✎ Wick Rotation [The Physics Travel Guide]

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====== Wick Rotation ======

<blockquote>
Field Theory and Statistical Mechanics are closely
connected. A Wick rotation t ➝ i /(kT) will take you from one
to the other.

<cite>http://jfi.uchicago.edu/~leop/TALKS/Phase%20TransitionsV2.4Dirac.pdf</cite>
</blockquote>

<blockquote>
This procedure (Wick rotation), which relates classical statistical mechanics and quantum field theory, is, however, somewhat subtle

<cite>https://arxiv.org/pdf/hep-th/9802035.pdf</cite>
</blockquote>

* [[http://jakob.physicsnotes.org/quantum_field_theory/methods/non_perturbative_qft#why_imaginary_time|Tunneling phenomena]] are described most easily by performing a Wick rotation. 
  * To classify all [[http://jakob.physicsnotes.org/the_standard_model/poincare_group#representations_of_the_lorentz_group|irreducible representations of the Lorentz group]], we must perform a Wick rotation. In this context, the Wick rotation is often called "Weyl's unitary trick".
  * Path integrals only make sense after we perform a Wick rotation:

<blockquote>
In flat space-time, the situation is well-understood: if your Hamiltonian has good positivity properties you can analytically continue to imaginary values of time, and **when you do this you end up with “Euclidean” path integrals, which actually make sense, unlike QFT path integrals expressed on Minkowski space, which don’t.** You can see the problem even in free field theory: the propagator is given by an integral that goes through two poles, so is ill-defined. The correct way to define it to get causal propagation for a theory with positive energies is to go above one pole, below the other, which is equivalent to “Wick rotating” the integration contour 90 degrees to lie on the imaginary time axis.

<cite>https://www.math.columbia.edu/~woit/wordpress/archives/000160.html</cite>
</blockquote>

<note tip>
Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.
</note>
  
<tabbox Student>

<blockquote>
This is widely used to convert quantum mechanics problems into statistical mechanics problems by means of Wick rotation, which essentially means studying the unitary group exp(−itH/~) by studying the semigroup exp(−βH) and then analytically continuing β to imaginary values.

<cite>https://arxiv.org/pdf/1311.0813.pdf</cite>
</blockquote>

--> Wick Rotation in Classical Mechanics#

<blockquote>
One of the stranger aspects of Lagrangian dynamics is how it turns into statics when we replace the time coordinate t by it — or in the jargon of physicists, when we ‘Wick rotate’ to ‘imaginary time’! People usually take advantage of this to do interesting things in the context of quantum mechanics, but the basic ideas are already visible in classical mechanics.

<cite>http://math.ucr.edu/home/baez/classical/spring.pdf</cite>
</blockquote>

<--
<tabbox Researcher> 
<blockquote>Unfortunately, relatively little is
known about Yang-Mills fields on Minkowski spacetime and, worse yet, the
basic objects of interest in quantum field theory (Feynman path integrals)
are extraordinarily difficult to make any sense of in this indefinite context.
The minus sign in the Minkowski inner product is rather troublesome. Not
to be deterred by such a minor inconvenience, the physicists do the only
reasonable thing under the circumstances—they change the sign! To lend
an air of respectability to this subterfuge, however, they give it a name.
Introducing an imaginary time coordinate τ = it is designated a Wick ro-
tation and has the laudable effect of transforming Minkowski spacetime into
R 4 (x 1 x 2 + y 1 y 2 + z 1 z 2 − t 1 t 2 = x 1 x 2 + y 1 y 2 + z 1 z 2 + τ 1 τ 2 ). What more could
you ask? Well, of course, a pedant might ask whether or not any physics
survives this transformation. This is a delicate issue and not one that we
are prepared to address. The answer would seem to be in the affirmative,
but the reader will have to consult the physics literature to learn why (see
Section 13.7 of [Guid]). Whether or not there is any physics in this positive
definite context is quite beside the point for mathematics, of course. It is
only in the positive definite case that (anti-) self-dual connections exist and
it is an understanding of the moduli space of these that pays such handsome
topological dividends.<cite>page 377 in Topology, Geometry and Gauge fields by Naber</cite></blockquote>

--> Example1#

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--> Example2:#

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<tabbox History>

</tabbox>

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