So much for Bitcoin, beloved of libertarians, drug dealers and, for a few golden years, the private sector currency that was set to displace the hated money of central banks. A week of sharp falls in its value has caused investors to question whether it is even viable as a financial asset, let alone a global currency that would one day replace the dollar or euro. Perhaps the next time you look at that Bitcoin ad, you should ask yourself: “What’s it worth in pet tortoise skeletons?” For everything else, there are euros.