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What is "now"?
It is an idea that physics treats as a mere illusion, yet it is something we are all familiar with. We tend to think of it as this current instant, a moment with no duration. But if now were timeless, we wouldn't experience a succession of nows as time passing. Neither would we be able to perceive things like motion. We couldn't operate in the world if the present had no duration. So how long is it?
That sounds like a metaphysical question, but neuroscientists and psychologists have an answer.
In recent years, they have amassed evidence indicating that now lasts on average between 2 and 3 seconds. This is the now you are aware of - the window within which your brain fuses what you are experiencing into a "psychological present". It is surprisingly long. But that's just the beginning of the weirdness. There is also evidence that the now you experience is made up of a jumble of mini subconscious nows and that your brain is choosy about what events it admits into your nows. Different parts of the brain measure now in different ways. What's more, the window of perceived now can expand in some circumstances and contract in others.
Now is clearly a slippery concept. Nevertheless, it would be good to pin it down because it could tell us something about the bigger picture of how the brain tracks time. Not just that, the perception of the present is also crucial to how we experience the world. If events appear simultaneous when they aren't, that has implications for our understanding of what causes what. "Your sense of nowness underpins your entire conscious experience," says Marc Wittmann at the Institute for Frontier Areas of Psychology and Mental Health in Freiburg, Germany. Understanding now even helps us address the question of whether we have free will.
We have long known that
the brain contains structures that use cycles of light and dark to set its daily clock. How it tracks the passing of seconds and minutes is much less well understood. At this level, there are two broad types of timing mechanism, an implicit and an explicit one. The explicit one relates to how we judge duration -
something we're surprisingly good at. The implicit mechanism is the timing of "now" - it is how the brain defines a psychological moment and so structures our conscious experience.
Comment: See also: The illusion of time: Carlo Rovelli's book 'The Order of Time' posits that reality is simply a complex network of events