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The big question

  1. Jul 23, 2007 #1
    hi, not sure that this is the right forum, but i will ask anyway.
    i've been told that when the earth was 'created' all the molecules or whatever that were ever going to be on earth were here.
    no matter what happens there has never been any more or any less.

    for example when you die you are burried, then the bugs eat some of you and the flowers and grass are fertalised by you, then the bugs are eaten by birds, and the grass is eaten by cowes and horses, when the go to the 'bathroom' it once again gets eaten by bugs and fertalises plants and grass.

    is this in fact true, because supposedly humans have been to the moon and obviously not all of the stuff they took out has come back
     
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  3. Jul 23, 2007 #2

    G01

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    Uhhh, I guess the fuel burnt in space by the rockets/space shuttle, space probes, etc. don't come back, but that is only a very small amount of matter. Most of the matter on the Earth is stays here, unless we talk about hydrogen gas and the like, that is not held on earth by the gravitational field.

    To sum up: A very small amount of the Earth's overall mass has been able to escape the planet.
     
  4. Jul 23, 2007 #3

    Danger

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    Welcome to PF, Melz.
    What you have been told is incorrect.
    To start with, though, if you can ever come up with a system to make cows use a bathroom you'll become rich beyond your wildest dreams. :rolleyes:
    For clarification, it's only incorrect in regard to Earth or our Solar system in general. Taken from the perspective of the Universe, it's right.
    Several hundred tonnes of extraterrestrial matter falls to Earth every year in the form of meteorites. Some is lost as well, by both escaping atmosphere and the stuff that we throw out there as GO1 pointed out. On the whole, Earth is getting fatter by the minute.
     
  5. Jul 23, 2007 #4

    G01

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    For some reason meteorites didn't enter my mind Danger! Thanks for pointing that out. Yeah, they would way much more overall than what the atmosphere looses and what we throw out up there. Yup, the earth is getting fatter! It's gotta cut down on those late-night meteorite snacks.:biggrin:
     
  6. Jul 24, 2007 #5
    Not quite. Conservation of matter and conservation of energy just means that you still have to have as much as you started with; this does not mean that it has to stay a molecule or a person, for that matter. So, we can break and make molecules, but we still have to use matter to make it. "There's no free lunch."

    - Bryan
     
  7. Jul 24, 2007 #6

    Danger

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    Good point, Kanzure. I was thinking along the line of Sagan's famous "We are all star-stuff." Everything that exists in the Universe has existed since it began, but I wasn't accounting for matter/energy conversion.
     
  8. Jul 29, 2007 #7
    Comets are supposed to have provided much of the water on earth and perhaps other planets. The Earth is not a closed system, as evidenced primarily by radiation from the Sun, but in many cases (like the activity of life) is practically so.
     
  9. Jul 29, 2007 #8

    mgb_phys

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    It's even more interesting if you go back a step, only the hydrogen on earth was created in the big bang. Earth is about 50% Oxygen and 25% silicon, these were created in earlier generations of short-lived massive stars, any elements heavier than iron must have been created when some of these early stars went supernovae.
     
  10. Jul 30, 2007 #9

    Danger

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    Quite right. I should also mention that unlike Earth, the sun is converting about 4.5 million tonnes of matter into energy every second. I don't know how much infall there is to alleviate that, but it's not enough to maintain the mass.
     
  11. Jul 30, 2007 #10

    LURCH

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    Melz 9,
    In the midst of all this new revalation, beware! Now that you have heard that Earth's overall mass is increasing as ET material falls in, someone will surely introduce you to the "Expanding Earth" theory. This model of planetary developement proposes that the Earth is twice as big (or four times, ten times, etc.) as it was when The Egyptians built the Pyramids, or when Acid Rock roamed the Eight-Tracks Primieval. Although based on a verifiable fact, this model is "fringe science" at best (it may even deserve the title of "crackpottery," but I hate to judge). It greatly overestimates the amount of matter falling to Earth per time period. There is no evidence that the Earth is growing anywhere near that fast.
     
  12. Jul 30, 2007 #11

    Danger

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    Man, I've never heard of that one. I certainly hope that nobody took that as what I was implying by saying that Earth is getting 'fatter'. :surprised
    It was just a sloppy way of indicating that there is actually a net gain of matter. It's pretty insignificant compared to the total mass, just as that 4 1/2 million tonnes of Sun that vanishes per second is insignificant. It's sort of like one of us putting a grain of sand in his pocket.

    Of course, if the Earth had been half as big during the time of the Egyptians, it would make the pyraminds less of a mystery. There would have been a lot less gravity. :rolleyes:
     
  13. Jul 30, 2007 #12

    mgb_phys

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    I think the mass of dust, cometry fragments, and particles from the solar wind is somewhere between 10^8-10^9 kg/day.
    Still pretty insignificant compared to teh mass of the earth.
    The rate was probably much higher in the early solar system when there where a lot more comets being swept up.
     
  14. Jul 30, 2007 #13
    What was the source of water in comets?
     
  15. Jul 30, 2007 #14

    Danger

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    Hydrogen and oxygen, the same as anywhere else. Both are abundant in the Universe. They can combine in interplanetary (or interstellar) space as easily as they can anywhere else. Of course, it immediately froze. It arrived on Earth as ice, which vaporized upon entering the atmosphere (which at the time was primarily methane and ammonia).
     
  16. Jul 31, 2007 #15
    Melz 9,

    this is what one of the greatest scientists of all times, Ludwig Bolzmann, said in 1875 :

    "The general struggle for existence of animate beings is not a struggle for raw materials – these, for organisms, are air, water and soil, all abundantly available – nor for energy which exists in plenty in any body in the form of heat, but a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth."

    You say :

    "i've been told that when the earth was 'created' all the molecules or whatever that were ever going to be on earth were here."


    You should reply to those who have told you this "monstruosity", that :

    a) they are probably assuming that the earth is an isolated thermodynamic system
    b) and that within this isolated system, the second law of thermodynamics does not apply

    Without even talking of meteorites, ask them if they have ever heard of cosmic rays and gamma rays ?
     
  17. Jul 31, 2007 #16
    Danger,

    Is the Oort cloud particularly rich in water, and if so, why?
     
  18. Jul 31, 2007 #17

    LURCH

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    LOL! Yeah, I think it explains their exitance in Central America, too. The continents were much closer together then, and the Atlantic was just a ditch running through the middle of town.
    (That may not be very funny right now, but later today some of you are going to look at a globe and just laugh your butts off!)
     
  19. Jul 31, 2007 #18

    Danger

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    I agree about the cosmic rays, which are protons, electrons, and alpha particles (helium nuclei), but gamma rays have no mass. They're just highly energetic photons.

    As far as I know, almost all comets consist of water ice, rocks, carbon, and maybe methane or ammonia ice. That's actually something that you should ask of Space Tiger or one of the other Astronomy dudes. If it's correct, then it certainly is rich in water. As has been mentioned in PF several times, Science doesn't ask 'why'... just 'how'. The 'how' of water is pretty much just that hydrogen and oxygen like each other a lot. If you put them in the same place with any sort of energy input, such as the sun, they'll join.

    It didn't take that long. :biggrin:
     
  20. Jul 31, 2007 #19
    You could take an electric lamp, install it in an air-tight terrarium, seal that in a vacuum thermos, plug it in and use it as a space-heater. Eventually it would stabilise and you would find: life is progressing inside the terrarium (it could maintain a plant and animal ecosystem) despite that no matter (atoms) are entering or leaving the terrarium, and the energy entering the system (from the lamp's power point) is exactly cancelled by the amount of energy leaving the system (radiated, conducted or convected away from the thermos outer casing) so there is also zero net energy flow (although the entropy current into the lamp is smaller than the entropy current out into the room).

    And to excellent approximation (as Boltzmann understood) this describes the Earth. You can be confident that your own atoms were already atoms on Earth back at the time of the dinosaurs (and that none came from the Apollo mission). It is likely that much of you was algae at some point. You can list the few technical exceptions (like the one new helium atom which was produced when another of your atoms changed its form a moment ago, or the very tiny fraction of material which the planet still continues sweeping up from space), but doing so seems to miss the point (after all, there are exceptions to everything).
     
  21. Jul 31, 2007 #20

    Danger

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    Nice post, Froggie. I can't remember the exact number, but with every breath that you take, you inhale something like a couple of million atoms that were once part of Julius Caesar's body (or anybody else's, for that matter).
     
    Last edited: Aug 1, 2007
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