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Just recently I learnt about Einstein's and Thomsom's explanation of the tea leaf paradox.

In my mind I always thought that the leaves move to the center when stirring the tea due to the fact that water is denser than the leaves (hence the leaves float). Therefore, the leaves would move inwards because the water moves outwards, just like a helium balloon in a car moves backwards when the car brakes.

enter image description here source: What happen when a helium balloon is floating inside a moving train?

It's beyond obvious that Einstein and other physicists are correct, so my question is: why is my reasoning wrong?

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  • $\begingroup$ In another way of putting my question, wikipedia says "The tea leaf paradox is a phenomenon where tea leaves in a cup of tea migrate to the center and bottom of the cup after being stirred rather than being forced to the edges of the cup, as would be expected in a spiral centrifuge". Would that be really expected? If the water is denser than the leaves, I'd expect that the centrifugation would move the water to the edges, not the leaves, isn't that correct? $\endgroup$
    – Megaptera novaeangliae
    Commented Oct 5, 2022 at 2:16
  • $\begingroup$ the wiki article is quite clear on the explanation. en.wikipedia.org/wiki/Tea_leaf_paradox Your two dimensional argument igrnores the three dimension kinematics of flow as explained in the link. $\endgroup$
    – anna v
    Commented Oct 5, 2022 at 3:39
  • $\begingroup$ @annav my question is exactly about the explanation, which I read but I'm not understanding: "Therefore, the anticipation is that tea leaves would, because of their mass, move to the edge of the cup". Once again, my question is: since the water is denser than the leaves, shouldn't the water, not the leaves, move to the edge of the cup? $\endgroup$
    – Megaptera novaeangliae
    Commented Oct 5, 2022 at 3:46
  • $\begingroup$ Where did you find the statement? $\endgroup$
    – Debanjan Biswas
    Commented Oct 5, 2022 at 4:00
  • $\begingroup$ I see. Even in two dimensions the leaves cannot be considered independent of the flow, it is the water that is stirred, being lighter they are carried by the water, as a part of the water, and do not interact the way you think. The leaves are too light in mass to affect the momentum conservation of the flow. They are an indicator of the flow. $\endgroup$
    – anna v
    Commented Oct 5, 2022 at 4:09

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In the video on Wikipedia you can see that the leaves are not floating: the leaves are denser than the tea, opposite to what you assume. Although dry tea leaves start by floating, they eventually sink, though some rise again.

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