"Because we live in a universe with a pronounced arrow of time, we treat the past and future not just as different from a practical perspective, but as deeply and fundamentally different things. The past has already happened, while the future is still up for grabs in some sense—we can sketch out alternative possibilities, but we don’t know which one is real. More particularly, when it comes to the past, we have recourse to memories and records of what happened. Our records may have varying degrees of reliability, but they fix the actuality of the past in a way that isn’t available when we contemplate the future.

Think of it this way: A loved one says, “I think we should change our vacation plans for next year. Instead of going to Cancún, let’s be adventurous and go to Rio.” You may or may not go along with the plan, but the strategy, should you choose to implement it, isn’t that hard to work out: you change plane reservations, book a new hotel, and so forth. But if your loved one says, “I think we should change our vacation plans for last year. Instead of having gone to Paris, let’s have been adventurous and have gone to Istanbul,” your strategy would be very different you’d think about taking him or her to the doctor, not rearranging your past travel plans. The past is gone, it’s in the books, there’s no way we can set about changing it. So it makes perfect sense to us to treat the past and future on completely differently.

That distinction between the fixedness of the past and the malleability of the future is nowhere to be found in the known laws of physics. The deep-down microscopic rules of nature run equally well forward or backward in time from any given situation. If you know the exact state of the universe, and all of the laws of physics, the future as well as the past is rigidly determined beyond John Calvin’s wildest dreams of predestination. The way to reconcile these beliefs—the past is once-and-for-all fixed, while the future can be changed, but the fundamental laws of physics are reversible - ultimately comes down to entropy. If we knew the precise
state of every particle in the universe, we could deduce the future as well as the past. But we don’t; we know something about the universe’s macroscopic characteristics, plus a few details here and there. With that information, we can predict certain broad-scale phenomena (the sun will rise tomorrow), but our knowledge is compatible with a wide spectrum of specific future occurrences. When it comes to the past, however, we have at our disposal our knowledge of the current macroscopic state of the universe, plus the fact that the early universe began in a low-entropy state. That one extra bit of information, known simply as the “past hypothesis,” gives us enormous leverage when it comes to reconstructing the past from the present.

The punch line is that our notion of free will, the ability to change the future by making choices in a way that is not available to us as far as the past is concerned, is only possible because the past has a low entropy and the future has a high entropy. The future seems open to us, while the past seems closed, even though the laws of physics treat them on an equal footing.

The major lesson of this overview of entropy and the arrow of time should be clear: the existence of the arrow of time is both a profound feature of the physical universe, and a pervasive ingredient of our everyday lives."