Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Distance analogy - am I being accurate?

  1. May 28, 2009 #1
    Hi guys,

    I am giving a schools talk and would like your opinion on whether or not you think the following reasoning is accurate and roughly scientific.

    Lets we assume that on average there is only one hydrogen atom per cubic metre on average in the universe. The diameter of a hydrogen atom is 1.7e-15m.

    Scaling up to the size of a human, can I say that the distance between hydrogen atoms in the universe is equivalent to:

    0.3 (diameter of a small human) / 1.7e-15 = 1.76e14 m = 1100 AU

    So the distance between hydrogen atoms in the universe is roughly equivalent to one of the kids standing the classroom and another kid standing 1000 times further away than we are to the sun (or if you can think of a better landmark to use I'd appreciate it! Mine is quite clumsy)

    Does that logic hold, or is it outright wrong?


  2. jcsd
  3. May 28, 2009 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    That's not right. More like an angstrom.
  4. May 28, 2009 #3
    Wikipedia says 2.4 A, so I'm wrong either way. But aside from that - can I use this as a useful analogy?
  5. May 28, 2009 #4


    User Avatar
    Science Advisor

    I think what you used for diameter of the hydrogen atom is more like diameter of a proton.

    The analogy makes sense, but I'm not sure how useful it is. These kind of analogies can be helpful because they put astronomically small or large numbers in some type of context humans have experience with. Even after your scaling, your factor is still 1100AU, which no child (or person, for that matter) has an intuitive grasp of.

    Try using something smaller, like a pea or a grain of salt. Maybe then you'll get a distance comparable to that between cities or something. That would greatly increase the usefulness, in my opinion.
  6. May 29, 2009 #5
    I think you have a very good point - there's no point taking an ungraspable analogy and replacing it with another one! I'll see what numbers come out of smaller things like a pea.

    Thanks for your advice - I really appreciate it!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook