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B If we *had* to make a perfect vacuum...

  1. Jan 4, 2017 #1
    Suppose a green alien came down to Earth in his big spaceship. Suppose that alien was the alien who brought life on Earth and created humanity. Suppose that alien is a bit dissatisfied with us. So basically God, if you're a christian (only green instead). So, what does he do? He asks us to create a perfect vacuum.

    "You have a couple of centuries to create a perfect vacuum" he says. "No need for anything fancy, all I care is to have a space at least a tenth of a cubic centimeter that is completely devoid of any atoms, I don't care much about neutrinos and photons and other silly stuff. But it has to remain like that for at least another century, otherwise I'll bring my death beam and kill you all. If you do it, I'll solve world hunger, give you the cure to cancer, tell you the meaning of life and abolish pop up ads".

    Yes, I know, "We can't create a perfect vacuum, blah blah blah..." but what if we really had to? And what if we could concentrate all of humanity's efforts on that? According to Wikipedia, concentrations of about 100 molecules per cubic centimeter have already been reached. For a space of 1/10cm^3 it gives us about 10 molecules. I know there are probably many problems other than pumping molecules out that should be addressed, such as diffusion, but these can probably be solved.

    So, can it be done? If yes, how?

    TL;DR: Make perfect vacuum, how?
  2. jcsd
  3. Jan 4, 2017 #2
    One approach could be something like a scaled up version of a Mercury thermometer.
    The vacuum above the Mercury would i guess have some small but detectable amount of Mercury vapor though.
  4. Jan 4, 2017 #3
    Yes, I thought of that, but the vapor is a big issue...
  5. Jan 4, 2017 #4


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  6. Jan 4, 2017 #5

    Vanadium 50

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    Why can't people ask a simple question like "Can one make a vacuum so good there is less than one molecule per cubic centimeter?" without getting all cutsie-pie and bringing in aliens and death beams and God? If I were to answer a question with green aliens, you'd be annoyed at me for wasting your time.

    So you're saying, "I am not a vacuum expert, but I am sure those of you who are aren't doing as well as you could." Do you have any idea how offensive this sounds? Especially coupled with the cutsie. Here's a suggestion - if you want an answer from an expert, don't talk down to them.

    Your problem is that you had to build your box out of something. That something will outgas when you start pumping its interior down. When you get to a very good vacuum you can't really pump any more - the pump has nothing left to pump on. Furthermore, you are left with so little gas than any tiny bit of trapped gas will ruin your vacuum. Tiny like the little bits of gas trapped between screw threads. (Which is why you don't use screws)
  7. Jan 4, 2017 #6


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    I see only one way of interpreting this as a sensible question: you are asking whether there are techniques that might produce better vacuums than we have produced so far, but that we haven't developed for economic reasons. You're constructing a "cost is no objection" hypothetical to eliminate any hypothetical economic objections.

    If that's what you're asking, let's start over again in another thread. This one is closed.
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