Time ordered product vs. commutator in path integral

In summary, the conversation discusses the possibility of defining quantum field theory using a path integral and how to show that the fields and local operators commute at space-like separation. It is suggested that an operator product expansion and additional assumptions may be needed to establish this.
  • #1
kharranger
17
0
Suppose I want to bypass the entire Hamiltonian formulation of quantum field theory and define the theory using a path integral. Thus all I can calculate are Green's function which are time ordered products of local operators. Given only these (no expansions of the field in creation anihilation operators etc.), how can I show that the fields and local operators commute at space-like separation? [\phi(x),\phi(y)]=0, when x,y are spacelike separated?

KH
 
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  • #2
Operator product expansion maybe?
 
  • #3
I think you will need extra assumtions since the fields have different order in the two parts of the commutator and the time ordered product will give only one of the parts.

So you will need an assumption connecting the expectations values of phi(x)phi(y) and phi(y)phi(x). Those must depend on x-y and y-x, due to homogeneity of spacetime (unless you have inhomogeneous external source in the theory).
 

1. What is the difference between a time ordered product and a commutator in the path integral method?

The time ordered product and commutator are two mathematical operations used in the path integral approach to quantum mechanics. The time ordered product orders the operators in a specific time sequence, while the commutator measures the non-commutativity of two operators. In other words, the time ordered product takes into account the temporal ordering of operators, while the commutator measures how the operators interact with each other.

2. How do the time ordered product and commutator affect the Feynman propagator in the path integral method?

The Feynman propagator, also known as the Green's function, is a key component of the path integral method. The time ordered product and commutator affect the Feynman propagator by altering the way operators are ordered and interact with each other. The time ordered product ensures that operators are ordered according to their respective times, while the commutator accounts for any non-commutativity between operators.

3. What is the significance of using a time ordered product in the path integral method?

The use of a time ordered product in the path integral method is crucial for accurately calculating quantum mechanical amplitudes. By ordering operators in a specific time sequence, the time ordered product takes into account the temporal evolution of a system, which is necessary for calculating amplitudes in quantum mechanics.

4. Can the commutator be expressed in terms of the time ordered product?

Yes, the commutator can be expressed in terms of the time ordered product. This is known as the Baker-Campbell-Hausdorff formula, which allows for the expansion of the commutator in terms of nested time ordered products. This relationship is useful for simplifying calculations in the path integral method.

5. How do the time ordered product and commutator relate to Heisenberg's uncertainty principle?

The Heisenberg uncertainty principle states that there is an inherent uncertainty in the simultaneous measurement of certain pairs of physical quantities, such as position and momentum. The time ordered product and commutator are related to this principle in that they measure the non-commutativity of operators, which is a fundamental property of quantum mechanics. This non-commutativity is the basis for the uncertainty principle and plays a crucial role in the path integral method.

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