Why Does Time Have a Direction? Sean Carroll on Spacetime and Entropy

For centuries, the world operated under the framework established by Isaac Newton, who viewed space and time as two separate, absolute stages where the universe's events played out. In this classical view, a mile is a mile and a second is a second for everyone, regardless of where they are or how fast they are moving. This classical mechanics was so successful that it allowed humanity to predict planetary motions and eventually land rockets on the moon. It provided a sense of a 'preferred' standard of rest that most people still intuitively feel today.

However, this comfort was disrupted in the 1800s by the study of electromagnetism. Scientists like James Clerk Maxwell, building on the work of Faraday and Ampere, discovered that light travels at a constant speed. This created a logical crisis: if Newton was right that velocities are relative, then the speed of light should change depending on the observer's motion. Yet, Maxwell's equations insisted it was a constant of nature. This discrepancy meant that either our understanding of light was wrong, or our understanding of space and time was fundamentally flawed.

In 1905, Albert Einstein resolved the light-speed paradox by proposing that the speed of light is indeed constant, but at the cost of abandoning absolute time. Two years later, his former professor Hermann Minkowski realized that the mathematical implication was that space and time are no longer separate. They are unified into a four-dimensional manifold known as spacetime. Different observers moving at different speeds are simply 'slicing' this four-dimensional block in different ways, leading to differences in what they perceive as 'now'.

The unification of space and time means that time is more like a distance traveled than a universal clock. Just as two people taking different routes between two cities will cover different distances, two observers taking different paths through spacetime will experience different amounts of elapsed time. This is famously illustrated by the Twin Paradox, where a space-traveling twin returns younger than their stationary sibling. This isn't a trick of the clock; it is a fundamental property of the path taken through the universe.