On Saturday the price of one Bitcoin reached an all-time high of US$2000; at the time of writing, two days later, it is nudging US$2100, or A$2,800, and is not far off double the price of that more traditional alternative currency, gold.

Count me wide-eyed and nonplussed. What exactly is this thing that now costs the same as quite a nice TV — more than a return ticket to New York? And is it a bubble?

Bubbles are notoriously hard to identify in real time. When the item in question has fallen by 50 per cent, or better still returned to zero, it can safely be declared to have been a bubble. In the meantime it is merely a puzzle.