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I have a strange question... a different way of calculating the atomic weight

  1. Jan 12, 2016 #1
    I'm writing a piece of fiction that, if my understanding of the subject is correct, will include a bit of chemistry knowledge. (In explaining my question, I'm going to spell out some obvious things. I'm terrible at math, and hence even worse at chemistry) For example:

    Atomic weight is calculated using a specific formula. Using this specific formula, we determine that:

    Hydrogen has an atomic weight of 1.0079.
    Helium has an atomic weight of 4.0026.

    The ratio between these two atomic weights is 3.97122730429606 : 1 (or 0.25181132264028).

    Now let's say there was a different way of calculating the atomic weight of an element, maybe using something instead of Carbon 12. Maybe we use Carbon 14 instead. That would of course change the numerical expression of the atomic weight.

    However, if that were the case, wouldn't the ratio between the two (new) atomic weights for hydrogen and helium be the same? The actual numbers would be different, but the ratio between them would be constant regardless of the formula used or the units of measure.

    The reason I ask is that I'm trying to find a way for a fictional extraterrestrial signal to communicate to us various elements. It seems that by using the ratio BETWEEN elements, it wouldn't matter what formula was used or what units of measure were used - the ratio would always be the same if the other fictional formula was used consistently for all elements.

    Am I completely off base? Should I re-enroll in high school? : )

  2. jcsd
  3. Jan 12, 2016 #2
    You are absolutely correct in that the ratios won't change no matter what units you use. During division the units cancel out. 5 murgs divided by 3 murgs is 5/3. Just plain 5/3, not 5/3 murgs. It is called a dimensionless number.

    I don't know that the elements would have the same atomic weights on some extraterrestrial planet. I know that the atomic weight of uranium changes slowly. It gets heavier because the lighter isotopes decay faster. You could work that into the plot: we'd be able to tell how old their planet is by the ratios.

    The atomic weight of carbon decreases slowly since the radioactive isotopes are heavier.

    All and all I think it is a clever idea. Never thought of that one before.
    Last edited: Jan 12, 2016
  4. Jan 12, 2016 #3
    I'd forgotten about the decrease of atomic weight. That's what eventually turns uranium into lead, isn't it?

    Basically, what I'm trying to do is write a more accurate version of "Contact," with less whiz-bang science fiction, like the mysterious billionaire. It's kind of fun trying to figure out what an alien culture might use to try to communicate. Thanks for your help!

  5. Jan 13, 2016 #4


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    Staff: Mentor

    Actually in the past we used other basis for calculations - oxygen was assumed to be 16. You are right that switching to another basis shouldn't change ratios.

    However, using ratios is not a good idea (not bad, but not perfect). Samples of an element from different sources can have different isotope composition so their average atomic masses can be different (and the ratios would be not constant). I believe especially for some heavy elements it can produce ambiguity. Using atomic number (nucleus charge, Z) is unambiguous and much simpler to transmit, as these are small integer numbers.
  6. Jan 13, 2016 #5
    That's way too simple. ; ) Actually, because of my inexperience with chemistry and having seen these other more complex numbers that make up atomic weight, I figured the atomic number was an artificial man-assigned number - way too simple to be "real world." (High school chemistry class was way too many semesters ago.) Thanks for the help! You've made my writing life much simpler.

  7. Jan 14, 2016 #6
    I would try it this way:

    1. Create a simple system of binary codes for elements and total formulas. For example an element could be coded by an element-symbol and the atomic number. A molecular formula could consist of a formula-symbol and elemental codes with their particular frequencies.

    2. Send a series of elemental codes using the emission spectra of the particular element.

    3. Send a series of simple chemical formulas with the emission spectra of the particular compound.

    4. Send your message using this code with a frequency of your choice.

    Any civilization which is able to identify elements and compounds by spectral analysis of astronomical observations and can deal with simple binary codes should be able to understand such a message.
  8. Jan 14, 2016 #7


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    Staff: Mentor

    That requires coding also spectra, so you need quite elaborate code for that.
  9. Jan 14, 2016 #8
    I'm talking about the spectra of the carrier signal used for the transmission.
  10. Jan 14, 2016 #9


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    Staff: Mentor

    Ah, OK. Interesting idea.

    I wonder how realistic it is. I mean - some wavelengths are better for interstellar communication as there is less noise from natural sources.
  11. Jan 14, 2016 #10


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    Last edited: Jan 14, 2016
  12. Jan 15, 2016 #11
    As these spectra would just be used for the transmission of the code and not for the message itself this shouldn't be a problem. Short symbols could be sent at a very low bit rate.
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