The amazing physics of time

In this Big Think interview, theoretical physicist Sean Carroll discusses the concept of time and the mysteries surrounding its properties. He points out that while we use the word “time” frequently in everyday language, the real puzzles arise when we consider the properties of time, such as the past, present, and future, and the fact that we can affect the future but not the future. . past.

Carroll also discusses the concept of entropy, which is a measure of how disordered or random a system is, and the second law of thermodynamics, which states that there is a natural tendency for things in the Universe to move from a state of low entropy to high entropy, in other words, from least disorganized to most disorganized. He explains that the arrow of time, or the perceived difference between the past and the future, arises due to the influence of the Big Bang and the fact that the Universe began in a low entropy state.

Carroll also touches on the possibility of time travel and the concept of the multiverse.

SEAN CARROLL: Lexicographers will tell you that time, the word “TIME”, is the most commonly used noun in the English language. We can’t go through the day without talking about the weather all the time. I think about how we use time, when we actually talk about it. If you say, “see you at 7 pm,” no one panics. No one says, “Oh my gosh, what are you talking about with these esoteric concepts around 7 pm?” We all know what to do operationally. Time, in a sense, is just a label about different events in the Universe. The Universe happens over and over again in different things that we call moments, and time helps us differentiate between one moment and another. So what time is it, I don’t think that’s the problem. The problem, the real enigmas, arise when we talk about the properties that time has. We have a past, we have a present, we have a future. What is the difference between them? Are we moving through it? We have memories of the past, but we have no memories of the future. Why is that? Where does that asymmetry come from? Why are we all born young? Why do we all age inevitably? Why do we think we can affect the future but not the past? Could we possibly travel back to it? Anyway, there are a lot of questions about the nature of time that are really confusing and a lot of them we don’t know the answer to, but I don’t think what time is is one of them.

One of the most remarkable features of time is that it has a direction, right? That there is a difference between the past and the future. Sometimes we think of this as an intrinsic feature of reality. As if the past has already happened, it’s on the books, the future is at stake. It hasn’t happened yet, and the present is where we live. But then, physics arrives. And what people notice about our best theories in physics is that those theories don’t distinguish between the past and the future. But in our everyday life, nothing is more obvious. It really takes a bit of mental discipline to say, “Well, time could exist without an arrow.” And one way to think about that is that there’s no intrinsic arrow of space, but there’s still space, okay? We live in a three-dimensional world – up, down, left, right, forward, back – at the level of the fundamental laws of physics, there is no special direction in space. And how you perceive that is to imagine that you are an astronaut: you are flying in your little spacesuit. There would be no difference between whichever direction you could look. There is no experiment you can do in physics that points a direction in the universe, but space still exists. Similarly, time would continue to exist even if there was no arrow. But here on Earth, we have an arrow from space. If I pick up a coffee cup and drop it, it will always fall over. Clearly there is a distinction between above and below. No one is tempted to think that this is a fundamental feature of the Universe. It’s not because depression is embedded in the laws of physics. It is because we live in the vicinity of an influential object: the Earth. The arrow of time is exactly the same. We, in our daily lives, perceive an arrow of time because we live after an influential event: the Big Bang.

And that brings us to the realm of the concept of ‘entropy’. Entropy is how disordered, how disorganized, how random a system is. When things are good, clean and tidy, they are low entropy. When they’re all messed up and all over the place, they’re high entropy. And there is a natural tendency for 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 did the world have always low entropy to begin with? Why was the world of lower entropy yesterday than today? The explanation is not entirely satisfactory, to be honest. The explanation is the following: because the entropy was even less the day before yesterday. And why did the Universe have an even lower entropy the day before yesterday? Because the entropy was even lower the day before. And this chain of reasoning goes back 14 billion years to the Big Bang, to the origin of our observable universe; in a hot and dense state, a state of very low entropy, and the Universe has been increasing in entropy ever since. And this is called the ‘past hypothesis’ by philosophers – David Albert, who is 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 previous hypothesis, that the entropy of the universe started out very low, then you can explain everything.” what happened after that. There’s a way of talking about human life and entropy, which I think is wrong, is that we should think about life. You know, literally living, being a biological organism, eating food and everything , like a fight against increasing entropy. I think that’s wrong. I think we owe our lives to entropy increasing, because what would it mean if entropy didn’t increase? It would mean nothing happens. Nothing interesting happens. taking place. Without an increase in entropy, there is no memory of the past. Without an increase in entropy, there is no causal effect that we would have in the future. You would just be in what we call “thermal equilibrium”. Everything would be the same everywhere parts. It would be the universe or extremely boring. But what we do have as a scientific question is: “Why do complex and complicated structures come into being?” It is clear that they need increasing entropy to exist, because if the entropy were already at the maximum, there would be no complexity, but that does not mean that they have to come into existence.

Think of a famous example there: perfume is all in one little bottle. He is in a big room. You open it up and everything floats around 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 everything is spread out around 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 endpoint, that there’s a huge space of possibilities where things can be complex. There is more perfume here than there. There may be eddies caused by the movement of the wind in the room, etc. The Universe is like that. Our Universe started out simple and low entropy. In the future, stars will die, black holes will evaporate. It will be dark, empty, and again simple, but high entropy. It is in the middle that things like us, complicated and intricate systems that feed on the increasing entropy of the Universe, can and do come to exist. We don’t know the whole story there. I think it’s a very fun and active area of ​​scientific inquiry: why did complex structures like living things arise, and exactly the way we did? What is the role of information? What is the realistic chemistry going on here? What is the geology going on here? Could it happen on other planets? Very interesting questions, but one thing I do know is that if the entropy hadn’t increased along the way, none of that would have happened.