- Good ghosts Faddeev–Popov ghosts Goldstone bosons
- Bad ghosts Ghost condensate Landau ghost
- See also
- References
- External links
In the terminology of quantum field theory, a ghost, ghost field, or gauge ghost is an unphysical state in a gauge theory. Ghosts are necessary to keep gauge invariance in theories where the local fields exceed a number of physical degrees of freedom.[1]
{{quote|For example in electrodynamics, in order to maintain manifest Lorentz invariance, one uses a four component vector potential
, whereas the photon has only two polarizations. Thus, one needs a suitable mechanism in order to get rid of the unphysical degrees of freedom. Introducing fictitious fields, the ghosts, is one way of achieving this goal. |Ludvig Faddeev}}Good ghosts
Faddeev–Popov ghosts
{{main|Faddeev–Popov ghost}}Faddeev–Popov ghosts are extraneous fields which are introduced to maintain the consistency of the path integral formulation. They are named after Ludvig Faddeev and Victor Popov.[2][3]
Faddeev–Popov ghosts are sometimes referred to as "good ghosts".
Goldstone bosons
Goldstone bosons are sometimes referred to as ghosts. Mainly, when speaking about the vanishing bosons of the spontaneous symmetry breaking of the electroweak symmetry through the Higgs mechanism. These good ghosts are artefacts of gauge fixing. The longitudinal polarization components of the W and Z bosons correspond to the Goldstone bosons of the spontaneously broken part of the electroweak symmetry SU(2)⊗U(1), which, however, are not observable. Because this symmetry is gauged, the three would-be Goldstone bosons, or ghosts, are "eaten" by the three gauge bosons (W± and Z) corresponding to the three broken generators; this gives these three gauge bosons a mass, and the associated necessary third polarization degree of freedom.[4]Bad ghosts
"Bad ghosts" represent another, more general meaning of the word "ghost" in theoretical physics: states of negative norm,[5] or fields with the wrong sign of the kinetic term, such as Pauli–Villars ghosts, whose existence allows the probabilities to be negative thus violating unitarity.[6]
Ghost particles could obtain the symmetry or break it in gauge fields. The "good ghost" particles actually obtain the symmetry by unchanging the "gauge fixing Lagrangian" in a gauge transformation, while bad ghost particles break the symmetry by bringing in the non-abelian G-matrix which does change the symmetry, and this was the main reason to introduce the gauge covariant and contravariant derivatives.
Ghost condensate
{{multiple issues|{{Refimprove section|date=January 2017}}{{expert needed|1=Physics|date=May 2015}}|section=yes
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A ghost condensate is a speculative proposal in which a ghost, an excitation of a field with a wrong sign of the kinetic term, acquires a vacuum expectation value. This phenomenon breaks Lorentz invariance spontaneously. Around the new vacuum state, all excitations have a positive norm, and therefore the probabilities are positive definite.
We have a real scalar field φ with the following action
where a and b are positive constants and
using the sign convention in the (+, −, −, −) metric signature.
The theories of ghost condensate predict specific non-Gaussianities of the cosmic microwave background. These theories have been proposed by Nima Arkani-Hamed, Markus Luty, and others.[7]
Unfortunately, this theory allows for superluminal propagation of information in some cases and has no lower bound on its energy. This model doesn't admit a Hamiltonian formulation (the Legendre transform is multi-valued because the momentum function isn't convex) because it is acausal. Quantizing this theory leads to problems.
Landau ghost
{{main|Landau pole}}The Landau pole is sometimes referred as the Landau ghost. Named after Lev Landau, this ghost is an inconsistency in the renormalization procedure in which there is no asymptotic freedom at large energy scales.[8]
See also
- No-ghost theorem, related to bad ghosts
References
2. ^{{Cite journal|last=Faddeev|first=Ludwig D.|author-link=Ludvig Faddeev|last2=Popov|first2=Victor N.|author-link2=Victor Popov|date=1967|title=Feynman diagrams for the Yang-Mills field|url=http://linkinghub.elsevier.com/retrieve/pii/0370269367900676|journal=Physics Letters B|volume=25|issue=1|pages=29–30|doi=10.1016/0370-2693(67)90067-6|issn=0370-2693|via=|bibcode=1967PhLB...25...29F}}
3. ^{{Citation|author=W. F. Chen.|arxiv=0803.1340v2|title=Quantum Field Theory and Differential Geometry|journal=Int. J. Geom. Methods Mod. Phys.|year=2008|volume=10|issue=4|pages=1350003|doi=10.1142/S0219887813500035 }}
4. ^{{Cite book|url=https://www.worldcat.org/oclc/19468842|title=Introduction to elementary particles|last=Griffiths|first=David J.|date=1987|publisher=Wiley|year=|isbn=0471603864|location=New York|pages=|oclc=19468842|author-link=David J. Griffiths}}
5. ^{{Cite journal|last=Hawking|last1=|first=Stephen W.|first1=|author-link=Stephen Hawking|last2=Hertog|first2=Thomas|author-link2=Thomas Hertog|date=|year=2001|title=Living with Ghosts|url=|journal=Physical Review D|volume=65|issue=10|pages=|arxiv=hep-th/0107088|bibcode=2002PhRvD..65j3515H|doi=10.1103/PhysRevD.65.103515|via=}}
6. ^{{cite book|title=Extra Dimensions in Space and Time |author=Itzhak Bars, John Terning |page=70}}
7. ^{{Cite journal|last=Arkani-Hamed|first=Nima|last2=Cheng|first2=Hsin-Chia|last3=Luty|first3=Markus A.|last4=Mukohyama|first4=Shinji|date=2004-05-29|title=Ghost Condensation and a Consistent Infrared Modification of Gravity|journal=Journal of High Energy Physics|volume=2004|issue=05|pages=074–074|doi=10.1088/1126-6708/2004/05/074|issn=1029-8479|arxiv = hep-th/0312099 |bibcode = 2004JHEP...05..074H }}
8. ^{{Cite book|url=http://oxfordindex.oup.com/view/10.1093/oi/authority.20110803100049732|title=A Dictionary of Physics|last=|first=|date=2009|publisher=Oxford University Press|others=Landau Ghost entry|year=|isbn=9780199233991|editor-last=Daintith|editor-first=John|edition=6th|location=Oxford|pages=|oclc=244417456}}
External links
- {{cite web|last=Copeland|first=Ed|authorlink=Edmund Copeland|title=Ghost Particles|url=http://www.sixtysymbols.com/videos/ghost_particles.htm|work=Sixty Symbols|publisher=Brady Haran for the University of Nottingham|archive-url=|archive-date=|dead-url=|access-date=|author2=Padilla, Antonio|date=26 October 2011}}
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