Let's say that we have 2 tennis balls in space, one being in motion (say, pushed by an astronaut), and the other one still. If we could take a snapshot of both tennis balls, would there be any evidence that could suggest that one is moving and the other one is still? Is there anything happening,...

For example, if a ball is attached to a string and released from a vertical height and then pivots around a point to initiate circular motion, tension is equal to centripetal motion. If, on the other hand, a ball hands from a string and it’s hit in such a way that it travels in a vertical circle...

Let $\mathcal E\in\mathrm{T}(\mathcal X,\mathcal Y)$ be a quantum channel (i.e. a completely positive, trace-preserving linear map sending states in $\mathrm{Lin}(\mathcal X,\mathcal X)$ into states in $\mathrm{Lin}(\mathcal Y,\mathcal Y)$). It is well known that any such map can be written in t...