I'm trying to solve this exercise:
Suppose an arbitrary theory (Flat space-time?) with a single field (Is a scalar field?) invariant under dilations, i.e.
$x\mapsto b x$ and $\phi \mapsto \phi$. Show that the stress energy
tensor is traceless.
Writing the transformations as $x\mapsto e^\theta x$, $\phi\mapsto e^{\omega\theta}\phi$, and $\partial_\mu\phi\mapsto e^{(\omega-1)\theta}\partial_\mu\phi$, for $\omega=0$.
I get the variations as $\delta x_\mu=\theta x_\mu$, $\delta\phi=\omega\theta\phi=0$, and $\partial_\mu\phi=(\omega-1)\theta\partial_\mu \phi=-\theta\partial_\mu\phi$; and
I've tried to get some useful expression bassed on the variation of lagrangian:
$$
\delta L=\frac{\partial L}{\partial\phi}\delta \phi+\frac{\partial L}{\partial(\partial_\mu\phi)}\delta({\partial_\mu\phi})=-\theta\frac{\partial L}{\partial(\partial_\mu\phi)}\partial_\mu\phi.
$$
On the other hand, the trace of the tensor takes the form
$$
T_\mu^\mu=\eta_{\mu\nu}T^{\mu\nu}=\eta_{\mu\nu}(-\eta^{\mu\nu}L+\frac{\partial L}{\partial(\partial_\mu\phi)}\partial^\nu\phi)=-2L+\frac{\partial L}{\partial(\partial_\mu\phi)}\partial_\mu\phi
$$
Thus,
$$
T_\mu^\mu=-2L-\frac{\delta L}{\theta}.
$$
Obviously, if the lagrangian is invariant then the Tensor isn't traceless. So I don't have idea how to proceed.
Best Answer
"Arbitrary theory" probably means
So basically use equations that are general and apply to anything within the Lagrangian formalism.
For instance, the stress energy tensor can be generally written as:
$$ T_{\mu\nu} = \frac{-2}{\sqrt{-g}}\frac{\delta S}{\delta g^{\mu\nu}},$$
where $S$ is the action.
Scaling transformations are a special case of conformal transformations where $$ \delta g^{\mu\nu} = \epsilon g^{\mu\nu},$$ in your specific example $\epsilon = b^2$.
Inverting the formula to single out the variation of the action: $$ \delta S \propto T_{\mu\nu}\delta g^{\mu\nu} = \epsilon T_{\mu\nu} g^{\mu\nu} = T^\mu_\mu,$$
where the last step is the trace!
Since the action must be minimised, $\delta S =0$, you must have $T^\mu_\mu=0$, i.e. a traceless stress-energy tensor.