Tuesday, October 24, 2017
Sven Carlsson
The physics department at Massey University has provided a possible explanation for last Sunday night’s bang that continues to generate questions around Opotiki.
Responding from Stuttgart in Germany, where he is working on theoretical explanations for subtle quantum phenomena in ultracold gases at the Max Planck Institute for Solid State Research, professor Joachim Brand offered a couple of potential explanations for the strange sound phenomenon.
“This appears to be a startling natural phenomenon that may have a simple explanation by the laws of physics,” he said.
“Since I don’t have detailed measurement data or first-hand knowledge of the local conditions, I can only speculate about the possible mechanisms that may be at play here.”
Professor Brand summarised the situation as follows.
“I understand that the fire was lit in a three-metre deep hole and started with the help of petrol causing a ‘woosh’ as observed at close distance, but recognised as a ‘large bang’ by observers located hundreds of metres up to kilometres away,” he said.
The professor said that several physical mechanisms could have caused or contributed to this effect.
“The first idea that comes to mind is focusing,” he said,
“Sound waves being reflected by structures like nearby buildings, trees, or even the hole itself could concentrate in a spot much further away.”
Professor Brand said this would yield a higher intensity at certain locations than at close distance, in a similar way as a magnifying glass could concentrate sunlight in a very bright spot or as a glass of water in the sunlight could cause spots and arcs of bright light in the shadows.
“These phenomena are generally known as caustics in optics and closely related to a field of mathematics called catastrophe theory,” he said.
The fact that it was a clear and calm night after sunset further suggested that an inversion layer might have formed.
“The interface between the air layers of different temperature and density might well have acted like a giant mirror reflecting off sound waves, which could have contributed to causing caustics at the locations where residents experienced the loud bang,” Professor Brand said.
A different type of explanation could be that a soliton was triggered.
A soliton was a phenomenon best known in the form of tsunamis in the ocean, where waves could transport destructive amounts of energy over distances of thousands of kilometres.
“Although, in principle, such phenomena could occur in acoustics, I am sceptical that conditions were suitable for this to happen to be the correct explanation for what was experienced in Opotiki,” Professor Brand said.