I was reading some of the first chapters of Zwiebach's book. It is explained how quantized relativistic strings turn out to describe an infinite set of possible particles and, therefore, that it is possible to consistently describe the results obtained in the Fock representation in QFT on a flat spacetime. However, in QFT there are scenarios in which particles are not the fundamental entities. This is the case in curved spacetimes where particles depend on the chosen reference frame, because there is not always a global definition of the time coordinate that provides a unique set of positive frequency modes to build the Fock representation. Applications such as Hawking radiation or the particle creation in an expanding cosmological background are examples of this. If I extrapolate what I have read so far about strings, it seams to me that the notion of string is also an observer dependent one. I am sure something is wrong with this view. Zwiebach shows how string theory can explain QFT in a flat background. How does it explain QFT on curved backgrounds? Before of moving on to more difficult references, I would like to get a feeling about this.