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## Main Question or Discussion Point

**How can curved spacetime creat particles ??**

I have heard that in curved spacetime the usual QFT theory does not hold and that the 'metric' (curvature) of spacetime produces particles how can it be ? for example given the Metric

[tex] -N(t)dt^{2}+ g_{ij}(X) dx^{i}dx^{j} [/tex] (Einstein summation is assumed on i and j)

what kind of particles are created by it ? , then if 2 observers move on a different metric would they observe a different "sea" of particles ??