) the interval between subsequent beeps would appear to get bigger and bigger (as measured by the outside observer) due to redshift. If the last beep happens just as the in-falling person crosses the horizon that last interval would suffer an infinite redshift which means that the outside observer would have to wait an infinite amount of time before he could observe that last beep. In other words, the in-falling person would appear to slow down as they approached the horizon. Dauto (talk) 03:35, 11 June 2009 (UTC)

I'm surprised no one seems to have mentioned this, but the gravitational potential energy is not free energy. Imagine Reason (talk) 00:31, 16 June 2009 (UTC)

Limit to Acceleration?

Is there a limit to how fast something can accelerate? I imagine there would be, based on the inertia of an electron. —Pie4all88 ^{T C} 22:16, 9 June 2009 (UTC)

Do you mean a theoretical limit like the fact that speed is limited to the speed of light? I don't believe so. If you apply an arbitrarily large force, you'll get an arbitrarily large acceleration (it isn't proportional once you take relativity into account, of course). --Tango (talk) 22:31, 9 June 2009 (UTC)

The limits are essentially the amount of energy you can apply (which in theory is something like the mass of the visible universe times the speed of light squared!) and the structural limits of whatever it is that you're accelerating. Of course this insane acceleration could not be sustained for very long because relativistic effects will increase the effective mass of the thing you're accelerating...and thereby reduce the acceleration that your energy budget allows you to apply. But I can't think of any 'hard' limits. SteveBaker (talk) 03:01, 10 June 2009 (UTC)

(Really? Two comments: First, I think that's a hand-wavey and probably totally incorrect estimate of the total energy in the universe. Second, energy is not related to acceleration - it is related to velocity. In the same way that you need a time-derivative of velocity to get an acceleration, you need a time-derivative of energy to get the power necessary for that acceleration. When's the last time you measured your Mini Cooper's "number of joules consumed for 0 to 60 in 2.8 seconds"? You need a horsepower and a torque and a mass to estimate the acceleration. Hypothetically, if you could harness the entire energy of the universe, there's still not any necessary inherent constraint on how quickly you can transfer that into a billiard-ball. I'm thinking that maybe a better estimate of maximum acceleration is something like, Speed of Light over Planck Time (asymptotic limit), but I'm just pulling this out of my own head. I've never heard of a maximum acceleration limit in any physics class I ever took.) Nimur (talk) 14:11, 10 June 2009 (UTC)

Following is a statement from the book "Motion Mountain" (The Adventure of Physics) by Christoph Schiller

We will discover that, just as special relativity is based on a maximum speed c,

general relativity is based on a maximum force (c**4)/4G or on a maximum power (c**5)/4G.

121.242.23.197 (talk) 07:05, 10 June 2009 (UTC)Vineet Chaitanya

That's his own work, rather than an accepted scientific result. I'm just reading his paper on the subject now, but it is very recent (2003) so I'm not sure the scientific community has had sufficient time to review it. --Tango (talk) 16:59, 10 June 2009 (UTC)

Upon further investigation, that paper doesn't even seem to have been published in a peer-review journal. That is a red flag to me... --Tango (talk) 17:10, 10 June 2009 (UTC)

Yes, that's very suspecious. Interestingly my estimate below gives a result in the same ball park of that paper, but I see that more as a practical limit than as a hard limit. Dauto (talk) 17:41, 10 June 2009 (UTC)

I understand the work has been published: International Journal of Theoretical Physics, vo. 44, pp. 1629-1647 (2005) 121.242.23.197 (talk) 06:49, 14 June 2009 (UTC)Vineet Chaitanya

I never heard of such a limit. I'm inclined to agree with Nimur. Another interesting point of view is that the maximum acceleration is the one imposed on a couple of eletrons placed at a planck distance from each other $a={\frac {e^{2}}{4\pi \epsilon _{0}l_{P}^{2}m_{e}}}$ Both of these estimates (Nimur's and mine) give absurdly high accelerations. Dauto (talk) 17:00, 10 June 2009 (UTC)

Very interesting discussion, guys! Thanks for all the responses so far. And yep, Tango, that was my line of thinking. I suppose there's no theoretically "largest" position, though, so you can't say that since velocity has a maximum value, acceleration does as well. At least, not without a major development in physics.  :) —Pie4all88 ^{T C} 17:15, 10 June 2009 (UTC)

Well, I partially take that back. I suppose you could consider the edge of the observable universe to be the "maximum" distance away from a central point. So it's be more like a radius. It doesn't seem like the universe itself has edges, so you can't use that value. Hmm. —Pie4all88 ^{T C} 17:20, 10 June 2009 (UTC)

Special relativity does put a limit on acceleration of an extended object (something with a size). If you pull on one end of an object of length L with a constant acceleration exceeding c²/L, it will break no matter what its composition. The reason is that uniform acceleration describes a hyperbola in spacetime (see hyperbolic motion), and anything beyond the asymptotes (which cross at a distance of c²/a) can't possibly catch up with you without exceeding c.

There is a Planck acceleration of c/t_P ≈ 10⁵² m/s² at which one might expect classical ideas about acceleration to break down. The Planck force c⁴/G and the Planck power c⁵/G are unusual in that they don't depend on h, so you might expect them to have a meaning in non-quantum general relativity. Whether they're maximums (as Motion Mountain claims) I don't know, but they might be. Acceleration is inversely proportional to radius of curvature in spacetime, and odd things might happen when your radius of curvature becomes smaller than your Schwarzschild radius, that is, when c²/a < 2Gm/c², that is, when ma > c⁴/2G. That's only off by a factor of two from Schiller's value. -- BenRG (talk) 21:43, 10 June 2009 (UTC)

As noted above, the work by Christoph Schiller mentioned further above has been published in a peer reviewed journal: . In his paper Schiller points out that his conclusions and the maximum force that he discusses in detail are based explicitly or in part on earlier work of ; and . So there is evidently authoritative agreement concerning the maximum force, F(max)=c^4/4G. In response to Schiller's paper, another paper (by R. Benish) has recently been submitted to the International Journal of Theoretical Physics, which provides an alternative, simpler derivation, based on Special Relativity, the equivalence principle and the inverse square law. An intermediate step in the latter derivation involves a maximum gravitational acceleration, which is inversely proportional to mass: g(max)=c^4/4GM. While the decision to publish or not by IJTH is pending, the submitted paper, can be accessed at . (User:Stoonroon) 8 September 2009

How to safely hang a suspended sign like this one

Hello, thanks for taking a look at my question. I have been doing a lot of searching and I can't find a suitable answer to this.
I have seen a few instances of a sign being suspended over a roadway, often as sort of an "entryway" into a city or neighborhood. An example is this one hanging over Highway US 199 in Grants Pass, Oregon.
My questions are:

1. Assuming the sign is held up by cable(s) hung between 2 vertical poles (one on either side of the road) with no other stays or trusses or guy wires, how would one calculate the needed materials for structural integrity? In other words, if the sign has a mass of X, and the distance between the poles is Y, then how would I find out how strong/thick the cables and poles must be? (We can assume the poles and cable are steel)
2. Within the United States, is there any national building code or regulation that would cover a sign such as this, or is it up to local building codes in each jurisdiction? (We can assume the sign is hanging over a local street, and not over a state or national highway)

Thanks for your help! John —Preceding unsigned comment added by JohnMGarrison (talk • contribs) 22:22, 9 June 2009 (UTC)

See this e.g.[5]. Remember you can't just rigidly hang something with cables at full tension. You have to allow for wind, heat expansion/contraction of the material and the like. One of those engineering headaches where you get something like the Tacoma Narrows Bridge if you overlook something or underestimate some effect. 71.236.26.74 (talk) 23:21, 9 June 2009 (UTC)

Thanks for the link to the Interstate sign guidelines. For my question, I am just wondering about suspending over a local city street - would that be a matter of local codes? As for the structure, I am mainly interested in just supporting the weight. We can assume for now that the cables will allow for heat expansion, etc. JohnMGarrison (talk) 23:31, 9 June 2009 (UTC)

Check what DOT is in charge for your area. They also have lower level county/city ones. I don't know who does what, but I know you can't just hang as sign by yourself. You have to get them to do it for you (For the above reasons). Unless we are talking private road. They'd probably be able to help you with that, too. 71.236.26.74 (talk) 23:56, 9 June 2009 (UTC) P.S. in my experience when calculating suspending things like signs, weight is the least worry when calculating cable gauge and tension. I've long since forgotten most of what I knew. I just work with the guys who do stuff like that somewhere down the line. Even they now just type their numbers into the software. :-) 71.236.26.74 (talk) 00:12, 10 June 2009 (UTC)

Two comments. First, you are asking a legal question, not a science question. We are prohibited from giving legal advice. You should contact the local government or a lawyer. Second, you cannot ignore wind. Even a "small" sign over a street will have a fairly large sail area, probably larger than the mainsail on a small sailboat. Look at the masts and rigging on even a small sailboat to see the effect. -Arch dude (talk) 14:00, 10 June 2009 (UTC)

I don't mean to be asking a legal question at all. I am really interested in the physics of how to support the weight of the sign. The second part of my question was more to see if anyone knows how the government handles this type of building project. Let's forget the second part of my question and just focus on the first. Where can I find an equation to tell me how strong the poles and cables must be in order to support the sign? Thanks, JohnMGarrison (talk) 17:10, 10 June 2009 (UTC)

I simplified this question and asked it again below. Please see below. JohnMGarrison (talk) 17:48, 10 June 2009 (UTC)