1.   Apparently, the collapsing bubbles generate miniature shock waves that squeeze gas within the bubble.

2.   Hence, Moss and his colleagues are trying to boost the temperature of collapsing bubbles by intensifying the sound waves.

3.   Lohse said he planned to see if the collapsing bubble also produced a flash of light.

4.   The crux of contention is whether fusion-energy bubbles are streaming out of the collapsing bubbles.

5.   The crackling noise comes from the sound waves given off by those collapsing bubbles.