There are reliably recurring events which are nonetheless distributed within very rough cycles. Take the ‘chrons’ of a half-million years, give or take a few hundred thousand, between the earth’s geomagnetic reversal, or the random but steady bursts of radioactive decay picked up by a Geiger counter. Then there are other events produced by stable systems which have regular periodicity. The even repetition of any time-based event of which we are consciously aware can be (but is certainly not always) perceived as a pulse. Two occurrences of identical or sufficiently similar things are all we need to raise our suspicions of the existence of a pulse: the brain goes ‘I just heard/felt/saw one of those!’ However, we need three occurrences to confirm it, since this creates two successive instances of an even-length gap. Despite the beat-event itself being the most consciously salient aspect of pulse, it’s the evenness of the gaps between those beats that confirm its existence. The gaps can be automatically compared and found symmetrical, because they have duration; sometimes the material content of the beat itself – take a snare drum hit in a dead room, for example – has negligible length. Additionally, that content may vary considerably in nature: a snare drum followed by a duck quack followed by a lonesome whistle will still be perceived as a pulse, if the gaps between them are of even length. Through pulse is borne ‘the passage of time’ – time, in its perceptual incarnation,1 being a phenomenon which is structured by the measurement of constant lengths. Through these fixed durations we both count our years, weeks, minutes and seconds, and organise their content.
These repeated beats can be very close, like a ticks of a clock or the drilling of a woodpecker, or – given enough salience – separated by hundreds or thousands of years, like the perigees of a comet. The limit of our auditory perception of a pulse, however, is around 10Hz or 600bpm, above which the beats are not heard as discrete events but as a continuous drone. This can be tested by twanging a ruler at the edge of a table, and lengthening it until the sound is low enough to hear the individual beats of which the notes are composed. If you were to keep going, and your ruler was thirty feet long, you would hear a different effect: any beats slower than 0.5Hz or 30bpm are hard to experience as a pulse at all without great conscious attention. I have no idea if it indicates something more fundamentally literal, but 30bpm also happens to coincide with the slowest viable human heartbeat. (The cyclist Miguel Indurain seems to hold the record for the slowest healthy human heart – at 29bpm.) So while the phenomenon of pulse is vast and various, its perceived aural bandwidth is considerably smaller, and roughly between the frequencies of a tenth of a second and a couple of seconds.
1 Some renegade physicists such as Julian Barbour feel this is time’s only existence – and that its place within physical law is apparitional; it is merely an illusion created by change, and our ability to recall and predict it. It may indeed be that time is a mere structure born of human intelligence – like mathematics – that we use to measure, sequence and compare events. But Mark Tegmark and others have argued eloquently that the external physical reality experienced by conscious beings in the cosmos rides on an underlying and verifiable mathematical reality; perhaps this place is similarly constituted by a temporal one. However, whether it is a ‘dimension’ or not (i.e. a thing ‘in which’ events can flow and things can move) it is patently unlike any other. Either way we appear to be too strongly composed of it to gain much perspective on its nature, deathbeds excepted. (The Zen Buddhist tradition of the ‘death poem’ explicitly takes advantage of this unique if fleeting prospect.)