#722 Why does time go forward not backward?

Why does time go forward not backward? Because of entropy.

Entropy is the measure of how random and dispersed the energy and mass in a system are distributed. It is described by the Second Law of Thermodynamics, which says that the entropy of a closed system increases over time. The system has to be closed because more matter entering from outside the system could affect entropy. Our universe, despite its size, is a closed system.

An apple spoiling and then rotting away on your kitchen counter is a good example of entropy. The apple is made entirely of atoms that comprise the different chemicals and structures in it. Those atoms have the right amount of energy to be where they are. However, if you leave the apple out on your counter, those atoms that are held together in the shape of the apple, start to lose energy to heat and some of them become gases, escaping the apple. Slowly, the molecules are released and the apple becomes dust. No energy has been lost because you cannot create or destroy energy, but the energy has been dispersed. Our universe couldn’t exist without entropy. There would be no chemical reactions, no heat transfer, and no changes of state. Things wouldn’t grow, change, or die.

And this is the reason why time goes forward and not backward. Entropy can only go in one direction. Heat energy will only ever move from a hot area to a cold area. It will not go the other way. If you have a bowl of hot water in a room at an average temperature, the heat energy will move from the bowl of water into the air. The opposite will not happen. The bowl of hot water will not take heat energy out of the air and become even hotter. That cannot happen. Entropy only goes in one direction. A good analogy is to get a pot of green paint and a pot of yellow paint and to mix them. That is entropy. The mixed paint will never unmix itself and become separate colors again, no matter how long you wait.

Entropy is moving through our universe in one direction only. We are moving from the state of low entropy that existed at the big bang to a state of high entropy that will exist at the end of the universe. The only way to reverse entropy is to add more energy into the system and this can’t be done in a closed system. At the very end of the universe all of the atoms will have equal energy and they will be spread out so far that there will be no interaction between them and no motion. The universe will have died. And, at that point, there would technically be no time because there would be no entropy.

So, the fact that entropy cannot go the other way is the reason why time goes forward rather than backward. However, if you look at most physical laws, they are time reversible. That means they work in any direction through time. For example, the attraction of gravity works the same if time is going forward and if it is going backward. In fact, all of the laws of physics, except for the Second Law of Thermodynamics, would work exactly the same if the universe ran backward. Einstein said that time is an illusion that moves relative to the observer. The closer to the speed of light you can travel, the slower time moves, until, theoretically, time stops when you are at light speed. Maybe at that point time is moving neither forward nor backward.

Also, if you watched a video of the motion of individual atoms vibrating, buzzing around, and colliding with each other, you would have no way of telling if the video was being played forward or backward. You could follow one molecule all the way to the heat death at the end of the universe or all the way back to just after the big bang and at no point along that line would you know which way through time you were going. It gets even more complicated when you get smaller. Aa discovery in 2022 showed that when you get down to the quantum level, quantum systems appear to be able to move forward and backward through time. So, either they are not bound by entropy, or the idea of time doesn’t work at such a small level. The only way to see that time is going forward is to zoom out and see the whole thing. And this is what I learned today.

Photo by Mat Brown: https://www.pexels.com/photo/close-up-photography-of-wristwatch-1034063/

Sources

https://www.bbc.com/future/article/20221003-why-does-time-go-forwards-not-backwards

eli5 the physics thing about how if you put an apple inside of a box that won’t let absolutely anything through, and you give it infinite time in the box how does the apple decay? isn’t even a little energy needed for hadrons to decay? also would it have a finite amount of “infinite” energy
byu/Piggy468 inexplainlikeimfive