Sometimes, the time-ordering operator T is included.
Depending on n (the number of inserted operators), the correlation functions are called one-point function (tadpole), two-point function, and so on. The correlation functions are often called simply correlators. Sometimes, the phrase Green's function is used not only for two-point functions, but for any correlators.
If the wave function collapses when the observation takes place, then he should describe the cat with a quantum state as well, in which the cat is part alive and desperately trying to get out of the box before the cyanide gets him, and part dead and lying in a heap on the floor.
Quantum mechanics predicts that the choice of the axis for the first measurement of spin will alter the results of measurement of spin of the second particle, in a manner which is not consistent with the notion that the two particles have separated and become independent.
Quantum mechanics seems to contradict the idea that, prior to measurement, a particle is a point-like object with an unknown position, and appears to say that the particle is actually a wave spread over space.
For stochastic processes, including those that arise in statistical mechanics and Euclidean quantumfieldtheory, a correlationfunction is the correlation between random variables at two different points in space or time.
Correlationfunctions used in astronomy, financial analysis, quantumfieldtheory and statistical mechanics differ only in the particular stochastic processes they are applied to.
A quantumfieldtheory is called renormalizable if this mapping has a fixed point which gives a quantumfieldtheory.
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