Entropy explained, with sheep (2016)

https://www.youtube.com/watch?v=caGX6PoVneU

What part of our understanding does it contradict? The second law of thermodynamics says that entropy increases with time; this seems entirely consistent with a low-entropy past.

One explanation for why the universe began in a low entropy state is that that state has a very low description length. (This is a bit of a truism, since description length is a measure of entropy). But basically, let's just imagine that the universe is a simulation, with an initial state described by an initialization routine that sets up the simulation to run. If the initial state has high entropy, that initialization routine would need to be very long and detailed to describe exactly the location of every electron, neutrino, etc. If the initial state has very low entropy, that initialization routine is very short. If there's a reason to think that a short program is "more probable" than any particular very long program, then that would explain a low-entropy initial condition.

Another explanation is that, if the universe random-walks through all possible configurations, the "past" will still always look lower-entropy than the "future", for any little life-form that occupies that universe, because that life-form's memories will be much more likely to be correlated with the lower-entropy state. (It would have been nice for the article to go into this detail, but it's rarely discussed).

Still another explanation is provided by Many-Worlds interpretation of QM. Again the "big bang" is akin to initializing the wavefunction of the universe to something very simple and compact like a constant function, which as a whole evolves unitarily; the complexity and increasing entropy arises within particular branches of that wavefunction, where an observer requires an ever-longer description length to identify their particular branch.

When I took engineering thermodynamics, the focus was on solving "practical" problems, and I never got a good understanding of what entropy was. The point of the class was not to teach physics, though. The point was to get engineers to solve thermodynamics problems (like heat engines and cycles). The primary thing I remember from that course was looking up things in the tables in the back of the book and converting between Btus and other units.

I only understood entropy after taking a statistic physics course. The course's explanation of entropy is basically the same explanation as the article. The point of that class was entirely different than the engineering course, so the professor spent several lectures going into detail about what entropy is and how it relates to everything else. I never had to do any table lookups in that class too.

Side note: you can easily tell what tradition you were taught in based on the sign convention in the first law of thermodynamics you learned:

https://en.wikipedia.org/wiki/First_law_of_thermodynamics#Si...

The only tiny change I'd like to make is to add a line or two near the end, something along the following lines:

  There's a lot fewer ways to arrange water molecules so that they form an ice cube than there are to arrange them as a liquid. Most arrangements of water molecules look like a liquid, and so that's the likely endpoint even if they start arranged as an ice cube.

  The same is true of more or less any macroscopic object: the thing that we recognize and name ("chair", "table", "pen", "apple") requires the atoms to remain in one of a fairly small set of particular arrangements. Compared to the vast number of other arrangements of the same atoms ("dust"), the ones where the atoms form the "thing" are quite unlikely. Hence over time it's more likely that we'll find the atoms in one of the other ("random", or "dust-like") arrangements than the one we have a name for. The reason things "fall apart" isn't that there's some sort of preference for it - it's that there are vastly more ways for atoms to be in a "fallen apart" state than arranged as a "thing".