- Location
- Earth
Short Answer: As long as Mat lets go before disappearing it, she's fine.
In general, it's a phase transition in a chaotic fluid. Which while technically a physical process, actually happens on the molecular level. The heat of cavitation is hot enough to melt just about anything, and I do mean, anything. It's something like 4.5 - 5k °C. The pressure at the event horizon is also silly and ridiculously loud. But... All the individual events are tiny. The implosion in a fluid isn't like an explosion from a solid. It's a chaotic collapse that produces randomly scattered tiny events. Like a group of fireflies.
In practice, with how heat, sound and shock-waves work, even something like 10cm is safe in terms of heat, and 50-100cm will do you in open water.
Quick Math:
Pressure works on surface area, and that's 4*Pi*r*r. With cavitation events usually smaller than 1mm, 10cm distance means a 125663 surface/power ratio. Which is something like 51 decibels. 100cm makes for 12566370 times less force, or about 71 decibels of difference.
For comparison 1-Ton TNT Bomb makes 210 dB and 140db is fine with hearing protection. This means that if you filled a 1mm ball with the sound of 1-Ton TNT at 100cm it would be no louder than a rock concert, and significantly weaker than a gunshot.
With the size of Mat's sword and this being a flowing river, 120-130 dB tops for the starting event. So Mat probably heard something between the volume of a shout and a vacuum cleaner.
