Of course you can invest in a fiat currency. I invested a few thousand dollars in a US current account because I thought the dollar would be more stable than Sterling during all the Brexit nonsense. I didn't buy bonds because bonds aren't as liquid, and have a greater risk because of the possibility of bank bail-ins. I've closed my dollar account, because I don't believe the dollar has a sustainable future any more. This is because of the massive debt mountain that is dragging it down. I've been transferring my dollar holding into .com domain names, and over the next few months I will be selling those for Bitcoin. I think Bitcoin is a much better long term investment, but you do need to be able to ride the coming "corrections". Some of those are likely to be fairly substantial in my opinion. But these will represent good opportunities to increase investment in Bitcoin.