Imaginary time
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Imaginary time is a mathematical representation of time which appears in some approaches to special relativity and quantum mechanics. It finds uses in connecting quantum mechanics with statistical mechanics and in certain cosmological theories.
Mathematically, imaginary time is real time which has undergone a Wick rotation so that its coordinates are multiplied by the imaginary root i. Imaginary time is not imaginary in the sense that it is unreal or madeup (any more than, say, irrational numbers defy logic), it is simply expressed in terms of what mathematicians call imaginary numbers.
Contents
Origins
Mathematically, imaginary time may be obtained from real time via a Wick rotation by in the complex plane: , where is the square root of minus one and is technically called an "imaginary" number.
Stephen Hawking popularized the concept of imaginary time in his book The Universe in a Nutshell.
“  One might think this means that imaginary numbers are just a mathematical game having nothing to do with the real world. From the viewpoint of positivist philosophy, however, one cannot determine what is real. All one can do is find which mathematical models describe the universe we live in. It turns out that a mathematical model involving imaginary time predicts not only effects we have already observed but also effects we have not been able to measure yet nevertheless believe in for other reasons. So what is real and what is imaginary? Is the distinction just in our minds?  ” 
— Stephen Hawking^{[1]} 
In fact, the names "real" and "imaginary" for numbers are just a historical accident, much like the names "rational" and "irrational":
“  ...the words real and imaginary are picturesque relics of an age when the nature of complex numbers was not properly understood.  ” 
— H.S.M. Coxeter^{[2]} 
In cosmology
In the Minkowski spacetime model adopted by the theory of relativity, time can be understood as multiplied by i^{[citation needed]}.
Spacetime may be represented as a fourdimensional surface or manifold. The equivalent of a distance in space is called an interval. Given an assumption that time is real, an interval in relativistic spacetime is given by the usual formula but with time negated:
where , and are distances along each spatial axis and a period of time. Mathematically this is equivalent to writing
In this context, may be accepted as a feature of the relationship between space and real time, as above, or it may be incorporated into time itself, as imaginary time, and the equation rewritten in normalised form:
Similarly its four vector may be written as
where distances are represented as , is the velocity of light and .
In physical cosmology, imaginary time may be incorporated into certain models of the universe which are solutions to the equations of general relativity. In particular, imaginary time can help to smooth out gravitational singularities, where known physical laws break down, to remove the singularity and avoid such breakdowns (see Hartle–Hawking state). The Big Bang, for example, appears as a singularity in ordinary time but, when modelled with imaginary time, the singularity can be removed and the Big Bang functions like any other point in fourdimensional spacetime. Any boundary to spacetime is a form of singularity, where the smooth nature of spacetime breaks down. With all such singularities removed from the Universe it thus can have no boundary and Stephen Hawking has speculated that "the boundary condition to the Universe may be that it has no boundary".
However the unclear nature of the relationship between real time and imaginary time required by such models has cast doubt on them.^{[3]}
In quantum statistical mechanics
It can be shown that at finite temperature , the Green's functions are periodic in imaginary time with a period of . Therefore, their Fourier transforms contain only a discrete set of frequencies called Matsubara frequencies.^{[clarification needed]}
Another way to see the connection between statistical mechanics and quantum field theory is to consider the transition amplitude between an initial state I and a final state F, where H is the Hamiltonian of that system. If we compare this with the partition function we see that to get the partition function from the transition amplitudes we can replace , set F = I = n and sum over n. This way we don't have to do twice the work by evaluating both the statistical properties and the transition amplitudes.
Finally by using a Wick rotation one can show that the Euclidean quantum field theory in (D + 1)dimensional spacetime is nothing but quantum statistical mechanics in Ddimensional space.
See also
References
Notes
Bibliography
 Stephen W. Hawking (1998). A Brief History of Time (Tenth Anniversary Commemorative ed.). Bantam Books. p. 157. ISBN 9780553109535.
 Hawking, Stephen (2001). The Universe in a Nutshell. United States & Canada: Bantam Books. pp. 58–61, 63, 82–85, 90–94, 99, 196. ISBN 055380202X.
Further reading
 Gerald D. Mahan. ManyParticle Physics, Chapter 3
 A. Zee Quantum field theory in a nutshell, Chapter V.2
External links
 The Beginning of Time — Lecture by Stephen Hawking which discusses imaginary time.
 Stephen Hawking's Universe: Strange Stuff Explained — PBS site on imaginary time.