The time-reverse of any causal theory is eternal noise

Understand

We consider a very general class of theories, process theories, which capture the underlying structure common to most theories of physics as we understand them today (be they established, toy or speculative theories).

• Amongst these theories, we will be focusing on those which are `causal', in the sense that they are intrinsically compatible with the causal structure of space-time -- as required by relativity.
• We demonstrate that there is a sharp contrast between these theories and the corresponding time-reversed theories, where time is taken to flow backwards from the future to the past.
• While the former typically feature a rich gamut of allowed states, the latter only allow for a single state: eternal noise.