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Ghosts are unphysical states that arise when quantizing gauge theories. Do not use this tag for 'ghosts' in the paranormal sense.

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What is the ontological status of Faddeev Popov ghosts?

If you fixed a gauge and got rid of unphysical degrees of freedom (sacrificing manifest gauge invariance), you would not have to muck around with ghosts. … As @RonMaimon says, ghosts are gauge dependent. It doesn't make sense to compare a state in one gauge (without ghost) to the same state in another gauge (with ghost). …
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Path integral quantization of bosonic string theory

Even in QFT when you do the Fadeev-Popov method and introduce ghosts, you have to quantize them so that diagrams with ghosts consistently cancel amplitudes of (some) diagrams with longitudinal gluons. …
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