An unsettling refreshment

You are an astronaut, it is a few hours before you are scheduled to launch for an intergalactic space mission, and you're sitting on the shore of the Atlantic Ocean near Cape Canaveral, savoring the view one last time. This mission is especially trying; because of the relativistic speed of your spaceship, Earth will be one billion years into the future upon your return.

Depressed beyond words, you decide, in one of your weaker moments, to pee into the Atlantic Ocean.

Flash forward: 1 billion years .

You return to Earth, expecting a hero's welcome, but instead you find that all of humanity has vanished. Instead, the Earth is run by a peaceful clan of telekinetic dolphins who made off with the lion's share of Bitcoins that were abandoned by the last humans as they uploaded their souls to the singularity server. In a disillusioned haze, you bend down and fill your astronaut survival cup with refreshing lake water, hoping for some clarity. After drinking the glass, you realize that this far into the future, all the Earth's water has been thoroughly well mixed since the time you took off on your mission.

Approximately how many molecules of your billion year old urine did you just consume?

Assumptions and details

  • You released 1 L \textrm{1 L} of urine into the Atlantic Ocean.
  • Approximate your urine as consisting of pure water.
0 250 2.5 25,000

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2 solutions

Radha Krishnan B
Jul 3, 2014

Weight of urine=1000L (1 gm/L)=1000 gms. Molar wt of urine(pure water)=18gms/mole. No of moles=1000/18 moles. No of molecules=1000/18 * Na=1000/18 6 10^23 molecules. volume of earth's sea water=1.33 10^21 L So, no of molecules per liter=1000/18 6 (10^23)/4 3 10^-21 molecules/liter. =1000/4*100=25000 molecules approx.

But it has been 1 million years so It would decompose into its constituents thus 0 should be the answer. Inorganic things like plastic take such long time, not organic

Vishwesh Ramanathan - 6 years, 11 months ago

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The problem tells you to assume your urine is pure water, so decomposition wouldn't be taken into account.

Samuel Rousser - 6 years ago

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The water molecules can be combined into minerals. H2O in minerals can come out as liquid water. Furthermore, H2O in the atmosphere can be broken down by UV and H2 lost to space.

Stephen Rasey - 3 years, 11 months ago

*1 billion years

Brian Wang - 5 years, 5 months ago

it says he drinks lake water, not water from the atlantic, so isn't your answer wrong?

Ronan Laker - 6 years, 11 months ago

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It also says all the water on the earth has been thoroughly mixed which is why he's taken into account all sea water on earth

Michael Lee - 6 years, 4 months ago

yh its obvious in 1 bln years everything will be mixed up properly

NYX 128 - 4 months, 2 weeks ago

It's based on the water cycle—the oceans will all have evaporated and precipitated somewhere else on the planet over that timescale!

Reid Chave - 2 months, 4 weeks ago

The person must be having a very large bladder... :)

Pankaj Joshi - 6 years, 11 months ago

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That's what I thought, as well!

Veselin Dimov - 4 months ago

The answer should be 0

Vivek Chawla - 6 years, 10 months ago

Do u guys have any idea that molecules can be dissociated after 1 yr, now u talk about a billion year.This question is data inadequate Natural phenomenon can be taken as negligible?? I know how big Na (avagadro number), still molecules can decompose.

Arya Haldar - 6 years, 10 months ago

Your answer tells you how many molecules from 1 liter would exist in an average liter of water thoroughly mixed with your estimate of the free water on earth in 1 billion years(which should also have been a given since it is merely a guess), but the problem states a sip was taken, not a liter. Therefore, your solution is wrong.

Thomas Kritzer - 6 years ago

How do I know the capacity of his mouth?

Kenny Lau - 6 years, 11 months ago

how large is his survival cup?

Oliver Padovan - 6 years, 11 months ago

is urine's Mr= 18 .... i think urine containts CO(NH2)2, NaCl, bilirubin, biliverdin etc. so it shouldn't be 18 ....

Roland Hartanto - 6 years, 9 months ago

@josh silverman how can you think of such a question?

Satvik Golechha - 6 years, 8 months ago

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@Satvik Golechha No idea. I would sooner touch the moon than try to figure out where this story came from.

Josh Silverman Staff - 6 years, 8 months ago

You did not provide the information on the remaining volume of seawater on earth after 1billion years! And he just sipped the water, he did not drink all the water from the cup.

Ronnie Cleofe - 6 years, 6 months ago

But he only takes one sip instead of 1 liter

Tobi Fehling - 4 years, 3 months ago

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Good point. I've changed it so that he drinks the full cup.

Josh Silverman Staff - 4 years, 3 months ago

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"survival cup" I would say is about 125 gms.

Jim Hobza - 3 years, 3 months ago

0, because homeopathy is useless.

Orlando Moreno - 3 years, 6 months ago

I think that the dolphins would kill him so could not drink any water so 0

sayantan mondal - 3 years, 3 months ago

Moreover urine is mostly just water, molecules of which would be indistinguishable from rest of the ocean water. So only a small fraction of molecules from that urine would be identifiable.

Saarth Jauhari - 2 years, 1 month ago

He did not drink 1 liter of water, he drank 1 cup for god's sake... The question was, how much urin did he drink, not how much urin is there in 1 l of current ocean water....

Stepan Šklarik - 3 months ago

The answer should be 0. I understand the point of fermi estimates... but realistically, all the urine would be filtered out of the water eventually.

F OsirisPhantom - 3 months ago

you wouldn't be alive after 1 billion years :P

David Lee - 6 years, 11 months ago

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You will, due to time dilation, your body would not develop too much.

William Nathanael Supriadi - 4 years, 6 months ago
Michael Holt
Aug 3, 2018

Ratio of urine volume to volume of all water (assuming it has all been made into liquid form) on earth= 1L/(1.26x10^21 L) Source: howstuffworks

Number of molecules in a cup of water: 8.36 x 10^24 molecules of water Source: username another_someone

The sources were not tested for reliability at all.

(Number of H2O molecules in a cup of water) * (Ratio of urine volume to volume of all water) = 6634.920634(repeating) This is closest in orders of magnitude to 25,000.

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