Two scientists at CERN:
- Bob, look at the data, it's scary! These two particle fields are anti-commuting but they have 0-spin. I am afraid, Bob. I am telling you, there must be ghosts in this lab. Run away!
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| Picture is taken from http://www.toonpool.com/cartoons/God%20particle%20found_172865 and is modified. |
Background
Only theoretical physicists can get it right away. Everybody else should go through a standard textbook of quantum field theory and then read the joke again.
However, we will attempt to give a glimpse of theory. According to the spin-statistics theorem, the spin of a particle is related to the statistics it follows. For example, particles with half-integer spin follow the Fermi-Dirac statistics and they are categorised as Fermions. Furthermore, particles with integer spin follow the Bose-Einstein statistics and they are named Bosons.
Now, in quantum field theory, the quantisation of a gauge field through the path-integral formalism requires the exclusion of the all gauge-equivalent contributions to the path integral. This is achieved by the so called Faddeev-Popov trick, introduced by Faddeev and Popov. This trick gives rise to fictitious fields, which are anti-commuting (i.e. obey Fermi-Dirac statistics) and have 0-spin.
We emphasize that these fields are fictitious, they do not correspond to real field (particles) and have been introduced as part of a mathematical trick, they can't appear in any observation and carry no physical content or information. This is probably the reason that have been given the provocative name ghosts.
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