Atoms Fall "Down"

  • I'm not sure how applicable this might be, but just watched a Vsauce video on the meaning of "Down". Since Epicurus maintained that atoms fall "down" unless they "swerve" I found Michael's explanation of"down" interesting. I never put this much thought into it... Which makes the Vsauce channel so enjoyable!

    Here you go:

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  • Haven't had a chance to watch this yet, but I've always had a slight confusion on the "down" issue. Epicurus seemed to think that the "original" motion of any given atom was "down" until it either swerved or hit another atom and ricocheted. Except that the cosmos was beginningless, so I'm not sure when this "original" downward motion happened. Lucretius is clear that an atom in motion is governed by inertial force at a uniform rate of speed in any given direction until they swerve or are acted upon by an outside force. These atoms, once moving in another direction, are not affected by any downward pull. Do I have that right?

  • That's my take. Epicurus's whole "down" thing never made much sense to me. Once you have swerving and collisions, you've messed up any original parallel trajectories. Okay, an atom moves in a straight line unless acted on (not sure if that's accurate, but I'm not too concerned about that).

    In any case, I found the Vsauce video thought-provoking and enjoyable. It's a fun YouTube channel, and we got to see Michael do a program once with Adam Savage of Mythbusters. A fun night of popular science stuff! :)

  • I hadn't heard of that one! I've heard of CGPGrey from other sources but never dug into their work. This is a good opportunity. Thanks for the recommendation [runs off to podcast app to search and subscribe...]

  • The video nicely illustrates what we know today. (Note that turning the balance upside down is just a joke and still just shows the force on us as the counterforce from hard enough material to prevent us from breaking or sinking through the balance and not directly the force with which we pull up Earth. We know from theory which is confirmed by other experiments that we pull up Earth with the same force.)


    Knowing this helps to get clear how wrong or misleading Epicurus' reference to down is. Some people have even used this wrong usage to claim that Epicurus was a flat-Earther. Epicurus use of down is so much against his own physics that he probably meant "down" in a different way but I have not yet seen and could not figure out a meaningful way. Maybe the ancient Greek word had a broader or different meaning than our "down".

  • Thanks for those insights, Martin ! In light of what you said, I went back to Diogenes to see what the Letter to Herodotus said:

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    60] "Further, we must not assert `up' or `down' of that which is unlimited, as if there were a zenith or nadir. As to the space overhead, however, if it be possible to draw a line to infinity from the point where we stand, we know that never will this space --or, for that matter, the space below the supposed standpoint if produced to infinity--appear to us to be at the same time `up' and `down' with reference to the same point ; for this is inconceivable. Hence it is possible to assume one direction of motion, which we conceive as extending upwards ad infinitum, and another downwards, even if it should happen ten thousand times that what moves from us to the spaces above our heads reaches the feet of those above us, or that which moves downwards from us the heads of those below us. None the less is it true that the whole of the motion in the respective cases is conceived as extending in opposite directions ad infinitum. [61] "When they are travelling through the void and meet with no resistance, the atoms must move with equal speed. Neither will heavy atoms travel more quickly than small and light ones, so long as nothing meets them, nor will small atoms travel more quickly than large ones, provided they always find a passage suitable to their size, and provided also that they meet with no obstruction. Nor will their upward or their lateral motion, which is due to collisions, nor again their downward motion, due to weight, affect their velocity. As long as either motion obtains, it must continue, quick as the speed of thought, provided there is no obstruction, whether due to external collision or to the atoms' own weight counteracting the force of the blow.

    It seems a bit more nuanced than I remembered. He seems to say that atoms can move any direction but their downward motion is due to their "weight" which, in a manner of speaking, is correct! The fact that he also says that "Neither will heavy atoms travel more quickly than small and light ones," took me by surprise. I didn't remember that. And that's correct (in a vacuum), right? How or why would he intuit that? I think I need to revisit that Letter.

  • Epicurus use of down is so much against his own physics that he probably meant "down" in a different way


    I strongly applaud that point of view. Surely Epicurus could make mistakes as much as anyone else can, but it's really inconceivable that he would accept and state something clearly contradictory within his core views, so either our texts are corrupted or he had some other understanding of the issue that would reconcile what we see as contradictory.

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    And that's correct (in a vacuum), right? How or why would he intuit that? I think I need to revisit that Letter.

    I don't have a citation to hand, but see Lucretius on this point. I think he says that in the absence of air-resistance a ball of wool and a ball of lead will fall at the same speed.

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    And that's correct (in a vacuum), right? How or why would he intuit that? I think I need to revisit that Letter.

    I don't have a citation to hand, but see Lucretius on this point. I think he says that in the absence of air-resistance a ball of wool and a ball of lead will fall at the same speed.

    This?

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    Which but for voids for bodies to go through 'Tis clear could happen in nowise at all. Again, why see we among objects some Of heavier weight, but of no bulkier size? Indeed, if in a ball of wool there be As much of body as in lump of lead, The two should weigh alike, since body tends To load things downward, while the void abides, By contrary nature, the imponderable. Therefore, an object just as large but lighter Declares infallibly its more of void; Even as the heavier more of matter shows, And how much less of vacant room inside. That which we're seeking with sagacious quest Exists, infallibly, commixed with things- The void, the invisible inane.

  • I might be misremembering slightly! That does look like the right passage.

    I thought you were kidding at first, JJElbert ! LOL I didn't remember the ball of wool and lead!! That's amazing that he had that kind of insight about mass! Thanks for calling my attention to that!