The Nature of Time: Understanding the Past, Present, and Future

Time, the word “T-I-M-E,” is the most used noun in the English language. We can’t get through the day without talking about it all the time. But what is time, really? And how do we understand its properties, such as the past, present, and future? These are questions that have puzzled philosophers and scientists for centuries.

One of the most noticeable features of time is that it has a direction, a difference between the past and future. Sometimes we think about this as just an intrinsic feature of reality – the past has already happened, it’s in the books, the future is up for grabs, and the present is where we live. But along comes physics, and what people notice about our best theories of physics is that these theories do not distinguish between the past and the future. This is a phenomenon known as the “time symmetry” of physical laws.

But in our everyday lives, nothing is more obvious than the direction of time. It really requires a bit of mental discipline to say, “Well, time could exist without an arrow.” One way of thinking about this is that there is no intrinsic arrow of space, but there’s still space. We live in a three-dimensional world – up, down, left, right, forward, backward – and at the level of the fundamental laws of physics, there’s no special direction in space. And how you perceive that is imagine you’re an astronaut: you’re flying around in your little spacesuit. There wouldn’t be any difference between any direction you could look. There’s no experiment you could do in physics that would point out a direction in the universe, but space still exists. Likewise, time would still exist even if there wasn’t an arrow.

But here on Earth, we do have an arrow of time. If I pick up a coffee cup and let it go, it will always fall down. There’s clearly a distinction between up and down. No one is tempted to think that’s a fundamental feature of the Universe. It’s not because “downness” is embedded in the laws of physics. It’s because we live in the vicinity of an influential object – the Earth. The arrow of time is exactly the same way. We in our everyday lives, perceive an arrow of time because we live in the aftermath of an influential event: the Big Bang.

And this brings us to the concept of “entropy.” Entropy is a measure of how messy, disorganized, and random a system is. When things are nice and neat and tidy, they are low entropy. When they’re all messy and all over the place, they’re high entropy. And there’s a natural tendency of things in the Universe to go from low entropy to high entropy. This is called the “second law of thermodynamics.”

The real question is, why was the world ever low entropy to begin with? Why was the world lower entropy yesterday than it is today? The explanation is not completely satisfying, to be honest. The explanation is the following: because it was even lower entropy the day before yesterday. And why was the Universe even lower entropy the day before yesterday? Because it was even lower entropy the day before that. And this chain of reasoning goes back 14 billion years to the Big Bang, to the origin of our observable universe; in a hot, dense state, a very low-entropy state, and the Universe has been increasing in entropy ever since. And this is called the “Past hypothesis” by philosophers – David Albert, who’s a philosopher of physics, gave it this name.

So now we say, “If you know that the world is full of diversity and beauty. From the vast expanses of the ocean to the towering peaks of the mountains, there is always something new and exciting to discover. The natural world is a constant source of inspiration and wonder, and it never fails to amaze me how much there is to explore and learn. Whether you’re fascinated by the intricacies of a rainforest or the majesty of the night sky, there is always something to discover and appreciate. And it’s not just the physical world that is full of diversity and beauty – the same can be said for the people and cultures that populate it. Each and every individual is unique and special in their own way, and it’s a privilege to be able to learn from and appreciate the different perspectives and experiences that they bring to the table. In short, the world is a vast and endlessly fascinating place, and there is always more to explore and discover.