|Bead Panel from a baby carrier, Ngaju people. Borneo 20th century, 41 x 26 cm. From the Teo Family collection, Kuching. Photograph by D Dunlop.|
The hypothesis of temporal isotropy is a presumption that almost all of the particles in a description have phase symmetry. That is, we assume that the quarks in nearly every particle can be divided into two sets that are distinguished from each other only by their phase. This condition is easily satisfied for protons and electrons. The hypothesis is useful because temporal isotropy implies that even if there is any confusion or mix-up about the phase, the overall description of a particle is unaffected. And if almost all particles share this symmetry, then we can greatly simplify analysis by usually ignoring the phase. The phase of an event can be interpreted as representing achromatic visual perceptions. So invoking this hypothesis, and disregarding the phase, is a way of objectifying a description of sensation; we stop describing events as black, white, bright or dark. A blind physicist who does not use the second hypothesis can nonetheless consider a particle with phase symmetry in much the same way as anyone else. And recall that if the Earth is used as a frame of reference, then phase indicates whether an event is diurnal or nocturnal. So ignoring the phase frees us from having to pay attention to whether events occur in the daytime or at night. The hypothesis implies that reversing the order of day and night does not affect a description, and that is why it is called the hypothesis of temporal isotropy.