I think that one thing to keep in mind is that we are only capable of observing the passage of time along one vector because our perception relies upon entropic biological processes.
We cannot observe in any subset of possible universes where entropy is not present or is working backwards-Ergo those possibilities are wholly out of our direct perception.
That does not mean that causality cannot run in reverse, however, only that we cannot interact with those mechanisms in a way that would preclude our existence or observation.
But could we observe the effects? Can there be particles which have mass and therefore exhibit gravitation but which are subject to reverse entropy? We would observe these for example as an unexplainable increase in gravitational pull on observable matter without an observable source of that gravitation and without a preceding cause.
Are photons themselves stratling this entropic boundary since they travel at the speed of light and within their own reference frame are not subject to the passage of time?
Wouldn't questioning those pre-existing notions be the first step towards constructing such a machine? It sounds silly to dismiss the idea time could go backwards because we don't have any machines that work that way, since that typically how all ideas work before we implement them.
We cannot observe in any subset of possible universes where entropy is not present or is working backwards-Ergo those possibilities are wholly out of our direct perception.
That does not mean that causality cannot run in reverse, however, only that we cannot interact with those mechanisms in a way that would preclude our existence or observation.