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The Order of Time

A trip through time with a leading theoretical physicist

By Carlo Rovelli
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The Order of Time by Carlo Rovelli

The Order of Time (2017) unpacks the latest research in physics to turn our everyday concept of time on its head. What we perceive and experience as a linear movement, from past to present and into the future, is little more than a trick of the mind. The reality, Carlo Rovelli shows, is a whole lot more interesting and bizarre.

Key idea 1 of 8

Time doesn’t move uniformly and it is fundamentally related to heat.

Appearances can be deceptive. That’s one of the first lessons of the sciences. After all, if you trust your eyes alone, you’re likely to end up believing the world is flat. The same goes for time. In everyday life, we see time as a uniform forward movement – something that just happens, like the ticking of an eternal clock entirely beyond our control. But that’s a false assumption.

In reality, time passes at different speeds in different places. Compare two clocks, where one is placed at sea level and the other high up in the mountains, and you’ll find the latter runs faster. Placing one clock on the floor and another on a table has the same effect: the differences are minuscule, and you’d need a precision timepiece to prove it, but the second timepiece will always run faster.

It’s not just time that slows down when measured at a lower level – all processes do. Take a simple thought experiment – two friends of the same age part company. One of them goes to live on a beach and the other at the top of a mountain. Years later, they meet. The result? The mountain-dweller will have aged more and lived longer than his pal from the flatlands. Even his houseplants will have grown more!

It sounds impossible, but there’s no such thing as one objective or “true” measure of time that can be applied both in the mountains and at sea level. That’s because times are relative to one another. Each point on a map has its own time. That, to put it in simple terms, was the central insight of Albert Einstein’s theory of general relativity.

As if that weren’t baffling enough, heat also plays a part in this dynamic. In fact, heat and time share a fundamental similarity – they can both only travel in one direction. Time moves from past to future, while heat always moves from hotter to colder objects.

In both cases, reversing that movement is impossible. But here’s where things get really interesting – we can only tell the past and future apart because of heat.

Let’s break that down. The past is distinguished from the future by change. But change is only possible if there’s motion. And if you really get down to it, motion is simply heat – the movement of molecules at a microscopic level. Without heat, in other words, nothing would move, and past, present and future would be little more than an indistinguishable mass!

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