Wick rotation

Wick rotation is motivated by the observation that the Minkowski metric in natural units (with metric signature (−1, +1, +1, +1) convention)

${\displaystyle ds^{2}=-\left(dt^{2}\right)+dx^{2}+dy^{2}+dz^{2}}$

and the four-dimensional Euclidean metric

${\displaystyle ds^{2}=d\tau ^{2}+dx^{2}+dy^{2}+dz^{2}}$

are equivalent if one permits the coordinate t to take on imaginary values. The Minkowski metric becomes Euclidean when t is restricted to the imaginary axis, and vice versa. Taking a problem expressed in Minkowski space with coordinates x, y, z, t, and substituting t = −iτ sometimes yields a problem in real Euclidean coordinates x, y, z, τ which is easier to solve. This solution may then, under reverse substitution, yield a solution to the original problem.

Wick rotation connects statistical mechanics to quantum mechanics by replacing inverse temperature ${\displaystyle 1/(k_{\text{B}}T)}$ with imaginary time ${\displaystyle it/\hbar }$. Consider a large collection of harmonic oscillators at temperature T. The relative probability of finding any given oscillator with energy E is ${\displaystyle \exp(-E/k_{\text{B}}T)}$, where k_B is Boltzmann's constant. The average value of an observable Q is, up to a normalizing constant,

${\displaystyle \sum _{j}Q_{j}e^{-{\frac {E_{j}}{k_{\text{B}}T}}},}$

where the j runs over all states, ${\displaystyle Q_{j}}$ is the value of Q in the j-th state, and ${\displaystyle E_{j}}$ is the energy of the j-th state. Now consider a single quantum harmonic oscillator in a superposition of basis states, evolving for a time t under a Hamiltonian H. The relative phase change of the basis state with energy E is ${\displaystyle \exp(-Eit/\hbar ),}$ where ${\displaystyle \hbar }$ is reduced Planck's constant. The probability amplitude that a uniform (equally weighted) superposition of states

${\displaystyle |\psi \rangle =\sum _{j}|j\rangle }$

evolves to an arbitrary superposition

${\displaystyle |Q\rangle =\sum _{j}Q_{j}|j\rangle }$

is, up to a normalizing constant,

${\displaystyle \left\langle Q\left|e^{-{\frac {iHt}{\hbar }}}\right|\psi \right\rangle =\sum _{j}Q_{j}e^{-{\frac {E_{j}it}{\hbar }}}\langle j|j\rangle =\sum _{j}Q_{j}e^{-{\frac {E_{j}it}{\hbar }}}.}$

Wick rotation relates statics problems in n dimensions to dynamics problems in n − 1 dimensions, trading one dimension of space for one dimension of time. A simple example where n = 2 is a hanging spring with fixed endpoints in a gravitational field. The shape of the spring is a curve y(x). The spring is in equilibrium when the energy associated with this curve is at a critical point (an extremum); this critical point is typically a minimum, so this idea is usually called "the principle of least energy". To compute the energy, we integrate the energy spatial density over space:

${\displaystyle E=\int _{x}\left[k\left({\frac {dy(x)}{dx}}\right)^{2}+V{\big (}y(x){\big )}\right]dx,}$

where k is the spring constant, and V(y(x)) is the gravitational potential.

The corresponding dynamics problem is that of a rock thrown upwards. The path the rock follows is that which extremalizes the action; as before, this extremum is typically a minimum, so this is called the "principle of least action". Action is the time integral of the Lagrangian:

${\displaystyle S=\int _{t}\left[m\left({\frac {dy(t)}{dt}}\right)^{2}-V{\big (}y(t){\big )}\right]dt.}$

We get the solution to the dynamics problem (up to a factor of i) from the statics problem by Wick rotation, replacing y(x) by y(it) and the spring constant k by the mass of the rock m:

${\displaystyle iS=\int _{t}\left[m\left({\frac {dy(it)}{dt}}\right)^{2}+V{\big (}y(it){\big )}\right]dt=i\int _{t}\left[m\left({\frac {dy(it)}{dit}}\right)^{2}-V{\big (}y(it){\big )}\right]d(it).}$

Taken together, the previous two examples show how the path integral formulation of quantum mechanics is related to statistical mechanics. From statistical mechanics, the shape of each spring in a collection at temperature T will deviate from the least-energy shape due to thermal fluctuations; the probability of finding a spring with a given shape decreases exponentially with the energy difference from the least-energy shape. Similarly, a quantum particle moving in a potential can be described by a superposition of paths, each with a phase exp(iS): the thermal variations in the shape across the collection have turned into quantum uncertainty in the path of the quantum particle.

The Schrödinger equation and the heat equation are also related by Wick rotation. However, there is a slight difference. Statistical-mechanical n-point functions satisfy positivity, whereas Wick-rotated quantum field theories satisfy reflection positivity.

Wick rotation is called a rotation because when we represent complex numbers as a plane, the multiplication of a complex number by i is equivalent to rotating the vector representing that number by an angle of π/2 about the origin.

Wick rotation also relates a quantum field theory at a finite inverse temperature β to a statistical-mechanical model over the "tube" R³ × S¹ with the imaginary time coordinate τ being periodic with period β.

Note, however, that the Wick rotation cannot be viewed as a rotation on a complex vector space that is equipped with the conventional norm and metric induced by the inner product, as in this case the rotation would cancel out and have no effect.

Wick rotations can be seen as a useful trick that works because of the similarity between the equations of two seemingly distinct fields of physics. Anthony Zee remarks:[1]

Surely you would hit it big with mystical types if you were to tell them that temperature is equivalent to cyclic imaginary time. At the arithmetic level this connection comes merely from the fact that the central objects in quantum physics exp(−iH T) and in thermal physics exp(βH) are formally related by analytic continuation. Some physicists, myself included, feel that there may be something profound here that we have not quite understood.

Dirk Schlingemann proved that a more rigorous link between Euclidean and quantum field theory can be constructed using the Osterwalder–Schrader theorem.[2]

• Complex spacetime
• Imaginary time
• Schwinger function

• Wick, G. C. (1954). "Properties of Bethe-Salpeter Wave Functions". Physical Review. 96 (4): 1124–1134. Bibcode:1954PhRv...96.1124W. doi:10.1103/PhysRev.96.1124.

Wikiquote has quotations related to Wick rotation.

• A Spring in Imaginary Time — a worksheet in Lagrangian mechanics illustrating how replacing length by imaginary time turns the parabola of a hanging spring into the inverted parabola of a thrown particle
• Euclidean Gravity — a short note by Ray Streater on the "Euclidean Gravity" programme.