Background tension in QFT

Standard QFT is built on a fixed classical spacetime (Minkowski with Poincaré symmetry). The field is a section of a bundle over this spacetime, and upon quantization its excitations are particles.

But QFT's own ontology says that particles have no position or trajectory — a photon is a delocalized excitation with momentum and helicity, not a worldline from A to B. Classical spacetime still supplies the labels "A" and "B," the metric, the light cones — everything that the quantum description undermines.

The tension:

Methodology: QFT assumes a classical background (Poincaré group, Lorentz metric, points as labels)
Ontology: QFT's particles have no classical trajectories or positions
The stage is classical, but the actors are quantum in a way that makes the stage's own concepts (worldlines, points) ontologically suspect

This is not a formal contradiction, but a conceptual instability — a mixed ontology. The background is treated as classical because we don't know how to quantize it consistently. This motivates quantum gravity approaches (loop quantum gravity, causal set theory, ER=EPR) where spacetime itself becomes quantum.