We can simply derive KG equation from Dirac equation. More generally ,even the Bargmann-Wigner equation which describes the higher spins can also lead to KG equation, but the fai function in the equation are totally different which result in the corresponding spin quantum numbers.
Thanks for your reply. I’ve read the paper many times, but I still can’t understand the main idea. There are many assumptions just as you quoted which I think improper either. If QFT is neither an orthodox quantum theory nor a field theory, everything we have done seems meaningless.
I also...
How should I understand the procedure of canonical quantization in quantum field theory. Do we really quantize the field by regarding the field as dynamics variables ?What’s the physical essence of quantization?