1,641 reputation
525
bio website
location
age 29
visits member for 2 years, 1 month
seen Aug 25 at 8:54

As time goes on, I grow more disillusioned with quantum field theory.


2d
awarded  Popular Question
Aug
12
answered Why can the Euler beta function be interpreted as a scattering amplitude?
Aug
9
answered Lagrangian and grassmann numbers
Aug
9
comment Lagrangian and grassmann numbers
Is $\Psi_R$ a two-component spinor like you say in the first paragraph, or is it a four-component one, as implied by how your $C$ is represented in terms of 4-dimensional gamma matrices?
Aug
9
comment A question about the asymptotic series in perturbative expansion in QFT
An asymptotic series happens when you expand a function around a singular point (that's why it has zero radius of convergence). The point of zero coupling is usually a branch point, which is singular. The reason for the appearance of the branch point is that for negative coupling the vacuum is unstable because the vacuum will create pairs of particles and antiparticles and "they will fly apart" forever. This leads to an imaginary part (i.e. a branch cut) in correlation functions. That's why (in your language) "things flying apart will lead to asymptotic series."
Aug
8
comment Distance between Newton's Rings fringes is does not seem linear
I wonder how you discovered this... do you have a picture?
Jul
30
answered Does spin-0 or spin-2 describe massive or massless particles?
Jul
15
comment How to replace $T$-product with retarded commutator in LSZ formula?
I believe there is a related mistake: in integrating by parts, you didn't include the surface terms. Especially important is from the time variable e.g. $\langle\varphi^\dagger(y)\varphi(x)\rangle\big|_{x^0=-\infty}^{x^0=+\infty} = \langle\varphi^\dagger(y)\big(\varphi_\text{out}(x)-\varphi_\text{in}(x)\big) \rangle$. Neglecting it, you can show all amplitudes vanish. See the remark on page 206, line 9-10 of Itzykson and Zuber.
Jul
15
comment How to replace $T$-product with retarded commutator in LSZ formula?
Thanks! I am working through the details, but can’t understand why $q_1^0$ and $\omega_{q,A}$ etc are distinguished. They must be equal since $A$ is an external particle on its mass shell.
Jul
14
revised How to replace $T$-product with retarded commutator in LSZ formula?
set equation number in tag.
Jul
12
revised Electrostatics with Yukawa mass
Corrected a typo in the last equation.
Jul
12
comment Electrostatics with Yukawa mass
Yes that's right, thanks! This final piece of information implies $C=-\big[\rho e^{-\mu R} (\mu R+1)\big]/\big[2 \mu^3\big]$.
Jul
12
asked Electrostatics with Yukawa mass
Jul
11
comment Fourier expansion of the Klein-Gordon field
Also, it is easiest to quantize decoupled oscillators. By going to Fourier space, all the modes are decoupled, and the expansion coefficients are easily seen to satisfy the standard SHO algebra.
Jul
10
awarded  Yearling
Jul
9
revised How to replace $T$-product with retarded commutator in LSZ formula?
edited body
Jul
9
comment How to replace $T$-product with retarded commutator in LSZ formula?
This can't be right. Firstly, I don't understand why you say that in my case I have $x^0 < y^0$. This ordering was not simply chosen on a whim; it is somehow fixed by the fact that $q_1$ and $q_2$ are in the forward light-cone, but the reason eludes me. Secondly, the fields appearing in the reduction formula eq 5-169 above are full Heisenberg-picture fields. I have a problem with your use of Wick's theorem in that they only apply to interaction-picture fields (where they evolve according to the free part of the Hamiltonian), and is applicable in perturbation theory exclusively.
Jul
8
comment Why Are Even and Odd Regge Trajectories Degenerate?
possible duplicate of Why are the even and odd Regge trajectories degenerate?
Jul
7
revised How to replace $T$-product with retarded commutator in LSZ formula?
fixed gramatical error
Jul
6
accepted Nuclear Compton Scattering Data