How do we know time exists and isn’t just an imaginary measurement?
The simple answer is we don’t. We cannot prove the concept of time at all, but nothing makes sense without it do we fudge stuff to apply time to science.
At quantum levels time is even excluded as a variable in equations. There was even a chap called Rovelli who published articles about how time does not exist at all and its merely human perception.
However, from a practical perspective try and spend a week without acknowledging time as a thing and you’ll come unstuck pretty fast.
It *is* an imaginary measurement, all measurements are. They’re arbitrary words we assigned to concepts so that we can communicate from one person to another, “Yes, I am referring to this specific amount of this specific thing.” Inches, pounds, minutes, miles, they don’t “exist” on their own. It’s language as tools.
This question is often asked as a thought exercise in philosophy classes as well because we can’t prove it even though we all live by the same convention that we think of as time.
If you’re talking specifically for physics, then “time”, as in a direction of development, is an equivalence to both causality and to the limited (non-infinite) speed of light. Because information can only travel at a fixed speed, and we observe that things do in fact travel, this necessitates that events have an “order” (although that order itself is subjective).
We can then use thermodynamics to define the positive direction of time to be the direction of increasing entropy, which we use because that also coincides with how we mentally interpret time.
We don’t, not in a “we are absolutely sure and can prove it” sort of way. What we do know is that we seem to have both a personal and shared experience of time, and (at least at the macro or emergent level) our physical laws incorporate time. Ultimately, that’s about it. There’s nothing we can touch or point to and say, “See that? That right there is ‘time’.”
But go one step further, how do we know ANYTHING exists and isn’t imaginary beyond our personal and apparently shared experiences? And ultimately even if nothing “exists” independent of our/your imagining of it, isn’t that still a form of existence? My imagination exists, my thoughts exist, maybe that’s all there is?