The first question is “What is entropy?” One answer I found on the internet was “The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity.” In other words, matter overall tends to become more chaotic over time. But what has that got to do with the direction of time?
Certainty
The universe has a different level of certainty of events forwards and backwards. Going forward, everything is very uncertain. You can predict the future but there’s only a very small chance of you being right. Meanwhile going backwards is much more certain - you can memories and evidence that events have occurred. The reason for this is entropy. As we move forwards in time, things tend to become more chaotic. If you have a fire you can safely say the atoms in the fire won’t spontaneously combine to cause a book to fly out of the fire. If there is something complicated and structured, there was a cause to it, and you can probably work out what that cause is.
When looking at the future however, any cause has a large number of possible results. It’s easier to work back from the result to that cause than the cause to the result. You can see this in the butterfly effect, where a flap of a butterfly’s wings causes the future to change drastically, but you never know what that change will be. This means at any point in the present, the past is much clearer than the future.
Evolution
As a species, we evolved through natural selection. Random patterns come together and those that survive the longest are more noticable, and may evolve into other species. So how did the concept of memory and the idea of action in one specific direction of time come from? Species that try to keep themselves alive and reproduce will be the ones that survive. So species that can take actions to increase the chance of surviving long will also survive. Let’s say it cools down before it rains, and the species is vunerable to water. They would sense the temperature drop, and move to a place they won’t be hit by rain. As a result, they are not killed by the rain.
This creature stays alive by seeing a change in its surroundings, and taking an action. But why isn’t there another species which does this all backwards? Couldn’t natural selection give rise to a species who sees events occur, and works backwards? Let’s say there was an organism vunerable to cold, who will sense it rains, and move out of the way to a warm place before the rain stops and it becomes cold. You could indeed have such an organism. But then, at the end of that it’ll be in the warm place. Why is it in the warm place?
When moving backwards, the future is extremely predictable and orderly. Sand will turn into sandcastles and books will appear from fires. This backwards species must have some extremely predictable result after going into the warm place, such as being cold. But in reality it’s completely unpredictable. Furthermore, the species that can survive the longest will not be the species that can ensure it stays alive, but the species that can completely explain it’s existance as a result of the Big Bang. As the goal of life is so different going backwards, natural selection and the whole process of action and reaction is exclusive to forwards time.
Conclusion
The reason we percieve time in the direction we do is entirely because our actions have unpredictable consequences, while the past is almost completely predictable. And this is a result of entropy. Forwards, the universe becomes more chaotic meaning anything can happen, while backwards, events must lead perfectly back to the Big Bang. Entropy defines both the direction of time, and our ability to make decisions at all.
