One time I was driving with a bunch of helium balloons in the passenger seat to my right. I took a left-hand turn and was surprised when I got attacked by a swarm of party balloons, seemingly accelerating into the turn. It took me a while to realize that all the air heavier than the helium was sloshing to the right, pushing the balloons out of the way and into my head.
There’s a cute paper* that models this by exploiting an accidental symmetry of Newton’s laws. If we arbitrarily consider the air in the car to have zero mass, then the helium balloons must have negative mass, and the dynamics are simpler to model. For example, gravity’s F=mg with negative mass immediately shows the balloons feel a force upward. But it’s just a conceptual simplification. The results are, of course, the same as if we used the actual masses of air and helium everywhere.
As Relativity and QM bring more constraints on mass than Newton has, I don’t think the model carries over to neutrinos. (Like, what would a negative wavelength even mean?) But for balloons, at least, it’s fun to think about. And your post reminded me of it.
Anyway, if the paper had been written a few years earlier, I would have been less surprised. And put the balloons in the back seat.
* https://pubs.aip.org/aapt/ajp/article/82/10/997/1039402/The-concept-of-negative-mass-and-its-application
Ray Lee
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2025-09-04 11:30:02 UTC
in reply to nevent1q…smdl
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