Maybe this is trivial, but the action of a conformally coupled scalar is $$ S = \frac{1}{2} \int d^Dx \sqrt{g} (g^{ab} \partial_a \phi \partial_b \phi + \xi R \phi^2),$$ where $\xi = (D-1)/4D$. Does this imply that... ... a theory of conformally coupled scalars is always massive (i.e., there exi...

I've had this question in mind for a while, but this recent post reminded me to ask. The post is interesting because it demonstrates how a very naive application of labeling states (without understanding unitary time evolution) with braket notation can lead to false conclusions. My question is, h...

I noticed that whenever I bunch up aluminum foil (into a ball), it becomes extremely hard to compress. If I use another piece of the same amount of aluminum foil, and keep folding it in, I arrive at a much smaller volume. It is full of air and much lighter than the same shape of solid aluminum, ...

I was told and convinced that gravity is not a force, and in free fall you're an inertial frame and experience no force, and when on the surface of Earth you would be accelerating upwards. What I admit I can't understand is Earth accelerating, why so? what property of mass explains this? Is there...