There's nothing especially mysterious about a glass of water. 10 thousand trillion or so water molecules, vibrating like crazy, dancing around on the smallest of scales, but contained by the walls of the glass, a little bit of gravity, and some hydrogen bonding.
It fits our expectations about the universe very nicely. And there's nothing surprising about watching a drop of ink swirl and spread through the fluid, dissipating from a cloud of color for an even hue. It's only the manner in which we expect things to go.
If we dripped that drop 100, 1000, or even a million times, we'd not only expect to see the same thing happen. The same thing would happen.
Probably. Thing is, there's nothing about the laws of physics that govern how all those molecules move around that prevents the whole thing running in reverse.
Then what makes this moment any different from the past? We realize that today differs from yesterday. But why? Physicists say that the universe tends toward disorder. An excellent arrow of time points in the direction of a future in which entropy is greater than it was in the past.
Probably. This is the 2nd law of thermo dynamics. Just, it is not a law like Newton's gravity. It's a law of probability. Flip a coin a million times, and will you ever get a million heads? No. But could you? Sure. There's no law of mathematics that says it can't happen. There's just so many more ways for it not to happen. Sure, we may get ten heads in a row, or maybe even 100 along the way. But it all evens out over time. This is the reason that bombs don't unexplode. It's the reason that we have waterfalls, and not water-ups. And thankfully for you and me, this is the reason that all the air in the room doesn't suddenly decide to hang out in one corner. The reason today is different from yesterday, and tomorrow will be different than today, is that since the very beginning, since the Big Bang itself, everything has been getting messier.
We know that entropy increases. We know that times moves in one direction. Probably. But WHY was the universe ever ordered to begin with? Why was entropy so small at the beginning? Why is it, 13.7 billion years ago, the universe could flip a coin one million heads in a row? We don't know. It's one of the great unsolved mysteries in physics. When we solve it, we'll finally have an answer for one of the strangest questions in science. It's strange because we all already intuitively know the answer. The universe matches our expectations simply because our expectations were written by the universe. Will we ever solve the mystery of order? Who knows? Tomorrow's another day, different from all the rest, and full of surprises. Probably.