In physics, the term "ghost" often refers to a concept in the context of quantum field theory, particularly in gauge theories and theories involving quantum gravity. Ghosts are typically unphysical states that can arise in the quantization of certain theories, particularly in the process of fixing gauge invariance. 1. **Gauge Theories**: In many quantum field theories, particularly those describing fundamental forces (like electromagnetism or the weak force), gauge invariance is a crucial symmetry.
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