This video by A Capella Science (surprisingly not One Direction) touches on many key implications of the “Arrow of Time” predicted by the laws of thermodynamics but still not totally understood:
The 2nd Law of Thermodynamics can be stated in several ways, but the most intuitive formulation is that we can immediate discern that this video is being played backwards, since it shows many processes that we know from everyday experience only happen in one direction, like scrambling an egg, dissolving a sugar cube, or burning a match. Hot water and cold water mix together to produce warm water – you can wait as long as you want watching a puddle of warm water, the reverse simply won’t happen.
The basic idea is that, in a closed system, the amount of disorder – the entropy – can only increase or, at best, stay the same. This defines an “arrow” of entropic time that points from an era of lower disorder to higher disorder. But many deeper implications from Cosmology are mentioned in the video, particularly those popularized by Sean Carroll, like in this video
And in his book From Eternity To Here: The Quest for the Ultimate Theory of Time
For example, as natural as it may seem for some processes to be irreversible, the laws of nature are, as best as we can tell, don’t distinguish between forward and backward in time. How can irrreversibility emerge from time-symmetric equations? And how did we end up with an mostly ordered Universe it the first place? That is, even though ceramic shards will not spontaneously jump back onto a table and reform a cup, at least we started with a intact cup in the first place. We live in an era of very low entropy, all in all.
One possibility put forward is that our Universe is just a rather large random fluctuation out of equilibrium. While very unlikely, if you postulate an eternal Universe, everything will happen if you wait long enough. The argument against this is the (rather unsettling) idea of Boltzmann brains. That is, if everything experienced is just a passing fluctuation, it is much more probable that a conscious observer would just be a single brain.
As mentioned in the video, Sean Carroll suggests that a better theory involves multiple big-bangs, each creating a new Universe with its own arrow of time. At the beginning, the entropy is incredibly low, and increases as time evolves.
So how was the video make? *Spoiler Alert* Someone spent a long time learning to sing backwards.