How the Big Bang gave us time

A great video (and transcript) about time and, one of my favorite subjects, entropy.

Entropy is how messy, how disorganized, how random a system is. When things are nice and neat and tidy, they are low entropy. When they’re all messy and all over the place, they’re high entropy. And there’s a natural tendency of things in the Universe to go from low entropy to high entropy. This is called the ‘second law of thermodynamics.’ The real question is: Why was the world ever low entropy to begin with? Why was the world lower entropy yesterday than it is today? The explanation is not completely satisfying, to be honest. The explanation is the following: because it was even lower entropy the day before yesterday. And why was the Universe even lower entropy the day before yesterday? Because it was even lower entropy the day before that. And this chain of reasoning goes back 14 billion years to the Big Bang, to the origin of our observable universe; in a hot, dense state, a very low-entropy state, and the Universe has been increasing in entropy ever since. And this is called the ‘Past hypothesis’ by philosophers- David Albert, who’s a philosopher of physics, gave it this name. So now we say, “If you know that the world is made of atoms, and you know what entropy is, in terms of rearranging all those atoms, and you know the past hypothesis- that the entropy of the universe started really low- then you can explain everything that happened after that. There’s a way of talking about human life and entropy, which I think is misguided, which is that we should think about life. You know, literally living, being a biological organism, taking in food and everything, as a fight against increasing entropy. I think that’s wrong. I think that we owe life to the fact that entropy is increasing, because what would it mean if entropy were not increasing? It would mean that nothing is happening. Nothing interesting is taking place. Without entropy increasing, there’s no memory of the past. Without entropy increasing, there’s no causal effect that we have on the future. You’d just be in what we call ‘thermal equilibrium.’ Everything would be the same everywhere. It would be the maximally boring universe. But what we do have as a scientific question is: ‘Why do complicated complex structures come into existence at all?’ It’s clear that they need increasing entropy to exist, because if entropy were already maxed out, there would be no complexity. But that doesn’t mean they have to come come into existence.

Think about a famous example there: The perfume is all in little bottle. It’s in a big room. You open it, and it all floats through the room. The entropy of the perfume increases. But if you think about it, when the perfume is all in the bottle, it’s very simple. Once it’s all spread through the room it’s also very simple. It went from low entropy to high entropy, but it went from simple to simple. It’s the journey from the simple, low-entropy starting point to the simple, high-entropy ending point, that there’s a large space of possibilities where things can be intricate. There’s more perfume here over there. There can be swirls caused by the motion of the wind in the room and so forth. The Universe is just like that. Our Universe started out simple and low entropy. In the future, the stars will die, the black holes will evaporate. It’ll be dark, empty, and again, simple, but high entropy. It’s in between that things like us- complicated, intricate systems that feed off of the increasing entropy of the Universe- can and do come into existence. We don’t know the whole story there. I think it’s a very fun, active, scientific research area: Why did complex structures like living beings come into existence and exactly the way we did? What is the role of information? What is the down-to-Earth chemistry that is going on here? What is the geology that is going on here? Could it happen on other planets? Very interesting questions- but one thing I do know is that if entropy weren’t increasing along the way, none of it would’ve come to pass.

The Big Think: How the Big Bang gave us time, explained by theoretical physicist Sean Carroll