The very fact that space is mostly dark and empty, with a speckling of hot bright stars that radiate outwards , is blatantly time-asymmetric, so the time arrow of radiation appears to be cosmological in origin. This fact about the universe is crucial to life as we know it, since all life on earth is powered by the outgoing radiation of the sun, and the earth in turn dumps its waste heat into the blackness of space. Here one wants to imagine oneself, the experimenter, as being able to do whatever one wants with the source , and see what it does to the field . There is only the need to choose the state that best matches what we observe. The time arrow of thermodynamics is perhaps the most famous aspect of time reversal symmetry - so I will treat it very briefly here.
Disorder seems to be always on the increase.
That is, it expresses a time asymmetry of the state of a system, rather than an asymmetry in the dynamical laws. It is also worth noting that the vast majority of states typically have quite large and . As with the time arrow of radiation, in the last analysis it appears to be nothing but a raw experimental fact that the entropy of our universe is increasing. In a sense this is not surprising, because pondering chemistry and biology a bit it becomes apparent that life as we know it requires the entropy to be changing monotonically, rather than staying about the same. Processes like remembering and planning, which define the psychological notions of future and past, are only able to occur in the direction of increasing entropy.