The theory of purchasing power parity as the basis of exchange rates: is it relevant in the XXI century?

Nowadays the exchange rate of countries is used to express the ratio between their monetary units, and often between their economic capacities. In the absence of currency stability, purchasing power parity theory could be used in international calculations as the basis for currency fluctuations. However, the feasibility of using this theory in the twenty-first century is controversial, but relevant for consideration.