What actually makes the popping sound when you crack your knuckles?

Settled in 2018 by G. Kawchuk et al. (2015) showed the sound coincides with cavity/bubble formation; V. Chandran Suja & M. Barakat (2018) mathematical model reconciled this with partial bubble collapse.

For decades two rival explanations competed: that the pop is caused by a gas bubble suddenly forming in the joint fluid as the joint is stretched (tribonucleation), or by an existing bubble suddenly collapsing. Kawchuk and colleagues' 2015 PLOS ONE study used real-time MRI to show the pop coincides with the rapid formation of a gas cavity, and that the resulting bubble does not fully collapse but can persist in the joint fluid for many minutes -- which is also why you cannot re-crack the same knuckle again immediately, since the gas must first redissolve. Chandran Suja and Barakat's 2018 Scientific Reports mathematical model reconciled the apparently conflicting camps by showing the sound is best explained by a rapid but only partial collapse of the newly formed cavity, unifying the formation-based MRI observations with the classic bubble-collapse acoustic theory.

A sound almost everyone has made with their own hands took nearly a century of competing hypotheses, an MRI scanner, and a fluid-dynamics model to finally pin down.

Explanations that circulated

• The sound is caused by bones grinding or ligaments snapping over each other
• The sound is simply the complete implosion of a single pre-existing bubble, with formation of the gas cavity playing no role

References

• Real-time visualization of joint cavitation, G. N. Kawchuk, J. Fryer, J. L. Jaremko, H. Zeng, L. Rowe, R. Thompson, PLOS ONE, 2015
• A mathematical model for the sounds produced by knuckle cracking, V. Chandran Suja, M. Barakat, Scientific Reports, 2018