Wikipedia:Reference desk/Archives/Science/2013 December 9

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December 9

Lake Drummond

According to the article about the Great Dismal Swamp, local legends about Lake Drummond "tell of a giant firebird that made a nest of fire in the swamp that later filled with rain". Which begs the question: Is there any evidence of volcanism in the area? Because this sounds very much like the formation of a volcanic caldera. 2601:9:3200:467:5527:C9C7:A04:2CD5 (talk) 03:16, 9 December 2013 (UTC)

Not really, see Will-o'-the-wisp. Swamps produce copious amounts of combustible gases, as well as featuring creatures known to have bioluminescence. Such phenomena have been associated with swamps all over the world since time immemorial, and it probably isn't vulcanism. Read that article for some probably explanations. --Jayron32 03:37, 9 December 2013 (UTC)

I've also heard tell of haunted pirate treasure and Blackbeard's ghost...! One cannot believe every legend one hears about the Carolina coast.

That National Park Service site also says: "The beaches along Cape Hatteras National Seashore sparkle at night. When you kick the sand, you disturb tiny dinoflagellates like seasparkle, magnified in the picture to the left. A chemical reaction causes them to glow with a blue-green light." Now, that's a phenomenon to which I can personally attest! Nimur (talk) 05:33, 9 December 2013 (UTC)

Honestly that sounds a lot more like a meteorite impact to me. And in fact the trust responsible gives that explanation for its origin. [1] Our article on Lake Drummond speaks of an underground peat fire as the explanation for the legend. It is not terribly improbable that it is based on truth, though if the lake really is 3500 years old or more, that is a most remarkable oral tradition indeed! Wnt (talk) 19:03, 9 December 2013 (UTC)

What size tree would I be ?

...if we assume I'd need the same amount of energy as a tree that I do as a human. So, around 2000 kilocalories. StuRat (talk) 05:31, 9 December 2013 (UTC)

Wow! That's a good question! And I don't see any direct answers to it either. Let's collect some back-of-envelope facts:

• We know that photosynthesis is between 0.1% to 8% efficient in converting sunlight to usable energy.
• Mean "insolation" is somewhere around 5kWh/m² per day.
• A mature tree has around 200,000 leaves.
• An average leaf is about 10 square centimeters.
• One kWh is 860 kcal.
• You probably need around 2,000 kcal of food each day to survive.

So...

• The typical area of light collectors on a tree is about 200,000 x 10cm² = 200m².
• Which is hit by about 200x5 = 1,000 kWh/day.
• Which produces between 0.001x1000 and 0.08x1000 = 1 to 80 kWh of useful energy per day.
• Which is between 1x860 and 80x860 = 860 and 69,000 kcal.
• Which is (amazingly) around 400 to 3,500 times 40% to 35 times as many calories as you eat over the same period!

So I think you'd be a very tiny shrub! smallish inefficient tree or an impressive hightly efficient one.

This seems wildly too high to me...but I don't see a lot of sources of error in there. Sure, some leaves shadow other leaves - but most of the light they don't absorb travels through them, so even the leaves in shadow are generating energy.

Someone please tell me where I screwed up?

SteveBaker (talk) 07:14, 9 December 2013 (UTC)

Steve, I'm not sure about your exact numbers, but your conclusion seems reasonable: you often hear similar factoids based on scientific details about how many pounds of grain it takes to make a single pound of beef, and so on. Animals - heterotrophs in general - use energy very differently from autotrophs, so it might be expected that a tree is a few orders of magnitude "more efficient." Trees also don't expend energy on locomotion, or homeostasis, and they spend very little energy on higher thought processes... there is no really good way to compare their energy budget to the energy budget of a large mammal. Nimur (talk) 07:22, 9 December 2013 (UTC)

Yes, it's hard to comprehend that a hummingbird has to eat it's entire weight every day and that an Alligator can survive for an entire year without eating. We can get by with eating around 2% of our body weight each day and are in trouble if we don't eat for a week. The hummingbird is vastly more active than we are - and the alligator is cold-blooded, has a tiny brain and spends most of it's time staying completely still. Even if we stay completely still, the demands of being warm-blooded and having a large brain that we can't turn off are large compared to most animals. SteveBaker (talk) 15:07, 9 December 2013 (UTC)

I think you missed a factor 1000 somewhere: 860 kcal isn't 400 times 2000 kcal. And you seem to have lost a zero: from 400 to 3500 is a factor 8 or 9, not 80. Ssscienccce (talk) 08:45, 9 December 2013 (UTC)

You're fine until the last line. 860/2000 = 0.4, 70,000/2000 = 3.5. So you'd probably be a tree, rather than a bush, but we'd need better estimates to determine how you range on the scale from mighty oak to sapling... MChesterMC (talk) 09:32, 9 December 2013 (UTC)

Thanks! I knew I'd gotten something wrong. I've amended the numbers and conclusion above. SteveBaker (talk) 14:56, 9 December 2013 (UTC)

35, not 3.5 . Photosynthetic efficiency is on average about 1%, which would make Stu a tree with a "solar energy capturing surface area" (isn't there a specific term for that?) of 40 to 50 m² Ssscienccce (talk) 09:35, 9 December 2013 (UTC)

...and if we consider a hemispherical tree which collects sunlight on its curved surface only, this gives the Stu-tree a diameter/span of 5 to 6 m. Gandalf61 (talk) 11:23, 9 December 2013 (UTC)

• I'd bypass SteveBaker's argument entirely, and simply compare CO2 produced by respiration by a plant of a certain living mass and a (propbably resting) human over a daily period. Respiration, producing CO2 from sugar and oxygen, is the same chemical process in animals and plants, producing the same ATP output. The CO2 figures are available for plants that have been kept in darkness and their output measured. Comparing the CO2 output would give a comparison living plant mass to average adult human. Then you'd need to account for the fact that most of the wood of a tree is dead, and get an estimate for the dead to live tissue ration in whatever kind of tree you want to use. μηδείς (talk) 18:22, 9 December 2013 (UTC)

Now for the main question: What kind of tree would you be? Clarityfiend (talk) 22:32, 9 December 2013 (UTC)

Finally, someone asks the obvious question. And it's important to point out that Stu is well on his way already, as he has roots, a trunk, limbs and sometimes leaves. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:43, 10 December 2013 (UTC)

I wasn't going to get into it, but the generous assumption is an oak. The roots for the words tree and oak overlap in many languages. μηδείς (talk) 02:31, 10 December 2013 (UTC)

I don't think StuRat is one, but the only kind of tree I've ever compared anyone to is a Caucasian wingnut. Katie R (talk) 13:12, 10 December 2013 (UTC)

The biggest problem in answering this well is the wide range of efficiency numbers for photosynthesis. 0.1% to 8% is what our article says - which means there is an 80:1 size ratio between an inefficient plant that consumes 2000 calories/day and an inefficient one. That's a bigger error bar by far than the other assumptions about the area of the leaves exposed to sunlight - or the amount of sunlight available. With best assumptions for the other numbers, that efficiency range covers everything from a small tree to a really big one - so without knowing the species of tree (and finding the photosynthetic efficiency of that particular species) - we can't tell what size of tree StuRat would best compare to. Absent that information, all we can really say is that a tree of some kind is about the right comparison. You're not like a blade of grass nor like an entire forest. SteveBaker (talk) 14:10, 10 December 2013 (UTC)

Again, photosynthesis is irrelevant. Plants don't consume energy by photosynthesis, they do so by respiration, just like animals, producing CO2, just like animals. The "energy they need" is produced by respiration. μηδείς (talk) 19:21, 10 December 2013 (UTC)

I don't understand your distinction. The energy to drive the mechanisms that make up a tree need a certain amount of energy to drive them. Since trees don't absorb energy-producing chemicals - their only energy source is sunlight. They use sunlight to accumulate carbohydrates from CO2 and water during the day - which is a handy form of energy storage. They sustain themselves at night by reversing the reaction to make CO2 and water, thereby extracting energy just like animals do. But to measure JUST the energy generated from that reaction is not capturing all of the energy the plant requires. Animals consume carbohydrates and generate CO2, missing out the stage where the carbohydrates are created. So comparing the CO2 cycle energy output of a tree to the CO2 output of a human is missing out all of the energy that the trees consume for other reasons...such as building the structure of leaves, fruit, flowers and branches and that humans consume to do things like creating skin cells that we eventually shed everywhere.

Look at it this way: A plant grabs some amount of energy from sunlight. It uses this to manufacture carbohydrates. Some of the carbohydrates make trunk and leaves - while the rest is converted back into CO2 during the night. The TOTAL energy consumption is the daylight photosynthesis...and only a smaller amount is eventually involved in respiration. However, animals eat carbohydrates and convert it to CO2 without ever manufacturing the carbohydrates in the first place. Their energy intake is still not quite the same as the output because they too are creating structures as they grow and shedding hair and skin cells that took energy to create. But the balance is much closer for animals than plants. So using respiration to estimate the total energy consumption of animals is a pretty reasonable estimate - but not so for plants...and especially not for those that produce structures that animals eat (fruit, for example) or that are deliberately shed (leaves, in deciduous trees).

So, no - I don't agree with you. Photosynthesis is the only energy input that plants have - and food is the only energy input that humans have - comparing them is the correct way to answer StuRat's question. SteveBaker (talk) 22:08, 10 December 2013 (UTC)

There's no argument to be had. Addressing photosynthesis is being too clever by half. The energy a plant consumes is produced by oxidative respiration, producing CO2, H2O and ATP from sugar and O2. That's the energy a plant uses. The process is identical to that in animals, and it is a very simple matter to compare the CO2 output of an animal and a plant (kept in the dark). That the sugar a plant uses comes from photosynthesis is an interesting, but in regard to this question, entirely irrelevant fact. There's not even any direct correlation between the sugar a plant produces in a day and how much energy it uses in a day.

There are other questions, like what number of working chloroplasts of a certain output and at a certain light level would produce sufficient O2 to sustain a man, and what size plant would have that many chloroplasts. But that's not the question that's been asked. It's entirely possible Stu is interested in knowing what size tree would produce all the Oxygen and sugar he would need to survive on if he could consume its entire output, or just its excess output. Those are interesting questions too, but questions yet to be put. μηδείς (talk) 22:42, 10 December 2013 (UTC)

Another question is how much carbohydrates are stored in the plant? (i.e. how "fat" it is). A human stores many times their daily caloric intake. What is the ratio of stored to consumed sugars? --DHeyward (talk) 22:39, 11 December 2013 (UTC)

Good point. Since deciduous trees must survive for a good 6 months or so without any energy coming in, they must store at least 6 months worth of energy, although presumably they spend very little energy in winter, but there must be a large usage of energy in spring, when all the new growth begins, before there's much photosynthesis possible due to a lack of large leaves. People, on the other hand, seem to only store a few weeks worth or energy, since we are near death if we don't eat for that long. StuRat (talk) 13:50, 12 December 2013 (UTC)

Mathematical model of geographical determinism

Not sure if this question shall be in science reference desk. Anyway, has anyone ever tried to build up precise mathematical models out of geographic determinism theories, like Guns, Germs and Steel (excluding models of infectous diseases)? I wasn't been able to find anything with Google, but AFAIK it seems that some of those theories should be coherent enough to be tested with computer simulations. Maybe more than one mathematical interpretation is possible, but I was surprised I wasn't able to find one with search engines.--Nickanc (talk) 09:24, 9 December 2013 (UTC)

Mathematical models require a large enough sample size, which is not really available here, with one planet, and only two to four samples, if you want to count Africa, Eurasia, Australia and the Americas. Diamond does give comparative counts of domesticated animals for the various areas. But there's not really any way to rerun history and see the results of the simulation. μηδείς (talk) 18:14, 9 December 2013 (UTC)

Certainly we can't rerun history, but maybe there could be simplified (not too simplified) models doing less precise and accurate predictions but needing less data, no? That's what I am looking for. Of course, such a mathematical construction won't be crucial in deciding whether Diamond is right or not, but if we find that some things work under some simplified models, it is a way to strengthen his arguments and, if some things fail even in a simplified environment, isnt it a way to falsify some of his arguments? That's why I thought there could be something on the topic, but maybe I am too optimistic on science predictions. Thank you!--Nickanc (talk) 21:12, 9 December 2013 (UTC)

Producing dark energy

Supposedly most of the matter in the cosmos is dark energy. Popularly, the universe has been likened to a "perpetual motion machine" that generates dark energy to fuel its expansion. Question: is there any physical reason to expect we can't produce dark energy, if we figure out what it is? And if we produce dark energy, could the reaction have any useful real-world uses like free power, antigravity, reactionless drive, warp drive, whatever? Wnt (talk) 18:54, 9 December 2013 (UTC)

Dark Energy is a misnomer. What is called dark energy is simply the cosmological term in Einstein equations. You can not use it for anything. Ruslik_Zero 19:29, 9 December 2013 (UTC)

If dark energy is just the cosmological term, does that mean that pie chart we see in the article (and everywhere else dark energy is mentioned) is meaningless? Wnt (talk) 22:33, 9 December 2013 (UTC)

It's not meaningless, but may not mean what you think it means. According to E=mc² a kilogram of dirt "contains" about 9×10¹⁶ J of energy, which would be worth billions of dollars if it could be converted into electrical energy. But there are conservation laws (not of energy but of baryon number) that prevent that conversion, and if there weren't, all of the matter would have decayed long ago and we wouldn't exist to begin with. Although almost nothing is known about the dark energy, you can make a similar argument – the fact that it hasn't decayed for over 13 billion years implies that there's no easy way to convert it into something else. Even if you did find a way to catalyze the conversion, you would very likely end up releasing more of it than you wanted to, destroying life as we know it. This is similar to the problem of trying to extract stored energy from an inflated balloon when you live on the balloon. -- BenRG (talk) 10:22, 10 December 2013 (UTC)

Well not even that is known. Conservation of energy in observed systems suggests that this is not easy. Perhaps you could generate gravitational waves with wavelengths of under a millimeter in huge quantities, this may look like dark energy, but I don't know how you would do it. Normal thermal vibrations of matter would emit some but just doesn't make enough of it. Graeme Bartlett (talk) 20:00, 9 December 2013 (UTC)

Is it possible to say what would happen if you could? For example, I know that generating ordinary gravitational waves would consume real energy and therefore is a lousy path to perpetual motion. But dark energy has some really strange attributes ("negative pressure" that somehow forces space to expand, I still don't get that) ... I don't know what to expect. Wnt (talk) 22:37, 9 December 2013 (UTC)

"Negative pressure" = tension. I don't know why so many of the accounts call it negative pressure. -- BenRG (talk) 10:22, 10 December 2013 (UTC)

• Rupert Sheldrake is widely considered a quack [2] and dark matter and dark energy are two different concepts. But science fiction author Larry Niven has a short story in his The Draco Tavern compilation about how Einstein's Constant is the result of the use of a warp drive that exploits the free potential energy of space. μηδείς (talk) 20:30, 9 December 2013 (UTC)

It took me a while to figure out that you mentioned Rupert Sheldrake because Wnt linked him in the original question. I'm still not sure why you mentioned Larry Niven. Neither of them know anything about cosmology or particle physics. -- BenRG (talk) 10:22, 10 December 2013 (UTC)

I'll lay the blame for your confusion at Wnt's feet, piping "likened" to Sheldrake indeed!. I explicitly mentioned Niven wrote a science fiction story--I assumed it was clear that was of tangential possible interest. μηδείς (talk) 19:15, 10 December 2013 (UTC)

Are we likely to be able to produce dark energy? Quite possibly, once we figure out what it actually is though I can't help but wonder if that question is like an alchemist asking if we would ever produce [Aether (classical element)|aether]. Would this be a free source of energy? Probably not, everything we've see so far suggests that energy is conserved, so you're very unlikely be able to get more energy out of creating dark energy than you put in. MChesterMC (talk) 10:01, 10 December 2013 (UTC)

Dark energy is just a convenient name for a completely unknown phenomenon. We can see that there is something missing in the equations that govern the expansion of the universe - and we don't have any clue whatever as to what it is. We attach this name to it - but that in no way indicates that this really is "energy" - let alone that we could produce it, measure it, use it...whatever. So anything comment that are stronger than "We Don't Know" may be safely ignored until/unless mainstream physics and cosmology comes up with some solid evidence for what is causing the anomalous data we're seeing. SteveBaker (talk) 13:51, 10 December 2013 (UTC)