“时间”存在吗?Does Time Exist?

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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 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 obviousof 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 travelling 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 backwards. 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? We currently have two sets of equations that govern physics.

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