Cracking knuckles has something to do with air bubbles, but would it work in the same way if there is no gravity? If not, how would it be different? Would there still be a popping sound and/or sensation?

  • 2
    $\begingroup$ what a ridiculous & awesome question! re: sound, whilst 'there's no sound in space' is a common (para)phrase, in almost all situations where people say it.. there would in fact be sound - whales don't sing in the air. $\endgroup$
    – Giu Piete
    Mar 5 '19 at 10:43
  • 4
    $\begingroup$ Should be moved to space.se as it's about a real-world phenomena. $\endgroup$
    – vsz
    Mar 5 '19 at 12:00
  • $\begingroup$ Note that OP specify Zero gravity which do not exist anywhere in known universe, free fall is often mistaken for no gravity thus this question seems fine I suppose. $\endgroup$
    – user6760
    Mar 5 '19 at 15:45
  • $\begingroup$ I agree with Gryphon that just because it is on-topic somewhere, does not make it off-topic here. Now, that being said, I don't see how that actually help building any world. $\endgroup$ Mar 12 '19 at 9:01

From Wikipedia

The cracking mechanism and the resulting sound is caused by carbon dioxide cavitation bubbles suddenly partially collapsing inside the joints.

The collapsing of the bubbles is due to pressure differential between the inside and the outside of the bubble.

When we consider the pressure in a liquid, we can distinguish two components:

  1. pressure due to the proper weight of the fluid above the measuring point
  2. pressure exerted from outside the fluid (i.e. atmosphere)

In microgravity 1 would be 0, but 2 will still be present (else any liquid in microgravity would immediatly evaporate).

Therefore, if the person cracking knuckles is in microgravity but still under some atmospheric pressure, the pressure differential will be of the same order of magnitude. Therefore it is reasonable to expect the same phenomena to happen.

  • 4
    $\begingroup$ As the cracking happens in fluid reservoirs on the mm-scale, the weight of the fluid is immaterial. All that needs to be considered is the external pressure, which on the ISS is sea level earth. As the partial pressure of carbon dioxide inside the body would also not be specific to space stations vs earth, we can assume knuckle cracking in space to happen. $\endgroup$
    – bukwyrm
    Mar 5 '19 at 10:18

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