Time is something we experience every day, and it seems like something we’d be familiar with. But when you dig into it, it gets weird. So what exactly is it? And how does it work?

Saint Augustine, the Christian monk was once asked what is time, and his answer was, “If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.”

The ancient Egyptians measured time with obelisks that served as sundials, water clocks that flowed at a steady rate, and of course hourglasses.

It was the Babylonians that first divided the day into hours made up of 60 minutes each with 60 seconds per minute, this was all the way back in 1800 BCE.

It was the great Dutch astronomer and physicist Christiaan Huygens who invented the first pendulum clock in 1656, which used a weight that swung at specific intervals to keep time. This would be the most precise way to keep time for nearly 300 years.

1927 saw the invention of the quartz crystal clock, which uses the piezoelectric properties of quartz which vibrates at a rate of 32,768 Hz, giving it an accuracy of 6 parts per million.

But the most accurate measure of time we’ve been able to conceive so far is the atomic clock, which uses the natural resonance of a cesium atom.

Ss the scientific revolution began to take hold, the main argument around time was had by the Absolutists and Relationalists.

Absolutists believed in absolute time. The idea that time was an independent, absolute constant of the universe, that it wasn’t affected by our perception of it or by the interaction of matter.

Sir Isaac Newton was one of these Absolutists.

The Relationalists saw time as a measure of change. Basically arguing that the only reason time exists is because of the changing state of matter.

Newton’s second law of thermodynamics states that in a closed system, entropy will always increase over time. Entropy being a movement toward a state of equilibrium.

By the way, another really interesting way to look at entropy is through a statistical mechanics model, which was championed by Ludwig Boltzmann.

There’s a more philosophical debate on the nature of time that has to do with how it actually progresses, between what they call tensed and tenseless theories of time.

The tensed theory of time states that the future doesn’t exist yet, that it’s a tree with branches that spread as the now passes through the tree.

The tenseless theory of time suggest that the past, present, and future are all equally real and what we experience as time is just the now passing down the timeline.

Einstein stated in his 1905 paper on special relativity that the laws of physics and the speed of light must be the same for all uniformly moving observers. And for this to be true, space and time can no longer be independent. This was the only way for the speed of light to be constant for all observers.

But it was Minkowski three years later who came up with the idea that space and time could be seen as a single four-dimensional spacetime fabric. He closed out his paper on the subject by saying, “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality”.