system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What would happen if we used a global currency that was worth the same everywhere.
gpt
We will get into big problems, as seen in the Euro crisis: 

Countries with a low productivity can't create enough income to pay all their imports - they will get broke rather fast and also have their population impoverished (this happened in Greece). Those low productivity countries usually devaluated their currencies to fight this. Imports will become prohibitively expensive, while exports become much cheaper (as you pay nominally the same too workers, which is "worth" far less internationally). If one has a rather strict inflation control it also avoids the poverty problem - You can't buy the expensive imported goods, but the local produce is still affordable, which also strengthens the local economy.

The high productivity countries however will largely benefit, as everybody who buys from them has enough "hard cash" - at least in the beginning. Effectively concentrating capital there, however there is no mechanism to slow that effect down (i.e. a higher valuation of their currency, making products more expensive). So there is no real incentive to increase productivity or innovate any more...

In the worst case you will get something like the world of the "Hunger Games". Some rich decadent centers with a lot of impoverished countries around that. 

However keep in mind that this just one theory (which is partly proven by the current state of the EU) and it will only apply with the mostly free globalized trade we have now. There may be a lot of measures to counteract this outcome (customs, subsidizing "low productivity" countries, immigration policies etc.) and nobody ever tried it. (The past experience with the global gold standard is IMHO not applicable, as economies where more separated, due to a lack of technologies back in the day)