Simple (but hard to find) science questions thread

[DRBUZZ0]

I'm starting this thread (which may or may not die) as a place to post quick and relatively simple questions which you just cannot find the answer to. I've looked for these on google, wiki and all the other places, but no dice.

They don't really seem to justify their own thread, so here a couple someone may happen to know.


1. Did the SR-71 make much of a sonic boom (at ground level) when flying at full speed and altitude? It was going hella fast, but it was so high up, and the air up there is very thin and bad at transmitting sound, so it seems like it might not be very noticable at ground level.

2. What's the limiting factor for the depth a human can dive to? In saturation diving, divers live at high pressures for long periods of time. Why not simply feed them gasses pressurized enough to allow for respiration at super deep depths? Eventually, I suppose the oxygen and even helium would become liquid, but that would have to be super high pressure. I've heard of it done at up to a couple thousand feet. But what is the limit?

3. What is the smallest possible nuclear fission reactor which can be made with natural (unenriched) uranium, using heavy water, graphite or some other highly efficient moderator? I could calculate it, but I know that it will not be acurate because of all the variables involving geometry, fission products "poisoning," breeding products and other factors. So it's more of a practical question than theoretical: What is the smallest natural isotope-ratio fission reactor built?

[Brian Jackson]

Could a person survive an extended period exposed in zero atmospheric pressure... for example in near Earth orbit? Provided of course there was a long depressurising process beforehand to prevent the bends. And of course a radiant heat source & a source of pure oxygen.

Consider that you can't form a perfect vacuum in our atmosphere. Why? It's impossible to pull a theoretically perfectly sealed piston from a cylinder, even though the only atmospheric force acting on it would be on the underside surface of the piston. At 1 atmosphere @ sea level (approx. 14 lbs. per square inch,) a piston with a bottom surface area of 1 sq. in. should only require 14 lbs. to pull out. But this is not the case. So I ask: would a suction cup then work on outer space? You can see how these 2 scenarios relate.

[slyjoe]

1. For the SR-71, yes it did. I was pretty much right over it when I was involved testing some low earth orbit satellite gear. NASA owed us money, and loaned us the SR-71 to fly some equipment for a day.

Suction cups work in outer space, but only inside a pressurized spacecraft. Not outside.

For the others I don't have the right background.

[tracer]

3. What is the smallest possible nuclear fission reactor which can be made with natural (unenriched) uranium, using heavy water, graphite or some other highly efficient moderator?
None. No nuclear fission reactor can operate on completely unenriched Uranium. The naturally-occurring isotope ratio for Uranium is something like 99.3% 238U and 0.7% 235U; even the most forgiving nuclear reactor designs, such as the CANDU reactor, require fuel that's at least 1+% 235U.

[JoeTheJuggler]

What about naturally-occurring nuclear reactors, like in Okla, Gabon?

BTW--I don't know that the questions are very hard to find! :)

[toddjh]

Could a person survive an extended period exposed in zero atmospheric pressure... for example in near Earth orbit? Provided of course there was a long depressurising process beforehand to prevent the bends. And of course a radiant heat source & a source of pure oxygen.

Conventional wisdom is that the person needs some kind of pressure suit (some science fiction stories postulate a lightweight elastic "wetsuit" instead of a full-on spacesuit; I'm not sure if that would work in reality). Usually the rationale is that the vacuum will prevent blood from flowing or distributing oxygen properly, or that the pressure differential will cause severe lung damage.

Other possibilities that leap immediately to mind are a lot of nasty internal hemorrhaging in tissues that can't stand the pressure buildup inside the body, and rapid evaporative cooling as sweat/saliva/tears/etc. boil away into the vacuum.

Of course, all that is speculation. The only real-life incident I could dig up where a person had enough air, but had another part of his body exposed to very low pressure was a high-altitude balloon passenger in 1960 whose glove lost pressure on the way up to over 100,000 feet.

At 43,000 feet:"My right hand does not feel normal. I examine the pressure glove; its air bladder is not inflating. The prospect of exposing the hand to the near-vacuum of peak altitude causes me some concern. From my previous experiences, I know that the hand will swell, lose most of its circulation, and cause extreme pain.... I decide to continue the ascent, without notifying ground control of my difficulty."


At 100,000 feet:"Circulation has almost stopped in my unpressurized right hand, which feels stiff and painful."


Another observer:"his right hand was twice the normal size... He tried to release some of his equipment prior to landing, but was not able to as his right hand was still in great pain. He hit the ground 13 min. 45 sec. after leaving Excelsior. Three hours after landing his swollen hand and his circulation were back to normal."


Sounds like it would be pretty nasty even for short periods of time, and fatal pretty darn quick, even if you had air (and your lungs worked properly).

[andyandy]

ok a quickie, from this thread on philosophy and inductive logic (http://forums.randi.org/showthread.php?t=80846)

is time truly continuous? I ask, because planck time....

The Planck time is the time it would take a photon travelling at the speed of light to across a distance equal to the Planck length. This is the ‘quantum of time’, the smallest measurement of time that has any meaning, and is equal to 10-43 seconds. No smaller division of time has any meaning. With in the framework of the laws of physics as we understand them today, we can say only that the universe came into existence when it already had an age of 10-43 seconds.http://www.physlink.com/Education/AskExperts/ae281.cfm?CFID=15863276&CFTOKEN=68885559

would seem to suggest that it's discrete....but is it? :)

[CapelDodger]

Conventional wisdom is that the person needs some kind of pressure suit (some science fiction stories postulate a lightweight elastic "wetsuit" instead of a full-on spacesuit; I'm not sure if that would work in reality). Usually the rationale is that the vacuum will prevent blood from flowing or distributing oxygen properly, or that the pressure differential will cause severe lung damage.

You'd have to hold your breath or have a pressurised breathing apparatus for your lungs to stay inflated. They'd reinflate in a pressurised atmosphere but it can't be good :) . The skin is an all-over elastic wetsuit which will maintain internal pressure. It's the valves you have to worry about, what with the screaming at one end and the sphincter-control issues at the other. As to erectile tissue I'd rather not speculate.

[DRBUZZ0]

None. No nuclear fission reactor can operate on completely unenriched Uranium. The naturally-occurring isotope ratio for Uranium is something like 99.3% 238U and 0.7% 235U; even the most forgiving nuclear reactor designs, such as the CANDU reactor, require fuel that's at least 1+% 235U.

I believe CP-1, the original experimental pile built by Enrico Fermi and others used metalic uranium which was NOT enriched. It was a large amount of uranium in a graphite "pile" Also CANDU reactors can run on enriched uranium, but they are vastly more efficient on even low level enriched uranium.

[DRBUZZ0]

Could a person survive an extended period exposed in zero atmospheric pressure... for example in near Earth orbit? Provided of course there was a long depressurising process beforehand to prevent the bends. And of course a radiant heat source & a source of pure oxygen.

Consider that you can't form a perfect vacuum in our atmosphere. Why? It's impossible to pull a theoretically perfectly sealed piston from a cylinder, even though the only atmospheric force acting on it would be on the underside surface of the piston. At 1 atmosphere @ sea level (approx. 14 lbs. per square inch,) a piston with a bottom surface area of 1 sq. in. should only require 14 lbs. to pull out. But this is not the case. So I ask: would a suction cup then work on outer space? You can see how these 2 scenarios relate.

A mechanical vacuum pump will get you a decent vacuum for testing biological types of things. Diffusion pumps are necessary when the vaccum has to be near perfect, such as in an accelerator, where even a few atoms bouncing around could cause you problems. But you can easily get the pressure near to zero as far as your body is concerned.

My best understanding of the effects (shortterm): Full Body Hickey.

As to whether or not you could breathe? Possibly very low pressure gas, which would obviously have to be 100% oxygen, but it may be very difficult to exhale. Perhaps some sort of thight "girdle" like thing on your chest could provide enough resistance to the outward pressure to allow you to breathe more easily.

My guess is you could not last a real long time before some major life-threatening damage starts happening, be it from cooling, or lack there of, or circulatory problems.

shesh... a girdle and a full body hickey... that's starting to sound downtight kinky

[RecoveringYuppy]

What about naturally-occurring nuclear reactors, like in Okla, Gabon?
It ran on enriched uranium. That was over a billion years ago IIRC and natural uranium hadn't decayed as much as today's has.

[RecoveringYuppy]

Could a person survive an extended period exposed in zero atmospheric pressure... for example in near Earth orbit? Provided of course there was a long depressurising process beforehand to prevent the bends. And of course a radiant heat source & a source of pure oxygen.
No. Your lungs can't maintain a high enough pressure. Apparently surviving in a vacuum for even a short period requires that you not hold your breath or something will rupture (similar to a rapid ascent in diving).

Marshall Savage wrote a book that summarizes a lot of ambitious ideas about space colonization. The references are fantastic. One of the ideas he investigated was minimal space suits. I'll post some references later since I don't have the book in front of me now.

[Ziggurat]

No. Your lungs can't maintain a high enough pressure. Apparently surviving in a vacuum for even a short period requires that you not hold your breath or something will rupture (similar to a rapid ascent in diving).

Indeed. There's about a thousand pounds of force exerted by 1 atmosphere of pressure on the front of your chest.

[Gord_in_Toronto]

1. For the SR-71, yes it did. I was pretty much right over it when I was involved testing some low earth orbit satellite gear. NASA owed us money, and loaned us the SR-71 to fly some equipment for a day.
. . .



My experience is a bit contrary to this. Some years ago (10?) I was at the last day of the appearance of the Blackbird at the CNE (Toronto) airshow. After a couple of low level, low speed passes, the pilot stood the aircraft on end and accelerated straight up until it was out of sight on a day with only a trace of cirrus cloud. It was one of the most impressive sights I have ever seen but there was no boom heard on the ground. "He'll be back in California in two hours," was the comment from the show announcer.

:eye-poppi

[RecoveringYuppy]

Indeed. There's about a thousand pounds of force exerted by 1 atmosphere of pressure on the front of your chest.
And my understanding is that even the 1/10 to 1/5 atmosphere you'd need if breathing pure oxygen would still be too much.

[Dilb]

Consider that you can't form a perfect vacuum in our atmosphere. Why? It's impossible to pull a theoretically perfectly sealed piston from a cylinder, even though the only atmospheric force acting on it would be on the underside surface of the piston. At 1 atmosphere @ sea level (approx. 14 lbs. per square inch,) a piston with a bottom surface area of 1 sq. in. should only require 14 lbs. to pull out. But this is not the case. So I ask: would a suction cup then work on outer space? You can see how these 2 scenarios relate.

If we can somehow ignore the friction of the piston seal, yes, it will only take 14 lbs to pull it, and it would be a constant 14 lbs, as opposed to our usual experience of things getting progressively harder to pull on. But a perfect seal can't be ignored. From a high-vacuum perspective a good seal can't be pulled on anyway, as that would break the seal. The real reason you can't get zero vacuum is seals leak and surfaces outgas. Absolutely perfect vacuum really requires not having anything around at all, which is achievable enough in space depending on who you ask (http://hypertextbook.com/facts/2002/MimiZheng.shtml), but obviously isn't possible on earth.

The other issue with your cylinder is that if we suppose we can ignore the seal, and we want a perfectly clean surface to pull on, then the surface will bond to itself. You'd be pulling on a solid lump of metal.

[davefoc]

So is the consensus that except for breathing, a human could survive for a short time in the vacuum of space?

So the scene in 2001 where the guy is in space for a short period of time without a suit is plausible?

One comment on the man with the swollen hand at 100,000 feet:
It sounds like the rest of his body was pressurized inside the suit so one would expect a whole lot of bulging on the hand. According to a chart that I just looked at the air pressure at 100,000 feet is about 1/100 of sea level pressure. Seems like if the hand could survive exposure to that the whole body might be able to withstand the vacuum of space (except of course for the whole breathing thing and the staying warm thing).

[JoeTheJuggler]

So is the consensus that except for breathing, a human could survive for a short time in the vacuum of space?

So the scene in 2001 where the guy is in space for a short period of time without a suit is plausible?

More importantly, the Hitchhiker's Guide to the Galaxy is correct, and Arthur Dent & Ford Prefect did in fact survive!

[Yllanes]

is time truly continuous? I ask, because planck time....


In current theories time is continuous.

However, most models of quantum gravity imply that spacetime is discrete.

[andyandy]

In current theories time is continuous.

However, most models of quantum gravity imply that spacetime is discrete.

pesky gravity messing up everyone's theories :)

Can the two be reconciled - or does something have to give? Do you have any links to articles on this?

[Cuddles]

pesky gravity messing up everyone's theories :)

Can the two be reconciled - or does something have to give? Do you have any links to articles on this?

That's pretty much the biggest question in physics at the moment. I'm not sure it really counts as a simple question.

[Yllanes]

I'm with Cuddles andyandy, that was the short answer. If you want something a little more elaborate I think it would be better to start a new thread.

[andyandy]

I'm with Cuddles andyandy, that was the short answer. If you want something a little more elaborate I think it would be better to start a new thread.

alright then (http://forums.randi.org/showthread.php?p=2565567#post2565567):D

[Schneibster]

1. Did the SR-71 make much of a sonic boom (at ground level) when flying at full speed and altitude? It was going hella fast, but it was so high up, and the air up there is very thin and bad at transmitting sound, so it seems like it might not be very noticeable at ground level.It didn't make much of one flying at its cruising altitude because of inverse-square law attenuation. 100,000 feet is 20 miles. The thickness (or thinness) of the atmosphere didn't make much difference.

2. What's the limiting factor for the depth a human can dive to? In saturation diving, divers live at high pressures for long periods of time. Why not simply feed them gasses pressurized enough to allow for respiration at super deep depths? Eventually, I suppose the oxygen and even helium would become liquid, but that would have to be super high pressure. I've heard of it done at up to a couple thousand feet. But what is the limit?I don't believe there is one, at least here on Earth; the Trieste went to the bottom of the Challenger Depth in the Marianas Trench, which is some 35,000 feet. That's the deepest a human has been, and it's the deepest place in the ocean as far as anyone knows. I'm not sure whether they used saturation, or the crew compartment was armored. I think it might have been a combination of both.

3. What is the smallest possible nuclear fission reactor which can be made with natural (unenriched) uranium, using heavy water, graphite or some other highly efficient moderator? I could calculate it, but I know that it will not be acurate because of all the variables involving geometry, fission products "poisoning," breeding products and other factors. So it's more of a practical question than theoretical: What is the smallest natural isotope-ratio fission reactor built?Most likely the ones the Russians were using in their RORSATs. Remember the one that fell to orbital decay a decade or so back, and all the antinuke idiots had a fit about it and nothing happened? It'll be that or one used in a nuclear submarine. Somebody was talking about nuclear powered planes at one point, there might have been some pretty small designs for that, but I doubt even a prototype was ever build.

[Brian Jackson]

Indeed. There's about a thousand pounds of force exerted by 1 atmosphere of pressure on the front of your chest.

True, but there is an equal and opposite force INSIDE your chest, so it's all relative. That's my whole point... Even in jet liners the "pressurized" cabin is typically less than one atmosphere. This has a tendency to make you slightly lethargic (less oxygen per breath I'm guessing) and your senses of taste and smell diminish, but obviously your chest doesn't rupture... (that would be bad PR for the airlines.)

Even at extremely high altitudes blood flows just fine because the heart "pressurizes" the veins and arteries. You should only experience the "full body hickey" if the depressurization was rapid and gasses didn't have time to escape, which is why deep sea divers get the bends.

That is why I mentioned that if one was allowed to depressurize over a very LONG PERIOD OF TIME, is it possible to survive for a few hours with no ill effects, provided you were heated and oxygenated.

[Brian Jackson]

If we can somehow ignore the friction of the piston seal, yes, it will only take 14 lbs to pull it, and it would be a constant 14 lbs, as opposed to our usual experience of things getting progressively harder to pull on. But a perfect seal can't be ignored. From a high-vacuum perspective a good seal can't be pulled on anyway, as that would break the seal. The real reason you can't get zero vacuum is seals leak and surfaces outgas. Absolutely perfect vacuum really requires not having anything around at all, which is achievable enough in space depending on who you ask (http://hypertextbook.com/facts/2002/MimiZheng.shtml), but obviously isn't possible on earth.

The other issue with your cylinder is that if we suppose we can ignore the seal, and we want a perfectly clean surface to pull on, then the surface will bond to itself. You'd be pulling on a solid lump of metal.

Then imagine pulling two suction cups, bonded to each other, apart. The cups will break before any "space" is created inside. At 1" surface area you'd think 14 lbs. would be enough to create such space... The freaky thing I'm grappling with is that if you had just a "few" air molecules between them they could be stretched a distance. But when no air molecules are present it makes it impossible. Why? I know that nature abhores a vacuum, but geeze! Is it simply impossible to create empty space?

[slyjoe]

My experience is a bit contrary to this. Some years ago (10?) I was at the last day of the appearance of the Blackbird at the CNE (Toronto) airshow. After a couple of low level, low speed passes, the pilot stood the aircraft on end and accelerated straight up until it was out of sight on a day with only a trace of cirrus cloud. It was one of the most impressive sights I have ever seen but there was no boom heard on the ground. "He'll be back in California in two hours," was the comment from the show announcer.

:eye-poppi

You probably didn't hear it because the plane wasn't supersonic yet.

http://www.dfrc.nasa.gov/DTRS/1995/HTML/TM-104310/index.html.orig

Also, research on SR-71 sonic booms up to Mach 3.0 and 24384 metres is available in technical report NASA TN-D-6823 (I don't have direct access to this report).

[RecoveringYuppy]

So is the consensus that except for breathing, a human could survive for a short time in the vacuum of space?

So the scene in 2001 where the guy is in space for a short period of time without a suit is plausible?
That's my opinion. Mt Everest is 2/3 of the way to hard vacuum. 11 km is an altitude where some pilots have flown unpressurized planes with suplemental oxygen, that is 80% of the way to hard vacuum. The hard vacuum of space is going to dry out your mucus membranes a bit faster and there is slightly greater risk of embolism, but for a few seconds to get back in to your ship it will likely work.

[RecoveringYuppy]

Here's a link for my earlier claim that the natural reactor in Gabon had uranium that was comparable to enriched uranium.

http://www.spacedaily.com/news/early-earth-04n.html

[toddjh]

So is the consensus that except for breathing, a human could survive for a short time in the vacuum of space?

So the scene in 2001 where the guy is in space for a short period of time without a suit is plausible?

Yeah, I think it was pretty plausible. There have been some accidents in pressure suit tests where people were exposed to a near-vacuum for short periods of time (and the Nazis, of course, conducted their own less-accidental tests...). From what I can tell, it looks like you lose consciousness very quickly (within 10 or 15 seconds), but as long as pressure is restored within a minute or so you'll be more or less fine once you wake up.

In 2001 I think Bowman stayed conscious for quite a bit more than fifteen seconds, but after all he was an astronaut in peak physical shape, presumably with a lot of training for that kind of thing. Plus he knew it was coming and had time to prepare for it.

[Ziggurat]

True, but there is an equal and opposite force INSIDE your chest, so it's all relative.

Actually, I was talking about the outward pressure from gas held inside your chest, and what it would do to you if you tried to hold it in when exposed to vacuum. In such a case the only balancing force would be from your chest wall.

[JJM]

{snip} 2. What's the limiting factor for the depth a human can dive to? In saturation diving, divers live at high pressures for long periods of time. Why not simply feed them gasses pressurized enough to allow for respiration at super deep depths? Eventually, I suppose the oxygen and even helium would become liquid, but that would have to be super high pressure. I've heard of it done at up to a couple thousand feet. But what is the limit?
{snip}I haven't looked it up, but I Know the greatest underwater pressure (Marianas Trench (sp?)) can't liquefy helium, and I seriously doubt it will liquefy oxygen.

However, although I don't know the number, there is a maximum depth (pressure) you can reach with oxygen. The reason you need to replace nitrogen with helium is nitrogen acts as a narcotic at sufficient partial pressure (aka- depth rapture). Oxygen, also, becomes toxic at sufficient depth (pressure). Replacing nitrogen with helium significantly extends the depths that can be explored; but not indefinitely.

[Ziggurat]

The freaky thing I'm grappling with is that if you had just a "few" air molecules between them they could be stretched a distance. But when no air molecules are present it makes it impossible. Why? I know that nature abhores a vacuum, but geeze! Is it simply impossible to create empty space?

I'm not sure where you're getting this idea that it becomes impossible to separate two objects if there's a perfect vacuum between them.

As a mater of practicality, you can't really FORM such a perfectly evacuated piston, because even solid steel actually has gas absorbed into its surface which outgasses when exposed to vacuum. But that's a problem of getting a gas-free situation, NOT a problem of not being able to pull apart a perfect vacuum.

There is additionally a practical issue of cold wielding (http://en.wikipedia.org/wiki/Cold_welding) of the piston parts together, but again, that's got nothing to do with a perfect vacuum, cold welding can happen in imperfect vacuums as well, and using different materials for the different parts can avoid the problem.

[JJM]

So is the consensus that except for breathing, a human could survive for a short time in the vacuum of space?

So the scene in 2001 where the guy is in space for a short period of time without a suit is plausible? {snip}It is not.

[Gord_in_Toronto]

You probably didn't hear it because the plane wasn't supersonic yet.

That makes sense. I don't know if the Blackbird can reach Mach 1 in vertical flight from a "standing start". My estimate (based on the cloud base and the size of the plane relative to a commercial jet) was that it ceased to be visible at 30000 feet.

http://www.dfrc.nasa.gov/DTRS/1995/HTML/TM-104310/index.html.orig


Interesting paper. Thanks.

Also, research on SR-71 sonic booms up to Mach 3.0 and 24384 metres is available in technical report NASA TN-D-6823 (I don't have direct access to this report).

I have enough info. :D

[666]

...24384 metres...
I was wondering why NASA chose this particularly precise altitude for the study and then realised that it was a translation of 80000 feet.

[toddjh]

I was wondering why NASA chose this particularly precise altitude for the study and then realised that it was a translation of 80000 feet.

Things like that make me wish our mathematical conventions gave us an easy way to show significant digits before the decimal point. Like, we could use "80,000" to denote a number that's between 79,999 and 80,001, and "80,XXX" to denote a number that's between 79,000 and 81,000.

[Wat Tyler]

Things like that make me wish our mathematical conventions gave us an easy way to show significant digits before the decimal point. Like, we could use "80,000" to denote a number that's between 79,999 and 80,001, and "80,XXX" to denote a number that's between 79,000 and 81,000.

I'd be tempted to suggest that you use '80k' - but to be honest, it'd be an approximation for figures between 79,500 and 80,499, rather than the numbers you cited.

Or, you could use scientific notation: 8.0 x10^4 (2SF)
(Actually, that suffers from the same problem - meh!)

I'll get me coat....

[Yllanes]

I'd be tempted to suggest that you use '80k' - but to be honest, it'd be an approximation for figures between 79,500 and 80,499, rather than the numbers you cited.

Or, you could use scientific notation: 8.0 x10^4 (2SF)
(Actually, that suffers from the same problem - meh!)

I'll get me coat....

It doesn't suffer from the same problem. You can write 8.0·104 and 8.0000·104 and everyone understands the first has two significant digits and the second 5.

[RecoveringYuppy]

It doesn't suffer from the same problem. You can write 8.0·104 and 8.0000·104 and everyone understands the first has two significant digits and the second 5.
Yes, but the range 79,000 to 81,000 requires about .9 (or is that 1.9) significant digits, that's the problem.

[RecoveringYuppy]

I'd be tempted to suggest that you use '80k' - but to be honest, it'd be an approximation for figures between 79,500 and 80,499, rather than the numbers you cited.

Or, you could use scientific notation: 8.0 x10^4 (2SF)
(Actually, that suffers from the same problem - meh!)

I'll get me coat....
You probably know this but, jic, if you're trying to express some definite degree of uncertainty you do it with the "Plus or minus" notation followed by the margin of error.

[Yllanes]

Yes, but the range 79,000 to 81,000 requires about .9 (or is that 1.9) significant digits, that's the problem.

If you want to write ranges so precisely, you can add an error term.

8.32566 ± 0.00021 = 8.32566(21)

EDIT: Sorry, I hadn't seen your last message (page break)

[Victor Meldrew]

How much truth is in this?

Global Warming
Carbon dioxide, methane and nitrous oxide are naturally occurring gases in the atmosphere. They act like the glass of a greenhouse by trapping the sun’s heat and reflecting it back to earth. This phenomenon is what makes the world habitable, keeping the atmosphere about 33ºC/92ºF higher than it would otherwise be. But animal agriculture adds significantly to global warming. Scientific American (9/97) reported that growing feed for livestock requires intense use of synthetic fertiliser, releasing nitrous oxide - a far stronger greenhouse gas than CO2. Producing feed and heating buildings that house animals uses fossil fuels, emitting CO2; decomposition of liquid manure releases larger amounts of methane into the atmosphere as well as forming nitrous oxide (37).
Better out than in? Maybe not. More cattle also means more belching and this is now the second largest contributor to global warming after fossil fuel burning. Worldwide, livestock accounts for 16 per cent of all global warming emissions of methane (38). Methane is 20 times more effective at warming the globe than CO2, which it joins above the earth (39).

From
http://www.viva.org.uk/guides/planetonaplate.htm#GlobalWarming

[Wat Tyler]

Yes, but the range 79,000 to 81,000 requires about .9 (or is that 1.9) significant digits, that's the problem.

And 79,000 falls outside the 2 significant figure range, as does 81,000.

I meant that 8.0 x104 covers the range from 79,500 to 80,499, and thus does not describe what he wanted (shorthand for 79,000 to 81,000).

I should have made that clearer, sorry guys.

ETA: and I should also have remembered the 'Margin of Error' notation. D'oh!
(in my defence, Your Honour, it is over 15 years since I last used Scientific Notation)

[CapelDodger]

Better out than in? Maybe not. More cattle also means more belching and this is now the second largest contributor to global warming after fossil fuel burning.

Second, and trailing by how much? A close second or a very distant second?

Worldwide, livestock accounts for 16 per cent of all global warming emissions of methane[/quote]

There's no distinction between global-warming methane emissions and some other kind. Methane is methane, and does what methane does.

So let's just consider methane emissions. If the livestock-farting (lets get the valves right; if I ever belch methane I hope I'm not smoking at the same time) element is only 16% of that and it's the second-largest contributor the other 84% of methane emissions must be sliced into many smaller contributors. Even at 15% each that's six other contributions. Which is best-case and still seems a lot.

Methane is 20 times more effective at warming the globe than CO2, which it joins above the earth.

The "20 times" doesn't really say anything without reference to the relative concentrations. Twenty times more than jack is still jack, after all.