Great explanation. What are the top theories for why the universe began in a low entropy state? The mere fact of that seems to contradict our current understanding of entropy. That implies that there is something very fundamental about the universe which we don't understand.
> The mere fact of that seems to contradict our current understanding of entropy
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.
A higher entropy state has a longer description length.
For example, let's say I have a magic electron microscope that can scan and record the exact position and velocity of each particle in some 1-cubic-micron volume, to within Heisenberg uncertainty limits and some finite digitization precision.
If my sample is a 1-cubic-micron volume of flawless monocrystalline silicon at 0 Kelvin, I can 'zip' my recording and transmit that description in a much shorter sentence (in fact, I just sent it to you!) than if my sample is a cubic micron of room-temperature saltwater (whose macrostate I just described, but whose microstate I did not).
If you care about describing the details, you can compress your description better if it's a low-entropy state.
But of course, cosmology is full of more mundane explanations about how the limit of the possible entropy of the universe can grow with time, so a high-entropy state suddenly has a lot of room to increase even further.
There is a fantastic book by Leonard Susskind [0] that tackles this questions. His answer - rooted in string theory - is based on a "landscape of possibilities" and we happen to be in the place where all elementary particles (more specifically the Higgs) had the right energy configuration.
Where did those particles come from? Is there any scientific theory for the beginning of the beginning? A friend who studied physics told me that there's no reason that matter/energy couldn't have just existed forever. I don't find that explanation satisfying. If "entropy always increases" is a universal rule, then there must have been a time where it went from zero to non-zero. If "matter cannot be created or destroyed" is supposed to be a universal rule, then the mere existence of matter seems to contradict the rule, rendering it a non-universal tautology with vast but nonetheless bounded application.
No. In a forever-existing universe, as you go back in time, entropy could asymtotically approach zero (or some other limit) without ever actually reaching it. It doesn't have to be zero at some point in the past.
In fact, if it is zero at some point in the past, then what happened before that? Did it not increase before that point, or was it less than zero before that point?
I get your point about the asymptote, but I should have clarified: I was synthesizing two rules, "entropy always increases" and "matter cannot be created or destroyed"
Beyond that, metaphysically, how does something increase without having an origin? "Everything just always was" seems to conveniently handwave away a very important line of inquiry. This line of inquiry might be uncomfortably close to religious thought, but that shouldn't be a reason to terminate it.
As time goes on, we are able to explain more and more things scientifically, and yet, arbitrarily, we are supposed to be satisfied with "energy and matter were just always there, any other explanation is arbitrarily religious" without a sense of irony. That belief is held onto with religious conviction, sometimes used to dismiss alternate ideas with religious fervor, and backed with a religious rather than scientific standard of evidence.
Here's a premise for a (maybe horrible) book idea: God manipulates the minds of top scientists so that they do not definitively prove his existence, which would ruin the point of his simulation. Each time they get close, he finds an idiosyncratic way to make them forget about it or dismiss the idea. The people who discover God's tricks get brownie points in the afterlife. The people who discover God's tricks but try to publicize them, hence ruining the illusion, get...taken care of. Perhaps you can say the entropy of their body would increase.
Or it wasn't and came into existence with a 'big bang'. So how did it come into existence? There's no scientific or religious answer for it. You'll still be skirting the question with the old 'who made the gods' problem. I know what you're getting at and trying to conflate science with religion. Spoilers here: Scientists don't know these answers and just because we don't know doesn't mean the thousands of denominations of thousands of gods of thousands of religions of thousands of years has a true answer either.
>we are supposed to be satisfied
No one said you should be satisfied by it. That's what's good about science, you're expected to not be satisfied and to survey and question, it's not infallible nor claim omniscience.
>God manipulates the minds of top scientists so that they do not definitively prove his existence, which would ruin the point of his simulation. Each time they get close, he finds an idiosyncratic way to make them forget about it or dismiss the idea.
And of course it's the gods, or all created thereof, who are behind helping or hurting belief in gods. Ask a god believer and it's always god's plan.
>then there must have been a time where it went from zero to non-zero.
Take a step towards the door. On the next step, take half that step. Keep taking half the step as before and you'll never get to the door. Perhaps the multi/universe is so infinite that there's no end or beginning as we know it. You can fill in the blank with deities, anthropomorphic or what have you, but that's where science ends and religion begins.
Cosmology works a lot around that question. There are a few theories that claim that the maximum entropy of the universe is growing with time, so what was high-entropy on the past gets room to keep growing.
