Q: When does three plus three equal ten?
A: When waves meet up in experiments to simulate monstrous freak waves that snap ships in half or topple oil drilling rigs.
Freak waves are rare, but satellite images and sailors' reports of waves in the open ocean rising up to tower over the surrounding surf show that they can reach the height of a twelve story building. They've caused numerous shipping disasters over the centuries, even appearing in epics of ancient mariners.
Until recently freak waves have defied physicists in search of an explanation of how they form. A study by Miguel Onorato of the University of Torino in Italy suggests we are closing in on an answer. It turns out that waves don't always obey simple math. When two waves combine, their total height is usually just the sum of the individual wave heights. When they start to get large and approach each other at just the right angle, however, they add up to more than the sum of their parts. In physics jargon, they combine nonlinearly.
Read more about it in Physical Review Focus.