- #1
noir1993
- 33
- 16
I am studying quantum field theory from [David Tong's lecture notes][1] and I am stuck at a particular place.
In Page 52., under the heading *3.1.1 Dyson's Formula*, Tong introduces an unitary operator
[tex]U(t, t_0) = T \exp(-i\int_{t_0}^{t}H_I(t') dt')[/tex]
He then introduces the usual definition of time ordered products and goes on to expand [tex]U(t,t_0)[/tex]. I am not able to follow how he expanded the time ordered product of operators in the second-order term of the Taylor expansion of the exponential. In particular, I am unable to follow the limits being used and why both integrals are being put in the front. Should we not get product of two integrals involving HI?
The expansion of U(t,t_0) is given by
[tex]1 - i\int_{t_0}^{t}dt'H_I(t') + \frac{-i^2}{2}[\int_{t_0}^{t}dt'\int_{t'}^{t}dt''H_I(t'')H_I(t')+\int_{t_0}^{t}dt'\int_{t_0}^{t'}dt''H_I(t')H_I(t'')]+... [/tex]
Link to Course Page - [David Tong: Lectures on Quantum Field Theory][2] [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf
[2]: http://www.damtp.cam.ac.uk/user/tong/qft.html
In Page 52., under the heading *3.1.1 Dyson's Formula*, Tong introduces an unitary operator
[tex]U(t, t_0) = T \exp(-i\int_{t_0}^{t}H_I(t') dt')[/tex]
He then introduces the usual definition of time ordered products and goes on to expand [tex]U(t,t_0)[/tex]. I am not able to follow how he expanded the time ordered product of operators in the second-order term of the Taylor expansion of the exponential. In particular, I am unable to follow the limits being used and why both integrals are being put in the front. Should we not get product of two integrals involving HI?
The expansion of U(t,t_0) is given by
[tex]1 - i\int_{t_0}^{t}dt'H_I(t') + \frac{-i^2}{2}[\int_{t_0}^{t}dt'\int_{t'}^{t}dt''H_I(t'')H_I(t')+\int_{t_0}^{t}dt'\int_{t_0}^{t'}dt''H_I(t')H_I(t'')]+... [/tex]
Link to Course Page - [David Tong: Lectures on Quantum Field Theory][2] [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf
[2]: http://www.damtp.cam.ac.uk/user/tong/qft.html