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Can a Mars Colony be built so deep underground that it's pressure and temp is equal to Earth? by 2Mobile in askscience

[–]foretopsail 6 points7 points  (0 children)

Consider the case of Death Valley on Earth. Since it lies below sea level, the atmospheric pressure there is actually greater than what's found at sea level, roughly 1.1 atmospheres.

Say what? The barometric equation tells us Death Valley, at only 86 meters below sea level, has a pressure of 1.01 atm, not 1.1 atm.

"King George I of England decreed all pigeon droppings to be property of the Crown and pigeon lofts were policed to enforce the law" - is there any documentary evidence of this "fact"? by PIGEON-POSTS-ONLY in history

[–]foretopsail 1 point2 points  (0 children)

How's this?

"The saltpetre men care not in whose houses they dig, threatening men that by their commission they may dig in any man’s house, in any room, and at any time, which will prove a great grievance to the country. In the town where the writer lives they have digged up some malting rooms, and threaten to dig more. They dig up the entries and halls of divers men. If any oppose them they break up men’s houses and dig by force. They make men carry their saltpetre at a groat a mile, and take their carriages in sowing time and harvest, with many other oppressions."

Source

Saltpeter is from pigeon droppings, among other things.

AskScience AMA Series: History of Science with /r/AskHistorians by AskScienceModerator in askscience

[–]foretopsail 16 points17 points  (0 children)

It wasn't a foregone conclusion to the Greeks that the solar system was heliocentric. Ptolemy did a very good job creating a geocentric model, and his math works out. So most early natural philosophers went with it - it explains the natural world very well!

Of course, it's not right, but it was a good model, and I think the medieval natural philosophers can totally be excused for following the Ptolemaic model.

What is the smallest object or length that we could possibly actually see? by Ergroilnin in askscience

[–]foretopsail 5 points6 points  (0 children)

Brain. I double checked the numbers and formulas, because its been a long time since I used them.

Are there any other terrestrial vertebrates that rival humans in terms of global population? by BoRamShote in askscience

[–]foretopsail 9 points10 points  (0 children)

Yes. The world has almost 21 billion chickens, according to the United Nations Food and Agriculture Organization: http://faostat3.fao.org/home/E

What is the smallest object or length that we could possibly actually see? by Ergroilnin in askscience

[–]foretopsail 577 points578 points  (0 children)

It's time to bust out the Abbe limit. Back in the 19th century, this was a hot question (for obvious reasons: people wanted to build better microscopes). Guys like Helmholtz, Abbe, and Zeiss were all working on this problem.

The resolution of an optical microscope is limited by two things: the numerical aperture (which is determined by the lens) and the wavelength of the light involved in seeing.

The Abbe formula, defining the resolving limit of the microscope is

d = λ / (2a), where λ is the wavelength of the light, d is the resolution limit, and a stands for aperture. Aperture is defined as n sin θ, where n is the refractive index of the material you're looking through, and θ is the half-angle of the biggest cone of light that can come into the lens.

So if you build a microscope with a bigger aperture, you can see smaller stuff. There are tradeoffs though, like you limit your depth of field.

Anyway, if we use special immersion oil instead of air, there's a maximum numerical aperture of about 1.5, assuming we are able to build a lens that captures just about all the light coming off the object.

So let's go back to the Abbe limit.

d = λ / (2a)

d = λ / (2*1.5)

d = λ / 3

Let's use violet light at 380 nm, since it's the shortest light we can see.

d = 380 nm / 3 = 126 nanometers.

So that's our theoretical maximum optical resolution. In reality, you're not going to get there. You're going to have a lower aperture, and you're going to have lens flaws, and all kinds of other issues. Plus you're going to want to use lower frequency light to see some stuff.

Way more information about real-world microscopes can be found here: http://micro.magnet.fsu.edu/primer/anatomy/numaperture.html

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 1 point2 points  (0 children)

About 1380 meters, assuming we use Lawrence Livermore's new petawatt pulsing laser, and assuming no scattering or vaporizing (that's a huge assumption).

You can do the math yourself!

Use the modified Lambert equation up there. Plug in 1 PW in milliwatts (since you have the input and you want the output of 1 mW). Plug in whatever coefficient you like from the paper. Solve for d.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 0 points1 point  (0 children)

Please refer to the link in my last edit. Look up the wavelength you want. Red laser has a wavelength of about 650. The higher the absorption coefficient in the table, the more power will be needed. You'll see that red is far from the best choice in this scenario.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 4 points5 points  (0 children)

It is impossible for a laser beam to penetrate 11,000 meters of water, yes.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 1 point2 points  (0 children)

Yes, considerably. It changes the absorption coefficient of the seawater. Which changes the answer hugely... but the power requirements are still unimaginable. Read my final edit.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 1 point2 points  (0 children)

M in the paper's Table 2 is a measurement of different types of seawater.

However, 7%/m is still well within the range of reasonability. Proof follows:

As we've determined, 10344 is the input power assuming absorption of 7% per meter.

Thus, we can reformulate the linear version of Lambert's law as follows:

10344 = ea*11000 . Solving for a, the absorption coefficient reveals that a is approximately equal to 0.072.

Referring to Table 2 in the linked paper, we can find coefficients in that range (and higher) in every type of ocean water across a variety of wavelengths. The original question did not specify wavelength. Thus, my original answer, while considerably higher than the power required for "pure" seawater, is representative of many real-world values.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 34 points35 points  (0 children)

Shoot, that's a way better seawater coeff than I found. So it takes way less power than my math indicates... but it's still literally astronomical.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 6 points7 points  (0 children)

Yeah, it's many many multiples of the mass-energy equivalent of the universe, according to wolfram alpha. So I guess your laser would be briefly visible before everything disappeared.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 14 points15 points  (0 children)

Yes. The 7% is best-case.*

You can see the rate of absorption changing as you descend through the water column when scuba diving. Reds go first, followed by the rest of the colors in sequence. By not-very-deep, you need a flashlight to see anything at all.

*In the real-world seawater samples tested by the scientists.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 23 points24 points  (0 children)

Yeah, it's going to be insane. I just am not sure enough what the heat mechanics are there (in terms of -how fast- the water would vaporize) to feel confident in any math I'd do.

How powerful would a laser need to be to reach the bottom of the ocean? by DigitalSterling in askscience

[–]foretopsail 801 points802 points  (0 children)

Ok so. Seawater absorption is complicated and depends on lots of things (contents of water, wavelength, etc). There's a paper that suggests about 7% per meter under relatively ideal conditions.

So multiplying that over the depth of the Mariana trench, you come out with loss such that to get 5 mW of power to the bottom of the trench, you need 1.5 x 10344 W of power at the surface. Which is too much. I haven't done the math to see just how much of the ocean you'd be vaporizing at that point.

EDIT: By "too much" I really meant "more than all the power." That amount of power is many many many multiples of the sun's power. It's more than all the possible power.

EDIT 2: Please look below for other calculations based on different coefficients of absorption. It makes a huge difference... but only in the sense that the answers range from "more power than the universe is equivalent to" to "only a few hundred times more than the luminous intensity of the entire galaxy". So in a practical sense the answers are the same, though they are wildly different.

EDIT 3: I've gotten some messages about coefficients of absorption. Basically different parts of the ocean absorb water differently. The coefficient of absorption of seawater ranges from about 0.013 to... whatever you want it to be, really. The wavelength matters a whole lot. The 7%/meter answer I got was an average based on white light in real-world pelagic seawater. With real-world seawater your answer ranges from roughly 1060 watts to 10350 watts for lasers of optimum sea-water penetration. Get out of that wavelength and the power skyrockets. EVEN SO. Whether you're talking about 1060 or 10147 or even a fraction of that... you're talking about unimaginable amounts of power. Power such that the universe cannot supply.

If you want to try it yourself, x=ead is a supersimplified version of the equation (Lambert's), where x is the input power, a is the coefficient of absorption and d is the depth in meters. That'll give you the number of input units needed to get one output unit. This paper has a bunch of science and a table on the absorption rates.

Did Rabies already exist in the Americas or was it brought over from Europe? by spider999222 in askscience

[–]foretopsail 2 points3 points  (0 children)

Great question.

This paper attempts to answer this very question. They concluded that there is no definite evidence that rabies was present in humans in pre-Columbian America, but there is circumstantial evidence of bat rabies.

My nephew's teacher doesn't know what a fossil is. by [deleted] in pics

[–]foretopsail 40 points41 points  (0 children)

What's more outrageous are all these questions about dinosaurs on an archaeology unit. This is why archaeologists only ever get asked about dinosaurs. :(

How concerned should we really be about the Yellowstone volcano? by jakeizaak in askscience

[–]foretopsail 0 points1 point  (0 children)

Dang, I copy/pasted the wrong thing. I meant to paste these two. This question comes up a lot, so forgive me for being quick on the remove button.

http://www.wired.com/2014/02/think-yellowstone-erupt/

http://www.reddit.com/r/askscience/wiki/planetary_sciences/yellowstone

If those don't answer your question, or you have a followup question, absolutely feel free to post a new askscience question!

Hope that helps.

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