- #1
Frank Castle
- 580
- 23
In quantum field theory (QFT), the requirement that physics is always causal is implemented by the microcausality condition on commutators of observables ##\mathcal{O}(x)## and ##\mathcal{O}'(y)##, $$\left[\mathcal{O}(x),\mathcal{O}'(y)\right]=0$$ for spacelike separations. Intuitively, I've always understood this as the requirement that a measurement of the observable ##\mathcal{O}(x)## at the point ##x^{\mu}## cannot influence a measurement of the observable ##\mathcal{O}'(y)## at the point ##y^{\mu}## is the two points are spacelike separated.
Now, consider a scalar field ##\phi(x)##. Is it true that it always commutes with itself regardless of the spacetime separation? i.e. is $$\left[\phi(x),\phi(y)\right]=0 $$ true ##\forall\;x^{\mu},\,y^{\mu}##?
If so, how does one interpret this physically? Is it analogous to ordinary quantum mechanics (QM) in which the different position operators ##\hat{x}^{i}## all commute among themselves (since a particle can be in a simultaneous eigenstate of its 3-position)?
Now, consider a scalar field ##\phi(x)##. Is it true that it always commutes with itself regardless of the spacetime separation? i.e. is $$\left[\phi(x),\phi(y)\right]=0 $$ true ##\forall\;x^{\mu},\,y^{\mu}##?
If so, how does one interpret this physically? Is it analogous to ordinary quantum mechanics (QM) in which the different position operators ##\hat{x}^{i}## all commute among themselves (since a particle can be in a simultaneous eigenstate of its 3-position)?