The earliest time measurements were observations of cycles of the natural world, using patterns of changes from day to night and season to season to build calendars. More precise time-keeping, like sundials and mechanical clocks, eventually came along to put the time in more convenient boxes.
But what exactly is it that we’re measuring?
Is time something that physically exists, or is it just in our heads? At first, the answer seems obvious—of course, time exists; it constantly unfolds all around us, and it’s hard to imagine the universe without it. But our understanding of time started getting complicated thanks to Einstein.
His theory of relativity tells us that time passes for everyone but doesn’t always pass at the same rate for people in different situations, like those traveling close to the speed of light or orbiting a supermassive black hole.
Einstein resolved the malleability of time by combining it with space to define space-time, which can bend, but behaves in consistent, predictable ways Einstein’s theory seemed to confirm that time is woven into the very fabric of the universe. But there’s a big question it didn’t fully resolve: why is it we can move through space in any direction, but through time in only one?
No matter what we do, the past is always, stubbornly, behind us. This is called the arrow of time. When a drop of food coloring is dropped into a glass of water, we instinctively know that the coloring will drift out from the drop, eventually filling the glass. Imagine watching the opposite happen. Here, we’d recognize time as unfolding backward. We live in a universe where the food coloring spreads out in the water, not a universe where it collects together.
In physics, this is described by the Second Law of Thermodynamics, which says that systems will gain disorder, or entropy, over time. Systems in our universe move from order to disorder, and it is that property of the universe that defines the direction of time’s arrow. So if time is such a fundamental property, it should be in our most fundamental equations describing the universe, right?
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We currently have two sets of equations that govern physics. General relativity describes the behavior of very large things, while quantum physics explains the very small. One of the biggest goals in theoretical physics over the last half-century has been reconciling the two into one fundamental “theory of everything."
There have been many attempts—none yet has proven—and they treat time in different ways. Oddly enough, one contender called the Wheeler-DeWitt equation doesn’t include time at all. Like all current theories of everything, that equation is speculative. But as a thought experiment, if it or a similarly time-starved equation turned out to be true, would that mean that time doesn’t exist, at the most fundamental level?
Could time just be some sort of illusion generated by the limitations of the way we perceive the universe? We don’t yet know, but maybe that’s the wrong way of thinking about it. Instead of asking if time exists as a fundamental property, maybe it could exist as an emergent one. Emergent properties are things that don’t exist in individual pieces of a system but do exist for the system as a whole. Each individual water molecule doesn’t have a tide, but the whole ocean does.
A movie creates change through time by using a series of still images that appear to have a fluid, continuous change between them. Lipping through the images fast enough, our brains perceive the passage of time from the sequence of still images. No individual frame of the movie changes or contains the passage of time, but it’s a property that comes out of how the pieces are strung together.
The movement is real, yet also an illusion. Could the physics of time somehow be a similar illusion? Physicists are still exploring these and other questions; we’re far from a complete explanation. At least for the moment.