One of the biggest mysteries in modern physics is something that we experience literally every moment of our lives. Physics describes the world by means of formulas that tell us how things vary as a function of time, but it does not explain what time is. We can write formulas that tell us how things vary in relation to their position, or how the taste of a risotto varies as a function of the ‘variable quantity of butter’. Something about time is different, though. Time seems to flow, whereas the quantity of butter or its location in space does not flow. Where does the difference come from?
Another way of posing the problem is to ask: what is the present? We say that only the things of the present exist; the past no longer exists and the future doesn’t exist yet. But in physics there is nothing that corresponds to the notion of the ‘now’. Compare ‘now’ with ‘here’. ‘Here’ designates the place where a speaker is. For two different people, ‘here’ points to two different places. Consequently ‘here’ is a word whose meaning depends on where it is spoken. The technical term for this kind of utterance is ‘indexical’. ‘Now’ also points to the instant in which the word is uttered and is also classed as indexical.
No one would dream of saying that things ‘here’ exist, whereas things that are not ‘here’ do not exist. So then why do we say that things that are ‘now’ exist and that everything else doesn’t? Is the present something that is objective in the world, that flows, and that makes things exist one after the other, or is it only subjective, like ‘here’?
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